A tough aluminum alloy plate with a heterogeneous structure and a preparation method thereof

Abstract

The present invention provides a tough aluminum alloy plate with a heterogeneous structure and a preparation method thereof. The aluminum alloy plate is a heat-treatable aluminum alloy, and the heat-treatable aluminum alloy further contains Zr and Sc elements. The contents of the Zr and Sc elements are in mass percentage, 0.1 wt% ≤ Zr + Sc ≤ 0.26 wt%, and 0.86 ≤ Zr / Sc ≤ 1.86. The present invention adopts a low-frequency electromagnetic casting technology and a low-temperature slow extrusion method, and further optimizes the alloy formula and the preparation method to obtain a high-strength and high-toughness aluminum alloy plate with a specific heterogeneous structure.

Description

Technical Field

[0001] The present invention relates to the technical field of aluminum alloy preparation, and particularly relates to a strong and tough aluminum alloy plate with a heterogeneous structure and a preparation method thereof. Background Art

[0002] Heat treatment strengthened alloys, such as typical 2XXX series, 6XXX series, and 7XXX series aluminum alloys, have higher strength compared to non-heat treatment strengthened alloys such as 3XXX series. However, as a common problem of materials, with the pursuit of high strength, the obvious consequence is a significant decrease in elongation and toughness. Based on this problem, the inventor has conducted a large number of studies on the organizational structure, hoping to improve the fracture toughness of the material by optimizing and improving the organizational structure while ensuring high strength.

[0003] Currently, in order to provide means for the strength and toughness of aluminum alloys, composition control and precipitation phase control of the strengthenable alloy in the structure are usually selected. On the one hand, the second-phase element components that cause brittleness are removed, and on the other hand, precipitation phase control with relatively little influence on brittleness and obvious strengthening effect is selected.

[0004] For example, in the prior art, appropriate amounts of Sc and Zr elements are added on the basis of 2XXX series, 6XXX series, and 7XXX series alloys, or both are added. Usually, in order to make the precipitation phase as a whole fine and dispersed, the precipitation phase is reduced to enhance toughening. However, the grains of the plates prepared by the prior art preparation process are usually equiaxed grains or non-equiaxed grain structures, that is, they are all fully recrystallized structures. Although high-strength plates can be obtained, their toughness is too low, and it is still difficult to improve the toughness.

[0005] Although the strength of age-hardened alloys is relatively ideal, the insufficient toughness of the materials has always been the primary problem restricting the application of such materials. For the existing high-strength and high-toughness improvement methods of heat-treatable aluminum alloys, continuous exploration and research are still ongoing, and it is of great significance to provide an aluminum alloy with relatively higher strength and toughness. Summary of the Invention

[0006] In view of the problems existing in the prior art, the present invention provides a strong and tough aluminum alloy plate with a heterogeneous structure and a preparation method thereof. A certain amount of Sc and Zr elements are introduced into the alloy, and the primary Al3(Sc,Zr) phase and the secondary Al3(Sc,Zr) phase are used to regulate the tissue morphology of the aluminum alloy plate in combination with the corresponding process, so as to obtain a high-strength and high-toughness aluminum alloy plate with a specific heterogeneous structure. The preparation method uses low-frequency electromagnetic casting technology and low-temperature slow extrusion method to regulate and obtain a heterogeneous structure with specific structural parameters, and realizes the preparation of strong and tough aluminum alloy plates that are more convenient for industrialization in terms of process.

[0007] To achieve this purpose, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a tough aluminum alloy plate with a heterogeneous structure. The material of the aluminum alloy plate is a heat-treatable aluminum alloy, and the heat-treatable aluminum alloy contains Zr element and Sc element; the contents of the Zr element and the Sc element are in mass percentage, 0.1 wt% ≤ Zr + Sc ≤ 0.26 wt% and 0.86 ≤ Zr / Sc ≤ 1.86;

[0009] The average size of the primary Al3(Sc,Zr) phase ≤ 1.2 μm; the average size of the secondary Al3(Sc,Zr) phase ≤ 20 nm;

[0010] The heterogeneous structure includes a recrystallized structure and a fibrous structure existing in the microstructure of the aluminum alloy plate. The minimum distribution unit of the recrystallized structure is a recrystallized structure cluster, and the recrystallized structure cluster is formed by the convergence of individual recrystallized grains; the recrystallized structure clusters are randomly distributed in the fibrous structure.

[0011] It should be noted that the present invention expects to regulate the crystal structure through the primary Al3(Sc,Zr) phase and the secondary Al3(Sc,Zr) phase formed by Zr and Sc elements, and obtain a heterogeneous structure in which the recrystallized structure and the fibrous structure are alternately distributed in the longitudinal section of the aluminum alloy plate. It is found in the research that under the same type of aluminum alloy formula system, this kind of heterogeneous structure has better strengthening and toughening effects compared with the traditional fully recrystallized structure (or equiaxed grain structure).

[0012] The invention mechanism of the present invention is:

[0013] Under normal circumstances, the heat treatment process of solid solution strengthening will lead to recrystallization, eventually forming a fully recrystallized structure, resulting in a decrease in the strength and toughness of the material. When we control the formation of primary Al3(Sc,Zr) phase in some areas during the casting process, the position where this phase precipitates has less precipitation of secondary Al3(Sc,Zr) phase during the subsequent homogenization heat treatment process, becoming a scarcity area of secondary Al3(Sc,Zr) phase. In the scarcity area, the number of secondary Al3(Sc,Zr) phase is less, and the dislocation density pinned by it is higher. First, sub-grain boundaries are formed, and then continuously absorb dislocations and change from small-angle grain boundaries to large-angle grain boundaries, and finally become fine recrystallized grains; in the remaining areas during the homogenization heat treatment process, it is easier to precipitate secondary Al3(Sc,Zr) phase dispersedly, which is called the enrichment area of secondary Al3(Sc,Zr) phase. In this area, only small-angle sub-grain boundaries exist after extrusion, and the grains are overall fibrous. And during the subsequent heat treatment process, the pinning effect of secondary Al3(Sc,Zr) phase on the grain boundaries can inhibit the nucleation of recrystallization, so that the fibrous structure can be retained. Therefore, by controlling the precipitation ability of primary Al3(Sc,Zr) phase to determine the precipitation ability of subsequent secondary Al3(Sc,Zr) phase, a heterogeneous structure can be obtained.

