A heat treatment method for improving the strength and toughness and shrinkage deformation of a rheo-cast Al-Mg-Zn alloy mobile phone middle frame

Abstract

The application discloses a heat treatment method for improving the strength and toughness and shrinkage deformation of a rheochemical casting Al-Mg-Zn alloy mobile phone middle frame, and comprises the following steps: (1) pre-aging the rheochemical casting mobile phone middle frame casting; (2) double-stage solid solution treatment is conducted on the casting, first heating to T1 at an average heating rate V1, and then heating to T2 at an average heating rate V2; (3) the casting after the solid solution treatment is cooled to T3 at an average rate V3; (4) natural aging treatment is conducted on the quenched casting; (5) the casting after the natural aging treatment is subjected to leveling treatment; and (6) after the leveling treatment, double-stage artificial aging is conducted at T4 and T5 temperatures in sequence. Under the premise of controlling the low-melting-point phase overburning and shrinkage deformation, the microstructure is uniform, the size deformation is small, the ideal matching of the strength and toughness of the casting and the size control precision are achieved after the heat treatment.

Description

Technical Field

[0001] This invention relates to a heat treatment method for improving the strength, toughness, and shrinkage deformation of the middle frame of a rheological die-cast Al-Mg-Zn alloy mobile phone, belonging to the field of heat treatment of metallic materials. Background Technology

[0002] Aluminum alloys are characterized by their light weight, high specific strength, ease of processing, and low cost, making them widely used in aerospace, transportation, and 3C electronics industries. Taking aluminum structural components for mobile phones as an example, the current main method is profile machining, with the mid-frame and mid-plate formed by welding. This process is lengthy and costly. With the strict implementation of dual-carbon emission standards, pursuing extremely low costs has become a consensus in the mobile phone industry. Therefore, the application of new materials and processes is increasingly evident.

[0003] Al-Mg-Zn alloys are a rapidly developing new type of alloy in recent years. Due to their high Mg and Zn content, they exhibit significant shrinkage during solidification, increasing the tendency for hot cracking during casting. Therefore, their application is currently mainly concentrated in profiles. Patent document CN114438356A discloses a method for preparing a high-strength, corrosion-resistant, and high-toughness Al-Mg-Zn-Ag(-Cu) aluminum alloy. This method involves high Mg (4.0–6.5%) and Zn (3.0–5.5%) content, requiring slow cooling to prepare ingots, followed by deformation treatment to achieve high mechanical properties. This process is time-consuming and costly. Patent document CN112877554A discloses a method for preparing an Al-Mg-Zn-Cu alloy, introducing T-phase strengthening to increase the alloy strength to the level of 7000 series alloys. However, the introduction of an intermediate deformation heat treatment process complicates the alloy manufacturing process, further limiting the application of Al-Mg-Zn alloys. If a new forming process can be adopted to overcome the casting defects of Al-Mg-Zn alloy, it will surely accelerate the widespread application of this type of alloy.

[0004] High-solids rheocasting technology involves obtaining a semi-solid, semi-liquid slurry (solid ratio 30-70%) by suspending some primary aluminum grains in the liquid phase through external field treatment, followed by die casting to form a near-spherical structure different from dendrites. Products prepared using high-solids rheocasting technology have the advantages of low injection temperature, small shrinkage deformation, and gas expulsion during mold filling of the solid-liquid slurry, significantly reducing casting defects. Furthermore, the formed castings can be directly subjected to solution aging treatment, overcoming the limitation of traditional die casting which cannot perform solution treatment, and achieving comprehensive mechanical properties close to those of profiles. Patent document CN 114381640 A discloses a high-strength aluminum alloy material for rheocasting and its application method. The alloy has a high content of Zn (7-9%) and Si (6-6.5%), but the mechanical properties of the cast alloy are significantly better than those of traditional cast aluminum alloys. However, current research on rheocasting alloy systems mainly focuses on Al-Si alloys, limiting their application scope. On the other hand, it also shows that high solid phase rheodynamic die casting technology has unique advantages in preparing high Zn alloys.

