Preparation method of composite reinforced high-strength high-formability aluminum-magnesium alloy

Abstract

This invention relates to the field of aluminum alloy preparation technology, specifically to a method for preparing a high-strength, high-formability aluminum-magnesium alloy with composite strengthening. The method comprises two parts: precise alloy composition design and gradient deformation heat treatment. By constructing a composite strengthening mechanism of solid solution strengthening, precipitation strengthening, grain refinement strengthening, and subgrain boundary strengthening, a high-strength, high-formability aluminum-magnesium alloy is prepared. This solves the core problems of existing aluminum-magnesium alloys, such as limited strengthening effect, difficulty in balancing strength and formability, lack of a synergistic composite strengthening mechanism, and inability to meet the comprehensive material requirements of 3C products. Through precise composition design and gradient deformation heat treatment, a four-fold composite strengthening mechanism is constructed, significantly improving alloy strength and achieving synergistic optimization of strength and formability. The alloy possesses a high hardening index and plasticity hardening ratio, excellent cold stamping formability, and the process is simple with low raw material costs, making it suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy preparation technology, specifically to a method for preparing a composite-strengthened, high-strength, high-formability aluminum-magnesium alloy. Background Technology

[0002] As is well known, aluminum alloys have become a core material for high-end manufacturing and electronic products due to their low density, good thermal conductivity, and significant advantages in lightweighting. Among them, 5XXX aluminum-magnesium alloys are widely used in the field of complex stamping parts for 3C products due to their good plasticity, corrosion resistance, and weldability. However, this series of alloys is not heat-treatable and mainly relies on cold work hardening to improve strength. The conventional tensile strength is only 173-305MPa and the yield strength is ≥65-210MPa. Facing the increasingly complex structural forming requirements of electronic products, the strength and impact resistance are obviously insufficient. At the same time, surface steps and serrated deformation bands are easily generated during deep drawing, making it difficult to control the surface quality.

[0003] While the higher-strength 6XXX and 7XXX series aluminum alloys possess excellent strength, their processing in ultra-thin dimensions (below 0.2mm) presents significant technical challenges and high production costs, making them unsuitable for the thin-walled requirements of 3C products. Existing technologies for alloy improvement primarily involve the addition of single elements or simple process adjustments, resulting in limited strengthening effects and difficulty in balancing strength and formability. Furthermore, they lack a composite strengthening mechanism that integrates solid solution, precipitation, and fine grain refinement, failing to meet the comprehensive requirements of 3C products for high strength, high formability, and thin-size processing. Therefore, a composite-strengthened high-strength, high-formability aluminum-magnesium alloy preparation method is needed. Summary of the Invention

[0004] Technical problems to be solved To overcome the shortcomings of existing aluminum-magnesium alloy preparation methods, such as limited strengthening effect, difficulty in balancing strength and formability, and the lack of a composite strengthening mechanism that combines solid solution, precipitation, and fine grain, which makes it difficult to meet the requirements of 3C products for high strength, high formability, and thin-gauge machinability, this invention provides a composite-strengthened high-strength and high-formability aluminum-magnesium alloy preparation method.

[0005] Technical solution To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a composite-strength and highly formable aluminum-magnesium alloy, comprising two parts: precise alloy composition design and gradient deformation heat treatment process. By constructing a composite strengthening mechanism of solid solution strengthening, precipitation strengthening, fine grain strengthening and subgrain boundary strengthening, a high-strength and highly formable aluminum-magnesium alloy is prepared. The alloy, by mass percentage, consists of 0.1% < Cu ≤ 0.5%, 0 < Zn ≤ 0.1%, 0 < Si ≤ 0.2%, 0 < Fe ≤ 0.3%, 0 < Mn ≤ 1.0%, 5.7% < Mg ≤ 6.3%, 0.05% < Cr ≤ 0.25%, 0 < V ≤ 0.10%, and the balance of Al and inevitable impurities. The single content of inevitable impurities is ≤ 0.05 wt% and the total content is ≤ 0.15 wt%. The preparation method successively includes: S1: Melting and casting. Weigh raw materials according to the mass percentages designed in the above composition. Put industrial pure aluminum ingots into a graphite crucible and heat them in an electric resistance furnace. After the aluminum ingots are completely melted, successively add Al-Mn10, Al-Cr5, and Al-V5 master alloys. Stir evenly and keep warm to fully dissolve the alloying elements. Then lower the temperature, add magnesium ingots and Al-Cu50, Al-Zn10 master alloys, stir quickly, and pour the refined melt into a preheated cast iron mold by gravity casting. The mold is cooled by water cooling. S2: Homogenization treatment. Remove the surface oxide scale and risers of the cast ingot after casting, and put it into a box-type electric resistance furnace for homogenization treatment. After the heat preservation is completed, cool it in the furnace to room temperature. S3: Hot working. Heat and keep the cast ingot warm, then send it to a two-high hot rolling mill for multi-pass hot rolling, and finally roll it into a hot rolled plate. Heat and keep the cast ingot warm, then send it to an extruder for hot extrusion, and finally extrude it into an extruded plate. S4: The first cold rolling. Clean the surface of the plate after hot working to remove the oxide scale and oil stain, and then send it to a four-high cold rolling mill for the first cold rolling. S5: Intermediate annealing treatment. Put the plate after the first cold rolling into a box-type electric resistance furnace for intermediate annealing treatment. S6: The second cold rolling. Clean the surface of the plate after intermediate annealing to remove the oxide scale generated during annealing, and then send it to a four-high cold rolling mill for the second cold rolling. S7: Final heat treatment. Put the ultra-thin plate after the second cold rolling into a box-type electric resistance furnace for final heat treatment. After the heat preservation is completed, cool it in the furnace to room temperature. After the heat treatment is completed, straighten the plate to achieve the coordinated optimization of the alloy strength and formability.

[0006] Preferably, the preferred composition of the alloy, by mass percentage, is: Cu 0.2 - 0.4 wt%, Zn 0.03 - 0.08 wt%. The contents of the remaining elements are the same as those in claim 1. Through this preferred composition, the synergistic solid solution and precipitation strengthening of Cu and Zn are achieved, and the comprehensive performance of the alloy is improved.

