Method for Manufacturing Al-Zn-Mg-Cu Series Aluminum Alloy Plate, and Aluminum Alloy Plate

The present disclosure 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.

It's well known that the density of aluminum is about ⅓ of that of steel. It is currently the most widely used lightweight material. Aluminum alloy materials are lightweight materials that have been used early and the technology of which is becoming increasingly mature. In recent years, the use of aluminum alloy materials in automobiles has shown a trend of continuous growth.

Compared with steel materials, aluminum alloy materials have many advantages such as high thermal conductivity, good corrosion resistance, and excellent processing performance. Although their strength is not as good as high-strength steel, aluminum alloy materials can fully meet the strength requirement of lightweight automobiles when they are modified technologically. In addition, the energy absorption performance of aluminum alloy materials is about twice that of steel, which can improve the collision safety of automobiles effectively. Therefore, in the automotive field, the use of aluminum alloy materials instead of traditional steel materials is an important trend of developing the automotive lightweight technology.

In the current existing technology, aluminum alloys for vehicle bodies mainly include 2000 series (Al—Cu series), 5000 series (Al—Mg series), 6000 series (Al—Mg—Si series), and a small amount of 7000 series (Al—Zn—Mg series or Al—Zn—Mg—Cu series). Among them, Al—Zn—Mg—Cu series aluminum alloy, also called 7000 series aluminum alloy material, can acquire very high strength and toughness after quenching and aging treatment. Because of its low density, many automobile manufacturers have begun to consider using this 7000 series aluminum alloy material to replace high-strength steel plates to manufacture some automobile parts, such as B-pillars in automobiles and reinforcement ribs in shock absorbers. For example, in the prior art, there is a 7000 series aluminum alloy that can be used to manufacture automobile safety devices. The strength of this alloy is twice as high as that of the existing bumper aluminum alloy. Compared with high-strength steel, this 7000 series aluminum alloy material can ensure the safety of passengers to the greatest extent while reducing the vehicle body weight.

However, since the 7000 series aluminum alloy in the quenched state has poor plasticity at room temperature, it exhibits high hardening capacity, and it is difficult to be formed directly into complex parts using ordinary forming processes. Therefore, it is usually necessary to anneal the 7000 series aluminum alloy plate to increase the plasticity of the material, and then perform quenching and aging treatment after forming.

It's found by research that this processing method used currently is very complicated, and the subsequent heat treatment takes a long time, so that the requirements of mass production of parts in the automotive industry cannot be satisfied. Moreover, the material is also easy to deform, which has a certain impact on the size of the parts.

At the same time, the current research on warm forming conducted by scientific researchers is mainly focused on aluminum alloy materials of 5000 series, 6000 series and the like that do not need heat treatment, and on some magnesium alloy materials, while there are fewer studies on warm forming of heat-treatable 7000 series aluminum alloy materials. Moreover, the fewer studies are all limited to warm forming experiments, the mechanical theories of warm forming, the warm processing performances, and the simulation of the warm forming process. In the prior art, there is still no process technology that is seen clearly for 7000 series aluminum alloy plates used for automobiles.

Therefore, in order to solve the problems that the current 7000 series aluminum alloys for automobiles have poor forming performance at room temperature and the formed specimens are prone to deformation in heat treatment, the inventors have designed and obtained a new method for manufacturing an Al—Zn—Mg—Cu series aluminum alloy plate, which belongs to a 7000 series aluminum alloy plate. The process principle of the manufacturing method is not only applicable to this Al—Zn—Mg—Cu aluminum alloy material, but also applicable to all other heat-treatable strengthened aluminum alloys, such as 2000 series, 6000 series and other 7000 series aluminum alloy materials.

One of the objects of the present disclosure is to provide a new method for manufacturing an Al—Zn—Mg—Cu series aluminum alloy plate. A reasonable process design is applied to the method for manufacturing an Al—Zn—Mg—Cu series aluminum alloy plate. The method can improve the formability of the aluminum alloy plate and meet the use requirement and lightweight requirement of automotive plates. At the same time, the Al—Zn—Mg—Cu series aluminum alloy plate prepared has high tensile strength, yield strength and elongation, which can meet the requirements of automotive plates for material strength and toughness, thereby overcoming the deficiencies existing in the prior art.

