Anodized die cast aluminum alloy and method of making and using same

Abstract

The application discloses anodized pressure-cast aluminum alloy and a preparation method and application thereof, and belongs to the technical field of pressure casting molding. The aluminum alloy contains, in mass percentage, Si≤0.6%, Mg 2%-4%, Zn 4%-6.5%, impurities≤0.3% in total, and the balance is Al; the average grain size of the pressure-cast state is 20-50 mu m (standard deviation≤10 mu m), and there are not more than 1 pore with an area of≥20 mu m 2 in any 1000 mu m 2 square region; the application solves the industry problem that a low-silicon 7-series aluminum alloy is difficult to be completely pressure-cast molded due to a narrow solidification interval and high thermal cracking sensitivity by means of low-silicon component design and microstructure regulation, so that the production of high-strength anodized structural parts is free from the dependence on traditional plates and CNC precise machining, appearance defects such as flow lines and black lines are effectively eliminated while the high strength is maintained, and the appearance quality is close to that of rolled plates.

Description

Technical Field

[0001] This invention belongs to the field of die casting technology, specifically relating to an anodized die-cast aluminum alloy, its preparation method, and its application. Background Technology

[0002] Aluminum alloy die casting, as a highly efficient and near-net-shape metal parts manufacturing process, has been widely used in high-end manufacturing fields such as consumer electronics casings, automotive structural parts, and robotic mechanical components due to its advantages such as high production efficiency and the ability to form complex structures. In these applications, the appearance quality and mechanical properties of the product are often equally important. Especially for high-end consumer electronics products, anodizing has become a key process in aluminum alloy surface treatment. By forming a dense alumina film on the aluminum alloy surface, not only is the product endowed with excellent wear resistance and corrosion resistance, but it can also achieve rich and uniform decorative coloring effects. However, the quality of the anodized film is highly dependent on the purity of the matrix material and the uniformity of its microstructure. Any microscale component segregation, porosity defects, or uneven distribution of the second phase will be significantly amplified during the electrochemical oxidation process, manifesting as visible appearance defects such as color difference, flow lines, black streaks, or glitter.

[0003] 7-series aluminum alloys (Al-Zn-Mg series, such as 7075), as representatives of high-strength aluminum alloys, have tensile strengths exceeding 500 MPa, far surpassing conventional die-cast aluminum alloys. They offer significant advantages in applications requiring both lightweight and high strength. However, 7-series aluminum alloys still face technical bottlenecks in die casting and anodizing. Specifically, to achieve good die-casting fluidity and avoid defects such as cold shuts and undercasting, traditional die-cast aluminum alloys typically require the addition of high silicon content. Silicon significantly reduces melt viscosity and widens the solidification temperature range; however, silicon has a significant negative impact on anodizing performance. During anodizing, the silicon phase is difficult to participate in the formation of the alumina film, leading to a sharp increase in local porosity and noticeable color differences and uneven coloring on the surface. More seriously, silicon reacts chemically with sulfuric acid anodizing solution to generate insoluble silicate precipitates, which not only contaminate the electrolyte and reduce its conductivity but also form difficult-to-remove residues on the oxide film surface, resulting in a dull oxide film color and poor uniformity. Therefore, it is difficult for current 7-series aluminum alloys to simultaneously achieve both die-casting performance and anodizing performance.

[0004] On the other hand, the rapid solidification characteristic unique to die casting further exacerbates the forming difficulty of 7-series aluminum alloys. Under high-speed cooling conditions, alloying elements such as Zn and Mg in the melt are prone to segregation at grain boundaries; at the same time, the coarse dendritic structure and the unevenly distributed strengthening phase will cause inconsistent oxide film growth rates due to local differences in electrochemical activity during the subsequent anodizing process, resulting in macroscopic surface defects.

[0005] Current technologies have not effectively solved the problem of synergistic control of die-casting formability and microstructure uniformity of 7-series aluminum alloys under low-silicon conditions. Therefore, there is an urgent need to develop a 7-series low-silicon die-casting aluminum alloy that combines high mechanical properties, good die-casting formability, and excellent anodizing performance.

[0006] It should be noted that this part of the present invention only provides background technology related to the present invention, and does not necessarily constitute prior art or known technology. Summary of the Invention

[0007] This invention provides an anodized die-cast aluminum alloy and its preparation method, which at least solves the technical problem that existing 7-series aluminum alloys cannot simultaneously possess high mechanical properties, good die-casting formability, and excellent anodizing performance. Furthermore, it also solves the technical problem that high-strength appearance parts made of low-silicon 7-series aluminum alloys have long relied on sheet metal machining.

[0008] To achieve the above objectives, in a first aspect, the present invention provides an anodized die-cast aluminum alloy, wherein the aluminum alloy comprises, by mass percentage, the following elements: not more than 0.6% Si, 2% to 4% Mg, 4% to 6.5% Zn, the total content of unavoidable impurities not more than 0.3%, and the balance being Al; Before heat treatment, the average grain size of aluminum alloy is 20μm~50μm, and the standard deviation of the grain size is no greater than 10μm; at any 1000μm in aluminum alloy 2 Within the observation area, the area is not less than 20 μm 2 The number of pores is no more than 1.

[0009] Preferably, before heat treatment, the aluminum alloy is subjected to heat treatment at any 1000μm depth. 2 Within the observation area, there is no area not less than 20 μm 2 pores.

[0010] Preferably, before heat treatment, the solid solution concentrations of Zn and Mg at the grain boundaries of the aluminum alloy vary in a banded pattern with a width of more than 0.05 mm along the width direction of the alloy, and the concentration difference between adjacent bands is less than 0.20% by mass.

[0011] Preferably, after heat treatment, the aluminum alloy contains two MgZn2 intermetallic compounds, D and E, wherein the equivalent circle diameter of intermetallic compound D is 1 μm to 3 μm and the equivalent circle diameter of intermetallic compound E is 0.1 μm to 1 μm; in any observation region of the aluminum alloy, the ratio (d / e) of the area ratio d of intermetallic compound D to the area ratio e of intermetallic compound E is 0.1 to 0.5.

[0012] Preferably, in any observation area of ​​the aluminum alloy, the sum of the area ratio d of intermetallic compound D and the area ratio e of intermetallic compound E is not less than 2.5%.

[0013] Preferably, the intermetallic compound D is dispersed in particulate form, and the intermetallic compound E is precipitated in fine needle-like forms.

[0014] Preferably, after anodizing, the aluminum alloy has an anodized film with a thickness of 5μm to 20μm on its surface, the area ratio of Al-Zn-Mg particles in the anodized film is 5% to 15%, the arithmetic mean roughness Ra of the anodized film is 0.1μm to 0.5μm, and the maximum height roughness Rz is 1μm to 5μm.

[0015] Preferably, the color uniformity of the anodic oxide film satisfies ΔE < 1.0.

[0016] Preferably, when observed under a spotlight of 500 lux to 1000 lux, the incidence of surface defects in the anodic oxide film is less than 0.1%.

[0017] Preferably, the aluminum alloy comprises no more than 0.4% Si by mass percentage.

[0018] Preferably, the aluminum alloy comprises no more than 0.1% Si by mass percentage.

[0019] Preferably, the aluminum alloy further comprises 2.5% to 3.5% Mg and 5% to 6% Zn by weight percentage.

[0020] Preferably, the solidification point of the aluminum alloy is in the range of 475℃ to 500℃.

[0021] Preferably, the heat treatment includes T4, T5, T6 or T651 heat treatment.

[0022] In a second aspect, the present invention provides a method for preparing the anodized die-cast aluminum alloy of the first aspect, comprising the following steps: Step S102: After melting the metal alloy to form a molten liquid, the molten liquid includes the following elements by mass percentage: no more than 0.6% Si, 2%~4% Mg, 4%~6.5% Zn, the total content of unavoidable impurities is no more than 0.3%, and the balance is Al. The molten liquid is kept at a first temperature, and the mold is kept at a second temperature lower than the first temperature. The temperature difference between the first and second temperatures is 300℃~350℃. Step S104: The molten metal is injected into the mold cavity through the inlet of the mold cavity. After the mold cavity is filled, the pressure is maintained and the mold is opened to obtain the die-cast aluminum alloy. The speed at which the molten liquid is injected into the mold cavity shall not exceed 2 m / s, and the moving speed of the molten liquid surface relative to the mold cavity shall not exceed 0.5 m / s; The cross-sectional area of ​​the flow channel varies within ±10% between the position where the liquid inlet contacts the mold cavity and the position where it is far away from the mold cavity; The ratio between the overflow rate of the molten filling and the volume of the mold cavity shall not be less than 0.1; Step S106: Heat-treat and anodize the die-cast aluminum alloy to obtain anodized die-cast aluminum alloy.

[0023] Preferably, the ratio between the overflow of the melt filling and the volume of the mold cavity is in the range of 0.1 to 0.5.

[0024] Preferably, the overflow rate of the molten metal filling and the volume of the mold cavity satisfy the following relationship:

[0025] In the formula, Overflow volume for molten filling; This refers to the volume of the mold cavity; This is the first proportionality constant, and its value ranges from 0.1 to 0.2; The temperature difference between the first temperature and the second temperature. The second temperature; The speed at which molten liquid is injected into the mold cavity.

[0026] Preferably, the molten filling overflow is discharged through an overflow port on the mold cavity; there are multiple overflow ports, which are arranged circumferentially on the side wall of the mold cavity, and the area of ​​the overflow ports accounts for 15% to 25% of the surface area of ​​the side wall of the mold cavity.

[0027] Preferably, the heat treatment is T6 heat treatment, and the specific steps include: solution treatment at 460℃~480℃ for 1 hour to 4 hours, water quenching, and then aging treatment at 110℃~130℃ for 12 hours to 24 hours.

[0028] Preferably, the anodizing treatment includes: in a sulfuric acid electrolyte with a concentration of 150 g / L to 200 g / L, at a temperature of 18°C ​​to 22°C, at an anode flow rate of 1.0 A / dm³. 2 ~1.5A / dm 2 DC anodizing is performed at a current density of 20 to 40 minutes.

[0029] Thirdly, the present invention provides an aluminum alloy die casting made of an anodized die casting aluminum alloy of the first aspect, or an anodized die casting aluminum alloy prepared by the preparation method of the second aspect.

[0030] Fourthly, the present invention provides an application of the aluminum alloy die-casting part as described in the third aspect in the manufacture of structural parts for electronic devices, transportation vehicles, robots, medical devices, industrial equipment, or new energy equipment.

[0031] The beneficial effects of this invention are as follows: The anodized die-cast aluminum alloy provided by this invention, through the synergistic effect of low silicon content design and precise microstructure control, successfully enables 7-series aluminum alloys to simultaneously possess high mechanical properties, good die-casting formability, and excellent anodizing performance. Traditional 7-series aluminum alloys (such as 7075) are difficult to use directly for die casting due to their narrow solidification temperature range and high sensitivity to hot cracking. If the fluidity is improved by increasing the silicon content, the silicon element will seriously interfere with the formation process of the alumina film. The silicon phase is difficult to participate in the construction of the anodic oxide film during anodizing, resulting in a significant increase in the porosity of the film layer. At the same time, silicon reacts with sulfuric acid electrolyte to form silicate precipitates, which reduce the conductivity of the electrolyte and contaminate the surface of the film layer, ultimately manifesting as a dull oxide film color, significant color difference, and uneven color defects. This invention strictly controls the silicon content, eliminating the destructive effect of silicon on the quality of the anodized film from the compositional perspective. This enables 7-series aluminum alloys to maintain high strength while achieving the feasibility of high-quality anodizing of die-cast aluminum alloys. After anodizing, the color difference is extremely low, and the surface is free of macroscopic defects such as watermarks, black lines, and glitter. The appearance quality reaches or even approaches the level of rolled sheet metal.

[0032] Furthermore, this invention regulates the solidification thermodynamic behavior of the alloy through a synergistic design of low silicon content and magnesium-zinc ratio. Under low silicon conditions, controlling the solidification point range of the alloy effectively widens the mushy region and significantly reduces the tendency for hot cracking. Simultaneously, combined with strategies such as temperature difference control between the molten metal and the mold, slow filling, and large overflow in the die-casting process, the growth rate of the edge cooling layer can be regulated, avoiding dendrite coarsening and element segregation caused by rapid cooling in traditional die casting. The resulting die-cast microstructure exhibits highly uniform fine-grained characteristics, fundamentally suppressing the formation of macroscopic defects. Crucially, this microscale compositional uniformity effectively prevents appearance defects caused by localized differences in electrochemical activity during anodizing.

[0033] Furthermore, after heat treatment, the MgZn2 strengthening phase precipitated in the alloy of this invention exhibits a unique dual-morphology distribution. This specific morphology and distribution of the strengthening phase structure provides excellent mechanical properties to the alloy material, while its fine and dispersed distribution avoids the formation of local current concentration points during anodizing. During anodizing, the moderately retained Al-Zn-Mg particles in the oxide film form a good interfacial bond with the matrix alumina, which not only does not disrupt the continuity of the film but also optimizes the surface optical properties by controlling the light scattering behavior, ultimately obtaining a high-quality oxide film without striped texture or scintillation defects, meeting the stringent requirements of high-end consumer electronics products for visible defects.

[0034] To achieve a balance between mechanical properties and aesthetic quality in 7-series aluminum alloys, this invention provides a preferred process coordination control system. This system enables the stable production of 7-series aluminum alloy structural parts suitable for high-end anodizing using the highly efficient, near-net-shape die-casting process. It overcomes the industry challenge of completing the die-casting process for low-silicon 7-series aluminum alloys due to their narrow solidification range and high susceptibility to hot cracking, achieving near-net-shape forming of high-strength, low-silicon die-cast aluminum alloys. This technical solution breaks the traditional manufacturing convention that high-end anodized structural parts must rely on CNC precision machining of rolled or forged sheets. Parts formed directly using this die-casting process can meet stringent appearance and performance requirements after anodizing, resolving the technical bottleneck of the mutual constraint between the die-casting formability and anodizing performance of 7-series aluminum alloys. This technological breakthrough provides a cost-effective and performance-advantageous manufacturing solution for lightweight, high-strength structural parts such as mobile phone frames and laptop casings, potentially replacing some forged or rolled 7-series aluminum alloy parts that require CNC precision machining. It significantly reduces material and processing costs while maintaining a high-end product feel, possessing significant industrial value and market prospects. Attached Figure Description

[0035] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 The image shows the metallographic structure of the aluminum alloy sample after T6 heat treatment in Example 1 of this invention. Figure 2 The image shows the metallographic structure of the aluminum alloy in the die-cast state in Embodiment 1 of the present invention; Figure 3 The image shows the metallographic structure of the aluminum alloy sample after T6 heat treatment in Example 8 of this invention. Figure 4The image shows the metallographic structure of the aluminum alloy in the die-cast state in Example 8 of this invention. Figure 5 This is a photograph of the appearance of an aluminum alloy die-casting part made of aluminum alloy after anodizing, as shown in Embodiment 1 of the present invention. Figure 6 This is a photograph of the appearance of the aluminum alloy die casting made of aluminum alloy in Comparative Example 2 of the present invention after anodizing. Figure 7 This is a photograph of the appearance of the aluminum alloy die casting made of aluminum alloy, Comparative Example 8 of the present invention, after anodizing. Figure 8 This is a schematic diagram of the gateless structure provided in Embodiment 1 of the present invention; Figure 9 This is a schematic diagram of the narrow gate structure provided in Comparative Example 4 of the present invention; Figure 10 This is a three-dimensional structural diagram of the mold provided in Embodiment 1 of the present invention; Figure 11 This is a side view of the mold provided in Embodiment 1 of the present invention.

[0037] Explanation of reference numerals in the attached figures: 10. Mold cavity; 20. Liquid inlet; 30. Overflow outlet; 40. Injection mechanism. Detailed Implementation

[0038] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0039] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. The terms "optional" and "discretionary" mean that they may or may not be included (or may or may not be present).

[0040] In a first aspect, the present invention provides an anodized die-cast aluminum alloy, wherein the aluminum alloy comprises, by mass percentage, not more than 0.6% Si, which can be 0.6%, 0.55%, 0.5%, 0.45%, 0.4%, 0.35%, 0.3%, 0.25%, 0.2%, 0.15%, 0.1%, 0.05%, 0%, and any value between therewith. Preferably, the aluminum alloy comprises not more than 0.4% Si by mass percentage. More preferably, the aluminum alloy comprises not more than 0.1% Si by mass percentage.