[0005] In the production of typical castings (rectangular hollow castings with a thickness of 1-5mm and a length of 80-120mm) exemplified by high solids rheology die casting of Al-Mg-Zn alloy mobile phone frames, the high dimensional accuracy requirements necessitate ensuring both the material's strength and toughness during solution-aging heat treatment, while simultaneously controlling dimensional deformation within a certain range to meet mass production demands. Therefore, there is an urgent need to develop novel heat treatment processes and dimensional control methods to lay the foundation for the mass production of high solids rheology die casting Al-Mg-Zn alloy frame products. Summary of the Invention

[0006] To address the aforementioned problems in the prior art, this invention provides a heat treatment method for improving the strength, toughness, and shrinkage deformation of Al-Mg-Zn alloy mobile phone frames in rheological die casting, overcoming the challenge of large fluctuations in mechanical properties and dimensions during the heat treatment of mobile phone frame castings.

[0007] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0008] A heat treatment method for improving the strength, toughness, and shrinkage deformation of a mobile phone frame made of rheological die-cast Al-Mg-Zn alloy includes the following steps:

[0009] (1) Perform pre-aging treatment on the rheological die-cast mobile phone frame casting at 50-70℃ for 2-6 hours;

[0010] (2) The pre-aged castings are subjected to two-stage solution treatment. The first stage heating rate is 10℃ / min≤V1≤20℃ / min to the first stage solution temperature of 400℃≤T1≤440℃, and the temperature is maintained for 8h≤t1≤24h. Then the second stage heating rate is 0.5℃ / min≤V2≤5℃ / min to the second stage solution temperature of 450℃≤T2≤500℃, and the temperature is maintained for 8h≤t2≤24h.

[0011] (3) The casting after solution treatment is quenched and cooled from point T2 to T3 at an average cooling rate V3, where 60℃ / s≤ V3 ≤ 100℃ / s, 50℃ ≤ T3 ≤ 80℃, and the quenching transfer time of the casting is ≤ 20s;

[0012] (4) The quenched castings are subjected to natural aging treatment for a time of 24h≤t3≤128h;

[0013] (5) The castings after natural aging are leveled, and the flatness is controlled within 0.03mm≤Δh≤0.12mm;

[0014] (6) The castings after leveling are subjected to two-stage aging treatment. The first-stage aging temperature is 60℃≤T4≤90℃ and the holding time is 12h≤t4≤96h. The second-stage aging temperature is 120℃≤T5≤160℃ and the holding time is 6h≤t5≤36h.

[0015] Preferably, in step (1), the pre-aging treatment of the rheological die-cast mobile phone frame is preferably held at 55-65℃ for 3-5 hours.

[0016] Preferably, in step (2), the rheological die-cast mobile phone frame casting is placed in a solution furnace and heated to the first-stage solution temperature for heat preservation treatment. The temperature error of the solution furnace is ±1℃. The first-stage heating rate is preferably 10℃ / min≤V1≤15℃ / min.

[0017] Preferably, in step (2), the primary solution temperature is 400℃≤T1≤430℃, and the heat preservation time is 8h≤t1≤16h.

[0018] Preferably, in step (2), the secondary heating rate of the casting from T1 to T2 is preferably 1℃ / min≤V2≤3℃ / min, the secondary solution temperature is preferably 470℃≤T2≤475℃, and the holding time is preferably 8h≤t2≤16h.

[0019] Preferably, in step (3), the quenching process is further preferably characterized by 70℃ / s≤V3≤80℃ / s and 60℃≤T3≤70℃.

[0020] Preferably, in step (4), the natural aging time is 24h≤t3≤96h.

[0021] Preferably, in step (5), during the leveling process, the flatness is preferably controlled within 0.05mm≤Δh≤0.10mm.

[0022] Preferably, in step (6), the first-stage aging temperature is 80℃≤T4≤90℃, and the heat preservation time is 24h≤t4≤48h.

[0023] Preferably, in step (6), the secondary aging temperature is preferably 130℃≤T5≤150℃, and the heat preservation time is preferably 12h≤t5≤24h.

