A preparation method of a surface-hardened aluminum alloy gradient structure based on precipitation phase regulation

Abstract

The application relates to a preparation method of a surface-hardened aluminum alloy gradient structure based on precipitation phase regulation, and the preparation method comprises the following steps: S1: Al-Mg-Sc-Zr alloy smelting, homogenization heat treatment, hot rolling and annealing treatment, so as to obtain a supersaturated plate containing micron Al3(Sc, Zr) phases; S2: a gradient thermal field is constructed in the thickness direction of the aluminum alloy plate by using friction stir progressive forming; micron phases are broken and nanophase dynamic precipitation is induced under high temperature and high strain of the surface layer, recrystallized grain growth is inhibited through a double pinning effect, and surface nanocrystalline is locked; coarse grains are retained in the core under low temperature and low strain; and S3: after the aluminum alloy plate is cooled, post-treatment is carried out, so that the surface-hardened aluminum alloy gradient structure is obtained. The application utilizes the differential evolution mechanism of double-scale precipitation phases, effectively solves the problem that surface fine grains of an aluminum alloy gradient structure are prone to thermal coarsening, realizes high strength and plasticity synergy without subsequent heat treatment, and has the advantages of a short process, low energy consumption and shape-property integrated manufacturing.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, and in particular to a method for preparing a surface-hardened aluminum alloy gradient structure based on precipitate phase regulation. Background Technology

[0002] Aluminum-magnesium alloys (5xxx series aluminum alloys) possess low density, excellent corrosion resistance, and good weldability, making them important structural materials for key load-bearing components in aerospace, rail transportation, and other fields. However, as typical non-heat-treatable aluminum alloys, their yield strength is usually low (<200 MPa), making it difficult to meet the stringent requirements of modern industry for lightweight and high-safety equipment.

[0003] Existing technologies primarily employ intense plastic deformation processes such as equal channel angular pressing (ECAP) and high pressure torsion (HPT) to introduce large strains, or to refine the microstructure by adding trace elements (such as Sc and Zr) to induce heterogeneous nucleation, thereby enhancing the strength of aluminum-magnesium alloys. While these processes effectively improve material strength, excessive grain refinement often leads to the depletion of dislocation storage capacity, resulting in decreased work hardening capacity and significantly reduced elongation. This typical "strength-plasticity inversion" severely limits the engineering applications of aluminum-magnesium alloys.

[0004] In recent years, gradient structures (GS) have been considered an effective strategy to overcome the "strength-plasticity inversion" relationship. By constructing a heterogeneous structure of "surface nanocrystalline hardened layer + core coarse-grained tough layer," and utilizing geometrically necessary dislocations (GNDs) at the interface to generate back stress strengthening, a synergistic effect of strength and plasticity can be achieved in metallic materials. Currently, technologies such as surface mechanical attrition treatment (SMAT) and ultrasonic surface rolling process (USRP) have been widely used in the preparation of gradient structures for materials such as stainless steel, nickel-based alloys, and high-entropy alloys. However, these processes mainly rely on intense plastic deformation at low temperatures. Although this mechanism can obtain a surface nanolayer through grain refinement, it introduces extremely high dislocation density on the surface, causing the material's work hardening potential to be prematurely exhausted. Under service stress, early necking is prone to occur, thus limiting the material's plasticity.

[0005] To overcome the plastic loss caused by the aforementioned processes, researchers have attempted to employ hot deformation techniques such as friction stir processing (FSP) to refine the surface grains of materials using a dynamic recrystallization mechanism to obtain a gradient structure. However, for high fault energy materials like aluminum-magnesium alloys, their microstructure is extremely sensitive to heat input, and the grains refined by drastic deformation are prone to coarsening during dynamic recrystallization. Furthermore, the lack of effective pinning of grain boundaries by thermally stable second-phase particles makes it difficult to obtain stable nanoscale (<500 nm) grain structures on the surface of aluminum alloys using conventional hot working. CN119426462A discloses a device and method for preparing thin-walled formed parts based on a dual-gradient structure, which prepares a dual-gradient structure including a positive grain gradient and a negative dislocation gradient through thermo-mechanical coupling progressive forming technology, but it has not yet solved the stability problem of nanocrystals on the surface of high fault energy aluminum alloys under a thermo-mechanical coupling field.

