Method for manufacturing al-zn-mg-cu series aluminum alloy sheet and aluminum alloy sheet

Abstract

The application discloses a manufacturing method of Al-Zn-Mg-Cu system aluminum alloy plate, which comprises the following steps: (1) preparing Al-Zn-Mg-Cu system aluminum alloy ingot; (2) sequentially performing homogenization treatment, hot rolling, cold rolling, solid solution quenching treatment and artificial aging treatment on the Al-Zn-Mg-Cu system aluminum alloy ingot to obtain T6 state aluminum alloy plate; (3) performing heating, warm forming, in-mold quenching, pre-aging treatment and baking varnish treatment on the T6 state aluminum alloy plate to obtain finished aluminum alloy plate. In addition, the application further discloses an Al-Zn-Mg-Cu system aluminum alloy plate prepared by the above manufacturing method. The production process and process parameter of the Al-Zn-Mg-Cu system aluminum alloy plate are standardized, the Al-Zn-Mg-Cu system aluminum alloy plate prepared by the manufacturing method has high tensile strength, yield strength and elongation, and can be applied to automobiles, and has good popularization prospect and application value.

Description

Technical Field

[0001] The present invention relates to an aluminum alloy plate and a manufacturing method thereof, and in particular to a 7000 series aluminum alloy plate and a manufacturing method thereof. Background Art

[0002] As we all know, the density of aluminum is about 1 / 3 of that of steel. It is currently the most widely used lightweight material. Aluminum alloy materials are lightweight materials that have been used earlier and the technology is becoming increasingly mature. In recent years, the use of aluminum alloy materials in automobiles has shown a continuous growth trend.

[0003] Compared to steel, aluminum alloys offer numerous advantages, including high thermal conductivity, excellent corrosion resistance, and superior processability. While not as strong as high-strength steel, through technological advancements, aluminum alloys can meet the strength requirements of lightweight vehicles. Furthermore, aluminum alloys have an energy absorption capacity roughly twice that of steel, significantly improving vehicle crash safety. Therefore, replacing traditional steel with aluminum alloys is a key trend in lightweighting technology within the automotive industry.

[0004] In current technology, aluminum alloys used in vehicle bodies primarily include the 2000 series (Al-Cu), 5000 series (Al-Mg), 6000 series (Al-Mg-Si), and a smaller amount of the 7000 series (Al-Zn-Mg or Al-Zn-Mg-Cu). Al-Zn-Mg-Cu aluminum alloys, also known as 7000 series aluminum alloys, achieve high strength and toughness after quenching and aging treatment. Their low density has led many automakers to consider replacing high-strength steel with these 7000 series aluminum alloys in the manufacture of automotive parts, such as B-pillars and shock absorber reinforcements. For example, a 7000 series aluminum alloy is currently used in automotive safety devices, boasting twice the strength of current bumper aluminum alloys. Compared to high-strength steel, these 7000 series aluminum alloys reduce vehicle weight while maximizing passenger safety.

[0005] However, due to the poor plasticity of 7000 series aluminum alloys in the quenched state at room temperature, they exhibit strong brittleness and are difficult to directly form into more complex parts using conventional forming methods. Therefore, it is usually necessary to anneal this 7000 series aluminum alloy sheet to increase the material's plasticity, and then perform quenching and aging treatments after forming.

[0006] Research has found that the current treatment method is very complicated and the subsequent heat treatment time is long, which cannot meet the requirements of mass production of parts in the automotive industry. It is also prone to deformation and has a certain impact on the size of the parts.

[0007] At the same time, current research on warm forming by scientific researchers mainly focuses on 5000 series, 6000 series aluminum alloy materials that do not require heat treatment and some magnesium alloy materials, while there are fewer studies on warm forming of heat-treatable 7000 series aluminum alloy materials. Although there are individual studies, they are all limited to warm forming experiments and their mechanical theory, warm processing performance research, and simulation research of warm forming processes. In the current existing technology, there is still a blank in the clear process technology for 7000 series aluminum alloy sheets for automobiles.

[0008] To address the poor room-temperature formability of current 7000-series aluminum alloys for automotive applications, as well as the tendency for deformation in heat-treated specimens after forming, the inventors have designed and developed a novel method for manufacturing Al-Zn-Mg-Cu aluminum alloy sheet materials, specifically 7000-series aluminum alloy sheet materials. The process principles underlying this manufacturing method are applicable not only to this Al-Zn-Mg-Cu aluminum alloy material but also to all other heat-treatable aluminum alloys, such as 2000-series, 6000-series, and other 7000-series aluminum alloys. Summary of the Invention

[0009] One of the objectives of the present invention is to provide a novel method for manufacturing Al-Zn-Mg-Cu aluminum alloy sheet material. This method utilizes a rational process design to improve the formability of the aluminum alloy sheet material, meeting the requirements for automotive sheet material use and lightweighting. Furthermore, the resulting Al-Zn-Mg-Cu aluminum alloy sheet material exhibits high tensile strength, yield strength, and elongation, meeting the strength and toughness requirements of automotive sheet material, thereby overcoming the shortcomings of the prior art.

[0010] In order to achieve the above object, the present invention proposes a method for manufacturing an Al-Zn-Mg-Cu series aluminum alloy plate, which comprises the following steps:

[0011] (1) preparing an Al-Zn-Mg-Cu aluminum alloy ingot;

[0012] (2) subjecting the Al-Zn-Mg-Cu series aluminum alloy ingot to homogenization treatment, hot rolling, cold rolling, solution quenching treatment, and artificial aging treatment in sequence to obtain a T6 aluminum alloy plate;

[0013] (3) The T6 aluminum alloy sheet is heated, warm-formed, in-mold quenched, pre-aged, and painted to obtain a finished aluminum alloy sheet.

