Process for improving the anodizing effect of aluminum-silicon alloy die castings

Abstract

The present application relates to a kind of process methods for improving the anodizing effect of aluminum-silicon alloy die casting, by controlling the opening temperature of aluminum-silicon alloy die casting and performing chilling or rapid cooling, the second phase precipitation on the surface of aluminum-silicon alloy die casting is reduced, so that anodizing can be performed, and the effect of anodizing is excellent. The overall process is economic and environmentally friendly, with low carbon emissions, energy saving and short process flow.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloys, and more particularly to a process method for improving the anodizing effect of aluminum-silicon alloy die castings. Background Technology

[0002] To overcome the defects of aluminum alloy in terms of surface hardness and wear resistance, expand its application range, and extend its service life, surface treatment technology has become an indispensable part of the use of aluminum alloys. Anodizing technology is the most widely used and most successful. Anodizing is the process of using aluminum or aluminum alloy as the anode and placing it in an electrolyte solution for electrolysis to artificially form a protective film with decorative and corresponding functions on the aluminum surface. The process utilizes electrolysis to form an aluminum oxide film on the surface.

[0003] With the continuous development of industrialization, the demand for high-performance cast aluminum alloys is increasing. Eutectic aluminum-silicon alloys are widely used in engineering, and cast aluminum alloys, especially near-eutectic aluminum-silicon alloys with high Si content, are receiving increasing attention compared to similar castings. Near-eutectic Al-Si alloys have a high Si content, and silicon is the second phase in the alloy's microstructure, improving its casting performance. However, as the silicon content increases, the difficulty of anodizing also increases. The high silicon content in the aluminum-silicon alloys used results in greater hardness but relatively weaker corrosion resistance. Metal coloring processes can not only improve the appearance of experimental specimens but also enhance their corrosion resistance. Currently, research on near-eutectic aluminum-silicon alloys is relatively limited. Oxides generated through metal coloring often have a certain color. Since the silicon content of cast aluminum alloys is as high as 10%–12%, most of the silicon in the matrix exists in elemental form, dispersed in the eutectic microstructure. The anodic oxide film generated by conventional anodizing process parameters has inconsistent pore sizes and is difficult to color. Even if the metal is successfully colored, the resulting oxide has a relatively uniform color, poor aesthetics, and poor corrosion resistance.

[0004] Die-cast aluminum alloys have a high silicon content (between 6-12%), which hinders the flow of electric current, making it difficult to form an oxide film on the surface using traditional anodizing methods. With existing oxidation processes, a large amount of silicon precipitates into the oxide film, forming a matte gray film, which fails to achieve a satisfactory anodizing effect.

[0005] Traditional aluminum die-casting products, such as aluminum-silicon alloys and aluminum-silicon-copper alloys, cannot be oxidized and are unevenly colored. Their composition contains silicon and copper, typically with high silicon content (6%-12%) and high copper content. High silicon content causes the oxide film to turn gray, while copper causes it to turn reddish and compromises electrolyte quality. Excessive iron content in the alloy can also cause black spots on the oxide film.

[0006] As can be seen from die-cast parts, high-silicon aluminum alloys exhibit a large number of coarse needle-like or lamellar silicon phases at grain boundaries, whose chemical reactivity is far weaker than that of the aluminum matrix. During the entire coloring process, the aluminum matrix on the sample surface is corroded first upon contact with a strong acid solution, and the oxide film cannot completely cover the base metal surface. Even after further electrolytic coloring, unreacted silicon phase "frameworks" remain on the sample surface. This is the fundamental reason why the blackness coefficient is difficult to further improve and why coloring is uneven under certain process conditions. Therefore, to obtain a more complete film layer for coloring treatment on the surface of high-silicon aluminum alloys, it is necessary to start by changing the morphology of the matrix structure through reasonable and effective melt treatments: such as purification, grain refinement, and modification processes to improve the internal microstructure of the aluminum alloy. Eliminating the silicon phase framework allows the silicon phase and other alloying elements to be evenly distributed in the matrix, enabling the generated oxide film to uniformly and completely cover the matrix surface, thereby improving the coloring effect of the film layer.

[0007] Existing anodizable die-cast aluminum alloys do not contain silicon and copper as their main components. For example, although Chinese invention patent application number CN202010213524.4 provides an anodizable die-cast aluminum alloy to obtain various colors, such as black and colored, its fluidity and hardness are poor. Although the hardness of anodizable die-cast aluminum alloys can be improved by adding trace metal elements such as molybdenum, zirconium, and chromium, or by adding rare earth elements such as Ce or La, the cost is very high and it is not suitable for widespread use.