[0006] In summary, the existing techniques for preparing gradient structures in aluminum alloys using friction stir incremental forming (FST) still face challenges, including coarsening of surface nanocrystals and poor thermal stability of the gradient structures due to a lack of effective microstructure control methods. Therefore, a novel technique is urgently needed to prepare stable gradient structures in situ using FST in aluminum-magnesium alloys. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of poor thermal stability of aluminum alloy gradient structures in the prior art by providing a method for preparing surface-hardened aluminum alloy gradient structures based on precipitate phase regulation.

[0008] The objective of this invention can be achieved through the following technical solutions: This invention first provides a method for preparing a surface-hardening aluminum alloy gradient structure based on precipitated phase regulation, the method comprising the following steps: S1: Weigh each metal raw material for Al-Mg-Sc-Zr alloy according to the composition ratio, and perform casting, homogenization treatment, hot rolling and annealing treatment in sequence to obtain aluminum alloy plate with micron-sized Al3(Sc, Zr) precipitates. S2: The annealed aluminum alloy sheet is subjected to friction stirring progressive forming to construct a gradient temperature field and gradient strain field along the thickness direction; under the action of the gradient temperature field and gradient strain field, the micron phase is broken and the nano phase is dynamically precipitated, and the recrystallization grain growth is inhibited by the double pinning effect to form a gradient structure with surface nanocrystals and core coarse grains. S3: After the aluminum alloy sheet cools down, post-processing is performed to obtain the surface-hardened aluminum alloy gradient structure.

[0009] Furthermore, in step S1, the contents of Sc and Zr in the Al-Mg-Sc-Zr alloy are both greater than 0.10 wt.%. This step aims to utilize the high content of Sc and Zr elements to form micron-sized Al3(Sc, Zr) precipitates during solidification.

[0010] Furthermore, by mass percentage, the Al-Mg-Sc-Zr alloy contains 3-7 wt.% Mg, 0.1-1.5 wt.% Sc, 0.1-1.2 wt.% Zr, and the balance is Al.

[0011] Further, in step S1, the specific casting operation is as follows: weigh high-purity aluminum ingots, high-purity magnesium ingots, Al-Sc master alloy, and Al-Zr master alloy according to the target composition ratio; heat the high-purity aluminum ingots until completely melted, then add the aforementioned master alloy and magnesium ingots sequentially and stir evenly; use argon gas for degassing and slag removal until no bubbles escape from the surface of the melt; after the melt cools down, pour it into a preheated mold, and then water-cool it to obtain the ingot. The casting process aims to obtain Al-Mg-Sc-Zr alloy ingots by controlling the melting and pouring temperatures and the cooling method, ensuring uniform alloy composition and effectively reducing casting defects.

[0012] Further, in step S1, the specific operation of the homogenization treatment is as follows: the ingot obtained from melting and casting is heated to 520-560 ℃, held at that temperature for 20-30 h, and then water-cooled. The homogenization treatment process aims to eliminate elemental segregation and retain the micron-sized Al3(Sc, Zr) precipitates formed during solidification.

[0013] Further, in step S1, the specific operation of hot rolling is as follows: the homogenized ingot is forged into a billet, preheated, and then rolled in multiple passes until an aluminum alloy sheet with a thickness of 1-3 mm is obtained. The hot rolling process aims to eliminate casting defects such as shrinkage cavities, porosity, and coarse dendrites through hot deformation.

[0014] Further, in step S1, the specific operation of the annealing treatment is as follows: the hot-rolled aluminum alloy sheet is heated to 500-540 ℃, held at that temperature, and then air-cooled. The annealing process aims to eliminate the work hardening introduced by hot rolling, while retaining the supersaturation of Sc and Zr elements in the matrix, reserving chemical potential energy for dynamic precipitation in the subsequent forming process.

[0015] Further, in step S2, the specific operation of the friction stir progressive forming is as follows: the annealed aluminum alloy sheet is clamped and fixed, and a tool head is used to progressively form the aluminum alloy sheet along a spiral trajectory on the surface of the sheet. Friction stir progressive forming utilizes a high-speed rotating tool head to apply a stirring friction effect to the surface of the sheet, establishing deformation conditions that decay from the surface to the core within the sheet.