In order to achieve the above object, the present disclosure proposes a method for manufacturing an Al—Zn—Mg—Cu series aluminum alloy plate, comprising the following steps:

In the current prior art, the traditional aluminum alloy plate forming process involves subjecting the aluminum alloy plate to solid solution quenching, aging treatment, and then hot stamping to obtain a finished product. This process is not good at forming, and it is not suitable for Al—Zn—Mg—Cu series alloys.

In the present disclosure, in order to solve the problems that 7000 series aluminum alloys have poor forming performance at room temperature and the formed specimens are prone to deformation in heat treatment, the inventors have discovered by extensive research that a new process of integrated warm forming and quenching (Solution Heat treatment-Forming-cold die Quenching), referred to as HFQ process, can be used to treat 7000 series aluminum alloys.

The warm forming process is a process that combines hot forming and heat treatment processes. It can be used to form a structural part having a complex shape and high strength from an aluminum alloy plate. It is beneficial to improving the formability of the aluminum alloy. If a 7000 series high-strength aluminum alloy is to be used in the automotive field, production efficiency needs to be improved greatly. The warm forming process is expected to become an optimal process for the production of a 7000 series high-strength aluminum alloy for use in the automotive manufacturing field.

However, in the ordinary warm forming process available nowadays, a solid-dissolved aluminum alloy plate (in a W state) is generally subjected to warm forming and die quenching. This warm forming process exhibits good forming ability, but still cannot stretch the strength to its limit. This is because part of the supersaturated solid solution decomposes during the die quenching process, affecting subsequent precipitation of an aging strengthening phase.

Therefore, different from the existing warm forming process mentioned above, in the present disclosure, the inventors have creatively designed and sequentially carried out the following process steps for the prepared Al—Zn—Mg—Cu series (i.e., 7000 series) aluminum alloy ingot: homogenization treatment, hot rolling, cold rolling, solid solution quenching treatment, artificial aging treatment, heating, warm forming, die quenching, pre-aging treatment and paint baking treatment, so as to obtain the finished Al—Zn—Mg—Cu series aluminum alloy plate.

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

In the warm forming process used in the present disclosure, the Al—Zn—Mg—Cu series alloy plate prepared by cold rolling is first heated to the solid solution treatment temperature, and then held at the solid solution treatment temperature for a period of time so that the solute atoms are fully dissolved into the a aluminum matrix; after full solid dissolution, the Al—Zn—Mg—Cu series alloy plate is quickly transferred to a die for stamping, and then quenched in the die under pressure. Accordingly, after the solid solution quenching treatment is completed, the formed part is finally subjected to artificial aging treatment to control formation of precipitates, thereby ensuring its strength.

There are two main reasons for pressure quenching in the die: first, rapid quenching prevents formation of coarse precipitates, especially at the grain boundaries; second, deformation of the formed part during the quenching process is avoided.

The novel warm forming process designed by the present disclosure not only improves the formability of aluminum alloy materials, but also reduces the resilience of aluminum alloy materials. It can meet the requirements for producing aluminum alloy components of the external surface of a vehicle body with high precision, high strength and complex shapes.

It should be noted that after completing step (2), in step (3), the (T6 state) aluminum alloy plate obtained after the artificial aging treatment needs to be heated rapidly to achieve solid dissolution, followed by warm forming and die quenching. This process not only exhibits good forming ability, but also can stretch the strength of the Al—Zn—Mg—Cu series alloy to its limit. This is because the Al—Zn—Mg—Cu series alloy still includes some fine strengthening phases after rapid heating and solid dissolution. These strengthening phases have a reinforcing function in the subsequent rapid pre-aging treatment process.

It should be noted that, in the present disclosure, the purpose of the artificial aging treatment is to keep the unstable supersaturated solid solution of the quenched profile at a certain temperature for a certain period of time, so that the supersaturated solid solution decomposes, causing a significant increase in the strength and hardness of the alloy.

For the Al—Zn—Mg—Cu series aluminum alloy, when simply pursuing high strength, a single-stage aging system can be used to obtain the T6 state. After the aging treatment, the main strengthening phases are the GP zones and a small amount of transition phase (n′ phase), and the strength can reach a peak value.