[0008] Meanwhile, the presence of second-phase particles causes current loss because Si particles or reinforcing phases such as Al2Cu and Mg2Si (θ phase) are areas where current accumulates, leading to localized burn-out of the parts. Currently, research on aluminum-silicon alloy anodizing both domestically and internationally mainly focuses on hard anodizing, with less research on traditional anodizing methods. Compared to traditional anodizing, hard anodizing utilizes higher current densities, larger applied voltages, and lower solution temperatures (below 5°C), resulting in films with greater hardness and wear resistance.

[0009] Currently, anodizing of cast aluminum alloys mainly focuses on three directions: additive selection, modification of electrical parameters, and composite anodizing. While significant progress has been made in the anodizing of cast aluminum alloys, compared to the anodizing of wrought aluminum alloys, the following problems remain:

[0010] (1) Under normal temperature conditions, the film formation rate of cast aluminum alloys during anodizing is relatively low. The highest film formation rate recorded in the literature is 0.833 μm / min at room temperature; while the film formation rate of wrought aluminum alloys during anodizing at room temperature is mostly greater than 1 μm / min. This indicates that the anodizing efficiency of cast aluminum alloys is relatively low.

[0011] (2) Although anodizing at temperatures below 5°C increases the film formation rate (up to 2.33 μm / min), the high temperature requirement increases energy consumption and is not conducive to the production and utilization of cast aluminum alloy anodizing.

[0012] Chinese invention patent application number CN201510543657.7 discloses an efficient and energy-saving anodizing treatment method for die-cast aluminum alloys. It achieves a good film formation rate at room temperature through pretreatment, anodizing treatment and sealing. However, its overall pretreatment process is long and the process is relatively complex.

[0013] Chinese invention patent application CN202111390429.2 discloses an anodized die-cast aluminum alloy and its preparation method. It optimizes the chemical composition and second phase by adding appropriate amounts of Mn, Zn, Mg, Si, Ti, and RE elements to the aluminum alloy, resulting in a fine and uniformly distributed second phase structure, thus exhibiting excellent die-casting performance, anodizing performance, and mechanical properties. With increasing Si content, the alloy's strength gradually increases. During die-casting, a small amount of Al-Si eutectic phase is still generated. Adding appropriate amounts of rare earth elements can transform the Al-Si eutectic phase from coarse rod-shaped second phase to fine spherical phase, which helps reduce the detrimental effect of Al-Si on the anodizing effect. This makes it less likely for large-sized rhombic Al-Mn phase and large-sized rod-shaped Al-Si phase to be generated during die-casting, resulting in a finer and more uniform distribution of the second phase in the die-cast part. This improves the alloy's elongation while reducing the potential difference between the second phase and the matrix in the die-cast part, making the produced oxide film more uniform and continuous with lower color difference values, thereby improving the anodizing performance. The anodized die-cast aluminum alloy material provided by this patent incorporates a large amount of Zn, which fully realizes the solid solution strengthening effect and simultaneously refines the Al-Mn phase, thereby improving both alloy strength and anodizing performance. The addition of appropriate amounts of Si and rare earth elements further refines the Al-Si phase, reducing the adverse effects of coarse Al-Si phases on anodizing and elongation, thus improving the strength, elongation, anodizing performance, and die-casting performance of the anodized die-cast alloy. However, this method cannot eliminate the influence of the second phase on the surface of the die-cast aluminum alloy during anodizing. It can only make the oxide film more uniform by refining the second phase, but color uniformity still exists, and it is difficult to achieve a stable and dense oxide film where the second phase is present, thus affecting the quality of anodizing. Summary of the Invention

[0014] (a) Technical problems to be solved

[0015] To address the aforementioned problems in the prior art, this invention provides a process method for improving the anodizing effect of aluminum-silicon alloy die-cast parts.

[0016] (II) Technical Solution

[0017] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0018] A process for improving the anodizing effect of aluminum-silicon alloy die-cast parts, characterized by comprising the following steps:

[0019] S1: The mold opening temperature of the aluminum-silicon alloy die casting is determined based on the nose tip temperature corresponding to the precipitation of the second phase in the aluminum-silicon alloy die casting; the mold opening temperature is higher than the nose tip temperature.

