Corrosion-resistant Sb-Bi composite microalloyed high-strength aluminum alloy and preparation method thereof

Abstract

The application discloses a kind of corrosion-resistant Sb-Bi composite micro-alloying high-strength aluminum alloy and preparation method thereof, belong to high-strength aluminum alloy material and its structure and performance control technical field.The aluminum alloy is Al-Zn-Mg-Cu alloy as matrix, its chemical composition includes by mass percentage:Zn:5.0~8.0 wt.%;Mg:0.5~2.5 wt.%;Cu:1.0~2.0 wt.%;Sc:0.1~0.5 wt.%;Zr:0.05~0.5 wt.%;Sb:0.1~0.6 wt.%;Bi:0.1~0.5wt.%;balance is Al and inevitable impurity element.For the problem that the corrosion resistance of existing Sc, Zr modified 7xxx series aluminum alloy is limited, the application is micro-alloyed by Sb and Bi, cooperates and controls grain boundary structure and precipitate distribution, reduces the connectivity of grain boundary continuous precipitate, makes corrosion change from local corrosion along grain boundary to more uniform corrosion form.Compared with alloy without Sb-Bi composite micro-alloying, the corrosion rate of the prepared aluminum alloy in simulated seawater or salt spray environment is significantly reduced, and the corrosion resistance is significantly improved.

Description

Technical Field

[0001] This invention belongs to the technical field of high-strength aluminum alloy materials and their microstructure and performance regulation, specifically relating to a corrosion-resistant Sb-Bi composite microalloyed high-strength aluminum alloy and its preparation method. Background Technology

[0002] 7xxx series aluminum alloys (represented by Al-Zn-Mg-Cu alloys) are widely used in aerospace, shipbuilding, and high-end transportation equipment due to their high specific strength and good machinability. However, these alloys are quite sensitive to corrosive environments under high strength conditions, especially in chlorine-containing media or marine environments, where they are prone to intergranular corrosion, exfoliation corrosion, and stress corrosion cracking, which seriously affect the service safety and service life of structural components.

[0003] To improve the microstructure stability and mechanical properties of 7xxx series aluminum alloys, researchers have developed a 7A33 aluminum alloy that balances strength and corrosion resistance, expanding the application of 7-series aluminum alloys in the marine field. However, further breakthroughs are needed in strength and marine corrosion resistance. Existing technologies commonly employ the method of adding trace amounts of elements such as Sc and Zr. Research and patent literature indicate that Sc and Zr can form a dispersed Al3(Sc,Zr) phase in the alloy, thereby refining the grains, inhibiting recrystallization, and improving thermal stability. For example, Chinese patent literature CN107299236A (publication date: 2010) discloses a method for modifying Al-Zn-Mg-Cu alloys by adding Sc and Zr, which significantly improves the alloy's strength and microstructure stability.

[0004] However, existing Sc and Zr modification techniques mainly focus on grain refinement and mechanical property improvement, with limited improvement on the corrosion resistance of alloys in highly corrosive environments. Numerous studies have shown that even with the addition of Sc and Zr, 7xxx series aluminum alloys still tend to form continuous precipitates at grain boundaries, thus providing rapid propagation channels for corrosive media, resulting in a still significant tendency for grain boundary corrosion and stress corrosion cracking (e.g., J. Mater. Sci., 2016, 51: 1234-245).

[0005] To further improve the overall performance of 7A33 aluminum alloy, conventional alloying methods are unlikely to significantly alter its properties. Our research group discovered that antimony (Sb), as a trace alloying element, can influence the morphology and distribution of precipitated phases in aluminum alloys and, to some extent, improve grain boundary structure and corrosion resistance. However, existing technologies primarily focus on the single addition of Sb, whose effectiveness is significantly limited by the addition amount window, and still falls short in achieving a balance between high strength and high corrosion resistance.

[0006] On the other hand, bismuth (Bi), as a low-melting-point trace element, tends to segregate at grain boundaries in aluminum alloys. Studies have shown that Bi's grain boundary segregation behavior can alter grain boundary energy states and influence the corrosion process (Corros.Sci., 2018, 139: 295-304). However, in existing publicly available technologies, Bi is mostly used as an auxiliary or incidental dopant element, and there is limited research on its systematic application in high-strength 7xxx series aluminum alloys, lacking clear process design and performance evaluation. Our research group discovered that the interaction between Bi and Mg forms the MgBi phase, which can regulate the morphology distribution of MgZn, thereby affecting strength, toughness, and corrosion resistance.

[0007] In summary, the existing technology has the following main shortcomings:

[0008] (1) Although Sc and Zr modification can improve the strength and microstructure stability of 7xxx series aluminum alloys, their effect on inhibiting grain boundary corrosion and stress corrosion cracking is limited.

[0009] (2) The microalloying effect window of a single trace element (such as Sb or Bi) is narrow, making it difficult to achieve significant improvement in corrosion resistance under high strength conditions at the same time.

[0010] (3) There is still a lack of systematic technical solutions for synergistic regulation of grain boundary structure, precipitate morphology and corrosion behavior through Sb and Bi composite microalloying.

[0011] Therefore, there is an urgent need to develop a new technical method that, through composite microalloying, can significantly improve the grain boundary stability and corrosion resistance of Al-Zn-Mg-Cu-Sc-Zr alloys while maintaining their high strength characteristics, in order to meet the service requirements of high-end equipment in complex corrosive environments. Summary of the Invention

[0012] To address the problems existing in the prior art, this invention provides a corrosion-resistant Sb-Bi composite microalloyed high-strength aluminum alloy and its preparation method. This invention introduces a composite microalloying design of Sb and Bi into the Al-Zn-Mg-Cu-Sc-Zr alloy system, synergistically controlling the grain boundary structure and precipitate distribution of the alloy, thereby significantly improving corrosion resistance while maintaining high strength.

