An ultra-high-strength, coarse-grain-free 7-series aluminum alloy and a method for preparing the same

Abstract

The application discloses a kind of superhigh strength, no coarse crystal 7 series aluminum alloy and preparation method thereof, belong to high-strength aluminum alloy material and its preparation processing technical field.It includes Zn 8.0%~11.5% by mass percentage;Mg 1.5%~3.2%;Cu 0.5%~1.8%;Er 0.05%~0.35%;Cr 0.05%~0.30%;Zr 0.05%~0.25%;Sc 0.05%~0.35%;Al and inevitable impurities, balance.The application introduces appropriate Sc in 7 series aluminum alloy, and cooperates with Zr, Er, Cr and other micro-alloy elements, forms heat stable L12-Al3 (Sc, Zr) dispersion phase, produces strong pinning effect to dislocation, subgrain boundary and migration grain boundary, to inhibit recrystallization and abnormal grain growth, obtain uniform structure without obvious surface coarse crystal layer.

Description

Technical Field

[0001] This invention belongs to the field of high-strength aluminum alloy materials and their preparation and processing technology, specifically relating to an ultra-high strength, coarse-grain-free 7-series aluminum alloy and its preparation method. Background Technology

[0002] 7-series aluminum alloys possess advantages such as low density and high specific strength / stiffness, making them important lightweight structural materials for high-end equipment in aerospace and other fields. These alloys typically achieve high strength by relying on elements such as Zn, Mg, and Cu to form high-density GP zones, η' phase, and η phase during the aging process. Their excellent comprehensive mechanical properties lead to their widespread application in various high-end load-bearing components.

[0003] Extruded profiles are one of the main applications of 7-series aluminum alloys in high-end equipment. Currently, extruded 7-series aluminum alloy profiles commonly suffer from a peripheral coarse grain layer (PCG) defect. During hot extrusion, the friction between the billet and the die causes the profile surface to undergo severe shear deformation, resulting in significant strain, deformation energy storage, and temperature gradients in the cross-section. These gradients drive rapid migration of surface grain boundaries and abnormal grain growth during subsequent solution treatment. The coarse grain layer causes inhomogeneity in the profile's cross-sectional microstructure and properties, reducing its plasticity, fatigue performance, corrosion resistance, and service reliability.

[0004] Existing technologies primarily suppress surface coarse grains through extrusion and heat treatment processes or alloy composition optimization. However, simply adjusting extrusion parameters is insufficient to eliminate the existing cross-sectional energy gradient. On the other hand, low-temperature or short-duration solution treatment leads to insufficient dissolution of alloying elements, weakening the subsequent aging strengthening effect, while increasing the solution temperature or extending the holding time exacerbates surface grain growth. While staged solution treatment can balance phase re-dissolution and coarse grain suppression, it is complex, time-consuming, and energy-intensive. Regarding composition control, existing technologies typically add elements such as Mn, Cr, and Zr to form dispersed phases, pinning grain boundaries and inhibiting recrystallization and grain growth. However, the size, number density, and thermal stability of these dispersed phases still fail to meet the microstructure stability requirements under high-temperature solution treatment conditions. Therefore, existing technologies cannot simultaneously achieve surface coarse grain suppression, cross-sectional microstructure homogenization, full solid solution of alloying elements, ultra-high strength, and efficient production of 7-series aluminum alloy profiles. There is an urgent need to develop a microstructure control method that can actively regulate the deformation energy storage state and grain boundary migration behavior of the profile surface, weaken the microstructure gradient of the profile cross-section, and coordinate with subsequent solid solution and aging strengthening processes. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide an ultra-high strength, coarse-grain-free 7-series aluminum alloy and its preparation method.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows.