[0007] Furthermore, the smelting and casting steps are as follows: raw materials are weighed according to the composition ratio, and after smelting, refining and removing impurities, they are cast into ingots. The refining process ensures that the raw materials are fully fused, effectively controls the impurity content, and avoids the adverse effects of impurities on the alloy structure and properties.

[0008] Furthermore, the homogenization process involves placing the ingot in a heating environment of 520–580°C for 4–12 hours. This process eliminates component segregation in the ingot, refines the as-cast microstructure, and provides a uniform microstructure basis for subsequent hot working and cold rolling.

[0009] In a further embodiment, the hot processing step is as follows: the homogenized ingot is hot rolled or hot extruded, the billet temperature is controlled at 480-550℃, and the final processing temperature is not lower than 300℃, so as to avoid material hardening and processing cracking caused by low temperature processing, and to ensure that the material after hot processing has good plasticity.

[0010] Based on the aforementioned scheme, the first cold rolling step is as follows: the hot-processed material is cold-rolled with a cold rolling reduction rate of not less than 30%. This step initially introduces work hardening, improves the basic strength of the material, and introduces dislocation density for subsequent secondary cold rolling.

[0011] Based on the aforementioned scheme, the intermediate annealing process is further described as follows: the material after the first cold rolling is annealed at 350-450°C and held for 1-5 hours, preferably at 400-450°C, to achieve complete recrystallization of the material, eliminate the processing stress generated by cold rolling, optimize the grain morphology, and avoid surface defects in subsequent processing.

[0012] Based on the aforementioned scheme, the second cold rolling step is as follows: the material after intermediate annealing is subjected to cold rolling treatment, and the cold rolling reduction rate is greater than the reduction rate of the first cold rolling, preferably more than twice the reduction rate of the first cold rolling, to further improve the work hardening effect, introduce high dislocation density, and strengthen the mechanical properties of the alloy.

[0013] Based on the aforementioned scheme, the final heat treatment step is as follows: the material after the second cold rolling is annealed at 150-250℃ and held for 1-5 hours to achieve alloy stabilization treatment, fix the microstructure of the alloy, eliminate residual stress, and ensure the long-term stability of the alloy's mechanical properties and formability.

[0014] Furthermore, based on the aforementioned scheme, the aluminum-magnesium alloy prepared by this method has a tensile strength ≥450MPa and a yield strength ≥350MPa, and has a high hardening index n value and a high plasticity hardening ratio r value. It has excellent cold stamping formability and maintains good processability and performance stability even in ultra-thin specifications of 0.2~0.6mm. It is suitable for the preparation of thin-walled structural parts of electronic products such as precision brackets, VC heat sinks, and shielding parts in the digital 3C industry. Moreover, the preparation process is simple and the raw material cost is low, making it suitable for large-scale industrial production.

[0015] Beneficial effects This composite-strengthened high-strength, high-formability aluminum-magnesium alloy preparation method solves the core problems of existing 5XXX aluminum-magnesium alloys, such as limited strengthening effect, difficulty in balancing strength and formability, lack of synergistic composite strengthening mechanism, and difficulty and high cost in processing ultra-thin specifications of 6XXX and 7XXX series alloys, which cannot meet the comprehensive material requirements of 3C products. Through precise composition design and gradient deformation heat treatment, a four-fold composite strengthening mechanism is constructed, which significantly improves the alloy strength, with tensile strength ≥450MPa and yield strength ≥350MPa, achieving synergistic optimization of strength and formability. The alloy has a high hardening index and plasticity hardening ratio, excellent cold stamping formability, and maintains good processability and stability even in ultra-thin specifications of 0.2~0.6mm, which is suitable for the needs of 3C thin-walled structural parts. Moreover, the process is simple and the raw material cost is low, making it suitable for large-scale industrial production. Attached Figure Description

[0016] Figure 1 This is a flowchart of the overall preparation process of the aluminum-magnesium alloy of the present invention; Figure 2 This is a precise design drawing of the aluminum-magnesium alloy composition for this invention; Figure 3 This is a diagram illustrating the quadruple composite reinforcement mechanism and synergistic relationship of the present invention. Figure 4 This is a comparison chart of core parameters for key processes in the gradient deformation heat treatment of this invention. Figure 5 This is a diagram showing the relationship between the performance indicators of the aluminum-magnesium alloy of the present invention and its application scenarios. Detailed Implementation

[0017] 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 some embodiments of the present invention, and not all embodiments. 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.

[0018] See Figures 1-5, A preparation method of a composite-strengthened high-strength and high-formability aluminum-magnesium alloy. The core lies in the coordinated cooperation of precise alloy composition design and gradient deformation heat treatment process to construct a quadruple composite strengthening mechanism of solid solution strengthening, precipitation strengthening, fine grain strengthening, and sub-grain boundary strengthening. Finally, a 5XXX series aluminum-magnesium alloy suitable for the thinning requirements of 3C products is prepared. This method not only solves the defects of insufficient strength and the difficulty in balancing formability and strength in traditional 5XXX series alloys but also avoids the problems of high processing difficulty and high cost in ultra-thin specifications of 6XXX and 7XXX series alloys, achieving a comprehensive balance of high strength, high formability, and thin-specification processability.

[0019] The alloy composition design of this invention is based on the 5XXX series aluminum-magnesium alloy. By precisely regulating the content of main alloy elements, introducing synergistic strengthening elements, and strictly controlling the impurity content, it lays a compositional foundation for the formation of the composite strengthening mechanism. The specific composition and design basis of the alloy are as follows by mass percentage: (I) Core alloy elements and content range Mg: 5.7% < Mg ≤ 6.3% Mg is the core strengthening element of the 5XXX series aluminum-magnesium alloy. Its content directly determines the solid solution strengthening effect of the alloy. Mg forms an α-Al solid solution with Al, and through solid solution strengthening, the basic strength of the alloy is improved. After Mg atoms are incorporated into the Al lattice, lattice distortion occurs, hindering dislocation movement, thus significantly increasing the hardness and strength of the alloy.

