A heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys

Abstract

This invention provides a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance, relating to the technical field of aluminum alloy preparation. The heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance includes alloy melting, homogenization treatment, thermoplastic deformation, solution treatment, natural aging pretreatment, and single-stage aging treatment. After treatment, the number density of intragranular precipitates in the alloy microstructure increases by more than 15%, and their distribution is more uniform; the size of grain boundary precipitates is reduced to less than half of that in the peak aging state, and the presence of precipitate-free zones (PFZs) at grain boundaries is almost unobservable. This results in a tensile strength higher than that of the traditional T6 peak aging process, while reducing the stress corrosion susceptibility factor by more than 40%. It is suitable for applications in aerospace, transportation, and marine engineering fields that require lightweight and long-life high-end equipment.

Description

Technical Field

[0001] This invention relates to the technical field of aluminum alloy preparation, and in particular to a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance, which can meet the requirements of lightweight and long service life for high-end equipment in aerospace, transportation and marine engineering fields. Background Technology

[0002] Al-Zn-Mg-Cu alloys are key structural materials in the manufacturing of high-end equipment in modern aerospace, transportation, and marine engineering due to their excellent specific strength, corrosion resistance, and good machinability. Among these, the alloy's strength and stress corrosion resistance are crucial indicators determining the lightweight nature and service life of high-end equipment. Achieving simultaneous improvement in both strength and stress corrosion resistance has always been a core issue in the research and development of high-end aluminum alloys.

[0003] However, there is often a trade-off between strength and stress corrosion resistance. Peak aging treatment can produce fine and dispersed intragranular precipitates, which is beneficial to improving the strength of the alloy, but it will lead to phenomena such as grain boundary element segregation and grain boundary PFZ (a region without precipitates), resulting in a decrease in stress corrosion resistance [Shi Feng, Zhang Zhichao, Wang Xu, et al. Study on aging treatment process of 7XXX series aluminum alloys [J]. Hot Working Technology, 2017, 46(2): 6-10.]; Over-aging and re-aging treatment can promote the discontinuous distribution of grain boundary precipitates and improve the stress corrosion resistance of the alloy, but it will increase the size of intragranular precipitates, reduce the aging strengthening effect, and lead to a decrease in alloy strength [Wen Kai. Study on the influence of Zn / Mg ratio on the microstructure and properties of high Zn content Al-Zn-Mg-Cu alloy [D]. Beijing: Beijing General Research Institute of Nonferrous Metals, 2017.]. In addition, by adopting a high-temperature pre-precipitation method, the precipitates preferentially precipitate at the grain boundaries and coarsen, which can improve the stress corrosion resistance of the alloy. However, this method will also reduce the supersaturation of the alloy after solution treatment, reduce the volume fraction of subsequent precipitates, and lead to a decrease in strength [Huang Lanping, Chen Kanghua, Li Song, et al. Effect of high-temperature pre-precipitation on stress corrosion fracture of Al-Zn-Mg alloy plates [J]. Rare Metals Materials and Engineering, 2006, 12: 1943-1948.].

[0004] Chinese patent CN104894496A discloses a heat treatment method for Al-Zn-Mg-Cu aluminum alloys. This method employs a bimodal aging process using multi-stage strengthening solid solution combined with high-energy high-frequency ultrasonic grain refinement technology. This process can effectively shorten the bimodal aging time and ensure that the alloy exhibits high strength, high toughness, and high resistance to SCC at the second peak aging time. However, this method has drawbacks, such as uneven energy distribution in large and complex components, difficulty in determining and controlling the bimodal aging time, high energy consumption, and uneven component performance.

[0005] Chinese patent CN110438377A discloses a high-strength, stress-corrosion-resistant Al-Zn-Mg-Cu alloy and its preparation method. The preparation method involves gravity casting, three-stage homogenization treatment, deformation treatment, two-stage solution treatment, and three-stage aging treatment. However, this method suffers from complex heat treatment processes, high operational difficulty, and high costs. Furthermore, although the prepared material possesses good stress corrosion resistance, its tensile strength is less than 450 MPa, which is insufficient to meet the strength requirements of high-end equipment.

[0006] Therefore, the aforementioned typical heat treatment processes are all insufficient to effectively and simultaneously improve the strength and stress corrosion resistance of high-end aluminum alloys with complex compositions. Summary of the Invention

[0007] To address the technical problem in existing technologies where improving the stress corrosion resistance of Al-Zn-Mg-Cu alloys for large-scale industrial production and application often leads to a decrease in strength, this invention proposes a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves both strength and stress corrosion resistance, thereby resolving the aforementioned technical problems. The technical solution is as follows:

[0008] A heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys includes alloy melting, homogenization treatment, thermoplastic deformation, solution treatment, natural aging pretreatment, and single-stage aging treatment.

[0009] Optionally, the heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys specifically includes the following steps:

[0010] S1. Alloy smelting: Weigh the raw materials of Al-Zn-Mg-Cu alloy according to industrial production conditions, smelt and cast to obtain alloy ingots;

[0011] S2. Homogenization treatment: The alloy ingot of S1 is subjected to a two-stage heating and holding homogenization treatment, and then air-cooled to room temperature after being taken out of the furnace to obtain a homogenized alloy ingot.

[0012] S3, thermoplastic deformation: The ingot after homogenization treatment of S2 is hot rolled or hot extruded to obtain alloy billet;

[0013] S4. Solution treatment: The alloy billet of S3 is subjected to a two-stage heating and holding solution treatment, followed by quenching in water at room temperature to obtain a solution-treated alloy billet.

[0014] S5. Natural aging pretreatment: The alloy billet with solid solution in S4 is subjected to natural aging pretreatment to obtain a naturally aged alloy billet.