[0016] Furthermore, the tool head is a flat-headed carbide tool head with a body diameter of 15-25 mm, and the transition area between the flat head and the body has a rounded corner with an arc radius of 4-6 mm. The tool head used in this invention has a cylindrical body with a hemispherical or planar end face, wherein the planar tool head has a rounded corner at the transition area between it and the body.

[0017] Furthermore, the tool head is made of ceramic, tungsten carbide, or cemented carbide to ensure sufficient high-temperature wear resistance and structural rigidity during intense friction.

[0018] Furthermore, the downward pressure of the tool head is 0.1-0.2 mm.

[0019] Furthermore, the rotational speed of the progressive forming is 1500-2500 rpm.

[0020] Furthermore, the feed rate for the incremental forming is 1200-1800 mm / min.

[0021] In the aforementioned friction stir progressive forming process, key process parameters such as tool head rotation speed, feed rate, indentation, and forming angle need to be controlled. This step aims to generate intense combined strains such as shearing, stretching, and bending, as well as high frictional heat in the surface layer through the contact between the high-speed rotating tool head and the sheet surface. At the same time, heat conduction is used to establish a lower temperature and strain environment in the core, thereby providing thermodynamic and kinetic conditions for the differentiated evolution of micron-sized Al3(Sc, Zr) precipitates on the surface and in the core.

[0022] Further, in step S2, the specific characteristics of the gradient temperature field are as follows: the peak temperature on the surface of the plate is controlled within the recrystallization temperature range of 300-600 ℃, and the peak temperature in the core of the plate is 50-250 ℃ lower than that on the surface. This step aims to ensure sufficient dynamic recrystallization nucleation on the surface and dynamic precipitation of nanoscale Al3(Sc, Zr) phases in the matrix, while the core temperature is maintained within the range where recovery or partial recrystallization mainly occurs, avoiding coarsening of the micron-scale Al3(Sc, Zr) precipitates.

[0023] Further, in step S2, the specific characteristics of the gradient strain field are as follows: the strain mode of the plate surface consists of severe shear, tensile, and bending strains, with a high equivalent plastic strain; the strain mode of the plate core mainly consists of tensile and bending strains, with an equivalent plastic strain much lower than that of the plate surface. This step aims to utilize the severe shear strain of the surface layer to ensure the sufficient fragmentation and refinement of the micron-sized Al3(Sc, Zr) precipitates, providing high-density particles for the subsequent formation of fine recrystallized grains on the pinned surface; at the same time, the low strain of the core preserves the integrity of the micron-sized phases, preventing interface damage between the precipitates and the matrix caused by severe deformation, thereby reducing the initiation of microcracks induced by local stress concentration and avoiding the loss of core toughness.

[0024] During the above forming process, a gradient structure of "surface nanocrystals + core coarse grains" was induced in situ. Accompanying step S2, under the combined effect of gradient temperature and strain field, the grain size was controlled by the differentiated evolution behavior of the dual-scale precipitates.

[0025] Furthermore, on the surface of the substrate, intense shearing forces mechanically break down and refine the existing micron-sized Al3(Sc, Zr) precipitates. Simultaneously, the supersaturation of supersaturated Sc and Zr elements retained in the matrix is ​​released under frictional heat, inducing in-situ dynamic precipitation of dispersed nano-sized Al3(Sc, Zr) phases. This step aims to utilize the highly thermally stable nano-sized precipitates and micron-sized phase fragments to generate a strong Zener pinning effect on grain boundaries, effectively suppressing the recrystallization and growth of dynamically recrystallized grains under thermo-mechanical coupling, thereby locking in an ultrafine grain structure with nanoscale grain size.

[0026] Furthermore, in the core of the sheet, lower temperature and strain conditions are used to maintain the structural integrity of the micron-sized Al3(Sc, Zr) precipitates and ensure a good interfacial relationship with the matrix. Simultaneously, due to the low temperature and low strain conditions in the core, the original coarse grains are elongated but only undergo dynamic recovery, preserving the high toughness of the sheet core.

[0027] Furthermore, in step S3, after the forming is completed, the gradient structure aluminum alloy component is air-cooled to room temperature, and the surface grease and trace debris are cleaned.