[0014] During the low-frequency electromagnetic casting process, the temperature field of the aluminum alloy melt is relatively uniform, and the residence time in the solidification temperature range is extended. The aluminum alloy melt containing Sc element undergoes a peritectic reaction, L+Al3Zr→α(Al)+Al3(Sc,Zr) (L is the liquid, referring to the Al melt), forming the primary Al3(Sc,Zr) phase. The solid solution amounts of Sc and Zr elements in the Al matrix around this phase decrease. During the subsequent homogenization heat treatment process, less secondary Al3(Sc,Zr) phase precipitates, becoming a scarcity area of secondary Al3(Sc,Zr) phase dispersion; while in the area where the primary Al3(Sc,Zr) phase is not formed, during the homogenization heat treatment process, the number density of precipitated secondary Al3(Sc,Zr) dispersed phase is relatively high, becoming an enrichment area of secondary Al3(Sc,Zr) phase dispersion. During the extrusion process, in the enrichment area of secondary Al3(Sc,Zr) phase dispersion, the dispersed secondary Al3(Sc,Zr) phase pins dislocations, forming sub-grain boundaries inside the grains; in the scarcity area of secondary Al3(Sc,Zr) phase dispersion, the secondary Al3(Sc,Zr) phase pins dislocations to form sub-grain boundaries, but due to the extremely small number of its particles, the sub-grain boundaries continuously absorb dislocations and change from small-angle grain boundaries to large-angle grain boundaries, thus forming recrystallized grains; on the other hand, the scarcity area of secondary Al3(Sc,Zr) phase dispersion is also the enrichment area of primary Al3(Sc,Zr) phase, and the primary Al3(Sc,Zr) phase particles contribute to the formation of fine recrystallized grains. After extrusion, the interior of the metal presents a heterogeneous structure with recrystallized grains uniformly distributed in the fibrous structure, and this structure shows excellent effects on the comprehensive improvement of the strength and toughness of the material.

[0015] In the present invention, in the tough aluminum alloy plate with a heterogeneous structure, by mass percentage: 0.1 wt% ≤ Zr + Sc ≤ 0.26 wt%. For example, it can be 0.1 wt%, 0.15 wt%, 0.20 wt%, 0.22 wt%, 0.24 wt% or 0.26 wt% etc., but it is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0016] It should be noted that if the content of Zr + Sc is too high, the primary Al3(Sc,Zr) phase generated during solidification is coarse. For example, in the traditional primary Al3(Sc,Zr) phase, when the total addition amount of Zr and Sc exceeds 0.26%, a coarse phase is formed during solidification, and the size ranges from 5 - 20 μm. The hard phases within this size range will affect the toughness of the material. The primary Al3(Sc,Zr) phase described in the present invention is generated under specific process conditions and has a relatively small size, and its own influence on toughness is very small. On the other hand, if the content of Zr + Sc is too low, the system cannot obtain a heterogeneous structure.

[0017] In the present invention, in the tough aluminum alloy plate with a heterogeneous structure, by mass percentage: 0.86 ≤ Zr / Sc ≤ 1.86. For example, it can be 0.86, 0.90, 0.95, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6 or 1.7 etc., but it is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0018] It should be noted that in the present invention, the Zr / Sc ratio is controlled within the range of 0.86 - 1.86. Because if the Zr / Sc ratio is too high or too low, the sizes of the primary phase and secondary phase in the final plate become larger, and the size distribution is too wide; if the Zr / Sc is too high, the size stability of the primary Al3(Sc,Zr) phase formed during solidification is poor, and it coarsens during homogenization heat treatment. The coarsening of the primary Al3(Sc,Zr) phase leads to an increase in the dispersion - phase - depleted region, thereby causing an increase in the recrystallized grain clusters. If the recrystallized grain clusters are too large, they will act as soft regions and preferentially deform and fracture during loading, resulting in a decrease in strength and toughness; if the Zr / Sc is too low, the size stability of the secondary Al3(Sc,Zr) phase formed during homogenization is poor, and it coarsens during homogenization. The coarsening of the secondary Al3(Sc,Zr) phase leads to a decrease in its quantity in the dispersion - phase - enriched region, resulting in the fact that the fibrous structure is prone to partial recrystallization during solution treatment and it is difficult to form a heterogeneous structure.

[0019] In the present invention, the average size of the primary Al3(Sc,Zr) phase ≤ 1.2 μm; the average size of the secondary Al3(Sc,Zr) phase ≤ 20 nm.

[0020] The existence forms of the Al3(Sc,Zr) compound are divided into primary Al3(Sc,Zr) phase and secondary Al3(Sc,Zr) phase. The primary Al3(Sc,Zr) phase is generated during the solidification of the alloy and forms fine recrystallized grains during the extrusion process; the secondary Al3(Sc,Zr) phase is generated during the homogenization process and determines the fibrous structure in the heterogeneous structure. The phase sizes of the primary Al3(Sc,Zr) phase and the secondary Al3(Sc,Zr) phase themselves will affect the properties of the material; when there is an oversized primary Al3(Sc,Zr) phase in the aluminum alloy plate, it serves as a crack source during the deformation process, causing cracking and resulting in problems such as a decrease in toughness and strength; when the size of the secondary Al3(Sc,Zr) phase in the aluminum alloy plate is too large, its number density will definitely decrease, which is not conducive to maintaining the fibrous structure.

[0021] In the present invention, in the tough and strong aluminum alloy plate with a heterogeneous structure, the heterogeneous structure is that recrystallized tissue clusters and fibrous tissues are distributed at intervals on the longitudinal section of the aluminum alloy plate; wherein, the longitudinal section refers to the L-ST surface of the plate.

[0022] It should be noted that neither the pure fibrous structure nor the pure dynamic recrystallized structure has shown a significant contribution to the improvement of toughness. However, the heterogeneous structure composed of randomly distributed recrystallized structures with a certain size and density in the fibrous structure shows an obvious improvement in toughness. The reason for the toughening effect of the heterogeneous structure composed of the fibrous structure and the recrystallized structure is that: compared with the fibrous structure, the recrystallized structure is a soft zone, randomly distributed around the fibrous structure, which can provide high tensile strain and high work hardening ability, effectively inhibit the stress concentration and constriction of the fibrous structure during the deformation process, and increase the stress threshold for crack initiation and propagation, thereby improving the toughness and plasticity of the material.