[0015] S6. Single-stage aging treatment: The naturally aged alloy billet of S5 is subjected to a single-stage aging treatment to obtain the final state sample of the Al-Zn-Mg-Cu alloy.

[0016] Optionally, the composition of the Al-Zn-Mg-Cu alloy in S1, by mass percentage, is as follows: Zn 6.0%-12.0%, Mg 1.5%-3.0%, Cu 1.0%-2.8%, and microalloying elements containing Cr, Mn, Zr, and Ti are added as needed, with the total amount of microalloying elements <0.5wt%, the total amount of impurity elements containing Fe and Si <0.3wt%, and the balance being Al and unavoidable impurity elements.

[0017] Optionally, the homogenization treatment of the two-stage heating and holding in S2 is to first heat to 380-420℃ and hold for 10-30h, and then heat to 450-480℃ and hold for 10-30h.

[0018] Optionally, the heating temperature of the aluminum alloy ingot in S3 is 380-420℃, and the heating time is 2-4h; the extrusion ratio is 7-50 during extrusion, and the total deformation during hot rolling is 50%-95%.

[0019] Optionally, the solution treatment in S4 involves first heating to 400-450℃ and holding for 1-2 hours; then heating to 450-480℃ and holding for 1-2 hours.

[0020] Optionally, the natural aging pretreatment in S5 involves storing the product at room temperature (10-35℃) for 4-14 days.

[0021] Optionally, the single-stage aging treatment in S6 involves heating to 110-130℃ and holding for 8-36 hours.

[0022] Optionally, compared with the T6 peak-aged state, the final state sample of the Al-Zn-Mg-Cu alloy treated by the present invention shows that the number density of intragranular precipitates in the alloy microstructure increases by more than 15%, and the distribution is more uniform. Among them, the spherical GPI region accounts for 20%-30%, the disc-shaped GPII region accounts for 40%-50%, the disc-shaped η' phase accounts for 20%-25%, and the disc-shaped η phase accounts for less than 5%.

[0023] Optionally, the final-state samples of the Al-Zn-Mg-Cu alloy in S6 have a tensile strength (UTS) of 530-800 MPa, a yield strength (YS) of 470-750 MPa, an elongation after fracture (EL) of 9.5%-20.0%, and a stress corrosion susceptibility factor (ISSRT) of 15%-25%; correspondingly, the peak-aged alloy has a tensile strength (UTS) of 520-780 MPa, a yield strength (YS) of 450-740 MPa, an elongation after fracture (EL) of 9.0%-18.0%, and a stress corrosion susceptibility factor (ISSRT) of 30%-50%.

[0024] Optionally, the heat treatment process of the present invention significantly reduces grain boundary segregation and the size of grain boundary non-precipitation zones while ensuring that the intragranular precipitates do not coarsen. This achieves a tensile strength higher than that of the conventional T6 peak aging process, while reducing the stress corrosion susceptibility factor by more than 40%.

[0025] Technical principle of the invention:

[0026] (1) The present invention adds a natural aging pretreatment before the single-stage aging process. A large number of fine and dispersed GPI phases are formed during the natural aging pretreatment stage. These GPI phases serve as precursors for the subsequent GPII and η′ phases, increasing the number density and distribution uniformity of intragranular precipitates formed in the subsequent single-stage aging, thereby improving the tensile strength of the alloy;

[0027] (2) In the natural aging pretreatment stage, a stable GPI phase will be formed in advance near the grain boundary, which will weaken the segregation of grain boundary elements in the subsequent single-stage aging process, reduce or even eliminate the grain boundary non-precipitation zone (PFZ), and also reduce the potential difference between the grain boundary and the grain, which helps to improve the stress corrosion resistance of the alloy.

[0028] (3) It is worth noting that short natural aging pretreatment will cause the formed GPI phase to dissolve during the artificial aging stage, which will reduce the number density of precipitates and is not conducive to the improvement of strength; while excessive natural aging treatment will cause the grain boundary precipitates to coarsen prematurely, hindering the improvement of stress corrosion resistance.

[0029] The above technical solution has at least the following advantages compared with the existing technology:

[0030] The present invention proposes a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance, which can solve the technical problem that improving the stress corrosion resistance of Al-Zn-Mg-Cu alloys in existing large-scale industrial production and application often leads to a decrease in strength.

[0031] This invention addresses the challenge of simultaneously improving strength and stress corrosion resistance in existing heat treatment processes for Al-Zn-Mg-Cu alloys. It proposes a novel heat treatment process that ensures that the alloy's tensile strength is higher than that of the traditional T6 peak aging process while reducing the stress corrosion susceptibility factor by more than 40%.

[0032] This invention employs a conventional casting → two-stage heating and holding homogenization treatment → hot extrusion / hot rolling → two-stage heating and holding solution treatment → natural aging pretreatment → single-stage aging treatment process. Compared with traditional over-aging and re-aging treatment processes, this invention ensures simultaneous improvement in strength and stress corrosion resistance while having a simpler process, lower economic cost, and is suitable for large-scale industrial production.

[0033] This invention employs natural aging pretreatment, which is beneficial to the actual production process. It reduces the impact of uncertain steps such as parking and transportation in industrial production on alloy properties, making the peak response of the single-stage aging stage more controllable, the performance distribution of the final product more concentrated, the batch-to-batch consistency stronger, and the yield rate higher. It is suitable for the manufacturing of aviation and rail transportation equipment with high safety requirements.