[0020] S2: The aluminum-silicon alloy die-casting part that has reached the mold opening temperature is taken out and subjected to quenching after a preset time; the cooling rate corresponding to the quenching is higher than the critical cooling rate corresponding to the absence of precipitation of the second phase.

[0021] S3: Perform anodizing treatment.

[0022] Furthermore, the nose tip temperature is determined by the TTT curve of the aluminum-silicon alloy die casting; the critical cooling rate is determined by the CCT curve of the aluminum-silicon alloy die casting.

[0023] Furthermore, the mold opening temperature is 10-50°C above the nose tip temperature.

[0024] Furthermore, the mold opening temperature is 280-360℃.

[0025] Furthermore, the preset time is 0.1-5 seconds.

[0026] Furthermore, the chilling specifically includes immersing the aluminum-silicon alloy die-cast part in a coolant at 0.5-20°C.

[0027] Furthermore, the soaking time is 5-70 seconds.

[0028] Furthermore, the coolant is water.

[0029] Furthermore, the quenching specifically includes immersing the aluminum-silicon alloy die-cast part in liquid nitrogen for cooling.

[0030] Furthermore, the cooling rate corresponding to the quenching is 0.5-100℃ / s.

[0031] (III) Beneficial Effects

[0032] The beneficial effects of this invention are as follows: By employing rapid cooling technology on the aluminum-silicon alloy die-cast parts after mold opening, the precipitation of second-phase particles such as Al2Cu and Mg2Si is prevented during the cooling process. This eliminates the influence of second-phase particles in the surface structure, allowing for the production of a high-quality aluminum oxide film using an appropriate anodizing process. Furthermore, because the process yields aluminum-silicon alloy die-cast parts with less and more uniformly distributed second-phase precipitation, it is suitable for conventional anodizing processes. It eliminates the need for low-temperature conditions and high current density requirements, achieving high film formation rates at room temperature and conventional current densities, effectively reducing costs, making it more economical and environmentally friendly, and reducing carbon emissions.

[0033] The core of this invention is twofold: first, to ensure the mold opening temperature during die casting is higher than the nose temperature corresponding to the precipitation of the second phase in the aluminum-silicon alloy die casting, thereby preventing the precipitation of the second phase after mold opening; second, to further suppress the precipitation of the second phase on the surface of the aluminum-silicon alloy die casting through rapid cooling, resulting in more uniform particles. Rapid cooling also rapidly seals the intergranular spaces of the second phase, preventing precipitation, which is equivalent to the second phase being completely encapsulated by the aluminum matrix. This gives the aluminum-silicon alloy die casting a perfectly anodized surface metallographic structure, enabling it to undergo room temperature anodizing with excellent results.

[0034] The process principle of this invention differs from traditional solution treatment. Traditional solution treatment increases process costs, and aluminum-silicon alloy die castings typically cannot undergo solution heat treatment because a large amount of gas is inevitably entrained in the molten aluminum during die casting. After the casting solidifies, this gas is sealed inside the casting. When the die casting is reheated to a certain temperature, this gas inside the casting expands, causing blistering on the surface. This makes it impossible to improve the anodizing effect of the die casting using traditional solution treatment. Even if the amount of gas injected during die casting is small enough to complete the solution treatment without blistering, the main purpose of traditional solution treatment—after heating, holding, cooling, and aging—is to homogenize the second phase and reduce its precipitation.

[0035] This invention involves directly cooling the die-cast part on the worktable using a cooling device for a certain period of time after die casting, allowing a dense layer to form on the surface. Due to the extremely rapid cooling rate, the precipitation of the second phase is effectively suppressed. Unlike solution casting, this process eliminates the need for heating; instead, the second phase is controlled through controlled cooling, avoiding problems such as bubbling that may occur in traditional solution casting. This is more economical and environmentally friendly, with a simpler process that effectively reduces energy consumption and carbon emissions. Die-cast parts produced by this process exhibit uniform anodized color and excellent corrosion resistance. After 1056 hours of neutral salt spray testing, the surface showed no ripples or corrosion marks, exceeding current industry standards by more than 5 times. Figure 13The technology has significant improvement effects and has broad application prospects in the anodizing industry of die-cast parts. Attached Figure Description

[0036] 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 of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0037] Figure 1 This is the TTT curve of the cast aluminum alloy of this invention;

[0038] Figure 2 This is the CCT curve of the cast aluminum alloy of this invention;

[0039] Figure 3 This is a schematic diagram of anodizing motorcycle cooling components using traditional methods.