[0020] Limiting the Mg content to the range of 5.7% - 6.3% is mainly based on two considerations. On the one hand, when the Mg content ≤ 5.7%, the concentration of Mg atoms in the solid solution is insufficient, and the solid solution strengthening effect is limited. It is difficult for the tensile strength of the alloy to reach the target of 450 MPa. On the other hand, when the Mg content > 6.3%, brittle phases such as β-Al3Mg2 are likely to precipitate in the alloy. These phases are continuously distributed in a network shape at the grain boundaries, which will seriously reduce the plasticity and formability of the alloy, resulting in defects such as cracking and surface steps during deep drawing processing. The narrow range of 5.7% - 6.3% is selected in this invention, which not only ensures sufficient solid solution strengthening effect but also avoids excessive precipitation of brittle phases, achieving a preliminary balance between strength and plasticity.

[0021] Cu: 0.1% < Cu ≤ 0.5%, preferably 0.2 - 0.4 wt% Cu is the key element for achieving precipitation strengthening in this invention. Its content design directly affects the final strength of the alloy. Cu has a certain solubility in the Al-Mg alloy. During subsequent heat treatment, it will form fine and dispersed precipitation phases such as Al2CuMg (S phase) and Al6CuMg4 (T phase) with Al and Mg. These precipitation phases can effectively pin dislocations and hinder dislocation slip, thereby achieving significant precipitation strengthening.

[0022] When the Cu content is less than 0.1%, the number of precipitated phases is too small and the precipitation strengthening effect is weak. When the Cu content is greater than 0.5%, excessive Cu will cause the coarsening and aggregation of precipitated phases. Not only is the strengthening effect saturated, but it will also reduce the plasticity and corrosion resistance of the alloy. The preferred Cu content is 0.2 - 0.4 wt%, because within this range, Cu can form a synergistic effect with the subsequent Zn element - the atomic radius difference between Cu and Zn is relatively small, and the addition of Zn can promote the solid solution and uniform precipitation of Cu in the Al matrix, avoiding the local enrichment of Cu elements to form coarse inclusions, and further improving the uniformity and effectiveness of precipitation strengthening.

[0023] Zn: 0 < Zn ≤ 0.1%, preferably 0.03 - 0.08 wt% Zn is the synergistic strengthening element of the present invention. Its main function is to assist Cu to achieve better solid solution and precipitation strengthening. The solid solubility of Zn and Al is relatively high, and it can preferentially dissolve into the Al matrix, reducing the solid solution activation energy of Cu in Al, promoting the uniform dispersion of Cu atoms in the matrix, and avoiding the segregation of Cu at grain boundaries. At the same time, the introduction of Zn atoms will slightly change the electron density of the Al lattice, optimizing the nucleation and growth conditions of precipitated phases, making precipitated phases such as Al2CuMg and Al6CuMg4 finer and more dispersed, and further enhancing the precipitation strengthening effect.

[0024] The Zn content is strictly controlled to be ≤ 0.1% because excessive Zn will reduce the corrosion resistance of the alloy and may form the brittle phase MgZn2 with Mg, affecting the formability of the alloy. The preferred range of 0.03 - 0.08 wt% can not only ensure the synergistic strengthening effect with Cu but also have no negative impact on the plasticity and corrosion resistance of the alloy.

[0025] Mn: 0 < Mn ≤ 1.0% Mn is the key element for refining grains and improving the processing performance of the alloy. Mn will form the Al6Mn dispersed phase in the Al-Mg alloy, which can act as a heterogeneous nucleation core, hindering grain growth during crystallization and recrystallization processes, achieving fine grain strengthening. At the same time, the Al6Mn phase can also pin grain boundaries, improving the thermal stability of the alloy and avoiding grain coarsening during hot processing.

[0026] The Mn content ≤ 1.0% is because excessive Mn will cause the coarsening of the Al6Mn phase, instead reducing the effect of refining grains, and will also increase the melting difficulty and work hardening rate of the alloy, affecting the formability. An appropriate amount of Mn (the best practice range is 0.3% - 0.8%) can ensure fine grain strengthening while improving the stress corrosion resistance of the alloy, meeting the requirements of complex stamping processing of 3C products.

[0027] Cr: 0.05% < Cr ≤ 0.25% Similar to Mn, Cr is also an element that achieves fine grain strengthening by forming dispersed phases. In the Al-Mg alloy, Cr forms the Al7Cr dispersed phase. The stability of this phase is higher than the Al6Mn phase and can maintain a fine and dispersed state at higher temperatures (such as during homogenization treatment and hot working processes), continuously hindering grain growth and further refining the alloy microstructure.

[0028] When the Cr content is < 0.05%, the amount of the Al7Cr phase is insufficient and the refining effect is not obvious. When the Cr content > 0.25%, coarse Cr-rich inclusions will be formed, becoming stress concentration sources in the alloy and reducing the plasticity and fatigue properties of the alloy. In the content range of 0.05% - 0.25%, the Al7Cr phase and the Al6Mn phase can act synergistically to form a denser heterogeneous nucleation core, refining the alloy grain size to below 10 μm and providing a structural basis for fine grain strengthening.

[0029] V: 0 < V ≤ 0.10% V is an additional auxiliary refining element in this invention, and its role is to further optimize the fine grain strengthening effect. The solubility of V in Al is extremely low, mainly forming the Al3V dispersed phase. The size of this phase is smaller than the Al6Mn and Al7Cr phases (usually 50 - 200 nm), which can more efficiently pin dislocations and grain boundaries, hinder grain growth and dislocation movement, and at the same time can also increase the density of sub-grain boundaries in the alloy, strengthening the sub-grain boundary strengthening mechanism.

[0030] The V content is controlled ≤ 0.10% because excessive V will cause aggregation of the Al3V phase, and the raw material cost of V is relatively high. Excessive addition will increase production costs. Appropriate V (0.03% - 0.08%) can form a synergistic refining effect with Mn and Cr, making the grains of the alloy more uniform and finer, while increasing the hardening index n value of the alloy and improving the formability.

[0031] Control of impurity elements: Si ≤ 0.2%, Fe ≤ 0.3%, individual impurity ≤ 0.05 wt%, total impurity ≤ 0.15 wt% Si and Fe are common impurity elements in aluminum-magnesium alloys. Excessive Si will form the brittle Mg2Si phase with Mg, and Fe will form the Al3Fe inclusion with Al. These phases and inclusions will reduce the plasticity, formability and corrosion resistance of the alloy, and also cause surface defects (such as scratches and pitting) during the processing.