[0024] The composition of the Al-Mg-Zn alloy mobile phone frame to which the heat treatment method of the present invention is applicable, by mass percentage, is: Mg 5.5-7.5%, Zn 1.5-3.0%, Cu 0-0.5%, Fe 0-0.2%, Si 0-0.2%, Ti 0.08-0.12%, Be 0.003-0.009%, with the balance being aluminum.

[0025] Compared with the prior art, the heat treatment method for the rheo-die-cast Al-Mg-Zn alloy mobile phone frame of the present invention has the following advantages and beneficial effects:

[0026] (1) By adding a pre-aging process before solution heat treatment, a optimized two-stage solution treatment process is designed. By controlling the heating rate, solution temperature, holding time, quenching method, and transfer time, a large amount of low-melting-point eutectic T phase can be dissolved back, avoiding overheating. At the same time, the alloy atoms are dissolved to the maximum extent, providing favorable conditions for subsequent aging. Natural aging can promote the uniform nucleation of nanophases in the early stage of aging. The subsequent leveling process can achieve precise control of the casting dimensions after solution heat treatment, and the introduced compressive stress further accelerates the uniform precipitation of nanophases. Through the control of the two-stage aging process, the optimal match between the strength, toughness, and dimensional accuracy of the frame in rheological die casting is finally achieved.

[0027] (2) The method of the present invention is accurate, reliable, economical and practical; it fully considers the structural characteristics of the Al-Mg-Zn alloy frame in rheological die casting, which helps to significantly improve the qualification rate of the frame products prepared by the new materials and new processes, and is conducive to large-scale industrial application. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of parameter process control in the heat treatment method of the present invention.

[0029] Figure 2 These are metallographic photographs of the microstructure of the rheological die-cast frame in Embodiment 2 of the present invention.

[0030] Figure 3 This is a metallographic photograph of the microstructure of the rheological die-cast frame in Comparative Example 2 of this invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0032] Figure 1 The diagram shown illustrates the parameter process control in the heat treatment method of this invention. The rheological die-cast Al-Mg-Zn alloy mobile phone frame casting sequentially undergoes pre-aging, two-stage solution treatment, quenching, natural aging, leveling, and two-stage artificial aging. Figure 1 T0 is the pre-aging temperature. Then, a two-stage solution treatment is performed. The first-stage heating rate is V1 and the first-stage solution temperature is T1. The second-stage heating rate is V2 and the second-stage solution temperature is T2. The average cooling rate of quenching is V3. The temperature is reduced to T3, and then natural aging and leveling treatments are performed. Finally, two-stage artificial aging is performed at T4 and T5 temperatures in sequence.

[0033] Comparative Example 1

[0034] The alloy chemical composition (by mass percentage, wt%) is: Mg 7.5, Zn 3.0, Cu 0.3, Fe < 0.05, Si < 0.05, Ti 0.08, Be 0.004, with the balance being aluminum. The rheo-die-cast mobile phone frame casting underwent pre-aging treatment at 50℃ for 6 hours; followed by solution treatment at a heating rate of 5℃ / min and a solution temperature of 450℃. After 8 hours of holding, the casting was quenched. It was cooled to 70℃ at an average cooling rate of 60℃ / s, with a quenching transfer time ≤ 20s. The quenched casting underwent natural aging treatment for 24 hours. After natural aging, the casting was leveled, with the flatness Δh controlled within 0.10mm. Finally, the leveled casting underwent aging treatment at 200℃ for 6 hours.

[0035] Comparative Example 2

[0036] The alloy chemical composition (by mass percentage, wt%) is: Mg 7.5, Zn 3.0, Cu 0.3, Fe < 0.05, Si < 0.05, Ti 0.08, Be 0.004, with the balance being aluminum. The rheo-die-cast mobile phone frame casting underwent pre-aging treatment at 55℃ for 3 hours; followed by a two-stage solution treatment. During the solution heat treatment, the first-stage heating rate was 10℃ / min, and the first-stage solution temperature was 420℃. After 8 hours of holding, the casting was quenched. The casting was cooled to 70℃ at an average cooling rate of 70℃ / s, with a quenching transfer time ≤ 20s. The quenched casting underwent natural aging treatment for 48 hours. After natural aging, the casting was leveled, with the flatness Δh controlled within 0.11mm. Finally, the leveled casting underwent aging treatment, with the first-stage aging temperature at 70℃ and a holding time of 24 hours.