[0034] In summary, compared with other traditional methods, the method of the present invention can synergistically improve the strength and stress corrosion resistance of the alloy by performing two-stage heating and holding homogenization heat treatment, thermoplastic deformation, two-stage heating and holding solution treatment, natural aging pretreatment, and single-stage aging treatment on the smelted ingot. The method is simple to operate, environmentally friendly, low in cost, short in process, and highly efficient, which is conducive to large-scale industrial production and promotion. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a morphology diagram of the intragranular precipitates of an Al-9.80Zn-2.23Mg-1.38Cu-0.11Cr-0.10Zr alloy after heat treatment, which simultaneously improves the strength and stress corrosion resistance of an Al-Zn-Mg-Cu alloy according to Embodiment 2 of the present invention.

[0037] Figure 2 The image shows the intragranular precipitate morphology of the Al-9.80Zn-2.23Mg-1.38Cu-0.11Cr-0.10Zr alloy after T6 process treatment.

[0038] Figure 3This is a grain boundary precipitate morphology diagram of an Al-9.80Zn-2.23Mg-1.38Cu-0.11Cr-0.10Zr alloy after heat treatment process to simultaneously improve the strength and stress corrosion resistance of an Al-Zn-Mg-Cu alloy according to Embodiment 2 of the present invention.

[0039] Figure 4 This is a grain boundary precipitate morphology diagram of the Al-9.80Zn-2.23Mg-1.38Cu-0.11Cr-0.10Zr alloy after T6 process treatment. Detailed Implementation

[0040] The technical solution of the present invention will now be described with reference to the accompanying drawings.

[0041] In embodiments of the present invention, words such as "exemplarily," "for example," etc., are used to indicate that something is an example, illustration, or description. Any embodiment or design described as "exemplary" in the present invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of the word "exemplary" is intended to present the concept in a concrete manner. Furthermore, in embodiments of the present invention, the meaning expressed by "and / or" can be both, or either one.

[0042] In the embodiments of the present invention, the terms "image" and "picture" may sometimes be used interchangeably. It should be noted that when the distinction is not emphasized, their intended meanings are consistent.

[0043] In this embodiment of the invention, sometimes a subscript such as W1 may be written in a non-subscript form such as W1. When the difference is not emphasized, the meaning they express is the same.

[0044] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0045] A heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys includes alloy melting, homogenization treatment, thermoplastic deformation, solution treatment, natural aging pretreatment, and single-stage aging treatment.

[0046] Specifically, the heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys includes the following steps:

[0047] S1. Alloy smelting: Weigh the raw materials of Al-Zn-Mg-Cu alloy according to industrial production conditions, smelt and cast to obtain alloy ingots;

[0048] S2. Homogenization treatment: The alloy ingot of S1 is subjected to a two-stage heating and holding homogenization treatment, and then air-cooled to room temperature after being taken out of the furnace to obtain a homogenized alloy ingot.

[0049] S3, thermoplastic deformation: The ingot after homogenization treatment of S2 is hot rolled or hot extruded to obtain alloy billet;

[0050] S4. Solution treatment: The alloy billet of S3 is subjected to a two-stage heating and holding solution treatment, followed by quenching in water at room temperature to obtain a solution-treated alloy billet.

[0051] S5. Natural aging pretreatment: The alloy billet with solid solution in S4 is subjected to natural aging pretreatment to obtain a naturally aged alloy billet.

[0052] S6. Single-stage aging treatment: The naturally aged alloy billet of S5 is subjected to a single-stage aging treatment to obtain the final state sample of the Al-Zn-Mg-Cu alloy.

[0053] Specifically, the composition of the Al-Zn-Mg-Cu alloy in S1, by mass percentage, is as follows: Zn 6.0%-12.0%, Mg 1.5%-3.0%, Cu 1.0%-2.8%, with appropriate amounts of microalloying elements including Cr, Mn, Zr, and Ti added as needed, the total of microalloying elements <0.5wt%, the total of impurity elements including Fe and Si <0.3wt%, and the balance being Al and unavoidable impurity elements.

[0054] Specifically, the homogenization treatment in S2, which involves two stages of heating and holding, is as follows: first, the temperature is raised to 380-420℃ and held for 10-30 hours, and then the temperature is raised to 450-480℃ and held for 10-30 hours.

[0055] Specifically, the heating temperature of aluminum alloy ingots in S3 is 380-420℃, and the heating time is 2-4h; the extrusion ratio is 7-50 during extrusion, and the total deformation during hot rolling is 50%-95%.

[0056] Specifically, in S4, the solution treatment involves first heating to 400-450℃ and holding for 1-2 hours; then heating to 450-480℃ and holding for 1-2 hours.

[0057] Specifically, the natural aging pretreatment in S5 involves standing at room temperature (10-35℃) for 4-14 days.

[0058] Specifically, the single-stage aging treatment in S6 involves heating to 110-130℃ and holding for 8-36 hours.

[0059] In particular, compared with the T6 peak-aged state, the final state sample of the Al-Zn-Mg-Cu alloy in S6 showed that the number density of intragranular precipitates in the alloy microstructure treated by the present invention increased by more than 15%, and the distribution was more uniform. Among them, the spherical GPI region accounted for 20%-30%, the disc-shaped GPII region accounted for 40%-50%, the disc-shaped η' phase accounted for 20%-25%, and the disc-shaped η phase accounted for less than 5%.

[0060] In particular, compared with the conventional peak aging state, after the heat treatment process of the present invention, the number density of intragranular precipitates in the alloy structure increases and the distribution is more uniform; the size of grain boundary precipitates is reduced to less than 1 / 2 of that in the peak aging state, and the presence of grain boundary precipitate-free zones (PFZs) is almost unobservable.