[0007] In a first aspect, the present invention provides an ultra-high strength, coarse-grain-free 7-series aluminum alloy, comprising, by weight percentage, the following components: Zn: 8.0%~11.5%; Mg: 1.5%~3.2%; Cu: 0.5%~1.8%; Er: 0.05%~0.35%; Cr: 0.05%~0.30%; Zr: 0.05%~0.25%; Sc: 0.05%~0.35%; Al and unavoidable impurities: balance.

[0008] Preferably, the ultra-high strength, coarse-grain-free 7-series aluminum alloy is composed of the following components by mass percentage: Zn: 10%~11%; Mg: 2%~3%; Cu: 1.4%~1.7%; Er: 0.2%~0.3%; Cr: 0.1%~0.2%; Zr: 0.1%~0.2%; Sc: 0.2%~0.3%; Al and unavoidable impurities: balance.

[0009] Preferably, the ultra-high strength, coarse-grain-free 7-series aluminum alloy is composed of the following components by mass percentage: Zn: 10.81%; Mg: 2.81%; Cu: 1.51%; Er: 0.24%; Cr: 0.19%; Zr: 0.18%; Sc: 0.24%; Al and unavoidable impurities: balance.

[0010] A second aspect of the present invention provides a method for preparing the ultra-high strength, coarse-grain-free 7-series aluminum alloy described in the first aspect, comprising: The raw materials are prepared according to the target alloy composition, and then smelting, melt purification and casting are carried out in sequence to obtain ingots; The ingots were subjected to homogenization treatment, hot extrusion treatment, solution treatment, water quenching treatment, and aging treatment in sequence to obtain ultra-high strength, coarse grain-free 7-series aluminum alloys.

[0011] Preferably, the raw materials used in the formulation include aluminum ingots, Al-Cu master alloy, Al-Zr master alloy, Al-Sc master alloy, Al-Cr master alloy, Al-Er master alloy, metallic Zn, and metallic Mg.

[0012] Preferably, the melting temperature is 700-850℃.

[0013] Preferably, the homogenization treatment is performed at a temperature of 460-470℃ for 20-30 hours.

[0014] Preferably, the ingot has a size of Φ60-120 mm × 300 mm, the hot extrusion temperature is 380-420℃, the extrusion exit speed is 0.4-0.6 mm / s, and an extruded bar with a diameter of 10-25 mm is obtained after hot extrusion.

[0015] Preferably, the solution treatment is performed at a temperature of 470-480°C for 1-2 hours.

[0016] Preferably, the aging treatment is performed at a temperature of 110-130℃ for 18-22 hours.

[0017] The beneficial effects of this invention are as follows: This invention provides an ultra-high strength, coarse-grain-free 7-series aluminum alloy. It employs sc and Zr synergistic microalloying modification of the 7-series aluminum alloy, effectively solving the technical challenges of existing processes that cannot simultaneously achieve ultra-high strength and microstructure uniformity, and are difficult to eradicate the coarse-grained layer on the surface of profiles. Through the synergistic effect of sc and Zr in the alloy system and the coordinated control of hot working, solution treatment, water quenching, and aging processes, this invention can form a highly thermally stable L12-Al3(Sc, Zr) dispersed phase within the alloy. This dispersed phase effectively pins dislocations, subgrain boundaries, and migrating grain boundaries, fundamentally inhibiting recrystallization behavior and abnormal grain growth on the surface during hot extrusion and solution treatment, completely avoiding the formation of a coarse-grained layer on the outer periphery of the profile. This effectively solves the technical drawback of traditional processes that can only alleviate coarse-grained defects but cannot eradicate them. Based on this, this invention significantly improves the overall microstructure uniformity of extruded profiles and bars from the core to the surface, enabling the alloy to maintain a fine and stable deformation substructure after solution treatment, eliminating the grain size gradient in the cross-section, and significantly improving the overall microstructure stability of the alloy. Simultaneously, the synergistic modification of Sc and Zr can effectively optimize the spatial distribution of alloy precipitates, improving the problems of coarsening of precipitates, uneven phase structure, and inconsistent strengthening effects caused by differences in deformation energy storage and diffusion conditions between the surface and core of traditional 7-series aluminum alloys. This allows strengthening phases such as η' to precipitate uniformly and at high density throughout the entire cross-section of the profile, significantly reducing the inhomogeneity of the alloy's mechanical properties. Ultimately, this invention achieves a synergistic improvement in the ultra-high strength and uniformity of microstructure and properties of 7-series aluminum alloys. The alloy's tensile strength can reach over 800 MPa, and its yield strength can reach approximately 780 MPa. While significantly improving the matrix strength, it greatly reduces the differences in mechanical properties across the profile cross-section, simultaneously improving the material's plasticity and engineering service stability. This breakthrough overcomes the technical bottleneck of existing 7-series aluminum alloys, which struggle to achieve both high strength and homogeneity, meeting the high-quality service requirements of lightweight, high-strength components in high-end equipment in aerospace and other fields. Attached Figure Description