[0028] Based on the successful construction of the above preparation method, the present invention also provides a gradient structure aluminum alloy component prepared by any of the above preparation methods.

[0029] Furthermore, the gradient structure aluminum alloy component has a gradient structure of surface nanocrystals and core coarse grains, wherein the surface layer forms a nanocrystal hardened layer with a grain size of less than 1 μm (e.g., 250-350 nm), and the core forms a coarse grain structure with a grain size of more than 10 μm (e.g., 12-20 μm).

[0030] Furthermore, the tensile strength of the gradient structure aluminum alloy component is not less than 350 MPa.

[0031] Furthermore, the elongation of the gradient structure aluminum alloy component is not less than 10%.

[0032] Compared with existing technologies, this invention focuses on the Al-Mg-Sc-Zr alloy system and creatively introduces a dual metallurgical control mechanism of "mechanical fragmentation of micron-sized phases and in-situ dynamic precipitation of nano-sized phases." Utilizing the gradient thermodynamic field during the forming process, it induces the in-situ dynamic precipitation of high-density nano-phases from supersaturated Sc / Zr elements, generating a strong Zener pinning effect that "locks" the grain boundaries. During the forming process, the high temperature and high strain conditions of the sheet metal cause intense dynamic recrystallization, resulting in an ultrafine crystalline layer. Simultaneously, extreme thermomechanical conditions cause the micron-sized Al3(Sc, Zr) precipitates to break down and disperse in the surface region, inducing the in-situ dynamic precipitation of nano-Al3(Sc, Zr) phases from the supersaturated matrix. By locking the surface nanocrystals through the strong pinning effect of the dual-size precipitates, the problem of easy coarsening of fine grains on the surface of high fault-energy aluminum alloys is solved, achieving surface hardening of thin-walled aluminum alloy components. Furthermore, due to the low temperature and low strain conditions in the core, the original coarse grains only undergo dynamic recovery after being elongated, preserving the high toughness of the core.

[0033] Existing technologies (such as CN119426462A) mainly focus on providing general forming equipment and macroscopic process paths, but they do not solve the common problem that high fault energy aluminum alloys are prone to grain coarsening during severe hot deformation, making it difficult to maintain a stable nanocrystalline structure on the surface. This invention fundamentally breaks through the bottleneck of general processes in terms of thermal stability, and realizes the one-step preparation of a high-performance gradient structure that combines "thermally stable nanocrystalline surface and tough coarse grains in the core" without subsequent heat treatment.

[0034] Compared with the prior art, the present invention has the following technical advantages: (1) This invention first prepares an initial Al-Mg-Sc-Zr aluminum alloy sheet containing Al3(Sc, Zr) precipitates and maintaining the supersaturation characteristics of Sc and Zr elements in the matrix through composition design and heat treatment. Subsequently, an extreme gradient temperature field and strain field are constructed in the thickness direction of the sheet using a thermo-coupled incremental forming process. During the forming process, the high temperature and high strain conditions of the sheet cause violent dynamic recrystallization, thereby forming an ultrafine crystalline layer. At the same time, the extreme thermomechanical conditions cause the micron-sized Al3(Sc, Zr) precipitates to break down and disperse in the surface area, inducing the in-situ dynamic precipitation of nano-Al3(Sc, Zr) phases in the supersaturated matrix. By locking the surface nanocrystals through the strong pinning effect of the dual-size precipitates, the problem of easy coarsening of fine grains on the surface of high fault energy aluminum alloys is solved, and the surface hardening of aluminum alloy thin-walled components is achieved. At the same time, due to the low temperature and low strain conditions in the core, the original coarse grains only undergo dynamic recovery after being stretched, retaining the high toughness of the core, and successfully realizing the high-performance forming of complex gradient aluminum alloy components.

[0035] (2) This invention breaks through the bottleneck of thermal stability of nanocrystals on the surface of high fault-energy aluminum alloys. It innovatively constructs high-density Zener pinning points in situ in the thermo-mechanical coupling field of friction stir processing through a dual mechanism of "micron-phase mechanical fragmentation" and "nanophase dynamic precipitation". Compared with traditional hot working processes, this invention utilizes the dynamic precipitation behavior during the forming process to make the pinning effect of the precipitates on dislocations and grain boundaries stronger, effectively suppressing the coarsening tendency of nanocrystals on the surface of aluminum alloys and realizing the stable preparation of surface nanocrystals.