[0014] In the current existing technology, the traditional aluminum alloy sheet forming process is to subject the aluminum alloy sheet to solid solution quenching and aging treatment and then hot stamping to obtain the finished product. This process has poor formability and is not suitable for Al-Zn-Mg-Cu alloys.

[0015] In the present invention, in order to solve the problem that 7000 series aluminum alloys have poor room temperature forming performance and are prone to deformation when heat treated after forming, the inventors have discovered through extensive research that a new process integrating warm forming and quenching (Solution Heat Treatment-Forming-Cold Die Quenching), referred to as the HFQ process, can be used to treat 7000 series aluminum alloys.

[0016] The warm forming process combines hot forming with heat treatment. It can be used to form complex, high-strength aluminum alloy sheet structural parts, improving the formability of aluminum alloys. The automotive application of 7000 series high-strength aluminum alloys requires significant improvements in production efficiency. Warm forming is expected to become the optimal process for producing 7000 series high-strength aluminum alloys in the automotive industry.

[0017] However, the current common warm forming process typically involves warm forming and in-die quenching of solutionized (W-state) aluminum alloy sheets. While this warm forming process offers good formability, it still fails to achieve its ultimate strength. This is because the in-die quenching process partially decomposes the supersaturated solid solution, hindering the subsequent precipitation of age-strengthening phases.

[0018] Therefore, unlike the above-mentioned existing warm forming process, in the present invention, the inventors creatively designed and sequentially carried out the following process steps for the prepared Al-Zn-Mg-Cu series (i.e., 7000 series) aluminum alloy ingots: homogenization treatment, hot rolling, cold rolling, solution quenching treatment, artificial aging treatment, heating, warm forming, in-mold quenching, pre-aging treatment and paint treatment to obtain the finished Al-Zn-Mg-Cu series aluminum alloy sheet.

[0019] In the present invention, the combination of steps (2) and (3) makes the process of this case far superior to the above-mentioned prior art process. Compared with the existing warm forming process, the process designed by the present invention further adds an artificial aging process after the solid solution quenching treatment.

[0020] In the present invention, the warm forming process first heats the cold-rolled Al-Zn-Mg-Cu alloy sheet to a solution treatment temperature, then holds it at this solution treatment temperature for a period of time to allow the solute atoms to fully dissolve into the α-aluminum matrix. The fully dissolved Al-Zn-Mg-Cu alloy sheet is then quickly transferred to a die for stamping and quenching within the die. Following the solution quenching treatment, the formed part undergoes a thermal aging treatment to control the formation of precipitates and thus ensure its strength.

[0021] Among them, there are two main reasons for pressure-holding quenching in the mold: first, rapid quenching prevents the formation of coarse precipitates, especially at the grain boundaries; second, it avoids deformation of the formed part during the quenching process.

[0022] The novel warm forming process designed by the present invention not only improves the formability of aluminum alloy materials, but also reduces the springback of aluminum alloy materials. It can meet the production requirements of aluminum alloy body exterior surface parts with high precision, high strength and complex shapes.

[0023] It should be noted that after completing step (2), the aluminum alloy plate (T6 state) after artificial aging treatment needs to be rapidly heated and solid-solution treated, and then warm formed and in-die quenched. This process not only has good formability, but also may bring out the strength limit of Al-Zn-Mg-Cu alloy. This is because after rapid heating and solid-solution treatment, Al-Zn-Mg-Cu alloy still has some fine strengthening phases, which will play a reinforcing role in the subsequent rapid pre-aging treatment process.

[0024] It should be noted that, in the present invention, the purpose of artificial aging treatment is to decompose the unstable supersaturated solid solution of the quenched profile by keeping it at a certain temperature for a certain period of time, thereby causing a significant increase in the strength and hardness of the alloy.

[0025] For Al-Zn-Mg-Cu aluminum alloys, a single-stage aging system can be used to obtain the T6 state when simply pursuing high strength. After aging treatment, the main strengthening phases are the GP zone and a small amount of transition phase (η' phase), and the strength can reach the peak.

[0026] Furthermore, in the manufacturing method of the present invention, the mass percentage ratio of the chemical elements in the Al-Zn-Mg-Cu aluminum alloy ingot is:

[0027] Cu: 1.6-2.2%, Mg: 1.8-2.4%, Zn: 6.0-8.6%, Zr: 0.10-0.16%, 0<Ti≤0.06%, 0<Mn≤0.05%, 0<Cr≤0.04%, and the balance is Al and inevitable impurities.

[0028] In the present invention, the Al-Zn-Mg-Cu series aluminum alloy ingot is optimized and designed, and the design principles of the various chemical elements are as follows:

[0029] Cu: The addition of Cu to the Al-Zn-Mg-Cu aluminum alloy ingot described in the present invention can improve the alloy's stress corrosion resistance, cracking performance, strength, fatigue resistance, and processing properties, enhance the alloy's fluidity, strengthen the strengthening effect of the second stage of two-stage aging, reduce processing defects, and reduce the crack propagation rate of the alloy in corrosive media. The dissolution of Cu into the GP zone can make the GP zone more stable and delay its aging precipitation. In addition, Cu atoms can dissolve into η and η', reducing the potential difference between the grain boundary and the grain boundary, thereby improving the alloy's corrosion resistance. Furthermore, an increase in Cu content increases the material's tendency to weld hot cracking, resulting in a decrease in welding performance. Therefore, when designing the composition of the Al-Zn-Mg-Cu aluminum alloy, various performance indicators of the alloy were comprehensively considered, and an appropriate Cu content was selected, with the mass percentage of Cu controlled between 1.6-2.2% to balance the alloy's welding performance.

[0030] Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Cu element can be further controlled to be between 1.8-2.2%.