[0061] Specifically, the final-state samples of the Al-Zn-Mg-Cu alloy in S6 exhibit tensile strength (UTS) of 530-800 MPa, yield strength (YS) of 470-750 MPa, elongation after fracture (EL) of 9.5%-20.0%, and stress corrosion susceptibility factor (ISSRT) of 15%-25%. Correspondingly, the peak-aged alloy exhibits tensile strength (UTS) of 520-780 MPa, yield strength (YS) of 450-740 MPa, elongation after fracture (EL) of 9.0%-18.0%, and stress corrosion susceptibility factor (ISSRT) of 30%-50%.

[0062] In particular, the heat treatment process of this invention significantly reduces grain boundary segregation and grain boundary-free zones while ensuring that intragranular precipitates do not coarsen. This achieves a tensile strength higher than that of the conventional T6 peak aging process, while reducing the stress corrosion susceptibility factor by more than 40%.

[0063] Example 1

[0064] This embodiment presents a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance. The composition of the Al-Zn-Mg-Cu alloy, by mass percentage, is: Zn 6.58%, Mg 2.28%, Cu 2.33%, Zr 0.08%, Fe 0.01%, Si 0.01%, with the balance being Al. The specific steps are as follows:

[0065] S1. Alloy smelting: Weigh the raw materials of Al-Zn-Mg-Cu alloy according to industrial production conditions, smelt and cast to obtain an alloy ingot with a diameter of 76mm × length of 500mm.

[0066] S2. Homogenization treatment: The alloy ingot of S1 is subjected to a two-stage heating and holding process for homogenization. First, the temperature is raised to 400℃ and held for 24 hours, then raised to 472℃ and held for 20 hours. After that, it is taken out of the furnace and air-cooled to room temperature to obtain a homogenized alloy ingot.

[0067] S3. Hot plastic deformation: The ingot after homogenization treatment in S2 is hot rolled. The heating temperature of the aluminum alloy ingot is 400-420℃ and the heating time is 2h. The total deformation during hot rolling is 90%, and an alloy billet with dimensions of 4000mm×60mm×5mm is obtained.

[0068] S4. Solution treatment: The S3 alloy billet is subjected to a two-stage solution treatment of heating and holding. First, the temperature is raised to 430℃ and held for 1 hour; then the temperature is raised to 475℃ and held for 1 hour. Finally, it is quenched in water at room temperature to obtain a solution-treated alloy billet.

[0069] S5. Natural aging pretreatment: The alloy billet of S4 solution solution is subjected to natural aging pretreatment at room temperature of 30℃ for 5 days to obtain naturally aged alloy billet.

[0070] S6. Single-stage aging treatment: The naturally aged alloy billet of S5 is subjected to a single-stage aging treatment, heated to 110℃ and held for 26h to obtain the final state sample of Al-Zn-Mg-Cu alloy.

[0071] Compared with the T6 peak aged state, the final state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment shows that the number density of intragranular precipitates in the alloy microstructure treated by the present invention increases by 18% and the distribution is more uniform. Among them, the spherical GPI region accounts for 28%, the disk-shaped GPII region accounts for 47%, the disk-shaped η' phase accounts for 23%, and the disk-shaped η phase accounts for 2%.

[0072] Compared with the traditional peak aging state, after the heat treatment process of this embodiment, the number density of intragranular precipitates in the alloy structure increases and the distribution is more uniform; the size of grain boundary precipitates is reduced to 47% of that in the peak aging state, and the presence of grain boundary precipitate-free zones (PFZs) is almost unobservable.

[0073] The final-state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment has a tensile strength (UTS) of 536±4 MPa, a yield strength (YS) of 488±3 MPa, an elongation after fracture (EL) of 16.6±0.4%, and a stress corrosion susceptibility factor (ISSRT) of 18.1±1.8%. The corresponding peak-aged alloy has a tensile strength (UTS) of 521±8 MPa, a yield strength (YS) of 465±4 MPa, an elongation after fracture (EL) of 14.9±0.5%, and an ISSRT of 32.6±2.6%.

[0074] The heat treatment process in this embodiment significantly reduces grain boundary segregation and grain boundary-free zones while ensuring that intragranular precipitates do not coarsen. This achieves a tensile strength higher than that of the traditional T6 peak aging process, while reducing the stress corrosion susceptibility factor by 45%.

[0075] Example 2

[0076] This embodiment presents a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance. The composition of the Al-Zn-Mg-Cu alloy, by mass percentage, is: Zn 9.70%, Mg 2.23%, Cu 1.38%, Cr 0.11%, Zr 0.10%, Fe 0.02%, Si 0.01%, with the balance being Al. The specific steps are as follows:

[0077] S1. Alloy smelting: Weigh the raw materials of Al-Zn-Mg-Cu alloy according to industrial production conditions, smelt and cast to obtain an alloy ingot with a diameter of 78mm × length of 540mm.

[0078] S2. Homogenization treatment: The alloy ingot of S1 is subjected to a two-stage heating and holding process for homogenization. First, the temperature is raised to 400℃ and held for 24 hours, then raised to 470℃ and held for 24 hours. After that, it is taken out of the furnace and air-cooled to room temperature to obtain a homogenized alloy ingot.

[0079] S3. Hot plastic deformation: The ingot after homogenization treatment in S2 is subjected to hot extrusion. The heating temperature of the aluminum alloy ingot is 400-420℃ and the heating time is 2h. The temperature of the extrusion cylinder, die and pad is 400-440℃ and the heating time is 12h. The extrusion ratio is 9, and an alloy billet with a diameter of 25mm × length of 4800mm is obtained.