[0036] (3) This invention utilizes a dual mechanism of “micron-phase fragmentation + nano-phase dynamic precipitation” on the surface to introduce high-density precipitated phase strengthening particles while forming an ultrafine crystalline layer. This combined effect of “fine-grain strengthening” and “precipitation strengthening” greatly enhances the surface hardness and strength of the component, achieving effective surface hardening.

[0037] (4) The gradient aluminum alloy component prepared by the present invention has both a surface hardening layer and a core toughness layer. The ultra-fine grain layer on the surface provides ultra-high yield strength and tensile strength, while the uniform coarse grain layer formed by dynamic recovery in the core provides excellent work hardening ability and fracture toughness, which significantly overcomes the defect of "strength-plasticity inversion" in traditional aluminum alloy materials and realizes the excellent strength-plasticity synergy of aluminum alloy.

[0038] (5) This invention abandons the time-consuming independent aging heat treatment process (usually requiring tens of hours) in traditional processes, and cleverly utilizes the frictional heat during the stirring friction process to achieve "in-situ dynamic precipitation" of nanophases. This not only completes precipitation strengthening and grain refinement strengthening in a one-step forming operation, greatly shortening the preparation cycle and reducing energy consumption, but also avoids the problem of coarsening of precipitated phases caused by aging before processing.

[0039] (6) This invention realizes the "form-property integration" manufacturing of complex thin-walled aluminum alloy components, deeply integrating microstructure control into the macro-forming process. While completing the progressive forming of complex geometric shapes (such as deep cavities and curved surfaces) of components, a gradient grain structure is induced in situ. This invention effectively solves the problems of separation of forming and modification in traditional processes and the easy destruction of strengthening phases during secondary processing, and achieves the simultaneous improvement of component geometric accuracy and mechanical properties. Attached Figure Description

[0040] Figure 1 This is a schematic flowchart of the preparation method of the present invention.

[0041] Figure 2 This is a schematic diagram of the overall scheme of an embodiment of the present invention.

[0042] Figure 3 This is a microstructure diagram of the aluminum-magnesium alloy after recrystallization annealing in an embodiment of the present invention.

[0043] Figure 4 This is a schematic diagram of the gradient structure aluminum-magnesium alloy component prepared by friction stirring in this invention.

[0044] Figure 5 This is a microstructure diagram of the pinning effect of nano-precipitated phases in the aluminum-magnesium alloy sheet in an embodiment of the present invention.

[0045] Figure 6 This is a microstructure diagram showing the effect of micron-sized precipitates promoting recrystallization in the aluminum-magnesium alloy sheet in an embodiment of the present invention.

[0046] Figure 7 This is a microstructure diagram of the gradient structure aluminum-magnesium alloy component obtained in an embodiment of the present invention.

[0047] Figure 8 This is a comparison diagram of the tensile properties of the aluminum-magnesium alloy gradient structure plate and the original plate in an embodiment of the present invention. Detailed Implementation

[0048] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0049] Example: The purpose of this embodiment is to provide a complete process for preparing surface-hardened aluminum alloy gradient structures based on precipitated phase-induced gradient structures. This aims to solve the technical problem that, in the process of preparing gradient structures of aluminum alloys using the friction stir progressive forming process, the lack of effective microstructure control methods leads to coarsening of surface nanocrystals and poor thermal stability of the gradient structure.

[0050] To address the aforementioned issues, this embodiment provides a complete process for fabricating aluminum alloy components with surface-hardened high-performance gradient structures using a friction stir incretion process that leverages dual-scale precipitates to synergistically control grain size distribution. This method encompasses the entire process from alloy composition design and precipitate pretreatment to the preparation of gradient microstructures via friction stir incretion.

[0051] Specifically, such as Figure 2 As shown, this embodiment aims to prepare surface-hardened gradient aluminum-magnesium alloy thin-walled components with excellent strength-plasticity synergy. Al-4.9Mg-0.6Sc-0.4Zr alloy is selected as the experimental object, and the shape and properties of the aluminum-magnesium alloy thin-walled components are prepared in an integrated manner through a complete process chain.