[0023] As a preferred technical solution of the present invention, the heat-treatable aluminum alloy is any one of 2XXX series, 6XXX series or 7XXX series aluminum alloys.

[0024] Preferably, the contents of the Zr element and the Sc element are calculated by mass percentage, 0.18 wt% ≤ Zr + Sc ≤ 0.26 wt%, preferably 0.2 wt% ≤ Zr + Sc ≤ 0.24 wt%.

[0025] If the content of Zr + Sc is 0.18 wt% ≤ Zr + Sc ≤ 0.26 wt%, the precipitation amount level of primary particles is better during solidification, the density of recrystallized structure is relatively good during subsequent extrusion, the recrystallized grains are not likely to grow too large during the extrusion deformation process, and it will not cause the coarsening of recrystallized grains during the subsequent processing, and the strength and toughness of the obtained material are better. Moreover, it is further worth noting that by controlling the total amount of Zr + Sc within the above range, the aluminum alloy plate exhibits relatively optimal strength and toughness.

[0026] Preferably, the size of the recrystallized structure clusters is ≤ 200 μm 2 , for example, it can be 200 μm 2 , 190 μm 2 , 185 μm 2 , 180 μm 2 , 170 μm 2 , 160 μm 2 or 150 μm 2 etc.

[0027] If the recrystallized structure clusters are too large, it will lead to poor toughness.

[0028] Preferably, the distribution density of the recrystallized structure clusters is 200 - 220 pieces / mm 2 , for example, it can be 200 pieces / mm 2 , 202 pieces / mm 2 , 204 pieces / mm 2 , 205 pieces / mm 2 , 206 pieces / mm 2 , 208 pieces / mm 2 , 210 pieces / mm 2 , 212 pieces / mm 2 , 214 pieces / mm 2 , 215 pieces / mm 2 , 218 pieces / mm 2 or 220 pieces / mm 2 etc.

[0029] Insufficient distribution density of the recrystallized structure clusters will affect toughness; when the distribution density of the recrystallized structure clusters is too large, it will lead to a significant decrease in strength; if the proportion of the distribution density of the recrystallized structure is too large or too small, it will break the balance of the regulated distribution of the heterogeneous structure, resulting in an insignificant toughening effect.

[0030] Preferably, the recrystallized structure clusters are evenly distributed in the fibrous tissue.

[0031] Uneven distribution of the recrystallized structure clusters will affect the strength and toughness values.

[0032] Preferably, the size of a single recrystallized grain in the heterogeneous structure is 7-12 μm 2 , for example, it can be 7 μm 2 , 7.5 μm 2 , 8 μm 2 , 8.5 μm 2 , 9 μm 2 , 9.5 μm 2 , 10 μm 2 , 10.5 μm 2 , 11 μm 2 or 12 μm 2 etc.

[0033] When the size of a single recrystallized grain is greater than 12 μm 2 , the recrystallized grain size is too coarse, resulting in a decrease in toughness and strength.

[0034] Under the same treatment conditions, for example, in the T74 state, the strength and toughness values of the tough and strong aluminum alloy plate with a heterogeneous structure are relatively improved compared with those of the aluminum alloy plate with a homogeneous structure in the same alloy formulation system, compared with the fully recrystallized structure, in terms of strength and toughness, or compared with the fully fibrous crystalline structure, in terms of strength and toughness.

[0035] In a second aspect, the present invention provides a method for preparing a tough and strong aluminum alloy plate with a heterogeneous structure as described in the first aspect, and the preparation method includes the following steps:

[0036] (1) According to the alloy composition ratio, melt the raw materials of the aluminum alloy plate and stir, and then successively carry out refining, slag skimming and low-frequency electromagnetic casting to obtain an ingot;

[0037] (2) Perform secondary homogenization heat treatment on the ingot obtained in step (1) to obtain a cast rod;

[0038] (3) Extrude the cast rod obtained in step (2) to obtain a semi-finished product plate;

[0039] (4) Perform secondary solution treatment on the semi-finished product plate obtained in step (3);

[0040] (5) Perform aging heat treatment on the plate after the secondary solution treatment in step (4) to obtain the tough and strong aluminum alloy plate with a heterogeneous structure.

[0041] The preparation method of the present invention combines low-frequency electromagnetic casting, secondary homogenization heat treatment, extrusion forming process and secondary solution treatment process to obtain an aluminum alloy plate with high strength and high toughness having a specific heterogeneous structure; the preparation method takes into account the realization of production efficiency and performance and has industrial feasibility.

[0042] The raw materials Sc and Zr described in step (1) can be added in the form of scandium-aluminum master alloy and zirconium-aluminum master alloy. For example, a refining agent can be added to refine the melt to eliminate gases and non-metallic inclusions in the melt, and slag skimming is carried out after refining to purify the melt; the refining agent is a conventional refining agent in the prior art and is not specifically limited herein. Preferably, a conventional aluminum-titanium grain refiner is added after slag skimming, and then standing is carried out. The specific composition of the aluminum-titanium grain refiner of the present invention is not specifically limited.

[0043] It should be noted that under the action of the induction magnetic field, the melt in the liquid cavity undergoes forced convection and generates a uniform temperature field, which prolongs the time of the metal in the crystallization temperature range, is conducive to the formation of primary Al3(Sc,Zr) phase, thereby reducing the solid solution amount of Zr and Sc elements in the surrounding melt, and becoming a secondary Al3(Sc,Zr) phase-deficient area after homogenization heat treatment.

[0044] Preferably, the magnetic field strength of the low-frequency electromagnetic casting described in step (1) is 1.2×10 4 ~3.6×10 4 a / M, and the current frequency is 15 - 30 Hz.

[0045] The coil, current, and effective magnetic path length jointly determine the magnetic field strength of the induction magnetic field; the calculation formula of the magnetic field strength: H = N×I / Le; where: H is the magnetic field strength, the unit is a / m; N is the number of turns of the exciting coil; I is the exciting current (measured value), the unit is A; Le is the effective magnetic path length of the test sample, the unit is m; the desired magnetic field strength can be obtained by adjusting the above parameters.