[0041] The aluminum alloy contains 2% to 4% Mg, which can be 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, or any value between them; and 4% to 6.5% Zn, which can be 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, or any value between them. Preferably, the aluminum alloy further comprises 2.5% to 3.5% Mg and 5% to 6% Zn by mass percentage.

[0042] The total content of unavoidable impurities shall not exceed 0.3%, and may be 0.3%, 0.25%, 0.2%, 0.15%, 0.1%, 0.05%, lower, or any value between them, with the balance being Al.

[0043] The aforementioned aluminum alloy composition design is the foundation for resolving the contradiction between the die-casting and anodizing performance of 7-series aluminum alloys in this invention. In the composition system of this invention, Si is considered an element that requires strict control. In traditional die-cast aluminum alloys, a relatively high content of silicon (e.g., 4% to 12%) is typically added to significantly improve the alloy's fluidity, reduce hot cracking tendency, and widen the solidification range, thereby achieving good filling ability and formability. However, high silicon content has a serious negative impact on the anodizing performance of 7-series aluminum alloys. During the anodizing process, silicon cannot effectively participate in the formation of the alumina film. The silicon phase (such as primary silicon or eutectic silicon) exists as an electrochemically inert phase in the matrix, hindering the continuous and uniform formation of alumina at the oxide film growth interface, leading to a loose structure and abnormally increased porosity in this region of the oxide film. More importantly, silicon undergoes a chemical reaction in the sulfuric acid anodizing electrolyte, generating insoluble silicate colloids or precipitates. These silicate products can contaminate the electrolyte, reducing its effective conductivity and stability. Furthermore, they can adhere to the surface or internal pores of the growing oxide film, leading to decreased gloss, dull color, and the appearance of white spots or a cloudy appearance. Simultaneously, the difference in dissolution rates between the silicon phase and the aluminum substrate during anodizing causes uneven current density distribution at the microscale, which is one of the root causes of macroscopic defects such as color differences (uneven surfaces), flow lines, and black streaks. Therefore, this invention strictly limits the Si content to an extremely low level, aiming to eliminate or minimize the destructive impact of silicon on the quality of the anodized film from the source, providing the prerequisite for obtaining a uniform, high-gloss, and color-difference-free anodized surface.

[0044] It should be noted that the Si element in this invention is not actively added, but is an unavoidable impurity brought in by the raw materials. The lower its content, the better it is for improving the quality of anodizing. In the most preferred case, the content of the Si element can be 0.

[0045] In the composition system provided by this invention, Mg and Zn mainly play the roles of solid solution strengthening and precipitation strengthening, and their content ratio directly affects the solidification behavior of the alloy and the precipitation kinetics of the strengthening phase. This invention controls the Mg and Zn content within a suitable range, which helps to form sufficient MgZn2 strengthening phase, thereby improving the mechanical properties of the aluminum alloy. Simultaneously, the synergistic low-silicon design can effectively adjust the crystallization temperature range of the alloy. This synergistic effect is beneficial for obtaining a fine equiaxed crystal structure during die casting solidification, avoiding the coarsening of secondary phases caused by solute element segregation, thus ensuring a uniform distribution of surface chemical activity during subsequent anodizing treatment. If the Mg content is too low, the number of MgZn2 strengthening phases that can be formed in the alloy will be insufficient, making it difficult to achieve the required tensile strength and failing to meet the strength requirements of structural components. At the same time, an excessively low Mg content will also result in an excessively narrow solidification temperature range and insufficient width of the pasty region, easily leading to defects such as cold shuts and undercasting during die casting. If the Zn content is too low, it will also lead to a decrease in tensile strength; at the same time, it will increase the liquidus temperature of the alloy, increase the temperature difference with the mold, intensify the edge chilling effect, and easily form coarse dendritic structures.

[0046] Unavoidable impurities mainly refer to harmful elements such as Fe and Cu. If the Fe content is too high, it easily forms coarse needle-like or blocky phases. These brittle phases not only impair the alloy's plasticity and toughness but also become localized corrosion initiation points during anodizing due to their large potential difference with the matrix, leading to localized peeling or color variations in the oxide film. While Cu can improve strength, excessive Cu content can also reduce the alloy's corrosion resistance and form copper-rich areas in the anodized film, affecting color uniformity. This invention, by controlling the total amount of impurities, helps to obtain a higher quality anodized film while ensuring mechanical properties.

[0047] It should be noted that the anodized die-cast aluminum alloy provided by this invention can be stably subjected to standard strengthening heat treatments, including T4 (natural aging after solution treatment), T5 (artificial aging after casting cooling), T6 (complete artificial aging after solution treatment), or T651 (stress relief and artificial aging after solution treatment), all of which can yield anodized die-cast aluminum alloys that meet the requirements of this invention. However, more preferably, the anodized die-cast aluminum alloy of this invention can achieve relatively superior microstructure and performance results after T6 heat treatment. Therefore, in the following specific embodiments, the heat treatment of this invention is illustrated using T6 heat treatment.

[0048] Before undergoing T6 heat treatment, the average grain size of the aluminum alloy is 20μm to 50μm, which can be 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm and any value between them; the standard deviation of the grain size is no greater than 10μm, which can be 10μm, 9μm, 8μm, 7μm, 6μm, 5μm, lower and any value between them.

[0049] Understandably, controlling the size and standard deviation of the die-cast grains is the microstructure basis for resolving the contradiction between the die-casting and anodizing performance of 7-series aluminum alloys in this invention.

[0050] Understandably, the “die-cast state” specifically refers to the original state of the alloy after it has been demolded from the mold without any subsequent heat treatment, and its microstructure directly reflects the thermodynamic and kinetic behavior of the melt during solidification.

[0051] Traditional aluminum alloys, due to their narrow solidification temperature range and high sensitivity to hot cracking, are prone to forming two extreme microstructures under rapid cooling conditions during die casting. One is the formation of abnormally fine grains in the edge regions due to the chilling effect. In this region, the segregation of solute elements (Zn, Mg) is intensified, and the concentration gradient at grain boundaries is abnormally increased. During anodizing, the difference in local electrochemical activity leads to excessive fluctuations in the oxide film growth rate, manifesting as flow-line defects. The second is the formation of coarse dendritic structures in the internal regions due to insufficient feeding. The difference in oxidation rate between different orientations of these coarse grains is significantly amplified, resulting in an excessively large standard deviation of oxide film thickness, macroscopically appearing as visible scintillation or streaking defects. When the grain size is too small, although the increased number of grain boundaries is beneficial for element diffusion, the excessively high nucleation rate can also lead to an excessively rapid advancement of the solidification front, exacerbating the microscopic segregation of Zn and Mg elements at grain boundaries, thus resulting in excessive color difference after anodizing. When the grain size is too large, the reduced total grain boundary area weakens the resistance to dislocation movement, thus decreasing tensile strength. Simultaneously, coarse grains, due to differences in crystal orientation during anodizing, can lead to increased dispersion in the oxide film growth rate and excessive film thickness fluctuations, manifesting as locally excessively thick or thin oxide films and the appearance of macroscopic discoloration on the surface. This invention controls the average grain size within a suitable range, ensuring the physical homogeneity of the surface layer and effectively balancing the performance contradictions between aluminum alloy die casting and anodizing.

[0052] Furthermore, this invention rationally controls the standard deviation of grain size, ensuring a high degree of concentration in grain size distribution and avoiding abrupt changes in local properties caused by the mixing of coarse and fine grains. Standard deviation is a key statistical parameter characterizing the degree of grain size dispersion; its physical meaning is the root mean square of the deviation of each grain size from the average value. When the standard deviation exceeds 10 μm, both excessively small fine grains and excessively large coarse grains inevitably exist simultaneously in the microstructure. During anodizing, the fine-grained region exhibits a rapid oxide film growth rate due to high-density grain boundaries, resulting in a thicker film with lower porosity; conversely, the coarse-grained region exhibits a thinner film with higher porosity. A significant film thickness gradient forms at the interface between the two, producing a significant difference in diffuse reflection under illumination, manifesting as visible banded texture. This invention's rational control of a uniform grain size distribution means stable thermodynamic conditions during solidification, low component segregation, and facilitates subsequent uniform precipitate distribution and consistent anodizing.

[0053] Optionally, the testing of grain size and its distribution is performed according to ASTM E112 standard. The specific method is as follows: cut a die-cast sample, mechanically grind and polish it, and then etch it with Keller's reagent. Observe the microstructure at least 5 different fields of view using a metallographic microscope at 500x magnification or a field emission scanning electron microscope. Automatically identify grain boundaries using image analysis software, calculate the equivalent circle diameter of each grain (i.e., the diameter of a circle with the same projected area as the grain), and calculate the arithmetic mean of all measured grains to obtain the average grain size. At the same time, calculate the standard deviation to characterize the dispersion of the size distribution.

[0054] In any 1000μm of aluminum alloy 2 Within the observation area, the area is not less than 20 μm 2 The number of pores is no more than 1, and can be 0 or 1. More preferably, before the aluminum alloy undergoes T6 heat treatment, any 1000μm of the aluminum alloy... 2 Within the observation area, there is no area not less than 20 μm 2 pores.

[0055] First, it is necessary to clarify the definition and detection method of "porosity" as described in this invention. The "porosity" described in this invention specifically refers to closed void defects formed inside die-cast aluminum alloys due to gas entrapment or solidification shrinkage, including gas pores (formed by gas entrapment in the melt, with an approximately spherical morphology and a major axis / minor axis ratio <1.5) and shrinkage cavities (formed by insufficient feeding, with an irregular morphology and a major axis / minor axis ratio ≥1.5). The pore area is determined by observing the polished and etched cross-section using a metallographic microscope (500x magnification) or scanning electron microscope. Image analysis software is used to automatically identify the pore contours and calculate their projected area. The area is not less than 20 μm. 2 Porosity is a macroscopic defect with significant harmful effects. It has a strong interference effect on the local current density distribution during the anodizing process, which may cause the oxide film growth rate to deviate greatly from the substrate region.

[0056] Area ≥ 20μm 2 The pore density is controlled to be ≤1 pore / 1000μm 2 The pore density is a key quantitative indicator for achieving high-quality anodized surfaces in this invention. When the pore density exceeds 1 pore / 1000 μm... 2During anodizing, the incidence of surface defects (dark spots, bright spots) increases dramatically. Regardless of the underlying principle, the mechanisms include: First, the curvature effect at the pore edges creates a localized electric field concentration, resulting in a higher current density than in flat substrate areas. This leads to excessively rapid oxide film growth, a loose structure, and a film thickness exceeding the normal range, manifesting as visible dark spots. Second, electrolyte easily accumulates inside the pores, and residual sulfuric acid slowly seeps out after oxidation, corroding the oxide film and forming corrosion spots. Third, pores act as stress concentration sources, easily inducing microcracks under the stress of oxide film growth, disrupting the film continuity. This invention, by strictly controlling the presence of pores, effectively avoids these problems.

[0057] A further preferred embodiment of the present invention requires complete elimination of an area ≥20 μm. 2 Porosity (i.e., 0 pores / 1000μm) 2 This method can further reduce the incidence of surface defects and color difference ΔE after anodizing, making it particularly suitable for consumer electronics products such as mobile phone frames and tablet computer back panels that have extremely high requirements for surface quality.

[0058] Preferably, before the aluminum alloy undergoes T6 heat treatment, the Zn and Mg solid solution concentrations at the grain boundaries of the aluminum alloy exhibit a banded variation along the alloy width direction with a width of 0.05 mm or more (which can be 0.05 mm, 0.06 mm, 0.07 mm, 0.08 mm, 0.09 mm, 0.10 mm, or any value between them), and the concentration difference between adjacent bands is less than 0.20% by mass, which can be 0.20%, 0.18%, 0.15%, 0.10%, 0.05%, or even lower, or any value between them.

[0059] This invention quantifies the uniformity of microstructure composition in die-cast 7-series aluminum alloys. The aim is to control the segregation of solute elements inherent in the rapid solidification process of die casting into a moderate and broad distribution pattern, rather than sharp and localized grain boundary enrichment, by defining the spatial structure and gradient intensity of the concentration distribution.

[0060] It should be clarified that the "banded variation" mentioned in this invention specifically refers to a macroscopic composition band formed by the arrangement and connection of multiple solute segregation regions at the grain boundaries along the width direction of the alloy. The macroscopic composition band has a width of more than 0.05 mm in the width direction of the alloy, and the solid solution concentration of Zn or Mg shows a continuous and gentle gradient change. The absolute difference between adjacent bands does not exceed 0.20 mass.

[0061] It should be noted that the above phenomena can be determined by line scanning analysis using electron probe microanalysis (EPMA) or a scanning electron microscope equipped with energy dispersive spectroscopy (EDS): On the polished and etched metallographic sample, a line scan with a length of not less than 0.3 mm is selected in a direction parallel to the width of the alloy and spanning multiple grains, with a step size set to 5 μm, to obtain the concentration distribution curves of Zn and Mg elements; a continuous interval with a concentration change of less than 0.03% by mass is defined as a "band", the concentration jump value between adjacent bands is the concentration difference, and the projection length of the concentration change region in space is the band width.

[0062] In traditional aluminum alloy die casting, due to the excessively rapid cooling rate and narrow solidification range, solute elements such as Zn and Mg are largely repelled by the rapidly advancing solidification front to the grain boundary region where they solidify last, forming sharp enrichment peaks with narrow widths. These enriched grain boundaries are arranged perpendicular to the filling flow direction (i.e., the alloy width direction), forming macroscopically visible segregation bands with significant local concentration variations. This steep compositional gradient can induce severe electrochemical inhomogeneities during anodizing. Specifically, the enriched regions exhibit enhanced electrochemical activity due to their high Zn and Mg content, resulting in a faster oxide film growth rate than the intragranular regions, leading to increased film thickness differences. Macroscopically, this manifests as flow lines or banded color difference defects distributed along the alloy width direction. This invention significantly mitigates the concentration gradient by controlling the width of the macroscopic segregation bands to above a threshold, reducing the difference in electrochemical activity between grain boundaries and intragranular regions to an acceptable range, and controlling the dispersion of the anodized film growth rate within a reasonable range, fundamentally eliminating the conditions for color difference formation.

[0063] Furthermore, the concentration difference directly determines the degree of difference in the growth rate of the oxide film in different regions. Therefore, this invention rationally controls the threshold of the concentration difference between adjacent bands based on the growth kinetics of anodic oxide films. When the concentration difference is too high, the difference in oxidation current density between the high-concentration region and the low-concentration region is too large, resulting in a significant increase in film thickness fluctuation. This film thickness difference will produce obvious interference color differences under a specific illumination angle, which macroscopically manifests as a flowing water ripple defect. It is particularly important to note that the band width and concentration difference have a synergistic effect. Even if the band width is sufficient, if the concentration difference is too large, the oxide film may still cause color differences due to abrupt changes in local electrochemical activity. Conversely, even if the concentration difference is small, if the band is too narrow, the enrichment region will be too localized, which may also form obvious electrochemical boundaries. This invention simultaneously controls both within a reasonable range, ensuring a high degree of uniformity in the microscopic composition field.

[0064] This invention constructs a specific dual-morphological strengthening phase distribution structure by controlling the size, quantity, and relative proportion of the MgZn2 intermetallic compounds precipitated after T6 heat treatment in multiple dimensions, which is beneficial for achieving high strength and excellent anodized appearance in 7-series die-cast aluminum alloys.

[0065] It should be noted that the "equivalent circle diameter" is a commonly used parameter in materials science to characterize the size of irregularly shaped particles. It is defined as the diameter of a circle with the same projected area as the target particle. This parameter avoids measurement bias caused by irregular shapes and can more scientifically characterize the spatial occupancy scale of precipitated phases. This invention clearly distinguishes intermetallic compounds into two categories based on their projected size on a two-dimensional metallographic observation surface.

[0066] It should be noted that the area ratio mentioned in this invention refers to the percentage of the area occupied by the target phase on the metallographic observation surface to the total observation area, which is an important two-dimensional parameter characterizing its volume fraction.

[0067] Preferably, after T6 heat treatment, the aluminum alloy contains two MgZn2 intermetallic compounds, D and E.

[0068] Preferably, the equivalent circle diameter of intermetallic compound D is 1 μm to 3 μm, and can be 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or any value between them. It should be noted that the lower limit of the equivalent circle diameter of intermetallic compound D includes 1 μm, that is, MgZn2 strengthening phase with an equivalent circle diameter of 1 μm belongs to intermetallic compound D.