[0018] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0019] Figure 1 This is a process flow diagram of the present invention.

[0020] Figure 2 The mechanical properties of the aluminum alloys obtained in Example 1 and Comparative Example 1 of this invention are shown in the diagram. Among them, (a) is the Vickers hardness distribution curve of the two alloy extruded profiles from the core to the surface section at different locations, and (b) is the stress-strain tensile curve of the two alloy profiles obtained by sampling at the core, 1 / 4 position and surface.

[0021] Figure 3The images show the electron backscattering diffraction (EBSD) patterns of coarse-grained surfaces during the solid solution process in Embodiment 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0022] To address the problems of existing 7-series high-strength aluminum alloys, such as the formation of a coarse-grained surface layer, uneven microstructure between the center and surface, and significant spatial differences in mechanical properties during hot extrusion and solution treatment, this invention proposes an ultra-high-strength, coarse-grain-free 7-series aluminum alloy and its preparation method.

[0023] A first typical embodiment of the present invention provides an ultra-high strength, coarse-grain-free 7-series aluminum alloy, which is composed of the following components by weight percentage: Zn: 8.0%~11.5%; Mg: 1.5%~3.2%; Cu: 0.5%~1.8%; Er: 0.05%~0.35%; Cr: 0.05%~0.30%; Zr: 0.05%~0.25%; Sc: 0.05%~0.35%; Al and unavoidable impurities: balance.

[0024] A coarse-grained surface layer is a serious microstructural defect in extruded 7-series aluminum alloys. During hot extrusion, intense friction exists between the billet and the die, resulting in significant strain, strain rate, and temperature gradients along the cross-sectional direction of the profile. The surface region typically experiences stronger shear deformation and higher deformation storage energy, making it prone to rapid grain boundary migration and abnormal grain growth during subsequent solution treatment or high-temperature holding. The formation of a coarse-grained layer not only disrupts the microstructural continuity between the material's surface and core but also leads to significant differences in mechanical properties along the cross-sectional direction, reducing the reliability of the component during service.

[0025] This invention introduces an appropriate amount of Sc element into 7-series aluminum alloys, which works synergistically with microalloying elements such as Zr, Er, and Cr to form a thermally stable L12-Al3(Sc, Zr) dispersed phase. This phase exerts a strong pinning effect on dislocations, subgrain boundaries, and migrating grain boundaries, thereby inhibiting recrystallization and aberrant grain growth, resulting in a uniform microstructure without a surface coarse-grained layer. Simultaneously, Sc element also promotes the homogenization of precipitation behavior from the center to the surface region, enabling the alloy to maintain ultra-high strength while achieving more stable plasticity and lower position dependence.

[0026] In this invention, the ultra-high strength, coarse-grain-free 7-series aluminum alloy is composed of the following components by mass percentage: Zn: 10%~11%; Mg: 2%~3%; Cu: 1.4%~1.7%; Er: 0.2%~0.3%; Cr: 0.1%~0.2%; Zr: 0.1%~0.2%; Sc: 0.2%~0.3%; Al and unavoidable impurities: balance.