[0013] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0014] A corrosion-resistant Sb-Bi composite microalloyed high-strength aluminum alloy, wherein the aluminum alloy uses an Al-Zn-Mg-Cu alloy as the matrix, and its chemical composition by mass percentage includes: Zn, Mg, Cu: the content of which conforms to the national standard composition range of 7xxx series aluminum alloys; Zn: 5.0~8.0 wt.%; Mg: 0.5~2.5 wt.%; Cu: 1.0~2.0 wt.%; Sc: 0.1~0.5 wt.%; Zr: 0.05~0.5 wt.%; Sb: 0.1~0.6 wt.%; Bi: 0.1~0.5 wt.%; the balance being Al and unavoidable impurity elements.

[0015] Through the above composition design, Sc and Zr form a dispersed stable phase in the alloy, which is used to refine the grains and inhibit recrystallization; Sb and Bi exist in the form of composite microalloying, which improves the grain boundary state and corrosion resistance through synergistic segregation and precipitation regulation.

[0016] A method for preparing the above-mentioned corrosion-resistant Sb-Bi composite microalloyed high-strength aluminum alloy, wherein the method comprises:

[0017] Step 1: Melting and Alloying: Heat high-purity aluminum to 630~750℃ and add Zn, Mg, and Cu sequentially to the melt to form an Al-Zn-Mg-Cu alloy matrix. While maintaining a stable melt temperature, add Sc and Zr to the melt to achieve the designed content, and stir thoroughly to ensure uniform element distribution. Then, add Sb and Bi simultaneously to the melt for composite microalloying treatment, and continue stirring to obtain a uniform alloy melt. Stir for 10~30 minutes after each element is added before adding the next.

[0018] Step 2: Casting and Homogenization Heat Treatment: The above alloy melt is poured into ingots and cooled to room temperature; the ingots are then subjected to homogenization annealing treatment.

[0019] Step 3: Plastic deformation processing: The ingot after homogenization annealing is hot rolled or warm rolled to introduce dislocations and refine the microstructure through plastic deformation, so as to obtain the required plate or profile.

[0020] Step 4: Heat treatment: The rolled alloy is subjected to solution treatment and aging treatment to obtain a stable microstructure and excellent comprehensive properties.

[0021] Furthermore, in step two, the annealing temperature is 400~480℃ and the holding time is 12~24 h, which can reduce the component segregation in the as-cast structure and stabilize the alloy microstructure.

[0022] Furthermore, in step three, the temperature of the rolls is controlled within the range of 100~200℃ during the rolling process.

[0023] Furthermore, in step three, the solution treatment temperature is 460~480℃ and the time is 0.5~2 h.

[0024] Furthermore, in step three, the aging treatment temperature is 120℃, and the aging time is set to 0~72h.

[0025] Compared with existing technologies, this invention introduces Sb-Bi composite microalloying into the Al-Zn-Mg-Cu-Sc-Zr alloy system, achieving a synergistic improvement in mechanical properties and corrosion resistance without increasing process complexity, and has the following significant effects and advantages:

[0026] (1) Significantly improves corrosion resistance while maintaining high strength: Addressing the limited improvement in corrosion resistance of existing Sc and Zr-modified 7xxx series aluminum alloys, this invention utilizes Sb-Bi composite microalloying to synergistically regulate grain boundary structure and precipitate distribution, reducing the connectivity of continuous precipitates at grain boundaries and transforming corrosion from localized corrosion extending along grain boundaries to a more uniform corrosion morphology. Compared to alloys without Sb-Bi composite microalloying, the prepared aluminum alloy exhibits a significantly reduced corrosion rate and significantly improved corrosion resistance in simulated seawater or salt spray environments.

[0027] (2) Improved overall mechanical properties and higher stability: Through the grain refinement of Sc and Zr and the synergistic regulation of precipitates by Sb-Bi, the alloy of this invention exhibits excellent mechanical properties in both the cast and rolled states. Compared with the Al-Zn-Mg-Cu-Sc-Zr alloy without composite microalloying, the tensile strength and yield strength of the prepared alloy are significantly improved, while maintaining good plasticity and microstructure stability, making it suitable for structural parts with high requirements for strength and reliability.

[0028] (3) Suppressing the tendency of grain boundary corrosion and stress corrosion cracking: In view of the problem that continuous precipitates are easily formed at the grain boundaries of high-strength 7xxx series aluminum alloys and that corrosive media are easily spread along the grain boundaries, this invention improves the energy state and chemical stability of the grain boundaries through the synergistic segregation effect of Sb-Bi, effectively weakens the rapid diffusion ability of corrosive media at the grain boundaries, thereby reducing the tendency of grain boundary corrosion and stress corrosion cracking, and improving the safety of the alloy in complex service environments.

[0029] (4) Clear composition design, stable performance window, and engineering repeatability: This invention clearly provides a composite microalloying scheme of Sb 0.4wt.% and Bi 0.3wt.%, which avoids the problems of narrow working window and large performance fluctuation in single microalloying, making the overall performance of the alloy more stable and easy to replicate and promote in engineering production.

[0030] (5) The process is simple and the cost increase is limited, making it suitable for industrial application: The present invention can achieve performance improvement by using conventional smelting, homogenization annealing and rolling processes, without the need to introduce additional complex processing or special equipment; the addition of Sb and Bi is low, which has little impact on raw material cost and processing performance, and has good economic and industrial applicability.