[0046] The magnetic field strength of the low-frequency electromagnetic casting, for example, can be 1.4×10 4 a / M, 1.6×10 4 a / M, 1.8×10 4 a / M, 2.0×10 4 a / M, 2.4×10 4 a / M, 2.8×10 4 a / M, 3.0×10 4 a / M, 3.2×10 4 a / M or 3.4×10 4 a / M, etc., but not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0047] If the magnetic field strength is too low, the melt flow rate is too low, and the temperature of the melt in the liquid cavity is uneven, resulting in uneven distribution of primary Al3(Sc,Zr) particles across the entire cross-section of the ingot, and further leading to uneven distribution of the recrystallized structure of the product, manifested as a large density difference between randomly selected two regions. The uneven distribution of the recrystallized structure will result in insignificant improvement in toughness.

[0048] The current frequency of the low-frequency electromagnetic casting can be, for example, 16 Hz, 18 Hz, 20 Hz, 22 Hz, 24 Hz, 26 Hz, or 28 Hz, etc., but is not limited to the listed values. Other unlisted values within the above value range are equally applicable.

[0049] The selection of the current frequency takes more into account the skin effect: under the influence of the skin effect, the melt flow is concentrated on the outer circle of the diameter, and the skin depth is inversely proportional to the current frequency, δ∝1 / f.

[0050] In the present invention, the selection of the current frequency range needs to be controlled. On the one hand, it ensures a relatively high skin depth, enabling the melt to flow sufficiently as a whole and generating a uniform temperature field; on the other hand, it considers manufacturing feasibility. When the current frequency decreases, the temperature field becomes uneven, the density of the homogenized primary phase is uneven, and the number of particles decreases. As a result, during the extrusion process, the distribution of the secondary phase deficiency regions is uneven and the quantity decreases, leading to uneven distribution and decreased density of the recrystallized tissue clusters. The uneven distribution and decreased density of the recrystallized tissue clusters will affect the strength and toughness. When the current frequency is high, the magnetic field is too high and it is not manufacturable.

[0051] For the preparation of heat-treatable alloys, homogenization heat treatment is necessary. Its purpose is to dissolve the non-equilibrium eutectic phase during solidification back into the aluminum matrix, avoiding the adverse effects of coarse non-equilibrium eutectic phases on the deformation ability of the ingot during extrusion and on the toughness of the final material; usually, primary homogenization heat treatment is required, and those skilled in the art can adjust the process of homogenization heat treatment according to the different materials and the dissolution situation of the specific non-equilibrium eutectic phase.

[0052] For example, for 7XXX series alloys, the secondary homogenization heat treatment process can be designed as follows: the first-stage holding temperature of the secondary homogenization heat treatment is 400 - 450 °C and the time is 4 - 8 h; the second-stage holding temperature of the secondary homogenization heat treatment is 460 - 480 °C and the time is 18 - 24 h.

[0053] Preferably, before the extrusion forming in step (3), the casting rod is heated and held at a temperature of 400 - 440 °C and a holding time of 2 - 4 h.

[0054] Preferably, the extrusion temperature of the extrusion forming in step (3) is 400 - 460 °C, the extrusion speed is 0.1 - 0.5 m / min, and it can also be determined according to the overburn temperature of the material and the surface quality of the extruded product; the extrusion ratio (the area before extrusion divided by the area after extrusion) is generally 20 - 23, and it can also be determined according to the size of the ingot and the shape and size of the extruded strip.

[0055] It should be noted that the extrusion speed in the prior art is usually 2 m / min, at least 1 m / min. The reason why the present invention chooses the above-mentioned low-speed extrusion method at the expense of processing efficiency to a certain extent is to regulate the dynamic recrystallization nucleation and growth around the primary Al3(Sc,Zr) phase in the tissue, so as to obtain a heterogeneous structure in which recrystallized tissue and fibrous tissue coexist. If the extrusion speed is too high, it will cause the original fine recrystallizations to merge with each other during the extrusion process; moreover, due to the too high extrusion speed, the energy storage of the fibrous tissue is too high, and the fibrous tissue undergoes complete recrystallization during the solution treatment, and finally becomes a completely recrystallized tissue morphology.

[0056] Preferably, after the extrusion molding in step (3) is completed, air cooling is carried out.

[0057] Preferably, the first-stage solution treatment temperature of the secondary solution treatment in step (4) is 350-400 °C, and the time is 2-4 h.

[0058] The first-stage solution treatment temperature of the secondary solution treatment, for example, can be 355 °C, 360 °C, 365 °C, 370 °C, 375 °C, 380 °C, 385 °C, 390 °C or 395 °C, etc., but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0059] The first-stage solution treatment time of the secondary solution treatment, for example, can be 2.2 h, 2.4 h, 2.6 h, 2.8 h, 3 h, 3.2 h, 3.4 h, 3.4 h, 3.6 h or 3.8 h, etc., but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0060] The second-stage solution treatment temperature of the secondary solution treatment in step (4) is the traditional high-temperature solution treatment temperature, which can be adjusted adaptively according to the specific materials. For example, generally, the 7XXX series alloy is 460-480 °C, and the time is 1-2 h; the 6XXX series alloy is 520-540 °C, and the time is 1-2 h.

[0061] The second-stage solution treatment temperature of the 7XXX series alloy, for example, can be 462 °C, 464 °C, 466 °C, 468 °C, 470 °C, 472 °C, 474 °C, 476 °C or 478 °C, etc., but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable. The second-stage solution treatment time, for example, can be 1.1 h, 1.2 h, 1.4 h, 1.6 h, 1.8 h or 1.9 h, etc., but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0062] For the 6XXX series alloy, the second-stage solution temperature of the secondary solution treatment can be, for example, 520°C, 522°C, 524°C, 525°C, 530°C, 532°C, 535°C or 540°C, etc., but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable; the time is 1 to 2 h, for example, it can be 1.1 h, 1.2 h, 1.4 h, 1.6 h, 1.8 h or 1.9 h, etc., but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0063] For heat-treatable aluminum alloys, the purpose of solution treatment is to redissolve the alloying elements in the alloy into the aluminum matrix again to form a supersaturated solid solution, so as to increase the number of precipitated phases during aging and thus increase the strength. Usually, a primary solution treatment at a high temperature can achieve the ideal strength. However, experiments have found that under the processing conditions of the primary high-temperature solution treatment, the heterogeneous structure alloy obtained during extrusion will undergo static recrystallization during the primary high-temperature solution treatment, resulting in an increase in the size of the recrystallized structure in the finally obtained aluminum alloy plate, and also resulting in too high energy storage in the fibrous structure. During the solution treatment, the fibrous structure undergoes complete recrystallization and finally becomes a fully recrystallized tissue morphology, and the higher strength and toughness expected by the present invention cannot be obtained.