[0027] In this invention, the ultra-high strength, coarse-grain-free 7-series aluminum alloy is composed of the following components by mass percentage: Zn: 10.81%; Mg: 2.81%; Cu: 1.51%; Er: 0.24%; Cr: 0.19%; Zr: 0.18%; Sc: 0.24%; Al and unavoidable impurities: balance.

[0028] The 7-series aluminum alloy provided by this invention has no obvious coarse grain layer on the surface, has a uniform structure and ultra-high strength, with a tensile strength of up to 800 MPa and a yield strength of up to 780 MPa. It is suitable for load-bearing structural components in high-end equipment in aerospace, rail transportation, new energy and other fields that require high specific strength and structural stability.

[0029] A second typical embodiment of the present invention provides a method for preparing the aforementioned ultra-high strength, coarse-grain-free 7-series aluminum alloy, comprising: The raw materials are prepared according to the target alloy composition, and then smelting, melt purification and casting are carried out in sequence to obtain ingots; The ingots were subjected to homogenization treatment, hot extrusion treatment, solution treatment, water quenching treatment, and aging treatment in sequence to obtain ultra-high strength, coarse grain-free 7-series aluminum alloys.

[0030] This invention successfully prepared a 7-series aluminum alloy with no surface coarse grain layer, uniform microstructure, and ultra-high strength by synergistically controlling microalloying composition design with hot working, solution treatment, water quenching, and aging processes. The resulting aluminum alloy is suitable for load-bearing structural components in high-end equipment in aerospace, rail transportation, and new energy fields that require high specific strength and microstructural stability.

[0031] In this invention, the raw materials used in the formulation include aluminum ingots, Al-Cu master alloy, Al-Zr master alloy, Al-Sc master alloy, Al-Cr master alloy, Al-Er master alloy, metallic Zn, and metallic Mg.

[0032] According to the above alloy composition, the alloy raw materials are weighed and then successively smelted, purified, and cast in mold or semi-continuously to obtain aluminum alloy ingots, which are then demolded and air-cooled.

[0033] In this invention, after smelting and melt purification (i.e., slag removal), the melt is poured into a metal mold preheated to 400±5℃ to obtain an aluminum alloy ingot with dimensions of Φ60-120 mm × 300 mm. The ingot is then subjected to homogenization treatment. The homogenization treatment temperature is 460-470℃, specifically 460℃, 461℃, 462℃, 463℃, 464℃, 465℃, 466℃, 467℃, 468℃, 469℃, 470℃, or any range of two values, preferably 465℃; the time is 20-30 h, specifically 20 h, 21 h, 22 h, 23 h, 24 h, 25 h, 26 h, 27 h, 28 h, 29 h, 30 h, or any range of two values, preferably 24 h. After homogenization, the ingot is air-cooled to room temperature. During the homogenization process, Sc and Zr elements form a thermally stable Al3(Sc, Zr) dispersed phase in the aluminum matrix, providing an organizational basis for grain boundary pinning during subsequent hot extrusion and solution treatment.

[0034] In this invention, the homogenized ingot is subjected to hot extrusion treatment.

[0035] The hot extrusion temperature is 380-420℃, specifically 380℃, 390℃, 400℃, 410℃, 420℃, or any combination of two values, preferably 400℃. The extrusion exit speed is 0.4-0.6 mm / s, and extruded bars with a diameter of 10-25 mm are obtained after hot extrusion. Specifically, before extrusion, the ingot, die, and extrusion cylinder are all preheated to 400℃. The extrusion process can be carried out using a 500-ton horizontal extrusion press, with an extrusion exit speed of 0.5 mm / s, resulting in extruded bars with a diameter of 15 mm. During hot extrusion, the billet is subjected to significant thermo-mechanical coupling, and different degrees of deformation structures are formed in the material center, 1 / 4 radius position, and surface area. The Al3(Sc, Zr) dispersed phase formed in the Sc-containing alloy can pin dislocations and subgrain boundaries during extrusion, reduce the degree of dynamic recrystallization, and reduce the grain size difference from the center to the surface area.