[0040] Figure 4 This is a schematic diagram of anodizing motorcycle heat sink components using the process of this invention;

[0041] Figure 5 This is a schematic diagram of lampshade 1 undergoing anodizing using traditional methods;

[0042] Figure 6 This is a schematic diagram of the lampshade 1 undergoing anodizing using the process of this invention;

[0043] Figure 7 This is a schematic diagram of lampshade 2 undergoing anodizing using traditional methods;

[0044] Figure 8 This is a schematic diagram of the lampshade 2 undergoing anodizing using the process of this invention;

[0045] Figure 9 It is a metallographic image of the microstructure taken from the surface of a motorcycle heat sink using traditional techniques at a magnification of 100.

[0046] Figure 10 This is a metallographic image of the microstructure taken from the surface of a motorcycle heat sink using the process of this invention at a magnification of 100x.

[0047] Figure 11 It is a metallographic image of the microstructure taken from the surface of a motorcycle heat sink using traditional techniques at a magnification of 200.

[0048] Figure 12 This is a metallographic image of the microstructure taken from the surface of a motorcycle heat sink using the process of this invention at a magnification of 200x.

[0049] Figure 13 This is a corrosion resistance test report for motorcycle heat sink components anodized using the process of this invention. Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to represent selected embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0051] Example 1:

[0052] like Figure 3 and Figure 4 The diagram shows a conventional anodizing process and anodizing of a motorcycle heat sink component using the process of this invention. The components of the motorcycle heat sink component are all composed of ADC12.

[0053] Figure 3 The specific process in / 5 / 7 is: the conventional die-casting process.

[0054] 1. Melting and holding: Place the ADC12 cast aluminum ingot in the furnace for melting, and start holding it at 650~680℃±5℃.

[0055] 2. Using a preheated cast iron ladle, scoop about 0.3 kg of molten aluminum from the furnace and pour it into the pressure chamber at a uniform speed over a period of 5 seconds.

[0056] 3. After the molten liquid aluminum is poured into the pressure chamber, it is formed under the action of low-speed (0.2m / S) injection stage and high-speed (4m / S) injection stage and compaction pressure (75MPa) in sequence, driven by the injection punch.

[0057] 4. After the liquid aluminum enters the mold cavity through the flow channel (solidifies), it is cooled for 6 seconds. After cooling, the die-casting machine opens the mold and the part is picked up by the set part picker or robot standby position. After the part is picked up, it is placed in the automatic conveying device and transported to the end material removal head and slag bag.

[0058] 5. Anodizing treatment:

[0059] Pre-treatment: Metal parts must first undergo some pre-treatment, such as deburring, grinding or polishing.

[0060] Cleaning: Before anodizing, the metal parts need to be thoroughly cleaned to remove surface grease, impurities, and naturally formed oxide films. The cleaning process typically uses alkaline or acidic solutions. This can improve the surface finish after anodizing to some extent.

[0061] Anodizing (voltage: 13-18V, anodizing time: 30-60min, anodizing tank temperature: 20±2℃, sulfuric acid concentration: 180±10g / l): The aluminum metal part is placed in the electrolyte (the composition of the electrolyte depends on the metal used and the desired surface treatment effect). After energizing, an oxide film is formed on the surface of the metal part. The thickness of the oxide film depends on the electrolyte used and the current / time parameters.

[0062] Dyeing treatment: Organic or inorganic dyes are adsorbed through the micropores in the anodic oxide film to produce various colors on metal parts, thereby improving their decorative properties;

[0063] Sealing (Temperature: 60-95℃, Time: 6-15min): By immersing in hot water or other sealants, the micropores of the oxide film are sealed, thereby enhancing its corrosion resistance and hardness. Cleaning and Drying: The treated parts need to be cleaned and dried to remove excess electrolyte and sealant.

[0064] Figure 4 The specific process is as follows:

[0065] 1. Melting and holding: Place the ADC12 cast aluminum ingot in the furnace for melting, and start holding it at 650~680℃±5℃.

[0066] 2. Using a preheated cast iron ladle, scoop about 0.3 kg of molten aluminum from the furnace and pour it into the pressure chamber at a uniform speed over a period of 5 seconds.