[0031] Mg and Zn: In the Al-Zn-Mg-Cu aluminum alloy ingot described herein, the alloying elements Zn and Mg precipitate from the alloy matrix to form a strengthening phase, η'(MgZn2), thereby improving the alloy's yield strength and fracture toughness. When the Zn content in the alloy is too low, the alloy's strength is insufficient. When the Zn content is too high, the alloy's toughness is low and its formability is poor. The inventors have discovered that the Zn and Mg content must be within a critical range to achieve an aging-hardening effect in the alloy matrix. If the Zn and Mg content exceeds the maximum value within this critical range, increasing the Zn and Mg content will not increase the aging-hardening effect. If the Zn and Mg content is below the minimum value within this critical range, there will be no aging-hardening effect. Therefore, a Zn / Mg ratio of 2.6-3.3 ensures a fine and disperse distribution of the alloy's aging precipitates, and the aging process proceeds rapidly. Therefore, in the present invention, the mass percentage of the Mg element is controlled between 1.8-2.4%, and the mass percentage of the Zn element is controlled between 6.0-8.6%.

[0032] Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Mg element can be further controlled between 2.0-2.4%, and the mass percentage of the Zn element can be further controlled between 6.1-7.8%.

[0033] Zr: In the Al-Zn-Mg-Cu aluminum alloy ingot described in the present invention, the fine dispersed precipitates formed by trace amounts of transition elements Mn, Cr, and Zr can improve the yield strength and tensile strength of the alloy. By inhibiting recrystallization, a fine grain structure containing deformation substructures is obtained, which is beneficial to improving the fracture toughness of the alloy, causing the alloy to undergo transgranular fracture, thereby improving toughness. Alloys containing Mn and Cr have significantly higher corrosion resistance than alloys without Mn and Cr. These elements help to increase the recrystallization temperature of the alloy, preventing the recrystallization process from proceeding during hot deformation and subsequent quenching and heating. Low contents of Cr and Mn will not form any harmful coarse phases. Of course, the most effective addition is Zr, which can increase the recrystallization temperature of the aluminum alloy, whether after hot deformation or cold deformation, making it possible to obtain a non-recrystallized structure after heat treatment. Therefore, in the present invention, in order to further improve the strength of the Al-Zn-Mg-Cu alloy, Zr is added as a required element, and the mass percentage of Zr is controlled between 0.10-0.16%.

[0034] Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Zr element can be further controlled to be between 0.10-0.13%.

[0035] Zr combines with Al to form an Al3Zr intermetallic compound. This intermetallic compound has two structural forms: one is the tetragonal Al3Zr structure precipitated directly from the melt, which significantly refines the alloy's cast grains; the other is the spherical particles precipitated during the homogenization of the ingot. These particles have an LI2 structure, coherent with the matrix, and strongly inhibit recrystallization during hot working. Adding trace amounts of Zr can improve the strength, fracture toughness, and stress corrosion resistance of aluminum alloys. Furthermore, due to its low quenching sensitivity, Zr can also improve the alloy's hardenability and weldability.

[0036] In summary, in the Al-Zn-Mg-Cu aluminum alloy designed by the present invention, the addition of trace amounts of Cr, Mn, Ti, and Zr has a strong grain refining effect, and the ingot structure of the Al-Zn-Mg-Cu aluminum alloy ingot obtained therefrom is uniform, fine equiaxed crystals. The refinement mechanism of this design is as follows: the atomic clusters containing Cr and Mn, which are completely coherent with α(Al), replace TiB as the "substrate" for the co-nucleation of Al3Ti and Al3Zr, allowing Ti and Zr to participate in the refinement process together. In the heterogeneous nucleation process, Al3Ti nucleates through the coherent atomic clusters, Al3(Ti, Zr) nucleates through Al3Ti, and α(Al) nucleates through Al3(Ti, Zr).

[0037] Furthermore, in the manufacturing method of the present invention, the mass percentage of chemical elements in the Al-Zn-Mg-Cu aluminum alloy ingot further satisfies at least one of the following:

[0038] Cu: 1.8-2.2%,

[0039] Mg: 2.0-2.4%,

[0040] Zn: 6.1-7.8%,

[0041] Zr: 0.10-0.13%.

[0042] Furthermore, in the manufacturing method described in the present invention, the inevitable impurities of the Al-Zn-Mg-Cu aluminum alloy ingot include at least one of the following: Si≤0.10%, Fe≤0.15%, and the total amount of other impurity elements≤0.100%.

[0043] In Al-Zn-Mg-Cu system aluminum alloy ingot casting of the present invention, above-mentioned Si, Fe are the impurity elements in the aluminum alloy.Impurity elements such as Si, Fe are the harmful elements that are difficult to avoid in the Al-Zn-Mg-Cu system aluminum alloy ingot casting melting process, and they can form the very high coarse brittle phase (as Al7Cu2Fe) of fusing point in alloy matrix, and these phases will be arranged in a string along the deformation direction in the processing deformation process.There is high-energy phase interface between them and the matrix, is difficult for coordinated deformation, easily produces microcrack after being stressed, and when stress lasts, microcrack generation polymerization grows up and develops into macro crack, increases crack propagation rate and reduces plasticity and the fracture toughness of alloy.

[0044] For example, Fe dissolves in Al to form FeAl3, which refines the recrystallized grains and improves the alloy's performance. However, due to the large potential difference between FeAl3 and the Al matrix, this can reduce the alloy's corrosion resistance. Another example is the addition of Mn to an aluminum alloy ingot, which forms (Fe, Mn)Al6. This reduces the potential difference between FeAl3 and Al, improving the alloy's corrosion resistance.

[0045] Therefore, in order to ensure the performance and quality of aluminum alloys, it is necessary to strictly control the mass percentage of the above-mentioned impurity elements, and control the total amount of other impurity elements to ≤0.100%, and the mass percentage of other individual impurity elements to ≤0.030%, so as to reduce the content of coarse second phases containing impurity elements such as Si and Fe in the alloy, and ultimately improve the fracture toughness of the alloy and reduce the crack growth rate.