[0031] (6) On the premise of maintaining the high strength and structural stability of Al-Zn-Mg-Cu-Sc-Zr high-strength aluminum alloy, the problem of further improving its corrosion resistance is: Although the existing Sc and Zr modification technology can effectively refine the grains and inhibit recrystallization, its inhibitory effect on grain boundary corrosion and stress corrosion cracking is limited. This invention improves the grain boundary structure and corrosion behavior through Sb-Bi composite synergistic regulation, thereby improving the service reliability of the alloy in the corrosive environment.

[0032] (7) Solving the problem of narrow window of action and limited improvement of comprehensive performance of single trace element microalloying: In the prior art, it is difficult to take into account both mechanical properties and corrosion resistance under high strength conditions by adding Sb or Bi alone. This invention uses the composite microalloying of Sb and Bi to give full play to the synergistic effect of the two and achieve synergistic control of grain boundaries and precipitates.

[0033] (8) Solving the problem that continuous precipitates at grain boundaries are easy to form and corrosion channels are difficult to block effectively: This invention uses composite microalloying to suppress the formation of continuous precipitates at grain boundaries or reduce their connectivity, thereby weakening the ability of corrosive media to extend along grain boundaries and improving the corrosion morphology of the alloy.

[0034] (9) A comprehensive performance optimization method that is simple to implement in the existing 7xxx series aluminum alloy production process is provided: without introducing complex metallurgical treatment or additional surface protection process, by reasonably designing the composite addition scheme of Sb and Bi, the high strength and high corrosion resistance can be synergistically improved to meet the needs of engineering applications. Attached Figure Description

[0035] Figure 1 For technology roadmap;

[0036] Figure 2 This is a schematic diagram of the rolling process.

[0037] Figure 3 This is an aging hardness diagram;

[0038] Figure 4 Transmission electron microscopy (TEM) image of the aged precipitation;

[0039] Figure 5 EDS elemental distribution diagram of the second phase of CB6 alloy;

[0040] Figure 6TEM and HRTEM images of the precipitated phases in CB6 alloy;

[0041] Figure 7 TEM images of CB6 and CB9 samples at different magnifications;

[0042] Figure 8 The image shows the cross-sectional morphology of intergranular corrosion in four gold-bearing composites.

[0043] Figure 9 SEM images of 7-day salt spray corrosion: (ad) Corrosion products not removed; (eh) After corrosion products were removed; (a, e) O alloy; (b, f) Bi3 alloy; (c, g) Sb4 alloy; (d, h) CB6 alloy.

[0044] Figure 10 Comparative images of the surface corrosion morphology of four alloys after 1–3 months of marine exposure. The four columns from left to right represent 0, Bi3, Sb4, and CB6. Detailed Implementation

[0045] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0046] Composite microalloying refers to the simultaneous introduction of two or more trace alloying elements into an alloy, which enable them to have a synergistic effect during the evolution of the microstructure, thereby achieving better microstructure control and performance improvement than single microalloying.

[0047] Synergistic effect refers to the synergistic regulatory effect of different trace alloying elements on grain structure, grain boundary state or corrosion behavior by influencing each other's solid solution, segregation or precipitation behavior. The overall effect is significantly better than the simple superposition of the effects of each element alone.

[0048] Grain boundary segregation refers to the enrichment of trace elements in the grain boundary region of an alloy. This behavior can significantly affect the grain boundary energy state, morphology of precipitated phases, and grain boundary corrosion behavior.

[0049] Continuous grain boundary precipitates refer to precipitates that are continuously distributed at grain boundaries. Their presence can easily become a channel for the rapid propagation of corrosive media, and is one of the important reasons for grain boundary corrosion and stress corrosion cracking in high-strength 7xxx series aluminum alloys.

[0050] Homogenization annealing refers to the heat treatment process performed on as-cast alloys. By holding the alloy at appropriate temperatures and times, the composition distribution in the alloy becomes more uniform, dendrite segregation and casting defects are reduced, and a stable microstructure is provided for subsequent plastic deformation processing.

[0051] Hot rolling / warm rolling refers to the rolling deformation process of an alloy at or near its recrystallization temperature. By introducing plastic deformation and dislocation structure, it promotes microstructure refinement and improves mechanical properties.

[0052] Corrosion resistance refers to the ability of an alloy material to resist corrosion damage in a specific corrosive environment (such as simulated seawater or salt spray environment), and is usually characterized by indicators such as corrosion rate, corrosion current density, and corrosion morphology.

[0053] Uniform corrosion refers to a corrosion pattern that occurs relatively evenly on the surface of a material. Compared to corrosion patterns that are concentrated along grain boundaries or locally, it has a smaller impact on the structural safety during service.

[0054] The core inventive point of this invention lies in: proposing a composition design scheme for Sb-Bi composite microalloying of aluminum alloys, which achieves synergistic control of grain boundary structure and precipitate morphology by simultaneously introducing trace elements of Sb and Bi, significantly different from existing single microalloying technologies; clearly providing a composite addition scheme of Sb and Bi (Sb 0.4wt.%, Bi 0.3wt.%), achieving synergistic improvement of mechanical properties and corrosion resistance in the Al-Zn-Mg-Cu-Sc-Zr alloy system; the above performance optimization can be achieved through conventional melting-homogenization annealing-rolling processes, without the need for additional complex treatments, and has good engineering applicability.

[0055] Example 1: Preparation method of Sb-Bi composite microalloyed high-strength corrosion-resistant aluminum alloy

[0056] This embodiment uses an Al-Zn-Mg-Cu-Sc-Zr aluminum alloy as the matrix and prepares an aluminum alloy material with both high strength and excellent corrosion resistance through composite microalloying of Sb and Bi.