[0080] S4. Solution treatment: The S3 alloy billet is subjected to a two-stage solution treatment of heating and holding. First, the temperature is raised to 442℃ and held for 1 hour; then the temperature is raised to 478℃ and held for 1.5 hours. Finally, it is quenched in water at room temperature to obtain a solution-treated alloy billet.

[0081] S5. Natural aging pretreatment: The alloy billet of S4 solution solution is subjected to natural aging pretreatment at room temperature of 20℃ for 7 days to obtain naturally aged alloy billet.

[0082] S6. Single-stage aging treatment: The naturally aged alloy billet of S5 is subjected to a single-stage aging treatment, heated to 120℃ and held for 20h to obtain the final state sample of Al-Zn-Mg-Cu alloy.

[0083] Compared with the T6 peak aged state, the final state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment shows that the number density of intragranular precipitates in the alloy microstructure treated by the present invention increases by 20% and the distribution is more uniform. Among them, the spherical GPI region accounts for 29%, the disc-shaped GPII region accounts for 46%, the disc-shaped η' phase accounts for 24%, and the disc-shaped η phase accounts for 1%.

[0084] Compared to the traditional peak aging state, after the heat treatment process of this embodiment, the number density of intragranular precipitates in the alloy microstructure increases and their distribution becomes more uniform, such as... Figure 1 and Figure 2 As shown, the size of the grain boundary precipitates is reduced to 46% of the peak aging state, and the presence of the grain boundary non-precipitate zone (PFZ) is almost unobservable. Figure 3 and Figure 4 As shown.

[0085] The final-state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment has a tensile strength (UTS) of 752±3 MPa, a yield strength (YS) of 707±3 MPa, an elongation after fracture (EL) of 10.9±0.2%, and a stress corrosion susceptibility factor (ISSRT) of 17.1±1.9%. The corresponding peak-aged alloy has a tensile strength (UTS) of 738±7 MPa, a yield strength (YS) of 692±5 MPa, an elongation after fracture (EL) of 10.2±0.4%, and an ISSRT of 34.2±3.1%.

[0086] The heat treatment process in this embodiment significantly reduces grain boundary segregation and grain boundary-free zones while ensuring that intragranular precipitates do not coarsen. This achieves a tensile strength higher than that of the traditional T6 peak aging process, while reducing the stress corrosion susceptibility factor by 50%.

[0087] Example 3

[0088] This embodiment presents a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance. The composition of the Al-Zn-Mg-Cu alloy, by mass percentage, is: Zn 10.20%, Mg 2.38%, Cu 1.40%, Zr 0.12%, Ti 0.08%, Fe 0.02%, Si 0.01%, with the balance being Al. The specific steps are as follows:

[0089] S1. Alloy smelting: Weigh the raw materials of Al-Zn-Mg-Cu alloy according to industrial production conditions, smelt and cast to obtain an alloy ingot with a diameter of 76mm × length of 520mm.

[0090] S2. Homogenization treatment: The alloy ingot of S1 is subjected to a two-stage heating and holding process for homogenization. First, the temperature is raised to 400℃ and held for 24 hours, then raised to 470℃ and held for 24 hours. After that, it is taken out of the furnace and air-cooled to room temperature to obtain a homogenized alloy ingot.

[0091] S3. Hot plastic deformation: The ingot after homogenization treatment in S2 is subjected to hot extrusion. The heating temperature of the aluminum alloy ingot is 380-420℃ and the heating time is 2h. The temperature of the extrusion cylinder, die and pad is 400-440℃ and the heating time is 12h. The extrusion ratio is 16, and an alloy billet with a diameter of 25mm × length of 4500mm is obtained.

[0092] S4. Solution treatment: The S3 alloy billet is subjected to a two-stage solution treatment of heating and holding. First, the temperature is raised to 450℃ and held for 1 hour; then the temperature is raised to 480℃ and held for 1 hour. Finally, it is quenched in water at room temperature to obtain a solution-treated alloy billet.

[0093] S5. Natural aging pretreatment: The alloy billet of S4 solution solution is subjected to natural aging pretreatment at room temperature of 15℃ for 12 days to obtain naturally aged alloy billet.

[0094] S6. Single-stage aging treatment: The naturally aged alloy billet of S5 is subjected to a single-stage aging treatment, heated to 120℃ and held for 18h to obtain the final state sample of Al-Zn-Mg-Cu alloy.

[0095] Compared with the T6 peak aged state, the final state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment shows that the number density of intragranular precipitates in the alloy microstructure treated by the present invention increases by 18% and the distribution is more uniform. Among them, the spherical GPI region accounts for 26%, the disc-shaped GPII region accounts for 48%, the disc-shaped η' phase accounts for 24%, and the disc-shaped η phase accounts for 2%.

[0096] Compared with the traditional peak aging state, after the heat treatment process of this embodiment, the number density of intragranular precipitates in the alloy structure increases and the distribution is more uniform; the size of grain boundary precipitates is reduced to 49% of that in the peak aging state, and the presence of grain boundary precipitate-free zones (PFZs) is almost unobservable.

[0097] The final-state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment has a tensile strength (UTS) of 787±5 MPa, a yield strength (YS) of 754±3 MPa, an elongation after fracture (EL) of 10.2±0.3%, and a stress corrosion susceptibility factor (ISSRT) of 19.1±2.1%. The corresponding peak-aged alloy has a tensile strength (UTS) of 774±9 MPa, a yield strength (YS) of 747±6 MPa, an elongation after fracture (EL) of 9.2±0.4%, and an ISSRT of 36.7±3.5%.

[0098] The heat treatment process in this embodiment significantly reduces grain boundary segregation and grain boundary-free zones while ensuring that intragranular precipitates do not coarsen. This results in an alloy with higher tensile strength than the traditional T6 peak aging process, while reducing the stress corrosion susceptibility factor by 48%.