Further, in the manufacturing method according to the present disclosure, the Al—Zn—Mg—Cu series aluminum alloy ingot comprises the following chemical elements in mass percentages:

In some embodiments, the content of Ti is 0<Ti≤0.06%. In some embodiments, the content of Ti is 0.01≤Ti≤0.10%. In some embodiments, the content of Ti is 0.04≤Ti≤0.10%. In some embodiments, the content of Ti is 0.04≤Ti≤0.06%.

In some embodiments, the content of Mn is 0.01≤Mn≤0.05%.

In some embodiments, the content of Cr is 0.005≤Ti≤0.04%. In some embodiments, the content of Cr is 0.005≤Ti≤0.01%.

In the present disclosure, the design of the Al—Zn—Mg—Cu series aluminum alloy ingot is optimized, and the chemical elements thereof are designed according to the following principles:

Cu: The addition of the Cu element to the Al—Zn—Mg—Cu series aluminum alloy ingot according to the present disclosure can improve the stress corrosion resistance, cracking performance, strength performance, fatigue resistance and processing performance of the alloy, enhance the fluidity of the alloy, enhance the strengthening effect of the second-stage aging in the two-stage aging process, reduce processing defects, and reduce the crack propagation rate of the alloy in a corrosive medium. The dissolution of the Cu element into the GP zone can make the GP zone more stable and delay its aging precipitation. In addition, Cu atoms can dissolve into n and n′, reducing the potential difference between the inside of a grain and the boundary of the grain, and improving the corrosion resistance of the alloy. On the other hand, an increase in the content of the Cu element will increase the tendency to hot cracking of the material during welding, leading to a decrease in the welding performance. For this reason, when designing the composition of the Al—Zn—Mg—Cu series aluminum alloy, the various performance indicators of the alloy have been given an overall consideration, and an appropriate Cu content is selected. The mass percentage of the Cu element is controlled in the range of 1.6-2.2%, so as to take into account the welding performance of the alloy.

Of course, in some preferred embodiments, in order to achieve better implementation effectiveness, the mass percentage of the Cu element may be further controlled in the range of 1.8-2.2%.

Mg, Zn: In the Al—Zn—Mg—Cu series aluminum alloy ingot according to the present disclosure, the alloying elements Zn and Mg can precipitate from the alloy matrix to form a strengthening phase η′ (MgZn) phase, thereby improving the yield strength and fracture toughness of the alloy. When the content of the Zn element in the alloy is too low, the strength of the alloy is insufficient. When the content of the Zn element in the alloy is too high, the toughness of the alloy is low, and the formability is poor. The inventors have discovered by research that the Zn and Mg elements can have an aging strengthening effect in the alloy matrix only when their contents are within critical value bounds. If the content exceeds the maximum value of the critical bound, increasing the content of the Zn or Mg element will not increase the aging hardening effect. If the Zn and Mg contents are lower than the minimum values of the critical value bounds, there will be no aging strengthening effect. Therefore, when the Zn/Mg ratio is in the range of 2.6-3.3, the aging precipitation phase of the alloy can be fine and distributed dispersively, and the aging process can proceed rapidly. Therefore, in the present disclosure, the mass percentage of the Mg element is controlled in the range of 1.8-2.4%, and the mass percentage of the Zn element is controlled in the range of 6.0-8.6%.

Of course, in some preferred embodiments, in order to achieve better implementation effects, the mass percentage of the Mg element may be further controlled in the range of 2.0-2.4%, and the mass percentage of the Zn element may be further controlled in the range of 6.1-7.8%.

Zr: In the Al—Zn—Mg—Cu series aluminum alloy ingot according to the present disclosure, the fine dispersive precipitate phase formed by trace amounts of transition elements Mn, Cr, and Zr can improve the yield strength and tensile strength of the alloy. It inhibits recrystallization to provide a fine grain structure including a deformation substructure. This structure is beneficial to improving the fracture toughness of the alloy, causing the alloy to undergo transgranular fracture and thus improving the toughness. The corrosion resistance of an alloy containing the Mn and Cr elements is significantly higher than that of an alloy without Mn and Cr. These elements are beneficial to increasing the recrystallization temperature of the alloy, and prevent the recrystallization process during hot deformation and subsequent quenching heating. Besides, low contents of Cr and Mn will not form any harmful coarse phase. Of course, adding Zr is the most effective measure, because it can increase the recrystallization temperature of the aluminum alloy, no matter it's added after heating deformation or after cold deformation, making it possible to obtain a non-recrystallized structure after the heat treatment. Therefore, in the present disclosure, in order to further improve the strength of the Al—Zn—Mg—Cu alloy, Zr is added as a necessary element, and the mass percentage of Zr is controlled in the range of 0.10-0.16%.

Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Zr element may be further controlled in the range of 0.10-0.13%. The Zr element can combine with the Al element to form an intermetallic compound AlZr. This intermetallic compound has two forms of structure: one is a tetragonal structure, which is the structure of AlZr precipitated directly from the melt and can refine the cast grains of the alloy significantly; and the other is an LI2 structure, which is the structure of spherical particles precipitated during the homogenization of the ingot, coherent with the matrix, and has a strong effect of inhibiting recrystallization during hot working. The addition of a trace amount of the Zr element can improve the strength, fracture toughness and stress corrosion resistance of the aluminum alloy. In addition, since Zr has a lower quenching sensitivity, Zr can also improve the hardenability and weldability of the alloy.

In summary, in the Al—Zn—Mg—Cu series aluminum alloy designed by the present disclosure, 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 series aluminum alloy ingot obtained is uniform and fine equiaxed crystals. The refining mechanism in this design is as follows: the atomic clusters containing Cr and Mn, which are completely coherent with α(A1), replace TiB as the “basis” for the common nucleation of AlTi and AlZr, allowing Ti and Zr to participate in the refinement process together. In the heterogeneous nucleation process, AlTi nucleates through coherent atomic clusters, Al(Ti, Zr) nucleates through AlTi, and α(A1) nucleates through Al(Ti, Zr).

Further, in the manufacturing method according to the present disclosure, the mass percentages of the chemical elements in the Al—Zn—Mg—Cu series aluminum alloy ingot further satisfy at least one of the following:

Further, in the manufacturing method according to the present disclosure, the unavoidable impurities in the Al—Zn—Mg—Cu series aluminum alloy ingot include at least one of the following: Si≤0.10%, Fe≤0.15%, and a total amount of other impurity elements≤0.100%.

In the Al—Zn—Mg—Cu series aluminum alloy ingot according to the present disclosure, Si and Fe mentioned above are both impurity elements in the aluminum alloy. The impurity elements such as Si and Fe are harmful elements that are difficult to avoid in the smelting process of the Al—Zn—Mg—Cu series aluminum alloy ingot. They can form coarse and brittle phases with very high melting points (such as AlCuFe) in the alloy matrix. These phases will be arranged in strings in the deformation direction during processing and deformation. There is a high-energy phase interface between them and the matrix, so that concerted deformation is difficult, and microcracks are prone to occur under stress. When the action of the stress continues, the microcracks aggregate and grow into macrocracks, increasing the crack propagation rate and reducing the plasticity and fracture toughness of the alloy.

For example, the Fe element dissolves in Al to form FeAl, which can refine the recrystallized grains and thus improve the performances of the alloy. However, due to the large electric potential difference between FeAland the Al matrix, the corrosion resistance of the alloy will be degraded. For another example, if Mn is added to an aluminum alloy ingot, (Fe, Mn)Alwill be formed in the alloy, which will reduce the electric potential difference between FeAland Al, and thus improve the corrosion resistance of the alloy.

Therefore, in order to ensure the performances and quality of the aluminum alloy, it is necessary to strictly control the mass percentages of the abovementioned impurity elements, and control the total amount of other impurity elements to be ≤0.100%, and the mass percentage of each of the other impurity elements to be ≤0.030%, so as to reduce the content of the coarse second phase containing impurity elements such as Si and Fe in the alloy, and ultimately improve the fracture toughness of the alloy and reduce the crack propagation rate.

When the technical conditions permit, in order to obtain an aluminum alloy having better performances and higher quality, the contents of the impurity elements in the Al—Zn—Mg—Cu series aluminum alloy ingot should be minimized.

In some preferred embodiments, in order to achieve better implementation effects and make the quality of the resulting Al—Zn—Mg—Cu series aluminum alloy ingot better, it's preferable to further control Si<0.08%, Fe<0.10%.