[0057] (1) Alloy composition design

[0058] The aluminum alloy, by mass percentage, comprises: Zn: 5.0~7.0 wt.%; Mg: 1.5~2.5 wt.%; Cu: 1.0~2.0 wt.%; Sc: 0.10~0.25 wt.%; Zr: 0.05~0.20 wt.%; Sb: 0.4 wt.%; Bi: 0.3 wt.%, with the balance being Al and unavoidable impurity elements. The introduction of Sb and Bi through a composite microalloying method is a key feature distinguishing this invention from existing technologies.

[0059] (2) Preparation process

[0060] The method for preparing the aluminum alloy includes the following steps:

[0061] Step S1: Raw material preparation and batching: Select industrial pure aluminum, Al-Zn, Al-Mg, Al-Cu, Al-Sc, Al-Zr, Al-Sb and Al-Bi master alloys, and batch them according to the above composition ratio.

[0062] Step S2: Melting and Alloying Treatment: Add industrial pure aluminum to a melting furnace and heat until completely melted. Then, add Zn, Mg, and Cu master alloys sequentially, stirring until homogeneous. Next, add Al-Sc, Al-Zr, Al-Sb, and Al-Bi master alloys sequentially, controlling the melting temperature at 700–750 °C to ensure complete dissolution and uniform distribution of all alloying elements. Stir for 10–20 minutes after each element is added before adding the next.

[0063] Step S3: Casting: After refining, degassing and slag removal, the melt is poured into a metal mold, cooled and shaped to obtain an ingot.

[0064] Step S4: Homogenization annealing: The ingot is subjected to homogenization annealing at a temperature of 450–480 °C and held for 12–18 h to eliminate component segregation and improve the uniformity of the microstructure.

[0065] Step S5: Rolling: The homogenized annealed ingot is hot-rolled and / or warm-rolled to obtain plates or profiles of the required thickness. The roll temperature is controlled within the range of 140–180°C during the rolling process.

[0066] Step S6: Heat Treatment: The rolled alloy is subjected to solution treatment and aging treatment to obtain a stable microstructure and excellent comprehensive properties. The solution treatment temperature is 460~480℃, and the time is 0.5~2 h. The aging treatment temperature is 120℃, and the aging time is 28~32 h.

[0067] The prepared Sb-Bi composite microalloyed aluminum alloy has fine grains, a uniform microstructure, and reduced continuity of grain boundary precipitates. While maintaining high tensile strength and high yield strength, its corrosion resistance is significantly better than that of similar alloys without Sb-Bi composite microalloying. The corrosion morphology changes from localized grain boundary corrosion to a more uniform corrosion morphology. It is suitable for marine environments, high-humidity environments, or structural components with high corrosion resistance requirements. The data obtained in Example 1 are shown in the table below:

[0068] Table 1 Comparison of tensile data

[0069]

[0070] Table 2 Comparison of intergranular corrosion data

[0071]

[0072] Table 3 Electrochemical test polarization data

[0073]

[0074] Table 4 Electrochemical test impedance data

[0075]

[0076] Example 2: Implementation method for replacing Sb and Bi content

[0077] Based on Example 1, the Sb content can be adjusted within the range of 0.2~0.6 wt.%, and the Bi content can be adjusted within the range of 0.1~0.5 wt.%, while still achieving good overall performance. When Sb and Bi are added in combination, the alloy exhibits a better synergistic effect in terms of corrosion resistance and microstructure stability compared to adding Sb or Bi alone.

[0078] Example 3:

[0079] An alloy based on 7A33, specifically Al-5Zn-2.5Mg-1.5Cu-0.3Sc-0.2Zr, was alloyed by adding different amounts of Bi and Sb. In Al-Zn-Mg-Cu-Sc-Zr alloys, the amount of trace elements added is generally less than 0.5 wt.%. Therefore, in the Al-5Zn-2.5Mg-0.5Cu-0.3Sc-0.2Zr alloy, five addition amounts of Bi and Sb were selected each within 0.5 wt.% (0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, and 0.5 wt%). To systematically reveal the microalloying of Sb and Bi and its combined effects, this invention first designed single microalloyed alloys of Sb and Bi, and determined the optimal content range for microstructure and properties through multi-content gradient experiments. Based on the performance window obtained from a single microalloying, an Sb-Bi composite microalloying system was constructed using a 3×3 content combination matrix. Two additional alloys were selected at the edge of the parameter space to verify the stability of the trend. A total of 22 alloys were designed, and their grouping and nominal chemical composition are shown in Table 5.

[0080] Table 5 Composite Microalloying Design Matrix

[0081]

[0082] Table 6. Design of Nominal Chemical Composition of Experimental Alloys (wt.%)

[0083]

[0084] Table 7 Overview of Alloy Systems and Grouping

[0085]

[0086] Experimental aluminum alloys with different chemical compositions were prepared using conventional casting processes. The raw materials used in the experiments were high-purity aluminum (99.99%), Al-50%Cu, Al-10%Mg, Al-2%Sc and Al-5%Zr master alloys, as well as pure Sb, Bi and Zn particles.

[0087] Step 1: Alloy melting is carried out in a low-frequency resistance furnace under an argon protective atmosphere to reduce high-temperature oxidation. The furnace temperature is 700–750°C. After the melt is fully melted and alloyed, trace elements are added in the final stage of melting. After the main alloying elements are completely dissolved and initially homogenized, the melt temperature is lowered to approximately 630°C, and then the trace elements are quickly added and thoroughly stirred. This process aims to shorten the residence time of trace elements at high temperatures, prevent their volatilization, and thus ensure their uniform distribution in the matrix. The alloy melt is then poured into a preheated steel mold at 200°C to form an ingot.