[0046] When technical conditions permit, in order to obtain aluminum alloys with better performance and higher quality, the content of impurity elements in Al-Zn-Mg-Cu aluminum alloy ingots should be reduced as much as possible.

[0047] In some preferred embodiments, in order to obtain better implementation effect, and to make the prepared Al-Zn-Mg-Cu series aluminum alloy ingot quality better, the step is preferably controlled as follows: Si < 0.08%, Fe < 0.10%.

[0048] Further, in the manufacturing method described in the present application, in step (2), the homogenization treatment adopts three-stage homogenization treatment, wherein the first-stage homogenization treatment is at a temperature of 418-430℃ for 5-8h, the second-stage homogenization treatment is at a temperature of 460-468℃ for 8-12h, and the third-stage homogenization treatment is at a temperature of 470-480℃ for 20-24h.

[0049] In the above step (2) of the present application, the purpose of homogenization treatment of the Al-Zn-Mg-Cu series aluminum alloy ingot is to eliminate dendritic segregation and composition segregation, to obtain a solid solution with uniform distribution of solute atoms, and to reduce the coarse second phase as the recrystallization PSN nucleation mechanism.

[0050] Although the second-stage homogenization treatment can obtain the best aging strengthening effect, the second-stage homogenization treatment system at a high temperature of 473℃ for a long time can cause a certain degree of aggregation and growth of the insoluble Fe-containing phase (Al7Cu2Fe) and S (Al2CuMg) phase, and these coarse and brittle second phases are not easy to deform, which can reduce the strength of the alloy and hinder the movement of dislocations, thereby reducing the plasticity of the alloy.

[0051] The purpose of the three-stage homogenization treatment of the present application is to obtain fine and dispersed Al3Zr particles. According to the dislocation bypass mechanism of the difficult-to-deform second phase, the smaller and more dispersed the radius and distribution interval of the Al3Zr particles that are not easy to deform, the greater the critical shear stress that the dislocations need to overcome to continue to move, the stronger the hindering effect of the Al3Zr particles on the movement of dislocations, and the higher the strength of the alloy. In addition, the fine and dispersed Al3Zr particles can also prevent recrystallization from occurring, retain the deformed substructure, and refine the grains, thereby shortening the distance of dislocation slip, reducing the strain concentration caused by the intersection of dislocations on different slip planes and the accumulation of dislocations at grain boundaries, and improving the plasticity of the aluminum alloy material. In addition, the three-stage homogenization treatment can also spheroidize the S (Al2CuMg) phase.

[0052] Further, in the manufacturing method described in the present application, in step (2), the hot rolling includes the steps of heating the ingot to 430-440℃ and holding for 90-120min, and then performing multi-pass hot rolling in a longitudinal and transverse alternating manner, with the total deformation amount of hot rolling controlled to be ≥85% and the final rolling temperature controlled to be >380℃.

[0053] In the above-mentioned technical solution, the hot rolling process employs an alternating longitudinal and transverse rolling method, i.e., the hot rolling direction is controlled to alternate along the length and width of the plate. This alternating longitudinal and transverse rolling method can achieve the plate dimensions required for subsequent warm forming while also exhibiting good through-thickness mechanical properties.

[0054] During the hot rolling process, the hot rolling start temperature is about 85-90% of the alloy melting point temperature, but the influence of low melting point phases such as S (Al2CuMg) phase and T (AlZnMgCu) phase in the Al-Zn-Mg-Cu system should also be considered. If the hot rolling temperature is too high, it is easy to cause coarse grains or melting of low melting points between crystals, resulting in overheating or overburning of the heated ingot, cracking or crushing during hot rolling; and if the hot rolling temperature is too low, it will cause uneven deformation of the ingot, increase the rolling load, increase the edge cracking tendency during ingot rolling, and affect normal rolling. It can be seen from this that the high or low hot rolling temperature has an impact on the heat resistance and room temperature mechanical properties of the material. Therefore, in order to ensure the performance of the aluminum alloy material, in the present invention, the hot rolling start temperature can be controlled between 430-440℃.

[0055] Accordingly, during the hot rolling process, the final rolling temperature is determined based on the alloy's type II recrystallization diagram. The final rolling temperature of Al-Zn-Mg-Cu aluminum alloys during hot slab rolling is generally controlled to be above the recrystallization temperature. Therefore, in the present invention, the final rolling temperature can be controlled to be greater than 380°C.

[0056] In addition, during the hot rolling process, the selection of the total rolling deformation should be combined with the characteristics of the Al-Zn-Mg-Cu aluminum alloy itself. The larger the total rolling deformation, the more uniform the material structure and the better the performance. When the total rolling deformation is controlled at more than 85%, the rolled plate with the best structure can be obtained.

[0057] In addition, during the hot rolling process, the selection of the pass processing rate should take into account the high-temperature performance of the Al-Zn-Mg-Cu alloy, the bite conditions, the product quality requirements, etc., and the pass processing rate varies in different rolling stages. In the initial rolling stage, a lower pass processing rate can be used for 3-5 passes; in the intermediate rolling stage, the pass processing rate can reach more than 45%; in the final rolling stage, the pass processing rate is generally reduced, so as to obtain good plate shape, thickness deviation and surface quality.

[0058] Furthermore, in the manufacturing method described in the present invention, in step (2), the cold rolling includes the steps of: first air-cooling the hot-rolled plate to room temperature, and then performing multiple cold rolling passes to control the total cold rolling deformation to be ≥75%.