[0036] In this invention, the hot-extruded aluminum alloy undergoes solution treatment at a temperature of 470-480℃, specifically 470℃, 471℃, 472℃, 473℃, 474℃, 475℃, 476℃, 477℃, 478℃, 479℃, 480℃, or any range of two values, preferably 475℃; the treatment time is 1-2 hours, specifically 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2 hours, or any range of two values, preferably 1.5 hours. Water quenching is performed immediately after solution treatment. During the solid solution process, the comparative alloy without Sc exhibited significant abnormal grain growth on its surface and formed a coarse-grained surface layer; while the alloy containing Sc, due to the presence of thermally stable Al3(Sc, Zr) dispersed phases, was able to continuously pin the migrating grain boundaries, inhibiting static recrystallization and subsequent grain growth, thereby avoiding the formation of a continuous coarse-grained surface layer.

[0037] In this invention, the aluminum alloy after solution quenching is subjected to peak aging treatment. The aging treatment temperature is 110-130℃, specifically 110℃, 115℃, 120℃, 125℃, 130℃, or any combination of two values, preferably 120℃; the aging time is 18-22 h, specifically 18, 19, 20, 21, 22 h, preferably 20 h. For the comparative alloy without Sc, the aging regime is 120℃ × 28 h. After aging treatment, a high-density nano-η' strengthening phase is formed in the alloy matrix. Compared with the alloy without Sc, the alloy containing Sc forms a more uniform fine η' precipitate in both the central and surface regions, which can suppress the accelerated coarsening of the precipitate in the surface region and the local transformation of η' to the equilibrium η phase, thereby improving the uniformity of the precipitate structure and the uniformity of mechanical properties in the cross-sectional direction.

[0038] After processing using the above-described method, the Sc-containing aluminum alloy of this invention exhibits a multi-scale synergistic strengthening structure: the matrix contains high-density fine η' strengthening precipitates, and thermally stable, coherent L12-Al3(Sc, Zr) dispersed phases are distributed within the grains. Relatively stable grain boundary precipitates and narrow grain boundary precipitate-free zones are formed near the grain boundaries. The Al3(Sc, Zr) dispersed phases are uniformly distributed in the central and surface regions, with average sizes of approximately 19.8 nm and 21.6 nm, respectively, indicating that these dispersed phases possess good thermal stability and size uniformity at different locations. This microstructure prevents the formation of a coarse-grained surface layer during hot extrusion and solution treatment, and maintains a highly uniform distribution of precipitates and mechanical properties from the center to the surface region after artificial aging.

[0039] The effects of this invention are as follows: 1) This invention achieves the formation of a thermally stable L12-Al3(Sc,Zr) dispersed phase in 7-series aluminum alloys through synergistic microalloying of Sc and Zr. This dispersed phase can exert a strong pinning effect on dislocations, subgrain boundaries, and migrating grain boundaries, significantly inhibiting recrystallization and abnormal grain growth during hot extrusion and solid solution processes, thereby effectively preventing the formation of a coarse-grained surface layer.

[0040] 2) This invention can significantly improve the microstructure uniformity from the center to the surface region of extruded profiles or bars. Compared with the control group without Sc, the Sc-containing alloy maintains a fine and stable deformation substructure during the solution treatment process, and no obvious coarse grain layer is formed on the surface. The grain size gradient is significantly reduced, thereby improving the overall microstructure stability of the component.

[0041] 3) This invention can improve the spatial distribution uniformity of precipitates. In Sc-free alloys, due to differences in deformation storage energy and diffusion conditions, the precipitates in the surface region are prone to coarsening, and may even transform from the η' phase to the η phase, resulting in different strengthening effects between the center and the surface. In Sc-containing alloys, the precipitates maintain a higher density and more uniform distribution in both the center and the surface, thereby reducing the differences in position-dependent strengthening.