[0088] Step 2: Homogenization and Rolling Process

[0089] According to the requirements for the homogenization process of 7X aluminum alloy in national standards (YS / T1624-2023) and (GJB1604A-2019), the heating rate and holding time of the homogenization process were determined by preparing alloy samples based on their shape, size, and thickness. The final homogenization process was determined to be 460℃ × 12h, with a heating rate of 5℃ / min. A schematic diagram of the rolling process is shown below. Figure 2 As shown.

[0090] The Al3(Sc,Zr) phase in the Al-Zn-Mg-Cu-Sc-Zr alloy can suppress recrystallization and improve the alloy's mechanical properties. At around 400℃, the Al3(Sc,Zr) precipitates exhibit a high nucleation rate. Therefore, the alloy undergoes a pretreatment step before rolling, with pretreatment parameters set at 400℃ for 1 hour. Subsequently, a multi-pass hot rolling process is used, with a reduction of approximately 10% per pass. After each rolling pass, the sample is immediately transferred to a holding furnace at 200℃ for 5 minutes to ensure uniform temperature distribution within the material. The total deformation is approximately 80%, ultimately yielding an alloy sheet with a thickness of approximately 1.5 mm.

[0091] The following four alloys are used as representative research objects for solution aging: matrix alloy (0), 0.3wt.%Bi alloy (Bi3), 0.4wt.%Sb alloy (Sb4), and 0.3wt.%Bi+0.4wt.%Sb composite alloy (CB6).

[0092] Step 3: Heat Treatment Process

[0093] (1) Solution Treatment: Based on the 7A33 alloy, this invention systematically explores the solution treatment process of a novel micro-alloyed Al-Zn-Mg-Cu-Sc-Zr alloy system. To determine a reasonable solution treatment temperature range, differential scanning calorimetry (DSC) analysis was performed on a representative rolled alloy to evaluate the dissolution behavior of the second phase and the characteristics of potential low-melting-point phases in the alloy. Based on the DSC curve characteristics and relevant literature reports, 460℃, 470℃, and 480℃ were selected as solution treatment temperatures for comparative studies. The solution treatment times were set to 0.5h, 1h, 1.5h, and 2h, respectively, and the effects of different holding times on the solution treatment effect and subsequent aging response behavior of the alloy were systematically investigated. All solution treatments were carried out in a box-type resistance furnace, and water quenching was performed immediately after solution treatment. The quenching transfer time was controlled within 10s to ensure a fully supersaturated solid solution structure.

[0094] (2) Aging Treatment: After solution treatment and obtaining a supersaturated solid solution microstructure, each alloy was subjected to artificial aging treatment to study the influence of microalloying elements on aging precipitation behavior and property evolution. Based on the typical aging regime of 7xxx series aluminum alloys and previous research, 120℃ was uniformly selected as the artificial aging temperature in this invention. The aging treatment was carried out in a constant temperature forced-air drying oven, with aging time set to 0~72h and time intervals of 4h. The changes in microstructure and properties of the alloy at different aging stages were systematically investigated by sampling at different times. After aging, the samples were air-cooled to avoid interference from additional heat on the aged microstructure.

[0095] Peak delivery

[0096] Based on the evolution of alloy mechanical properties with aging time during artificial aging, this invention defines the state in which the corresponding mechanical properties reach their peak value during artificial aging as the peak aging state. Through systematic testing of alloy property changes during aging at 120℃ for 0–72 h, the peak aging time range for each alloy was determined. Based on this, samples corresponding to the peak aging state were selected for subsequent experimental studies, including microstructure characterization, mechanical property testing, and corrosion behavior analysis.

[0097] The introduction of trace amounts of Sb and Bi significantly affects the microstructure evolution and performance of Al-Zn-Mg-Cu-Sc-Zr alloys, with distinct differences in their mechanisms of action. Sb primarily exists as the AlSb phase in the alloy, regulating the continuity of the grain boundary second phase. The addition of Sb reduces the proportion of continuous precipitates at grain boundaries, improves grain boundary structural stability, thereby weakening electrochemical inhomogeneities at grain boundaries and reducing intergranular corrosion susceptibility. Sb also has a certain inhibitory effect on grain boundary migration, contributing to maintaining microstructure stability. However, Sb's direct contribution to the precipitation strengthening system is relatively limited; its strengthening effect mainly stems from grain boundary structure optimization and interface stabilization. The results indicate that Bi's influence on the alloy is primarily manifested in the regulation of precipitation behavior. Bi can alter the local distribution and diffusion behavior of Mg, thus affecting the nucleation and growth process of the η′ phase. Within an appropriate content range, Bi helps increase the number density of precipitates and refine their size distribution, enhancing the strengthening effect. Therefore, Bi's contribution to improved mechanical properties mainly stems from the optimization of precipitation strengthening mechanisms, while its ability to regulate grain boundary continuity and interface structure is relatively weak. Based on the above results, Sb and Bi exhibit a clear division of labor in their microstructure regulation pathways. Sb focuses more on grain boundary structure regulation and interface stabilization, while Bi primarily influences the formation and evolution of precipitated phases. Based on this, this invention proposes a synergistic hypothesis: Sb reduces corrosion sensitivity by improving grain boundary structure stability, while Bi enhances strength by optimizing precipitation strengthening efficiency. The combined addition of both is expected to achieve a dual-scale synergistic regulation mechanism of grain boundary regulation + precipitation strengthening optimization, establishing a more balanced improvement path between strength and corrosion resistance.

