A high-strength and high-toughness Al-Zn-Mg-Cu series aluminum alloy plate and its preparation method

Abstract

The present invention provides a high-strength and high-toughness Al-Zn-Mg-Cu series aluminum alloy plate and a preparation method thereof. The Al-Zn-Mg-Cu series aluminum alloy plate contains the following elements according to mass percentage: 6.0 wt% - 7.0 wt% of Zn, 2.6 ≤ Zn / Mg ≤ 3.9, and 1.5 wt% - 2.0 wt% of Cu, and the balance is Al and inevitable impurities; the equivalent circular diameter of the η(MgZn2) phase size at the grain boundary of the Al-Zn-Mg-Cu series aluminum alloy plate is ≤ 0.2 μm, and the equivalent circular diameter of the Al7Cu2Fe phase size is ≤ 0.8 μm. The present invention obtains a high-strength and high-toughness Al-Zn-Mg-Cu series aluminum alloy plate by controlling the size of the combined phase at the grain boundary.

Description

Technical Field

[0001] The present invention relates to the technical field of aluminum alloy preparation, and particularly relates to an Al-Zn-Mg-Cu series aluminum alloy plate with high strength and high toughness and a preparation method thereof. Background Art

[0002] Al-Zn-Mg-Cu alloys have strong comprehensive properties and are widely used in the fields of aerospace, military, transportation, etc. As high-performance aluminum alloys, the core problem of Al-Zn-Mg-Cu alloys still focuses on the improvement of fracture toughness. High-toughness materials often also have high fatigue performance and crack propagation rate performance.

[0003] Considering from the microscopic structure of Al-Zn-Mg-Cu aluminum alloys, the main factors affecting their toughness are as follows: ① Coarse compounds on the grain boundaries, mainly including η(MgZn2) phase, S(Al2CuMg) phase and Al7Cu2Fe phase. During the loading process of the material, these particles act as crack sources due to stress concentration, resulting in rapid failure of the material; ② Coarse recrystallized grains. The Al7Cu2Fe phase is a refractory phase generated during the solidification process and is broken during subsequent processing. However, when its size is broken to 1-2 μm, it is in a stable state and cannot be further refined. Particles within the range of 1-2 μm undergo static recrystallization to form coarse recrystallized grains through particle-stimulated nucleation (PSN) during the heat treatment process after material deformation, leading to a decrease in the toughness of the material.

[0004] Currently, in the prior art, at the same strength level, the main methods for further improving the toughness of materials are: composition regulation and precipitation phase regulation. The conventional practice in the prior art is to add 8-10% of Zn element, about 2 wt% of Cu and about 2.5 wt% of Mg. Through three-stage aging, the area ratio of η(MgZn2) phase, S(Al2CuMg) phase and Al7Cu2Fe phase on the grain boundaries in the structure is regulated, and high-toughness and corrosion-resistant aluminum alloy profiles are produced. The tensile strength of the manufactured profiles can reach 680 MPa, but the fracture toughness is about 28 MPa·m 1 / 2 . The unreasonable element ratio in the preparation method leads to the generation of S(Al2CuMg) phase in the structure, which has a certain adverse effect on the improvement of the toughness and ductility of the product.

[0005] Currently, the improved methods of composition regulation and precipitation phase regulation still remain in the research and trial stage, making the corresponding improvement means inapplicable to large-scale production. Therefore, it is of great significance to study a 7XXX series aluminum alloy with good processing performance and relatively higher toughness. Summary of the Invention

[0006] In view of the problems existing in the prior art, the present invention provides a high-strength and high-toughness Al-Zn-Mg-Cu series aluminum alloy plate and a preparation method thereof. By controlling the contents of Zn and Cu in the Al-Zn-Mg-Cu series aluminum alloy plate and simultaneously defining the mass ratio of Zn / Mg, an η (MgZn2) phase with a circular equivalent diameter of ≤0.2 μm and an Al7Cu2Fe phase with a circular equivalent diameter of ≤0.8 μm are obtained at the grain boundaries of the Al-Zn-Mg-Cu series aluminum alloy plate, thereby improving the toughness and strength of the aluminum alloy plate simultaneously.

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

[0008] In the first aspect, the present invention provides a high-strength and high-toughness Al-Zn-Mg-Cu series aluminum alloy plate, and the Al-Zn-Mg-Cu series aluminum alloy plate includes the following elements according to the mass percentage content: Zn 6.0 wt% - 7.0 wt%, 2.6 ≤ Zn / Mg ≤ 3.9, and Cu 1.5 wt% - 2.0 wt%, and the balance is Al and unavoidable impurities.

[0009] In the present invention, Mg and Zn form a precipitation phase to play a strengthening role. It is necessary to match the control of the Mg element content with the content of the Zn element, and control the Zn / Mg ratio within the range of 2.6 - 3.9. This is because different Zn / Mg ratios correspond to different precipitation phases, including the η series and T phases, etc. When the Zn / Mg ratio is close to 1, it is easy to form a T-type phase, and the strengthening effect is weak; when the Zn / Mg ratio is between 2.5 and 7, an η-type phase is easily precipitated in the alloy, and there is a strong strengthening effect. The present invention controls the Zn / Mg ratio within the range of 2.6 - 3.9, which can effectively ensure the balance between the strengthening effect and the improvement of toughness.

[0010] The present invention controls the Cu element content within 2.0 wt%. The Cu element in the alloy can increase the supersaturation degree of the alloy and improve the strength of the material. However, with the increase of the Cu content, an S (Al2CuMg) phase will be formed, resulting in a decrease in the toughness of the material.

[0011] The circular equivalent diameter of the η (MgZn2) phase size at the grain boundaries of the Al-Zn-Mg-Cu series aluminum alloy plate is ≤0.2 μm; the circular equivalent diameter of the Al7Cu2Fe phase size at the grain boundaries of the Al-Zn-Mg-Cu series aluminum alloy plate is ≤0.8 μm.