[0099] Example 4

[0100] This embodiment presents a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance. The composition of the Al-Zn-Mg-Cu alloy, by mass percentage, is: Zn 8.60%, Mg 2.06%, Cu 1.10%, Zr 0.10%, Ti 0.10%, Fe 0.02%, Si 0.02%, with the balance being Al. The specific steps are as follows:

[0101] S1. Alloy smelting: Weigh the raw materials of Al-Zn-Mg-Cu alloy according to industrial production conditions, smelt and cast to obtain an alloy ingot with a diameter of 77mm × length of 510mm.

[0102] S2. Homogenization treatment: The alloy ingot of S1 is subjected to a two-stage heating and holding process for homogenization. First, the temperature is raised to 400℃ and held for 24 hours, then raised to 470℃ and held for 24 hours. After that, it is taken out of the furnace and air-cooled to room temperature to obtain a homogenized alloy ingot.

[0103] S3. Hot plastic deformation: The ingot after homogenization treatment in S2 is subjected to hot extrusion. The heating temperature of the aluminum alloy ingot is 380-420℃ and the heating time is 2h. The temperature of the extrusion cylinder, die and pad is 400-440℃ and the heating time is 12h. The extrusion ratio is 16, and an alloy billet with a diameter of 25mm × length of 4450mm is obtained.

[0104] S4. Solution treatment: The S3 alloy billet is subjected to a two-stage solution treatment of heating and holding. First, the temperature is raised to 444℃ and held for 1 hour; then the temperature is raised to 478℃ and held for 1 hour. Finally, it is quenched in water at room temperature to obtain a solution-treated alloy billet.

[0105] S5. Natural aging pretreatment: The alloy billet of S4 solution solution is subjected to natural aging pretreatment at room temperature of 15℃ for 10 days to obtain naturally aged alloy billet.

[0106] S6. Single-stage aging treatment: The naturally aged alloy billet of S5 is subjected to a single-stage aging treatment, heated to 120℃ and held for 17h to obtain the final state sample of Al-Zn-Mg-Cu alloy.

[0107] Compared with the T6 peak aging state, the final state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment shows that the number density of intragranular precipitates in the alloy microstructure treated by the present invention increases by 17% and the distribution is more uniform. Among them, the spherical GPI region accounts for 25%, the disc-shaped GPII region accounts for 49%, the disc-shaped η' phase accounts for 23%, and the disc-shaped η phase accounts for 3%.

[0108] Compared with the traditional peak aging state, after the heat treatment process of this embodiment, the number density of intragranular precipitates in the alloy structure increases and the distribution is more uniform; the size of grain boundary precipitates is reduced to 47% of that in the peak aging state, and the presence of grain boundary precipitate-free zones (PFZs) is almost unobservable.

[0109] The final-state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment has a tensile strength (UTS) of 686±6 MPa, a yield strength (YS) of 644±3 MPa, an elongation after fracture (EL) of 12.4±0.3%, and a stress corrosion susceptibility factor (ISSRT) of 18.1±1.6%. The corresponding peak-aged alloy has a tensile strength (UTS) of 673±4 MPa, a yield strength (YS) of 632±4 MPa, an elongation after fracture (EL) of 11.2±0.4%, and an ISSRT of 34.7±2.6%.

[0110] The heat treatment process in this embodiment significantly reduces grain boundary segregation and grain boundary-free zones while ensuring that intragranular precipitates do not coarsen. This results in an alloy with higher tensile strength than the traditional T6 peak aging process, while reducing the stress corrosion susceptibility factor by 48%.

[0111] Example 5

[0112] This embodiment presents a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance. The composition of the Al-Zn-Mg-Cu alloy, by mass percentage, is: Zn 7.60%, Mg 2.18%, Cu 1.40%, Zr 0.12%, Cr 0.09%, Fe 0.02%, Si 0.01%, with the balance being Al. The specific steps are as follows:

[0113] S1. Alloy smelting: Weigh the raw materials of Al-Zn-Mg-Cu alloy according to industrial production conditions, smelt and cast to obtain an alloy ingot with a diameter of 78mm × length of 500mm.

[0114] S2. Homogenization treatment: The alloy ingot of S1 is subjected to a two-stage heating and holding process for homogenization. First, the temperature is raised to 400℃ and held for 24 hours, then raised to 470℃ and held for 24 hours. After that, it is taken out of the furnace and air-cooled to room temperature to obtain a homogenized alloy ingot.

[0115] S3. Hot plastic deformation: The ingot after homogenization treatment in S2 is hot rolled. The heating temperature of the aluminum alloy ingot is 400-420℃ and the heating time is 2h. The total deformation during hot rolling is 90%, and an alloy billet with dimensions of 4000mm×60mm×5mm is obtained.

[0116] S4. Solution treatment: The S3 alloy billet is subjected to a two-stage solution treatment of heating and holding. First, the temperature is raised to 453℃ and held for 1.5 hours; then the temperature is raised to 479℃ and held for 1 hour. Finally, it is quenched in water at room temperature to obtain a solution-treated alloy billet.

[0117] S5. Natural aging pretreatment: The alloy billet of S4 solution solution is subjected to natural aging pretreatment at room temperature of 15℃ for 9 days to obtain naturally aged alloy billet.

[0118] S6. Single-stage aging treatment: The naturally aged alloy billet of S5 is subjected to a single-stage aging treatment, heated to 120℃ and held for 19 h to obtain the final state sample of Al-Zn-Mg-Cu alloy.