[0067] 3. After the molten liquid aluminum is poured into the pressure chamber, it is formed under the action of low-speed (0.2m / S) injection stage and high-speed (4m / S) injection stage and compaction pressure (75MPa) in sequence, driven by the injection punch.

[0068] 4. After the liquid aluminum enters the mold cavity through the flow channel (solidifies), it is cooled for 6 seconds; after cooling, the die-casting machine opens the mold, and the mold opening temperature is controlled at 280℃; the nose temperature corresponding to the precipitation of the second phase of ADC12 is 265℃, and the critical cooling rate corresponding to the precipitation of the second phase of ADC12 is 10℃ / S.

[0069] 5. Set the pick-up machine or robot to standby position for picking up parts. After mold opening, the pick-up device is set to pick up parts in 1 second. After picking up the parts, immediately immerse them in 5℃ cooling water for 6 seconds with a cooling rate of 46.6℃ / s.

[0070] 6. After immersion soaking, the material is automatically conveyed to the end discharge head and slag bag by an automatic conveying device.

[0071] 7. Anodizing treatment:

[0072] Pre-treatment: Metal parts must first undergo some pre-treatment, such as deburring, grinding or polishing.

[0073] Cleaning: Before anodizing, the metal parts need to be thoroughly cleaned to remove surface grease, impurities, and naturally formed oxide films. The cleaning process typically uses alkaline or acidic solutions. This can improve the surface finish after anodizing to some extent.

[0074] Anodizing (voltage: 13-18V, anodizing time: 30-60min, anodizing tank temperature: 20±2℃, sulfuric acid concentration: 180±10g / l): The aluminum metal part is placed in the electrolyte (the composition of the electrolyte depends on the metal used and the desired surface treatment effect). After energizing, an oxide film is formed on the surface of the metal part. The thickness of the oxide film depends on the electrolyte used and the current / time parameters.

[0075] Dyeing treatment: Organic or inorganic dyes are adsorbed through the micropores in the anodic oxide film to produce various colors on metal parts, thereby improving their decorative properties;

[0076] Sealing (Temperature: 60-95℃, Time: 6-15min): By immersing in hot water or other sealants, the micropores of the oxide film are sealed, thereby enhancing its corrosion resistance and hardness. Cleaning and Drying: The treated parts need to be cleaned and dried to remove excess electrolyte and sealant.

[0077] Figure 6 Specific processes and Figure 4 The difference in the process is that the part-removing device is set to remove parts in 2 seconds after the mold is opened.

[0078] Figure 8 Specific processes and Figure 4 The difference in the process is that the part-removing device is set to remove parts in 3 seconds after the mold is opened.

[0079] Figure 3 The oxide film formed on the anodized surface in / 5 / 7 is uneven in color, with the overall oxide film being grayish and accompanied by the formation of other colored oxide films. At the same time, striped protrusions can be clearly seen on the surface, and the oxide film thickness is inconsistent, resulting in a short salt spray test time.

[0080] After the mold is opened at the mold opening temperature, the die-cast part should be quickly removed and chilled. The removal time should preferably be controlled within 5 seconds. If the time is too long, it cannot be guaranteed that no second phase will precipitate on the surface of the die-cast part, which will affect the quality of the final anodizing.

[0081] In one embodiment of the present invention, the mold opening temperature is 290°C.

[0082] In one embodiment of the present invention, the mold opening temperature is 300°C.

[0083] In one embodiment of the present invention, the mold opening temperature is 310°C.

[0084] In one embodiment of the present invention, the mold opening temperature is 320°C.

[0085] In one embodiment of the present invention, the mold opening temperature is 330°C.

[0086] In one embodiment of the present invention, the mold opening temperature is 340°C.

[0087] In one embodiment of the present invention, the mold opening temperature is 350°C.

[0088] In one embodiment of the present invention, the preset time is 4 seconds.

[0089] In one embodiment of the present invention, the preset time is 5 seconds.

[0090] In one embodiment of the present invention, the chilling specifically includes immersing the aluminum-silicon alloy die-cast part in a cooling liquid at 0.5°C.

[0091] In one embodiment of the present invention, the chilling specifically includes immersing the aluminum-silicon alloy die-cast part in a cooling liquid at 10°C.

[0092] In one embodiment of the present invention, the chilling specifically includes immersing the aluminum-silicon alloy die-cast part in a cooling liquid at 15°C.

[0093] In one embodiment of the present invention, the chilling specifically includes immersing the aluminum-silicon alloy die-cast part in a cooling liquid at 20°C.

