A micro-alloyed high-strength and corrosion-resistant Al-Zn-Si alloy and its heat treatment process

Abstract

This invention relates to the field of aluminum alloy materials technology, specifically to a microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy and its heat treatment process. It comprises aluminum, zinc, silicon, manganese, chromium, copper, and impurities. During the high-temperature solid solution stage, manganese and chromium preferentially diffuse to the interface of the acicular iron-rich phase and undergo a substitution reaction, transforming the coarse acicular β-AlFeSi phase into spherical or blocky AlFeMnSi and AlFeCrSi phases. This replaces the originally continuously distributed harmful phases along the grain boundaries with fine, dispersed particles. While maintaining the alloy's yield strength, these dispersed particles eliminate the mechanical damage to the passivation film continuity caused by the acicular phases, effectively improving the alloy's resistance to pitting and intergranular corrosion. This not only resolves the temperature window contradiction between the morphology control of the iron-rich phase and the precipitation of age-strengthening phases but also avoids the loss of age-strengthening potential due to premature agglomeration of zinc and copper at high temperatures, thereby improving the service life of the microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy in corrosive environments.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy materials technology, and more specifically, to a micro-alloyed high-strength and corrosion-resistant Al-Zn-Si alloy and its heat treatment process. Background Technology

[0002] Currently, Al-Zn-Si aluminum alloys have attracted attention in lightweight structural components for aerospace, automotive, and marine engineering due to their excellent casting properties and high strength potential. Zn is the main strengthening element in these alloys, and the precipitation of metastable phases such as MgZn2 through aging significantly improves strength. The addition of Si improves melt flowability and wear resistance. In existing technologies, the addition of transition metals such as manganese and chromium to improve alloy microstructure is widely adopted. These elements can partially suppress the formation of acicular iron-rich phases under as-cast conditions, thereby reducing the damage of iron impurities to mechanical properties. Simultaneously, single-stage aging treatment allows elements such as zinc and copper to precipitate nano-strengthening phases, resulting in higher levels of tensile strength. However, there are still some shortcomings in addressing the contradiction between the temperature window for controlling the morphology of the iron-rich phase and the precipitation of the age-strengthening phase: the diffusion capacity of manganese or chromium at the conventional single-stage aging temperature is insufficient to fully spheroidize or eliminate the coarse needle-like iron-rich phase, causing these harmful phases to be distributed along grain boundaries or dendrites, which not only reduces the local corrosion resistance of the alloy but also causes cracks to propagate in the early stage of the needle-like phase; at the same time, in order to increase the solution temperature to improve the morphology of the iron phase, it is easy to induce the premature enrichment of elements such as zinc and copper at the grain boundaries, which interferes with the uniform precipitation process of the nano-strengthening phase during the aging stage and reduces the supersaturation in the matrix, weakening the precipitation strengthening effect and thus reducing the overall service performance of the microalloyed high-strength corrosion-resistant Al-Zn-Si alloy.

[0003] Therefore, there is an urgent need for a micro-alloyed high-strength and corrosion-resistant Al-Zn-Si alloy and its heat treatment process. Summary of the Invention

[0004] The purpose of this invention is to provide a micro-alloyed high-strength and corrosion-resistant Al-Zn-Si alloy and its heat treatment process to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, firstly, the present invention provides a micro-alloyed high-strength and corrosion-resistant Al-Zn-Si alloy, comprising the following raw material components by weight: Aluminum (Al) 80-90 parts; Zinc (Zn) 5-10 parts; Silicon (Si) 3-6 parts; Manganese (Mn) 0.3-0.8 parts; Chromium (Cr) 0.2-0.6 parts; Copper (Cu) 0.5-1.5 parts; Total impurities 0-0.2 parts; wherein: the manganese (Mn) and chromium (Cr) form a manganese / chromium-containing spherical or blocky dispersed phase (Al) with aluminum (Al) and iron (Fe) in the iron-rich phase (iron is an unavoidable impurity brought in from the furnace charge or mold during the casting process). The 1FeMnSi phase and AlFeCrSi phase can preferentially diffuse to the interface of the acicular iron-rich phase and undergo a displacement reaction during the high-temperature solid solution stage, transforming the coarse acicular phase into fine, dispersed particles that do not damage the integrity of the passivation film. Simultaneously, the copper (Cu) and zinc (Zn) remain completely dissolved in the aluminum matrix during the high-temperature solid solution stage and do not participate in the aforementioned iron-containing phase reaction. During the low-temperature aging stage, copper and zinc together form nanoscale GP regions, and further precipitate MgZn2 as the main component. The phase is used to maintain a coherent or semi-coherent relationship with the aluminum matrix, and is uniformly distributed inside the matrix to produce a precipitation strengthening effect.

[0006] Furthermore, the nanoscale The average diameter of the phase is 5-20 nm, and the average length is 30-80 nm.

[0007] During the high-temperature solid solution stage (480-520℃), manganese and chromium preferentially diffuse towards the interface of acicular iron-rich phases (such as β-AlFeSi), utilizing the strong affinity between manganese / chromium and iron to undergo a substitution reaction: β-AlFeSi + Mn / Cr → Al(Fe,Mn)Si or Al(Fe,Cr)Si (spherical or blocky). This not only transforms the originally coarse acicular phases continuously distributed along the grain boundaries into fine, dispersed particles that do not disrupt the integrity of the passivation film, eliminating their disruption to the corrosion continuity, but also, because zinc and copper have weaker affinity for iron and lower diffusion rates than manganese / chromium at this temperature window, they are completely dissolved in the aluminum matrix and do not participate in the above substitution reaction. After rapid quenching and entering the low-temperature aging stage (160-200℃), copper and zinc utilize the characteristic that their solid solubility in aluminum decreases sharply with decreasing temperature, with copper promoting GP zone nucleation and zinc participating in formation. The process involves the synergistic precipitation of nanoscale coherent / semi-coherent strengthening phases in the form of MgZn2, which are uniformly distributed within the matrix. This process, which activates different microalloying elements in stages according to temperature windows, can effectively reduce the disruption of the passivation film by acicular iron-rich phases, while avoiding the loss of aging strengthening potential due to premature segregation of zinc and copper at high temperatures. This process is beneficial for simultaneously improving the alloy's resistance to pitting and intergranular corrosion while maintaining a high yield strength.