[0052] The preparation process in this embodiment mainly includes two core stages: The first stage is the pre-regulation of the microstructure of the original sheet material. This stage, through melting, homogenization, hot rolling, and specific annealing processes, aims to obtain a special microstructure state, that is, to eliminate work hardening while retaining the micron-sized Al3(Sc,Zr) precipitates and maintaining the supersaturation of Sc and Zr elements in the matrix, thus reserving the material basis and chemical potential energy for subsequent processes.

[0053] The second stage involves the fabrication of gradient-structured aluminum alloys using a friction stir incremental forming process. This stage utilizes a high-speed rotating tool head to construct a gradient temperature and strain field within the sheet metal, in-situ stimulating the differentiated evolution of dual-scale precipitates: on the surface, micron-scale phase fragmentation and dynamic precipitation of nano-scale phases achieve nanocrystalline Zener pinning; in the core, dynamic recovery retains micron-scale coarse grains, ultimately achieving the fabrication of a gradient component with "surface nanocrystals + core coarse grains".

[0054] The specific preparation steps and parameter control in this embodiment are as follows: Step 1: Alloy smelting and casting.

[0055] Accurately weigh high-purity aluminum ingots, high-purity magnesium ingots, Al-2wt.%Sc master alloy, and Al-10wt.%Zr master alloy according to the target composition ratio. After the high-purity aluminum ingots are completely melted in a resistance furnace to 760 °C, the aforementioned master alloys and magnesium ingots are added sequentially, and thorough mechanical stirring is performed to ensure uniform composition. To obtain a dense ingot structure, argon gas is used for degassing and slag removal until no bubbles escape from the melt surface. When the melt temperature drops to 720 °C, it is smoothly poured into a steel mold preheated to 250 °C, followed by water cooling for demolding.

[0056] It should be noted that this step, by strictly controlling the solidification conditions, successfully pre-placed coarse micron-sized Al3(Sc, Zr) primary phases in the as-cast microstructure, thus providing a key material basis for subsequent microstructure control.

[0057] Step 2: Measure the chemical composition of the ingot.

[0058] The actual chemical composition (mass fraction, wt.%) of the alloy ingot was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) to be: Mg 4.96%, Sc 0.58%, Zr 0.38%, with the balance being Al.

[0059] Step 3: Homogenization process.

[0060] The ingot was placed in a box-type resistance furnace and heated to 540 ℃ at a rate of 5 ℃ / min, held for 24 h, and then rapidly water-cooled after the holding period.

[0061] It should be noted that this treatment aims to eliminate elemental segregation that occurs during the casting process. Furthermore, because the selected homogenization temperature of 540 °C is lower than the solution temperature of the micron-sized primary phase, the highly thermally stable micron-sized Al3(Sc,Zr) particles (approximately 2-10 μm in size) are completely preserved.

[0062] Step 4: Multi-pass hot rolling.

[0063] The homogenized ingot was forged into a square billet and preheated at 500 °C for 1 hour. Then, it was rolled in multiple passes on a two-roll reversible hot rolling mill. The reduction in each pass was controlled at 20%, and the billet was reheated for 15 minutes every three passes. Finally, the 8 mm thick square billet was rolled into a 1.5 mm thick plate.

[0064] It should be noted that the high-temperature, large-deformation process of hot rolling effectively welded the casting porosity defects, improved the initial processing performance of the material, and prepared the microstructure for subsequent forming.

[0065] Step 5: Critical annealing control.

[0066] The hot-rolled sheet was heated to 520 ℃, held at that temperature for 1 hour, and then air-cooled.

[0067] It should be noted that the selection of these annealing process parameters has a dual and crucial regulatory significance: on the one hand, this treatment is sufficient to eliminate most of the dislocation accumulation introduced by hot rolling, causing the matrix to recrystallize and soften (see...). Figure 3This process facilitates subsequent deformation; on the other hand, it avoids the premature precipitation and depletion of nanophases caused by traditional long-term aging, ensuring that a high concentration of Sc and Zr supersaturated solid solutions are still retained in the matrix, thus reserving sufficient chemical potential energy for "in-situ dynamic precipitation" during subsequent stirring and friction.