[0037] Comparative Example 3

[0038] The alloy chemical composition (by mass percentage, wt%) is: Mg 7.5, Zn 3.0, Cu 0.3, Fe < 0.05, Si < 0.05, Ti 0.08, Be 0.004, with the balance being aluminum. The rheo-deposited mobile phone frame underwent a two-stage solution heat treatment. During the solution heat treatment, the first stage heating rate was 10℃ / min, the first solution temperature was 430℃, and after 16 hours of holding, the second stage heating rate was 1℃ / min, the final solution temperature was 480℃, and after 12 hours of holding, the casting was quenched. The casting is cooled to 60℃ at an average cooling rate of 80℃ / s, with a quenching transfer time of ≤20s. The quenched casting is then subjected to natural aging treatment for 96 hours. After natural aging, the casting is leveled, with the flatness Δh controlled within 0.13mm. Finally, the leveled casting is subjected to a two-stage aging treatment: the first stage aging temperature is 50℃ with a holding time of 24 hours, and the second stage aging temperature is 150℃ with a holding time of 24 hours.

[0039] Example 1

[0040] The alloy chemical composition (by mass percentage, wt%) is: Mg 7.5, Zn 3.0, Cu 0.3, Fe < 0.05, Si < 0.05, Ti 0.08, Be 0.004, with the balance being aluminum. The rheo-die-cast mobile phone frame casting was pre-aged at 50℃ for 6 hours; then, a two-stage solution heat treatment was performed. During the solution heat treatment, the first stage heating rate was 20℃ / min, the first solution temperature was 400℃, and after 8 hours of holding, the second stage heating rate was 5℃ / min, the final solution temperature was 450℃, and after 8 hours of holding, the casting was quenched. The castings were cooled to 70℃ at an average cooling rate of 60℃ / s, with a quenching transfer time of ≤20s. The quenched castings were then subjected to natural aging treatment for 24 hours. The naturally aged castings were then leveled, with the flatness Δh controlled within 0.08mm. Finally, the leveled castings were subjected to a two-stage aging treatment: the first stage aging temperature was 40℃ with a holding time of 96 hours, and the second stage aging temperature was 200℃ with a holding time of 6 hours.

[0041] Example 2

[0042] The alloy chemical composition (by mass percentage, wt%) is: Mg 7.5, Zn 3.0, Cu 0.3, Fe < 0.05, Si < 0.05, Ti 0.08, Be 0.004, with the balance being aluminum. The rheo-die-cast mobile phone frame casting was pre-aged at 55℃ for 3 hours; then subjected to a two-stage solution heat treatment. During the solution heat treatment, the first stage heating rate was 10℃ / min, the first solution temperature was 420℃, and after 8 hours of holding, the second stage heating rate was 2℃ / min, with a final solution temperature of 490℃. After 16 hours of holding, the casting was quenched. The castings were cooled to 70℃ at an average cooling rate of 70℃ / s, with a quenching transfer time of ≤20s. The quenched castings were then subjected to natural aging treatment for 48 hours. The naturally aged castings were then leveled, with the flatness Δh controlled within 0.04mm. Finally, the leveled castings were subjected to a two-stage aging treatment: the first stage aging temperature was 70℃ with a holding time of 24 hours, and the second stage aging temperature was 150℃ with a holding time of 36 hours.