[0064] In the present invention, the solution treatment is divided into two stages. The first-stage solution treatment with a lower temperature is used to enable the metal to undergo a recovery process to release part of the energy storage, and then a higher second-stage solution temperature is used for further solution treatment; to prevent static recrystallization and recrystallization growth from occurring under the traditional one-time high-temperature solution process and retain the heterogeneous structure after extrusion.

[0065] Preferably, the aging heat treatment described in step (5) includes T4, T6 or T74 processes.

[0066] The numerical ranges described in the present invention not only include the above-listed point values, but also include any point values between the above-listed numerical ranges that are not listed. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the described ranges.

[0067] Compared with the prior art, the present invention has at least the following beneficial effects:

[0068] (1) Compared with the recrystallized structure of the traditional heat-treatable aluminum alloy plate provided by the present invention, the heat-treatable aluminum alloy plate has a unique heterogeneous structure: the fibrous structure is the hard zone, and the recrystallized structure is the soft zone, and the soft zone and the hard zone are alternately distributed; the heterogeneous structure not only facilitates the activation of various deformation mechanisms during the deformation process, but also promotes the multiplication of dislocations and generates back stress strengthening, effectively improving the strength of the material. At the same time, the soft zone is a fully recrystallized structure, which can provide high tensile strain and high work hardening ability, effectively inhibit the stress concentration and constriction of the hard zone during the deformation process, extend the crack initiation and propagation, improve the toughness and plasticity of the material, and compared with the prior art, at the same strength level, the fracture toughness value can be increased by about 30% or more;

[0069] (2) The present invention introduces rare earth Sc elements and Zr elements, and adopts the low-frequency electromagnetic casting technology and the method of low-temperature slow extrusion to control the structural morphology of the internal structure of the aluminum alloy plate, improve the comprehensive level of strength and toughness of the aluminum alloy plate material, and has better strength and toughness than similar alloys. The present invention uses the primary Al3(Sc,Zr) phase and the secondary Al3(Sc,Zr) phase to control the obtaining of the recrystallized structure and the fibrous structure. By combining the low-frequency electromagnetic casting technology with the method of low-temperature slow extrusion, a heterogeneous structure with specific structural parameters can be controlled and obtained. The process is simple, stable and reliable, and is a more convenient industrial preparation means. Description of the Drawings

[0070] Figure 1 It is a schematic diagram of the heterogeneous structure in the aluminum alloy plate obtained in Example A1.

[0071] Description of the reference numerals: 1 - recrystallized structure cluster, 2 - fibrous structure. Detailed Embodiments

[0072] The technical solution of the present invention will be further described below in conjunction with the drawings and specific embodiments.

[0073] The present invention will be further described in detail below. However, the following examples are only simple examples of the present invention, and do not represent or limit the scope of the patent protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

[0074] 1. Random terms

[0075] Uniform distribution: It means that in the internal structure of the aluminum alloy plate with the same area, the distribution probability of the crystal structure clusters is equally possible; theoretically, it is required that the difference in the distribution density of the randomly sampled recrystallized structure clusters is controlled within 30 / mm 2 within.

[0076] Primary Al3(Sc,Zr) phase: It refers to the Al3(Sc,Zr) compound that is first formed from the liquid phase during solidification.

[0077] Secondary Al3(Sc,Zr) phase: It refers to other Al3(Sc,Zr) compounds that precipitate subsequently during the heat treatment after the formation of the primary Al3(Sc,Zr) phase.

[0078] 2. Preparation process and product

[0079] The following examples and comparative examples provide a method for preparing an aluminum alloy plate, including the following steps:

[0080] (1) According to the alloy composition ratio, the raw materials of the aluminum alloy plate are melted and stirred, and then refined, slag skimmed, and low-frequency electromagnetic casting are carried out in sequence to obtain an ingot.

[0081] (2) The ingot obtained in step (1) is subjected to secondary homogenization heat treatment to obtain a casting rod.

[0082] (3) The casting rod obtained in step (2) is extruded into a semi-finished product plate.

[0083] (4) The semi-finished product plate obtained in step (3) is subjected to secondary solution treatment.

[0084] (5) The plate after the secondary solution treatment in step (4) is subjected to aging heat treatment to obtain the strong and tough aluminum alloy plate with a heterogeneous structure.

[0085] To study the influence of each variable affecting the heterogeneous structure of the product by unifying the processing variables, some conventional parameters are uniformly defined below. The following is only a typical definition case and does not limit the protection scope of the present invention.

[0086] Examples A1 - A9 and Comparative Examples B1 - B7

[0087] The above examples and comparative examples use 7050 - T74 heat - treatable alloy as the raw material. The specific alloy formula is: Cu: 2.5wt%, Mg: 2.0wt%, Zn: 6.2wt%. Except for the adjusted mass ratios of Zr and Sc, the other elements remain unchanged.

[0088] In the above examples and comparative examples, the first - stage temperature of the secondary homogenization heat treatment in step (2) is 420°C and the time is 6h; the second - stage end - point temperature of the secondary homogenization heat treatment is 460°C and the time is 24h. Before the extrusion forming in step (3), the casting rod is heated and kept warm. The heating temperature is 420°C and the holding time is 3h. The aging heat treatment method in step (5) is selected as T74.

[0089] The settings of the remaining process parameters and component ratios are shown in Table 1.