[0068] Step 6: Selection of process parameters for friction stirring progressive forming process.

[0069] The annealed sheet metal was fixed onto the tooling fixture of the three-axis CNC milling machine using a special fixture. A flat-head carbide tool with a diameter of 20 mm and an arc radius of 5 mm was selected, and the rotational speed was set to 2000 RPM, the feed rate to 1500 mm / min, and the indentation to 0.15 mm. The material was then shaped along a preset spiral trajectory.

[0070] Step 7: Prepare surface-hardened aluminum alloy gradient structures by friction stir progressive forming.

[0071] The three-axis CNC milling machine is started, and the high-speed rotating tool head rotates along a preset trajectory on the surface of the sheet metal. At the same time, a significant gradient temperature field and strain field are established in the thickness direction of the sheet metal, inducing a completely different microstructure evolution mechanism.

[0072] It should be noted that, as Figure 4 As shown, in the high-temperature, high-deformation coupling region on the surface (high temperature of approximately 450℃ and shear strain exceeding 20), the residual micron-sized precipitates are further broken down and refined. Simultaneously, frictional heat induces the in-situ dynamic precipitation of a large number of fine (20-50 nm) nano-precipitates in the matrix (see...). Figure 5 The grain boundaries are strongly locked by the Zener pinning effect. Meanwhile, in the core heat conduction region (relatively low temperature and low strain environment), the micron-sized precipitates remain relatively intact at the matrix interface, and dynamic recovery eliminates the deformation structure caused by grain elongation (see...). Figure 6 ).

[0073] Step 8: Cooling and Shaping and Result Verification After the forming process is completed, the component is naturally cooled to room temperature. After cleaning the surface oil, a gradient structure aluminum-magnesium alloy thin-walled component with integrated form and properties is finally obtained.

[0074] Step 9: Characterization of the microstructure and mechanical properties of the gradient components.

[0075] Thin-walled components were wire-cut to obtain tensile and microscopic specimens. After the surfaces were polished to the relevant experimental standards, electron backscatter diffraction (EBSD) characterization and uniaxial tensile testing were performed to verify the gradient structure and mechanical properties of the aluminum alloy components.

[0076] Depend on Figure 7The EBSD results show that the aluminum alloy material prepared in this embodiment has a typical gradient structure characteristic of transitioning from surface nanocrystals to core coarse grains. At the same time, a nanocrystalline hardened layer with a grain size of about 300 nm is formed on the surface, and a coarse grain structure with a grain size of about 16 μm is formed in the core.

[0077] Depend on Figure 8 The mechanical property test results show that the tensile strength of the gradient structure aluminum alloy component prepared in this embodiment can be increased to 460 MPa, which is about 47% higher than that of the original plate, while still maintaining an excellent elongation of up to 17%.

[0078] In summary, this embodiment fully verifies the effectiveness of the entire preparation method of the present invention. By pre-setting micron-level primary phases and retaining matrix supersaturation during the sheet preparation stage (first stage), combined with gradient deformation condition control during the forming stage (second stage), the present invention successfully induces a dual pinning mechanism of "micron-level phase fragmentation + in-situ dynamic precipitation of nano-phases" on the surface of aluminum alloys. This effectively overcomes the technical bottleneck that surface grains of high fault energy aluminum alloys are prone to thermal coarsening during hot deformation, effectively achieving surface hardening of the aluminum alloy. At the same time, excellent toughness is retained in the core using a dynamic recovery mechanism. The final component possesses excellent comprehensive properties of "high surface strength + high core toughness" without subsequent heat treatment, realizing efficient and green manufacturing of complex thin-walled components with integrated form and properties.

[0079] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for preparing a surface-hardening aluminum alloy gradient structure based on precipitate-controlled precipitation, characterized in that, The preparation method includes the following steps: S1: Weigh each metal raw material for Al-Mg-Sc-Zr alloy according to the composition ratio, and perform casting, homogenization treatment, hot rolling and annealing treatment in sequence to obtain aluminum alloy plate with micron-sized Al3(Sc, Zr) precipitates. S2: The annealed aluminum alloy sheet is subjected to friction stirring progressive forming to construct a gradient temperature field and gradient strain field along the thickness direction; under the action of the gradient temperature field and gradient strain field, the micron phase is broken and the nano phase is dynamically precipitated, and the recrystallization grain growth is inhibited by the double pinning effect to form a gradient structure with surface nanocrystals and core coarse grains. S3: After the aluminum alloy sheet cools down, post-processing is performed to obtain the surface-hardened aluminum alloy gradient structure.