[0059] Furthermore, in the manufacturing method described in the present invention, in step (2), the solution quenching treatment adopts a two-stage solution treatment, wherein the first stage solution treatment is kept at a temperature of 445-450°C for 20-30 minutes, the second stage solution treatment is kept at a temperature of 475-478°C for 10-20 minutes, and then directly water quenching is performed.

[0060] In the above technical solution of the present invention, the purpose of the two-stage solution treatment process is to obtain an aluminum alloy matrix with less and more uniform second phase. Before over-burning occurs in the solution treatment of Al-Zn-Mg-Cu series aluminum alloy, the higher the solution temperature, the greater the concentration of alloying elements dissolved in the solution, the higher the concentration of supersaturated solid solution in the alloy after quenching, and the higher the strength after aging. However, with the second-stage solution treatment, the holding time at a temperature of 475-478°C is prolonged, and the microstructure of the aluminum alloy will gradually grow. The growth of the grains will reduce the strength of the alloy. Therefore, in order to improve the degree of solid solution of the alloy, the holding time during the second solution treatment should not be too long.

[0061] In the two-stage solution treatment process provided by the present invention, the lower solution temperature in the first stage promotes the diffusion of non-equilibrium phases, while the higher solution temperature in the second stage increases the element concentration in the alloy structure, increasing the concentration of the supersaturated solid solution in the solution treatment. This allows more coarse secondary phases in the alloy to dissolve into the aluminum matrix, ultimately producing an alloy with improved mechanical properties. Accordingly, the solution treatment time should not be too long, as this can lead to greater recrystallization and grain growth, affecting material properties.

[0062] In the above step (2) of the present invention, the Al-Zn-Mg-Cu alloy elements can be dissolved into the matrix as much as possible through solid solution treatment, and the residual primary phase in the alloy and the second phase formed during homogenization annealing or deformation can be eliminated as much as possible, so that the microstructure of the product after solid solution contains a large amount of substructure and fine grain structure; by optimizing the solid solution system, the aluminum alloy can obtain excellent strength and toughness properties, and the strength and toughness of the aluminum alloy product can be further enhanced through precipitation aging treatment.

[0063] Correspondingly, quenching is the process of rapidly cooling an alloy from a solid solution state to room temperature, preserving the alloy's high-temperature structure in a metastable state. For all heat treatments that involve structural changes during cooling, the ideal structure can be achieved by properly controlling the cooling rate.

[0064] In the present invention, a certain quenching rate should be maintained during the quenching cooling process of the Al-Zn-Mg-Cu aluminum alloy. Rapid quenching can inhibit the nucleation and growth of precipitates during the quenching process, allowing solute atoms to remain in the solid solution and prevent the formation of a second phase, thereby ensuring a higher material strength. Therefore, in the present invention, water quenching can be used for quenching, and the quenching transfer time can be controlled to within 10 seconds.

[0065] Furthermore, in the manufacturing method described in the present invention, in step (2), the quenching transfer time (i.e., the transfer time from the heat treatment furnace to the cooling water) is controlled to be within 10 seconds.

[0066] Furthermore, in the manufacturing method of the present invention, in step (2), the artificial aging treatment is carried out at a temperature of 185-205° C. for 30-60 minutes.

[0067] It should be noted that, in the prior art, the aging temperature of 7000 series aluminum alloy in the T6 state is generally controlled to be 100-150° C., and the holding time is generally 8-36 hours.

[0068] In the present invention, the microstructure and performance of the high-strength Al-Zn-Mg-Cu aluminum alloy are closely related. The performance of the alloy is mainly affected by the size, type, and distribution of the precipitated phases within the alloy structure. The microstructure of the Al-Zn-Mg-Cu aluminum alloy mainly consists of three parts: intragranular precipitated phase (mpt), grain boundary precipitated phase (GBP), and precipitate-free zone (PFZ) around the grain boundary. Among them, the intragranular precipitated phase (mpt) plays a decisive role in the strength of the alloy. The GP zone and η' phase precipitated by aging treatment have a better strengthening effect than the coarse equilibrium phase η phase.

[0069] It is generally believed that the aging precipitation sequence of 7000 series ultra-high-strength aluminum alloys is: supersaturated solid solution (ss) - GP zone - η' transition phase (MgZn2) - η equilibrium phase (MgZn2). Therefore, considering the impact of the amount, distribution, and size of GP zone, η' phase, η phase, T phase, and other secondary phases on alloy properties, how to control the amount, distribution, and size of these phases is the key to artificial aging treatment.

[0070] During the aging process, the aging temperature significantly influences the precipitation of the alloy's precipitates. The critical nucleation size, type, and aggregation growth rate of the precipitates vary at different aging temperatures. When the aging temperature is low, the precipitates initially precipitate rapidly, resulting in a significant aging strengthening effect. Later, the precipitation and growth of the precipitates slow. While low-temperature aging can yield higher strength, it takes a longer time to reach peak hardness. As the aging temperature increases, the diffusion coefficient of solute atoms increases, accelerating the precipitation rate of the precipitates. Furthermore, high aging temperatures favor the formation of the transition phase η' and the equilibrium phase η. As the aging temperature gradually increases, the time it takes for the alloy to reach peak hardness decreases.

[0071] Therefore, in the present invention, when optimizing the artificial aging process, a high-temperature, short-time aging system can be adopted, where the aluminum alloy plate is kept at a temperature range of 185-205°C for 30-60 minutes. This system allows for rapid peak aging and optimizes the strength of the aluminum alloy plate.

[0072] Furthermore, in the manufacturing method of the present invention, in step (3), the heating is carried out at a temperature of 460-477° C. for 5-10 minutes.

[0073] In the above technical solution of the present invention, the heating solution temperature is set in the range of 460-477°C. On the one hand, this is due to the fact that the dissolution temperatures of the S (Al2CuMg) phase and the T (AlZnMgCu) phase in the Al-Zn-Mg-Cu series aluminum alloy are within this range; on the other hand, this is due to the fact that the problem of overburning caused by excessively high temperature is avoided during rapid solution.