[0012] Zn and Mg are the main strengthening elements of Al-Zn-Mg-Cu series aluminum alloy plates. In the present invention, the Zn element can improve the strength of the alloy through solid solution strengthening. When the Mg element is added simultaneously in the alloy, after solution treatment and aging treatment, the Zn and Mg elements will form strengthening phases such as GP zones, η'(MgZn2) phase and η(MgZn2) phase, which play a role in precipitation strengthening. In the present invention, the Zn element is limited to 6.0wt%-7.0wt%. When the Zn content is low, the number of precipitation phases is insufficient, resulting in a significant reduction in the strengthening effect, and the alloy will not reach the level of ultra-high strength aluminum alloy (yield strength higher than 500MPa). However, when the content of the Zn element is further increased, it will lead to a significant decrease in toughness. We found that too high Zn content leads to coarsening of the η(MgZn2) phase precipitated at the grain boundary, which will induce crack initiation and promote crack propagation during the material loading process. Therefore, it is very necessary and effective to control the circular equivalent diameter of the η(MgZn2) phase at the grain boundary of the Al-Zn-Mg-Cu series aluminum alloy plate to be 0.2-0.5μm for obtaining high-strength and high-toughness Al-Zn-Mg-Cu series aluminum alloy plates.

[0013] On the other hand, the Fe element in the aluminum alloy will inevitably form the Al7Cu2Fe phase. The Al7Cu2Fe phase is a coarse, brittle and insoluble phase, which has an adverse effect on the toughness of the aluminum alloy. It should be noted that in the traditional deformation process, the size of the Al7Cu2Fe phase is broken to within the range of 1-2μm and reaches a stable state, and it is difficult to continue to be broken. However, in the present invention, by controlling the size of the Al7Cu2Fe phase to ≤0.8μm, it is found that a certain small size of the Al7Cu2Fe phase can avoid coarse recrystallized grains, improve the toughness of the aluminum alloy plate, and at the same time avoid itself as a crack source to cause premature failure of the material.

[0014] The Al-Zn-Mg-Cu series aluminum alloy plate described in the present invention includes, by mass percentage: Zn 6.0wt%-7.0wt%, for example, it can be 6.0wt%, 6.1wt%, 6.2wt%, 6.3wt%, 6.4wt%, 6.5wt%, 6.6wt%, 6.7wt%, 6.8wt%, 6.9wt% or 7.0wt% etc.

[0015] In the Al-Zn-Mg-Cu series aluminum alloy plate described in the present invention, 2.6≤Zn / Mg≤3.9 by mass percentage. The value of Zn / Mg can be, for example, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8 or 3.9 etc.

[0016] In the Al-Zn-Mg-Cu series aluminum alloy plate described in the present invention, by mass percentage, it includes: Cu 1.5 wt% - 2.0 wt%, for example, it can be 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt% or 2.0 wt%, etc.

[0017] In the Al-Zn-Mg-Cu series aluminum alloy plate described in the present invention, the circular equivalent diameter of the η(MgZn2) phase size at the grain boundary is ≤ 0.2 μm, for example, it can be 0.05 μm, 0.10 μm or 0.15 μm, etc.

[0018] In the Al-Zn-Mg-Cu series aluminum alloy plate described in the present invention, the size of the Al7Cu2Fe phase at the grain boundary is ≤ 0.8 μm, for example, it can be 0.8 μm, 0.7 μm, 0.6 μm, 0.5 μm or 0.4 μm, etc.

[0019] Preferably, in the Al-Zn-Mg-Cu series aluminum alloy plate, by mass percentage: the content of Zn element is 6.6 - 6.8 wt%.

[0020] In the present invention, the content of Zn element is further preferably 6.6 - 6.8 wt%. In this range, it shows relatively the best strength and toughness.

[0021] Preferably, in the Al-Zn-Mg-Cu series aluminum alloy plate, it further includes the following elements by mass percentage: Ti 0.01 wt% - 0.06 wt%, for example, it can be 0.01 wt%, 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt% or 0.06 wt%, etc.

[0022] The Ti element has the effect of refining grains. The Ti element is beneficial to the improvement of strength; if the content of Ti is too small, the effect cannot be achieved; if the content of Ti is too high, the number of TiB2 particles is large and agglomerates are formed, resulting in the reduction of the toughness of the material.

[0023] Preferably, the Al-Zn-Mg-Cu series aluminum alloy plate further includes Zr and Sc elements. By mass percentage, 0.86 ≤ Zr / Sc ≤ 1.86 and 0.1 wt% ≤ Zr + Sc ≤ 0.26 wt%.

[0024] The average size of the primary Al3(Sc,Zr) phase is ≤ 1.2 μm; the average size of the secondary Al3(Sc,Zr) phase is ≤ 20 nm.

[0025] 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, which is formed by the aggregation of individual recrystallized grains; the recrystallized structure clusters are randomly distributed in the fibrous structure.

[0026] 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 thickness direction of the aluminum alloy plate. In the research, it is found that under the same type of aluminum alloy formulation system, this heterogeneous structure has better strengthening and toughening effects compared with the traditional fully recrystallized structure (or equiaxed grain structure).

[0027] Generally, the heat treatment process of solid solution strengthening will cause recrystallization and finally form a fully recrystallized structure, resulting in a decrease in the strength and toughness of the material. When we control the formation of the primary Al3(Sc,Zr) phase in some areas during the casting process, the position where this phase precipitates has less precipitation of the secondary Al3(Sc,Zr) phase during the subsequent homogenization heat treatment process, becoming a secondary Al3(Sc,Zr) phase-deficient area. In the deficient area, the number of secondary Al3(Sc,Zr) phases is small, and the dislocation density pinned by them is high. First, sub-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 the secondary Al3(Sc,Zr) phase diffusely, which is called the secondary Al3(Sc,Zr) phase-rich area. In this area, only small-angle sub-boundaries exist after extrusion, and the grains are overall fibrous. Moreover, during the subsequent heat treatment process, the pinning effect of the secondary Al3(Sc,Zr) phase on the grain boundaries can inhibit the nucleation of recrystallization and retain the fibrous structure. Therefore, controlling the precipitation ability of the primary Al3(Sc,Zr) phase determines the precipitation ability of the subsequent secondary Al3(Sc,Zr) phase, and thus a heterogeneous structure is obtained.

[0028] During the low-frequency electromagnetic casting process, the temperature field of the aluminum alloy melt is relatively uniform, and the 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 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, fewer secondary Al3(Sc,Zr) phases precipitate, becoming a scarce area of the 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 the precipitated secondary Al3(Sc,Zr) dispersion phases is relatively high, becoming an enrichment area of the secondary Al3(Sc,Zr) phase dispersion. During the extrusion process, in the enrichment area of the secondary Al3(Sc,Zr) phase dispersion, the dispersed secondary Al3(Sc,Zr) phases pin dislocations, forming sub-boundaries inside the grains; in the scarce area of the secondary Al3(Sc,Zr) phase dispersion, the secondary Al3(Sc,Zr) phases pin dislocations to form sub-boundaries, but due to the extremely small number of its particles, the sub-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 scarce area of the secondary Al3(Sc,Zr) phase dispersion is also an enrichment area of the 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 comprehensively improving the strength and toughness of the material.

