A non-isothermal aging heat treatment method of an Al-Mg-Zn-Sc-Zr alloy with high Zn content

Abstract

This invention relates to the field of heat treatment technology for high-strength and corrosion-resistant aluminum alloys, specifically to a non-isothermal aging heat treatment method for high-Zn-content Al-Mg-Zn-Sc-Zr alloys. The alloy composition by mass percentage is 7.05–7.50 wt.% Mg, 5.12–5.24 wt.% Zn, 0.32–0.35 wt.% Sc, 0.28–0.32 wt.% Zr, with unavoidable impurities ≤0.1 wt.%, and the balance being Al. The non-isothermal aging treatment consists of: a first stage where the temperature is increased from room temperature to 135–145°C at a rate of 5–10°C / h; and a second stage where the temperature is increased to 180–190°C at a rate of 10–20°C / h. After reaching the final temperature, the alloy is immediately air-cooled to room temperature. After treatment using this invention, the intracrystalline precipitates are fine and dispersed, enabling the alloy to simultaneously achieve high strength, good plasticity, and excellent corrosion resistance within a relatively short aging time.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for high-strength and corrosion-resistant aluminum alloys, specifically relating to a non-isothermal aging heat treatment method for Al-Mg-Zn-Sc-Zr alloys with high Zn content. Background Technology

[0002] Al-Mg alloys are widely used in aerospace and transportation applications where stringent corrosion resistance and mechanical performance requirements are present, due to their excellent corrosion resistance, good weldability, and moderate mechanical properties. However, with the rapid development of modern industry towards lightweight and high load-bearing capacity, the strength levels achieved by traditional Al-Mg alloys, primarily through solid solution strengthening and work hardening, are no longer sufficient to meet the service requirements of high-end structural components. Therefore, there is an urgent need to further improve the overall performance of Al-Mg alloys.

[0003] Introducing Zn into Al-Mg alloys promotes the formation of the T phase and enhances precipitation strengthening potential. For example, as reported in the literature Jiang L., Zhang Z., Bai Y., et al. Design of novel Al-Mg-(Zn-Sc) alloys with enhanced mechanical properties and corrosion resistance[J]. Journal of Alloys and Compounds, 2023, 969: 172425, the addition of Zn promotes the precipitation of the T phase in Al-Mg-Zn alloys, reducing the β-Al3Mg2 phase, which is prone to intergranular corrosion in traditional Al-Mg alloys, to a certain extent. This reduces the grain boundary phase size from approximately 10 μm to approximately 5 μm, thus improving the precipitation strengthening potential, mechanical properties, and resistance to intergranular corrosion.

[0004] For Al-Mg alloys with high Zn content, mismatch between heat treatment regime and composition design can easily lead to the segregation and increased density of grain boundary precipitates. This may also exacerbate grain elongation and the streamlined distribution of precipitates along the processing direction, thus increasing microstructure inhomogeneity. Mechanically, it may reduce alloy plasticity and enhance mechanical anisotropy, resulting in high strength and low plasticity. Therefore, increasing Zn content presents new challenges for the performance control of Al-Mg-Zn alloys. For high Zn content Al-Mg-Zn-Sc-Zr alloys, it is urgent to develop heat treatment processes that match their precipitation behavior to achieve effective control of microstructure and properties. Summary of the Invention

[0005] To address the challenges of balancing strength, plasticity, and corrosion resistance in existing high-Zn-content Al-Mg-Zn-Sc-Zr alloys, and the lack of a non-isothermal aging process that matches their precipitation behavior, this paper proposes a non-isothermal aging heat treatment method for high-Zn-content Al-Mg-Zn-Sc-Zr alloys. This method achieves synergistic optimization of the mechanical properties and intergranular corrosion resistance of such alloys, providing technical support for their application in related equipment fields. By designing specific component ratios and combining them with a non-isothermal aging process, the morphology, size, and distribution of intragranular and grain boundary precipitates are synergistically controlled, thereby facilitating the achievement of a better match between strength, plasticity, and corrosion resistance. The non-isothermal aging process employs a two-stage differentiated heating method to dynamically control the heat treatment temperature: the first stage is a slow heating section in the low-temperature range, with a heating rate of 5-10℃ / h to 135-145℃, promoting the uniform diffusion and redistribution of solute atoms in the alloy, forming a high-density, uniformly distributed precipitate precursor, providing sufficient sites for the synchronous nucleation of subsequent strengthening phases. It should be noted that the precipitate precursor in this invention refers to atomic clusters or GP regions formed by the diffusion of solute atoms (Mg, Zn, etc.) during the low-temperature continuous heating stage (135-145℃), typically with a size of 1-5 nm, and not yet evolved into a stable T phase; this precursor further transforms and grows into a nanoscale T phase during the second-stage heating process, thereby achieving precipitation strengthening.