[0069] This invention controls the equivalent circle diameter of intermetallic compound D within the range of 1 μm to 3 μm, which is beneficial for balancing mechanical properties and the appearance quality of anodizing. The source of intermetallic compound D may include original coarse second-phase particles that were not completely dissolved during solution treatment; or precipitates that preferentially nucleate and grow rapidly at high-energy sites (such as grain boundaries, subgrain boundaries, and dislocation entanglement regions) in the early stages of aging.

[0070] Intermetallic compounds D, as a relatively large-scale hard second phase, can effectively disrupt the planarity of dislocation slip, forcing dislocations to circumvent or form dislocation loops around them. This not only contributes to strength but, more importantly, increases strain hardening capacity, which is beneficial for improving the uniform plastic deformation capacity of the alloy. Furthermore, these intermetallic compounds D distributed near grain boundaries or subgrain boundaries can mechanically pin them to the grain boundaries, inhibiting abnormal grain growth at high temperatures (such as solution treatment or subsequent service temperatures), thereby stabilizing the fine-grained structure. In addition, during crack propagation, intermetallic compounds D can serve as sites for crack tip passivation or crack deflection, consuming additional fracture energy and positively contributing to improving the fracture toughness of the material.

[0071] When the size is less than 1 μm, the phase falls within the size range of intermetallic compound E, making it difficult to distinguish effectively using conventional metallographic methods. Furthermore, its strengthening mechanism shifts from the Orowan bypass mechanism to a cutting mechanism, significantly reducing its contribution to strength. When the size exceeds 3 μm, intermetallic compound D may have the following negative effects during anodic oxidation: First, the large electrochemical potential difference between the coarse MgZn2 phase and the aluminum matrix leads to the formation of significant microgalvanic corrosion cells in the sulfuric acid electrolyte, resulting in a reduced oxide film growth rate and increased film thickness variation, forming macroscopic surface defects around the precipitated phase. Second, under the stress of oxide film growth, the excessively large precipitated phase is prone to interfacial debonding from the matrix, forming microcrack sources, reducing film adhesion, and may also become the origin of internal cracks, damaging the mechanical properties and fatigue life of the matrix.

[0072] Preferably, the equivalent circle diameter of the intermetallic compound E is 0.1 μm to 1 μm, and can be 0.1 μm, 0.2 μm, 0.4 μm, 0.6 μm, 0.8 μm, 1 μm, or any value between them. It should be noted that the upper limit of the equivalent circle diameter of the intermetallic compound E does not include 1 μm.

[0073] The equivalent circle diameter of intermetallic compound E is limited to 0.1 μm to 1 μm, which is beneficial for achieving a synergistic improvement in high strength and high appearance quality at the microscopic level. This size range covers the most effective precipitate size for precipitation strengthening of aluminum alloys, such as the size of the transition phase from GP zone to η' or the stable η phase (MgZn2). These phases are the result of uniform nucleation within the matrix and diffusion-controlled growth, which is beneficial for improving precipitation strengthening effect.

[0074] Intermetallic compound E, as the main reinforcing phase, exhibits unique advantages in the anodizing process due to its small size. On the one hand, its size is much smaller than the wavelength of visible light, resulting in uniform and diffuse light scattering without causing macroscopic optical inhomogeneities. On the other hand, its small size allows intermetallic compound E to be partially retained and uniformly embedded in the alumina matrix during oxidation, forming Al-Zn-Mg particles. These particles have good bonding with the alumina interface, and by regulating light scattering behavior, they optimize the surface optical properties, giving the oxide film a soft and uniform luster.

[0075] When the equivalent circle diameter of intermetallic compound E is <0.1 μm, dislocations can directly pass through the precipitated phase via a shearing mechanism, significantly reducing the strengthening efficiency. Furthermore, this size is close to the GP region scale, making it highly susceptible to complete dissolution during anodic oxidation and unable to provide beneficial control over the oxide film structure. Setting the upper limit of intermetallic compound E's size to less than 1 μm ensures good dispersibility and clearly distinguishes it from the lower limit of intermetallic compound D, ensuring a clear definition of its dual-morphological distribution.

[0076] Preferably, the ratio (d / e) of the area ratio d of intermetallic compound D to the area ratio e of intermetallic compound E is 0.1 to 0.5, and can be 0.1, 0.2, 0.3, 0.4, 0.5, or any value between them. When the ratio (d / e) of the area ratio d of intermetallic compound D to the area ratio e of intermetallic compound E is 0.1 to 0.5, the aluminum alloy helps to balance mechanical properties and anodizing performance.

[0077] When d / e < 0.1, it means that the proportion of intermetallic compound D (coarse phase) to intermetallic compound E (fine phase) is extremely low, approaching a single-morphology fine and dispersed structure. Although this structure is very favorable for anodizing, as mentioned earlier, it may lose some of the beneficial effects of the coarse phase, and under certain heat treatment conditions, it may indicate the onset of over-aging, with the fine phase beginning to coarsen and merge. When d / e > 0.5, it means that the proportion of coarse phase is too high. This not only impairs anodizing performance as mentioned earlier, but is also uneconomical from the perspective of strengthening efficiency, because the strength increase per unit volume of coarse phase is far lower than that of fine phase. More importantly, an excessively high d / e value is often associated with uneven precipitation, which may indicate insufficient solid solution or improper aging process, leading to excessive precipitation of the precipitate at grain boundaries and insufficient precipitation within the grains. This unevenness itself is a cause of anodizing color difference.

[0078] By controlling the d / e ratio within a suitable range of 0.1 to 0.5, the microstructure is predominantly composed of fine, dispersed intermetallic compounds E, which provide core strength, thus facilitating the achievement of high strength in aluminum alloys. Simultaneously, it allows for the presence of a small, controllable amount of coarse intermetallic compounds D, which can act as sacrificial or buffering phases. During anodizing, although these slightly larger phases do react, their small number and large spacing prevent them from forming macroscopically visible defects.

[0079] This specially designed dual-morphology precipitate structure exhibits unique advantages during the anodizing process. The tiny intermetallic compounds E, due to their size being close to or smaller than the initial barrier layer thickness of the oxide film, can be gradually encapsulated, assimilated, or uniformly dissolved by the growing oxide film. Their disruption to the continuity of the oxide film is negligible, and their uniform distribution results in uniform light scattering, contributing to a consistent surface gloss. While the controlled amount of intermetallic compounds D is large in size, their scarcity means that, although there may be slight variations in local thickness or structure, they cannot form discernible color differences or patterns on a macroscopic optical scale. Furthermore, this precipitate distribution helps alleviate internal stress caused by volume expansion during anodizing, reducing the formation of microcracks and resulting in a denser and more durable oxide film.

[0080] Preferably, in any observation area of ​​the aluminum alloy, the sum of the area ratios d of intermetallic compound D and e of intermetallic compound E is not less than 2.5%, and can be 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3.0%, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4.0%, higher, and any value between them; more preferably, the sum of the area ratios d of intermetallic compound D and e of intermetallic compound E is not less than 3.0%.

[0081] Furthermore, the sum of the area ratios is no greater than 4.2%.

[0082] It should be noted that the "sum of area ratios" mentioned in this invention specifically refers to the percentage of the total area of ​​the projected areas of intermetallic compound D and intermetallic compound E on the metallographic observation surface. This two-dimensional parameter can effectively characterize the total volume fraction of the MgZn2 strengthening phase precipitated after heat treatment and is a microstructure indicator that relates alloy composition, heat treatment process and final mechanical properties.

[0083] This invention controls the sum of the area ratios to be no less than 2.5% to ensure sufficient precipitation of strengthening phases to improve mechanical properties. For 7-series Al-Zn-Mg alloys, their yield strength mainly comes from the precipitation strengthening contribution of fine, dispersed MgZn2 precipitates (i.e., intermetallic compound E), while coarse phases (intermetallic compound D) mainly play a role in grain boundary pinning and crack deflection. When the sum of the area ratios is less than 3%, it means that the total amount of effective strengthening phase per unit volume is insufficient, and the alloy may not be able to achieve the strength level expected for 7-series die-cast aluminum alloys used as structural components. This is usually due to excessively high solution treatment temperature or time, resulting in the complete dissolution of most of the MgZn2 phase, or insufficient aging treatment, resulting in a small number of precipitates. In addition, an excessively low total amount of strengthening phases will also weaken the electrochemical buffering capacity of the alloy during anodizing, leading to increased dispersion in the oxide film growth rate, which macroscopically manifests as local color differences or patchy defects with uneven brightness. Therefore, controlling the sum of area ratios to a lower limit of 3% helps to give 7-series die-cast aluminum alloys both high strength and excellent anodized appearance.

[0084] Furthermore, based on the principle of mass conservation and precipitation kinetics, the upper limit of the sum of area ratios in this invention is no greater than 4.2%. Specifically, the amount of MgZn2 intermetallic compound formed is limited by the total content of Zn and Mg elements in the alloy and their stoichiometric relationship. According to the chemical formula of MgZn2, by controlling the total content of Zn and Mg elements in this invention, the limiting area ratio of MgZn2 intermetallic compounds in the aluminum alloy can be calculated to be approximately 4.4%. Further considering that some Zn / Mg will dissolve in the aluminum matrix during actual aging, this invention specifies an upper limit of 4.2% for the sum of area ratios, which should be understood by those skilled in the art.

[0085] Preferably, in any observation region of the aluminum alloy, the area ratio d of the intermetallic compound D satisfies 0.3%≤d≤1.3%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, and any value between them.

[0086] It should be noted that the specific value of d mentioned above needs to be selected in conjunction with the area ratio e of the intermetallic compound E to ensure that the constraint of d / e ratio of 0.1 to 0.5 and the sum of area ratios of 3.0% to 4.2% is met.

[0087] For intermetallic compounds D, if d is less than 0.3%, it means that coarse secondary phases are extremely rare. This may stem from near-perfect solution treatment, which, while more beneficial to matrix purity and electrochemical homogeneity, loses the pinning effect of coarse secondary phases on grain boundary migration and the deflection effect on crack propagation paths, potentially adversely affecting the alloy's heat resistance, fracture toughness, and ability to inhibit recrystallization. If d is greater than 1.3%, it means that there are too many coarse secondary phases with micron-sized components in the microstructure. These phases are not only potential crack initiation sources, but more importantly, during anodizing, they act as heterogeneous phases with different electrochemical properties from the aluminum matrix, inducing severe localized corrosion or selective dissolution. Since micron-sized components are much larger than the initial growth scale of the oxide film, these phases may completely detach in the electrolyte or form deep pits around them, resulting in discontinuous and uneven oxide film thickness at these locations, macroscopically manifesting as unremovable pitting, dark spots, or color differences. Therefore, d is controlled within the range of 0.3% to 1.3% to retain its limited positive contribution to microstructure stability and toughness, while suppressing its adverse effects on surface treatment to an acceptable level.

[0088] Preferably, in any observation area of ​​the aluminum alloy, the area ratio e of the intermetallic compound E satisfies 2.0%≤e≤3.7%, and can be 2.0%, 2.2%, 2.4%, 2.6%, 2.8%, 3.0%, 3.2%, 3.4%, 3.6%, 3.7%, and any value between them.

[0089] It should be noted that the specific value of e mentioned above needs to be selected in conjunction with the area ratio d of the intermetallic compound D to ensure that the constraint of d / e ratio of 0.1 to 0.5 and the sum of area ratios of 3.0% to 4.2% is met.

[0090] For intermetallic compounds E, this invention controls their area fraction e between 2.0% and 3.7%, which is beneficial for aluminum alloys to achieve high strength (especially high yield strength). For 7-series aluminum alloys, sufficient nano / submicron-scale reinforcing phases are required to achieve sufficiently high tensile strength. When e is below 2.0%, the amount of reinforcing phase is insufficient, and the strength may be difficult to achieve; when e is above 3.7%, although the strength may continue to increase, the precipitated phases are too dense, which may lead to narrowing of matrix channels, a significant decrease in plasticity, and also increase the number of heterogeneous phases per unit area during anodizing, which is detrimental to the overall electrochemical uniformity. Controlling e within the range of 2.0% to 3.7% can simultaneously achieve high strength and good plasticity, and obtain a uniform anodized surface.

[0091] It should be noted that the intermetallic compounds D and E defined in this invention both refer to the MgZn2 phase (including the equilibrium phase η and the transition phase η'). Intermetallic compound D is dispersed in a granular manner under a metallographic microscope, while intermetallic compound E precipitates as fine needles or rods. The difference in morphology between the two stems from their different crystal growth habits and precipitation kinetics.

[0092] It should be noted that the aspect ratio mentioned in this invention refers to the ratio of the longest axis dimension (major axis) to the largest dimension (minor axis) perpendicular to the direction of the precipitate observed by scanning electron microscopy (SEM) or transmission electron microscopy (TEM). During the test, no less than 10 fields of view should be selected at different positions on the die casting, and the aspect ratios of no less than 50 precipitates of the same type should be counted in each field of view and the average value should be calculated.

[0093] Preferably, the intermetallic compound D is in particulate form with an aspect ratio of 1.0 to 2.5, which can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5 and any value between them.

[0094] For intermetallic compound D, this invention controls its aspect ratio to be between 1.0 and 2.5, meaning that these larger phases tend to be approximately equiaxed or slightly elliptical. This shape helps reduce its sensitivity as a stress concentration source, which is beneficial to toughness. More importantly, the approximately equiaxed shape makes the dissolution or reaction rate relatively uniform in all directions during anodizing, preventing sharp edges from causing a sharp increase in local current density, thus promoting uniform growth of the oxide film around it and reducing local optical distortion.

[0095] Preferably, the intermetallic compound E is in the form of needles with an aspect ratio of 3.0 to 10.0, which can be 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0 and any value between them.

[0096] For intermetallic compounds E, this invention controls their aspect ratio to be between 3.0 and 10.0, meaning that these fine strengthening phases are mostly needle-like, rod-like, or short fibrous. These fine precipitates with high aspect ratios are typical morphological characteristics of high-strength phases (such as the η' phase) in 7-series aluminum alloys after optimized aging. They are extremely effective at hindering dislocation movement and are the main source of high strength (especially yield strength). Due to their very small size, even when needle-like, their absolute length is very short, resulting in uniform light scattering, and their distribution and transformation behavior in the oxide film are less affected by their shape. Controlling the aspect ratio of the E phase within this range is a key indicator of achieving a specific high strength level while ensuring its diffuse distribution in the matrix.

[0097] Preferably, after anodizing, the aluminum alloy has an anodized film with a thickness of 5μm to 20μm on its surface, which can be 5μm, 8μm, 10μm, 12μm, 15μm, 18μm, 20μm and any value between them.

[0098] The present invention controls the thickness of the anodic oxide film to be 5μm~20μm, which helps to balance the protective performance, appearance quality and process economy of aluminum alloy materials.

[0099] When the film thickness is <5μm, the barrier layer thickness is insufficient, the probability of pore penetration increases significantly, leading to a sharp decline in corrosion resistance. Simultaneously, the film's ability to cover microscopic defects in the substrate (such as grain boundaries and micropores) is insufficient; minor component fluctuations remaining in the substrate are directly reflected on the film surface, manifesting as visible watermarks or slight color differences. When the film thickness is >20μm, the accumulated volumetric expansion stress during growth easily exceeds the critical value, readily inducing microcracks within the film. These microcracks may lead to localized darkening or poor sealing. Furthermore, excessive film thickness increases porosity, enhancing light scattering, resulting in a hazy appearance, decreased gloss, and increased brittleness, making the film prone to cracking during bending deformation. A thickness range of 5μm to 20μm allows the oxide film to maintain excellent corrosion and wear resistance while also exhibiting very stable surface gloss, meeting the decorative and protective requirements of high-end consumer electronics products.

[0100] It should be noted that this thickness range can be determined by eddy current thickness gauge or metallographic sectioning method: the eddy current method is suitable for rapid inspection of finished parts with an accuracy of ±0.5μm; the metallographic sectioning method requires the sample to be mounted, ground and polished before directly measuring the cross-sectional thickness of the film layer under a scanning electron microscope with an accuracy of ±0.1μm.

[0101] Preferably, the area ratio of Al-Zn-Mg particles in the anodic oxide film is 5% to 15%, which can be 5%, 7%, 9%, 11%, 13%, 15%, or any value between them.