[0029] In the present invention, in the tough and strong aluminum alloy plate with a heterogeneous structure, by mass percentage: 0.1wt% ≤ Zr + Sc ≤ 0.26wt%, for example, it can be 0.12wt%, 0.19wt%, 0.20wt%, 0.22wt%, 0.24wt% or 0.26wt% etc., but is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0030] It should be noted that if the content of Zr + Sc is too high, the primary Al3(Sc,Zr) phase generated during the solidification process is coarse. For example, the traditional primary Al3(Sc,Zr) phase is a coarse phase formed during the solidification process when the total addition amount of Zr and Sc exceeds 0.3%, and its size ranges from 5 to 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 will not be able to obtain a heterogeneous structure.

[0031] 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. Other unlisted values within the above numerical range are equally applicable.

[0032] It should be noted that in the present invention, the Zr / Sc ratio is controlled within the range of 0.86 - 1.86. If the Zr / Sc ratio is too high or too low, the sizes of the primary phase and secondary phase in the final plate will become larger, and the size distribution will be too wide. When Zr / Sc is too high, the size stability of the primary Al3(Sc,Zr) phase formed during the solidification process is poor, and it coarsens during the homogenization heat treatment. The coarsening of the primary Al3(Sc,Zr) phase leads to an increase in the dispersion phase-deficient region, thus causing an increase in the recrystallized tissue clusters. If the recrystallized tissue clusters are too large, they will preferentially deform and fracture as soft regions during the loading process, resulting in a decrease in strength and toughness. When Zr / Sc is too low, the size stability of the secondary Al3(Sc,Zr) phase formed during the homogenization process is poor, and it coarsens during the homogenization process. 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 partial recrystallization of the fibrous tissue during the solution treatment and making it difficult to form a heterogeneous structure.

[0033] 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.

[0034] The existence forms of the Al3(Sc,Zr) compound are divided into the primary Al3(Sc,Zr) phase and the secondary Al3(Sc,Zr) phase. The primary Al3(Sc,Zr) phase is generated during the alloy solidification process 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 tissue 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 are too large primary Al3(Sc,Zr) phase or secondary Al3(Sc,Zr) phase in the aluminum alloy plate, they will act as crack sources during the deformation process, causing cracking and resulting in problems of decreased toughness and strength.

[0035] In the present invention, in the tough aluminum alloy plate with a heterogeneous structure, the heterogeneous structure is that the recrystallized tissue clusters and the fibrous tissue are distributed at intervals in the thickness direction of the aluminum alloy plate; wherein, the thickness direction refers to the direction perpendicular to the rolling direction of the plate.

[0036] It should be noted that neither pure fibrous tissue nor pure dynamic recrystallization tissue shows a significant contribution to the improvement of toughness. However, a heterogeneous structure composed of randomly distributed recrystallized tissues of a certain size and density in the fibrous tissue shows obvious toughness improvement. The reason for the toughening effect of the heterogeneous structure composed of fibrous tissue and recrystallized tissue is that: compared with the fibrous tissue, the recrystallized structure is a soft zone, randomly distributed around the fibrous tissue, which can provide high tensile strain and high work hardening ability, effectively inhibit the stress concentration and constriction of the fibrous tissue during the deformation process, increase the stress threshold for crack initiation and propagation, and thus improve the toughness and plasticity of the material.

[0037] Preferably, in the Al-Zn-Mg-Cu series aluminum alloy plate, by mass percentage: 0.18 wt% ≤ Zr + Sc ≤ 0.26 wt%, preferably 0.2 wt% ≤ Zr + Sc ≤ 0.24 wt%.

[0038] If the content of Zr + Sc is within 0.18 wt% ≤ Zr + Sc ≤ 0.26 wt%, the precipitation amount level of primary particles is better during the solidification process, the density of the recrystallized tissue is relatively good during the subsequent extrusion process, 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 shows relatively optimal strength and toughness.

[0039] Preferably, the recrystallized tissue clusters are evenly distributed in the fibrous tissue.

[0040] The uneven distribution of the recrystallized tissue clusters will affect the strength and toughness values.

[0041] Preferably, the distribution density of the recrystallized tissue clusters in the heterogeneous structure 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.

[0042] Insufficient distribution density of recrystallized tissue clusters will affect toughness; when the distribution density of recrystallized tissue clusters is too high, it will lead to a significant decrease in strength; if the proportion of the distribution density of recrystallized tissue clusters 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.

[0043] Preferably, the size of the recrystallized tissue clusters in the heterogeneous structure 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. Too large recrystallized tissue clusters will result in poor toughness.

[0044] 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.

[0045] 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.

[0046] In a second aspect, the present invention provides a method for preparing a high-strength and high-toughness Al-Zn-Mg-Cu series aluminum alloy plate as described in the first aspect. The preparation method includes the following steps:

[0047] (1) According to the component ratio of the Al-Zn-Mg-Cu series aluminum alloy plate, melt the raw materials of the Al-Zn-Mg-Cu series aluminum alloy plate and stir, and then carry out casting to obtain an ingot;

[0048] (2) Perform secondary homogenization heat treatment on the ingot obtained in step (1) to obtain a bar;

[0049] (3) Perform pre-extrusion and secondary extrusion on the bar obtained in step (2) to obtain a semi-finished product plate;

[0050] (4) Solution treat the semi-finished product sheet obtained in step (3) to obtain the solution-treated sheet.

[0051] (5) Perform three-stage aging heat treatment on the solution-treated sheet obtained in step (4) to obtain the Al-Zn-Mg-Cu series aluminum alloy sheet.

[0052] In the preparation method of the present invention, by controlling the processes of pre-extrusion and secondary extrusion, the size of the Al7Cu2Fe phase at the grain boundaries of the obtained Al-Zn-Mg-Cu series aluminum alloy sheet is ≤0.8 μm, and the circular equivalent diameter of the η(MgZn2) phase at the grain boundaries of the Al-Zn-Mg-Cu series aluminum alloy sheet is ≤0.2 μm.

[0053] The raw materials in step (1) include pure aluminum ingots, magnesium ingots, zinc ingots, aluminum-copper master alloys, etc. 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 performed 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 statically settled. The specific composition of the aluminum-titanium grain refiner is not specifically limited herein.