[0032] This invention strictly controls Si ≤ 0.2%, Fe ≤ 0.3%, and limits individual impurity ≤ 0.05 wt% and total impurity ≤ 0.15 wt%. This is mainly achieved by optimizing the melting and refining processes (see the subsequent melting and casting steps for details). The low impurity content can ensure the purity of the alloy matrix, provide a good matrix environment for the full play of mechanisms such as solid solution strengthening and precipitation strengthening, and avoid the weakening of the strengthening effect by impurity phases.

[0033] (II) Synergistic Principle of Composite Reinforcement Mechanism The core innovation of this invention lies in constructing a four-fold combined strengthening mechanism of solid solution strengthening, precipitation strengthening, grain refinement strengthening, and subgrain boundary strengthening. These strengthening mechanisms do not act independently, but rather achieve synergistic effects through compositional design and process control. Solid solution strengthening is fundamental: elements such as Mg, Cu, and Zn are incorporated into the Al matrix to form solid solutions, causing lattice distortion and providing basic strength for subsequent strengthening mechanisms. At the same time, supersaturated alloying elements in the solid solution provide compositional reserves for precipitation strengthening.

[0034] Precipitation strengthening is the core: through gradient deformation heat treatment, supersaturated Cu, Zn and Mg elements in the solid solution precipitate to form fine and dispersed second phases (Al2CuMg, Al6CuMg4, etc.). These precipitates pin dislocations, significantly improving the alloy strength, and the fine and dispersed characteristics of the precipitates avoid excessive damage to plasticity.

[0035] Fine grain strengthening is the key to ensuring that the dispersed phases (Al6Mn, Al7Cr, Al3V) formed by elements such as Mn, Cr, and V hinder grain growth, thus refining the alloy grains to below 10μm. According to the Hall-Page relationship, grain refinement can simultaneously improve the strength and plasticity of the alloy, thus resolving the contradiction that the traditional strengthening method must sacrifice plasticity to improve strength.

[0036] Subgrain boundary strengthening is a supplement: gradient cold rolling introduces high dislocation density, intermediate annealing and final heat treatment cause dislocation reorganization to form subgrain boundaries. Subgrain boundaries can both hinder dislocation movement and improve strength, and improve plasticity through the coordinated deformation of subgrains, further optimizing the balance between strength and formability.

[0037] The synergistic effect of the quadruple composite strengthening mechanism enables the alloy to achieve high strength indicators of tensile strength ≥450MPa and yield strength ≥350MPa, while maintaining a high hardening index n value (≥0.25) and a high plasticity hardening ratio r value (≥0.8), meeting the stringent requirements of cold stamping forming of 3C products.

[0038] II. Detailed Process Flow of Gradient Deformation Heat Treatment The gradient deformation heat treatment process of the present invention includes seven steps in sequence: melting and casting → homogenization treatment → hot working → first cold rolling → intermediate annealing treatment → second cold rolling → final heat treatment. Each step is progressive and corresponds to different stages of the composite strengthening mechanism, ultimately achieving synergistic optimization of alloy properties.

[0039] (I) Step S1: Smelting and casting – laying the foundation for compositional uniformity 1. Operating Procedures (1) Raw material preparation: Weigh the raw materials according to the mass percentage designed above. Among them, Al uses industrial pure aluminum ingots with a purity ≥99.7%, Mg uses magnesium ingots with a purity ≥99.9%, and Cu, Zn, Mn, Cr, V and other elements are added in the form of intermediate alloys (such as Al-Cu50, Al-Zn10, Al-Mn10, Al-Cr5, Al-V5 intermediate alloys) to avoid burning or agglomeration of pure metal elements. (2) Melting process: Place the industrial pure aluminum ingots into a graphite crucible and heat it in a resistance furnace to 720-750℃. After the aluminum ingots are completely melted, add Al-Mn10, Al-Cr5, and Al-V5 intermediate alloys in sequence. Stir evenly and keep warm for 20-30 minutes to fully dissolve the alloy elements. Then lower the temperature to 680-700℃ and add magnesium ingots and Al- Cu50, Al-Zn10 intermediate alloy, stir quickly for 10-15 min to prevent Mg volatilization and Cu and Zn agglomeration. (3) Refining and impurity removal: Add 0.1%-0.2% hexachloroethane (C2Cl6) refining agent to the melt, stir evenly and keep warm for 15-20 min. Use the Cl2, HCl and other gases generated by the decomposition of the refining agent to remove hydrogen and oxide inclusions in the melt. After refining, let stand for 20-30 min to allow the inclusions to float to the surface of the liquid. Skim off the scum. (4) Casting process: Control the temperature of the refined melt at 670-690℃ and inject it into a preheated cast iron mold at 200-250℃ by gravity casting. The mold is cooled by water cooling at a rate of 10-20℃ / s. Finally, an ingot with Φ120-150mm is obtained.

[0040] 2. Basis for process parameter control (1) Melting temperature: The aluminum ingot is melted at a high temperature of 720-750℃ to ensure that the intermediate alloy is fully dissolved. The Mg ingot and Cu and Zn intermediate alloy are added at a low temperature of 680-700℃ to reduce the volatilization loss of Mg (Mg has a boiling point of 1090℃ and volatilizes severely at high temperatures) and to avoid excessive oxidation of Cu and Zn elements. (2) Refining agent dosage: The dosage of 0.1%-0.2% hexachloroethane can effectively remove hydrogen and inclusions, and will not cause Cl element residue due to excessive use, which will affect the corrosion resistance of the alloy. (3) Cooling rate: The cooling rate of 10-20℃ / s can suppress the segregation of alloy elements, make the ingot composition uniform, refine the as-cast grains, avoid the formation of coarse columnar crystals, and lay a good foundation for subsequent homogenization treatment and processing technology.

[0041] 3. Process Objectives By precisely controlling the melting temperature, refining process, and cooling rate, ingots with uniform composition, low impurity content, and fine grains can be prepared, avoiding problems such as cracking and surface defects in subsequent processing due to component segregation, excessive impurities, or coarse grains, thus providing a pure and uniform matrix for the formation of the composite strengthening mechanism.

[0042] (ii) Step S2: Homogenization treatment – ​​eliminating segregation and refining the microstructure 1. Operating Procedures After casting, the surface oxide scale and risers are removed from the ingots, and the ingots are cut into billets with a diameter of 120-150 mm and a diameter of 300-400 mm. The billets are then placed in a box-type resistance furnace for homogenization treatment. The heating rate is controlled at 5-10℃ / min. After heating to 520-580℃, the temperature is held for 4-12 hours. After the holding period, the ingots are cooled to room temperature with the furnace at a rate of 15-25℃ / min.