2. The method for preparing a surface-hardening aluminum alloy gradient structure based on precipitate phase regulation according to claim 1, characterized in that, In step S1, the contents of Sc and Zr in the Al-Mg-Sc-Zr alloy are both greater than 0.10 wt.%.

3. The method for preparing a surface-hardening aluminum alloy gradient structure based on precipitate phase regulation according to claim 1, characterized in that, In step S1, the specific casting operation is as follows: Weigh high-purity aluminum ingots, high-purity magnesium ingots, Al-Sc master alloys, and Al-Zr master alloys according to the target composition ratio; Heat the high-purity aluminum ingots until they are completely melted, then add the above-mentioned intermediate alloy and magnesium ingots in sequence and stir evenly; Argon gas is used for degassing and slag removal until no more bubbles escape from the surface of the melt. After the melt cools down, it is poured into a preheated mold, and then water-cooled to demold and obtain an ingot.

4. The method for preparing a surface-hardening aluminum alloy gradient structure based on precipitate phase regulation according to claim 1, characterized in that, In step S1, the specific operation of the homogenization process is as follows: The ingots obtained from melting and casting are heated to 520-560 ℃, held at that temperature for 20-30 h, and then water-cooled.

5. The method for preparing a surface-hardening aluminum alloy gradient structure based on precipitate-controlled precipitation according to claim 1, characterized in that, In step S1, the specific operation of hot rolling is as follows: the homogenized ingot is forged into a billet, preheated and then rolled in multiple passes until an aluminum alloy sheet with a thickness of 1-3 mm is obtained. The specific operation of the annealing process is as follows: heat the hot-rolled aluminum alloy sheet to 500-540℃, hold it at that temperature, and then air cool it.

6. The method for preparing a surface-hardening aluminum alloy gradient structure based on precipitate phase regulation according to claim 1, characterized in that, In step S2, the specific operation of the friction stir progressive forming is as follows: the annealed aluminum alloy sheet is clamped and fixed, and a tool head is used to progressively form the aluminum alloy sheet along a spiral trajectory on the surface of the sheet. The tool head is a flat-head carbide tool head with a body diameter of 15-25 mm and a rounded corner with an arc radius of 4-6 mm at the transition area between the flat head and the body. The downward pressure of the tool head is 0.1-0.2 mm; The progressive forming rotation speed is 1500-2500 rpm, and the feed rate is 1200-1800 mm / min.

7. The method for preparing a surface-hardening aluminum alloy gradient structure based on precipitate phase regulation according to claim 1, characterized in that, In step S2, the specific characteristics of the gradient temperature field are as follows: The peak temperature on the surface of the board is controlled within the recrystallization temperature range of 300-600 ℃, while the peak temperature in the core of the board is 50-250 ℃ lower than that on the surface.

8. The method for preparing a surface-hardening aluminum alloy gradient structure based on precipitate phase regulation according to claim 1, characterized in that, In step S2, the specific characteristics of the gradient strain field are as follows: The strain mode of the surface layer of the board consists of severe shear, tensile and bending strains, with a high equivalent plastic strain; the strain mode of the core layer of the board mainly consists of tensile and bending strains, with a lower equivalent plastic strain than that of the surface layer.

9. A gradient structure aluminum alloy component, characterized in that, Prepared by the preparation method according to any one of claims 1-8; The gradient structure aluminum alloy component has a gradient structure of surface nanocrystals and core coarse grains, wherein the surface layer forms a nanocrystal hardened layer with a grain size of less than 1 μm, and the core forms a coarse grain structure with a grain size of more than 10 μm.

10. A gradient structure aluminum alloy component according to claim 9, characterized in that, The tensile strength of the gradient structure aluminum alloy component is not less than 350 MPa, and the elongation is not less than 10%.

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