[0006] The second stage is a mid-temperature range adaptation heating section, where the temperature is increased to 180-190℃ at a heating rate of 10-20℃ / h to promote controlled precipitation and moderate growth of the T phase, ultimately obtaining fine-sized, uniformly dispersed intragranular precipitates. Through the above-mentioned non-isothermal aging heat treatment, the process of this invention can suppress abnormal coarsening of precipitates and the tendency for continuous grain boundary precipitates, optimize the morphology, size, and distribution of grain boundary precipitates, improve the stability of the microstructure near grain boundaries, and thus simultaneously optimize the strength-toughness match while improving the alloy's strength and enhancing its resistance to intergranular corrosion.

[0007] To achieve the above objectives, the technical solution adopted by the present invention comprises two parts: A high Zn content Al-Mg-Zn-Sc-Zr alloy, wherein the alloy composition by mass percentage is Mg 7.05-7.50 wt.%, Zn 5.12-5.24 wt.%, Sc 0.32-0.35 wt.%, Zr 0.28-0.32 wt.%, unavoidable impurities ≤0.1 wt.%, and the balance is Al.

[0008] A non-isothermal aging heat treatment method for the high Zn content Al-Mg-Zn-Sc-Zr alloy includes the following steps: (1) Weigh the raw materials according to the mass percentage and smelt them to obtain alloy ingots; (2) The alloy ingot is subjected to a two-stage homogenization heat treatment, followed by air cooling to room temperature; (3) Hot rolling and cold rolling are performed on the homogenized alloy ingot to obtain rolled plates; (4) The rolled sheet is solution treated and then rapidly water-cooled; (5) Perform a two-stage non-isothermal aging heat treatment on the solution-treated plate; The preparation of Al-Mg-Zn-Sc-Zr alloy with high Zn content was completed.

[0009] Preferably, the non-isothermal aging treatment in step (5) includes the following steps: in the first stage, the temperature is raised from room temperature to a first temperature at a first heating rate; in the second stage, the temperature is raised to a second temperature at a second heating rate; and after reaching the second temperature, the temperature is immediately removed and air-cooled to room temperature. The first temperature is 135–145°C, the second temperature is 180–190°C, the first heating rate is 5–10°C / h, and the second heating rate is 10–20°C / h.

[0010] More preferably, in the first stage, the temperature is increased from room temperature to 140°C at a heating rate of 7°C / h, and in the second stage, the temperature is increased to 185°C at a heating rate of 15°C / h. After reaching 185°C, the temperature is immediately removed and air-cooled to room temperature.

[0011] Preferably, the two-stage homogenization treatment in step (2) includes the following steps: holding the alloy ingot at 300-400℃ for 8 hours, then directly raising the temperature to 450-470℃ and holding it for 7-9 hours, and finally air-cooling it to room temperature after homogenization. More preferably, the alloy ingot is held at 350℃ for 8 hours, then directly raised to 460℃ and held for 8 hours, and finally air-cooled to room temperature after homogenization. Preferably, the hot rolling process in step (3) includes the following steps: hot rolling the homogenized alloy ingot to 10-12 mm in multiple passes at 440-460°C.

[0012] Preferably, the cold rolling process in step (3) includes the following steps: cold rolling the hot-rolled alloy sheet to 2-3 mm at room temperature.

[0013] Preferably, step (4) solution treatment includes the following steps: holding the cold-rolled alloy sheet at 490-500℃ for 60-65 minutes, and then water-cooling it to room temperature. Preferably, the cold-rolled alloy sheet is held at 490℃ for 60 minutes.

[0014] Preferably, after the solution treatment in step (4) is completed, the transfer time from when the sample is taken out of the furnace to when it is completely immersed in the water-cooling medium does not exceed 10 seconds.

[0015] Preferably, after solution treatment and water cooling to room temperature in step (4), the process is transferred to the two-stage non-isothermal aging heat treatment in step (5) within 30 minutes.

[0016] A high-Zn-content Al-Mg-Zn-Sc-Zr alloy sheet, wherein the high-Zn-content Al-Mg-Zn-Sc-Zr alloy sheet is prepared by the method described above.

[0017] Compared with the prior art, the present invention has the following advantages: This invention, through specific alloy composition design and non-isothermal aging process, facilitates the synergistic regulation of intragranular and grain boundary precipitates in high-Zn-content Al-Mg-Zn-Sc-Zr alloys, thereby improving the matching relationship between alloy strength, plasticity, and resistance to intergranular corrosion.

[0018] Within the scope of testing in this invention, the specific component ratios show a good match with the non-isothermal aging window. Compared with the conventional two-stage isothermal aging, single-stage non-isothermal aging, and representative complex subsequent heat treatment routes involved in the comparative examples, this approach is more conducive to achieving a better overall performance balance.

[0019] When using the specific component ratios and non-isothermal aging windows described in this invention, excellent comprehensive performance is exhibited, with hardness not lower than 221.58 HV, tensile strength not lower than 629.45 MPa, elongation not lower than 17.89%, and maximum intergranular corrosion depth not higher than 23.50 μm. This indicates that the component range and process range defined by this invention can achieve synergistic optimization of strength, plasticity, and corrosion resistance.