[0043] 2. Basis for process parameter control (1) Homogenization temperature: The temperature range of 520-580℃ is close to the solidus of Al-Mg alloy (about 650℃), which can maximize the diffusion of elements such as Mg, Cu, and Zn that are segregated in the ingot, and eliminate compositional segregation. At the same time, this temperature range can partially dissolve or spheroidize the coarse second phase (such as Mg2Si, Al3Fe) in the as-cast structure, reducing the adverse effects of brittleness on subsequent processing. When the temperature is below 520℃, the element diffusion rate is slow and the segregation is not completely eliminated. When the temperature is above 580℃, the ingot is prone to overheating, leading to crystal growth. (2) Holding time: The holding time of 4 to 12 hours is adjusted according to the size of the ingot - small ingots (Φ120mm) only need to be held for 4 to 6 hours, and large ingots (Φ150mm) need to be held for 8 to 12 hours to ensure that the composition of the core and the surface is uniform. (3) Cooling rate: A furnace cooling rate of 15 to 25℃ / min can avoid thermal stress caused by rapid cooling, and at the same time make the dissolved alloying elements uniformly distributed in the matrix, providing a uniform composition basis for subsequent solid solution strengthening and precipitation strengthening.

[0044] 3. Process Objectives (1) Eliminate compositional segregation in ingots: promote the diffusion of alloying elements through high-temperature heat preservation, make the composition of each part of the ingot uniform and avoid local hardening or plasticity differences caused by compositional inhomogeneity during subsequent processing, and improve the processing uniformity of the alloy. (2) Refine the as-cast structure: refine and spheroidize the coarse grains and second phase in the as-cast state, reduce stress concentration during processing, and improve the plasticity and processing formability of the alloy. (3) Provide a uniform structure for subsequent processing: the uniform structure and fine grains of the ingot after homogenization can improve the deformation uniformity during hot working and cold rolling, avoid processing cracks, and lay the foundation for the subsequent introduction of cold work hardening and fine grain strengthening.

[0045] (III) Step S3: Hot working - roughing and shaping, preliminary grain refinement 1. Operating Procedures After homogenization, the ingot is hot-processed. Two methods can be selected: hot rolling or hot extrusion. The specific process is as follows: (1) Hot rolling process: The ingot is heated to 480-550℃ (billing temperature), held for 2-3 hours, and then sent to a two-roll hot rolling mill for multi-pass hot rolling. The pass reduction rate is controlled at 10%-15%, and the final rolling temperature is not lower than 300℃. Finally, it is rolled into a hot-rolled plate with a thickness of 6-10mm. (2) Hot extrusion process: The ingot is heated to 480-520℃, held for 2-3 hours, and then sent to an extruder for hot extrusion. The extrusion ratio is controlled at 15-25:1, the extrusion speed is 5-10mm / s, and the final extrusion temperature is not lower than 300℃. Finally, it is extruded into an extruded plate with a thickness of 8-12mm.

[0046] 2. Basis for process parameter control (1) Opening temperature: The temperature range of 480 to 550℃ can ensure that the alloy has good plasticity, reduce processing stress, and avoid hardening and cracking caused by cold working. At the same time, this temperature range can activate dynamic recrystallization, so that the dislocations generated during processing can be partially eliminated through recrystallization, and the grains can be refined. (2) Final processing temperature: The final processing temperature of not less than 300℃ is to avoid the alloy from working hardening at low temperature, and ensure that the material after hot working still has good plasticity, providing a basis for subsequent cold rolling process. If the final processing temperature is lower than 300℃, the alloy will have reduced plasticity due to work hardening, and defects such as edge cracks and surface peeling are likely to occur during subsequent cold rolling. (3) Reduction rate / extrusion ratio: The reduction rate of hot rolling pass is 10% to 15%, and the extrusion ratio is 15 to 25:1. This can ensure sufficient deformation, promote dynamic recrystallization to refine the grains, and prevent the processing stress concentration caused by excessive deformation, which can lead to cracking.

[0047] 3. Process Objectives (1) Billet shaping: The ingot is processed into a plate of a certain thickness to provide a suitable billet size for the subsequent cold rolling process. (2) Preliminary grain refinement: Through dynamic recrystallization during the hot working process, the alloy grains are refined to further enhance the grain strengthening effect. At the same time, hot working can break the coarse second phase in the cast structure, so that the second phase is more evenly distributed in the matrix. (3) Improved processing performance: The material after hot working has good plasticity and low processing stress, which can reduce the risk of cracking in the subsequent cold rolling process and improve the processing uniformity.

[0048] (iv) Step S4: First cold rolling – Introducing work hardening to improve basic strength 1. Operating Procedures After heat treatment, the sheet material is cleaned to remove oxide scale and oil stains, and then sent to a four-roll cold rolling mill for the first cold rolling: the cold rolling temperature is room temperature (20-30℃), the cold rolling reduction rate is not less than 30%, and finally rolled into a cold-rolled sheet with a thickness of 3-5mm. Rolling oil is used for lubrication during the cold rolling process, and the rolling speed is controlled at 5-10m / min to ensure the surface quality of the sheet material.

[0049] 2. Basis for process parameter control (1) Cold rolling reduction rate: A reduction rate of not less than 30% can effectively introduce work hardening - during the cold rolling process, the alloy matrix undergoes plastic deformation, dislocations multiply in large numbers, and the dislocation density increases from 10 after hot working. 12 ~10 13 m -2 Upgraded to 10 14 ~10 15 m -2 Dislocation interaction (crossing, entanglement) can significantly improve the basic strength of the alloy. If the reduction rate is less than 30%, the dislocation density proliferation is insufficient, the cold work hardening effect is weak, and the subsequent strengthening mechanism is difficult to achieve the target strength. (2) Rolling temperature: Room temperature cold rolling can retain dislocations to the maximum extent and avoid the dislocation recovery or recrystallization at high temperature, which will weaken the cold work hardening effect. At the same time, room temperature cold rolling can avoid alloy oxidation and ensure the surface quality of the plate. (3) Rolling speed and lubrication: A rolling speed of 5-10 m / min combined with rolling oil lubrication can reduce the friction between the roll and the plate, avoid the appearance of scratches, sticking to the roll and other defects on the plate surface, and ensure that the surface roughness Ra≤0.8μm meets the surface quality requirements of 3C products.