[0008] Secondly, according to Figure 1As shown, this invention provides a heat treatment process for microalloyed high-strength and corrosion-resistant Al-Zn-Si alloys, comprising the following steps: S1. Melting and Casting: Industrial aluminum, zinc, silicon, manganese, chromium and copper are melted in a medium-frequency induction melting furnace to obtain an alloy melt; it is then poured using a bottom-pouring gating system, and after naturally cooling to room temperature in air, it is demolded to obtain a cast Al-Zn-Si alloy ingot; this helps to reduce secondary oxidation and gas entrapment defects in the melt during the pouring process, while ensuring that the original distribution state of each microalloying element in the as-cast structure is completely preserved; S2. Homogenization pretreatment: The as-cast Al-Zn-Si alloy ingot is placed in a box-type resistance furnace and kept at a constant temperature to fully dissolve the non-equilibrium eutectic phase in the as-cast structure and reduce dendrite segregation. After cooling to room temperature in the furnace, it is taken out to obtain a pretreated billet with uniform composition. S3. Multi-stage heat treatment control: The pretreated billet is transferred to a controlled atmosphere heat treatment furnace for multi-stage heat treatment control to obtain an alloy billet with dispersed phase distribution and age-strengthening phase distribution. S4. Stress relief treatment: The alloy billet is transferred to a circulating air furnace for heat preservation and slow cooling to obtain a low-stress alloy billet, which is used to eliminate the residual thermal stress generated during the rapid quenching stage. S5. Surface passivation pretreatment: The stressed alloy billet is transferred to an acid-resistant immersion tank, and modified nitric acid passivation solution is added. After maintaining at room temperature for 10-20 seconds, it is removed. This process removes the machining oxide scale on the billet surface and the zinc-poor areas formed on the surface due to quenching segregation, exposing a uniform metallographic surface composed of spherical manganese / chromium dispersed phases and nano-aged phases. Then, it is rinsed in deionized water at a flow rate of 20-30 L / min for 40-60 seconds to remove residual nitrate ions and dissolved products. Next, it is placed in a centrifugal dehydrator and dehydrated at a speed of 500-800 r / min for 30-45 seconds. Finally, it is dried in a hot air drying oven at 60-80℃ for 5-10 minutes to obtain a surface-dried Al-Zn-Si alloy product without residue.

[0009] Furthermore, in S1, the melting steps of the medium-frequency induction melting furnace include: first, evacuating the furnace chamber to a pressure below 10 Pa, then refilling it with high-purity argon to atmospheric pressure to remove residual oxygen and moisture from the furnace; then heating it to 720-750°C at a heating rate of 15-20°C / min, and holding it at that temperature for 25-35 min under an argon protective atmosphere, so that all alloy raw materials are completely melted and form a uniform melt.

[0010] Furthermore, in S1, the casting operation of the bottom-pouring casting system includes: inserting a graphite rod into the melt for stirring, stirring once every 10 minutes for a total of three times, with each stirring lasting 20-30 seconds; after the third stirring, letting it stand for 10 minutes to allow non-metallic inclusions to float to the surface of the melt and be scraped off; then stopping heating and allowing the melt to cool naturally to 700-720°C, at which point bottom-pouring can be performed.

[0011] Furthermore, in step S2, the box-type resistance furnace heats the aluminum matrix to 380-420℃ at a heating rate of 2-5℃ / min and holds it at that temperature for 6-8 hours. This allows the solid solubility of zinc and copper in the aluminum matrix to gradually increase with increasing temperature. This is beneficial for the diffusion of soluble components in the non-equilibrium eutectic phases (such as α-Al+MgZn2 eutectic and α-Al+Si eutectic) formed during solidification into the aluminum matrix, reducing the compositional differences between the grains and grain boundaries. At the same time, this temperature window is lower than the starting temperature (above 450℃) at which manganese and chromium undergo significant substitution reactions at the iron-rich phase interface. Therefore, manganese and chromium only undergo preliminary homogenization at this stage and do not form the final dispersed phase prematurely. This is beneficial for retaining manganese and chromium in a solid solution or metastable state until the high-temperature solid solution stage, allowing them to preferentially diffuse to the needle-like iron-rich phase interface and complete the substitution reaction at 480-520℃. This avoids weakening their ability to clean harmful phases at high temperatures due to premature precipitation of coarse manganese or chromium-containing particles.

[0012] Furthermore, in step S3, the multi-stage heat treatment control includes the following steps: S3.1 High-temperature solution treatment: The billet is heated to 480-520℃ in a controlled atmosphere heat treatment furnace and held for 2-4 hours. At this temperature, manganese and chromium preferentially diffuse to the interface of the acicular β-AlFeSi phase, transforming the coarse acicular phase in the original cast state into spherical or blocky AlFeMnSi and AlFeCrSi phases through a substitution reaction. Zinc and copper have a weaker affinity for iron than manganese and chromium at this temperature window and do not participate in the above substitution reaction. Instead, they are completely dissolved in the aluminum matrix to form a supersaturated solid solution matrix. S3.2 Rapid quenching: The solid solution matrix is ​​transferred to a water bath at 20-40℃ within 10s, completely immersed and kept for 1-2min to reduce the billet temperature to room temperature. This is used to freeze the spherical dispersed phase morphology of manganese and chromium elements at high temperature, while zinc and copper elements are retained in the aluminum lattice in an atomically dispersed state to form a supersaturated aluminum solid solution with a high concentration of vacancy trapping ability. The tendency of zinc and copper to aggregate at grain boundaries in this supersaturated solid solution is suppressed, providing a uniform matrix condition for the uniform precipitation of new phases in the low-temperature aging stage. S3.3, Staged Aging: First, the supersaturated aluminum solid solution is held at 80-120℃ for 2-3 hours to promote the migration of copper atoms to vacant sites in the aluminum matrix and form atomic-scale GP regions. These GP regions serve as nucleation sites for subsequent precipitation. Then, the temperature is increased to 160-200℃ and held for 4-6 hours to allow zinc atoms to migrate cooperatively with the copper atoms in the previously formed GP regions, resulting in the precipitation of nano-sized atoms that maintain a coherent or semi-coherent relationship with the aluminum matrix based on the GP regions. The two nanophases, MgZn2 and copper-containing auxiliary strengthening phase, are distributed in a high-density uniform state within the crystal. Near the grain boundary, the spherical dispersed phase formed by manganese and chromium pre-occupies the grain boundary position, inhibiting the secondary segregation of zinc and copper towards the grain boundary. This results in an alloy billet with a uniformly distributed nano-strengthening phase within the crystal and the grain boundary pinned by the spherical dispersed phase.

[0013] Furthermore, in step S4, the circulating air furnace heats the furnace to 120-150°C at a heating rate of 30-50°C / min and holds it at that temperature for 1-2 hours. Then, it is cooled down to below 60°C at a cooling rate of 0.5-1°C / min and removed from the furnace. This allows the lattice distortion caused by the uncoordinated volume shrinkage during the rapid quenching stage to be released uniformly, while preventing the residual stress concentration area from becoming a preferential channel for anodic dissolution in a chloride-containing environment.