[0102] It should be noted that the Al-Zn-Mg particles described in this invention specifically refer to the micro-regional structure rich in Zn and Mg elements retained in the cross-section of the anodic oxide film and not completely oxidized. Their formation originates from the selective retention behavior of the dual-mode MgZn2 strengthening phase (especially intermetallic compound E) obtained through T6 heat treatment in the matrix during the anodic oxidation process. In the conventional 7-series aluminum alloy anodic oxidation process, the coarse and uneven MgZn2 phase completely dissolves in the electrolyte, forming localized current concentration areas. However, the fine and dispersed intermetallic compound E of this invention undergoes only surface oxidation transformation (forming a ZnO / MgO transition layer) during anodic oxidation, while retaining its metallic or partially oxidized state internally, ultimately embedding itself into the alumina matrix as Al-Zn-Mg particles. These particles form a good interfacial bond with the alumina matrix, with a transition layer (mainly composed of a mixed oxide of ZnO, MgO, and Al2O3) at the interface. This transition layer ensures the bonding strength between the particles and the matrix, avoiding the interface debonding problem caused by the traditional coarse second phase.

[0103] It should be noted that, during testing, a focused ion beam (FIB) can be used to prepare cross-sectional samples of the oxide film. Transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS) can be used to confirm the enriched regions of Zn and Mg elements. Image analysis software can be used to statistically analyze the area ratio of Al-Zn-Mg particles in the cross-section of the oxide film. The measurement area should include at least five fields of view, with each field having an area ≥ 50 μm. 2 .

[0104] The fact that the area ratio of Al-Zn-Mg particles is 5% to 15% is a significant feature of the microstructure design of the anodic oxide film in this invention, and its formation mechanism is directly related to the control of the microstructure of the substrate.

[0105] When the area ratio of Al-Zn-Mg particles is less than 5%, the optical properties of the oxide film are entirely dominated by the porous alumina structure, resulting in a monochromatic surface lacking metallic texture and depth. This is especially problematic when using darker colors, where the color appears thin and lacks saturation. When the area ratio is greater than 15%, excessive particle density may interfere with the orderly growth of alumina, increasing film porosity and reducing corrosion resistance. Simultaneously, the close proximity of particles leads to localized stress concentration, making the film prone to microcracks under bending or impact loads. Furthermore, excessive particles can cause localized film continuity damage during anodizing due to anisotropic dissolution, forming microporous channels and reducing the oxide film's corrosion resistance. A 5%–15% Al-Zn-Mg particle area ratio allows the oxide film to exhibit a soft and uniform metallic luster while maintaining film density and adhesion.

[0106] Preferably, the arithmetic mean roughness Ra of the anodic oxide film is 0.1 μm to 0.5 μm, and can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm and any value between them; the maximum height roughness Rz is 1 μm to 5 μm, and can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm and any value between them.

[0107] The present invention controls the arithmetic mean roughness Ra of the anodic oxide film to be 0.1 μm to 0.5 μm and the maximum height roughness Rz to be 1 μm to 5 μm, with the aim of regulating the microstructure of the oxide film and optimizing the visual quality of the product.

[0108] Ra (arithmetic mean roughness) represents the arithmetic mean of the deviation of the surface profile from the center line, reflecting the overall smoothness; Rz (maximum height roughness) represents the vertical distance between the highest peak and the lowest valley of the profile, reflecting the degree of local abrupt change.

[0109] When Ra < 0.1 μm, the surface is too smooth, and the surface quality requirements for the die-casting substrate are too high (requiring additional polishing), resulting in poor economic efficiency. Simultaneously, an excessively low Ra value means that the surface micro-undulations are insufficient to produce soft diffuse reflection, leading to excessive specular reflection, which may cause glare at specific lighting angles. When Ra > 0.5 μm, the surface micro-peaks and valleys produce irregular diffuse reflection of incident light, forming a visible haze or striped appearance at specific lighting angles, resulting in severe visual defects.

[0110] A lower limit of 1 μm for Ra ensures that the surface has no significant macroscopic undulations. When Rz > 5 μm, some deep valleys become electrolyte retention areas. After oxidation, residual sulfuric acid slowly seeps out and corrodes the oxide film, forming corrosion spots, and the defect incidence rate increases sharply. A Ra of 0.1 μm to 0.5 μm and a Rz of 1 μm to 5 μm work synergistically to make the surface profile exhibit uniform and fine microscopic undulations, which avoids specular reflection and eliminates visual defects caused by uneven diffuse reflection.

[0111] It should be noted that this roughness range can be determined by a contact surface profilometer: during measurement, a flat area of ​​the die casting is selected, and five parallel scans are performed along the direction perpendicular to the filling direction (scanning length 5mm, cutoff wavelength 0.8mm). The average value of the five measurements is taken as the final Ra and Rz values.

[0112] Preferably, the color uniformity of the anodic oxide film satisfies ΔE < 1.0, which can be 0.9, 0.8, 0.7, 0.6, 0.5, lower, or any value between them.

[0113] ΔE is a value calculated using the color difference formula defined by the International Commission on Illumination (CIE), used to quantify the perceptual difference of colors in the Lab color space. ΔE < 1.0 is generally considered a color difference threshold that is imperceptible to the human eye and is a standard for the appearance quality of high-end consumer electronics products. This invention controls the color uniformity of the product surface after anodizing to ΔE < 1.0. This comprehensively reflects the combined effects of multiple technical means, including the elimination of silicon interference by low silicon content, the provision of a consistent reaction substrate by a uniform fine-grained matrix, the avoidance of electrochemical anisotropy by controlled precipitation, and the elimination of porosity defects by high density. This means that the die-cast aluminum alloy products of this invention, after anodizing, exhibit highly consistent color throughout their surface, free from defects such as color variations, flow lines, and black streaks visible to the naked eye. The appearance quality reaches or even surpasses the level of some rolled sheet materials, meeting the stringent visual perfection requirements of high-end applications such as mobile phone frames and laptop casings.

[0114] It should be noted that when testing ΔE, a standard spectrophotometer can be used. Under specified lighting and observation conditions (usually a D65 light source and a 10° field of view), the L* (lightness), a* (red-green value), and b* (yellow-blue value) of different areas of the sample surface (e.g., center and edge, and between different batches of samples) are measured. Then, the ΔE value is calculated according to the CIE1976Lab color difference formula.

[0115] Preferably, when observed under a spotlight of 500 lux to 1000 lux (including 500 lux, 600 lux, 700 lux, 800 lux, 900 lux, 1000 lux and any value between them), the incidence of surface defects in the anodic oxide film is less than 0.1%, and can be 0.1%, 0.09%, 0.08%, 0.07%, 0.06%, 0.05%, lower and any value between them.

[0116] This invention also provides a more practical appearance inspection standard. Under spotlight illumination of 500 lux to 1000 lux (simulating the strong light environment of a product in a display case or during daily use), the anodized surface is visually observed, and the proportion of any visible defects (including but not limited to color difference, bright spots, dark spots, textures, and contamination spots) to the total surface area is counted, requiring it to be less than 0.1%. This is a visual yield requirement, which more comprehensively reflects the product's appearance under real lighting conditions than a simple color difference value. Strong light illumination amplifies any microscopic unevenness on the surface. Controlling the defect rate to below 0.1% means that the product is almost flawless even under lighting conditions, meeting the appearance quality standards of the consumer electronics industry.

[0117] Preferably, the solidification point of the aluminum alloy is in the range of 475℃ to 500℃, and can be 475℃, 480℃, 485℃, 490℃, 495℃, 500℃ and any value between them.

[0118] The solidification point is an important thermophysical property of an alloy. This invention controls the solidification point of the aluminum alloy to be within the range of 475°C to 500°C. This solidification point range can be understood as the liquidus temperature range of the alloy, or the temperature at which substantial solidification begins, as measured by thermal analysis. Controlling the solidification point within a suitable range means that the temperature at which the melt transitions from a liquid to a solid state is also appropriate. A suitable solidification point allows the melt to form a stable primary solidified shell more quickly after contacting the low-temperature mold wall, which helps suppress turbulence and gas entrapment, and quickly establish feeding channels. Simultaneously, this solidification point range matches the composition design with low silicon and specific Mg / Zn content. If the solidification point is too low, the actual temperature of the melt is relatively higher at the same superheat, and the release time of latent heat of solidification may be delayed, which is not conducive to quickly establishing a solidification sequence and may increase the risk of hot cracking. If the solidification point is too high, it may impose more stringent requirements on the melting temperature and mold temperature, and excessively rapid solidification may be detrimental to gas escape. Therefore, the freezing point range of 475℃ to 500℃ is the ideal thermodynamic range after composition optimization to achieve good filling, rapid shell solidification, and reduced defects.

[0119] It should be clarified that the core of the protection of the anodized die-cast aluminum alloy in this invention lies in its unique microstructural characteristics, rather than being entirely limited to a specific manufacturing process. Regardless of the manufacturing method used, any metal alloy possessing the specific microstructural characteristics described in this invention should be considered to fall within the scope of protection of this invention. Manufacturing methods include, but are not limited to: employing different melt filling strategies with varying timing or paths; using local pressurization methods based on different principles such as mechanical, hydraulic, or pneumatic pressure; controlling the solidification sequence through special mold cooling design combined with pressure intervention; and performing appropriate local solid-state thermomechanical treatment (such as warm forging or rolling) after conventional die casting to further eliminate edge defects, or combinations thereof. However, despite the existence of multiple possible implementation paths, this invention provides a preferred manufacturing method. After extensive experimental verification, an optimal process window has been determined, enabling the stable and efficient achievement of the aforementioned stringent microstructural control indicators. Below, to better realize and explain in detail how to efficiently and reliably prepare the metal alloy of this invention, an optimized and verified die-casting method embodiment is provided, along with its technical details and principles. This embodiment not only demonstrates the specific operational steps for realizing the product of the present invention, but also reveals the intrinsic relationship between process parameters and microstructure evolution, providing reproducible technical guidance for those skilled in the art.

[0120] In a second aspect, the present invention provides a method for preparing the anodized die-cast aluminum alloy of the first aspect, comprising the following steps: Step S102: After melting the metal alloy to form a molten liquid, the molten liquid includes the following elements by mass percentage: no more than 0.6% Si, 2%~4% Mg, 4%~6.5% Zn, the total content of unavoidable impurities is no more than 0.3%, and the balance is Al. The molten liquid is kept at a first temperature, and the mold is kept at a second temperature lower than the first temperature. The temperature difference between the first and second temperatures is in the range of 300℃~350℃, and can be 300℃, 305℃, 310℃, 315℃, 320℃, 325℃, 330℃, 335℃, 340℃, 345℃, 350℃, and any value between them.

[0121] Step S102 includes alloy smelting, composition control, and crucial temperature difference control. First, the raw materials are batched and smelted according to the aforementioned low silicon and specific magnesium-zinc ratio to ensure that the final melt composition meets the requirements and the total impurities are controlled to ≤0.3% to ensure the purity of the matrix.

[0122] After melting, the molten metal needs to be held at a first temperature. The first temperature is the holding temperature of the molten metal, preferably ranging from 650°C to 720°C, including 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, and any value between them. This temperature range is chosen to ensure the molten metal has sufficient superheat and good fluidity to cope with the potential decrease in fluidity caused by low silicon content, while avoiding excessively high temperatures that could lead to accelerated magnesium loss or severe gas absorption by the melt. The mold needs to be held at a second temperature lower than the first temperature. The second temperature is the mold temperature, preferably ranging from 300°C to 420°C, including 300°C, 320°C, 340°C, 360°C, 380°C, 400°C, 420°C, and any value between them.

[0123] This invention controls the temperature difference between the melt temperature (first temperature) and the mold temperature (second temperature) within the range of 300℃ to 350℃ based on solidification rate, surface defect formation, and internal microstructure uniformity. This creates optimized thermodynamic conditions for the solidification process of the melt, ensuring that the casting achieves a uniform microstructure and good surface quality. Specifically, the temperature difference is the main driving force for melt solidification. If the temperature difference is too small, the driving force is insufficient, resulting in slow overall cooling of the melt and prolonged solidification time. This not only reduces production efficiency but also provides time for grain growth, leading to the formation of coarse grain structures, which is detrimental to the improvement of mechanical properties. At the same time, an excessively low temperature difference may cause the solidified shell formed at the melt front to be too thin or lack strength, and it may still shift under the subsequent push of the melt. Conversely, if the temperature difference is too large, the melt will undergo severe supercooling upon contact with the mold, rapidly forming a thick and brittle primary solidified shell on the surface of the casting. This shell is extremely prone to cracking under the thermal stress and mechanical action of the subsequent molten metal. Its fragments are trapped in the molten metal or accumulate on the surface, resulting in surface defects such as black streaks. In addition, excessive temperature difference will generate strong temperature gradient and thermal stress inside the casting, which may not only cause deformation or thermal cracking, but also aggravate the segregation of solute elements and promote the directional growth of columnar crystals, which is not conducive to the formation of uniform and fine equiaxed crystals.

[0124] Controlling the temperature difference within the range of 300℃ to 350℃ effectively avoids the aforementioned problems. This temperature difference provides a suitable cooling intensity, ensuring the necessary solidification rate to obtain refined grains while avoiding surface defects and internal stress concentration caused by excessive cooling. Under these conditions, the molten metal forms an initial solidified layer of suitable thickness and strength upon contact with the mold. This relatively stable layer helps resist the slight erosion from subsequent molten metal, protecting surface quality. Simultaneously, the molten metal inside the casting has ample time for relatively uniform heat dissipation and solidification in this relatively mild cooling environment, which is conducive to the nucleation and uniform growth of equiaxed crystals.

[0125] Step S104: The molten metal is injected into the mold cavity through the inlet of the mold cavity. After the mold cavity is filled, the pressure is maintained and the mold is opened to obtain the die-cast aluminum alloy. The velocity of the molten metal injected into the mold cavity shall not exceed 2 m / s, and can be 2 m / s, 1.9 m / s, 1.8 m / s, 1.7 m / s, 1.6 m / s, 1.5 m / s, 1.4 m / s, 1.3 m / s, 1.2 m / s, 1.1 m / s, 1.0 m / s, 0.9 m / s, 0.8 m / s, 0.7 m / s, 0.6 m / s, 0.5 m / s, 0.4 m / s, 0.3 m / s, etc. The moving speed of the molten liquid surface within the mold cavity relative to the mold cavity is no greater than 0.5 m / s, and can be 0.5 m / s, 0.45 m / s, 0.4 m / s, 0.35 m / s, 0.3 m / s, 0.25 m / s, 0.2 m / s, 0.15 m / s, 0.1 m / s, 0.05 m / s, and any value between them.

[0126] Preferably, the velocity of the molten metal injected into the mold cavity is no greater than 1.0 m / s, and the moving velocity of the molten metal surface relative to the mold cavity is no greater than 0.4 m / s. More preferably, the velocity of the molten metal injected into the mold cavity is no greater than 0.5 m / s, and the moving velocity of the molten metal surface relative to the mold cavity is no greater than 0.3 m / s. Even more preferably, the velocity of the molten metal injected into the mold cavity is no greater than 0.3 m / s, and the moving velocity of the molten metal surface relative to the mold cavity is no greater than 0.2 m / s.

[0127] In this invention, the "velocity of molten metal injected into the mold cavity" specifically refers to the flow velocity of the molten metal at the inlet cross-section at the instant it enters the mold cavity through the inlet during filling. This invention controls the filling flow rate and the rising velocity of the molten metal surface within the mold cavity, effectively achieving laminar flow filling and eliminating turbulence and air entrapment. Traditional die-casting processes, in pursuit of filling efficiency, often employ high-speed jets (typically much higher than 2 m / s), causing the molten metal to generate strong inertial and shear forces upon entering the cavity, rapidly transforming the flow state into disordered turbulence. This turbulence not only tears apart the molten metal front, breaking up air within the cavity and entraining it into the melt to form bubbles that are difficult to expel, but also violently erodes the surface of the thin solidified layer that has already formed in contact with the low-temperature mold wall, causing this solidified layer to shift and accumulate, resulting in macroscopic black streaks and watermarks on the product surface. This invention sets the upper limit of the molten metal injection velocity into the mold cavity to no more than 2 m / s precisely to suppress the generation of this destructive flow from the inlet. When the molten metal enters the mold cavity smoothly at a speed not exceeding 2 m / s, its Reynolds number decreases significantly, allowing the flow pattern to transition from turbulent to laminar or steady flow. This velocity threshold makes subsequent smooth filling possible. Building upon this, the proposed optimal range reflects a deeper understanding of the low-speed concept. The lower the velocity, the weaker the inertial force of the molten metal, the gentler the flow, and the less disturbance it causes to the gas inside the mold cavity and the mold surface. For example, in fields with extremely high surface finish requirements, such as consumer electronics casings, using an extremely low filling speed of ≤0.3 m / s can almost completely eliminate surface defects caused by flow impact.