[0054] 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 to avoid the adverse effects of coarse non-equilibrium eutectic phases on the deformation ability of the ingot during extrusion and the toughness of the final material; usually, primary homogenization heat treatment is required, and those skilled in the art can adjust the homogenization heat treatment process according to the different materials and the dissolution situation of specific non-equilibrium eutectic phases.

[0055] The first-stage holding temperature of the secondary homogenization heat treatment in step (2) is 400 - 450 °C and the time is 4 - 8 h; preferably, the second-stage end temperature of the secondary homogenization heat treatment is 460 - 480 °C and the time is 18 - 24 h.

[0056] The extrusion ratio of the pre-extrusion in step (3) is 3 - 4, for example, it can be 3, 3.2, 3.4, 3.6, 3.8 or 4, etc. The extrusion ratio of the secondary extrusion is 10 - 12, for example, it can be 10, 10.2, 10.4, 10.6, 10.8, 11.0, 11.2, 11.4, 11.6, 11.8 or 12, etc.

[0057] The method of adopting pre - extrusion and secondary extrusion is because the Al7Cu2Fe phase will be broken to a certain extent after pre - extrusion. After the process of cooling to room temperature and then through the heating process of secondary extrusion, the interfacial strength between the Al7Cu2Fe phase and the aluminum matrix will decrease, which is beneficial to more complete fragmentation during the secondary extrusion process. At the same time, compared with single - stage extrusion, the metal flow has more dimensions and is more uniform in two - stage extrusion. The extrusion ratio of pre - extrusion needs to be controlled within the range of 3 - 4. If the extrusion ratio is too low, it is not conducive to the pre - fragmentation of the Al7Cu2Fe phase. If the extrusion ratio is too high, it will affect the fragmentation degree of secondary extrusion, both of which will lead to insufficient fragmentation of the Al7Cu2Fe phase and the phase size exceeding the target range. The extrusion ratio of secondary extrusion can also be determined according to the shape and size of the ingot blank after pre - extrusion and the final extruded strip.

[0058] It should be noted that in the traditional deformation process, the Al7Cu2Fe phase reaches a stable state when its size is broken to within the range of 1 - 2μm and it is difficult to continue to break. The present invention adopts the method of pre - extrusion and secondary extrusion, and utilizes the more uniform multi - dimensional flow of metal during the two extrusion processes to perform multi - dimensional processing and fragmentation on Al7Cu2Fe.

[0059] In step (3), the extrusion temperatures of the pre - extrusion and secondary extrusion are 400 - 430°C, and can also be determined according to the over - burning temperature of the material and the surface quality of the extruded product.

[0060] Preferably, the extrusion speeds of the pre - extrusion and secondary extrusion in step (3) are independently 0.1 - 0.5 m / min, for example, they can be 0.1 m / min, 0.15 m / min, 0.2 m / min, 0.25 m / min, 0.3 m / min, 0.35 m / min, 0.4 m / min, 0.45 m / min or 0.5 m / min, etc.

[0061] 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 is to make the entire cross - section of the ingot deform uniformly during extrusion to fully break the Al7Cu2Fe phase in the structure. If the extrusion speed is too high, it will cause the extrusion deformation to concentrate on the surface layer, and the Al7Cu2Fe in the core will not be fully broken. Moreover, in another study on heterogeneous structures, it was found that due to too high an extrusion speed, the stored energy in the tissue is too high, and the fibrous tissue undergoes complete recrystallization during the solution treatment process, and finally becomes a completely recrystallized tissue morphology.

[0062] In the Al - Zn - Mg - Cu series aluminum alloy plate of the present invention, the raw materials of aluminum, magnesium, and zinc are respectively selected from pure aluminum, pure magnesium, and pure zinc. Cu, Zr, Sc, and Ti respectively select aluminum - copper master alloy, aluminum - zirconium master alloy, aluminum - scandium master alloy, and aluminum - titanium master alloy as raw materials.

[0063] Preferably, before the extrusion forming in step (3), the cast rod is heated and held at a temperature of 400 - 430°C for 2 - 4 h; after the extrusion forming in step (3), air cooling is carried out. The heating temperature can be, for example, 400°C, 405°C, 410°C, 415°C, 420°C, 425°C or 430°C, etc.; the holding time can be, for example, 2 h, 2.2 h, 2.3 h, 2.5 h, 2.8 h, 3.0 h, 3.2 h, 3.5 h or 4.0 h, etc.

[0064] Preferably, the secondary aging temperature of the three - stage aging heat treatment in step (5) is 170 - 180°C, and can be, for example, 170°C, 171°C, 172°C, 173°C, 174°C, 175°C, 176°C, 178°C or 180°C, etc.

[0065] Preferably, the aging heat treatment in step (5) is to hold at 110 - 120°C for 22 - 24 h, followed by air cooling, then hold at 170 - 180°C for 20 - 30 min and water cooling; finally, hold at 110 - 120°C for 22 - 24 h again and air cooling.

[0066] After solution heat treatment, the aluminum alloy forms a supersaturated solid solution. Then, the purpose of aging is to precipitate precipitation phases to play a role in precipitation strengthening. The primary aging in the present invention is peak aging to obtain peak strength; the secondary aging is regression aging, and its purpose is to partially redissolve the coarse η(MgZn2) phase generated during the primary aging process to reduce its size. The temperature of the secondary aging is controlled at 170 - 180°C. If the temperature is too low, the redissolution degree of the coarse η(MgZn2) phase is small and its size cannot be sufficiently reduced. If the temperature is too high, the solubility of the GP zone and η’(MgZn2) phase, which have a stronger strengthening effect on the matrix, will increase, ultimately resulting in a decline in material properties; the tertiary aging is peak aging, enabling continuous precipitation of precipitation phases in the aluminum matrix to achieve the effect of high strength.

[0067] Preferably, when the Al - Zn - Mg - Cu series aluminum alloy plate further includes Zr and Sc elements, the casting in step (1) is low - frequency electromagnetic casting.

[0068] 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 a high - strength and high - toughness aluminum alloy plate with a specific heterogeneous structure; the preparation method takes into account the realization of production efficiency and performance and has industrial feasibility.

[0069] The raw materials Sc and Zr in step (1) can be added in the form of aluminum - scandium master alloy and aluminum - zirconium master alloy.