[0020] The process flow of this invention is relatively simple. Within the process range verified in the embodiments of this invention, the total aging time is only 14.3~26.5h. Compared with the tested traditional two-stage isothermal aging and representative complex subsequent heat treatment routes, it has better process simplification advantages and engineering application potential. Attached Figure Description

[0021] Figure 1 This is a flow chart of the non-isothermal aging heat treatment process for the high Zn content Al-Mg-Zn-Sc-Zr alloy described in this invention; Figure 2 This is a bright-field diagram of intragranular precipitates in an aged aluminum alloy obtained by the non-isothermal aging heat treatment process of Example 1. Figure 3 This is a bright-field diagram of intragranular precipitates in an aged aluminum alloy obtained through the non-isothermal aging heat treatment process of Example 2. Figure 4 This is a bright-field diagram of grain boundary precipitates in an aged aluminum alloy obtained through the non-isothermal aging heat treatment process of Example 1. Figure 5 This is a bright-field diagram of grain boundary precipitates in an aged aluminum alloy obtained through the non-isothermal aging heat treatment process of Example 2. Figure 6 The intergranular corrosion results are obtained from the non-isothermal aging heat treatment process of Example 1. Figure 7 The intergranular corrosion results are obtained from the non-isothermal aging heat treatment process of Example 2; Figure 8 The results show the intergranular corrosion of the aged aluminum alloy obtained through the non-isothermal aging heat treatment process of Comparative Example 1. Figure 9 The results show the intergranular corrosion of the aged aluminum alloy obtained through the single-stage aging heat treatment process in Comparative Example 2. Figure 10 These are the electrochemical corrosion polarization curves of Examples 1-3 and Comparative Examples 2-3; Figure 11 These are the self-corrosion current densities and self-corrosion potentials of Examples 1-3 and Comparative Examples 1-7. Detailed Implementation

[0022] To facilitate understanding of the present invention, the present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0023] All technical terms used in this document have the same meaning as commonly understood by those skilled in the art.

[0024] All the raw materials, reagents, instruments and equipment used in this invention can be purchased on the market.

[0025] In this invention, unless otherwise stated, room temperature refers to 25°C.

[0026] To verify the rationality of the specific component ratio of the high-Zn-content Al-Mg-Zn-Sc-Zr alloy described in this invention, the optimality of the non-isothermal aging process, and the synergistic matching relationship between the component ratio and the heat treatment process, Examples 1-3 and Comparative Examples 1-7 are provided in this section. Except for the explicitly stated adjustment parameters, the alloy smelting process parameters, two-stage homogenization heat treatment, hot rolling and cold rolling, solution cooling and water cooling processes and parameters of each example and comparative example are completely consistent, so as to compare the effects of different composition designs, process parameters and their cross-variations on microstructure and properties.

[0027] The technical effects of this invention are not independently determined by adjustments to a single composition or changes in a single heat treatment parameter, but rather by the synergistic matching between the contents of the main strengthening elements Mg and Zn, the level of composite microalloying of Sc and Zr, and the non-isothermal aging parameters. Within the compositional window defined by this invention, Mg and Zn provide the necessary solute basis for T-phase precipitation, while Sc and Zr facilitate the formation of dispersed phases and improve microstructural stability. Simultaneously, non-isothermal aging, through low-temperature slow nucleation and medium-temperature adaptive growth processes, regulates the size, morphology, and distribution of intragranular and grain boundary precipitates, thereby achieving comprehensive optimization of strength, plasticity, and corrosion resistance.

[0028] Example 1 This embodiment 1 discloses a high Zn content Al-Mg-Zn-Sc-Zr alloy composition, with the following weight percentages: Mg 7.1 wt.%, Zn 5.18 wt.%, Sc 0.34 wt.%, Zr 0.30 wt.%, and the balance being Al. Unavoidable impurities are ≤0.1 wt.%. The preparation method and non-isothermal aging heat treatment process of the high Zn content Al-Mg-Zn-Sc-Zr alloy described in this embodiment are as follows: Step 1, Alloy smelting and homogenization heat treatment: The required high-purity Al (99.9%), high-purity Mg (99.9%), high-purity Zn (99.9%), and Al-Sc and Al-Zr master alloys are prepared and smelted according to the mass percentage of each element.

[0029] Step 2: The ingot obtained from smelting undergoes a two-stage homogenization heat treatment at 350℃ / 8h + 460℃ / 8h. After homogenization, it is taken out and air-cooled to room temperature.

[0030] Step 3, Alloy Rolling and Solution Heat Treatment: The milled ingot is placed into a two-roll reversible hot rolling mill for hot rolling at a temperature of 450℃. It is hot rolled in multiple passes to 10mm, and finally cold rolled at room temperature to 2mm to obtain the rolled plate.

[0031] Step 4: Subsequently, solution heat treatment is performed at 490℃ for 60 min. After the solution treatment, the transfer time from removing the sample from the furnace to entering the water-cooling medium is controlled within 10 s. After water cooling to room temperature, the sample is transferred to a muffle furnace for non-isothermal aging treatment within 30 min.

[0032] Step 5, Non-isothermal aging heat treatment: Increase the temperature from room temperature to 140℃ at a rate of 7℃ / h; continue to increase the temperature to 185℃ at a rate of 15℃ / h. Once the temperature reaches 185℃, immediately remove the product and air cool it to room temperature.