[0098] Composition optimization: Based on the optimization of single microalloying, 0.3wt.%Bi-0.4wt.%Sb was further determined as the optimal composition for composite microalloying. After composite addition, the continuous second phase at the alloy grain boundaries was significantly reduced, and the distribution of intragranular precipitates became finer and more dispersed, achieving synergistic regulation of grain boundary corrosion-sensitive structures and intragranular precipitation strengthening behavior. In the rolled state, the yield strength and tensile strength of the alloy reached 405.51MPa and 432.85MPa, respectively, which were 33.56% and 22.13% higher than those of the base alloy; the corrosion current density decreased to 1.58×10⁻⁶. -7 A·cm -2 The charge transfer resistance was increased to 9300 Ω·cm 2 The resulting alloy exhibits a superior balance between strength and corrosion resistance. This indicates that Sb / Bi composite microalloying is not a simple superposition of single effects, but rather optimizes key microstructural parameters affecting the balance between alloy strength and corrosion resistance by simultaneously regulating grain boundary precipitation connectivity and intragranular precipitation strengthening behavior.

[0099] Heat treatment coupling: Based on DSC analysis and microstructure characterization, the optimal solution treatment regime for the alloy was determined to be 470℃ × 1h. Under single-stage aging conditions at 120℃, the peak aging time of the matrix alloy was approximately 28h, while the peak aging time was extended to approximately 32h after the addition of Bi, Sb, and Bi / Sb composite elements. In the peak-aged state, the yield strength and tensile strength of the 0.3wt.%Bi-0.4wt.%Sb composite microalloyed alloy reached 488.28MPa and 528.62MPa, respectively, and the corrosion current density decreased to 1.45 × 10⁻⁶. -7 A·cm -2 The charge transfer resistance was increased to 1.87 × 10⁻⁶. 4 Ω·cm 2 Statistical analysis based on TEM images shows that the peak-aged precipitates in the composite microalloyed alloy are smaller, denser, and more dispersed, with the PFZ width decreasing from 35 nm in the matrix alloy to 18 nm. Further analysis indicates that the AlSb and Mg3Bi2 phases form a composite encapsulation structure around the dispersed Al3(Sc,Zr) phase, which is closely related to grain refinement, recrystallization inhibition, and increased substructure ratio, thus providing a structural basis for multi-scale synergistic optimization of alloy microstructure parameters.

[0100] Based on the DSC endothermic peak temperature range (478~485℃) and the principle of avoiding grain boundary liquefaction due to proximity to the peak temperature, this invention initially selects 460℃, 470℃ and 480℃ as candidate solution temperatures, and further optimizes the solution regime by combining the degree of dissolution of the residual second phase after solution.

[0101] After determining the solution temperature to be 470℃, in order to further optimize the solution time and verify the applicability of this system to different microalloying systems, four alloys of 0, Bi3, Sb4 and CB6 were selected and subjected to solution treatment at 470℃ for 0.5h, 1h, 1.5h and 2h respectively. Subsequently, the dissolution of the residual second phase was compared and analyzed by SEM.

[0102] Based on a unified solution treatment regime (470℃×1h, water quenching), the four-component alloy of 0, Bi3, Sb4, and CB6 was subjected to a single-stage artificial aging treatment at 120℃ for aging times ranging from 0 to 72 hours. Microhardness was measured every 4 hours, and the hardness variation curve over aging time is shown below. Figure 3 As shown.

[0103] The results show that all four alloys exhibit typical unimodal age hardening characteristics, i.e., the hardness initially increases rapidly with increasing aging time, then gradually decreases after reaching a peak value. The matrix alloy (0) reaches its peak hardness at approximately 28 hours, with a peak hardness of 198.90 HV; the peak aging times of the Bi3, Sb4, and CB6 alloys are all delayed to approximately 32 hours, with corresponding peak hardnesses of 202.86 HV, 199.82 HV, and 206.46 HV, respectively.

[0104] Figure 4 The morphology of the precipitation free zone (PFZ) near the grain boundaries of the four alloys in the peak-aged state is shown. It can be clearly observed that the PFZ region is the widest in the matrix alloy, while the microalloyed alloys show a gradual narrowing trend, with the CB6 alloy having the narrowest PFZ.

[0105] Table 8 shows that the average PFZ width of the CB6 alloy is 18 nm, which is smaller than that of the 0 (35 nm), Bi3 (27 nm), and Sb4 (22 nm) alloys. The reduced PFZ width indicates a decrease in the degree of grain boundary solute depletion and a weakening of the difference between grain boundary and intragranular precipitation. This result is consistent with the low HAGB ratio and high LAGB content observed in EBSD. Bi-Sb composite microalloying effectively alleviates grain boundary solute segregation and reduces the degree of grain boundary weakening by inhibiting recrystallization and optimizing the grain boundary structure.

[0106] Table 8. Statistics on PFZ width of different microalloyed alloys at peak aging state

[0107]

[0108] In the peak-aging tensile curves, all four alloys exhibited typical deformation characteristics of precipitation-strengthened aluminum alloys, namely, entering a stable and uniform plastic stage after yielding, followed by necking until fracture. Compared with the matrix alloy, the microalloyed system significantly improved the strength level while maintaining good plasticity; specific mechanical property parameters are shown in Table 9. The yield strength of the matrix alloy was 395.93 MPa. The yield strength of the Bi3 alloy increased to 469.28 MPa, an increase of approximately 18.5%; the Sb4 alloy increased to 426.66 MPa, an increase of approximately 7.8%; and the CB6 alloy reached a yield strength of 488.28 MPa, an increase of approximately 23.3% compared to the matrix alloy, the highest among the four alloys. The CB6 alloy had the smallest average precipitate size (4.26 nm), the smallest spacing (7.3 nm), and the highest area fraction. The high-density fine precipitates significantly enhanced the dislocation hindering effect, thereby improving the yield strength.