[0090] Table 1

[0091]

[0092]

[0093] The products obtained from Examples A1 - A9 and Comparative Examples B1 - B7 are shown in Table 2. The schematic diagrams of the heterogeneous structures obtained from Examples A1 - A9 are as shown in Figure 1 shown. It can be seen from Figure 1 that the heterogeneous structure includes a recrystallized structure and a fibrous structure 2 existing in the microstructure of the aluminum alloy plate. The minimum distribution unit of the recrystallized structure is a recrystallized structure cluster 1, and the recrystallized structure cluster 1 is formed by the aggregation of individual recrystallized grains; the recrystallized structure clusters 1 are uniformly distributed in the fibrous structure 2.

[0094] In Comparative Example B4, the current frequency was too high, resulting in an excessive magnetic field, making it impossible to manufacture and obtain an aluminum alloy plate.

[0095] Table 2

[0096]

[0097]

[0098] The properties of the aluminum alloy plates prepared in Examples A1 - A9 and Comparative Examples B1 - B7 are shown in Table 3.

[0099] Table 3

[0100]

[0101] In Tables 1 - 3, " / " indicates no relevant data.

[0102] It can be seen from Tables 1 - 3 as follows:

[0103] (1) From a comprehensive view of Examples A1 - A9, it can be seen that in the present invention, by adding Zr element and Sc element to the heat - treatable strengthened alloy; and controlling the contents of the Zr element and Sc element by mass percentage, 0.1 wt% ≤ Zr + Sc ≤ 0.26 wt% and 0.86 ≤ Zr / Sc ≤ 1.86, for 7xxx series alloys, a strong and tough aluminum alloy plate can be prepared, with a tensile strength ≥ 520 MPa, an elongation rate ≥ 12.6%, and a fracture toughness ≥ 35.6 MPa×m 1 / 2 ;

[0104] Comparing Comparative Example A1 with Examples A4 to A6, it can be seen that the component contents of Zr + Sc in Examples A4 to A6 are 0.2%, 0.24%, and 0.26% respectively. The final elongation and fracture toughness of the aluminum alloy plates obtained in Examples A4 and A5 are higher than those in Examples A1 and A6. This shows that by preferably selecting 0.18 wt% ≤ Zr + Sc ≤ 0.26 wt%, the toughness of the aluminum alloy plate is improved, and further preferably selecting 0.2 wt% ≤ Zr + Sc ≤ 0.24 wt% further improves the toughness of the aluminum alloy plate;

[0105] (2) Recrystallized structure and its distribution in the heterogeneous structure

[0106] Compared with Example A1, the magnetic field strength in Example A2 is only 1×10 4 a / M, the magnetic field strength is too low, the melt flow rate is low, and the melt temperature in the liquid cavity is uneven, resulting in uneven distribution of primary Al3(Sc,Zr) particles across the entire cross-section of the ingot, and further leading to uneven distribution of the recrystallized structure of the product, resulting in a strength of only 528 MPa, an elongation of only 16.5%, and a fracture toughness of only 41 MPa×m 1 / 2 , which is lower in strength and toughness than that in Example A1;

[0107] Compared with Example A1, the current frequency in Example A3 is only 12 Hz, resulting in an uneven temperature field, uneven density of homogenized primary phases and a reduction in the number of particles. Further, during the extrusion process, the distribution of secondary phase depletion regions is uneven and the number decreases, making the distribution of recrystallized structure clusters uneven and the density decrease. Finally, the strength, fracture toughness, and elongation are only 523 MPa, 38.2 MPa×m 1 / 2 and 15.3% respectively;

[0108] Compared with Example A1, the typical product characteristics of Examples A2 and A3 are poor uniformity in the distribution of recrystallized structure clusters, with density deviations reaching 50 grains / mm 2 and 60 grains / mm 2 respectively, which makes the aluminum alloy plates in Examples A2 and A3 less excellent in strength and toughness than the aluminum alloy plate in Example A1.

[0109] (3) Size of a single recrystallized grain

[0110] Compared with Example A1, the sum of the mass fractions of Zr + Sc in Example A9 is 0.16%. The doping of Zr + Sc in Example A9 is too little, resulting in a coarse size of a single recrystallized grain in the obtained heterogeneous structure, and the size of a single recrystallized grain reaches 15.6 μm 2, although other parameters of the heterogeneous structure, as well as the sizes of the primary phase and secondary phase, are similar to those in Example A1, when the size of a single recrystallized grain in Example A1 is finer, better strength and toughness can be obtained.

[0111] (4) Heterogeneous structure, fully recrystallized structure, fibrous structure

[0112] Comparing Example A1, Comparative Example B5, Comparative Example B6, and Comparative Example B7, it can be seen that under the same alloy formula, by adjusting the process to different tissue structures, different strength and toughness values will be obtained. For Example A1 containing a heterogeneous structure, its strength and toughness are significantly better than the fully recrystallized structures of Comparative Example B5 and Comparative Example B6 and the fibrous structure of Comparative Example B7;

[0113] Compared with an extrusion speed of 0.5 m / min in Example A1, the extrusion speed in Comparative Example B5 is as high as 1 m / min. Due to the too high extrusion speed, the energy storage of the fibrous structure is too high, and the fibrous structure undergoes complete recrystallization during the solution treatment process and finally becomes a fully recrystallized tissue morphology, with the toughness dropping to 32.6 MPa×m 1 / 2 , and the elongation rate is only 9.6%. Moreover, the strength is lower than that in Example A1. This shows that only by selecting an appropriate extrusion speed can a heterogeneous junction structure be obtained in the present invention to improve strength and toughness;

[0114] Compared with Example A1 where the solution treatment is carried out in two stages, with the first-stage solution treatment temperature being 400 °C and the second-stage solution treatment temperature being 480 °C, in Comparative Example B6, only the first-stage solution treatment is set, and the solution treatment temperature is 480 °C, resulting in static recrystallization and recrystallization growth during the high-temperature solution treatment process, forming complete recrystallization, and only a slight decrease in toughness, tensile strength, and elongation rate. This shows that through two-stage solution treatment in the present invention, the toughness and strength of the aluminum alloy plate are improved;

[0115] Compared with Example A1, in Comparative Example B7, the non-electromagnetic casting method is used, and it is difficult to form a heterogeneous structure. Eventually, the structure in the aluminum alloy plate is a fibrous structure, and the corresponding fracture toughness drops to 28.4 MPa×m 1 / 2 , and the elongation rate is also only 9.6%.