[0014] Furthermore, in step S5, the amount of modified nitric acid passivation solution added is 15-25 L / m² based on the total surface area of ​​the billet. 2 .

[0015] Furthermore, in step S5, the preparation method of the modified nitric acid passivation solution includes the following steps: Prepare a 10% (v / v) primary nitric acid solution by mixing concentrated nitric acid (64-70% by mass) with deionized water at a volume ratio of 1:9. Place the primary nitric acid solution in a sealed container, stir at 300-400 rpm, and simultaneously introduce ozone gas at a rate of 0.5-1.0 L / min for 15-20 min to generate a nitrogen-containing reactive intermediate with micro-oxidation activity in the solution. Then, add citric acid to the solution to achieve a concentration of 0.05-0.10 mol / L in the modified nitric acid passivation solution, thus obtaining the modified nitric acid passivation solution. This solution facilitates the simultaneous completion of three layers of action within a 10-20 s contact time: oxide scale removal, selective dissolution of zinc-poor areas, and uniform micro-passivation of the billet surface, avoiding the local pitting caused by over-corrosion during conventional single treatment with dilute nitric acid.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In this microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy and its heat treatment process, manganese and chromium preferentially diffuse to the interface of the acicular iron-rich phase during the high-temperature solid solution stage and undergo a substitution reaction, transforming the coarse acicular β-AlFeSi phase into spherical or blocky AlFeMnSi and AlFeCrSi phases. This allows the harmful phases that were originally continuously distributed along the grain boundaries to be replaced by fine, dispersed particles. While maintaining the alloy's yield strength, these dispersed particles eliminate the mechanical damage to the continuity of the passivation film caused by the acicular phases, effectively improving the alloy's resistance to pitting and intergranular corrosion. This not only solves the temperature window contradiction between the morphology control of the iron-rich phase and the precipitation of age-strengthening phases, but also avoids the loss of age-strengthening potential due to premature agglomeration of zinc and copper at high temperatures, thereby improving the service life of the microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy in corrosive environments.

[0017] 2. In this microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy and its heat treatment process, through the hierarchical synergistic precipitation mechanism of copper and zinc during the low-temperature aging stage, copper atoms first form GP zones at a lower temperature as nucleation sites, and then zinc atoms precipitate nano-reinforcing phases that are coherent or semi-coherent with the aluminum matrix on the basis of the GP zones at a higher temperature, so that the reinforcing phases exhibit a high-density and uniform distribution state within the grains; at the same time, the spherical dispersed phases formed by the transformation of manganese and chromium at the grain boundaries pre-occupy the grain boundary positions, effectively suppressing the secondary segregation of zinc and copper to the grain boundaries; not only is the precipitation strengthening effect maximized, but also the local corrosion-sensitive areas caused by compositional segregation near the grain boundaries are eliminated, so that the alloy obtains high yield strength while the tendency of intergranular corrosion is simultaneously suppressed, thereby improving the alloy's resistance to intergranular corrosion under the condition of maintaining high yield strength. Attached Figure Description

[0018] Figure 1 This is a flowchart of the heat treatment process for the microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy of the present invention. Figure 2 This is a tree diagram showing the yield strength of the present invention; Figure 3 This is a tree diagram of the pitting potential difference of the present invention; Figure 4 This is a tree diagram showing the intergranular corrosion depth. Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1 Take 82 parts aluminum, 10 parts zinc, 5 parts silicon, 0.8 parts manganese, 0.54 parts chromium, 1.5 parts copper, and 0.16 parts total impurities. Place the above raw materials in a medium-frequency induction melting furnace, evacuate to a pressure below 10 Pa, then refill with high-purity argon to atmospheric pressure, heat to 750°C at a heating rate of 20°C / min, and hold at this temperature for 25 min under argon protection. Stir the melt with a graphite rod inserted into it, stirring once every 10 min for 30 s each time, for a total of three stirrings. After the third stirring, let it stand for 10 min, scrape off non-metallic inclusions from the surface, stop heating, and allow the melt to cool naturally to 720°C. Cast the ingot using a bottom-pouring casting system, allow it to cool naturally to room temperature in air, and then demold to obtain a cast alloy ingot. Place the cast alloy ingot in a box-type resistance furnace and heat to 420°C at a heating rate of 5°C / min, hold for 6 h, and cool to room temperature with the furnace. Transfer the pretreated billet to a controlled atmosphere heat treatment furnace and heat to 520°C. The billet was subjected to high-temperature solution treatment for 2 hours, then rapidly quenched by immersing it in a 40°C water bath for 2 minutes within 10 seconds. It was then subjected to first-stage aging at 120°C for 2 hours, followed by second-stage aging at 200°C for 4 hours. The aged billet was then transferred to a circulating air furnace and heated to 150°C at a rate of 50°C / min, held for 1 hour, and then cooled to below 60°C at a rate of 1°C / min. A 70% (w / w) concentrated nitric acid solution was mixed with deionized water at a volume ratio of 1:9, stirred at 400 rpm, and simultaneously purged with ozone gas at a rate of 1.0 L / min for 15 minutes. Citric acid was then added to achieve a concentration of 0.10 mol / L, yielding a modified nitric acid passivation solution. The solution was prepared by applying a solution with a surface area of ​​25 L / m². 2 Add passivation solution, keep at room temperature for 10 seconds, then remove and rinse with a spray at a flow rate of 30 L / min for 40 seconds. Then place in a centrifugal dehydrator and dehydrate at a speed of 800 r / min for 30 seconds. Finally, dry in an 80℃ hot air drying oven for 5 minutes to obtain the Al-Zn-Si alloy product.