[0043] Example 3

[0044] The alloy chemical composition (by mass percentage, wt%) is: Mg 7.5, Zn 3.0, Cu 0.3, Fe < 0.05, Si < 0.05, Ti 0.08, Be 0.004, with the balance being aluminum. The rheo-die-cast mobile phone frame casting was pre-aged at 60℃ for 4 hours; then subjected to a two-stage solution heat treatment. During the solution heat treatment, the first stage heating rate was 10℃ / min, the first solution temperature was 430℃, and after 16 hours of holding, the second stage heating rate was 1℃ / min, the final solution temperature was 480℃, and after 12 hours of holding, the casting was quenched. The casting is cooled to 60℃ at an average cooling rate of 80℃ / s, with a quenching transfer time of ≤20s. The quenched casting is then subjected to natural aging treatment for 96 hours. After natural aging, the casting is leveled, with the flatness Δh controlled within 0.05mm. Finally, the leveled casting is subjected to a two-stage aging treatment: the first stage aging temperature is 50℃ with a holding time of 24 hours, and the second stage aging temperature is 150℃ with a holding time of 24 hours.

[0045] Example 4

[0046] The alloy chemical composition (by mass percentage, wt%) is: Mg 7.5, Zn 3.0, Cu 0.3, Fe < 0.05, Si < 0.05, Ti 0.08, Be 0.004, with the balance being aluminum. The rheo-die-cast mobile phone frame casting was pre-aged at 65℃ for 5 hours; then subjected to a two-stage solution heat treatment. During the solution heat treatment, the first stage heating rate was 15℃ / min, the first solution temperature was 400℃, and after 16 hours of holding, the second stage heating rate was 3℃ / min, the final solution temperature was 450℃, and after 8 hours of holding, the casting was quenched. The castings were cooled to 70℃ at an average cooling rate of 80℃ / s, with a quenching transfer time of ≤20s. The quenched castings were then subjected to natural aging treatment for 24 hours. The naturally aged castings were then leveled, with the flatness Δh controlled within 0.06mm. Finally, the leveled castings were subjected to a two-stage aging treatment: the first stage aging temperature was 50℃, with a holding time of 48 hours, and the second stage aging temperature was 170℃, with a holding time of 12 hours.

[0047] Example 5

[0048] The alloy chemical composition (by mass percentage, wt%) is: Mg 7.5, Zn 3.0, Cu 0.3, Fe < 0.05, Si < 0.05, Ti 0.08, Be 0.004, with the balance being aluminum. The rheo-die-cast mobile phone frame casting was pre-aged at 65℃ for 4 hours; then, a two-stage solution heat treatment was performed. During the solution heat treatment, the first stage heating rate was 12℃ / min, the first solution temperature was 410℃, and after 12 hours of holding, the second stage heating rate was 1.5℃ / min, with a final solution temperature of 460℃. After 14 hours of holding, the casting was quenched. The castings were cooled to 60℃ at an average cooling rate of 80℃ / s, with a quenching transfer time of ≤20s. The quenched castings were then subjected to natural aging treatment for 72 hours. The naturally aged castings were then leveled, with the flatness Δh controlled within 0.08mm. Finally, the leveled castings were subjected to a two-stage aging treatment: the first stage aging temperature was 50℃ with a holding time of 36 hours, and the second stage aging temperature was 170℃ with a holding time of 24 hours.

[0049] Example 6

[0050] The alloy chemical composition (by mass percentage, wt%) is: Mg 7.5, Zn 3.0, Cu 0.3, Fe < 0.05, Si < 0.05, Ti 0.08, Be 0.004, with the balance being aluminum. The rheo-die-cast mobile phone frame casting was pre-aged at 50℃ for 2 hours; then subjected to a two-stage solution heat treatment. During the solution heat treatment, the first stage heating rate was 20℃ / min, the first solution temperature was 400℃, and after 8 hours of holding, the second stage heating rate was 5℃ / min, the final solution temperature was 450℃, and after 8 hours of holding, the casting was quenched. The casting is cooled to 80℃ at an average cooling rate of 100℃ / s, with a quenching transfer time of ≤20s. The quenched casting is then subjected to natural aging treatment for 24 hours. After natural aging, the casting is leveled, with the flatness Δh controlled within 0.03mm. Finally, the leveled casting is subjected to a two-stage aging treatment: the first stage aging temperature is 40℃ with a holding time of 12 hours, and the second stage aging temperature is 150℃ with a holding time of 6 hours.