[0033] Example 2 The alloy composition of Example 2 is 7.20 wt.% Mg, 5.24 wt.% Zn, 0.35 wt.% Sc, 0.32 wt.% Zr, with the balance being Al and unavoidable impurities ≤0.1 wt.%. The smelting, homogenization, rolling, and solution treatment processes of the alloy in this example are completely consistent with those in Example 1.

[0034] The only difference lies in the alloy composition and the non-isothermal aging parameters. The aging heat treatment process is as follows: the sample is placed in a muffle furnace and heated from room temperature to 135°C at a rate of 5°C / h, and then heated to 180°C at a rate of 10°C / h. After reaching 180°C, the sample is immediately taken out and air-cooled to room temperature.

[0035] Example 3 The alloy composition of Example 3 is 7.5 wt.% Mg, 5.12 wt.% Zn, 0.32 wt.% Sc, 0.28 wt.% Zr, with the balance being Al. Unavoidable impurities are ≤0.1 wt.%. The melting, homogenization, rolling, and solution treatment processes for this alloy are completely identical to those of Example 1. The only difference lies in the alloy composition and the non-isothermal aging parameters. The aging heat treatment process is as follows: the sample is placed in a muffle furnace, heated from room temperature to 145°C at a rate of 10°C / h, then heated to 190°C at a rate of 20°C / h. After reaching 190°C, the sample is immediately removed and air-cooled to room temperature.

[0036] Comparative Example 1 The alloy composition of Comparative Example 1 is Mg 4.5wt.%, Zn 2.8wt.%, Sc 0.12wt.%, Zr 0.11wt.%, with the balance being Al, and the unavoidable impurity content is ≤0.1wt.%. The melting, homogenization, rolling, solution treatment and aging heat treatment processes of the alloy in this comparative example are completely consistent with those of Example 1.

[0037] Comparative Example 2 The alloy composition of Comparative Example 2 is the same as that of Example 1. The melting, homogenization, rolling and solution treatment processes of the alloy in this comparative example are exactly the same as those in Example 1. The only difference is the aging heat treatment process, which is as follows: the temperature is raised from room temperature to 140°C and held for 24 hours. After the holding period, the temperature is air-cooled to room temperature.

[0038] Comparative Example 3 The alloy composition of Comparative Example 3 is the same as that of Example 1. The melting, homogenization, rolling and solution treatment processes of the alloy in this comparative example are exactly the same as those in Example 1. The only difference is the aging heat treatment process, which is as follows: the temperature is raised from room temperature to 90°C and held for 24 hours, then raised to 140°C and held for 24 hours. After the holding is completed, the temperature is air-cooled to room temperature.

[0039] Comparative Example 4 The alloy composition of Comparative Example 4 is completely consistent with that of Example 1. The melting, homogenization, rolling, and solution treatment processes of the alloy in this comparative example are exactly the same as those in Example 1. The only difference is the aging heat treatment process, which is as follows: the temperature is directly increased from room temperature to 185°C at a constant rate of 7°C / h, and then air-cooled to room temperature after reaching the final temperature.

[0040] Comparative Example 5 The alloy composition of Comparative Example 5 is the same as that of Example 1. The melting, homogenization, rolling and solution treatment processes of the alloy in this comparative example are exactly the same as those in Example 1. The only difference is the aging heat treatment process, which is as follows: in the first stage, the temperature is increased from room temperature to 140°C at a rate of 15°C / h, and in the second stage, the temperature is increased to 240°C at a rate of 30°C / h. After reaching the final temperature, the temperature is air-cooled to room temperature.

[0041] Comparative Example 6 Comparative Example 6 is a comparative alloy designed with reference to the composition of typical 2xxx series aluminum alloys. By weight percentage, it is Cu 4.4wt.%, Mg 1.5wt.%, Mn 0.6wt.%, Si≤0.5wt.%, Fe≤0.5wt.%, Zn≤0.25wt.%, Ti≤0.15wt.%, Cr≤0.10wt.%, with the balance being Al. The unavoidable impurity content is ≤0.1wt.%. The melting, homogenization, and rolling processes of the alloy are the same as those in Example 1. The solution treatment and aging heat treatment adopt its standard process: solution treatment at 505℃ for 1.5h, water quenching followed by holding at 190℃ for 12h, and aging followed by air cooling.

[0042] Comparative Example 7 Comparative Example 7 is a comparative alloy designed with reference to the composition of typical 7xxx series aluminum alloys. By weight percentage, it is Zn 5.6wt.%, Mg 2.5wt.%, Cu 1.6wt.%, Cr 0.30wt.%, Fe≤0.7wt.%, Si≤0.5wt.%, Mn≤0.3wt.%, Ti≤0.2wt.%, with the balance being Al. The unavoidable impurity content is ≤0.1wt.%. The melting, homogenization, and rolling processes of the alloy are the same as those in Example 1. The solution treatment and aging heat treatment adopt its standard process: solution treatment at 480℃ for 2 hours, water quenching followed by holding at 120℃ for 24 hours, and air cooling after aging. Comparative Example 8 The alloy composition of this comparative example is: Mg: 9.0 wt.%, Zn: 6.12 wt.%, Sc: 0.34 wt.%, Zr: 0.30 wt.%, with the balance being Al, and unavoidable impurities ≤ 0.1 wt.%. The ingot prepared with this composition in Example 1 is prone to edge cracking and splitting during rolling after homogenization treatment, and no subsequent heat treatment or other processes are performed.