[0128] Furthermore, the movement speed of the molten liquid surface relative to the mold cavity is a key parameter that complements the speed of molten liquid injection into the mold cavity. Understandably, this "movement speed" specifically refers to the rate of advancement of the molten liquid front within the mold cavity space, that is, the instantaneous speed at which the liquid level height or flow extension length corresponding to the volume of molten liquid filling the cavity changes per unit time. The movement speed of the molten liquid surface relative to the mold cavity describes how quickly the solid-liquid interface (i.e., the molten liquid front) advances within the cavity space during the filling process. Understandably, an excessively fast ascent speed means that the molten liquid front covers an excessively large area of ​​the mold per unit time, and the heat exchange process between the molten liquid and the cryogenic mold is too rapid. This leads to two main problems: first, a sudden drop in local temperature at the molten liquid front forms an unstable and unevenly thick initial solidified shell, which is easily broken by subsequent molten liquid, resulting in defects; second, air is rapidly compressed and expelled, unable to be completely discharged through the overflow port, increasing the risk of being trapped. Limiting the liquid surface rise velocity to below 0.5 m / s ensures that the solidification front advances at a uniform and controllable speed, allowing sufficient time for gas to be discharged to the overflow port and making the heat transfer process more uniform. This promotes sequential solidification and fundamentally avoids watermarks and black streaks caused by friction of the solidified layer. Similarly, its preferred range is for those seeking higher surface quality and internal purity.

[0129] Understandably, simultaneously controlling both the speed of molten metal injection into the mold cavity and the speed of molten metal surface movement is essential for ensuring high-quality filling. The speed of molten metal injection into the mold cavity reflects the initial flow state of the molten metal as it enters the cavity from the inlet, while the speed of molten metal surface movement describes the overall behavior of the solid-liquid interface advancing within the cavity. These two parameters are physically different and are influenced by different factors; therefore, they must be controlled collaboratively to achieve the desired filling effect. Controlling only the speed of molten metal surface movement without controlling the speed of molten metal injection into the mold cavity will not effectively solve the problem. If the flow velocity at the inlet is too high, the molten metal will form a divergent and disordered flow front in the initial stage of entering the cavity, disrupting the stability of the entire filling process. Even if the overall surface rise speed is slow, the high-speed injection of molten metal will create severe turbulence in the area near the inlet, directly eroding the thin solidified layer formed on the mold surface, producing black streaks and surface defects. At the same time, the turbulence at the inlet will disrupt the flow order of the molten metal, rendering subsequent control of the surface movement speed meaningless.

[0130] The cross-sectional area of ​​the flow channel varies within ±10% between the position where the liquid inlet contacts the mold cavity and the position where it is far from the mold cavity. It can be ±10%, ±9%, ±8%, ±7%, ±6%, ±5%, ±4%, ±3%, ±2%, ±1%, 0, and any value between them.

[0131] This invention requires that the cross-sectional area variation of the flow channel between the position where the liquid inlet contacts the mold cavity and the position where it is far from the mold cavity be controlled within ±10%. Controlling the above parameters is the physical prerequisite for this method to achieve a "gateless" structure and ensure that subsequent low-speed and stable filling can be achieved. This feature defines the geometry of the entire molten metal delivery path from the output end of the injection mechanism to the mold cavity inlet.

[0132] Understandably, "the position in contact with the mold cavity" refers to the interface where the end of the runner directly connects to the mold cavity, i.e., the inlet section where the molten metal finally enters the cavity. "The position away from the mold cavity" refers to any reference section upstream of the runner, typically the starting point where the injection cylinder or pressure chamber connects to the runner, or any section of the runner with a relatively stable cross-sectional area. The "interval runner" refers to a continuous channel segment connecting the two positions along the direction of molten metal flow, not just the section at the endpoints. "Cross-sectional area variation within ±10%" means that, over the entire length of the runner between these two positions, the maximum positive or negative deviation of the area of ​​any cross-section compared to a reference cross-sectional area (usually the inlet section or the starting section) does not exceed 10% of that reference cross-sectional area. Understandably, the inlet section here is usually the section where the inlet runner and the injection chamber in the injection mechanism directly connect. This means that the flow channel is designed as a pipe with a constant or nearly constant cross-section, or a channel where the cross-sectional area changes only very gently and gradually, without any artificially created necks or gates with drastically reduced cross-sectional area. Based on this, the constraint ensures that there are no local throttling points inside the flow channel, thereby eliminating the structural basis for passively amplifying the flow velocity due to abrupt changes in cross-section.

[0133] Understandably, setting the limit for cross-sectional area variation to ±10% is a critical threshold determined through fluid dynamics analysis and numerous comparative experiments. In traditional die casting processes, the cross-sectional area at the gate is typically designed to be 3% to 9% of the upstream runner's cross-sectional area. This drastic cross-sectional shrinkage (area reduction of 91% to 97%) forces the molten liquid to increase its velocity sharply as it flows through, forming a high-speed jet. This invention completely eliminates this design. When the runner's cross-sectional area variation is strictly limited to a tiny range of ±10%, the molten liquid's velocity will not undergo abrupt, uncontrollable forced acceleration as it flows through the entire inlet runner. This allows the injection punch's movement speed to be directly controlled in a highly linear and predictable manner, directly controlling the initial velocity of the molten liquid entering the cavity, providing a reliable physical basis for precise low-speed filling.

[0134] Understandably, if the cross-sectional area of ​​the runner changes beyond this range, for example, if a local shrinkage of more than 10% occurs, even if the shrinkage ratio is less than that of a conventional gate, a significant Venturi effect will still occur at that location, leading to an increase in local flow velocity. This local acceleration will disrupt the smooth flow state of the molten liquid, potentially initiating turbulence prematurely within the runner or upon entering the cavity. This undermines the laminar or stable flow filling intent pursued by this invention, rendering all subsequent process controls based on the low-speed assumption (such as liquid surface movement speed control and dynamic overflow control) meaningless. Therefore, the strict ±10% limit eliminates the geometric conditions that could cause uncontrollable acceleration from the structural source of fluid transport, and is crucial to ensuring that the molten liquid can smoothly enter the cavity at its original or near-original velocity.

[0135] Understandably, this smooth runner design eliminates the "gate" as a separate structural component with a specific shrinkage function within the mold, thus physically achieving a "gateless" system. It changes the fundamental mode of molten metal injection, shifting from traditional high-pressure, high-speed jetting to a smooth, stable introduction. This is not merely a structural simplification, but a fundamental change in flow control philosophy. It ensures that the kinetic energy of the molten metal entering the cavity is known and controllable, creating the prerequisite for smooth, laminar flow filling throughout the entire cavity.

[0136] The ratio between the overflow of the melt filling and the volume of the mold cavity shall not be less than 0.1, and may be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1.0, higher, and any value between them.

[0137] Preferably, the ratio between the overflow rate of the molten filling and the volume of the mold cavity is in the range of 0.1 to 0.5, and can be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, and any value between them. More preferably, the ratio between the overflow rate of the molten filling and the volume of the mold cavity is in the range of 0.1 to 0.2.

[0138] Understandably, this invention, based on the flow dynamics and solidification behavior of 7-series low-silicon aluminum alloy melt within the mold cavity, determines the critical proportional relationship between overflow and mold cavity volume. This proportional limitation helps improve the internal density of die-cast parts and reduce anodized surface defects.

[0139] Specifically, the overflow flow rate serves to directionally expel the cold melt, oxide inclusions, and entrained gas formed at the filling front due to contact with the low-temperature mold wall during the final stage of filling, using continuously injected clean melt. For the low-silicon (Si≤0.6%) 7-series aluminum alloy described in this invention, although the reduction in silicon content significantly improves anodizing performance, it also leads to a decrease in melt fluidity compared to traditional high-silicon die-casting alloys. The melt front is more prone to forming high-viscosity oxide scale or cold shut-off layers due to temperature drop. If these cold shut-off layers remain inside the casting body or near the surface, the difference in electrochemical behavior between the oxide scale and the substrate will be significantly amplified during subsequent anodizing. Furthermore, the segregation of Zn and Mg elements in the cold shut-off layer of the 7-series alloy will exacerbate the uneven local electrochemical activity, manifesting as visible black streaks, flow lines, or color difference defects.

[0140] Understandably, in order to minimize the removal of the cold contaminant layer at the leading edge of the mold cavity, this invention controls the lower limit of the overflow flow rate to the mold cavity volume to be no less than 0.1. During the die casting filling process, no matter how smoothly the liquid surface velocity is controlled (e.g., ≤0.5m / s as specified in this invention), heat exchange will inevitably occur at the moment the molten front edge contacts the mold, forming a solidified or semi-solidified layer with a lower temperature, poorer fluidity, and rich in oxides. If the overflow flow rate ratio is too low, it means that the volume of molten liquid used for flushing and replacement is insufficient, and this portion of the cold contaminant molten liquid at the leading edge cannot be completely pushed into the overflow channel. If the residual cold contaminant molten liquid remains inside the casting body or in the near-surface area, it will cause defects such as cold shuts, flow marks, and porosity in the casting, and significantly reduce the mechanical properties and density of the casting. Especially in the gateless, gentle runner design adopted in this invention (runner cross-sectional area variation within ±10%), although turbulent air entrapment is reduced from the source, sufficient overflow flow rate is still required to ensure the purity of the filling leading edge. Therefore, unlike existing technologies, this invention significantly increases the overflow flow rate, thereby promoting thorough cleaning of the mold cavity, obtaining high-quality castings, and providing a defect-free matrix for subsequent anodizing.

[0141] Building upon this, the present invention further provides a preferred overflow ratio range of 0.1 to 0.5. This range is designed to ensure an optimal balance between process effectiveness, mold design rationality, and production costs. When the overflow ratio is too low, as mentioned earlier, the amount of molten metal available for venting gas and cold slag is insufficient, making it difficult to consistently ensure adequate purification of the cavity under all process conditions. This increases the risk of porosity or inclusions in the casting, leading to a decrease in yield. Conversely, when the overflow ratio is too high, although a larger overflow may technically contribute to the purification effect, it leads to a significant decrease in metal utilization and an increase in raw material costs. Furthermore, an excessively large overflow requires the design of a larger overflow channel (cavity), which not only places higher demands on the spatial layout and structural strength of the mold design, increasing the difficulty and cost of mold manufacturing, but may also increase the processing burden of subsequently removing overflow waste, reducing production efficiency. Therefore, the range of 0.1 to 0.5 defines a reasonable operating range that is technically effective in ensuring quality and economically feasible in production.

[0142] Based on this, the present invention further specifies a preferred ratio range of 0.1 to 0.2. Under the condition of satisfying other core process parameters of the present invention (such as a molten metal temperature difference of 300℃ to 350℃ between the molten metal and the mold, and an injection speed ≤ 2m / s), controlling the overflow ratio within the range of 0.1 to 0.2 can more stably obtain high-quality castings with low porosity and uniform microstructure. This is because under the low-speed, stable filling strategy of the present invention, most of the gas in the cavity has been orderly discharged; the main task of the overflow stage is to fill the remaining space and perform overflow flushing. An overflow rate of 0.1 to 0.2 is sufficient to discharge the small amount of cold-staining molten metal generated in the final stage, while avoiding energy consumption and material waste caused by excessive overflow. This preferred range reflects the optimized choice between high quality and high efficiency in the process of the present invention, which helps to achieve large-scale stable production.

[0143] It is important to note that maintaining a ratio of at least 0.1 between the overflow rate of the molten metal and the volume of the mold cavity is a mandatory requirement for achieving the desired technical effect in the preferred preparation method of this invention, constituting the fundamental safety boundary of the process scheme. The formula provided below regarding the specific functional relationship between the overflow rate and the mold cavity volume represents a preferred dynamic control strategy. In other words, those skilled in the art, when implementing the preferred preparation method of this invention, must first ensure that the overflow rate ratio meets the mandatory requirement of at least 0.1. Based on this, to obtain better process matching and casting quality consistency, a more precise overflow rate ratio can be determined using this formula according to specific process conditions (such as temperature difference and flow rate). If the ratio calculated by the formula is lower than 0.1, the lower limit of 0.1 should be followed to ensure that the process remains within a safe process window. This control logic, which uses absolute values ​​as a baseline and empirical formulas as optimization guidance, effectively improves the universality and reliability of the method of this invention.

[0144] Preferably, the overflow rate of the molten metal filling and the volume of the mold cavity satisfy the following relationship:

[0145] In the formula, Overflow volume for molten filling; This refers to the volume of the mold cavity; The first proportionality constant has a value range of 0.1 to 0.2, and can be 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, and any value between them; The temperature difference between the first temperature and the second temperature. The second temperature; The speed at which molten liquid is injected into the mold cavity.

[0146] This invention derives the above-mentioned empirical model based on the thermodynamic and fluid dynamic mechanisms of the filling process. This model quantitatively describes the intrinsic relationship between the minimum theoretical overflow ratio required to obtain high-quality castings and the temperature difference between the molten metal and the mold, the absolute temperature level of the mold, and the speed at which the molten metal is injected into the mold cavity.

[0147] The core of the relationship proposed in this invention lies in determining the theoretical minimum overflow ratio related to process conditions. The value calculated on the right side of the formula represents the critical lower limit of the ratio of overflow to cavity volume that needs to be achieved under specific temperature differences, mold temperatures, and the rate at which molten metal is injected into the mold cavity, in order to ensure that the cold molten metal at the filling front and the entrained gas can be effectively discharged from the cavity, thereby obtaining a casting with a dense interior and intact surface.

[0148] Understandably, the use of a greater than or equal to relationship in the formula is to clearly define that, in the actual implementation of the method of this invention, the actual overflow ratio must be at least equal to or greater than the value calculated by this formula. As long as this condition is met, the most basic process requirement for cavity purification through overflow is achieved. This ensures that under different combinations of temperature differences, mold temperatures, and molten metal injection speeds into the mold cavity, the overflow rate dynamically meets the verified minimum standard. This invention does not impose an upper limit on the actual overflow ratio. Technically speaking, as long as the overflow system capacity of the mold allows, increasing the overflow rate generally will not damage but will only more thoroughly remove impurities. Therefore, the overflow ratio in actual production can be selected and optimized based on a trade-off between technical effectiveness and economic costs (metal loss, energy consumption).

[0149] Understandably, the constant in the relation This was determined through extensive repeatable die-casting tests under baseline and adjacent process conditions, and statistical analysis of the actual overflow ratio corresponding to the obtained high-quality castings. Under the premise of satisfying other core process conditions of this invention, By controlling the quality within this range, the goals of low porosity, no surface defects, and uniform microstructure can be achieved more stably.

[0150] Understandably, the temperature term in the relational expression adopts... The form of represents the physical significance of relative thermal shock intensity. Among them, molecules... Represents the driving force for heat exchange between the molten metal and the mold; the denominator The mold temperature level was introduced as a benchmark.

[0151] Understandably, the overflow rate is related to the temperature difference between the first and second temperatures. The direct correlation exists because a greater temperature difference results in a higher heat flux density at the moment of contact between the molten metal and the mold, a faster cooling rate at the solidification front, and a thicker layer of cold, viscous molten metal (which may be rich in oxides) formed per unit time. To effectively remove this increased cold molten metal from the casting body, a corresponding increase in the overflow rate for flushing and displacement is required. Overflow rate and the second temperature... The inverse relationship exists because mold temperature not only affects the temperature difference but also independently influences the flow behavior of the molten liquid. When the second temperature... At lower temperatures, even the temperature difference between the first and second temperatures Similarly, the cooling effect of the mold on the molten metal is more pronounced. Low-temperature molds cause a rapid increase in the viscosity of the molten metal front, increasing flow resistance and thus increasing the force required to push the cold contaminant out. Furthermore, the lower secondary temperature... This means the mold's thermal capacity is at a low level, resulting in higher heat absorption efficiency and exacerbating the temperature drop at the molten front. Therefore, it implies that at the second temperature... Under lower operating conditions, the present invention requires a larger overflow to replenish the heat loss at the front edge with sufficient superheated melt, avoid incomplete filling, and provide stronger hydrodynamics to ensure the discharge of impurities.

[0152] Understandably, the reason for using the ratio of temperature difference to mold temperature is... Rather than a single parameter, this ratio constructs a dimensionless thermal index. It takes into account the temperature difference between the first and second temperatures. Second temperature A single temperature difference cannot fully reflect the intensity of cooling, and the same temperature difference has different effects on the melt viscosity at different mold temperatures. By using a ratio, the formula can adaptively evaluate the actual cooling intensity under different mold temperature settings, thereby achieving precise matching of overflow.