[0051] Example 7

[0052] The alloy chemical composition (by mass percentage, wt%) is: Mg 7.5, Zn 3.0, Cu 0.3, Fe < 0.05, Si < 0.05, Ti 0.08, Be 0.004, with the balance being aluminum. The rheo-die-cast mobile phone frame casting was pre-aged at 70℃ for 2 hours; then, a two-stage solution heat treatment was performed. During the solution heat treatment, the first stage heating rate was 10℃ / min, the first solution temperature was 440℃, and after 24 hours of holding, the second stage heating rate was 0.5℃ / min, with a final solution temperature of 500℃. After 24 hours of holding, the casting was quenched. The castings were cooled to 50℃ at an average cooling rate of 60℃ / s, with a quenching transfer time of ≤20s. The quenched castings were then subjected to natural aging treatment for 128 hours. The naturally aged castings were then leveled, with the flatness Δh controlled within 0.12mm. Finally, the leveled castings were subjected to a two-stage aging treatment: the first stage aging temperature was 60℃ with a holding time of 12 hours, and the second stage aging temperature was 150℃ with a holding time of 36 hours.

[0053] Figure 2 The image shown is a metallographic photograph of the microstructure of the rheological die-cast frame in Example 2. Figure 3 The image shown is a metallographic photograph of the microstructure of the rheo-cast frame in Comparative Example 2. From... Figure 2 It can be seen that after two-stage solution treatment and two-stage aging, a large amount of black eutectic phase on the grain boundaries is dissolved back, and the discontinuous dot-like precipitates on the grain boundaries produce a significant strengthening effect. Figure 3 The microstructure after only one stage of solution treatment and subsequent aging shows a large amount of residual black eutectic phase, while the matrix structure has relatively few precipitated phases. The low concentration of solid solution elements results in a poor strengthening effect. Furthermore, the residual micron-sized black phase significantly degrades the anodizing effect. Table 1 shows a comparison of the mechanical properties and flatness of the Al-Mg-Zn alloy mobile phone frame castings obtained in the comparative examples and embodiments above.

[0054] The mechanical performance tests in Table 1 are performed in accordance with GB / T 228.1-2021, and the flatness test is performed by three-coordinate measuring machine according to GB / T 1958-2017.

[0055] Table 1. Mechanical properties and flatness of Al-Mg-Zn alloy mobile phone frame castings obtained from different comparative examples and embodiments.

[0056] Table 1, a comparison of the mechanical properties of the alloys, shows that the lack of primary solution treatment and primary aging significantly reduces the strength and toughness of the alloy. This is because the low-melting-point eutectic phase can cause overheating, and the uneven growth of the nanophase at high temperatures during primary aging significantly reduces mechanical properties. Furthermore, the presence of coarse eutectic phase residues makes it difficult to control flatness during leveling, and the second phase is easily crushed during the aging process, leading to microcracks. Comparing Comparative Example 2 and Example 2, the lack of primary solution treatment and primary aging further exacerbates the decrease in mechanical properties. This is mainly due to the low primary solution temperature, the large amount of eutectic phase residue, and the fact that only primary aging occurs, leaving the nanophase in a uniform nucleation stage without significant strengthening effect. Comparing Comparative Example 3 and Example 3, the lack of pre-aging processes prevents a significant reduction in residual stress in the casting, leading to deformation during subsequent solution treatment and heating. Even after leveling, the flatness remains poor. Meanwhile, since pre-aging can promote the early nucleation of a large number of nanophases, the two-stage solution treatment followed by aging is conducive to the uniform precipitation of nanophases. Therefore, the strength and toughness of Example 3 are better than those of Comparative Example 3. The best comprehensive mechanical properties in Table 1 are those of Example 3, mainly due to the optimal combination of two-stage solution treatment and two-stage aging process. All eutectic phases are dissolved back, and the nanophases are uniformly precipitated under the first-stage aging process. When the temperature is raised to the second-stage aging process, the nanophases grow uniformly, resulting in a significant match between strength and toughness.