[0074] In certain embodiments, the heating method may be contact heating.

[0075] Furthermore, in the manufacturing method of the present invention, in step (3), the pre-aging treatment is carried out at a temperature of 75-100° C. for 30-60 minutes.

[0076] In the present invention, the Al-Zn-Mg-Cu alloy plate is subjected to a pre-aging process to prevent the aluminum alloy from naturally aging. The alloy after solution treatment must be pre-aged within 30 minutes. The pre-aging process hinders the natural aging process by essentially hindering the formation of atomic clusters and GP zones in the Al-Zn-Mg-Cu alloy. At the same time, pre-aging can also promote the nucleation of low-temperature precipitation phases. The subsequent strength improvement in the paint baking stage depends on the nucleation of these precipitation phases.

[0077] Furthermore, in the manufacturing method of the present invention, in step (3), the paint baking treatment is carried out at a temperature of 170-190° C. for 20-40 minutes.

[0078] In the present invention, paint baking is equivalent to artificial aging. After T4P+PB (pre-aging+paint baking), a large number of fine and dense precipitates can be obtained. The atomic clusters and GP areas in the precipitates increase significantly. Compared with the temperature of natural aging, the paint baking temperature is higher, and the unstable phase in the T4P state will continue to nucleate and precipitate, so that the strength is further improved, that is, obvious paint hardening is shown.

[0079] In the above technical solution, the preferred design of the paint baking process is based on the paint baking process requirements on the one hand, and on the other hand, it is set in combination with the process characteristics of Al-Zn-Mg-Cu aluminum alloy reaching peak aging at high temperature and in a short time.

[0080] Correspondingly, another object of the present invention is to provide an Al-Zn-Mg-Cu series aluminum alloy plate, which is easy to produce and has high tensile strength, yield strength and high elongation, can meet the requirements of automotive plates for material strength and toughness, can be effectively used in the vehicle manufacturing industry, meet the requirements of vehicle lightweighting, and has a very broad application prospect.

[0081] In order to achieve the above-mentioned object, the present invention proposes an Al-Zn-Mg-Cu series aluminum alloy plate, which is manufactured by the above-mentioned manufacturing method of the Al-Zn-Mg-Cu series aluminum alloy plate of the present invention.

[0082] Furthermore, in the Al-Zn-Mg-Cu series aluminum alloy plate of the present invention, its performance meets the following requirements: tensile strength of 650-680 MPa, yield strength of 580-630 MPa, and elongation ≥15.0%.

[0083] Compared with the prior art, the manufacturing method of the Al-Zn-Mg-Cu series aluminum alloy plate of the present invention has the following advantages and beneficial effects:

[0084] In order to solve the problems that the current 7000 series aluminum alloys for automobiles have poor room temperature forming performance and the heat treatment specimens after forming are prone to deformation, the inventors designed and obtained a new manufacturing method for Al-Zn-Mg-Cu series aluminum alloy sheets.

[0085] The Al-Zn-Mg-Cu series aluminum alloy sheet manufacturing method adopts a reasonable process design, with standardized production process and process parameters, and can significantly improve the formability and comprehensive mechanical properties of the produced Al-Zn-Mg-Cu series aluminum alloy sheet to meet the use requirements of automobile sheets, thereby overcoming the shortcomings of the existing technology.

[0086] The Al-Zn-Mg-Cu aluminum alloy sheet is easy to produce and has high tensile strength, yield strength and high elongation, which can meet the requirements of automotive sheet materials for material strength and toughness. It can also be effectively used in the vehicle manufacturing industry to meet the requirements of vehicle lightweighting and has very broad application prospects.

[0087] In certain embodiments, the mechanical properties of the Al-Zn-Mg-Cu aluminum alloy sheet meet the following requirements: tensile strength of 650-680 MPa, yield strength of 580-630 MPa, and elongation ≥15.0%. The mechanical properties of the Al-Zn-Mg-Cu aluminum alloy sheet produced using this manufacturing method are significantly improved compared to existing 7000 series aluminum alloy sheets.

[0088] In addition, the process principle provided by this manufacturing method is not only applicable to this Al-Zn-Mg-Cu aluminum alloy material, but also to all other heat-treatable and strengthened aluminum alloys, such as 2000 series, 6000 series and other 7000 series aluminum alloy materials. It has good promotion prospects and application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Figure 1 The process flow chart of the method for manufacturing the Al-Zn-Mg-Cu series aluminum alloy plate of the present invention is schematically shown. DETAILED DESCRIPTION

[0090] The Al-Zn-Mg-Cu aluminum alloy sheet manufacturing method and the aluminum alloy sheet of the present invention will be further explained and illustrated below in conjunction with specific embodiments and the accompanying drawings. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0091] Examples 1-6 and Comparative Examples 1-2

[0092] In the present invention, the chemical compositions of the Al-Zn-Mg-Cu series aluminum alloy plates of Examples 1-6 are shown in Table 1. Table 1 lists the chemical compositions of the Al-Zn-Mg-Cu series aluminum alloy plates of Examples 1-6.

[0093] Table 1. (Remainder is Al)

[0094]

[0095] Accordingly, based on the above chemical composition design, in the present invention, the Al-Zn-Mg-Cu aluminum alloy sheets of Examples 1-6 are all prepared by the following steps:

[0096] (1) according to the chemical composition shown in Table 1, and smelting, refining treatment in a smelting furnace, and then casting into Al-Zn-Mg-Cu aluminum alloy ingot.