[0094] In one embodiment of the present invention, the quenching specifically includes immersing the aluminum-silicon alloy die-cast part in liquid nitrogen for cooling.

[0095] In one embodiment of the present invention, the soaking time is 10 seconds.

[0096] In one embodiment of the present invention, the soaking time is 15 seconds.

[0097] In one embodiment of the present invention, the soaking time is 20 seconds.

[0098] In one embodiment of the present invention, the soaking time is 30 seconds.

[0099] In one embodiment of the present invention, the soaking time is 40 seconds.

[0100] In one embodiment of the present invention, the cooling rate corresponding to the quenching is 15°C / s.

[0101] In one embodiment of the present invention, the cooling rate corresponding to the quenching is 50°C / s.

[0102] In one embodiment of the present invention, the cooling rate corresponding to the quenching is 60°C / s.

[0103] In one embodiment of the present invention, the cooling rate corresponding to the quenching is 70°C / s.

[0104] In one embodiment of the present invention, the cooling rate corresponding to the quenching is 80°C / s.

[0105] Figure 9 , 11 This is a motorcycle cooling plate product, made of aluminum alloy ADC12 (ADC12 is recycled aluminum). The metallographic images obtained using conventional methods (100x and 200x magnification) show that the grain uniformity is very disordered, with coarse branched crystals (red rectangles) and second phase precipitation (green circles).

[0106] Figure 10 , 12 This is a motorcycle cooling plate product, made of aluminum alloy ADC12 (ADC12 is recycled aluminum from waste aluminum). The metallographic structure obtained by the method of this invention (100x and 200x) shows that the grains are uniform and very fine, resulting in the same structure as after strontium or sodium modification treatment, and can be anodized.

[0107] The inherent defects of die-cast aluminum alloys have the most fundamental impact on the anodizing process. This is mainly reflected in the influence of alloying elements on the anodizing process, alloy composition, die-casting process, and cooling method.

[0108] (1) The effect of alloying elements on the anodizing process:

[0109] Die-cast aluminum alloys often incorporate various alloying elements to meet die-casting requirements, achieve higher mechanical properties, and other comprehensive performance characteristics. These elements play different roles in die-casting and anodizing processes, often exhibiting opposite effects. For example, in die casting: silicon improves the fluidity of the molten alloy; manganese reduces the harmful effects of iron; copper enhances strength and tensile force; an iron content between 0.7% and 1.2% improves demolding performance; magnesium increases the alloy's mechanical properties; and titanium significantly refines the grain structure of the aluminum alloy, reducing its tendency to hot cracking. Regarding the anodizing process, in alloys: silicon causes the oxide film to turn gray, especially when its content exceeds 4.5%, the effect is more pronounced; manganese causes the oxide film to turn brownish-blue, and the surface color after oxidation changes from brownish-blue to dark brown; copper causes the oxide film to turn reddish, damages the electrolyte quality, and increases oxidation defects; iron, due to its inherent characteristics, exists as black spots after anodizing; magnesium and aluminum alloys containing more than 5% titanium can obtain a colorless and transparent oxide film after oxidation. The process of this invention significantly improves the surface uniformity of die-cast aluminum alloy parts, with no second-phase precipitation and minimal or no segregation on the surface, thus providing a uniform and dense surface structure for anodizing. This reduces the influence of alloying elements on anodizing, thereby improving anodizing quality and salt spray resistance. Furthermore, rapid cooling can form fine grains, which also improves material properties.

[0110] (2) Alloy composition

[0111] Besides the matrix and strengthening phases, aluminum alloys contain many impurities, whose chemical components can affect the physicochemical properties and thickness of the resulting oxide film. Under the same oxidation conditions, different compositions of the aluminum alloy will result in different oxide film performance characteristics. The composition of aluminum alloy anodic oxide films includes aluminum oxide, anions in solution, and some alloying elements existing in the aluminum alloy in oxide, intermetallic compound, or elemental states. Whether the second-phase intermetallic compounds in the aluminum alloy dissolve in the electrolyte or enter the oxide film depends on the potential difference between them and the aluminum matrix. Only when the potential of the second phase is more positive than that of the aluminum alloy or the potential difference between the two is negligible can the second phase enter the oxide film; otherwise, most of it will dissolve in the electrolyte.