[0070] It should be noted that under the action of the induced magnetic field, the melt in the liquid cavity undergoes forced convection and generates a uniform temperature field, which prolongs the residence time of the metal in the crystallization temperature range, facilitating the formation of primary Al3(Sc,Zr) phase. As a result, the solid solution amounts of Zr and Sc elements in the surrounding melt are reduced, and it becomes a secondary Al3(Sc,Zr) phase-deficient region after homogenization heat treatment.

[0071] Preferably, the magnetic field strength of the low-frequency electromagnetic casting is 1.2×10 4 ~3.6×10 4 a / M, and the current frequency is 15 - 30 Hz.

[0072] The magnetic field strength of the induced magnetic field is jointly determined by the coil, the current, and the effective magnetic path length; the calculation formula for the magnetic field strength: H = N×I / Le; where: H is the magnetic field strength, with the unit of a / m; N is the number of turns of the excitation coil; I is the excitation current (measured value), with the unit of A; Le is the effective magnetic path length of the test sample, with the unit of m; the desired magnetic field strength can be obtained by adjusting the above parameters.

[0073] The magnetic field strength of the low-frequency electromagnetic casting can be, for example, 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 is not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0074] 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.

[0075] 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, and other unlisted values within the above value range are equally applicable.

[0076] 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 diameter of the straight line, and the skin depth is inversely proportional to the current frequency, δ∝1 / f.

[0077] In the present invention, it is necessary to control the selection of the current frequency range. 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 the 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, which in turn leads to uneven distribution and reduction in the number of secondary phase deficiency regions during the extrusion process, resulting in uneven distribution and reduced density of the recrystallized tissue clusters. The uneven distribution and reduced 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.

[0078] 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 specific non-equilibrium eutectic phases.

[0079] Preferably, when the Al-Zn-Mg-Cu series aluminum alloy plate further includes Zr and Sc elements, the solution treatment in step (4) includes secondary solution treatment.

[0080] Preferably, the first-stage solution temperature of the secondary solution treatment is 350-400°C, and the time is 2-4h.

[0081] The first-stage solution temperature of the secondary solution treatment can be, for example, 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.

[0082] The first-stage solution time of the secondary solution treatment can be, for example, 2.2h, 2.4h, 2.6h, 2.8h, 3h, 3.2h, 3.4h, 3.4h, 3.6h or 3.8h, etc., but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0083] Preferably, the second-stage solution temperature of the secondary solution treatment is 460-480°C, and the time is 1-2h.

[0084] The second-stage solution temperature of the two-stage solution treatment can be, for example, 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. Other unlisted values within the numerical range are equally applicable.

[0085] The second-stage solution time of the two-stage solution treatment can be, for example, 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. Other unlisted values within the numerical range are equally applicable.

[0086] For heat-treatable aluminum alloys, the purpose of solution treatment is to redissolve alloying elements in the alloy into the aluminum matrix again to form a supersaturated solid solution, so as to increase the number of precipitates during aging and thus improve the strength. Usually, a single-stage solution at a high temperature can achieve the desired strength. However, experiments have found that under the processing conditions of a single-stage high-temperature solution, the heterogeneous structure alloy obtained during extrusion will undergo static recrystallization during the high-temperature single-stage solution process, resulting in an increase in the size of the recrystallized grains in the finally obtained aluminum alloy plate, and also leading to too high energy storage in the fibrous structure. During the solution process, the fibrous structure undergoes complete recrystallization and finally becomes a fully recrystallized tissue morphology, and it is impossible to obtain the higher strength and toughness expected in the present invention.

[0087] In the present invention, the solution treatment is divided into two stages. The first-stage solution treatment with a lower temperature is used to make the metal undergo a recovery process to release part of the stored energy, 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 single high-temperature solution process and retain the heterogeneous structure after extrusion.

[0088] The numerical range described in the present invention not only includes the above-listed point values, but also includes any point values between the above unlisted numerical ranges. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0089] Compared with the prior art, the present invention has at least the following beneficial effects:

[0090] (1) Compared with traditional aluminum alloy plates, the Al-Zn-Mg-Cu series aluminum alloy plates provided by the present invention control the sizes of η (MgZn2) phase and Al7Cu2Fe phase at the grain boundaries of the aluminum alloy plate within a specific range through extrusion process control, thereby improving the strength and toughness.

[0091] (2) The Al-Zn-Mg-Cu series aluminum alloy plate provided by the present invention has a heterogeneous structure with unique characteristics compared with the structure of traditional aluminum alloy plates: the fibrous structure is the hard zone, and the recrystallized structure is the soft zone, and the soft zones are distributed in the hard zones; 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 zones are completely recrystallized structures, which can provide high tensile strain and high work hardening ability, effectively suppressing the stress concentration and constriction in the hard zones during the deformation process, prolonging the crack initiation and propagation, and improving the toughness and plasticity of the material. Compared with the prior art, at the same strength level, the fracture toughness value can be significantly increased; the prepared Al-Zn-Mg-Cu series aluminum alloy plate combines the high strength of AA7050 alloy and the high toughness of AA7475 alloy, and has higher plasticity, with a strength ≥ 630 MPa, an elongation ≥ 14.3%, and a fracture toughness ≥ 34.2 MPa·m 1 / 2 。 BRIEF DESCRIPTION OF THE DRAWINGS

[0092] Figure 1 is a schematic diagram of the heterogeneous structure.

[0093] Description of the reference numerals in the drawings: 1 - recrystallized tissue cluster, 2 - fibrous tissue. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0094] The technical solutions of the present invention will be further described below in conjunction with the drawings and through specific embodiments.

[0095] 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.

[0096] 1. Glossary Explanation

[0097] Uniform distribution: It means that in the internal tissues of the aluminum alloy plate with the same area, the distribution probability of the crystal tissue clusters is equally possible; theoretically, the difference in the distribution density of the randomly sampled recrystallized tissues is required to be controlled within 30 per mm 2 or less.

[0098] Primary Al3(Sc,Zr) phase: It refers to the Al3(Sc,Zr) compound that is first formed from the liquid phase during the solidification process.

[0099] Secondary Al3(Sc,Zr) phase: It refers to other Al3(Sc,Zr) compounds that precipitate subsequently during the heat treatment process after the formation of the primary Al3(Sc,Zr) phase.