[0021] Example 2 Take 85 parts aluminum, 8 parts zinc, 5 parts silicon, 0.5 parts manganese, 0.4 parts chromium, 1.0 part copper, and 0.1 parts total impurities. Place the above raw materials in a medium-frequency induction melting furnace, evacuate to a pressure below 10 Pa, then refill with high-purity argon to atmospheric pressure, heat to 735°C at a heating rate of 18°C / min, and hold at this temperature for 30 min under argon protection. Stir the melt with a graphite rod inserted into it, stirring once every 10 min for 25 s each time, for a total of three stirrings. After the third stirring, let it stand for 10 min, scrape off non-metallic inclusions from the surface, stop heating, and allow the melt to cool naturally to 710°C. Cast the ingot using a bottom-pouring casting system, allow it to cool naturally to room temperature in air, and then demold to obtain a cast alloy ingot. Place the cast alloy ingot in a box-type resistance furnace and heat to 400°C at a heating rate of 3°C / min, holding for 7 h, then cool to room temperature with the furnace. Transfer the pretreated billet to a controlled atmosphere heat treatment furnace and heat to 500°C, holding for 3 h. High-temperature solution treatment was performed, followed by rapid quenching by immersion in a 30°C water bath for 1.5 minutes within 10 seconds. The billet was then subjected to first-stage aging at 100°C for 2.5 hours, and then further aging at 180°C for 5 hours. After aging, the billet was transferred to a circulating air furnace and heated to 135°C at a rate of 40°C / min, held for 1.5 hours, and then cooled to below 60°C at a rate of 0.8°C / min. A 67% (w / w) concentrated nitric acid solution was mixed with deionized water at a volume ratio of 1:9, stirred at 350 rpm, and simultaneously infused with ozone gas at a rate of 0.8 L / min for 18 minutes. Citric acid was then added to achieve a concentration of 0.08 mol / L, yielding a modified nitric acid passivation solution. The solution was then applied at a concentration of 20 L / m² based on the total surface area of ​​the billet. 2 Add passivation solution, keep at room temperature for 15s, remove and rinse with spray at a flow rate of 25L / min for 50s, then place in a centrifugal dehydrator and dehydrate at a speed of 650r / min for 38s, and finally dry in a 70℃ hot air drying oven for 8min to obtain Al-Zn-Si alloy finished product.

[0022] Example 3 Take 89 parts aluminum, 6 parts zinc, 3.8 parts silicon, 0.45 parts manganese, 0.2 parts chromium, 0.5 parts copper, and 0.05 parts total impurities. Place the above raw materials in a medium-frequency induction melting furnace, evacuate to a pressure below 10 Pa, then refill with high-purity argon to atmospheric pressure, heat to 720°C at a heating rate of 15°C / min, and hold at this temperature for 35 min under argon protection. Stir the melt with a graphite rod inserted into it, stirring once every 10 min for 20 s each time, for a total of three stirrings. After the third stirring, let it stand for 10 min, scrape off non-metallic inclusions from the surface, stop heating, and allow the melt to cool naturally to 700°C. Cast the ingot using a bottom-pouring casting system, allow it to cool naturally to room temperature in air, and then demold to obtain a cast alloy ingot. Place the cast alloy ingot in a box-type resistance furnace and heat to 380°C at a heating rate of 2°C / min, hold for 8 h, and cool to room temperature with the furnace. Transfer the pretreated billet to a controlled atmosphere heat treatment furnace and heat to 480°C. The billet was subjected to high-temperature solution treatment for 4 hours, then rapidly quenched by immersing it in a 20°C water bath for 1 minute within 10 seconds. It was then subjected to first-stage aging at 80°C for 3 hours, followed by second-stage aging at 160°C for 6 hours. The aged billet was then transferred to a circulating air furnace and heated to 120°C at a heating rate of 30°C / min, held for 2 hours, and then cooled to below 60°C at a cooling rate of 0.5°C / min. A 64% (w / w) concentrated nitric acid solution was mixed with deionized water at a volume ratio of 1:9, stirred at 300 rpm, and simultaneously purged with ozone gas at a rate of 0.5 L / min for 20 minutes. Citric acid was then added to achieve a concentration of 0.05 mol / L, yielding a modified nitric acid passivation solution. The solution was then applied at a concentration of 15 L / m² based on the total surface area of ​​the billet. 2 Add passivation solution, keep at room temperature for 20s, remove and rinse with spray at a flow rate of 20L / min for 60s, then place in a centrifugal dehydrator and dehydrate at a speed of 500r / min for 45s, and finally dry in a 60℃ hot air drying oven for 10min to obtain Al-Zn-Si alloy product.

[0023] To verify that the microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy prepared in the embodiments of the present invention has good strength retention and corrosion resistance, the following experimental examples are used to illustrate the microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy provided in the embodiments of the present invention.

[0024] Test case The purpose of this experimental group is to investigate the effects of different component ratios on microalloyed high-strength and corrosion-resistant Al-Zn-Si alloys, and to test the yield strength, pitting corrosion resistance (characterized by self-corrosion potential and pitting potential difference) and intergranular corrosion resistance (characterized by electrochemical impedance spectroscopy and intergranular corrosion depth) of the microalloyed high-strength and corrosion-resistant Al-Zn-Si alloys of this invention.

[0025] Experimental Objective: Experimental groups A, B, and C adopted the composition ratios of the microalloyed high-strength and corrosion-resistant Al-Zn-Si alloys provided in Examples 1-3, respectively; the control examples included control groups A, B, C, D, E, and F, wherein: Control group A Take 85 parts aluminum, 8 parts zinc, 5 parts silicon, 0.5 parts manganese, 0.4 parts chromium, 1.0 part copper, and 0.1 parts total impurities. Place the above raw materials in a medium-frequency induction melting furnace, evacuate to a pressure below 10 Pa, then refill with high-purity argon to atmospheric pressure, heat to 735°C at a heating rate of 18°C / min, and hold at this temperature for 30 min under argon protection. Stir the melt with a graphite rod inserted into it, stirring once every 10 min for 25 s each time, for a total of three stirrings. After the third stirring, let it stand for 10 min, scrape off any non-metallic inclusions on the surface, stop heating, and allow the melt to cool naturally to 710°C. Pour the melt using a bottom-pouring casting system, and demold after allowing it to cool naturally to room temperature in air. The as-cast alloy ingot was obtained; the as-cast alloy ingot was placed in a box-type resistance furnace and heated to 400℃ at a heating rate of 3℃ / min and held for 7 hours, then cooled to room temperature in the furnace; subsequently, the billet was directly heated to 180℃ and held for 8 hours for single-stage aging treatment, without high-temperature solution treatment, rapid quenching, and graded aging steps; the aged billet was transferred to a circulating air furnace and heated to 135℃ at a heating rate of 40℃ / min and held for 1.5 hours, then cooled to below 60℃ in the box at a cooling rate of 0.8℃ / min and removed; the billet was then sized based on a total surface area of ​​20 L / m². 2 Add a 10% (w / w) dilute nitric acid solution, keep at room temperature for 15 seconds, then remove and rinse with a spray at a flow rate of 25 L / min for 50 seconds. Then place it in a centrifugal dehydrator and dehydrate at a speed of 650 r / min for 38 seconds. Finally, dry in a 70℃ hot air drying oven for 8 minutes to obtain the Al-Zn-Si alloy product.