[0153] Understandably, the exponent of the temperature term in the formula is 0.5 (square root), reflecting the nonlinear effect of the temperature difference on the rate of formation of the cold sludge at the solidification front. This aligns with the fundamental laws of heat conduction and solidification kinetics. According to Fourier's law of heat conduction and the theory of solidified layer growth, the thickness of the solidified shell or the cooling depth is usually proportional to the square root of the temperature difference. Since one of the main functions of the overflow is to displace the cold sludge layer formed during cooling, the required overflow volume is closely related to the thickness of this cold sludge layer. Given that the amount of cold sludge layer formed is related to the square root of the temperature difference, the proportion of overflow used for flushing should naturally also be positively correlated with the square root of the temperature difference. This exponent setting makes the formula more consistent with physical reality, avoiding overestimation or underestimation that might result from a linear relationship.

[0154] Understandably, the exponent of the second term in the equation is -1, reflecting another mechanism by which the speed of molten metal injection into the mold cavity affects the filling process and overflow requirements. When the speed of molten metal injection into the mold cavity decreases (i.e., filling slows down), the molten metal flow is more stable, and air entrapment within the cavity is significantly reduced, which is advantageous. However, slower filling also means a longer residence time of the molten metal in the cavity, increasing its overall heat loss, and potentially causing a more pronounced drop in the temperature of the molten metal at the leading edge. To maintain sufficient hydrodynamics during the longer filling time, pushing the potentially increased viscous resistance due to temperature drop and the continuously generated small amount of cold sludge at the leading edge towards and out of the overflow port, a sufficient molten metal flow rate is needed to provide continuous flushing. Therefore, the overflow ratio needs to be compensated for by a negative correlation with the speed of molten metal injection into the mold cavity; that is, the slower the filling, the larger the required minimum overflow ratio.

[0155] Understandably, the "2" in the second term of the relation is a normalization reference value, corresponding to... The characteristic process parameter value. The unit of "2" is m / s, which is taken from the upper limit of the velocity of the melt injection into the mold cavity of the present invention (≤2m / s), and is used as the normalization benchmark for this parameter, so that the term in the formula concerning the velocity of the melt injection into the mold cavity becomes a dimensionless factor.

[0156] Using this formula, those skilled in the art can calculate the required minimum overflow ratio based on specific process conditions. For example, when die-casting an aluminum alloy structural part, the process parameters are set as follows: Second temperature The first temperature is 700℃, and the temperature difference between the first and second temperatures is... The speed at which molten liquid is injected into the mold cavity Substitute the values ​​into the formula to calculate ( This yields a minimum a / b ratio of 0.15, meaning the minimum theoretical overflow ratio is approximately 0.15. The minimum total filling amount of the molten metal is approximately 115% of the cavity volume, which helps to obtain higher quality die-cast parts. If the second temperature is increased to... The temperature difference between the melt and the mold With other parameters remaining constant, the calculated minimum a / b ratio is approximately 0.13. This represents the ratio of temperature difference to mold temperature. As a relative indicator of thermal shock intensity, it can more accurately characterize overflow demand than a single temperature parameter. In actual production, the final overflow rate can be determined based on different calculation results and economic considerations.

[0157] In summary, this relationship links core thermodynamic parameters with fluid dynamic parameters, jointly determining the minimum process parameters required to achieve effective cavity purification. It replaces the traditional, crude experience-based approach of fixing the overflow ratio, achieving adaptive and scientific matching of overflow volume with specific process conditions. This dynamic control model reflects the systematic and precise nature of the invention's process, and is more conducive to the stable production of high-quality die-cast parts under a wide range of process parameter combinations.

[0158] Preferably, the molten filling overflow is discharged through an overflow port on the mold cavity; the overflow port is circumferentially disposed on the side wall of the mold cavity, and the area of ​​the overflow port accounts for 15% to 25% of the surface area of ​​the side wall of the mold cavity, which can be 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25% and any value between them.

[0159] To better achieve the aforementioned overflow function, the overflow port design of this invention differs significantly in area from that of traditional die-casting processes. In the prior art, the main function of the overflow port is often understood as passively containing cold, contaminated metal and a small amount of gas; its total opening area is typically small, often accounting for less than 10% of the surface area of ​​the mold cavity sidewall. In contrast, this invention preferably requires the overflow port area to account for 15% to 25% of the surface area of ​​the mold cavity sidewall. This larger area percentage is more conducive to adapting to the core process of this invention.

[0160] Understandably, the overflow behavior in this invention is not a simple passive containment, but an active, quantitative flushing process. As mentioned above, this method controls the lower limit of the total overflow, and its lower limit (not less than 0.1) and its preferred implementation range are generally higher than the overflow of conventional processes. To accommodate this larger planned overflow, a preferred matching scheme is to provide it with an overflow channel with stronger flow capacity to ensure that at the end of the filling period, this portion of the melt rich in impurities and gases can be quickly, smoothly, and unimpededly discharged from the cavity, avoiding throttling or back pressure at the overflow port.

[0161] In this invention, the "area of ​​the overflow port" refers to the sum of the effective cross-sectional areas of the channels through which the molten liquid flows from inside the mold cavity to the external overflow system; the "surface area of ​​the sidewall of the mold cavity" refers to the actual total area of ​​the vertical or inclined surfaces around the mold cavity that form the product shape, excluding the bottom and top surfaces of the mold cavity (i.e., the plane where the parting surface is located). This invention controls the area of ​​the overflow port to account for 15% to 25% of the sidewall surface area of ​​the mold cavity, aiming to ensure that the flow capacity of the overflow system matches the cavity volume and heat dissipation surface area, thus meeting the requirements for rapid venting and slag removal under large overflow flow while ensuring the overall strength of the mold structure.

[0162] Specifically, this invention provides two typical mold structure implementation methods: The first embodiment is an independent overflow port structure, which can be a bi-splitting mold where the upper and lower mold mating surfaces are perpendicularly abutting. In this structure, the mold cavity is jointly enclosed by the upper and lower molds, and the outer walls of the upper and lower molds are usually set as vertical surfaces. When the mold is closed, the outer walls of the upper and lower molds abut each other perpendicularly to form a closed cavity sidewall. The overflow port is manifested as multiple independent through holes opened on the mold sidewall, preferably located in the upper region of the upper mold sidewall, so as to utilize the gas rising characteristics for venting. In this embodiment, the "area of ​​the overflow port" is the sum of the opening cross-sectional areas of each independent through hole at the inner wall of the cavity; the "surface area of ​​the mold cavity sidewall" corresponds to the total surface area of ​​the sidewalls of the cavity formed by the upper and lower molds after they are closed. Therefore, the area ratio in this embodiment refers to the ratio of the total cross-sectional area of ​​the independent overflow port through holes to the total surface area of ​​the cavity sidewalls formed by the upper and lower molds. This structure is easy to process and clean, and is suitable for castings with regular shapes and uniform overflow requirements.

[0163] The second implementation is a continuous overflow edge structure, which can be an insert-type mold with a vertical upper mold, an inclined lower mold, and the upper mold inserted into the lower mold. In this structure, the sidewall of the lower mold is designed as an inclined surface, and the sidewall of the upper mold is a vertical surface. When the mold is closed, the upper mold is inserted downward into the lower mold, and a fitting gap is formed between the end face of the upper mold and the inclined sidewall of the lower mold. At this time, the overflow port no longer appears as an independent through hole, but as a continuous overflow edge around the top of the lower mold (i.e., the fitting gap between the end face of the upper mold and the sidewall of the lower mold). After the molten metal is filled, the excess molten metal and gas overflow evenly from this ring gap at the top of the lower mold. In this implementation, the "area of ​​the overflow port" is the cross-sectional area of ​​the annular channel of the continuous overflow edge; the "surface area of ​​the mold cavity sidewall" mainly corresponds to the surface area of ​​the sidewall of the lower mold (because the main sidewall of the cavity is surrounded by the inclined surface of the lower mold, and the upper mold mainly forms the top). Therefore, the area ratio in this implementation refers to the ratio of the cross-sectional area of ​​the continuous overflow edge at the top of the lower mold to the surface area of ​​the sidewall of the lower mold cavity. This continuous overflow edge structure eliminates the dead zone between independent overflow ports, allowing the gas and cold molten sludge at the molten front to be discharged evenly and synchronously along the entire circumference of the cavity. It is particularly suitable for complex castings with extremely high requirements for airtightness and surface quality.

[0164] Regardless of the implementation method used, this invention controls the ratio of the overflow port area to the cavity sidewall surface area to be between 15% and 25%. For the first implementation method, if the ratio is below 15%, the total flow area of ​​the independent through holes is insufficient, which can easily lead to a throttling effect under large overflow flow processes, resulting in poor venting or back pressure at the overflow port. If the ratio is above 25%, the effective support area of ​​the upper and lower mold sidewalls decreases, weakening the mold strength and increasing the risk of mold expansion. For the second implementation method, if the ratio is below 15%, the gap between the continuous overflow edges is too narrow, which is not conducive to the smooth discharge of cold melt and can easily cause premature solidification and blockage of the overflow edges. If the ratio is above 25%, it means that the upper mold insertion depth is too shallow or the lower mold sidewall is too thin, which will also affect the guiding accuracy and structural rigidity of the mold. Therefore, the range of 15% to 25% is a balance point between fluid dynamics flow requirements and mold mechanical strength, which helps to ensure the stable implementation of the slow filling and large overflow process of this invention.

[0165] Step S106: Perform T6 heat treatment and anodizing on the die-cast aluminum alloy to obtain anodized die-cast aluminum alloy.

[0166] Step S106 involves subsequent strengthening and surface treatment of the die-cast blank to ultimately obtain the target product. First, a T6 heat treatment is performed, typically including solution treatment and artificial aging. Solution treatment aims to dissolve the soluble strengthening phases (mainly MgZn2 phase) in the die-cast microstructure into the aluminum matrix to the maximum extent, forming a supersaturated solid solution, preparing for subsequent aging precipitation. Artificial aging is performed at a lower temperature, promoting the orderly precipitation of fine, dispersed strengthening phases (mainly GP zone, η' phase, and η-MgZn2 phase) from the supersaturated solid solution, thereby significantly improving the alloy's strength. For this invention, the T6 heat treatment is not only for achieving high strength, but it also directly affects the size, morphology, quantity, and distribution of the final precipitated phases (the aforementioned D and E phases), thus influencing anodizing performance. Therefore, the T6 heat treatment of this invention may need to be optimized according to the specific composition of the alloy (especially the Mg and Zn content) to ensure the acquisition of the aforementioned dual-scale precipitated phase characteristics. Anodizing is then performed, an electrochemical process that generates a dense alumina film on the aluminum alloy surface. The anodizing process parameters affect the thickness, hardness, porosity, color, and uniformity of the oxide film. For the low-silicon 7-series die-cast aluminum alloy of this invention, due to its pure matrix and uniform structure, anodizing processes similar to those used for high-quality rolled sheets (such as sulfuric acid DC anodizing) can be employed to obtain a high-quality oxide film. Through the T6 heat treatment and anodizing process in step S106, the die-cast aluminum alloy blank is ultimately transformed into an anodized die-cast aluminum alloy product possessing high strength, high toughness, high corrosion resistance, and excellent appearance.

[0167] Preferably, the T6 heat treatment includes: solution treatment at 460℃~480℃ (including 460℃, 465℃, 470℃, 475℃, 480℃ and any value between them) for 1 hour to 4 hours (including 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h and any value between them), water quenching, and then aging treatment at 110℃~130℃ (including 110℃, 115℃, 120℃, 125℃, 130℃ and any value between them) for 12 hours to 24 hours (including 12h, 14h, 16h, 18h, 20h, 22h, 24h and any value between them).

[0168] This preferred embodiment provides a specific parameter range for T6 heat treatment applicable to the composition range described in this invention. The solution temperature is 460℃~480℃. For 7-series aluminum alloys, this temperature range is sufficient to dissolve most of the MgZn2 phase, but below its overheating temperature to avoid grain boundary melting. The solution time is 1~4 hours to ensure sufficient dissolution while considering production efficiency. Water quenching is used after solution treatment to achieve rapid cooling, preserving the supersaturated solid solution state at high temperature to room temperature and preventing the precipitation of coarse equilibrium phases during cooling. The artificial aging temperature is selected as 110℃~130℃, which is a typical artificial aging temperature range for 7-series aluminum alloys, beneficial for forming a strengthening structure dominated by the η' phase. The aging time is 12~24 hours to ensure sufficient precipitation process, reaching peak strength or an over-aged state. This invention, through the combination of solution and aging parameters, aims to achieve the aforementioned specific distribution of the D and E phases. For example, slightly lowering the solution temperature or shortening the solution time may intentionally retain a small amount of undissolved coarse second phase as the D phase; while precise control of the aging temperature and duration determines the size, quantity, and aspect ratio of the E phase (fine η' or η phase). These parameters, combined with the specific alloy composition and die-cast microstructure, are used to simultaneously meet the requirements of high strength and anodizing uniformity.

[0169] Furthermore, the die-cast microstructure obtained by the aforementioned die-casting process parameters (temperature difference control between melt and mold, low-speed stable filling, and large overflow design) provides an ideal matrix prerequisite for the precise precipitation of intermetallic compounds D and E during subsequent heat treatment. Specifically, the low silicon content and temperature difference control of 300℃~350℃ avoid excessive formation of coarse second phases, allowing Zn and Mg elements to be distributed in a wide, gentle banded pattern within a fine and uniform equiaxed crystal matrix; the low-speed laminar flow filling and large overflow ratio effectively eliminate air entrapment and the leading edge cold contamination layer, ensuring extremely high internal density and uniform solute distribution in the casting. Based on this highly homogeneous die-cast microstructure, a solution treatment at 460℃~480℃ allows Zn and Mg elements to fully and uniformly dissolve back into the aluminum matrix, forming a supersaturated solid solution. Simultaneously, based on phase dissolution kinetics, a very small amount of incompletely dissolved original coarse second phase or grain boundary segregated phase is precisely retained as heterogeneous nucleation sites for the subsequent intermetallic compound D. Subsequently, aging at a lower temperature of 110℃~130℃ causes a large number of fine η' / η phases (i.e., intermetallic compound E) to precipitate in a needle-like morphology within the grains, while the remaining nuclei preferentially grow along the original grain boundaries / subgrain boundaries to form a granular distribution (i.e., intermetallic compound D). The homogeneity and density of the die-cast microstructure directly determine the uniformity of the matrix composition after solution treatment, thereby precisely controlling the nucleation rate, growth kinetics, and final spatial distribution ratio of the D and E phases during aging. The strict low-temperature aging window effectively suppresses the Ostwald ripening effect, prevents the E phase from excessively coarsening and merging into the D phase, and ensures that the d / e value remains stable within the optimized range of 0.1 to 0.5, thereby achieving a synergistic effect of high-intensity precipitation enhancement and electrochemical uniformity of the anodic oxidation surface on a macroscopic level.

[0170] Preferably, the anodizing treatment comprises: in a sulfuric acid electrolyte with a concentration of 150 g / L to 200 g / L (inclusive of 150 g / L, 160 g / L, 170 g / L, 180 g / L, 190 g / L, 200 g / L and any value between therewith), at a temperature of 18°C ​​to 22°C (inclusive of 18°C, 19°C, 20°C, 21°C, 22°C and any value between therewith), at an A / dm². 2 ~1.5A / dm 2 (including 1.0A / dm) 2 1.1A / dm 2 1.2A / dm 2 1.3A / dm 2 1.4A / dm 2 1.5A / dm 2 DC anodizing is performed at a current density of 20 to 40 minutes (including 20, 25, 30, 35, 40 minutes and any value between them).

[0171] This preferred embodiment provides the anodizing process parameters applicable to the low-silicon 7-series die-cast aluminum alloys of this invention. Using sulfuric acid electrolyte is the most common and economical anodizing method. The concentration is controlled at 150 g / L to 200 g / L, which provides sufficient sulfate ions to participate in the reaction, forming a porous oxide film, which is beneficial for subsequent coloring or sealing. The electrolyte temperature is strictly controlled within the lower range of 18℃ to 22℃. Temperature has a significant impact on the formation rate, hardness, and pore structure of the oxide film. Too low a temperature may result in slow film formation and a hard but brittle film; too high a temperature will result in a loose film layer with low hardness and easy powdering. 18℃ to 22℃ is the ideal temperature range for obtaining a dense, hard, and uniform oxide film, especially for decorative anodizing with high appearance requirements. The current density is selected as 1.0 A / dm³. 2 ~1.5A / dm 2 The process employs a direct current (DC) mode. This current density range, matched to the aforementioned temperature and concentration, ensures the oxide film grows at a moderate and controllable rate. Excessive current density can lead to film scorching or generate excessive Joule heat, disrupting film uniformity; conversely, insufficient current density results in low film formation efficiency. An oxidation time of 20-40 minutes, combined with the aforementioned current density, can generate an oxide film of the target thickness (5-20 μm). Because the substrate material provided by this invention is pure and homogeneous, a high-quality oxide film can be obtained using this standard sulfuric acid anodizing process without the need for special additives to the electrolyte or complex power waveforms, demonstrating the excellent adaptability of the material itself to anodizing. After oxidation, subsequent processing steps such as water washing, coloring (if necessary), and sealing are typically required. These are conventional techniques in the field and can be performed according to specific product requirements.