[0100] 2. Preparation Process, Product and Performance

[0101] The following examples and comparative examples provide a method for preparing an aluminum alloy plate, comprising the following steps:

[0102] (1) According to the component ratio of the Al-Zn-Mg-Cu series aluminum alloy plate, the raw materials of the Al-Zn-Mg-Cu series aluminum alloy plate are melted and stirred, and then cast to obtain an ingot;

[0103] (2) The ingot obtained in step (1) is subjected to secondary homogenization heat treatment to obtain a bar;

[0104] (3) The bar obtained in step (2) is pre-extruded and then secondarily extruded to obtain a semi-finished product plate;

[0105] (4) The semi-finished product plate obtained in step (3) is subjected to secondary solution treatment to obtain a plate after secondary solution treatment;

[0106] (5) The plate after secondary solution treatment obtained in step (4) is subjected to three-stage aging heat treatment to obtain the Al-Zn-Mg-Cu series aluminum alloy plate.

[0107] 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 defined situation and does not limit the protection scope of the present invention.

[0108] In the following examples and comparative examples, neither Sc element nor Zr element is added. The casting temperature is 680 °C, the extrusion temperature is 430 °C, the first-stage holding temperature of the secondary homogenization heat treatment 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. The aging heat treatment is a three-stage heat treatment. In the first stage, it is held at 120 °C for 22 h, air-cooled, then held at a certain temperature (170 - 180 °C) for 20 min, water-cooled; finally, it is held at 120 °C for 22 h again and air-cooled. The above does not mean that the present invention can only adopt the above process parameters, and other temperature and time ranges are also feasible.

[0109] 2.1 Without Sc element and Zr element

[0110] Examples 1 - 2 and Comparative Examples 1 - 5

[0111] The components and other relevant process parameters in Examples 1 - 2 and Comparative Examples 1 - 5 are shown in Table 1.

[0112] Table 1

[0113]

[0114]

[0115] In Table 1, " / " indicates no relevant data or steps; "same as 1" indicates the same data as in Example 1.

[0116] The organizational structures and mechanical properties of the products in Examples 1 to 2 and Comparative Examples 1 to 5 are shown in Table 2.

[0117] Table 2

[0118]

[0119] It can be seen from Tables 1 to 2 as follows:

[0120] (1) From Examples 1 to 2, it can be seen that for the Al-Zn-Mg-Cu series aluminum alloy plate provided by the present invention, by controlling the circular equivalent diameter of the η(MgZn2) phase size at the grain boundary to ≤0.2 μm and the circular equivalent diameter of the Al7Cu2Fe phase size to ≤0.8 μm, an aluminum alloy plate with excellent strength and toughness properties can be obtained, where the tensile strength ≥630 MPa, the elongation rate ≥14.3%, and the fracture toughness ≥34.2 MPa×m 1 / 2 ;

[0121] (2) The η(MgZn2) phase size at the grain boundary

[0122] Compared with Example 1, the Zn content in Example 1 is 7%, while the zinc content in Comparative Example 1 is as high as 8%. The excessive zinc content results in an excessively large η(MgZn2) phase size at the grain boundary, only ≤0.36 μm, causing the elongation rate and fracture toughness of the aluminum alloy plate in Comparative Example 1 to drop to 12.9% and 31.7 MPa×m respectively 1 / 2 .

[0123] Compared with Example 1, the secondary aging temperature in Example 1 is 175 °C, while the secondary aging temperature in Comparative Example 5 is relatively low, only 160 °C, resulting in a small degree of re-dissolution of the coarse η(MgZn2) phase and the size not being able to be sufficiently reduced, and the obtained η(MgZn2) phase size is too large, and both the strength and toughness decrease.

[0124] This shows that the present invention can control the η(MgZn2) phase size to ≤0.2 μm by regulating the component content and the secondary aging temperature, and improve the strength and toughness.

[0125] (3) The Al7Cu2Fe phase size at the grain boundary

[0126] Compared with Example 1 where pre-extrusion and secondary extrusion are combined and the extrusion speed is set at 0.5 m / min, in Comparative Example 3, only one extrusion is carried out, and the extrusion ratio is 30, resulting in an excessively large Al7Cu2Fe phase size, and the fracture toughness and strength drop to 29.2 MPa×m respectively 1 / 2And 12.1%; while in Comparative Example 4, although pre-extrusion and secondary extrusion were carried out, the extrusion speed was too fast, up to 1 m / min, resulting in too large Al7Cu2Fe phase size, and the toughness and strength also decreased. Thus, it is shown that by regulating the extrusion process, the present invention controls the Al7Cu2Fe phase size to ≤0.8 μm, improving the toughness and strength of the aluminum alloy plate.

[0127] (4) Component regulation

[0128] Compared with the Cu element content of 1.5% in Example 1, the Cu content in Comparative Example 2 is as high as 2.2%. Although the sizes of η(MgZn2) phase and Al7Cu2Fe phase at the final grain boundaries are appropriate, due to the too high Cu content, S(Al2CuMg) phase will be formed, resulting in the decrease of the toughness of the aluminum alloy plate.

[0129] 2.2 Containing Sc element and Zr element

[0130] Examples 3 to 14

[0131] The components and other relevant process parameters in Examples 3 to 14 are shown in Table 3.

[0132] Table 3

[0133]

[0134]

[0135] In Table 3, " / " means no relevant data or steps; "same as 1" means the same as the data in Example 1, and "same as 3" means the same as the data in Example 3.

[0136] The microstructures and mechanical properties of the products in Examples 3 to 14 are shown in Table 4. The schematic diagrams of the heterogeneous structures obtained in the following examples are as Figure 1 shown, and 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 convergence of single recrystallized grains; the recrystallized structure cluster 1 is uniformly distributed in the fibrous structure 2.

[0137] Table 4

[0138]

[0139]

[0140] It can be seen from Tables 3 to 4 as follows:

[0141] (1) It can be seen from Comprehensive Examples 3 to 15 that the aluminum alloy plate provided by the present invention further improves the fracture toughness, tensile strength and elongation of the aluminum alloy plate by additionally adding Sc and Zr on the basis of Examples 1 to 2, wherein the tensile strength ≥ 637 MPa, the fracture toughness ≥ 36.7 MPa×m 1 / 2 , and the elongation ≥ 16.4%; the present invention further forms a heterogeneous structure by preferably regulating process conditions and component compositions, further improving the fracture toughness, tensile strength and elongation of the aluminum alloy plate;

[0142] Comparing Example 3 with Examples 5 to 7, it can be seen that the component contents of Zr+Sc in Examples 5 to 7 are 0.2%, 0.24% and 0.26% respectively. The final elongation of the aluminum alloy plates obtained in Examples 5 and 6 is higher than that in Examples 3 and 7. This shows that the present invention improves the toughness of the aluminum alloy plate by preferably selecting 0.18 wt% ≤ Zr+Sc ≤ 0.26 wt%, and further preferably selecting 0.2 wt% ≤ Zr+Sc ≤ 0.24 wt% to further improve the toughness of the aluminum alloy plate.