[0026] Control group B Take 85 parts aluminum, 8 parts zinc, 5 parts silicon, 0.5 parts manganese, 0.4 parts chromium, 1.0 part copper, and 0.1 parts total impurities. Place the above raw materials in a medium-frequency induction melting furnace, evacuate to a pressure below 10 Pa, then refill with high-purity argon to atmospheric pressure, heat to 735°C at a heating rate of 18°C / min, and hold at this temperature for 30 min under argon protection. Stir the melt with a graphite rod inserted into it, stirring once every 10 min for 25 s each time, for a total of three stirrings. After the third stirring, let it stand for 10 min, scrape off the non-metallic inclusions on the surface, stop heating, and allow the melt to cool naturally to 710°C. Cast the melt using a bottom-pouring casting system, allow it to cool naturally to room temperature in air, and then demold to obtain a cast alloy ingot. Place the cast alloy ingot in a box-type resistance furnace and heat to 400°C at a heating rate of 3°C / min. The pretreated billet was held at ℃ for 7 hours and then cooled to room temperature in the furnace. It was then transferred to a controlled atmosphere heat treatment furnace and heated to 500℃ for 3 hours for high-temperature solution treatment. Within 10 seconds, it was transferred to a 30℃ water bath and immersed for 1.5 minutes for rapid quenching. Subsequently, it was held at 100℃ for 2.5 hours for the first stage of aging, and then heated to 180℃ for 5 hours for the second stage of aging. The aged billet was then transferred to a circulating air furnace and heated to 135℃ at a heating rate of 40℃ / min, held for 1.5 hours, and then cooled in the furnace at a cooling rate of 0.8℃ / min to below 60℃ before removal. A 10% (w / w) dilute nitric acid solution was used as the passivation solution, with a concentration of 20 L / m² based on the total surface area of ​​the billet. 2 Add the product, keep it at room temperature for 15 seconds, then remove it and rinse it with a spray at a flow rate of 25 L / min for 50 seconds. Then place it in a centrifugal dehydrator and dehydrate it at a speed of 650 r / min for 38 seconds. Finally, dry it in a 70℃ hot air drying oven for 8 minutes to obtain the Al-Zn-Si alloy product.

[0027] Control group C Take 85 parts aluminum, 8 parts zinc, 5 parts silicon, 0.5 parts manganese, 0.4 parts chromium, 1.0 part copper, and 0.1 parts total impurities. Place the above raw materials in a medium-frequency induction melting furnace, evacuate to a pressure below 10 Pa, then refill with high-purity argon to atmospheric pressure, heat to 735°C at a heating rate of 18°C / min, and hold at this temperature for 30 min under argon protection. After melting, stop heating and allow the melt to cool naturally to 710°C. Directly pour the melt using a bottom-pouring casting system without stirring with graphite rods, allowing it to stand, or scraping off non-metallic inclusions. After naturally cooling to room temperature in air, demold to obtain a cast alloy ingot. Place the cast alloy ingot in a box-type resistance furnace and heat to 400°C at a heating rate of 3°C / min, hold for 7 h, and cool to room temperature with the furnace. Transfer the pretreated billet to a controlled atmosphere heat treatment furnace and heat to 500°C, hold for 3 h for high-temperature solution treatment, and transfer to a 30°C furnace within 10 s. The billet was rapidly quenched by immersing it in a water bath for 1.5 minutes; then, it was aged at 100℃ for 2.5 hours for the first stage of aging, and then heated to 180℃ and held for 5 hours for the second stage of aging. The aged billet was then transferred to a circulating air furnace and heated to 135℃ at a heating rate of 40℃ / min and held for 1.5 hours. It was then cooled in the furnace at a cooling rate of 0.8℃ / min to below 60℃ and removed. Concentrated nitric acid with a mass fraction of 67% and deionized water were mixed at a volume ratio of 1:9, stirred at 350 rpm, and ozone gas was simultaneously introduced at a rate of 0.8 L / min for 18 minutes. Citric acid was then added to achieve a concentration of 0.08 mol / L to obtain a modified nitric acid passivation solution. The solution was then applied at a concentration of 20 L / m² based on the total surface area of ​​the billet. 2 Add passivation solution, keep at room temperature for 15s, remove and rinse with spray at a flow rate of 25L / min for 50s, then place in a centrifugal dehydrator and dehydrate at a speed of 650r / min for 38s, and finally dry in a 70℃ hot air drying oven for 8min to obtain Al-Zn-Si alloy finished product.

[0028] Control group D Take 85.9 parts aluminum, 8 parts zinc, 5 parts silicon, 1.0 part copper, and 0.1 parts total impurities. Place the above raw materials in a medium-frequency induction melting furnace, evacuate to a pressure below 10 Pa, then refill with high-purity argon to atmospheric pressure, heat to 735°C at a heating rate of 18°C / min, and hold at this temperature for 30 min under argon protection. Stir the melt with a graphite rod inserted into it, stirring once every 10 min for 25 s each time, for a total of three stirrings. After the third stirring, let it stand for 10 min, scrape off non-metallic inclusions from the surface, stop heating, and allow the melt to cool naturally to 710°C. Cast the ingot using a bottom-pouring casting system, allow it to cool naturally to room temperature in air, and then demold to obtain a cast alloy ingot. Place the cast alloy ingot in a box-type resistance furnace and heat to 400°C at a heating rate of 3°C / min, holding for 7 h, then cool to room temperature with the furnace. Transfer the pretreated billet to a controlled atmosphere heat treatment furnace and heat to 500°C, holding for 3 h. High-temperature solution treatment was performed, followed by rapid quenching by immersion in a 30°C water bath for 1.5 minutes within 10 seconds. The billet was then subjected to first-stage aging at 100°C for 2.5 hours, and then further aging at 180°C for 5 hours. After aging, the billet was transferred to a circulating air furnace and heated to 135°C at a rate of 40°C / min, held for 1.5 hours, and then cooled to below 60°C at a rate of 0.8°C / min. A 67% (w / w) concentrated nitric acid solution was mixed with deionized water at a volume ratio of 1:9, stirred at 350 rpm, and simultaneously infused with ozone gas at a rate of 0.8 L / min for 18 minutes. Citric acid was then added to achieve a concentration of 0.08 mol / L, yielding a modified nitric acid passivation solution. The solution was then applied at a concentration of 20 L / m² based on the total surface area of ​​the billet. 2 Add passivation solution, keep at room temperature for 15s, remove and rinse with spray at a flow rate of 25L / min for 50s, then place in a centrifugal dehydrator and dehydrate at a speed of 650r / min for 38s, and finally dry in a 70℃ hot air drying oven for 8min to obtain Al-Zn-Si alloy finished product.