[0042] 4) This invention achieves a synergistic improvement in ultra-high strength and microstructure uniformity. The alloy in the example exhibits a tensile strength exceeding 800 MPa and a yield strength of 780 MPa, while significantly reducing the difference in tensile strength between different locations, demonstrating excellent uniformity in cross-sectional mechanical properties. Compared to the comparative example, this invention not only improves strength but also enhances plasticity and engineering stability.

[0043] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0044] Example 1 This embodiment provides an ultra-high strength, coarse-grain-free 7-series aluminum alloy, which is formulated by mass percentage with the following composition: Zn 10.0%, Mg 2.8%, Cu 1.5%, Zr 0.18%, Er 0.26%, Cr 0.20%, Sc 0.24%, with the balance being Al and unavoidable impurities.

[0045] The preparation process of this 7-series aluminum alloy is as follows: 1) Weigh high-purity aluminum, industrial-pure zinc, industrial-pure magnesium, aluminum-copper master alloy, aluminum-zirconium master alloy, aluminum-erbium master alloy, aluminum-chromium master alloy, and aluminum-scandium master alloy according to the above mass percentages, and obtain Al-Zn-Mg-Cu-Er-Cr-Zr-Sc alloy ingots through smelting, refining, degassing, slag removal, and casting.

[0046] After smelting and slag removal, the melt is poured into a metal mold preheated to 400±5℃ to obtain an aluminum alloy ingot with dimensions of Φ60 mm × 300 mm.

[0047] 2) The aluminum alloy ingot was homogenized to make the alloy elements more uniformly distributed and to promote the formation of fine and dispersed Al3(Sc, Zr) phases of Sc and Zr. The homogenization temperature was 465℃, the holding time was 24 h, and the ingot was air-cooled to room temperature after homogenization. After homogenization, the ingot was peeled off.

[0048] 3) Hot extrusion of the homogenized ingot. Before extrusion, the ingot, die, and extrusion cylinder were preheated to 400℃. The extrusion process was carried out using a 500-ton horizontal extrusion press with an extrusion exit speed of 0.5 mm / s, resulting in extruded bars with a diameter of 15 mm. During the extrusion process, the Al3(Sc, Zr) dispersed phase remained stable and pinned the dislocations and subgrain boundaries formed during deformation, thereby reducing the degree of dynamic recrystallization and improving the uniformity of the extruded microstructure.

[0049] 4) The extruded alloy was solution treated at 475℃ for 1.5 h, followed by rapid water quenching. After solution treatment, no obvious continuous coarse grain layer was observed on the surface of the Sc-containing alloy, indicating that the present invention can effectively suppress abnormal grain growth on the surface during the solution treatment process.

[0050] 5) The alloy after solution quenching was subjected to artificial aging treatment at 120℃ for 20 h to form high-density fine strengthening precipitates in the matrix. This resulted in an ultra-high strength 7-series aluminum alloy with no obvious coarse grain layer on the surface and excellent uniformity of microstructure from the center to the surface.

[0051] Example 2 This embodiment provides an ultra-high strength, coarse-grain-free 7-series aluminum alloy, which is formulated by mass percentage with the following composition: Zn 8%, Mg 1.5%, Cu 0.5%, Zr 0.05%, Er 0.05%, Cr 0.05%, Sc 0.05%, with the balance being Al and unavoidable impurities.

[0052] The preparation process of this 7-series aluminum alloy is completely consistent with that of Example 1.

[0053] Ultimately, a 7-series aluminum alloy with ultra-high strength, no obvious coarse grain layer on the surface, and good uniformity of microstructure from the center to the surface was obtained, but its strength was slightly lower than that of Example 1, while its elongation was slightly higher than that of Example 1.