Further, in the manufacturing method according to the present disclosure, in step (2), a three-stage homogenization process is used for the homogenization treatment, wherein the first stage homogenization treatment is held at a temperature of 418-430° C. for 5-8 hours, the second stage homogenization treatment is held at a temperature of 460-468° C. for 8-12 hours, and the third stage homogenization treatment is held at a temperature of 470-480° C. for 20-24 hours.

In step (2) in the present disclosure, the purpose of homogenizing the Al—Zn—Mg—Cu series aluminum alloy ingot is to eliminate dendrite segregation and component segregation, produce a solid solution with uniform distribution of solute atoms, and reduce the coarse second phase that contributes to the PSN nucleation mechanism of recrystallization.

Although a two-stage homogenization process can achieve the best aging strengthening effect, the two-stage homogenization treatment system will result in a certain degree of aggregation and growth of the insoluble Fe-containing phase (AlCuFe) and S(AlCuMg) phase during the high-temperature holding stage at 473° C. These coarse and brittle second phases are not easy to deform, which will reduce the strength of the alloy, hinder the mobility of dislocations, and reduce the plasticity of the alloy.

The reason why a three-stage homogenization process is utilized in the present disclosure is that fine AlZr particles distributed dispersively and uniformly can be obtained by the three-stage homogenization process. According to the dislocation bypassing mechanism by which the second phase that is difficult to deform impedes dislocation movement, when the AlZr particles that are not easy to deform have a smaller radius, are distributed with a smaller spacing, and are more dispersive, the critical shear stress that the dislocations have to overcome to continue moving will be larger, the impeding effect on dislocation movement will be stronger, and the alloy strength will be higher. In addition, the fine and dispersive AlZr particles can also prevent recrystallization, retain the deformation substructure, and refine the grains, thereby shortening the dislocation slip distance, 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 aluminum alloy material. In addition, the three-stage homogenization process can also cause spheroidization of the S(AlCuMg) phase.

Further, in the manufacturing method according to the present disclosure, in step (2), the hot rolling includes the steps of: heating the ingot to 430-440° C., holding for 90-120 minutes, and then performing multiple passes of hot rolling, wherein the hot rolling is carried out in longitudinal and transverse directions alternately, and the total hot rolling deformation rate is controlled to be ≥85%. The rolling-end temperature is controlled to be ≥380° C., such as 380-400° C.

In the above technical solution of the present disclosure, the hot rolling in longitudinal and transverse directions alternately means controlling the hot rolling direction so that the plate is rolled in the length direction and the width direction of the plate alternately. The alternating rolling in longitudinal and transverse directions can provide a plate size desired for subsequent warm forming, and the plate has good through-thickness mechanical performances.

During the hot rolling process, the rolling-start temperature of the hot rolling is about 85-90% of the melting point of the alloy. Nevertheless, the influence of low melting phases in the Al—Zn—Mg—Cu system such as S(AlCuMg) phase and T(AlZnMgCu) phase should also be considered. If the hot rolling temperature is too high, grain coarsening or melting of low melting phases between grains tends to occur, and the ingot will be overheated or overfired, or even cracked or crushed during the hot rolling. If the hot rolling temperature is too low, non-uniform deformation of the ingot will be resulted; the rolling load will be increased; and the tendency to edge cracking of the ingot during rolling will be increased, thereby affecting normal rolling. It can be seen that the hot rolling temperature has an influence on the heat resistance and the ambient temperature mechanical performances of the material. Therefore, in order to ensure the performances of the aluminum alloy material, in the present disclosure, the rolling-start temperature of the hot rolling may be controlled in the range of 430-440° C.

Accordingly, during the hot rolling process, the rolling-end temperature of the hot rolling is determined according to the type II recrystallization diagram of the alloy. The rolling-end temperature of the blooming rolling in the hot rolling process for the Al—Zn—Mg—Cu series aluminum alloy is generally controlled above the recrystallization temperature. Therefore, in the present disclosure, the rolling-end temperature may be controlled to be ≥380° C.

In addition, during the hot rolling process, the total rolling deformation rate should be selected in view of the characteristics of the Al—Zn—Mg—Cu series aluminum alloy itself. The larger the total rolling deformation rate, the more uniform the material structure, and the better the performances. When the total rolling deformation rate is controlled to be 85% or higher, a rolled plate with the best structure can be obtained.