[0112] (3) Die casting process

[0113] After the alloy is refined, the die-casting process becomes particularly important. Obtaining high-yield die-cast aluminum alloy parts requires adjusting many die-casting process parameters, such as: aluminum melt temperature, injection time, cooling time, injection speed, return hammer delay, and holding pressure. These die-casting process parameters not only affect the workpiece yield but also significantly impact the anodizing process. Generally speaking, well-filled and denser die-cast parts result in better anodizing effects. This invention, combined with a rapid cooling process, can effectively improve surface density, thereby enhancing the anodizing effect.

[0114] (4) Cooling method

[0115] The cooling method and rate of an alloy directly determine the diffusion and segregation of alloying elements. Choosing a suitable cooling method is beneficial for obtaining alloy parts with uniform elemental composition in all areas. Improper cooling can lead to segregation of alloy components in one area of ​​the workpiece, resulting in uneven distribution of alloying elements and inconsistent performance. The anodized film will also be discontinuous. Current technologies typically only control the cooling rate of the aluminum molten aluminum entering the mold cavity to reduce component segregation. After the casting solidifies, it is removed for finishing and then allowed to undergo natural aging or air cooling after correction. During these processes, slow cooling or natural aging can cause the precipitation of a second phase on the surface, forming an uneven surface metallographic structure, thus affecting the subsequent anodizing structure. This invention employs rapid further cooling after mold opening to ensure that the aluminum alloy casting has no second phase precipitation, little or no segregation, and improved surface quality before anodizing. This enhances the anodizing effect, avoids the influence of alloying elements on anodizing, and improves salt spray resistance. The rapid cooling process of aluminum alloy injection into the mold cavity during die casting results in an extremely high solidification rate, leading to numerous changes in the microstructure formed during solidification. This expands the solid solution limit of the alloy, allowing for the formation of a single-phase solid solution at the equilibrium eutectic point through rapid solidification. Due to the increased solid solubility, ultrafine grain size, and ultrafine and highly dispersed precipitates, the alloy exhibits high strength and high toughness in terms of mechanical properties. The expanded solid solution limit also prevents the precipitation of certain second phases that could seriously compromise performance. Further rapid cooling of the solidified aluminum alloy casting ensures that no second phase precipitates, providing a favorable surface quality for anodizing.

[0116] By employing rapid cooling technology on the aluminum-silicon alloy die-cast parts after mold opening, the precipitation of second-phase particles such as Al₂Cu and Mg₂Si is prevented during the cooling process. This eliminates the influence of second-phase particles in the surface microstructure. The effects of different elements in the alloy on anodizing are as follows:

[0117] (1) Silicon

[0118] The addition of silicon hinders the anodizing process. After degreasing and alkaline etching, aluminum-silicon alloys have a layer of ash insoluble in alkali, mainly composed of silicon. This ash is removed after acid washing, but during the oxidation stage, due to the dissolution of the film, the silicon is gradually exposed, covering the substrate surface and hindering further oxidation. Furthermore, the resulting oxide film turns gray due to the presence of silicon; when the content exceeds 4.5%, it becomes whitish-gray. Adding more silicon will prevent the formation of an anodized film.

[0119] (2) Copper

[0120] Studies using aluminum-copper alloys have revealed that when copper precipitates as a large Al₂Cu phase at the grain boundaries, it forms localized galvanic cells with the matrix, resulting in corrosion caused by the preferential dissolution of Al₂Cu. The precipitated Al₂Cu and the dissolved Cu aluminum matrix exhibit different anodic oxidation behaviors. Specifically, the Al₂Cu matrix is ​​anolyzed more quickly and subsequently dissolved. This leads to the formation of anodized films with numerous pores and defects. Furthermore, copper imparts a reddish tint to the oxide film, compromises electrolyte quality, and increases oxidation defects.

[0121] (3) Magnesium

[0122] Magnesium does not exhibit excessively adverse reactions during the anodizing process. Commercially available aluminum alloy profiles are primarily made of 6-series alloys, whose main components are Al-Mg. Generally, aluminum alloys with a magnesium content greater than 5% can produce a colorless and transparent oxide film after anodizing.

[0123] (4) Zinc

[0124] Zinc causes aluminum alloys to develop a milky color after anodizing. In basic studies, as the zinc content gradually increases, the color changes from red to purple after oxidation. When the zinc content reaches 11.6% of the aluminum alloy, the anodized color begins to become uneven.