[0172] Thirdly, the present invention provides an aluminum alloy die casting made of an anodized die casting aluminum alloy of the first aspect, or an anodized die casting aluminum alloy prepared by the preparation method of the second aspect.

[0173] The product of this invention is an aluminum alloy die-casting part, which is composed of the aforementioned anodized die-cast aluminum alloy material with specific composition, specific microstructure and specific surface characteristics, or produced by the aforementioned specific preparation method. This die-casting part inherits the advantages of the aluminum alloy of this invention, such as high strength, high density, and excellent anodized appearance. It can have various complex shapes, which is the advantage of near-net-shape forming in die-casting technology. For example, it can be a one-piece molded mobile phone frame, laptop shell, camera frame, drone parts, etc. These structural parts often have features such as reinforcing ribs, snap-fits, and threaded holes. Traditional sheet metal machining by CNC not only results in significant material waste, but also easily leads to color differences at the edges and corners during anodizing. The die-casting part of this invention can achieve one-piece molding of complex shapes with a uniform overall appearance.

[0174] Fourthly, the present invention provides an application of the aluminum alloy die-casting part as described in the third aspect in the manufacture of structural parts for electronic devices, transportation vehicles, robots, medical devices, industrial equipment, or new energy equipment.

[0175] Preferably, metal alloy die castings can be used in the manufacture of consumer electronics housings, precision automotive parts, humanoid robot mechanical components, or medical components.

[0176] The above are preferred application areas of the die-cast parts of this invention. These areas all have high requirements for material strength, lightweight, complex forming capabilities, and surface appearance. Application in consumer electronics casings (such as the mid-frame and back panel of smartphones, tablets, laptops, and smartwatches) is a core scenario. These products require thin yet robust casings that protect internal precision components, while their appearance (color, gloss, texture) is a key factor in determining product grade and consumer perception. The die-cast parts of this invention perfectly meet these needs. 7-series aluminum alloy provides high strength, enabling thinner wall designs; die casting allows for integrated molding of complex structures, reducing assembly parts; and excellent anodizing performance provides a uniform color and smooth surface comparable to or even better than sheet aluminum. Applications in automotive precision parts, such as structural supports, chassis components, trim strips, and battery pack housings, also benefit from their high strength, lightweight, good corrosion resistance, and consistent appearance. In humanoid robot mechanical components (such as joint connectors, skeletons, and shells), lightweight, high-strength, wear-resistant, and fatigue-resistant materials are required. The die-casting parts of this invention, through T6 heat treatment, can achieve high mechanical properties, meeting the needs of such dynamic load-bearing components. In medical components (such as medical device shells, stents, and non-implantable tools), there are requirements for the biocompatibility of materials (usually requiring low-toxicity metals), surface cleanliness, and disinfection resistance. Aluminum alloy anodized films have good chemical stability and a clean surface, and the high-purity, high-density die-casting parts of this invention provide the means for this. These examples illustrate the broad market prospects and industrialization value of the technical solution of this invention. Its core lies in enabling die casting, a high-efficiency, low-cost forming technology, to directly produce structural parts that meet the appearance and performance requirements of the high-end market through material and process innovation. It is expected to replace some of the complex processing paths of traditional forging + CNC + anodizing or rolled sheet + CNC + anodizing, bringing significant cost and efficiency advantages.

[0177] The present invention will be further described in detail below with reference to specific embodiments, but these are exemplary and do not limit the scope of protection of the present invention in any way.

[0178] Example 1 An anodized die-cast aluminum alloy is prepared by the following method: Step S102: Weigh the raw materials according to the following mass percentages: Si content 0.3%, Mg content 2.5%, Zn content 5%, other unavoidable impurities (Fe, Cu, etc.) ≤0.1%, and the balance is Al. After melting, a molten liquid is formed. The temperature of the molten liquid (first temperature) is controlled at 700℃ and held at this temperature. The mold is preheated to 380℃ (second temperature) and held at this temperature, with a temperature difference ΔT of 320℃.

[0179] Step S104: The molten liquid is injected into the mold cavity through the inlet of the mold cavity. The speed at which the molten liquid is injected into the mold cavity is 1.0 m / s, and the moving speed of the molten liquid surface in the mold cavity relative to the mold cavity is 0.3 m / s. After the cavity is filled, pressure is maintained and the mold is opened to obtain the die-cast aluminum alloy.

[0180] Among them, the cross-sectional area of ​​the flow channel changes 0 between the position where the liquid inlet contacts the mold cavity and the position where it is far away from the mold cavity; The ratio of the overflow volume of the molten filling to the volume of the mold cavity is 0.2; the overflowing molten filling is discharged through the overflow port on the mold cavity; the area of ​​the overflow port accounts for 20% of the side wall surface area of ​​the mold cavity, and the overflow port is a continuous overflow edge structure, which is set along the circumference of the top of the lower mold.

[0181] Step S106: Perform T6 heat treatment on the die-cast aluminum alloy: solution treatment at 470℃ for 2.5 hours, followed by water quenching; then age treatment at 120℃ for 18 hours.

[0182] Anodizing was then performed: in a sulfuric acid electrolyte with a concentration of 180 g / L, at 20 °C, at a rate of 1.2 A / dm³. 2 DC anodizing was performed at a certain current density for 30 minutes, followed by water washing and sealing to obtain anodized die-cast aluminum alloy products.

[0183] Example 2 The preparation method of this embodiment is the same as that of Example 1, except that the aluminum alloy composition contains 0.60% Si, 2.0% Mg, and 4.0% Zn.

[0184] Example 3 The preparation method of this embodiment is the same as that of Example 1, except that the Si content in the aluminum alloy is 0.40%, the Mg content is 3%, and the Zn content is 5.5%.

[0185] Example 4 The preparation method of this embodiment is the same as that of Example 1, except that the Si content in the aluminum alloy is 0.10%, the Mg content is 3.5%, and the Zn content is 6.0%.

[0186] Example 5 The preparation method of this embodiment is the same as that of Example 1, except that the Si content in the aluminum alloy is 0.05%, the Mg content is 4%, and the Zn content is 6.5%.

[0187] Example 6 The preparation method in this embodiment is the same as that in Example 1, except that the melt temperature is 650°C, the mold cavity temperature is 350°C, and the temperature difference is 300°C.

[0188] Example 7 The preparation method in this embodiment is the same as that in Example 1, except that the melt temperature is 720°C, the mold cavity temperature is 370°C, and the temperature difference is 350°C.

[0189] Example 8 The preparation method in this embodiment is the same as that in embodiment 1, except that: the molten liquid is filled into the mold cavity through the inlet of the mold cavity at a speed of 0.3 m / s, and the moving speed of the molten liquid surface in the mold cavity relative to the mold cavity is 0.15 m / s.

[0190] Example 9 The preparation method in this embodiment is the same as that in embodiment 1, except that: the molten liquid is filled into the mold cavity through the inlet of the mold cavity at a speed of 0.5 m / s, and the moving speed of the molten liquid surface in the mold cavity relative to the mold cavity is 0.25 m / s.

[0191] Example 10 The preparation method in this embodiment is the same as that in embodiment 1, except that: the molten liquid is filled into the mold cavity through the inlet of the mold cavity at a speed of 2 m / s, and the moving speed of the molten liquid surface in the mold cavity relative to the mold cavity is 0.5 m / s.

[0192] Example 11 The die-casting method in this embodiment is the same as that in Embodiment 1, except that the cross-sectional area of ​​the flow channel changes by -10% between the position where the liquid inlet contacts the mold cavity and the position where it is far away from the mold cavity.

[0193] Example 12 The die-casting method in this embodiment is the same as that in Embodiment 1, except that the cross-sectional area of ​​the flow channel changes by -5% between the position where the liquid inlet contacts the mold cavity and the position where it is far away from the mold cavity.

[0194] Example 13 The die-casting method in this embodiment is the same as that in Embodiment 1, except that the cross-sectional area of ​​the flow channel changes by +5% between the position where the liquid inlet contacts the mold cavity and the position where it is far away from the mold cavity.

[0195] Example 14 The die-casting method in this embodiment is the same as that in Embodiment 1, except that the cross-sectional area of ​​the flow channel changes by +10% between the position where the liquid inlet contacts the mold cavity and the position where it is far away from the mold cavity.

[0196] Example 15 The preparation method of this embodiment is the same as that of Example 1, except that the solution temperature of the T6 heat treatment is 460°C, the solution time is 4 hours, the aging temperature is 110°C, and the aging time is 24 hours.

[0197] Example 16 The preparation method of this embodiment is the same as that of Example 1, except that the solution temperature of the T6 heat treatment is 480°C, the solution time is 1 hour, the aging temperature is 130°C, and the aging time is 12 hours.

[0198] Example 17 The preparation method in this embodiment is the same as that in Example 1, except that the anodic oxidation treatment is carried out in a sulfuric acid electrolyte with a concentration of 150 g / L at a temperature of 18°C ​​and a current density of 1.0 A / dm² for 40 minutes.

[0199] Example 18 The preparation method in this embodiment is the same as that in Example 1, except that the anodic oxidation treatment is carried out in a sulfuric acid electrolyte with a concentration of 200 g / L at a temperature of 22°C and a current density of 1.5 A / dm² for 20 minutes.

[0200] Example 19 The preparation method in this embodiment is the same as that in Example 1, except that the ratio of the overflow of the melt filling to the volume of the mold cavity is 0.1.

[0201] Example 20 The preparation method in this embodiment is the same as that in Example 1, except that the ratio of the overflow of the melt filling to the volume of the mold cavity is 0.5.

[0202] Example 21 The preparation method in this embodiment is the same as that in Example 1, except that the ratio of the overflow of the melt filling to the volume of the mold cavity is 0.6.

[0203] Example 22 The preparation method in this embodiment is the same as in Example 1, except that the ratio of the overflow rate of the melt filling to the volume of the mold cavity is determined by calculation using a formula:

[0204] Among them, take The temperature difference between the first temperature and the second temperature Second temperature The speed at which molten liquid is injected into the mold cavity The calculated ratio is approximately 0.18.

[0205] Example 23 The preparation method of this embodiment is the same as that of embodiment 1, except that the area of ​​the overflow port accounts for 25% of the side wall surface area of ​​the mold cavity.

[0206] Example 24 The preparation method of this embodiment is the same as that of embodiment 1, except that the area of ​​the overflow port accounts for 15% of the side wall surface area of ​​the mold cavity.

[0207] Example 25 The preparation method of this embodiment is the same as that of embodiment 1. The difference is that the overflow port is an independent overflow port structure. Specifically, an overflow port is provided on the upper part of each side wall of the mold cavity. The overflow port is circular in shape and the area of ​​the overflow port accounts for 20% of the surface area of ​​the side wall of the mold cavity.

[0208] Comparative Example 1 This comparative example provides a conventional 7-series die-cast aluminum alloy with the following composition by mass percentage: 6.0% Si, 2.5% Mg, 5.6% Zn, 1.6% Cu, 0.3% Fe, and the balance Al (close to the composition of 7075 but with increased Si content to improve fluidity). The melt temperature is 700℃, the mold temperature is 250℃, and the temperature difference is 450℃. The melt injection velocity into the mold cavity is 4.5 m / s, without runner cross-sectional area control (conventional gate shrinkage reaches 95%). Subsequent T6 heat treatment and anodizing process parameters are the same as in Example 1.

[0209] Comparative Example 2 The preparation method of this comparative example is the same as that of Example 1, except that the speed at which the melt is injected into the mold cavity is 4.0 m / s, the speed at which the melt surface moves within the mold cavity is 2.0 m / s, and other parameters are the same as those of Example 1.

[0210] Comparative Example 3 The preparation method of this comparative example is the same as that of Example 1, except that: the melt temperature is 700℃, the mold cavity temperature is 200℃, the temperature difference is 500℃, and other parameters are the same as those of Example 1.

[0211] Comparative Example 4 The preparation method of this comparative example is the same as that of Example 1, except that the cross-sectional area shrinkage rate of the flow channel at the inlet reaches 92%, resulting in a velocity of 5m / s when the melt enters the cavity. Other parameters are the same as those of Example 1.

[0212] Comparative Example 5 The preparation method of this comparative example is the same as that of Example 1, except that the T6 heat treatment solution temperature is 440℃, the solution time is 0.5 hours, and other parameters are the same as those of Example 1.

[0213] Comparative Example 6 The preparation method of this comparative example is the same as that of Example 1, except that the T6 heat treatment aging temperature is 160℃ and the aging time is 36 hours, and other parameters are the same as those of Example 1.

[0214] Comparative Example 7 The preparation method of this comparative example is the same as that of Example 1, except that the die-cast aluminum alloy is directly subjected to the same anodizing treatment as in Example 1, without undergoing T6 heat treatment.

[0215] Comparative Example 8 The preparation method of this comparative example is the same as that of Example 1, except that the ratio of the overflow of the first filling and the second filling to the volume of the mold cavity is 0.05.

[0216] Test case The following performance tests were performed on the aluminum alloys obtained in all embodiments and comparative examples: 1. Microstructure analysis in the die-cast state: Grain size: After grinding, polishing and etching (Keller's reagent) of the die-cast sample, the average grain size and standard deviation of at least 5 fields of view were statistically analyzed using a metallographic microscope (500X) and image analysis software (according to ASTM E112 standard).

[0217] Pore ​​analysis: Ten 1000μm pores were randomly selected from the above metallographic samples. 2 The field of view was used to identify and statistically analyze areas ≥20μm using image analysis software. 2 The number of pores was determined, and their average value was calculated. Simultaneously, the presence of such pores was observed.

[0218] 2. Analysis of the T6 state strengthening phase: The samples after T6 heat treatment were ground, polished, and etched (using a mixed acid solution). They were then observed using a field emission scanning electron microscope (FESEM) in backscattered electron mode.

[0219] Using image analysis software, at least 5 different fields of view (each field of view area ≥ 500 μm) were analyzed. 2 In the study, the area ratio d of intermetallic compound D with an equivalent circle diameter of 1μm-3μm and the area ratio e of intermetallic compound E with an equivalent circle diameter of 0.1μm-1μm are distinguished and statistically analyzed, and the d / e value is calculated.

[0220] 3. Characterization of anodic oxide film: Film thickness: Multiple measurements were taken on the sample surface using an eddy current thickness gauge, and the average value was recorded. The thickness was then verified under a scanning electron microscope using metallographic sectioning.

[0221] Intra-film particles: Cross-sectional samples of oxide films were prepared using focused ion beam (FIB), and the area ratio of Al-Zn-Mg particles was observed and statistically analyzed using transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS).

[0222] Roughness: Using a contact surface profilometer, the arithmetic mean roughness Ra and the maximum height roughness Rz were measured in a flat area of ​​the sample. The average value was taken after 5 measurements.

[0223] Color difference ΔE: Using a standard spectrophotometer (D65 light source, 10° field of view), measure the L*, a*, b* values ​​of the central and edge areas of the sample surface, or between different samples in the same batch, and calculate ΔE according to the CIE1976Lab formula.

[0224] Appearance defect rate: Under an 800 lux spotlight, the surface of the anodized sample was visually inspected, and the proportion of the area with any visible defects (color difference, bright spots, dark lines, water flow lines, etc.) to the total surface area was counted and the average value was taken.

[0225] 4. Grain boundary composition distribution analysis: Electron probe microanalysis (EPMA) was used to perform line scan tests on the die-cast samples. On the polished and etched metallographic samples, a line scan with a length of not less than 0.3 mm was performed parallel to the alloy width direction and spanning multiple grains, with a step size of 5 μm, to obtain the concentration distribution curves of Zn and Mg. A continuous interval with a concentration variation less than 0.03% by mass was defined as a "band." This band is a macroscopic compositional band formed by the arrangement and connection of solute segregation regions at multiple grain boundaries. The concentration jump value between adjacent bands is the concentration difference, and the projected length of the macroscopic compositional band in the alloy width direction is the band width.