[0143] (2) Recrystallized structure and its distribution in the heterogeneous structure

[0144] Compared with Example 3, the magnetic field strength in Example 8 is only 1×10 4 a / M, the magnetic field strength is 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 resulting in uneven distribution of the recrystallized structure of the product, with an elongation of only 18.1% and a fracture toughness of only 40.9 MPa×m 1 / 2 , lower than the elongation and fracture toughness in Example 3;

[0145] Compared with Example 3, the current frequency in Example 9 is only 12 Hz, resulting in uneven temperature field, uneven density and reduced number of homogenized primary phases, and further resulting in uneven distribution and reduced number of secondary phase depletion regions during the extrusion process, making the cluster distribution of the recrystallized structure uneven and the density decreasing. Finally, the fracture toughness and elongation are only 40.4 MPa×m 1 / 2 and 17.9% respectively;

[0146] Compared with Example 3, the typical product characteristics of Examples 8 and 9 are poor uniformity of the distribution of recrystallized structure clusters, with density deviations reaching 56 grains / mm 2 and 59 grains / mm 2 respectively, which makes the aluminum alloy plates in Examples 8 and 9 less excellent in strength and toughness than the aluminum alloy plate in Example 3 in terms of performance.

[0147] (3) Size of a single recrystallized grain

[0148] Compared with Example 3, the sum of the mass fractions of Zr + Sc in Example 4 is 0.16%. The doping of Zr + Sc in Example 4 is too little, resulting in a coarse single recrystallized grain size in the obtained heterogeneous structure, and the single recrystallized grain size reaches 15.9 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 3, when the single recrystallized grain size in Example 3 is finer, better strength and toughness can be obtained.

[0149] (4) Heterogeneous structure, fully recrystallized structure, and fibrous structure

[0150] Comparing Example 3 and Example 10, 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 3 containing a heterogeneous structure, its strength and toughness are significantly better than the fully recrystallized structure of Example 10.

[0151] Compared with Example 3 where solution treatment is carried out in two stages, with the first-stage solution temperature being 400 °C and the second-stage solution temperature being 480 °C, in Example 10, only the first-stage solution is set, and the solution temperature is 480 °C, resulting in static recrystallization and recrystallization growth during the high-temperature solution treatment process, forming a fully recrystallized structure, and only a decrease in toughness, tensile strength, and elongation. This shows that the heterogeneous structure formed by two-stage solution treatment in the present invention improves the toughness and strength of the aluminum alloy plate.

[0152] (5) Sizes of primary phase and secondary phase

[0153] Compared with the sum of the mass fractions of Zr + Sc in Example 3 being 0.18%, and the sum of the mass fractions of Zr + Sc in Example 13 being 0.28%; the doping of Zr + Sc in Example 13 is too much, resulting in a coarse primary Al3(Sc,Zr) phase (reaching 1.52 μm) generated during solidification, and the final fracture toughness drops to 38.1 MPa×m 1 / 2 , and the elongation also drops to 16.9%. This shows 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;

[0154] Compared with Example 3, when the total amount of Zr + Sc remains the same, the Zr / Sc in Example 14 is only 0.8, while the Zr / Sc in Example 15 is as high as 2.00; the aluminum alloy plate of Example 14 shows poor size stability of the secondary Al3(Sc,Zr) phase formed during the homogenization process, coarsens during the homogenization process, and ultimately leads to partial recrystallization of the fibrous structure during the solution treatment, 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 36.7 MPa×m 1 / 2 , and the elongation rate is only 16.4%; the product of Example 15 shows poor size stability of the primary Al3(Sc,Zr) phase formed during the solidification process, coarsens during the homogenization heat treatment, and 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. The overly large recrystallized structure clusters will preferentially deform and fracture as soft zones during the loading process, causing the strength to drop to 638 MPa in Example 15, and the elongation rate and fracture toughness also decrease significantly.

[0155] 3. Test methods

[0156] (1) Strength characterization: The room temperature mechanical properties were tested according to the method disclosed in GB / T228.1-2010 "Tensile Testing of Metallic Materials". The testing instrument was a ZWICK universal material testing machine, and the testing indicators were the tensile yield strength Rp0.2, the tensile strength Rm, and the elongation rate A;

[0157] (2) Toughness characterization: The plane strain fracture toughness of metallic materials was tested according to the method disclosed in GB / T4161-2007 "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;

[0158] (3) Phase size characterization: Five 20 mm×20 mm samples were taken. After grinding and polishing the longitudinal section, the size of the η(MgZn2) phase at the grain boundaries of the Al-Zn-Mg-Cu series aluminum alloy plate was measured using an electron microscope. The size of 20 phases was randomly measured for each sample, and the equivalent circle diameter was calculated based on the area value. The equivalent circle diameters of 100 phases were statistically analyzed to obtain the equivalent circle diameter of the η(MgZn2) phase at the grain boundaries.

[0159] Al7Cu2Fe phase size: Five 20 mm×20 mm samples were taken. After grinding and polishing the longitudinal section, the size of the Al7Cu2Fe phase was measured using an electron microscope. The size of 20 phases was randomly measured for each sample, and the sizes of 100 phases were statistically analyzed to obtain the size of the Al7Cu2Fe phase.

[0160] Size of Al3(Sc,Zr) phase: Five samples of 20mm x 20mm were taken. After grinding and polishing the longitudinal section, the size of the primary Al3(Sc,Zr) phase was measured using an electron microscope. For each sample, 10 phase particle sizes were randomly measured, and 50 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. For each sample, 20 phase particle sizes were randomly measured, 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 satisfy the normal distribution, 3δ of the primary phase ≤ 0.2μm, and 3δ of the secondary phase ≤ 12nm.

[0161] Size of recrystallized grain clusters: Five samples of 20mm × 20mm were taken. After grinding and polishing the longitudinal section, vibratory polishing was carried out, and then EBSD analysis was performed using an electron microscope. For each sample, 10 regions of 1mm x 1mm were randomly measured, 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 represents the typical value of the cluster size.