[0125] (5) Manganese

[0126] Manganese is usually added as a manganese additive. The addition of manganese will cause a color change in the anodized film. Aluminum alloys containing 1%-2% manganese generally turn brownish-blue after oxidation. As the manganese content gradually increases, the surface color after oxidation changes from brownish-blue to dark brown.

[0127] (6) Titanium

[0128] Titanium and magnesium have similar effects. The addition of titanium has a negligible impact on the anodic oxide film. When the titanium content is greater than 5%, a colorless and transparent oxide film can be obtained after anodizing.

[0129] In the manufacturing process of die-cast aluminum alloys, various alloying elements are often added to achieve better casting performance, higher mechanical properties, and other comprehensive properties, thus forming the traditional cast aluminum alloy system. The presence of these alloying elements results in inhomogeneity in the internal chemical composition and microstructure of the aluminum alloy. Combined with residual stress from processing and machining, this makes die-cast aluminum alloys highly susceptible to micro-cell corrosion during use. (Micro-cell corrosion occurs when the material comes into contact with an electrolyte solution. Due to various factors such as material inhomogeneity and surface inhomogeneity, the electrochemical interaction on the metal surface becomes uneven, resulting in numerous extremely small electrodes that form micro-cells, leading to corrosion. In practice, many factors contribute to electrochemical inhomogeneity, such as uneven chemical composition, uneven microstructure, and incomplete surface films. Avoiding the presence of these electrochemical inhomogeneities can effectively reduce corrosion.) During anodizing of die-cast aluminum alloys, uneven element distribution, varying grain sizes, and grain boundaries create pits on the surface. These pits result in uneven coating during anodizing, leading to electrochemical corrosion under atmospheric moisture. The material matrix forms the anode, while impurities and heterogeneous phases form the cathode, creating micro-cells. Therefore, traditionally anodized die-cast aluminum alloy parts can only withstand 160-200 hours of salt spray testing, while anodized die-cast aluminum alloy parts processed by this invention can withstand over 1000 hours. The main reason is that the rapid cooling process after die casting prevents the precipitation of second phases on the die-cast surface, resulting in minimal or no segregation and a uniform elemental distribution. This leads to a more uniform coating during anodizing, significantly reducing the impact of micro-cell corrosion.

[0130] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A process for improving the anodizing effect of aluminum-silicon alloy die-cast parts, characterized in that, Includes the following steps: S1: The mold opening temperature of the aluminum-silicon alloy die casting is determined based on the nose tip temperature corresponding to the precipitation of the second phase in the aluminum-silicon alloy die casting; the mold opening temperature is higher than the nose tip temperature. S2: The aluminum-silicon alloy die-cast part that has reached the mold opening temperature is removed and subjected to quenching after a preset time; the cooling rate corresponding to the quenching is higher than the critical cooling rate corresponding to the precipitation of the second phase; the preset time is 0.1-5 seconds; S3: Perform anodizing treatment.

2. The process method for improving the anodizing effect of aluminum-silicon alloy die-castings according to claim 1, characterized in that: The nose tip temperature was determined by the TTT curve of the aluminum-silicon alloy die casting; the critical cooling rate was determined by the CCT curve of the aluminum-silicon alloy die casting.

3. The process method for improving the anodizing effect of aluminum-silicon alloy die-castings according to claim 1, characterized in that: The mold opening temperature is 10-50°C above the nose tip temperature.

4. The process method for improving the anodizing effect of aluminum-silicon alloy die-castings according to claim 1, characterized in that: The mold opening temperature is 280-360℃.

5. The process method for improving the anodizing effect of aluminum-silicon alloy die-castings according to claim 1, characterized in that: The quenching specifically involves immersing the aluminum-silicon alloy die-cast part in a cooling liquid at 0.5-20°C.

6. The process method for improving the anodizing effect of aluminum-silicon alloy die-castings according to claim 5, characterized in that: The soaking time is 5-70 seconds.

7. The process method for improving the anodizing effect of aluminum-silicon alloy die-castings according to claim 5, characterized in that: The coolant is water or an emulsion.

8. The process method for improving the anodizing effect of aluminum-silicon alloy die-castings according to claim 1, characterized in that: The quenching specifically involves immersing the aluminum-silicon alloy die-cast part in liquid nitrogen for cooling.

9. A process method for improving the anodizing effect of aluminum-silicon alloy die-castings according to any one of claims 1, 5, and 8, characterized in that: The cooling rate corresponding to the quenching is 0.5-100℃ / s.

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