[0043] 1. TEM observation Thin samples of 10mm × 10mm × 0.5mm were cut from the heat-treated specimens using wire electrical discharge machining. The samples were then mechanically ground to a thickness of 50–80 μm and punched into small circular pieces with a diameter of approximately 3mm. These pieces were then subjected to double-jet thinning in an electrolytic double-jet apparatus. The electrolyte consisted of 30% HNO3 + 70% CH3OH (volume fraction). The double-jet temperature was controlled at -25℃ to -30℃, and the operating voltage was 20–25V. After double-jet thinning, the sample surface was cleaned with anhydrous ethanol, dried, and the microstructure of the alloy was observed under a transmission electron microscope.

[0044] Figures 2-3 The figures show bright-field images of intragranular precipitates in the aluminum alloys after aging in Examples 1 and 2. As can be seen from the figures, within the compositional range and non-isothermal aging process range defined by this invention, nanoscale precipitates can be formed in Examples 1 and 2, indicating that this process window effectively controls the refinement and dispersion distribution of intragranular precipitates. Furthermore, statistical results show that the average sizes of the precipitates in Examples 1, 2, and 3 are 10.18 nm, 12.81 nm, and 34.26 nm, respectively. In Example 1, the precipitate size is smaller and the distribution is more uniform, indicating a better match between its composition ratio and heating parameters; Example 2 maintains a good dispersion precipitation state; in Example 3, the precipitate size increases, but it is still within the process window of this invention, and it still maintains good comprehensive mechanical properties. These results demonstrate that the non-isothermal aging window defined by this invention can control the size and distribution of precipitates under different endpoint conditions, thereby providing a microstructural basis for the alloy to achieve a comprehensive balance of high strength and high plasticity.

[0045] 2. Microhardness test The test was conducted using an HV-1000 microhardness tester with a test load of 0.5 kgf and a holding time of 15 s. To reduce test errors, at least 10 parallel measurements were performed on each alloy sample in different defect-free areas. The maximum and minimum values ​​were discarded, and the average value was taken as the microhardness value of the sample.

[0046] Table 1 Mechanical properties of Examples 1-3 and Comparative Examples 1-7

[0047] As shown in Table 1, the mechanical properties of the high Zn content Al-Mg-Zn-Sc-Zr alloy and related comparative alloys in the examples differ significantly under different component ratios and aging regimes. The performance advantages of the present invention are not solely due to the increase of a single component or the change of a single heat treatment method, but are closely related to the synergistic matching between a specific component ratio and a non-isothermal aging window.

[0048] Within the testing scope of this invention, the alloy composition ratios of Examples 1 to 3 all fall within the composition window defined in claim 1, namely Mg 7.05–7.50 wt.%, Zn 5.12–5.24 wt.%, Sc 0.32–0.35 wt.%, and Zr 0.28–0.32 wt.%, and all adopt non-isothermal aging process parameter windows, namely, the first stage is heated to 135–145 °C at a rate of 5–10 °C / h, and the second stage is heated to 180–190 °C at a rate of 10–20 °C / h. All three sets of examples exhibited excellent mechanical and corrosion resistance properties: the hardness, tensile strength, and elongation of Example 1 were 230.67 HV, 647.95 MPa, and 18.44%, respectively; those of Example 2 were 226.34 HV, 638.72 MPa, and 18.12%; and those of Example 3 were 221.58 HV, 629.45 MPa, and 17.89%. Meanwhile, the maximum intergranular corrosion depths of Examples 1-3 were only 11.03 μm, 12.07 μm, and 23.50 μm, respectively, all significantly lower than those of the comparative examples. These results demonstrate that within the composition and non-isothermal aging process window defined by this invention, the alloy can achieve a synergistic balance of high strength, high plasticity, and excellent resistance to intergranular corrosion.

[0049] When the composition window deviates from the composition design range described in this invention, the overall performance of the alloy decreases significantly. In Comparative Example 1, the contents of Mg, Zn, Sc, and Zr are all below the range defined in this invention. Under the same non-isothermal aging regime as Example 1, its hardness, tensile strength, and elongation are 182.35 HV, 552.74 MPa, and 14.62%, respectively. In Comparative Example 2, using a single-stage isothermal aging process, its hardness, tensile strength, and elongation are 178.64 HV, 546.28 MPa, and 14.75%, respectively. These results indicate that when the contents of the main strengthening elements Mg and Zn, and the level of composite microalloying of Sc and Zr, are below the window specified in this invention, the alloy cannot achieve precipitation strengthening and microstructure stabilization effects comparable to Examples 1-3, and the overall mechanical properties are significantly reduced.