[0116] (5) Sizes of primary phase and secondary phase

[0117] Compared with the sum of the mass fractions of Zr + Sc being 0.18% in Example A1, in Comparative Example B1, the sum of the mass fractions of Zr + Sc is 0.28%; too much doping of Zr + Sc in Comparative Example B1 leads to the coarsening of the primary Al3(Sc,Zr) phase (up to 1.4 μm) generated during solidification. Finally, the fracture toughness drops from 41.8 MPa×m in Example A1 1 / 2 to 33.5 MPa×m1 / 2 The elongation rate also decreased to 10.2%, indicating that the prerequisite for improving the toughness of the aluminum alloy plate by introducing Zr and Sc elements in the present invention is that the sizes of the primary phase and secondary phase should be controlled as follows: the average size of the primary Al3(Sc,Zr) phase ≤ 1.2 μm; the average size of the secondary Al3(Sc,Zr) phase ≤ 20 nm;

[0118] Compared with Example A1, when the total amount of Zr + Sc remains the same, the Zr / Sc in Comparative Example B2 is only 0.8, while the Zr / Sc in Comparative Example B3 is as high as 2.00; the product of Comparative Example B2 shows poor size stability of the secondary Al3(Sc,Zr) phase formed during the homogenization process, coarsening during the homogenization process, and finally resulting in partial recrystallization of the fibrous structure during the solution process, making it difficult to form a heterogeneous structure, but generating a recrystallized structure. The secondary phase in the final product shows an overly large average size, ultimately reducing the toughness to 30.4 MPa×m 1 / 2 The elongation rate is only 14.3%; the product of Comparative Example B3 shows poor size stability of the primary Al3(Sc,Zr) phase formed during the solidification process, coarsening during the homogenization heat treatment. The coarsening of the primary Al3(Sc,Zr) phase leads to an increase in the dispersion phase depletion zone, thereby increasing the recrystallized structure clusters. Excessively large recrystallized structure clusters will preferentially deform and fracture as soft zones during the loading process, causing a decrease in strength and toughness; and the average size of the primary phase in the final product is overly large, and the tensile strength decreases from 537 MPa in Example A1 to 508 MPa in Comparative Example B3, and the elongation rate and toughness also decrease significantly.

[0119] Examples C1 - C9 and Comparative Examples D1 - D7

[0120] The above examples and comparative examples use 6061 - T6 heat - treatable strengthened alloy as the raw material. The specific alloy formula is: Si: 0.6 wt%, Cu: 0.23 wt%, Mg: 1 wt%, Cr: 0.15 wt%. Except for the adjusted mass ratios of Zr and Sc, the other elements remain unchanged.

[0121] In the above examples and comparative examples, the first - stage temperature of the secondary homogenization heat treatment in step (2) is 420 °C and the time is 6 h; the second - stage end temperature of the secondary homogenization heat treatment is 460 °C and the time is 24 h. Before the extrusion forming in step (3), the cast rod is heated and held at a temperature of 420 °C for a holding time of 3 h. The aging heat treatment method in step (5) is selected as T6.

[0122] The settings of the remaining process parameters and component ratios are shown in Table 4.

[0123] Table 4

[0124]

[0125] The products obtained in Examples C1 - C9 and Comparative Examples D1 - D7 are shown in Table 5 as follows.

[0126] In Comparative Example D4, the current frequency was too high, resulting in an excessive magnetic field, making it impossible to manufacture and obtain an aluminum alloy plate.

[0127] Table 5

[0128]

[0129]

[0130] The properties of the aluminum alloy plates prepared in Examples C1 - C9 and Comparative Examples D1 - D7 are shown in Table 6 as follows.

[0131] Table 6

[0132]

[0133] In Tables 4 - 6, " / " indicates no relevant data.

[0134] The following points can be seen from Tables 4 - 6:

[0135] (1) From a comprehensive view of Examples C1 - C9, it can be seen that in the present invention, by adding Zr element and Sc element to the heat - treatable strengthening alloy, and controlling the contents of the Zr element and Sc element in mass percentage, 0.1wt% ≤ Zr + Sc ≤ 0.26wt% and 0.86 ≤ Zr / Sc ≤ 1.86, for the 6xxx series alloy, a strong and tough aluminum alloy plate can be obtained, with a tensile strength ≥ 322 MPa, an elongation rate ≥ 12.6%, and a fracture toughness ≥ 33.3 MPa×m 1 / 2 ;

[0136] (2) Heterogeneous structure, fully recrystallized structure, fibrous structure

[0137] By comparing Example C1 with Comparative Examples D5 - D7, it can be seen that under the same alloy formula, by adjusting the process to different microstructures, different strength and toughness values will be obtained. For Example C1 containing a heterogeneous structure, its strength and toughness are significantly better than the fully recrystallized structures of Comparative Examples D5 and D6 and the fibrous structure of Comparative Example D7; it can be seen that the effect of the heterogeneous structure on improving strength and toughness is also reproduced in the 6XXX series system.

[0138] (3) Regarding the influence of other heterogeneous structure parameters and phase size parameters on the material properties, the same technical effects as those in the 7XXX series alloy system are also reproduced in the 6XXX series system, and no further analysis will be carried out here.

[0139] 3. Test methods

[0140] (1) Strength characterization: The room temperature mechanical properties were tested according to the method disclosed in GB / T 228.1-2010 "Tensile Testing of Metallic Materials". The testing instrument was a ZWICK universal material testing machine, and the testing indicators were the tensile strength R m , and the elongation A;

[0141] (2) Toughness characterization: The plane strain fracture toughness of metallic materials was tested according to the method disclosed in GB / T 4161-2007 "Test Method for Plane Strain Fracture Toughness KIC of Metallic Materials". The testing instrument was an electro-hydraulic servo fatigue testing machine 8872MTL6641, and the testing indicator was the plane strain fracture toughness KIC;

[0142] (3) Phase size characterization: Five 20mm×20mm samples were taken. After grinding and polishing the longitudinal section, the size of the primary Al3(Sc,Zr) phase was measured using an electron microscope. 20 phase particle sizes were randomly measured for each sample, and 100 phase particle sizes were statistically analyzed to obtain the average size and standard deviation δ of the primary Al3(Sc,Zr) phase; the size of the secondary Al3(Sc,Zr) phase was measured using a transmission electron microscope. 20 phase particle sizes were randomly measured for each sample, and 100 phase particle sizes were statistically analyzed to obtain the average size and standard deviation δ of the secondary Al3(Sc,Zr) phase; in this embodiment, both the primary phase and the secondary phase satisfied the normal distribution, 3δ of the primary phase ≤ 0.2μm, and 3δ of the secondary phase ≤ 12nm.