[0109] Table 9. Peak-aged mechanical properties

[0110]

[0111] Unique performance verification

[0112] The marine field exposure corrosion results showed good agreement with the laboratory corrosion evaluation results. Compared with the matrix alloy, the intergranular corrosion depth of the composite microalloyed alloy decreased from 97 μm to 55 μm, the exfoliation corrosion grade improved from ED to EA, and the marine exposure corrosion rate decreased from 0.13 mm·year. -1 Reduced to 0.06 mm·year -1 It exhibits a lower corrosion rate and a slight tendency for grain boundary corrosion propagation. XPS fine spectral analysis shows that the Cl-related signal in the corrosion product film is weakened and the stability of the surface oxide film is enhanced after composite microalloying, indicating that the laboratory corrosion evaluation results can reflect the corrosion propagation trend of the alloy in the marine atmospheric environment to a certain extent.

[0113] Figure 5 The results show the elemental distribution in high-magnification EDS. It can be observed that the Al3Sc phase is encapsulated by Mg3Bi2 and AlSb phases, forming a multiphase composite structure. Al3Sc, as a typical dispersion-strengthening phase, possesses high thermal stability and interfacial bonding strength, and can remain stable during rolling. Its outer layer is surrounded by Mg3Bi2 and AlSb phases, indicating that there is a certain nucleation correlation or interfacial adsorption between the second phase when Bi and Sb are added. This "core-shell" multiphase structure is beneficial for improving the overall stability of the second phase and enhancing its pinning ability to grain boundaries and dislocations. Simultaneously, the large-size Mg3Bi2 and AlSb phases are preferentially distributed in the grain boundary region, significantly hindering grain boundary migration and thus suppressing the recrystallization process, which corroborates the aforementioned result of a reduced HAGB ratio.

[0114] Under the CB6 composition, the nano-MgZn2 precipitates exhibit the highest density and smallest spacing, while Al3Sc is encapsulated by Mg3Bi2 and AlSb to form a stable composite structure. This achieves a multi-level pinning effect at the microscale: the fine precipitates hinder dislocation movement, while the large-sized second phase suppresses grain boundary migration, jointly maintaining a high dislocation density and a low recrystallization ratio. This forms a consistent chain of evidence with the highest KAM value and the lowest HAGB ratio. Therefore, Figure 5 and Figure 6 The results further validated the aforementioned strengthening mechanism analysis from the perspective of interface structure and dislocation interaction mechanism. That is, the combined addition of Bi and Sb in an appropriate ratio can promote multiphase synergistic distribution, construct a stable multi-level strengthening system, and thus achieve synergistic optimization of tissue stability and strength enhancement.

[0115] To evaluate the grain boundary corrosion susceptibility of different microalloying systems at peak aging, intergranular corrosion (IGC) tests were conducted on alloys of 0, Bi3 (containing 0.3% Bi), Sb4 (containing 0.4% Sb), and CB6 (0.3wt.% Bi - 0.4wt.% Sb). Metallographic observation and corrosion depth statistics were performed on the corroded sections. Figure 8 It can be clearly observed that all four alloys exhibit corrosion morphologies that preferentially grow along grain boundaries, but the degree of corrosion varies significantly. The matrix alloy (0) forms obvious continuous corrosion grooves at the grain boundaries, and the corrosion extends through the grain boundaries, forming deep and continuous dark corrosion channels, indicating that its grain boundaries are highly sensitive to corrosion.

[0116] The introduction of Bi reduced the degree of grain boundary corrosion in the Bi3 alloy, but relatively obvious continuous corrosion paths were still observed, with both the depth and length of the corrosion trenches decreasing compared to the base alloy. In contrast, grain boundary corrosion in the Sb4 alloy was further reduced, with the continuous corrosion trenches significantly shortened and some areas exhibiting discontinuous corrosion characteristics. The CB6 alloy showed the best resistance to intergranular corrosion, with its grain boundary corrosion only exhibiting a shallow, locally discontinuous distribution, without forming through-type corrosion channels. The statistical results of intergranular corrosion depth are as follows: Figure 8 As shown.

[0117] To further evaluate the exfoliation corrosion susceptibility of different microalloying systems at peak aging, exfoliation corrosion (EXCO) tests were conducted on the four-part alloy, and the corrosion morphology of the RD-TD surface was observed using SEM. Figure 9 It can be observed that different microalloying systems exhibit significant differences in exfoliation corrosion behavior. The matrix alloy (0) shows obvious layered uplift structure on its surface, and the corrosion extends along the rolling direction, forming a continuous layered exfoliation area. Some areas show flaking traces, and the corrosion level reaches ED or EC level, which is a typical severe exfoliation corrosion morphology.

[0118] The exfoliation corrosion performance test of the alloy was conducted according to the national standard GB / T22639-2008 "Test Method for Exfoliation Corrosion of Aluminum Alloy Processed Products". Samples were taken from the middle of the rolled sheet to ensure the representativeness of the test results. Three samples were tested in parallel for each alloy condition. The sample dimensions were 2 mm thickness (ND direction), 12 mm length (RD direction), and 10 mm width (TD direction), with the test surface being the RD-TD section. Before the experiment, the test surfaces of the samples were ground and polished to remove the surface oxide layer and attached oil stains. They were then washed sequentially in anhydrous ethanol and distilled water and dried. Non-test surfaces were sealed and protected with rosin or paraffin. The exfoliation corrosion test was conducted in EXCO corrosion solution, with a composition of 234 g / L NaCl + 50 g / L KNO3 + 6.3 mL / L HNO3. The sample was completely immersed in the corrosion solution, with the ratio of the corrosion solution volume to the exposed sample area not less than 15 cm². 2The corrosion process was carried out in a constant-temperature water bath at 25±3℃. The sample immersion time was 24 hours. After corrosion, the sample was removed, rinsed with clean water, and its surface corrosion morphology was visually inspected and graded. Simultaneously, the surface morphology of the exfoliated corrosion was recorded using a digital camera. The evaluation criteria were performed according to the relevant provisions of GB / T22639-2008. After surface grading, the sample was rinsed with clean water, immersed in concentrated nitric acid for approximately 30 seconds, and then rinsed again.