[0050] When the aging process deviates from the non-isothermal aging process range described in this invention, the overall balance between the alloy's strength and plasticity is also affected. Comparative Example 3 used a traditional two-stage isothermal aging process of 90℃×24h+140℃×24h, and its hardness, tensile strength, and elongation were 197.42HV, 518.67MPa, and 10.58%, respectively. Comparative Example 4 used a single-stage non-isothermal aging process of directly heating from room temperature to 185℃ at a rate of 7℃ / h, and its hardness, tensile strength, and elongation were 192.45HV, 579.63MPa, and 13.26%, respectively, all of which were lower than the overall performance levels of Examples 1-3. The above results show that when deviating from the non-isothermal aging heat treatment process window defined in this invention, it is difficult to achieve an effective match between the low-temperature slow nucleation and the medium-temperature adaptive growth process, which easily leads to coarsening and uneven distribution of intragranular precipitates and continuity of grain boundary precipitates, thereby impairing the balance between strength and plasticity.

[0051] When the aging process or the matching relationship between the two deviates from the window described in this invention, Comparative Example 5 uses non-isothermal aging process parameters of 15℃ / h heating to 140℃ in the first stage and 30℃ / h heating to 240℃ in the second stage, resulting in a decrease in hardness to 125.14HV and tensile strength to 345.19MPa. Although its elongation is 17.20%, the strength level is significantly lower than that of Examples 1-3, indicating that when the composition and process parameters deviate from the window of this invention and do not form an effective match, the precipitation strengthening and microstructure stabilization effects are significantly weakened. Comparative Examples 6 (traditional 2xxx series) and 7 (traditional 7xxx series) use their respective standard aging processes, with performance values ​​of 142.30HV, 427.03MPa, and 5.00% and 175.36HV, 572.40MPa, and 11.00%, respectively, both of which are difficult to reach the comprehensive level of the embodiments of this invention, further confirming the excellent synergistic matching of specific components and processes in this invention.

[0052] In summary, when the alloy composition and non-isothermal aging process parameters both fall within the window defined by this invention, fine and dispersed intragranular T-phase can be obtained through a precipitation path of low-temperature slow nucleation + medium-temperature adaptive growth. This achieves a comprehensive performance match of 221.58–230.67 HV hardness, 629.45–647.95 MPa tensile strength, 17.89%–18.44% elongation, and a maximum intergranular corrosion depth of 11.03–23.50 μm. When the composition or process deviates from this window, the size, density, and distribution of the precipitated phase tend to deviate from the favorable state, leading to a significant decrease in the comprehensive match of the alloy's strength, plasticity, and corrosion resistance.

[0053] 3. Intergranular corrosion Intergranular corrosion (IGC) tests were conducted according to GB / T7998-2005. The corrosion solution was prepared using 10 mL of hydrochloric acid solution (concentration 1.19 g / mL), 30 g of NaCl, and 1 L of deionized water. The test temperature was kept constant at 35 ± 2℃. The sample surface was polished to a mirror finish using 240#, 400#, 800#, 1200#, and 2000# sandpaper, and then immersed in the corrosion solution for 24 hours before removal. After cleaning with anhydrous ethanol and drying, cross-sections of three parallel samples were observed using an optical microscope (CSM7003D). The maximum intergranular corrosion depth of each sample was measured, and the average value was taken as the result for that group.

[0054] Figures 4-5 Bright-field images of grain boundary precipitates in the aluminum alloys after aging in Examples 1 and 2 are shown. As can be seen from the figures, there are significant differences in the distribution morphology, size, and continuity of the grain boundary precipitates under different aging regimes. Specifically, the grain boundary precipitate sizes in Examples 1 and 2 are 56.38 nm and 89.39 nm, respectively. These results indicate that the non-isothermal aging process described in this invention can effectively suppress the abnormal coarsening and continuity tendency of grain boundary precipitates, thereby providing a microstructural basis for reducing intergranular corrosion susceptibility.

[0055] Table 2 Maximum intergranular corrosion depth of Examples 1-3 and Comparative Examples 1-7

[0056] Summary Table 2 and Figures 6-9 The representative intergranular corrosion morphologies shown indicate that the intergranular corrosion susceptibility of the alloy varies significantly under different aging process conditions and composition designs. Within the compositional window and non-isothermal aging process window for high Zn-content Al-Mg-Zn-Sc-Zr alloys defined in this invention, Examples 1-3 all exhibit excellent resistance to intergranular corrosion. The maximum intergranular corrosion depth in Example 1 is only 11.03 μm, in Example 2 it is 12.07 μm, and in Example 3 it is 23.50 μm, all significantly lower than that in Comparative Examples 1-7. These results demonstrate that the intergranular corrosion susceptibility of the alloy can be effectively suppressed within the compositional range and non-isothermal aging process range defined in this invention. Among them, Example 1 has the shallowest intergranular corrosion depth and can be considered as an embodiment with superior resistance to intergranular corrosion.