[0050] 3. Process Objectives (1) Introducing cold work hardening: The basic strength of the alloy is improved by the proliferation of a large number of dislocations, laying the foundation for the strengthening of the subsequent second cold rolling and heat treatment. (2) Reserve dislocations for precipitation strengthening: The high dislocation density introduced by cold rolling can provide sites for the nucleation of precipitates during subsequent heat treatment. Dislocation lines are the preferred sites for the nucleation of precipitates. High dislocation density can promote the uniform nucleation of precipitates, avoid coarsening of precipitates, and improve the precipitation strengthening effect. (3) Improving the dimensional accuracy of the plate: The thickness of the plate is precisely controlled by cold rolling, providing a dimensional basis for the subsequent ultra-thin specification processing.

[0051] (v) Step S5: Intermediate annealing treatment – ​​complete recrystallization, optimizing grain morphology 1. Operating Procedures The first cold-rolled sheet is placed in a box-type resistance furnace for intermediate annealing: the heating rate is controlled at 10-15℃ / min, and after heating to 350-450℃ (preferably 400-450℃), it is held for 1-5 hours. After the holding period, it is cooled to room temperature with the furnace at a cooling rate of 20-30℃ / min.

[0052] 2. Basis for process parameter control (1) Annealing temperature: The temperature range of 350-450℃ is the complete recrystallization temperature range of 5XXX series aluminum-magnesium alloys. When the temperature is below 350℃, recrystallization is incomplete, the processing stress generated by cold rolling cannot be completely eliminated, and the grain refinement effect is not good. When the temperature is above 450℃, the grains after recrystallization will coarsen, which will reduce the grain strengthening effect. 400-450℃ is preferred because this range can achieve complete recrystallization and obtain finer and more uniform recrystallized grains (grain size ≤8μm). (2) Holding time: The holding time of 1-5 hours is adjusted according to the thickness of the plate. For plates with a thickness of 3-4mm, the holding time is 1-2 hours, and for plates with a thickness of 4-5mm, the holding time is 3-5 hours to ensure that the core and surface of the plate can achieve complete recrystallization. (3) Cooling rate: The furnace cooling rate of 20-30℃ / min can avoid the thermal stress caused by rapid cooling and stabilize the grain morphology after recrystallization, avoiding abnormal grain growth.

[0053] 3. Process Objectives (1) Complete recrystallization: Eliminates the processing stress and work hardening generated by the first cold rolling, allowing dislocations to rearrange through recrystallization, forming fine and uniform recrystallized grains, significantly improving the fine grain strengthening effect. (2) Optimize grain morphology: The recrystallized grains are equiaxed and have good plasticity, which can avoid surface defects (such as serrated deformation bands and steps) in the subsequent second cold rolling process, improving formability. (3) Provide good plasticity for the second cold rolling: The plasticity of the plate after intermediate annealing is restored (elongation ≥ 25%), which can withstand a greater cold rolling reduction rate, laying the foundation for further improving the cold work hardening effect and subgrain boundary strengthening.

[0054] (vi) Step S6: Second cold rolling – High dislocation density introduction to enhance mechanical properties 1. Operating Procedures After intermediate annealing, the sheet material is surface-cleaned to remove the oxide scale generated during annealing, and then fed into a four-roll cold rolling mill for a second cold rolling: the cold rolling temperature is room temperature (20-30℃), and the cold rolling reduction rate is greater than that of the first cold rolling, preferably more than twice the reduction rate of the first cold rolling (i.e., reduction rate ≥ 60%). Finally, it is rolled into an ultra-thin cold-rolled sheet with a diameter of 0.2-0.6mm. Rolling oil lubrication is continued during the cold rolling process, and the rolling speed is controlled at 3-8m / min to ensure the dimensional accuracy and surface quality of the ultra-thin sheet material.

[0055] 2. Basis for process parameter control (1) Cold rolling reduction rate: A high rolling reduction rate of ≥60% is the key to achieving high strength in this invention - the second cold rolling is carried out on the basis of fully recrystallized fine grains, and the high rolling reduction rate can introduce a higher dislocation density (reaching 10). 15~10 16 m -2 ), The large-scale proliferation and entanglement of dislocations can significantly improve the cold work hardening effect. At the same time, the high rolling reduction rate can promote the reorganization of dislocations to form subgrain boundaries, providing a structural basis for subgrain boundary strengthening. If the reduction rate is less than the first cold rolling reduction rate, the dislocation density proliferation is insufficient and it is difficult to achieve the target strength. If the reduction rate is too high (≥80%), it will lead to excessive reduction of alloy plasticity and poor subsequent formability. (2) Rolling speed: Low-speed rolling of 3~8m / min can ensure the dimensional accuracy of ultra-thin plates (thickness tolerance ≤±0.02mm), avoid problems such as plate warping and uneven thickness caused by high-speed rolling. At the same time, low-speed rolling can reduce the frictional heat during the rolling process, avoid dislocation recovery caused by local heating of the alloy, and ensure the retention of high dislocation density. (3) Lubrication method: High viscosity rolling oil is used for lubrication, which can effectively reduce the friction between the roll and the plate, avoid scratches, tears and other defects on the surface of ultra-thin plates, and ensure that the surface roughness Ra≤0.5μm meets the requirements of precision stamping of 3C products.

[0056] 3. Process Objectives (1) Further enhance the cold work hardening effect: By introducing high dislocation density through high rolling reduction rate, the strength of the alloy is significantly improved, and the tensile strength of the alloy reaches more than 400MPa. (2) Form subgrain boundary structure: High dislocation density forms subgrain boundaries through recombination. Subgrain boundaries can hinder dislocation movement and further enhance strength. At the same time, the coordinated deformation of subgrains can improve plasticity and achieve a balance between strength and formability. (3) Prepare ultra-thin plate: Finally rolled into ultra-thin plates of 0.2 to 0.6 mm to meet the needs of thin-walled 3C products, such as precision brackets, VC heat sinks, shielding parts, etc.