[0097] (2) the obtained Al-Zn-Mg-Cu aluminum alloy ingot is sequentially subjected to homogenization treatment, hot rolling, cold rolling, solid solution quenching treatment and artificial aging treatment to obtain T6 state Al-Zn-Mg-Cu aluminum alloy plate:

[0098] Homogenization treatment: the aluminum alloy ingot is placed in a homogenization furnace for homogenization treatment, and three-stage homogenization treatment system is adopted, wherein the first stage homogenization treatment is at a temperature of 418-430℃ for 5-8h, the second stage homogenization treatment is at a temperature of 460-468℃ for 8-12h, and the third stage homogenization treatment is at a temperature of 470-480℃ for 20-24h, and then forced water cooling is selected to room temperature to obtain the homogenization treated aluminum alloy ingot;

[0099] Hot rolling: the aluminum alloy ingot is heated to 430-440℃ and kept for 90-120min, and then multi-pass hot rolling is carried out, the hot rolling is adopted in the way of longitudinal and transverse alternation, the total deformation amount of hot rolling is controlled to be ≥85%, and the final rolling temperature is >380℃, so as to obtain hot rolled plate with a final thickness of 6mm;

[0100] Cold rolling: the hot rolled plate is first air cooled to room temperature, and then multi-pass cold rolling is carried out to 1.5-2.5mm, and the total deformation amount of cold rolling is controlled to be ≥75%;

[0101] Solid solution quenching treatment: the cold rolled plate is subjected to two-stage solid solution treatment, wherein the first stage solid solution treatment is at a temperature of 445-450℃ for 20-30min, the second stage solid solution treatment is at a temperature of 475-478℃ for 10-20min, and then water quenching is directly carried out; wherein the quenching transfer time is controlled to be within 10s;

[0102] Artificial aging treatment: the solid solution quenched plate is transferred to an aging furnace for artificial aging treatment, and the plate is kept at a temperature of 185-205℃ for 30-60min to obtain T6 state Al-Zn-Mg-Cu aluminum alloy plate.

[0103] (3) the obtained T6 state Al-Zn-Mg-Cu aluminum alloy plate is subjected to heating, warm forming, in-die quenching, pre-aging treatment and baking finish treatment to obtain finished Al-Zn-Mg-Cu aluminum alloy plate:

[0104] Heating: the T6 state Al-Zn-Mg-Cu aluminum alloy plate obtained by artificial aging is kept at a temperature of 460-477℃ for 5-10min, and then warm forming and in-die quenching are directly carried out;

[0105] Pre-aging treatment: After completing warm forming and in-die quenching, the Al-Zn-Mg-Cu aluminum alloy sheet should be pre-aged in a short time to obtain the Al-Zn-Mg-Cu aluminum alloy sheet in T4P state. The pre-aging process is to control the sheet to be kept at a temperature of 75-100℃ for 30-60min, and then perform a paint treatment;

[0106] Baking varnish treatment: the plate is kept at a temperature of 170-190°C for 20-40 minutes to obtain a T4P+PB aluminum alloy plate.

[0107] In the present invention, the Al-Zn-Mg-Cu aluminum alloy plates designed in Examples 1-6 are all 7055 aluminum alloy plates, and the relevant manufacturing processes of the Al-Zn-Mg-Cu aluminum alloy plates in Examples 1-6 all meet the design specification requirements of the present invention.

[0108] Table 2-1, Table 2-2 and Table 2-3 list the specific process parameters of the Al-Zn-Mg-Cu aluminum alloy sheet of Examples 1-6 in the above process steps.

[0109] Table 2-1.

[0110]

[0111] Table 2-2.

[0112]

[0113] Table 2-3.

[0114]

[0115]

[0116] In order to further illustrate the mechanical properties of the Al-Zn-Mg-Cu aluminum alloy sheets of Examples 1-6 described in the present invention, the finished Al-Zn-Mg-Cu aluminum alloy sheets of Examples 1-6 can be sampled respectively, and the mechanical properties of the finished aluminum alloy sheets of each embodiment can be tested. The relevant mechanical property test results are listed in Table 3 below.

[0117] Relevant mechanical properties testing methods are as follows:

[0118] Tensile test: Tensile specimens were prepared according to GB / T 228.1-2010 standard to obtain the yield strength, tensile strength and elongation values ​​of the Al-Zn-Mg-Cu aluminum alloy sheet of each embodiment.

[0119] Accordingly, in order to prove that the Al-Zn-Mg-Cu series aluminum alloy plates of Examples 1-6 prepared by the manufacturing method of the present invention have excellent mechanical properties, the inventors further cited two 7000 series aluminum alloy materials in the prior art for comparison, namely Comparative Example 1 and Comparative Example 2.

[0120] Comparative Example 1 is taken from the patent technical document with publication number CN104862551A, published on August 26, 2015. Comparative Example 1 is an Al-Mg-Cu-Zn aluminum alloy plate, and its mechanical properties after T4P+artificial aging are shown in Table 3.

[0121] Comparative Example 2 is taken from CN107686954A, a patent technical document with a publication number of February 13, 2018. Comparative Example 2 is a 7075 aluminum alloy hot stamping formed plate, and its mechanical properties in the T6 state are shown in Table 3.

[0122] Table 3 lists the mechanical property test results of the finished Al-Zn-Mg-Cu aluminum alloy plates of Examples 1-6 and the finished aluminum alloy plates of Comparative Examples 1-2.

[0123] Table 3.

[0124]

[0125]

[0126] As shown in Table 3, in the present invention, the finished Al-Zn-Mg-Cu aluminum alloy plates of Examples 1-6 have relatively high mechanical properties, with a tensile strength Rm of 612-647 MPa, a yield strength Rp0.2 of 586-623 MPa, and an elongation A of 15.4-17.2%.