[0057] Intergranular corrosion resistance is closely related to the morphology, continuity, and microstructure near grain boundaries of the precipitated phases. Figures 4-5The representative grain boundary precipitate morphologies and related statistical results show that the average sizes of the grain boundary precipitates in Examples 1-3 are 56.38 nm, 89.39 nm, and 113.39 nm, respectively, all significantly lower than the 124.63 nm of Comparative Example 1. Furthermore, the grain boundary precipitates in Examples 1-2 exhibit a relatively discrete distribution, indicating that the non-isothermal aging process defined in this invention can suppress the coarsening and continuity tendency of grain boundary precipitates within the specified window. Correspondingly, the maximum intergranular corrosion depth in Examples 1-3 is controlled below 23.50 μm, significantly lower than that of the comparative examples. Further comparison shows that Example 1 has the smallest grain boundary precipitate size and better dispersion, thus exhibiting the lowest intergranular corrosion depth; Example 2 maintains a relatively good grain boundary precipitation state; although the grain boundary precipitate size in Example 3 is somewhat increased, it is still superior to the comparative example, indicating that it remains within the effective process window. The above results indicate that the composition window and non-isothermal aging window of the present invention can improve the stability of the microstructure near the grain boundary by controlling the morphology, size and distribution of the precipitated phase at the grain boundary, thereby improving the alloy's resistance to intergranular corrosion.

[0058] When the alloy composition deviates from the specific window of this invention, the resistance to intergranular corrosion is significantly weakened. In Comparative Example 1, the contents of Mg, Zn, Sc, and Zr were all below the limits specified in this invention, resulting in a maximum intergranular corrosion depth of 66.37 μm. In Comparative Example 2, using a single-stage isothermal aging process, the corrosion depth was further increased to 89.47 μm. These results indicate that when the contents of the main strengthening elements Mg and Zn are insufficient and the level of Sc and Zr composite microalloying is low, the precipitation state within and at grain boundaries is difficult to effectively control, and the grain boundary precipitates are more prone to coarsening or continuity, thereby increasing the susceptibility to intergranular corrosion.

[0059] When the aging process deviates from the process window of this invention, the corrosion resistance of the alloy also decreases significantly. Comparative Example 3 uses conventional two-stage isothermal aging, and Comparative Example 4 uses single-stage non-isothermal aging. Their maximum intergranular corrosion depths are 55.69 μm and 58.71 μm, respectively, both significantly higher than those of Examples 1-3. The above results indicate that without the low-temperature slow nucleation and medium-temperature adaptive growth process defined in this invention, the morphology, size, and distribution of grain boundary precipitates are difficult to effectively optimize, leading to a decrease in corrosion resistance. Comparative Example 5 uses extreme heating parameters, achieving a corrosion depth of 65.38 μm; Comparative Examples 6 and 7 are typical 2xxx and 7xxx series alloys, respectively, with corrosion depths of 56.24 μm and 65.03 μm, respectively, after treatment using their standard processes.

[0060] The above results fully demonstrate that when the alloy composition falls within the specific window of Mg 7.05~7.50wt.%, Zn 5.12~5.24wt.%, Sc 0.32~0.35wt.%, and Zr 0.28~0.32wt.%, and the non-isothermal aging process described in this invention is used (heating to 135~145℃ at 5~10℃ / h, and to 180~190℃ at 10~20℃ / h), the maximum intergranular corrosion depth can be controlled below 25μm by dispersing fine grain boundary precipitates and achieving a better grain boundary microstructure, thus realizing a synergy between high strength and excellent resistance to intergranular corrosion.

[0061] Potentiodynamic polarization measurements were performed using an electrochemical workstation (CHI660E). The test system was a typical three-electrode system, with the sample as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The non-working surface of the sample was encapsulated with epoxy resin, with an exposed area of ​​1 cm². 2 The test electrolyte was a 3.5 wt.% NaCl solution.

[0062] Table 3 Electrochemical polarization parameters of Examples 1-3 and Comparative Examples 1-7

[0063] Summary Table 3 and Figures 10-11 It can be seen that the electrochemical corrosion behavior of the alloy varies significantly under different aging process conditions and composition designs, and the corrosion pattern is basically consistent with the intergranular corrosion test results. Within the composition and process window defined by this invention, Examples 1 to 3 all exhibit low self-corrosion current densities, which are 0.628 μA / cm², respectively. 2 1.363 μA / cm 2 and 1.595μA / cm 2 The values ​​were all significantly lower than those of the comparative proportions, indicating that the composition range and non-isothermal aging process range defined by the present invention can effectively reduce the corrosion reaction rate of the alloy in 3.5 wt.% NaCl solution.

[0064] Among them, Example 1 exhibited the lowest self-corrosion current density, at only 0.628 μA / cm², indicating the lowest corrosion kinetic rate; its self-corrosion potential was -1.141 V, the most positive among all groups of samples, indicating a relatively low thermodynamic tendency for corrosion. It should be noted that the corrosion resistance of the alloy should be evaluated in conjunction with the self-corrosion current density, self-corrosion potential, and maximum intergranular corrosion depth. Based on the above results, Examples 1-3 all demonstrated excellent corrosion resistance, proving that the composition window and non-isothermal aging process window defined in this invention are effective in improving corrosion resistance. Therefore, Example 1, possessing both the lowest self-corrosion current density and the shallowest maximum intergranular corrosion depth, can be considered a preferred embodiment of this invention.