[0119] Water rinsing treatment.

[0120] After 7 days of salt spray corrosion, the corrosion of the base alloy was significantly accelerated. Large areas of corrosion and localized lamellar structures could be observed even without removing the corrosion products. Figure 9 After removing the corrosion products, multiple interconnected corrosion pits were observed, forming distinct groove-like corrosion channels. Local areas exhibited early characteristics resembling flaking. The Bi3 alloy showed more pronounced interconnected corrosion areas, with the grooves having less continuity and area than the base alloy. The Sb4 alloy at this stage was mainly characterized by localized interconnections, without forming large-area, continuous corrosion channels. The CB6 alloy primarily showed scattered and relatively shallow pitting corrosion, without obvious grooves or flaking corrosion structures. The evolution of salt spray corrosion morphology followed a development process from pitting initiation → pit expansion → interconnected corrosion, with the severity of corrosion consistently ranked as: 0 > Bi3 > Sb4 > CB6. This trend is consistent with the results of intergranular corrosion and exfoliation corrosion, and also with the results obtained from electrochemical testing. corr With R ct The complete matching of the order verifies the stable corrosion resistance advantage of Bi-Sb composite microalloying in complex corrosive environments.

[0121] Marine field exposure corrosion behavior

[0122] To evaluate the staged corrosion behavior of different microalloyed systems in a real marine environment, marine field exposure corrosion tests were conducted on peak-aged 0, Bi3, Sb4, and CB6 alloys for 1 month, 2 months, and 3 months. The test site was located at a typical open marine atmospheric exposure test base on the coast of South my country, belonging to the marine atmospheric environment of the South China Sea, characterized by high temperature, high humidity, high salt spray deposition, and periodic sea wind effects. The samples were exposed to this environment for 1 month, 2 months, and 3 months, respectively, and their corrosion behavior was evaluated based on weight loss, surface morphology, and corrosion product analysis. To compare the damage evolution characteristics of different alloys during marine exposure, this section first analyzes the surface corrosion morphology without removed corrosion products, then compares the corrosion rates based on weight loss results, and finally further evaluates the matrix damage degree of each alloy based on the surface morphology after removing corrosion products. Figure 10As shown, after 1 to 3 months of marine exposure, corrosion products of varying degrees were deposited on the surfaces of all four alloys. After 1 month of exposure, the surface of the matrix alloy 0 showed relatively obvious localized corrosion product accumulation, with relatively large product size and certain agglomeration characteristics; the surface of the Bi3 alloy had a large number of corrosion products, mainly in the form of dispersed particles; in contrast, the surface of the Sb4 and CB6 alloys had relatively fewer corrosion products, and their distribution was relatively sparse. The surface of CB6 was relatively smooth overall, with only a small amount of localized product deposition visible, indicating that it had a good inhibitory effect on corrosion product formation in the early stage of marine exposure.

Claims

1. A corrosion-resistant Sb-Bi composite microalloyed high-strength aluminum alloy, characterized in that: The aluminum alloy uses an Al-Zn-Mg-Cu alloy as its base, and its chemical composition by mass percentage includes: Zn: 5.0~8.0 wt.%; Mg: 0.5~2.5 wt.%; Cu: 1.0~2.0 wt.%; Sc: 0.1~0.5 wt.%; Zr: 0.05~0.5 wt.%; Sb: 0.1~0.6 wt.%; Bi: 0.1~0.5 wt.%; with the balance being Al and unavoidable impurity elements.

2. A method for preparing the corrosion-resistant Sb-Bi composite microalloyed high-strength aluminum alloy according to claim 1, characterized in that: The method is as follows: Step 1: Melting and Alloying: High-purity aluminum is heated and melted to 630~750℃. Zn, Mg, and Cu are added sequentially to the melt to form an Al-Zn-Mg-Cu alloy matrix. While maintaining a stable melt temperature, Sc and Zr are added to the melt to achieve the designed content, and the mixture is stirred thoroughly to ensure uniform element distribution. Subsequently, Sb and Bi are added to the melt simultaneously for composite microalloying treatment, and stirring is continued to obtain a uniformly composed alloy melt. Step 2: Casting and Homogenization Heat Treatment: The above alloy melt is poured into ingots and cooled to room temperature; the ingots are then subjected to homogenization annealing treatment. Step 3: Plastic deformation processing: The ingots after homogenization annealing are hot rolled or warm rolled to obtain plates or profiles of the required specifications; Step 4: Heat treatment: The rolled alloy is subjected to solution treatment and aging treatment.

3. The method according to claim 2, characterized in that: In step two, the annealing temperature is 400~480℃, and the holding time is 12~24 h.

4. The method according to claim 2, characterized in that: In step three, the temperature of the rolls is controlled within the range of 100~200℃ during the rolling process.

5. The method according to claim 2, characterized in that: In step three, the solution treatment temperature is 460~480℃ and the time is 0.5~2 h.

6. The method according to claim 2, characterized in that: In step three, the aging treatment temperature is 120℃, and the aging time is set to 0~72h.

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