[0029] Control group E Take 80 parts aluminum, 12 parts zinc, 5 parts silicon, 0.8 parts manganese, 0.6 parts chromium, 1.4 parts copper, and 0.2 parts total impurities. Place the above raw materials in a medium-frequency induction melting furnace, evacuate to a pressure below 10 Pa, then refill with high-purity argon to atmospheric pressure, heat to 735°C at a heating rate of 18°C / min, and hold for 30 min under argon protection. Stir the melt with a graphite rod inserted, stirring once every 10 min for 25 s each time, for a total of three stirrings. After the third stirring, let it stand for 10 min, scrape off non-metallic inclusions from the surface, stop heating, and allow the melt to cool naturally to 710°C. Cast the ingot using a bottom-pouring casting system, allow it to cool naturally to room temperature in air, and then demold to obtain a cast alloy ingot. Place the cast alloy ingot in a box-type resistance furnace, heat to 400°C at a heating rate of 3°C / min, hold for 7 h, and cool to room temperature with the furnace. Transfer the pretreated billet... The material is heated to 500℃ in a controlled atmosphere heat treatment furnace and held for 3 hours for high-temperature solution treatment. Within 10 seconds, it is transferred to a 30℃ water bath and immersed for 1.5 minutes for rapid quenching. Then, it is held at 100℃ for 2.5 hours for the first stage of aging, followed by a second stage of aging at 180℃ for 5 hours. The aged material is then transferred to a circulating air furnace and heated to 135℃ at a heating rate of 40℃ / min, held for 1.5 hours, and cooled to below 60℃ at a cooling rate of 0.8℃ / min. A 67% (w / w) concentrated nitric acid solution is mixed with deionized water at a volume ratio of 1:9, stirred at 350 rpm, and simultaneously purged with ozone gas at a rate of 0.8 L / min for 18 minutes. Citric acid is then added to achieve a concentration of 0.08 mol / L, yielding a modified nitric acid passivation solution. The solution is then applied at a concentration of 20 L / m² based on the total surface area of ​​the material. 2 Add passivation solution, keep at room temperature for 15s, remove and rinse with spray at a flow rate of 25L / min for 50s, then place in a centrifugal dehydrator and dehydrate at a speed of 650r / min for 38s, and finally dry in a 70℃ hot air drying oven for 8min to obtain Al-Zn-Si alloy finished product.

[0030] control group F Take 91 parts aluminum, 7.7 parts zinc, 1 part silicon, and 0.3 parts total impurities. Place the above raw materials in a conventional resistance melting furnace and heat to 740℃ in an atmospheric environment to melt them. Without vacuum degassing and argon protection, stir manually and pour directly into a metal mold. Cool in air and demold to obtain a cast alloy ingot. Place the cast alloy ingot in a box-type resistance furnace and heat to 400℃ for 2 hours for homogenization treatment. Water quench and cool to room temperature. Then heat to 180℃ and hold for 6 hours for single-stage aging treatment. Air cool to room temperature. Take a 10% dilute nitric acid solution and immerse it at room temperature for 15 seconds. Rinse with tap water and air dry to obtain the Al-Zn-Si alloy product.

[0031] Test methods: The yield strength, pitting corrosion resistance, and intergranular corrosion resistance of the microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy according to the present invention were tested respectively. The specific test methods are as follows: Yield strength test: Round bar tensile specimens with a diameter of 5 mm and a gauge length of 25 mm were cut from the alloy billets obtained in each embodiment and the control example along their length. Under room temperature conditions, an axial tensile load was applied using an electronic universal testing machine at a beam displacement rate of 1 mm / min. The force-displacement curves of the specimens during the tensile process were recorded. The force value at which the specimen underwent 0.2% plastic deformation was divided by the original cross-sectional area of ​​the specimen as the yield strength. Three parallel specimens were tested for each embodiment or control example, and the arithmetic mean was calculated as the final yield strength. The calculation formula is: ;in: Yield strength (unit: MPa). The tensile force (in N) is the value of the sample when it undergoes 0.2% plastic deformation. Original cross-sectional area of ​​the sample (unit: mm) 2 ).

[0032] Pitting resistance test: The alloy billets obtained in each example and the control example were cut into square test pieces of 10mm×10mm×3mm. After being progressively polished with silicon carbide sandpaper until the surface roughness Ra≤0.1μm, they were ultrasonically cleaned with acetone and alcohol for 5min each, and then immersed in a 3.5% sodium chloride aqueous solution at room temperature for 30min. Electrochemical measurements were performed using a three-electrode system, with a saturated calomel electrode as the reference electrode, a platinum electrode as the auxiliary electrode, and the test piece as the working electrode. After the open-circuit potential stabilized, a potential scan was performed at a scan rate of 0.5mV / s from a position relative to the open-circuit potential of -0.5V towards the anode, until the current density reached 100μA / cm². 2 Stop at a certain point; take the potential at which the current density on the polarization curve first shows a sustained increase (corresponding to an increase of more than one order of magnitude in corrosion current density) as the pitting potential. The difference between the pitting potential and the open circuit potential is the pitting potential difference, calculated using the formula ΔE=E_pit-E_ocp, where ΔE is the pitting potential difference (unit: V), E_pit is the pitting potential (unit: V), and E_ocp is the open circuit potential (unit: V). The larger the ΔE value, the higher the overpotential required for the passivation film to break down, and the less likely the material is to pit, i.e., the stronger its resistance to pitting corrosion. Three parallel specimens were tested for each embodiment or control example, and the arithmetic mean of the pitting potential difference was calculated.

[0033] Intergranular corrosion resistance test: The alloy billets obtained in each example and the control example were cut into rectangular samples of 15mm×10mm×5mm. After being polished stepwise with silicon carbide sandpaper until the surface roughness Ra≤0.2μm, they were immersed in an aqueous solution composed of 57g / L sodium chloride and 10mL / L hydrogen peroxide (mass fraction of 30%) and soaked in a constant temperature water bath at 35℃ for 6h. After being taken out, the samples were cut along the cross section and the corrosion morphology of the cross section was observed with a metallographic microscope. The maximum depth of the corrosion grooves at the grain boundaries was measured with a micrometer eyepiece. The maximum value of the corrosion depth of the sample in five different fields of view was taken as the intergranular corrosion depth. The smaller the depth, the stronger the resistance to intergranular corrosion. Three parallel samples were tested for each example or control example, and the arithmetic mean of the intergranular corrosion depth was calculated.

[0034] Specific testing indicators are shown in Table 1.

[0035] Table 1 Detection indicators of each sample according to Figures 2-4 As shown in Table 1, the yield strength, pitting potential difference, and intergranular corrosion depth of each embodiment and control example are analyzed as follows: The control group F, processed using ordinary melting without vacuum degassing and argon protection, and only undergoing single-stage aging treatment with dilute nitric acid passivation, exhibited a yield strength of only 256 MPa, a pitting potential difference of 0.09 V, and an intergranular corrosion depth of 96 μm. This was because oxygen and moisture were not removed during the casting process, leading to extensive oxidation of the aluminum melt and the inclusion of hydrogen and oxide film, resulting in dense shrinkage porosity and inclusions in the as-cast microstructure. The subsequent homogenization temperature and time were too low to eliminate dendritic segregation; the single-stage aging treatment failed to form a uniform nano-reinforcing phase; and the dilute nitric acid passivation could not remove the zinc-depleted areas and oxide scale on the surface. This control example showed the lowest performance across all tested samples, demonstrating that omitting any of the key steps in vacuum melting, homogenization pretreatment, staged aging, and modified nitric acid passivation would reduce the overall service performance of the alloy.