[0065] Compared to the examples, the self-corrosion current densities of Comparative Examples 1-7 showed an increasing trend, indicating that the corrosion reaction rate on the alloy surface increases when the alloy composition ratio or non-isothermal aging parameters deviate from the window of this invention. The self-corrosion current densities of Comparative Examples 6 and 7 reached 1.870 μA / cm², respectively. 2 and 1.950μA / cm 2 The results were all higher than those in Examples 1-3, further demonstrating that the composition window and process window defined by the present invention have a positive effect on improving electrochemical corrosion resistance.

[0066] In summary, within the range of samples tested in this invention, Examples 1-3 all fall within the compositional window and non-isothermal aging process window of the high Zn content Al-Mg-Zn-Sc-Zr alloy defined by this invention, and all achieved excellent comprehensive properties. Specifically, Examples 1-3 exhibited a hardness of 221.58-230.67 HV, a tensile strength of 629.45-647.95 MPa, an elongation of 17.89%-18.44%, a maximum intergranular corrosion depth of 11.03-23.50 μm, and a self-corrosion current density of 0.628-1.595 μA / cm². 2 The above results indicate that the composition and process ranges defined by the present invention are not only effective in a single embodiment, but can achieve a synergistic combination of high strength, high plasticity, and excellent corrosion resistance within the scope covered by Examples 1 to 3.

[0067] Building upon the synergistic effect of high strength, high plasticity, and excellent corrosion resistance achieved in Examples 1-3, Example 1 exhibits superior overall performance, with a hardness of 230.67 HV, a tensile strength of 647.95 MPa, an elongation of 18.44%, a maximum intergranular corrosion depth of 11.03 μm, and a self-corrosion current density of 0.628 μA / cm². 2 This can be considered a preferred embodiment of the present invention.

[0068] This demonstrates that, within the verified composition and process range of this invention, there is a clear matching relationship between the specific alloy composition ratio and the non-isothermal aging window. This comprehensive performance balance cannot be achieved simply by adjusting the Mg / Zn ratio, introducing Sc / Zr, or replacing it with other known aging routes, fully confirming that the technical solution of this invention has outstanding substantive features and significant progress compared to existing technologies.

[0069] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A non-isothermal aging heat treatment method for high Zn content Al-Mg-Zn-Sc-Zr alloys, characterized in that, The alloy composition, by mass percentage, is: Mg 7.05–7.50 wt.%, Zn 5.12–5.24 wt.%, Sc 0.32–0.35 wt.%, Zr 0.28–0.32 wt.%, unavoidable impurities ≤0.1 wt.%, with the balance being Al; The method includes the following steps: (1) Weigh the raw materials according to the mass percentage and smelt them to obtain alloy ingots; (2) The alloy ingot is subjected to a two-stage homogenization heat treatment, followed by air cooling to room temperature; (3) Hot rolling and cold rolling are performed on the homogenized alloy ingot to obtain rolled plates; (4) The rolled sheet is solution treated and then rapidly water-cooled; (5) Perform a two-stage non-isothermal aging heat treatment on the solution-treated plate; The preparation of high Zn content Al-Mg-Zn-Sc-Zr alloy was completed; In step (5), the non-isothermal aging treatment includes the following steps: in the first stage, the temperature is raised from room temperature to a first temperature at a first heating rate; in the second stage, the temperature is raised to a second temperature at a second heating rate; after reaching the second temperature, the temperature is immediately removed and air-cooled to room temperature. The first temperature is 135-145℃, the second temperature is 180-190℃, the first heating rate is 5-10℃ / h, and the second heating rate is 10-20℃ / h.

2. The method according to claim 1, characterized in that, The two-stage homogenization process in step (2) includes the following steps: the alloy ingot is kept at 300-400℃ for 8 hours, then directly heated to 450-470℃ and kept for 7-9 hours, and after the homogenization process is completed, it is air-cooled to room temperature.

3. The method according to claim 1, characterized in that, The hot rolling process in step (3) includes the following steps: hot rolling the homogenized alloy ingot to 10-12 mm in multiple passes at 440-460℃.

4. The method according to claim 1, characterized in that, The cold rolling process in step (3) includes the following steps: cold rolling the hot-rolled alloy sheet to 2-3 mm at room temperature.

5. The method according to claim 1, characterized in that, The solution treatment in step (4) includes the following steps: keeping the cold-rolled alloy sheet at 490-500℃ for 60-65 minutes, and then water-cooling it to room temperature.

6. The method according to claim 1, characterized in that, After the solution treatment in step (4) is completed, the transfer time from when the sample is taken out of the furnace to when it is completely immersed in the water-cooling medium shall not exceed 10 seconds.

7. The method according to claim 1, characterized in that, After solution treatment and water cooling to room temperature in step (4), the process is transferred to the two-stage non-isothermal aging heat treatment in step (5) within 30 minutes.

8. The method according to claim 1, characterized in that, The first stage involves heating from room temperature to 140°C at a heating rate of 7°C / h. The second stage involves heating at a heating rate of 15°C / h to 185°C. Once 185°C is reached, the temperature is immediately removed and air-cooled to room temperature.

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