[0226] 5. Freezing point test: The solidification point of the alloy was determined by differential scanning calorimetry (DSC). Approximately 50 mg of sample was taken and cooled from 700 °C to 400 °C at a cooling rate of 10 °C / min under argon protection. The exothermic curve was recorded, and the onset temperature of the exothermic peak was taken as the solidification point.

[0227] Based on the above testing methods, the embodiments and comparative examples were tested, and the core performance data obtained are summarized in Table 1.

[0228] Table 1

[0229] As shown in Table 1, comprehensive performance tests were conducted on the anodized die-cast aluminum alloys prepared in each embodiment and comparative example of the present invention. The test results clearly verify the effectiveness of the technical solution of the present invention. As can be seen from the data in Table 1, the aluminum alloys prepared in the embodiments all meet the requirements of the present invention in terms of microstructure parameters and anodizing performance indicators. Within the defined range of parameters, the alloys in the embodiments can obtain good microstructure uniformity and anodized appearance quality, proving the rationality of the process parameter ranges set in the present invention. In contrast, the comparative examples, due to deviation from the process conditions of the technical solution of the present invention, show a significant deterioration in all performance indicators. In summary, the data in Table 1 fully demonstrates that the present invention, through the combined effect of low-silicon composition design, precise microstructure control, and synergistic optimization of process parameters, enables 7-series die-cast aluminum alloys to achieve a significant improvement in anodized appearance quality while maintaining good mechanical properties. The appearance quality is close to that of rolled sheet metal, effectively solving the technical bottleneck of the prior art where the die-casting formability and anodizing performance of 7-series aluminum alloys are difficult to balance.

[0230] To further demonstrate the technical effectiveness of the present invention, the following analysis will be conducted in conjunction with specific experimental test results and images.

[0231] Figure 1 The image shows the metallographic structure of the aluminum alloy sample after T6 heat treatment according to Example 1 of this invention. The distribution characteristics of two different sizes and morphologies of MgZn2 strengthening phases can be observed in the image. The specific locations of intermetallic compounds D and E are clearly indicated by arrows and text labels in the image. The dispersed black granular structure represents intermetallic compound D (as indicated by the arrows and granular labels in the image), with an equivalent circle diameter of approximately 1 μm to 3 μm and an area fraction d of approximately 0.8%. Finer needle-like precipitates, i.e., intermetallic compound E (as indicated by the arrows and needle-like labels in the image), are dispersed within the matrix, with an equivalent circle diameter of approximately 0.1 μm to 1 μm, an area fraction e of approximately 3.2%, and a d / e ratio of approximately 0.25. The granular D phase is mainly distributed near grain boundaries or subgrain boundaries, providing good grain boundary pinning and crack deflection effects, which helps improve the fracture toughness of the alloy. The needle-like E phase precipitates uniformly within the grains, achieving efficient precipitation strengthening, and is the main source of the alloy's high strength. Under the process conditions of Example 1, the melt solidification process was stable, and the die-cast grain size was uniform, providing ideal matrix conditions for the preferential precipitation of the strengthening phase after T6 heat treatment. The strengthening phase is uniformly distributed in the figure, without local enrichment or depletion. The area ratio of the D phase to the E phase is within the preferred range of this invention, directly corresponding to the excellent appearance quality after anodizing. This demonstrates the success of the low-silicon composition design, low-speed stable filling process, and synergistic control of T6 heat treatment parameters in this invention, ensuring that the 7-series die-cast aluminum alloy achieves an anodized appearance quality close to that of rolled sheet metal while maintaining high strength.

[0232] Figure 2 This image shows the metallographic structure of the aluminum alloy in the die-cast state according to Embodiment 1 of the present invention. As can be observed from the image, the microstructure exhibits uniform and fine equiaxed grain characteristics, with an average grain size of approximately 35 μm and a standard deviation of approximately 5 μm. The grain size distribution is concentrated, with no obvious coarse dendrites or columnar crystal regions. No macroscopic pores with an area ≥20 μm² are observed within the field of view. In this embodiment, the low silicon content effectively reduces coarse segregation of the eutectic phase, avoiding the destructive impact of silicon on the quality of the anodic oxide film; the temperature difference between the melt and the mold is controlled within the preferred range of 300℃~350℃, avoiding surface defects and internal stress concentration caused by rapid cooling; the low-speed filling strategy eliminates turbulent gas entrapment, ensuring orderly gas discharge from the cavity; the gateless, gentle flow channel design ensures smooth melt introduction, avoiding high-speed jetting caused by traditional gate shrinkage; and the appropriate overflow ratio ensures effective discharge of the cold melt at the leading edge. The die-cast microstructure provides a uniform solute distribution basis for subsequent T6 heat treatment. The solid solution concentrations of Zn and Mg at the grain boundaries exhibit a gradual banded change, avoiding sharp compositional gradients and fundamentally eliminating the conditions for anodizing color difference. This ensures that the final product's appearance quality meets consumer electronics standards.

[0233] Figure 3 This image shows the metallographic structure of an aluminum alloy sample after T6 heat treatment in Example 8 of this invention. Example 8 employed a lower melt injection rate into the mold cavity and a lower liquid surface movement rate. The intermetallic compound D and intermetallic compound E are clearly distinguished in the image using arrows and text labels. It can be observed that, compared to Example 1, the distribution of intermetallic compounds D and E in Example 8 is more uniform, with the D phase area fraction being approximately 0.7%, the E phase area fraction being approximately 3.0%, and the d / e ratio being approximately 0.23. The extremely low filling rate further reduces thermal disturbance during solidification, resulting in more dispersed precipitation of the strengthening phase and more effective suppression of compositional segregation at grain boundaries. This provides a more uniform electrochemical reaction substrate for anodizing.

[0234] Figure 4 This is a metallographic image of the aluminum alloy in the die-cast state according to Example 8 of the present invention. As can be observed from the image, the microstructure exhibits extremely uniform and fine equiaxed grain characteristics, with an average grain size of approximately 37 μm and a standard deviation of approximately 5.5 μm. No areas ≥20 μm are observed within the field of view. 2 The macroscopic porosity indicates that the low-speed filling process controlled by this invention can effectively eliminate turbulent air entrapment. Combined with a suitable mold temperature difference, a high-density as-cast structure is obtained, avoiding anodizing black spot defects caused by porosity.

[0235] Figure 5This is a photograph of the anodized appearance of an aluminum alloy die-cast part made of aluminum alloy according to Embodiment 1 of the present invention. As can be observed from the image, the product surface has a uniform color and a soft metallic luster. No defects such as flow lines, black streaks, color differences, or bright spots are visible to the naked eye. This directly demonstrates that the present invention successfully solved the problem of poor anodized appearance quality of 7-series aluminum alloy die-cast parts through low silicon content, microstructure control, and synergistic process control, achieving the appearance standards for consumer electronics casings.

[0236] Figure 6 This is a photograph of the anodized appearance of the aluminum alloy die-casting part made of aluminum alloy, Comparative Example 2 of this invention. Comparative Example 2 used a higher molten metal injection speed into the mold cavity. It can be clearly observed from the image that the surface is rougher and has obvious color differences, with dark spots in some areas. This is because the high-speed filling causes turbulent air entrainment and oxide inclusions, resulting in uneven microstructure and porosity defects in the matrix. These defects are magnified during the anodizing process, forming macroscopic appearance defects that cannot meet the requirements of high-end applications.

[0237] Figure 7 This is a photograph of the anodized appearance of the aluminum alloy die casting made of aluminum alloy (Comparative Example 8). Comparative Example 8 used an excessively low overflow ratio. As can be observed in the image, scattered black spots and discoloration patches are distributed on the surface. This is because insufficient overflow prevented the cold molten metal and gas at the filling front from effectively escaping the mold cavity, remaining near the surface of the casting. During anodizing, this resulted in localized corrosion or discontinuous film layers, verifying the effectiveness of the overflow ratio control in this invention.

[0238] Figure 8 and Figure 9 The diagrams show a comparison between the gateless structure of this invention and the narrow gate structure of the prior art. For example... Figure 8 As shown, in this invention, the change in the cross-sectional area of ​​the flow channel between the inlet 20 and the injection mechanism 40 is controlled within ±10%. After the molten liquid is output from the injection mechanism 40, it directly enters the mold cavity 10 through the smooth flow channel, without any throttling parts with abrupt cross-sectional contraction. This gateless design ensures that the flow velocity of the molten liquid in the flow channel will not undergo abrupt forced acceleration, and the movement speed of the injection punch can be transmitted to the cavity in a highly linear manner, providing a structural basis for achieving low-speed laminar flow filling. In contrast, as... Figure 9 As shown, in traditional narrow gate structures, the cross-sectional area at the inlet 20 is deliberately designed to be 3% to 9% of the cross-sectional area of ​​the upstream flow channel. When the molten liquid flows through this part, it will be uncontrollably accelerated to a high-speed jet state (usually >2m / s), generating strong turbulence and air entrapment. This is the structural root cause of surface defects and internal porosity in traditional die castings. Figure 8 and Figure 9 The comparison clearly reveals the technical principle of this invention in eliminating turbulence-inducing factors from the structural source.

[0239] Figure 10 and Figure 11 The specific embodiments of the continuous overflow edge structure in the mold of the present invention are shown from both a three-dimensional perspective and a side view. For example... Figure 10 and Figure 11 As shown, the overflow port 30 is not a traditional independent through hole, but a continuous annular channel set on the top side of the lower mold. Multiple overflow ports 30 are connected around the circumference of the mold cavity 10 to form a complete overflow edge. When the upper and lower molds are closed, this continuous overflow edge cooperates with the upper mold to form a complete venting and slag removal channel around the top of the cavity. The advantages of this structural design are: First, it eliminates the venting dead angle that may exist between independent overflow ports, allowing the gas and cold molten sludge at the front of the molten metal to be discharged evenly and synchronously along the entire circumference of the cavity; Second, the overflow stalk formed on the casting by the continuous overflow edge is complete and continuous, which is convenient for subsequent one-time removal by special tooling, improving post-processing efficiency; Third, the large cross-sectional area of ​​the overflow edge ensures that the flow rate of the molten metal through the overflow port 30 can still be kept at a low level under high overflow flow process, maintaining a laminar or steady flow state, and achieving truly stable rinsing.

[0240] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. An anodized die-cast aluminum alloy, characterized in that, The aluminum alloy comprises the following elements by weight percentage: The total content of unavoidable impurities is no more than 0.6% Si, 2%~4% Mg, 4%~6.5% Zn, and the balance is Al. Before heat treatment, the aluminum alloy has an average grain size of 20μm to 50μm, and the standard deviation of the grain size is no greater than 10μm; in any 1000μm of the aluminum alloy... 2 Within the observation area, the area is not less than 20 μm 2 The number of pores is no more than 1.

2. The anodized die-cast aluminum alloy according to claim 1, characterized in that, Before heat treatment, the aluminum alloy is subjected to any 1000μm of heat treatment. 2 Within the observation area, there is no area not less than 20 μm 2 pores.

3. The anodized die-cast aluminum alloy according to claim 1, characterized in that, Before heat treatment, the solid solution concentrations of Zn and Mg at the grain boundaries of the aluminum alloy vary in a banded pattern with a width of more than 0.05 mm along the width direction of the alloy, and the concentration difference between adjacent bands is less than 0.20% by mass.

4. The anodized die-cast aluminum alloy according to claim 1, characterized in that, After heat treatment, the aluminum alloy contains two MgZn2 intermetallic compounds, D and E, wherein the equivalent circle diameter of intermetallic compound D is 1 μm to 3 μm and the equivalent circle diameter of intermetallic compound E is 0.1 μm to 1 μm; in any observation region of the aluminum alloy, the ratio (d / e) of the area ratio d of intermetallic compound D to the area ratio e of intermetallic compound E is 0.1 to 0.

5.

5. The anodized die-cast aluminum alloy according to claim 4, characterized in that, In any observation area of ​​the aluminum alloy, the sum of the area ratios d of intermetallic compound D and e of intermetallic compound E is not less than 2.5%.

6. The anodized die-cast aluminum alloy according to claim 4, characterized in that, The intermetallic compound D is dispersed in particulate form, while the intermetallic compound E precipitates as fine needle-like particles.

7. The anodized die-cast aluminum alloy according to claim 1, characterized in that, After anodizing, the aluminum alloy has an anodized film with a thickness of 5μm to 20μm on its surface. The area ratio of Al-Zn-Mg particles in the anodized film is 5% to 15%, the arithmetic mean roughness Ra of the anodized film is 0.1μm to 0.5μm, and the maximum height roughness Rz is 1μm to 5μm.

8. The anodized die-cast aluminum alloy according to claim 7, characterized in that, The color uniformity of the anodic oxide film satisfies ΔE<1.

0.

9. The anodized die-cast aluminum alloy according to claim 7, characterized in that, When observed under a spotlight of 500 lux to 1000 lux, the incidence of surface defects in the anodic oxide film is less than 0.1%.

10. The anodized die-cast aluminum alloy according to claim 1, characterized in that, The aluminum alloy comprises no more than 0.4% Si by weight percentage.

11. The anodized die-cast aluminum alloy according to claim 10, characterized in that, The aluminum alloy comprises no more than 0.1% Si by weight percentage.

12. The anodized die-cast aluminum alloy according to claim 1, characterized in that, The aluminum alloy also includes 2.5% to 3.5% Mg and 5% to 6% Zn by weight percentage.

13. The anodized die-cast aluminum alloy according to claim 1, characterized in that, The solidification point range of the aluminum alloy is 475℃~500℃.

14. The anodized die-cast aluminum alloy according to claim 1, characterized in that, The heat treatment includes T4, T5, T6 or T651 heat treatment.

15. A method for preparing anodized die-cast aluminum alloy as described in any one of claims 1 to 14, characterized in that, Includes the following steps: Step S102: After melting the metal alloy to form a molten liquid, the molten liquid includes the following elements by mass percentage: no more than 0.6% Si, 2%~4% Mg, 4%~6.5% Zn, the total content of unavoidable impurities is no more than 0.3%, and the balance is Al; The molten liquid is kept at a first temperature, and the mold is kept at a second temperature lower than the first temperature. The temperature difference between the first and second temperatures is 300℃~350℃. Step S104: The molten liquid is injected into the mold cavity through the inlet of the mold cavity. After the mold cavity is filled, pressure is maintained and the mold is opened to obtain the die-cast aluminum alloy. The speed at which the molten liquid is injected into the mold cavity is no greater than 2 m / s, and the moving speed of the molten liquid surface relative to the mold cavity is no greater than 0.5 m / s; The cross-sectional area of ​​the flow channel varies within ±10% between the position where the liquid inlet contacts the mold cavity and the position where it is far away from the mold cavity. The ratio between the overflow rate of the molten filling and the volume of the mold cavity is not less than 0.1; Step S106: Perform heat treatment and anodizing on the die-cast aluminum alloy to obtain the anodized die-cast aluminum alloy.

16. The preparation method according to claim 15, characterized in that, The ratio between the overflow rate of the molten filling and the volume of the mold cavity is in the range of 0.1 to 0.

5.

17. The preparation method according to claim 15, characterized in that, The overflow rate of the molten metal and the volume of the mold cavity satisfy the following relationship: In the formula, Overflow volume for molten filling; This refers to the volume of the mold cavity; This is the first proportionality constant, and its value ranges from 0.1 to 0.2; The temperature difference between the first temperature and the second temperature. The second temperature; The speed at which molten liquid is injected into the mold cavity.

18. The preparation method according to claim 15, characterized in that, The molten filling overflow is discharged through the overflow port on the mold cavity; the overflow port is arranged circumferentially on the side wall of the mold cavity, and the area of ​​the overflow port accounts for 15% to 25% of the surface area of ​​the side wall of the mold cavity.

19. The preparation method according to claim 15, characterized in that, The heat treatment is T6 heat treatment, and the specific steps include: solution treatment at 460℃~480℃ for 1 hour to 4 hours, water quenching, and then aging treatment at 110℃~130℃ for 12 hours to 24 hours.

20. The preparation method according to claim 15, characterized in that, The anodizing treatment includes: in a sulfuric acid electrolyte with a concentration of 150 g / L to 200 g / L, at a temperature of 18°C ​​to 22°C, at an A / dm³... 2 ~1.5A / dm 2 DC anodizing is performed at a current density of 20 to 40 minutes.

21. An aluminum alloy die-casting part, characterized in that, Made of anodized die-cast aluminum alloy according to any one of claims 1 to 14, or anodized die-cast aluminum alloy prepared by any one of claims 15 to 20.

22. The application of the aluminum alloy die-casting part as described in claim 21 in the manufacture of structural parts for electronic devices, transportation vehicles, robots, medical devices, industrial equipment, or new energy equipment.

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