[0036] Control group D omitted the two microalloying elements, manganese and chromium, retaining only aluminum, zinc, silicon, and copper. Its yield strength was 340 MPa, pitting potential difference was 0.19 V, and intergranular corrosion depth was 48 μm. Compared to experimental group B (yield strength 352 MPa, pitting potential difference 0.45 V, intergranular corrosion depth 15 μm), control group D had a slightly lower yield strength, but its pitting potential difference decreased by more than 57%, and its intergranular corrosion depth increased by more than two times. This is because alloys without manganese and chromium cannot transform the coarse needle-like β-AlFeSi phase into spherical AlFeMnSi or AlFeCrSi phases during the high-temperature solution treatment stage. The needle-like phase is continuously distributed along the grain boundaries, serving as a preferential anodic dissolution channel in the corrosive environment. Simultaneously, the sharp ends of these needle-like phases mechanically damage the integrity of the passivation film. This control example demonstrates that the combined addition of manganese and chromium and their displacement reaction during the high-temperature solution treatment stage are beneficial for simultaneously improving resistance to pitting corrosion and intergranular corrosion.

[0037] Control group E increased the zinc content to 12 parts, while also increasing the contents of manganese, chromium, and copper. Its yield strength reached 372 MPa, the highest among all samples, but the pitting potential difference was only 0.15 V, and the intergranular corrosion depth reached 72 μm, which is 4.8 times that of experimental group B (pitting potential difference 0.45 V, intergranular corrosion depth 15 μm). This is because the excessive zinc content caused the solid solubility of zinc in the aluminum matrix to exceed the limit during the high-temperature solid solution stage, and some zinc prematurely agglomerated at the grain boundaries to form a zinc-rich phase. These zinc-rich phases could not be redissolved during subsequent quenching and aging processes, resulting in locally excessive zinc concentration near the grain boundaries, forming an anodic dissolution sensitive zone. This control example demonstrates that when the zinc content is within 5-10 parts, the alloy can achieve high yield strength while maintaining excellent resistance to pitting and intergranular corrosion. When the zinc content exceeds the upper limit of this range, even if the strength is further improved, the integrity of the passivation film and the corrosion stability of the grain boundary region will be severely damaged due to the premature agglomeration of zinc at the grain boundaries to form a zinc-rich phase.

[0038] Control group C omitted the graphite rod stirring, settling, and non-metallic inclusion scraping operations during the melting process. Its yield strength was 298 MPa, pitting potential difference was 0.29 V, and intergranular corrosion depth was 36 μm. Compared with experimental group B, the yield strength decreased by about 15%, the pitting potential difference decreased by about 36%, and the intergranular corrosion depth increased by about 140%. This is because the melt without stirring and settling treatment contained a large number of non-metallic inclusions (such as alumina film and undissolved salts). These inclusions became stress concentration sources and crack initiation points in the as-cast structure. At the same time, hydrogen atoms were easily adsorbed at the interface between the inclusions and the aluminum matrix, forming micropores, which reduced the density of the alloy and the continuity of the passivation film. This comparative example demonstrates that the graphite rod stirring, settling, and non-metallic inclusion scraping operations performed sequentially in the bottom-pouring casting system can significantly reduce oxide film and micropore defects in the as-cast structure, thereby improving the density of the alloy while suppressing the preferential corrosion channels at the interface between inclusions and the matrix.

[0039] Control group B used ordinary dilute nitric acid solution instead of modified nitric acid passivation solution for surface treatment. Its yield strength was 349 MPa, pitting potential difference was 0.23 V, and intergranular corrosion depth was 42 μm. Compared with experimental group B (yield strength 352 MPa, pitting potential difference 0.45 V, intergranular corrosion depth 15 μm), the yield strengths were basically the same, but the pitting potential difference of control group B decreased by about 49%, and the intergranular corrosion depth increased by about 180%. This is because ordinary dilute nitric acid solution can only remove surface oxide scale and cannot selectively dissolve the zinc-depleted areas on the surface formed by quenching segregation. In contrast, the passivation film formed slowly and is porous due to the lack of nitrogen-containing active intermediates, making it susceptible to localized breakdown in chloride-containing environments. This comparative example demonstrates that the nitric acid passivation solution modified with ozone and citric acid can accelerate oxide scale removal and selectively dissolve zinc-poor areas on the surface through nitrogen-containing active intermediates, while simultaneously generating a continuous and firmly adhered passivation film on the billet surface. In contrast, ordinary dilute nitric acid solution, lacking the aforementioned active components, cannot densify the passivation film within the same contact time, resulting in pores in the surface film that become channels for corrosive media penetration.

[0040] Control group A, which used single-stage aging treatment instead of multi-stage aging, and omitted the high-temperature solution treatment and rapid quenching steps, achieved a yield strength of 315 MPa, a pitting potential difference of 0.18 V, and an intergranular corrosion depth of 58 μm. Compared with experimental group B, the yield strength decreased by approximately 10.5%, the pitting potential difference decreased by 60%, and the intergranular corrosion depth increased by approximately 287%. This is because without the high-temperature solution treatment, manganese and chromium cannot diffuse to the acicular iron-rich phase interface and undergo a substitution reaction, and the acicular β-AlFeSi phase is completely preserved at the grain boundaries. Without the rapid quenching step, zinc and copper prematurely precipitate coarse equilibrium phases during slow cooling, consuming solute atoms used for aging strengthening. Single-stage aging cannot achieve the goal of copper forming the GP zone first and zinc precipitating later on the GP zone. The hierarchical synergistic precipitation mechanism of the phases resulted in uneven distribution of the reinforcing phase and the appearance of precipitation-free zones at grain boundaries. This comparative example demonstrates that multi-stage heat treatment control enables manganese and chromium to independently complete the replacement reaction of harmful needle-like phases at high temperatures without interfering with the solid solution state of zinc and copper, while simultaneously enabling zinc and copper to form high-density nano-reinforcing phases at low temperatures through stepwise nucleation and precipitation mechanisms.