[0057] (vii) Step S7: Final heat treatment – ​​stabilization treatment to fix the microstructure 1. Operating Procedures The ultra-thin sheet after the second cold rolling is placed in a box-type resistance furnace for final heat treatment: the heating rate is controlled at 5-10℃ / min, and after heating to 150-250℃, it is held for 1-5 hours. After the holding period, it is cooled to room temperature with the furnace at a rate of 10-15℃ / min. After the heat treatment, the sheet is straightened to ensure that the flatness of the sheet is ≤0.2mm / m.

[0058] 2. Basis for process parameter control (1) Heat treatment temperature: The low temperature range of 150 to 250℃ is the stabilization treatment temperature of the alloy. This temperature range can promote the precipitation of supersaturated Cu, Zn and Mg elements in the solid solution to form fine and dispersed second phases (Al2CuMg, Al6CuMg4, etc.), thereby achieving precipitation strengthening. At the same time, low temperature treatment will not lead to recrystallization and can retain the subgrain boundary structure introduced by the second cold rolling, ensuring the subgrain boundary strengthening effect. When the temperature is below 150℃, the nucleation rate of the precipitated phase is slow and the precipitation strengthening effect is not obvious. When the temperature is above 250℃, the precipitated phase is prone to coarsening. (1) Aggregation, which reduces the strengthening effect and may cause the subgrain boundary to disappear. (2) Holding time: A holding time of 1 to 5 hours can ensure that the precipitate fully nucleates and grows, and at the same time eliminate the residual stress generated by the second cold rolling. If the holding time is too short, the precipitation is insufficient and the residual stress cannot be completely eliminated. If the holding time is too long, the precipitate coarsens and affects the strengthening effect. (3) Cooling rate: A furnace cooling rate of 10 to 15℃ / min can avoid the thermal stress caused by rapid cooling, and at the same time make the precipitate stably distributed in the matrix, ensuring the long-term stability of the alloy's mechanical properties.

[0059] 3. Process Objectives (1) Achieve precipitation strengthening: Promote the precipitation of fine and dispersed second phase, pin dislocations, further improve the strength of the alloy, and make the tensile strength reach more than 450MPa. (2) Eliminate residual stress: Eliminate the residual stress generated by the second cold rolling, avoid deformation and cracking of the plate due to stress release during subsequent stamping processing, and improve the forming stability. (3) Fix microstructure: Stabilize the subgrain boundary structure and the distribution of precipitated phases, ensure the long-term stability of the mechanical properties and formability of the alloy, and avoid performance degradation due to changes in microstructure during use.

[0060] III. Product Performance and Application Scenarios (I) Core performance indicators Through the above-described composition design and gradient deformation heat treatment process, the aluminum-magnesium alloy prepared by this invention possesses the following core properties: Mechanical properties: Tensile strength ≥450MPa, yield strength ≥350MPa, elongation ≥15%, significantly superior to traditional 5XXX series aluminum-magnesium alloys (tensile strength 173-305MPa, yield strength 65-210MPa). Formability: Hardening index n value ≥0.25, plastic hardening ratio r value ≥0.8, excellent cold stamping formability, capable of deep drawing of complex shapes, without surface steps, serrated deformation bands, or other defects. Thin-gauge machinability: Maintains good machinability and performance stability even at ultra-thin gauges of 0.2~0.6mm, thickness tolerance ≤±0.02mm, surface roughness Ra≤0.5μm, flatness ≤0.2mm / m. Corrosion resistance: No significant corrosion after 48 hours of neutral salt spray test (NSS), meeting the environmental requirements for 3C products.

[0061] ; ; (II) Application Scenarios Due to its combined advantages of high strength, high formability, thin profile, and low cost, this aluminum-magnesium alloy is mainly suitable for the following thin-walled structural components in the digital 3C industry: Precision brackets: such as the mid-frame brackets and camera brackets of mobile phones and tablets, which require high strength and complex formability.

[0062] VC heat sinks: such as VC heat sinks for laptops and game consoles, which require high thermal conductivity (Al-Mg alloy thermal conductivity ≥120W / (m・K)) and thin-size machinability.

[0063] Shielding components: such as electromagnetic shielding covers for 5G base stations and smart wearable devices, require high strength, good formability and corrosion resistance.

[0064] Other thin-walled structural components: such as the fuselage of drones, watch strap connectors for smartwatches, etc.

[0065] (III) Advantages of Industrialized Production Simple process: It adopts traditional smelting, rolling and annealing equipment, without the need for new special equipment, and can easily achieve large-scale industrial production.

[0066] Low raw material cost: Based on 5XXX series aluminum-magnesium alloys, the amount of added elements such as Cu, Zn, Mn, Cr, and V is small, and there are no rare precious metals, so the raw material cost is controllable.

[0067] High production efficiency: The parameters of each process step have a wide range, the operation error tolerance is high, and the production cycle is short (about 3 to 5 days from ingot to finished product), making it suitable for mass production.

[0068] IV. Core Differences from Existing Technologies Composite strengthening mechanism: This invention constructs a four-fold composite strengthening mechanism of solid solution, precipitation, grain refinement, and subgrain boundary, which is different from the traditional single cold work hardening or grain refinement strengthening of 5XXX series alloys, and achieves synergistic optimization of strength and formability.

[0069] Composition design: The content of elements such as Mg, Cu, and Zn is precisely controlled within a narrow range to achieve synergistic strengthening of Cu and Zn. At the same time, V element is introduced to help refine the grains, which is different from the single element addition of traditional 5XXX series alloys.

[0070] Gradient deformation heat treatment: The gradient process, which involves two cold rollings, intermediate annealing, and final stabilization, differs from the traditional single cold rolling and annealing process. It can precisely control dislocation density and grain size, and achieve subgrain boundary strengthening.

[0071] Thin-size adaptability: It can stably produce ultra-thin sheets of 0.2-0.6mm, which is different from the problems of difficult processing and high cost of ultra-thin sheets of 6XXX and 7XXX series alloys, and is fully adapted to the thin-wall requirements of 3C products.