[0057] The above description is only a more optimized specific embodiment of the present invention. However, it should be noted that the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention. All such modifications and substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A heat treatment method for improving the strength, toughness, and shrinkage deformation of a mobile phone frame made of rheological die-cast Al-Mg-Zn alloy, characterized in that, The applicable Al-Mg-Zn alloy mobile phone frame composition, by mass percentage, is: Mg 5.5-7.5%, Zn 1.5-3.0%, Cu 0-0.5%, Fe 0-0.2%, Si 0-0.2%, Ti 0.08-0.12%, Be 0.003-0.009%, with the balance being aluminum; the heat treatment method includes the following steps: (1) Perform pre-aging treatment on the rheological die-cast mobile phone frame casting at 50-70℃ for 2-6 hours; (2) The pre-aged castings are subjected to two-stage solution treatment. The first stage heating rate is 10℃ / min≤V1≤20℃ / min to the first stage solution temperature of 400℃≤T1≤440℃, and the temperature is held for 8h≤t1≤24h. Then the second stage heating rate is 0.5℃ / min≤V2≤5℃ / min to the second stage solution temperature of 450℃≤T2≤500℃, and the temperature is held for 8h≤t2≤24h. (3) The casting after solution treatment is quenched and cooled from point T2 to T3 at an average cooling rate V3, where 60℃ / s≤ V3 ≤ 100℃ / s, 50℃ ≤ T3 ≤ 80℃, and the quenching transfer time of the casting is ≤ 20s; (4) The quenched castings are subjected to natural aging treatment for a time of 24h ≤ t3 ≤ 128h; (5) The castings after natural aging are leveled, and the flatness is controlled within 0.03mm ≤ ∆h ≤ 0.12mm; (6) The castings after leveling are subjected to two-stage aging treatment. The first-stage aging temperature is 60℃ ≤ T4 ≤ 90℃ and the holding time is 12h ≤ t4 ≤ 96h. The second-stage aging temperature is 120℃ ≤ T5 ≤ 160℃ and the holding time is 6h ≤ t5 ≤ 36h.

2. The heat treatment method according to claim 1, characterized in that, In step (1), the casting undergoes a pre-aging treatment at 55-65℃ for 3-5 hours.

3. The heat treatment method according to claim 1, characterized in that, In step (2), the first-stage heating rate is 10℃ / min ≤ V1 ≤ 15℃ / min.

4. The heat treatment method according to claim 1, characterized in that, In step (2), the first-stage solution temperature is 400℃≤T1≤430℃, and the heat preservation time is 8h≤t1≤16h.

5. The heat treatment method according to claim 1, characterized in that, In step (2), the secondary heating rate of the casting from T1 to T2 is 1℃ / min ≤ V2 ≤ 3℃ / min.

6. The heat treatment method according to claim 1, characterized in that, In step (2), the secondary solution temperature is 450℃ ≤ T2 ≤ 490℃, and the holding time is 8h ≤ t2 ≤ 16h.

7. The heat treatment method according to claim 1, characterized in that, In step (3), 70℃ / s ≤ V3 ≤ 80℃ / s, 60℃ ≤ T3 ≤ 70℃.

8. The heat treatment method according to claim 1, characterized in that, In step (4), the natural time of effect is 24h ≤ t3 ≤ 96h.

9. The heat treatment method according to claim 1, characterized in that, In step (5), the flatness of the leveling process is controlled within 0.05mm ≤ ∆h ≤ 0.10mm.

10. The heat treatment method according to claim 1, characterized in that, In step (6), the first-stage aging temperature is 80℃ ≤ T4 ≤ 90℃, and the heat preservation time is 24h ≤ t4 ≤ 48h.

11. The heat treatment method according to claim 1, characterized in that, In step (6), the secondary aging temperature is 130℃ ≤ T5 ≤ 150℃, and the heat preservation time is 12h ≤ t5 ≤ 24h.

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