[0054] Example 3 This embodiment provides an ultra-high strength, coarse-grain-free 7-series aluminum alloy, which is formulated by mass percentage with the following composition: Zn 11.5%, Mg 3.2%, Cu 1.8%, Zr 0.25%, Er 0.35%, Cr 0.30%, Sc 0.35%, with the balance being Al and unavoidable impurities.

[0055] The preparation process of this 7-series aluminum alloy is completely consistent with that of Example 1.

[0056] Ultimately, a 7-series aluminum alloy with ultra-high strength, no obvious coarse grain layer on the surface, and good uniformity of microstructure from the center to the surface was obtained, but its strength was slightly higher than that of Example 1, while its elongation was slightly lower than that of Example 1.

[0057] Comparative Example 1 This comparative example provides a Sc-free 7-series aluminum alloy, formulated by mass percentage with the following composition: Zn 10.57%, Mg 2.76%, Cu 1.56%, Er 0.26%, Cr 0.21%, Zr 0.16%, with the balance being Al and unavoidable impurities.

[0058] By comparing the differences in microstructure and mechanical properties between alloys containing and without Sc in the extruded, solution-treated, and aged states, this study demonstrates the role of Sc addition in suppressing surface coarse grains, improving microstructure uniformity, and enhancing mechanical properties.

[0059] The preparation method differs from that of Example 1 in that the aging period is 120℃×28 h, while the remaining steps are completely consistent with those of Example 1.

[0060] The microstructure and properties of the alloys obtained in the examples and comparative examples were tested according to the method of GB / T 16865-2023, and the test results are shown in Table 1.

[0061] Table 1 Summary of alloy microstructure and property test results obtained from the examples and comparative examples

[0062] Comparative Example 1 used a 7-series aluminum alloy without Sc, whose main composition was similar to that of Example 1, but without the addition of Sc. After undergoing the same melting, casting, homogenization, hot extrusion, solution treatment, and aging, the comparative example was prone to abnormal grain growth in the surface region during the solution treatment process, forming a distinct coarse-grained layer. As the solution treatment time increased, the coarse-grained layer continued to extend from the surface to the interior, resulting in a significant difference between the surface and core microstructure.

[0063] Compared to Comparative Example 1, the Sc-containing alloy obtained in Example 1 of this invention did not form a significant coarse-grained layer under the same solid solution conditions; the surface layer still retained stable fine grains and substructures. This is because the Al3(Sc, Zr) dispersed phase has excellent thermal stability, is not easily dissolved during high-temperature solid solution treatment, and can continuously provide grain boundary pinning forces, inhibiting rapid grain boundary migration and abnormal grain growth. Therefore, this invention can avoid surface coarse-grained defects from the root cause of microstructure evolution.

[0064] Regarding precipitation behavior, in Comparative Example 1, the precipitated phase on the surface is more prone to coarsening than in the central region, and in some areas even a balanced η phase is formed, resulting in uneven strengthening effect along the cross-sectional direction. In the Sc-containing aluminum alloy obtained in Example 1 of this invention, both the central and surface regions exhibit a more uniform, fine, and high-density precipitated phase distribution, indicating that the addition of Sc can effectively regulate the local solute distribution and precipitation behavior, thereby improving the uniformity of mechanical properties from the center to the surface region.

[0065] Mechanical property test results show that the aluminum alloy obtained in Comparative Example 1 exhibits significant position dependence, with substantial differences in hardness and tensile strength between the surface and central regions. In contrast, the Sc-containing aluminum alloy obtained in Example 1 of this invention demonstrates a higher and more stable strength level, with a significantly reduced strength difference between different locations. The aluminum alloy obtained in Example 1 achieves an average tensile strength of 805 MPa and an average yield strength of 780 MPa, while also exhibiting a significantly improved elongation. This indicates that this invention achieves synergistic optimization of ultra-high strength, absence of coarse grains, and uniform microstructure.