[0162] Size of recrystallized grain clusters = (A1 + A2 + A3 + … + An) / n

[0163] Distribution density of recrystallized grain clusters: Five samples of 20mm × 20mm were taken. After grinding and polishing the longitudinal section, vibratory polishing was carried out, and then EBSD analysis was performed using an electron microscope. For each sample, 10 regions of 1mm × 1mm were randomly measured, and the number of recrystallized grain clusters was statistically analyzed. Finally, the average value of the number Bi of recrystallized grain clusters in 50 regions was taken 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 regions and the average value was 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.

[0164] Distribution density of recrystallized grain clusters = ∑Bi / 50;

[0165] Uniformity index of distribution density of recrystallized grain clusters = max(|∑Bi / 50 - Bi|).

[0166] Size of a single recrystallized grain: Five samples of 20mm × 20mm were taken. After grinding and polishing the longitudinal section, vibratory polishing was carried out, and then EBSD analysis was performed using an electron microscope. For each sample, 10 recrystallized grain clusters were randomly measured, and the area of each recrystallized grain was statistically analyzed to obtain the average size of a single recrystallized grain.

[0167] The applicant declares that the above description is only a specific implementation mode 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 high-strength and high-toughness Al-Zn-Mg-Cu series aluminum alloy plate, characterized in that, The Al-Zn-Mg-Cu series aluminum alloy plate contains the following elements by mass percentage: Zn 6.0wt%-7.0wt%, 2.6 ≤ Zn / Mg ≤ 3.9, and Cu 1.5wt%-2.0wt%. The Al-Zn-Mg-Cu series aluminum alloy plate also contains Zr and Sc elements. By mass percentage, 0.86 ≤ Zr / Sc ≤ 1.86 and 0.1wt% ≤ Zr + Sc ≤ 0.26wt%. The balance is Al and inevitable impurities; The equivalent circle diameter of the η(MgZn2) phase at the grain boundary of the Al-Zn-Mg-Cu series aluminum alloy plate is ≤ 0.2μm; the equivalent circle diameter of the Al7Cu2Fe phase at the grain boundary of the Al-Zn-Mg-Cu series aluminum alloy plate is ≤ 0.8μm.

2. The Al-Zn-Mg-Cu series aluminum alloy plate according to claim 1, characterized in that, In the Al-Zn-Mg-Cu series aluminum alloy plate, by mass percentage, the content of Zn element is 6.6 - 6.8wt%.

3. The Al-Zn-Mg-Cu series aluminum alloy plate according to claim 1 or 2, wherein The Al-Zn-Mg-Cu series aluminum alloy plate also contains the following elements by mass percentage: Ti 0.01wt%-0.06wt%.

5. The Al-Zn-Mg-Cu series aluminum alloy plate according to claim 1, characterized in that The average size of the primary Al3(Sc,Zr) phase is ≤ 1.2μm; the average size of the secondary Al3(Sc,Zr) phase is ≤ 20nm; The Al-Zn-Mg-Cu series aluminum alloy plate includes a heterogeneous structure. 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 single recrystallized grains; the recrystallized structure clusters are distributed in the fibrous structure.

5. The Al-Zn-Mg-Cu series aluminum alloy plate according to claim 4, characterized in that, The recrystallized structure clusters are evenly distributed in the fibrous structure.

6. The Al-Zn-Mg-Cu series aluminum alloy plate according to claim 4, characterized in that, The distribution density of recrystallized grain clusters in the heterogeneous structure is 200 - 220 grains / mm 2 .

7. The Al-Zn-Mg-Cu series aluminum alloy plate according to claim 4, characterized in that, The size of the recrystallized tissue clusters in the heterogeneous structure is ≤200 μm 2 .

8. The Al-Zn-Mg-Cu series aluminum alloy plate according to claim 4, wherein The size of a single recrystallized grain in the heterogeneous structure is 7-12 μm 2 .

9. The Al-Zn-Mg-Cu series aluminum alloy plate according to claim 4, characterized in that, In the Al-Zn-Mg-Cu series aluminum alloy plate, by mass percentage, 0.18wt% ≤ Zr + Sc ≤ 0.26wt%.

10. The Al-Zn-Mg-Cu series aluminum alloy plate according to claim 9, wherein In the Al-Zn-Mg-Cu series aluminum alloy plate, by mass percentage, 0.2wt% ≤ Zr + Sc ≤ 0.24wt%.

11. A method for preparing an Al-Zn-Mg-Cu series aluminum alloy plate with high strength and high toughness as described in any one of claims 1 to 10, characterized in that, The preparation method includes the following steps: (1) According to the component ratio of the Al-Zn-Mg-Cu series aluminum alloy plate, the raw materials of the Al-Zn-Mg-Cu series aluminum alloy plate are melted and stirred, and then cast to obtain an ingot; (2) The ingot obtained in step (1) is subjected to secondary homogenization heat treatment to obtain a bar; (3) The bar obtained in step (2) is subjected to pre-extrusion and secondary extrusion to obtain a semi-finished product plate; (4) The semi-finished product plate obtained in step (3) is subjected to solution treatment to obtain a solution-treated plate; (5) The solution-treated plate obtained in step (4) is subjected to three-stage aging heat treatment to obtain the Al-Zn-Mg-Cu series aluminum alloy plate.

12. The preparation method according to claim 11, wherein, In step (3), the extrusion ratio of the pre-extrusion is 3 - 4; the extrusion ratio of the secondary extrusion is 10 - 12.

13. The preparation method according to claim 11, characterized in that, The extrusion speeds of the pre-extrusion and the secondary extrusion are each independently 0.1 - 0.5m / min.

14. The preparation method according to claim 12, wherein, In step (5), the secondary aging temperature of the three-stage aging heat treatment is 170-180 °C.

15. The preparation method according to claim 14, characterized in that, When the Al-Zn-Mg-Cu series aluminum alloy plate further includes Zr and Sc elements, the casting in step (1) is low-frequency electromagnetic casting.

16. The preparation method according to claim 15, characterized in that, The magnetic field strength of the low-frequency electromagnetic casting is 1.2×10 4 ~3.6×10 4 a / M, and the current frequency is 15~30 Hz.

17. The preparation method according to claim 14, characterized in that, When the Al-Zn-Mg-Cu series aluminum alloy plate further includes Zr and Sc elements, the solution treatment in step (4) includes a two-stage solution treatment.

18. The preparation method according to claim 17, wherein The first-stage solution temperature of the two-stage solution treatment is 350-400 °C, and the time is 2-4 h.

19. The preparation method according to claim 17, wherein, The second-stage solution temperature of the two-stage solution treatment is 460-480 °C, and the time is 1-2 h.

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