[0041] In summary, the yield strengths of experimental groups A, B, and C reached 368 MPa, 352 MPa, and 331 MPa, respectively, all higher than those of all control examples except for control group E; the pitting potential differences reached 0.42 V, 0.45 V, and 0.48 V, respectively, and the intergranular corrosion depths were 18 μm, 15 μm, and 12 μm, respectively, demonstrating superior resistance to pitting and intergranular corrosion compared to all control examples. These data prove that the present invention, by limiting manganese and chromium to 0.3-0.8 parts and 0.2-0.6 parts, and zinc and copper to 5-10 parts and 0.5-1.5 parts, respectively, and employing a multi-stage heat treatment process combining high-temperature solution treatment, rapid quenching, and graded aging, can simultaneously improve the alloy's resistance to pitting and intergranular corrosion while maintaining high yield strength, thus resolving the temperature window contradiction between the regulation of iron-rich phase morphology and the precipitation of age-strengthening phases.

[0042] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A heat treatment process for microalloyed high-strength, corrosion-resistant Al-Zn-Si alloys, characterized in that, Includes the following steps: S1. Industrial aluminum, zinc, silicon, manganese, chromium and copper are placed in a medium frequency induction melting furnace for melting to obtain an alloy melt; it is poured using a bottom pouring system, and after natural cooling to room temperature, it is demolded to obtain a cast Al-Zn-Si alloy ingot. S2. Place the as-cast Al-Zn-Si alloy ingot in a box-type resistance furnace and keep it at a constant temperature to fully dissolve the non-equilibrium eutectic phase in the as-cast structure and reduce dendrite segregation. After cooling to room temperature in the furnace, take it out to obtain a pre-treated billet with uniform composition. S3. The pretreated billet is transferred to a controlled atmosphere heat treatment furnace for multi-stage heat treatment to obtain an alloy billet with dispersed phase distribution and age-strengthening phase distribution. S4. Transfer the alloy billet to a circulating air furnace for heat preservation and slow cooling to obtain a low-stress alloy billet. S5. Transfer the stressed alloy billet to an acid-resistant impregnation tank, add modified nitric acid passivation solution, keep it at room temperature for 10-20s, then remove it and rinse it in deionized water at a flow rate of 20-30L / min for 40-60s. Then place it in a centrifugal dehydrator and dehydrate it at a speed of 500-800r / min for 30-45s. Finally, dry it in a hot air drying oven at 60-80℃ for 5-10min to obtain the Al-Zn-Si alloy product.

2. The heat treatment process for microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy according to claim 1, characterized in that, In S1, the melting steps of the medium-frequency induction melting furnace include: first, evacuating the furnace chamber to a pressure below 10Pa, then refilling it with high-purity argon to atmospheric pressure to remove residual oxygen and moisture from the furnace; then heating it to 720-750℃ at a heating rate of 15-20℃ / min, and holding it at that temperature for 25-35min under an argon protective atmosphere, so that all alloy raw materials are completely melted and form a uniform melt.

3. The heat treatment process for microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy according to claim 1, characterized in that, In S1, the casting operation of the bottom-pouring casting system includes: inserting a graphite rod into the melt for stirring, stirring once every 10 minutes, for a total of three stirrings, each stirring lasting 20-30 seconds; after the third stirring, let it stand for 10 minutes, then stop heating and allow the melt to cool naturally to 700-720℃, at which point bottom-pouring can be performed.

4. The heat treatment process for microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy according to claim 1, characterized in that, In step S2, the box-type resistance furnace heats the material to 380-420℃ at a heating rate of 2-5℃ / min and holds it at that temperature for 6-8 hours.

5. The heat treatment process for microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy according to claim 1, characterized in that, In step S3, the multi-stage heat treatment control includes the following steps: S3.1 The billet is heated to 480-520℃ and held in a controlled atmosphere heat treatment furnace for 2-4 hours to form a supersaturated solid solution matrix; S3.2 Transfer the solid solution matrix to a water bath at 20-40℃ within 10s, immerse it completely and keep it for 1-2 minutes to allow the billet temperature to drop to room temperature, forming a supersaturated aluminum solid solution with high concentration of vacancy trapping ability; S3.3 First, the supersaturated aluminum solid solution is held at 80-120℃ for 2-3 hours to promote the migration of copper atoms to vacant positions in the aluminum matrix and form atomic-scale GP regions; then the temperature is raised to 160-200℃ and held for 4-6 hours to obtain the alloy billet.

6. The heat treatment process for microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy according to claim 1, characterized in that, In step S4, the circulating air furnace heats the air to 120-150°C at a heating rate of 30-50°C / min and holds it at that temperature for 1-2 hours. Then, it is cooled down to below 60°C with the furnace at a cooling rate of 0.5-1°C / min and removed from the furnace.

7. The heat treatment process for microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy according to claim 1, characterized in that, In step S5, the preparation method of the modified nitric acid passivation solution includes the following steps: Prepare a primary nitric acid solution with a volume concentration of 10% by mixing concentrated nitric acid (64-70% by mass) and deionized water at a volume ratio of 1:

9. Place the primary nitric acid solution in a sealed container, stir at 300-400 r / min, and simultaneously introduce ozone gas at a rate of 0.5-1.0 L / min for 15-20 min. Then add citric acid to the solution to achieve a concentration of 0.05-0.10 mol / L in the modified nitric acid passivation solution, thus obtaining the modified nitric acid passivation solution.

8. The heat treatment process for microalloyed high-strength and corrosion-resistant Al-Zn-Si alloy according to claim 1, characterized in that, In step S5, the amount of modified nitric acid passivation solution added is 15-25 L / m² based on the total surface area of ​​the billet. 2 .

9. A microalloyed high-strength corrosion-resistant Al-Zn-Si alloy prepared by the heat treatment process of the microalloyed high-strength corrosion-resistant Al-Zn-Si alloy according to any one of claims 1-8, characterized in that, It consists of the following raw materials: Aluminum 80-90 parts; Zinc 5-10 parts; Silicon 3-6 parts; Manganese 0.3-0.8 parts; Chromium 0.2-0.6 parts; Copper 0.5-1.5 parts; Total impurities 0-0.2 parts; wherein: the manganese and chromium form a manganese / chromium-containing spherical or blocky dispersed phase with aluminum and iron components in the iron-rich phase, which can preferentially diffuse to the interface of the needle-like iron-rich phase and undergo a displacement reaction during the high-temperature solid solution stage, transforming the coarse needle-like phase into fine dispersed particles that do not damage the integrity of the passivation film; at the same time, the copper and zinc remain completely dissolved in the aluminum matrix during the high-temperature solid solution stage, and together form nanoscale GP regions during the low-temperature aging stage, and further precipitate MgZn2 as the main component. Phases are used to maintain a coherent or semi-coherent relationship with the aluminum matrix.

10. The microalloyed high-strength, corrosion-resistant Al-Zn-Si alloy according to claim 9, characterized in that, The nanoscale The average diameter of the phase is 5-20 nm, and the average length is 30-80 nm.

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