[0066] Comparative Example 2 This comparative example provides a coarse-grain-free 7-series aluminum alloy, formulated by mass percentage with the following composition: Zn 10.0%, Mg 2.8%, Cu 1.5%, Zr 0.18%, Er 0.26%, Cr 0.20%, Sc 0.38%, with the balance being Al and unavoidable impurities.

[0067] The preparation process of this 7-series aluminum alloy is completely consistent with that of Example 1.

[0068] The aluminum alloy obtained in Comparative Example 2 has no coarse grain layer on the surface and its microstructure is uniform from the surface to the center. However, due to the excessive Sc content, coarse primary Al3Sc and Al3(Sc, Zr) phases are formed in the 7-series aluminum alloy during solidification. These phases are difficult to dissolve fully in subsequent homogenization and solution treatment. They not only consume Sc, Zr and Cu in the matrix and reduce the precipitation potential of fine coherent Al3(Sc, Zr) dispersed phases and η strengthening phases, but also cause stress concentration, interface debonding and microcrack initiation during deformation. This reduces the plasticity, fracture toughness, fatigue performance and hot working stability of the alloy, and may aggravate microstructure inhomogeneity and processing cracking.

[0069] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-strength, coarse-grain-free 7-series aluminum alloy, characterized in that, It consists of the following components by weight percentage: Zn: 8.0%~11.5%; Mg: 1.5%~3.2%; Cu: 0.5%~1.8%; Er：0.05%~0.35%； Cr：0.05%~0.30%； Zr：0.05%~0.25%； Sc: 0.05%~0.35%; Al and unavoidable impurities: balance.

2. The ultra-high strength, coarse-grain-free 7-series aluminum alloy as described in claim 1, characterized in that, It consists of the following components by weight percentage: Zn: 10%~11%; Mg: 2%~3%; Cu: 1.4%~1.7%; Er：0.2%~0.3%； Cr：0.1%~0.2%； Zr：0.1%~0.2%； Sc: 0.2%~0.3%; Al and unavoidable impurities: balance.

3. The ultra-high strength, coarse-grain-free 7-series aluminum alloy as described in claim 1, characterized in that, By weight percentage, it consists of the following components composition: Zn: 10.81%; Mg: 2.81%; Cu: 1.51%; Er：0.24%； Cr：0.19%； Zr：0.18%； Sc: 0.24%; Al and unavoidable impurities: balance.

4. A method for preparing the ultra-high strength, coarse-grain-free 7-series aluminum alloy according to any one of claims 1-3, characterized in that, include: The raw materials are prepared according to the target alloy composition, and then smelting, melt purification and casting are carried out in sequence to obtain ingots; The ingots were subjected to homogenization treatment, hot extrusion treatment, solution treatment, water quenching treatment, and aging treatment in sequence to obtain ultra-high strength, coarse grain-free 7-series aluminum alloys.

5. The preparation method according to claim 4, characterized in that, The raw materials used in the formulation include aluminum ingots, Al-Cu master alloy, Al-Zr master alloy, Al-Sc master alloy, Al-Cr master alloy, Al-Er master alloy, metallic Zn, and metallic Mg.

6. The preparation method according to claim 4, characterized in that, The melting temperature is 700-850℃.

7. The preparation method according to claim 4, characterized in that, The homogenization process is carried out at a temperature of 460-470℃ for 20-30 hours.

8. The preparation method according to claim 4, characterized in that, The ingot has dimensions of Φ60-120 mm × 300 mm, the hot extrusion temperature is 380-420℃, the extrusion exit speed is 0.4-0.6 mm / s, and the extruded bar with a diameter of 10-25 mm is obtained after hot extrusion.

9. The preparation method according to claim 4, characterized in that, The solution treatment is performed at a temperature of 470-480℃ for 1-2 hours.

10. The preparation method according to claim 4, characterized in that, The aging treatment is performed at a temperature of 110-130℃ for 18-22 hours.

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