[0072] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a composite-strength, high-formability aluminum-magnesium alloy, characterized in that, It includes two parts: precise design of alloy composition and gradient deformation heat treatment process. By constructing a composite strengthening mechanism of solid solution strengthening - precipitation strengthening - grain refinement strengthening - sub - grain boundary strengthening, a high - strength and high - formability aluminum - magnesium alloy is fabricated. The alloy, by mass percentage, consists of 0.1% < Cu ≤ 0.5%, 0 < Zn ≤ 0.1%, 0 < Si ≤ 0.2%, 0 < Fe ≤ 0.3%, 0 < Mn ≤ 1.0%, 5.7% < Mg ≤ 6.3%, 0.05% < Cr ≤ 0.25%, 0 < V ≤ 0.10% and the balance of Al and inevitable impurities. The single content of inevitable impurities is ≤ 0.05 wt% and the total content is ≤ 0.15 wt%. The preparation method successively includes: S1: Melting and casting. Weigh the raw materials according to the mass percentages of the above - mentioned composition design. Put the industrial pure aluminum ingot into a graphite crucible and place it in a resistance furnace for heating. After the aluminum ingot is completely melted, successively add Al - Mn10, Al - Cr5, and Al - V5 master alloys, stir evenly and keep warm to fully dissolve the alloying elements. Then lower the temperature, add magnesium ingot and Al - Cu50, Al - Zn10 master alloys, stir rapidly, and pour the refined melt into a pre - heated cast iron mold by gravity casting. The mold is cooled by water - cooling. S2: Homogenization treatment. Remove the surface oxide scale and risers of the cast ingot after casting, and put it into a box - type resistance furnace for homogenization treatment. After the heat preservation ends, cool it to room temperature with the furnace. S3: Hot working. Heat and keep the ingot warm, then send it into a two - high hot rolling mill for multi - pass hot rolling, and finally roll it into a hot - rolled sheet. Heat and keep the ingot warm, then send it into an extruder for hot extrusion, and finally extrude it into an extruded sheet. S4: First - pass cold rolling. Clean the surface of the sheet after hot working to remove the oxide scale and oil, and then send it into a four - high cold rolling mill for the first - pass cold rolling. S5: Intermediate annealing treatment. Put the sheet after the first - pass cold rolling into a box - type resistance furnace for intermediate annealing treatment. S6: Second - pass cold rolling. Clean the surface of the sheet after intermediate annealing to remove the oxide scale generated during annealing, and then send it into a four - high cold rolling mill for the second - pass cold rolling. S7: Final heat treatment. Put the ultra - thin sheet after the second - pass cold rolling into a box - type resistance furnace for final heat treatment. After the heat preservation ends, cool it to room temperature with the furnace. After the heat treatment is completed, straighten the sheet to achieve the coordinated optimization of the alloy strength and formability.

2. The method for preparing a composite-strength, high-formability aluminum-magnesium alloy according to claim 1, characterized in that, The preferred composition of the alloy, by mass percentage, is: Cu 0.2 - 0.4 wt%, Zn 0.03 - 0.08 wt%, and the contents of the other elements are the same as those in claim 1. Through this preferred composition, the synergistic solid solution and precipitation strengthening of Cu and Zn are achieved, improving the comprehensive properties of the alloy.

3. The method for preparing a composite-strength, high-formability aluminum-magnesium alloy according to claim 2, characterized in that, The melting and casting step is: Weigh the raw materials according to the composition ratio, melt, refine and remove impurities, and then cast them into ingots. During the refining process, ensure the full fusion of raw materials, effectively control the impurity content, and avoid the adverse effects of impurities on the alloy structure and properties.

4. The method for preparing a composite-strength, high-formability aluminum-magnesium alloy according to claim 3, characterized in that, The homogenization process involves placing the ingot in a heating environment of 520–580°C for 4–12 hours. This process eliminates component segregation in the ingot, refines the as-cast microstructure, and provides a uniform microstructure for subsequent hot working and cold rolling.

5. The method for preparing a composite-strength, high-formability aluminum-magnesium alloy according to claim 4, characterized in that, The hot processing step is as follows: the homogenized ingot is hot rolled or hot extruded, the billet temperature is controlled at 480-550℃, and the final processing temperature is not lower than 300℃, so as to avoid material hardening and processing cracking caused by low temperature processing, and to ensure that the material after hot processing has good plasticity.

6. The method for preparing a composite-strength, high-formability aluminum-magnesium alloy according to claim 5, characterized in that, The first cold rolling step is as follows: the hot-processed material is cold-rolled with a cold rolling reduction rate of not less than 30%. This step introduces work hardening to improve the basic strength of the material and introduces dislocation density for subsequent secondary cold rolling.

7. The method for preparing a composite-strength, high-formability aluminum-magnesium alloy according to claim 6, characterized in that, The intermediate annealing process involves annealing the material after the first cold rolling at 350–450°C and holding it at that temperature for 1–5 hours, with a preferred annealing temperature of 400–450°C. This process aims to achieve complete recrystallization of the material, eliminate the processing stress generated by cold rolling, optimize the grain morphology, and prevent surface defects from occurring during subsequent processing.

8. The method for preparing a composite-strength, high-formability aluminum-magnesium alloy according to claim 7, characterized in that, The second cold rolling step is as follows: the material after intermediate annealing is subjected to cold rolling treatment, and the cold rolling reduction rate is greater than the reduction rate of the first cold rolling, preferably more than twice the reduction rate of the first cold rolling, to further improve the work hardening effect, introduce high dislocation density, and strengthen the mechanical properties of the alloy.

9. The method for preparing a composite-strength, high-formability aluminum-magnesium alloy according to claim 8, characterized in that, The final heat treatment step is as follows: the material after the second cold rolling is annealed at 150-250℃ and held for 1-5 hours to achieve alloy stabilization treatment, fix the microstructure of the alloy, eliminate residual stress, and ensure the long-term stability of the alloy's mechanical properties and formability.

10. The method for preparing a composite-strength, high-formability aluminum-magnesium alloy according to claim 9, characterized in that, The aluminum-magnesium alloy prepared by this method has a tensile strength ≥450MPa and a yield strength ≥350MPa. It also has a high hardening index n value and a high plasticity hardening ratio r value, and excellent cold stamping formability. It maintains good processability and performance stability even in ultra-thin specifications of 0.2~0.6mm. It is suitable for the preparation of thin-walled structural parts of electronic products such as precision brackets, VC heat sinks, and shielding parts in the digital 3C industry. Moreover, the preparation process is simple and the raw material cost is low, making it suitable for large-scale industrial production.

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