[0143] Size of recrystallized grain clusters: Five 20mm×20mm samples were taken. After grinding and polishing the longitudinal section, vibratory polishing was carried out, and then EBSD analysis was performed using an electron microscope. 10 1mm×1mm areas were randomly measured for each sample, and the areas A1, A2, A3…An of each recrystallized grain cluster were statistically analyzed. Finally, the average value of the areas of all recrystallized grain clusters was taken to obtain the size value of the recrystallized grain clusters, which is the typical value representing the cluster size.

[0144] Size of recrystallized grain clusters = (A1 + A2 + A3 + … + An) / n

[0145] Distribution density of recrystallized grain clusters: Take 5 samples of 20 mm × 20 mm. After polishing the longitudinal section, perform vibratory polishing, and then conduct EBSD analysis using an electron microscope. Randomly measure 10 areas of 1 mm × 1 mm for each sample, count the number of recrystallized grain clusters in them, and finally take the average value of the number Bi of recrystallized grain clusters in 50 areas to obtain the distribution density of recrystallized grain clusters; the maximum value of the difference between the number of recrystallized grain clusters in any one of the 50 areas and the average value is defined as the uniformity index of the distribution density of recrystallized grain clusters, and the magnitude of this index represents the quality of the distribution uniformity.

[0146] Distribution density of recrystallized grain clusters = ∑Bi / 50;

[0147] Uniformity index of the distribution density of recrystallized grain clusters = max(|∑Bi / 50 - Bi|).

[0148] Size of a single recrystallized grain: Take 5 samples of 20 mm × 20 mm. After polishing the longitudinal section, perform vibratory polishing, and then conduct EBSD analysis using an electron microscope. Randomly measure 10 recrystallized grain clusters for each sample, count the area of each recrystallized grain in them, and obtain the average value of the size of a single recrystallized grain.

[0149] The applicant declares that the above description is only a specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A tough aluminum alloy plate with a heterogeneous structure, characterized in that, The material of the aluminum alloy plate is a heat-treatable aluminum alloy, and the heat-treatable aluminum alloy contains Zr element and Sc element; The contents of the Zr element and the Sc element are in mass percentage, 0.1wt% ≤ Zr + Sc ≤ 0.26wt% and 0.86 ≤ Zr / Sc ≤ 1.86; In the aluminum alloy plate, the average size of the primary Al3(Sc,Zr) phase ≤ 1.2μm; the average size of the secondary Al3(Sc,Zr) phase ≤ 20nm; The heterogeneous structure includes a recrystallized structure and a fibrous structure existing in the microstructure of the aluminum alloy plate. The minimum distribution unit of the recrystallized structure is a recrystallized structure cluster, and the recrystallized structure cluster is formed by the convergence of individual recrystallized grains; the recrystallized structure clusters are distributed in the fibrous structure; the size of the recrystallized structure clusters is ≤200 μm 2 ; the distribution density of the recrystallized structure clusters is 200 - 220 pieces / mm 2 .

2. The tough aluminum alloy plate with a heterogeneous structure according to claim 1, characterized in that, The heat-treatable aluminum alloy is any one of 2XXX series, 6XXX series or 7XXX series aluminum alloys.

3. The tough aluminum alloy plate with a heterogeneous structure according to claim 1, characterized in that, The contents of the Zr element and the Sc element are in mass percentage, 0.18wt% ≤ Zr + Sc ≤ 0.26wt%.

4. The tough aluminum alloy plate with a heterogeneous structure according to claim 3, characterized in that, The contents of the Zr element and the Sc element are in mass percentage, 0.2wt% ≤ Zr + Sc ≤ 0.24wt%.

5. The tough aluminum alloy plate with a heterogeneous structure according to claim 1, characterized in that, The recrystallized tissue clusters are evenly distributed in the fibrous tissue.

6. The tough aluminum alloy plate with a heterogeneous structure according to claim 1, characterized in that, The size of a single recrystallized grain in the heterogeneous structure is 7 to 12 μm 2 .

7. A preparation method of the tough aluminum alloy plate with a heterogeneous structure according to any one of claims 1 to 6, characterized in that, The preparation method includes the following steps: (1) According to the alloy composition ratio, melt and stir the raw materials of the aluminum alloy plate, and successively carry out refining, slag skimming and low-frequency electromagnetic casting to obtain an ingot; (2) Carry out secondary homogenization heat treatment on the ingot obtained in step (1) to obtain a casting rod; (3) Extrude the casting rod obtained in step (2) to obtain a semi-finished product plate; (4) Carry out secondary solution treatment on the semi-finished product plate obtained in step (3); (5) Carry out aging heat treatment on the plate after the secondary solution treatment in step (4) to obtain the tough and strong aluminum alloy plate with a heterogeneous structure.

8. The preparation method according to claim 7, characterized in that, The magnetic field strength of the low-frequency electromagnetic casting described in step (1) is 1.2×10 4 ~3.6×10 4 A / M, and the current frequency is 15~30Hz.

9. The preparation method according to claim 7, characterized in that, The extrusion temperature for the extrusion forming in step (3) is 400~460°C, and the extrusion speed is 0.1~0.5m / min.

10. The preparation method according to claim 7, characterized in that, The first-stage solution temperature for the secondary solution treatment in step (4) is 350~400°C, and the time is 2~4h.

11. The preparation method according to claim 7, characterized in that, When the aluminum alloy plate is a 7XXX alloy, the second-stage solution temperature for the secondary solution treatment in step (4) is 460~480°C, and the time is 1~2h.

12. The preparation method according to claim 7, characterized in that, When the aluminum alloy plate is a 6XXX alloy, the second-stage solution temperature for the secondary solution treatment in step (4) is 520~540°C, and the time is 1~2h.

13. The preparation method according to claim 7, characterized in that, The aging heat treatment in step (5) includes T4, T6 or T74 process.

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