[0119] Compared with the T6 peak aging state, the final state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment shows that the number density of intragranular precipitates in the alloy microstructure treated by the present invention increases by 16% and the distribution is more uniform. Among them, the spherical GPI region accounts for 26%, the disc-shaped GPII region accounts for 50%, the disc-shaped η' phase accounts for 22%, and the disc-shaped η phase accounts for 2%.

[0120] Compared with the traditional peak aging state, after the heat treatment process of this embodiment, the number density of intragranular precipitates in the alloy structure increases and the distribution is more uniform; the size of grain boundary precipitates is reduced to 41% of that in the peak aging state, and the presence of grain boundary precipitate-free zones (PFZs) is almost unobservable.

[0121] The final-state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment has a tensile strength (UTS) of 618±4 MPa, a yield strength (YS) of 571±3 MPa, an elongation after fracture (EL) of 13.8±0.5%, and a stress corrosion susceptibility factor (ISSRT) of 14.8±2.0%. The corresponding peak-aged alloy has a tensile strength (UTS) of 607±4 MPa, a yield strength (YS) of 556±2 MPa, an elongation after fracture (EL) of 13.2±0.2%, and an ISSRT of 33.9±1.8%.

[0122] The heat treatment process in this embodiment significantly reduces grain boundary segregation and grain boundary-free zones while ensuring that intragranular precipitates do not coarsen. This achieves a tensile strength higher than that of the traditional T6 peak aging process, while reducing the stress corrosion susceptibility factor by 56%.

[0123] Example 6

[0124] This embodiment presents a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance. The composition of the Al-Zn-Mg-Cu alloy, by mass percentage, is: Zn 9.60%, Mg 2.26%, Cu 1.36%, Zr 0.12%, Fe 0.01%, Si 0.02%, with the balance being Al. The specific steps are as follows:

[0125] S1. Alloy smelting: Weigh the raw materials of Al-Zn-Mg-Cu alloy according to industrial production conditions, smelt and cast to obtain an alloy ingot with a diameter of 75mm × length of 500mm.

[0126] S2. Homogenization treatment: The alloy ingot of S1 is subjected to a two-stage heating and holding process for homogenization. First, the temperature is raised to 400℃ and held for 24 hours, then raised to 470℃ and held for 24 hours. After that, it is taken out of the furnace and air-cooled to room temperature to obtain a homogenized alloy ingot.

[0127] S3. Hot plastic deformation: The ingot after homogenization treatment in S2 is subjected to hot extrusion. The heating temperature of the aluminum alloy ingot is 380-420℃ and the heating time is 2h. The temperature of the extrusion cylinder, die and pad is 400-440℃ and the heating time is 12h. The extrusion ratio is 16, and an alloy billet with a diameter of 25mm × length of 4500mm is obtained.

[0128] S4. Solution treatment: The S3 alloy billet is subjected to a two-stage solution treatment of heating and holding. First, the temperature is raised to 442℃ and held for 1 hour; then the temperature is raised to 478℃ and held for 1 hour. Finally, it is quenched in water at room temperature to obtain a solution-treated alloy billet.

[0129] S5. Natural aging pretreatment: The alloy billet of S4 solution solution is subjected to natural aging pretreatment at room temperature of 15℃ for 11 days to obtain naturally aged alloy billet.

[0130] S6. Single-stage aging treatment: The naturally aged alloy billet of S5 is subjected to a single-stage aging treatment, heated to 120℃ and held for 17h to obtain the final state sample of Al-Zn-Mg-Cu alloy.

[0131] Compared with the T6 peak aging state, the final state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment shows that the number density of intragranular precipitates in the alloy microstructure treated by the present invention increases by 18% and the distribution is more uniform. Among them, the spherical GPI region accounts for 27%, the disk-shaped GPII region accounts for 48%, the disk-shaped η' phase accounts for 22%, and the disk-shaped η phase accounts for 3%.

[0132] Compared with the traditional peak aging state, after the heat treatment process of this embodiment, the number density of intragranular precipitates in the alloy structure increases and the distribution is more uniform; the size of grain boundary precipitates is reduced to 46% of that in the peak aging state, and the presence of grain boundary precipitate-free zones (PFZs) is almost unobservable.

[0133] The final-state sample of the Al-Zn-Mg-Cu alloy prepared in this embodiment has a tensile strength (UTS) of 766±2 MPa, a yield strength (YS) of 725±3 MPa, an elongation after fracture (EL) of 10.4±0.4%, and a stress corrosion susceptibility factor (ISSRT) of 17.1±1.8%. The corresponding peak-aged alloy has a tensile strength (UTS) of 751±4 MPa, a yield strength (YS) of 714±5 MPa, an elongation after fracture (EL) of 9.2±0.4%, and an ISSRT of 36.2±2.6%.

[0134] The heat treatment process in this embodiment significantly reduces grain boundary segregation and grain boundary-free zones while ensuring that intragranular precipitates do not coarsen. This achieves a tensile strength higher than that of the traditional T6 peak aging process, while reducing the stress corrosion susceptibility factor by 53%.

[0135] The present invention proposes a heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance, which can solve the technical problem that improving the stress corrosion resistance of Al-Zn-Mg-Cu alloys in existing large-scale industrial production and application often leads to a decrease in strength.

[0136] This invention addresses the challenge of simultaneously improving strength and stress corrosion resistance in existing heat treatment processes for Al-Zn-Mg-Cu alloys. It proposes a novel heat treatment process that ensures that the alloy's tensile strength is higher than that of the traditional T6 peak aging process while reducing the stress corrosion susceptibility factor by more than 40%.