[0127] Comparing the finished Al-Zn-Mg-Cu aluminum alloy sheets of Examples 1-6 with those of Comparative Examples 1-2, it is readily apparent that the mechanical properties of the finished Al-Zn-Mg-Cu aluminum alloy sheets of Examples 1-6 are over 30% higher than those of the finished aluminum alloy sheet of Comparative Example 1. This is primarily due to the fact that the process employed in the present invention differs from that in Comparative Example 1. The fundamental reason for this is that the present invention utilizes warm forming after T6 heat treatment. This treatment provides a foundation for precipitation strengthening, which, combined with subsequent baking varnish reinforcement, results in the material possessing higher strength and toughness.

[0128] In summary, it can be seen that the production process and process parameters of the preparation method of the Al-Zn-Mg-Cu series aluminum alloy for automobiles designed by the present invention are highly standardized, and the comprehensive performance of the product produced is significantly improved, which can meet the requirements of automobiles for high-performance aluminum alloys.

[0129] The process principle provided by the present invention is not only applicable to the 7000 series Al-Zn-Mg-Cu aluminum alloy material, but also applicable to all other heat-treatable strengthened aluminum alloy materials, such as 2000 series, 6000 series and other 7000 series aluminum alloy materials.

[0130] Figure 1 The process flow chart of the method for manufacturing the Al-Zn-Mg-Cu series aluminum alloy plate of the present invention is schematically shown.

[0131] like Figure 1 As shown, in the present invention, after the Al-Zn-Mg-Cu aluminum alloy plate is smelted and cast according to the designed chemical element composition, the corresponding Al-Zn-Mg-Cu aluminum alloy ingot can be obtained. The Al-Zn-Mg-Cu aluminum alloy ingot is sequentially subjected to homogenization treatment, hot rolling, cold rolling, solution quenching treatment and artificial aging treatment to obtain T6 aluminum alloy plate.

[0132] Based on the obtained T6 state aluminum alloy sheet, further heating, warm forming, in-die quenching, pre-aging and baking paint treatment are performed to obtain the finished Al-Zn-Mg-Cu aluminum alloy thin plate in T4P+PB state.

[0133] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0134] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A method for manufacturing an Al-Zn-Mg-Cu series aluminum alloy plate, characterized in that: Including steps: (1) preparing an Al-Zn-Mg-Cu aluminum alloy ingot; the Al-Zn-Mg-Cu aluminum alloy ingot has the following chemical element weight percentage ratios: Cu: 1.6-2.2%, Mg: 1.8-2.4%, Zn: 6.0-8.6%, Zr: 0.10-0.16%, 0<Ti≤0.06%, 0<Mn≤0.05%, 0<Cr≤0.04%; the balance being Al and unavoidable impurities; (2) the Al-Zn-Mg-Cu aluminum alloy ingot is subjected to homogenization treatment, hot rolling, cold rolling, solution quenching treatment and artificial aging treatment in sequence to obtain a T6 aluminum alloy plate; the solution quenching treatment adopts a two-stage solution treatment, wherein the first stage solution treatment is at a temperature of 445-450°C and kept at a temperature of 20-30 minutes, the second stage solution treatment is at a temperature of 475-478°C and kept at a temperature of 10-20 minutes, and then directly water quenched; the artificial aging treatment is at a temperature of 185-205°C and kept at a temperature of 30-60 minutes; (3) The T6 aluminum alloy plate is heated, warm-formed, in-mold quenched, pre-aged, and painted to obtain a finished aluminum alloy plate; the pre-aging treatment is performed at a temperature of 75-100° C. for 30-60 minutes.

2. The manufacturing method according to claim 1, wherein The mass percentage of chemical elements in the Al-Zn-Mg-Cu aluminum alloy ingot further satisfies at least one of the following items: Cu: 1.8-2.2%, Mg: 2.0-2.4%, Zn: 6.1-7.8%, Zr：0.10-0.13％。 3. The manufacturing method according to claim 1, wherein: The inevitable impurities of the Al-Zn-Mg-Cu aluminum alloy ingot include at least one of the following: Si≤0.10%, Fe≤0.15%, and the total amount of other impurity elements≤0.100%.

4. The manufacturing method according to claim 1, wherein: In step (2), the homogenization treatment adopts a three-stage homogenization treatment, wherein the first stage homogenization treatment is to keep the temperature at 418-430°C for 5-8 hours, the second stage homogenization treatment is to keep the temperature at 460-468°C for 8-12 hours, and the third stage homogenization treatment is to keep the temperature at 470-480°C for 20-24 hours.

5. The manufacturing method according to claim 1, wherein: In step (2), the hot rolling includes the steps of heating the ingot to 430-440°C, keeping the temperature for 90-120 minutes, and then performing multiple hot rolling passes, wherein the hot rolling adopts a longitudinal and transverse alternating manner, controlling the total hot rolling deformation to be ≥85%, and the final rolling temperature to be >380°C.

6. The manufacturing method according to claim 1, wherein: In step (2), the cold rolling includes the steps of: first air cooling the hot-rolled plate to room temperature, and then performing multiple cold rolling, controlling the total cold rolling deformation to be ≥75%.

7. The manufacturing method according to claim 1, wherein: In step (2), the quenching transfer time is controlled within 10 s.

8. The manufacturing method according to claim 1, wherein: In step (3), the heating is carried out at a temperature of 460-477° C. for 5-10 minutes.

9. The manufacturing method according to claim 1, wherein: In step (3), the paint baking treatment is carried out at a temperature of 170-190° C. for 20-40 minutes.

10. An Al-Zn-Mg-Cu series aluminum alloy plate, produced by the manufacturing method according to any one of claims 1 to 9.

11. The Al-Zn-Mg-Cu aluminum alloy plate according to claim 10, wherein: Its performance meets the following requirements: tensile strength of 650-680MPa, yield strength of 580-630MPa, and elongation ≥15.0%.

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