[0137] This invention employs a conventional casting → two-stage heating and holding homogenization treatment → hot extrusion / hot rolling → two-stage heating and holding solution treatment → natural aging pretreatment → single-stage aging treatment process. Compared with traditional over-aging and re-aging treatment processes, this invention ensures simultaneous improvement in strength and stress corrosion resistance while having a simpler process, lower economic cost, and is suitable for large-scale industrial production.

[0138] This invention employs natural aging pretreatment, which is beneficial to the actual production process. It reduces the impact of uncertain steps such as parking and transportation in industrial production on alloy properties, making the peak response of the single-stage aging stage more controllable, the performance distribution of the final product more concentrated, the batch-to-batch consistency stronger, and the yield rate higher. It is suitable for the manufacturing of aviation and rail transportation equipment with high safety requirements.

[0139] In summary, compared with other traditional methods, the method of the present invention can synergistically improve the strength and stress corrosion resistance of the alloy by performing two-stage heating and holding homogenization heat treatment, thermoplastic deformation, two-stage heating and holding solution treatment, natural aging pretreatment, and single-stage aging treatment on the smelted ingot. The method is simple to operate, environmentally friendly, low in cost, short in process, and highly efficient, which is conducive to large-scale industrial production and promotion.

[0140] It should be understood that the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. Additionally, the character " / " in this article generally indicates an "or" relationship between the preceding and following related objects, but it can also represent an "and / or" relationship. Please refer to the context for a more accurate understanding.

[0141] In this invention, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of a single item or a plurality of items. For example, at least one of a, b, or c can represent: a, b, c, ab, ac, bc, or abc, where a, b, and c can be a single item or multiple items.

[0142] It should be understood that, in various embodiments of the present invention, the order of the above-mentioned process numbers does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0143] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and stress corrosion resistance, characterized in that, The heat treatment process for Al-Zn-Mg-Cu alloys that simultaneously improves strength and resistance to stress corrosion includes alloy melting, homogenization treatment, hot plastic deformation, solution treatment, natural aging pretreatment, and single-stage aging treatment. The specific steps of the heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys are as follows: S1. Alloy smelting: Weigh the raw materials of Al-Zn-Mg-Cu alloy according to industrial production conditions, smelt and cast to obtain alloy ingots; S2. Homogenization treatment: The alloy ingot of S1 is subjected to a two-stage heating and holding homogenization treatment, and then air-cooled to room temperature after being taken out of the furnace to obtain a homogenized alloy ingot. S3, thermoplastic deformation: The ingot after homogenization treatment of S2 is hot rolled or hot extruded to obtain alloy billet; S4. Solution treatment: The alloy billet of S3 is subjected to a two-stage heating and holding solution treatment, followed by quenching in water at room temperature to obtain a solution-treated alloy billet. S5. Natural aging pretreatment: The solution-treated alloy billet of S4 is subjected to natural aging pretreatment, which is to be left at room temperature of 10-35℃ for 4-14 days to obtain naturally aged alloy billet. S6. Single-stage aging treatment: The alloy billet that was naturally aged in S5 was subjected to a single-stage aging treatment to obtain the final state sample of the Al-Zn-Mg-Cu alloy. The composition of the Al-Zn-Mg-Cu alloy in S1, by mass percentage, is as follows: Zn 6.0%-12.0%, Mg 1.5%-3.0%, Cu 1.0%-2.8%. Microalloying elements containing Cr, Mn, Zr, and Ti are added as needed, with the total amount of microalloying elements <0.5wt%. The total amount of impurity elements containing Fe and Si is <0.3wt%, and the balance is Al and unavoidable impurity elements.

2. The heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys according to claim 1, characterized in that, The homogenization process in S2 involves two stages of heating and holding: first, heating to 380-420℃ and holding for 10-30 hours, then heating to 450-480℃ and holding for 10-30 hours.

3. The heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys according to claim 1, characterized in that, The heating temperature of S3 aluminum alloy ingots is 380-420℃, and the heating time is 2-4h; the extrusion ratio is 7-50 during extrusion, and the total deformation during hot rolling is 50%-95%.

4. The heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys according to claim 1, characterized in that, In S4, the solution treatment involves first heating to 400-450℃ and holding for 1-2 hours; then heating to 450-480℃ and holding for 1-2 hours.

5. The heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys according to claim 1, characterized in that, In S6, the single-stage aging treatment involves heating to 110-130℃ and holding for 8-36 hours.

6. The heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys according to claim 1, characterized in that, Compared to the T6 peak-aged state, the number density of intragranular precipitates in the treated alloy microstructure increases by more than 15%, and the distribution is more uniform. Among them, the spherical GPI region accounts for 20%-30%, the disc-shaped GPII region accounts for 40%-50%, the disc-shaped η' phase accounts for 20%-25%, and the disc-shaped η phase accounts for less than 5%. The size of the grain boundary precipitates is reduced to less than 1 / 2 of that in the peak-aged state, and the presence of the grain boundary non-precipitated region PFZ is almost unobservable, with a PFZ width of less than 3 nm.

7. The heat treatment process for simultaneously improving the strength and stress corrosion resistance of Al-Zn-Mg-Cu alloys according to claim 1, characterized in that, The final-state samples of the Al-Zn-Mg-Cu alloy in S6 have a tensile strength (UTS) of 530-800 MPa, a yield strength (YS) of 470-750 MPa, an elongation after fracture (EL) of 9.5%-20.0%, and a stress corrosion susceptibility factor (ISSRT) of 15%-25%. The corresponding peak-aged alloy has a tensile strength (UTS) of 520-780 MPa, a yield strength (YS) of 450-740 MPa, an elongation after fracture (EL) of 9.0%-18.0%, and an ISSRT of 30%-50%.

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