A method for preparing aluminum alloy profiles for high airtight battery structure systems in new energy vehicles

Abstract

This invention relates to the field of aluminum alloy forming technology, and in particular to an aluminum alloy profile for a high airtight battery structure system for new energy vehicles and its preparation method, comprising the following steps: Step (1) Batching and casting: Prepare aluminum alloy raw materials, melt and refine them, and then cast them into ingots; Step (2) Low-temperature long-term homogenization treatment: Heat the ingot and hold it at a certain temperature to allow Zr and Sc elements to precipitate in the form of Al3Sc·Zr nano-dispersed phases; Step (3) Gradient cooling extrusion forming: Heat the homogenized ingot and extrude it. A zoned temperature-controlled cooling device is set at the extrusion outlet to apply a differentiated cooling regime to the thin-walled and thick-walled areas according to the profile wall thickness; Step (4) Two-stage aging treatment: Perform two-stage aging on the extruded profile. The first stage is held at 80-95℃ for 12-24 hours, and the second stage is held at 155-165℃ for 8-12 hours. This significantly inhibits the recrystallization of the heat-affected zone of welding, thereby obtaining an aluminum alloy profile with excellent airtightness and welding performance.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy forming technology, and in particular to an aluminum alloy profile for a high airtight battery structure system for new energy vehicles and its preparation method. Background Technology

[0002] As a core component of the entire vehicle, the reliability of the battery pack's structural system directly affects vehicle safety. The battery tray / box profile serves as both a load-bearing component and a sealing boundary, requiring extremely high airtightness—seal failure will lead to moisture intrusion, reduced insulation, and even the risk of thermal runaway. Industry statistics show that the current seal failure rate of battery structural systems is approximately 4.4%, mainly due to micro-leakage at welds and joints after the profile is extruded.

[0003] Structural components such as battery trays are typically assembled from aluminum alloy profiles using friction stir welding. The microstructure evolution of the weld heat-affected zone (HAZ) is a key factor affecting airtightness: during welding, the HAZ undergoes high-temperature thermal cycling, resulting in recrystallization and grain growth. This leads to a decrease in the mechanical properties of the region and an increased tendency for micropore aggregation, making it a weak point in airtightness. Therefore, suppressing recrystallization in the weld HAZ is a core technical requirement for improving the airtightness of battery structural systems.

[0004] In existing technologies, adding trace elements such as Zr and Sc to form nanoscale dispersed phases pinning grain boundaries is a known method for suppressing recrystallization. For example, Chinese Patent Publication No. CN114318076A discloses an Al-Sc alloy vehicle battery pack and its production method, in which the Sc content is 0.2-0.4%, and a three-stage homogenization process is adopted (460-480℃ + 550-570℃ + 575-585℃). The precipitation of Al3Sc particles refines the grains and suppresses recrystallization. The Sc content is controlled at 0.2-0.4% to prevent insufficient Sc addition from failing to achieve the grain refinement effect.

[0005] However, the aforementioned technical solution contains an inherent contradiction that has not been fully recognized: the optimal pinning size of Al3Sc particles is 8-25 nm, while the homogenization process used in CN114318076A includes a high-temperature range of 575-585℃. Prolonged holding at such high temperatures will cause significant Ostwald ripening of the Al3Sc particles, coarsening their size to 50-100 nm and drastically weakening the pinning effect. In other words, although this technical solution adds the expensive element Sc, its homogenization process is precisely detrimental to maintaining the optimal size of the dispersed phase, and the fineness potential of Sc is not fully realized.

[0006] Furthermore, existing technologies for controlling the cooling rate at the extrusion exit are primarily based on quench sensitivity considerations. The selection of the cooling rate is usually aimed at "ensuring a supersaturated solid solution," without considering its impact on the retention of dispersed phase size. Regarding aging regimes, the first-stage temperature of existing two-stage aging processes is generally above 100°C. The design goal of the aging regime is usually to promote GP zone nucleation to improve room temperature mechanical properties, without addressing the synergistic effect with the dispersed phase.

[0007] Therefore, how to maintain the nanoscale size of Al3Sc·Zr particles during the entire hot working process so that they can continue to play a pinning role in the welding thermal cycle, while achieving better grain refinement effect with a more economical amount of Sc addition, is a technical problem that has not yet been effectively solved. Summary of the Invention

[0008] Therefore, to address the aforementioned problems, this invention proposes a method for preparing aluminum alloy profiles for high-airtightness battery structure systems in new energy vehicles. This invention achieves precise control of the size of the Al3Sc·Zr dispersed phase by employing a unique synergistic combination of low-temperature long-term homogenization, ultra-rapid gradient cooling, and low-temperature long-term pre-aging, while significantly reducing the amount of Sc added. This significantly suppresses recrystallization in the weld heat-affected zone, thereby obtaining aluminum alloy profiles with excellent airtightness and weldability.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: a method for preparing aluminum alloy profiles for high airtight battery structure systems in new energy vehicles, comprising the following steps: Step (1), Batching and casting: Prepare aluminum alloy raw materials containing Mg, Si, Mn, Cr, Zr and Sc, melt and refine them, and then cast them into ingots; Step (2), low temperature long-term homogenization treatment: heat the ingot to 480-510℃ and keep it at that temperature for 10-16h, so that Zr and Sc elements precipitate in the form of Al3Sc·Zr nano-dispersed phase, and control the average particle size of the dispersed phase in the range of 8-25nm. Step (3), gradient cooling extrusion forming: the homogenized ingot is heated to 490-510℃ for extrusion. A zoned temperature control cooling device is set at the extrusion outlet. Differentiated cooling regimes are applied to the thin-walled and thick-walled areas according to the wall thickness of the profile. The cooling rate of the thin-walled area is ≥100℃ / s and the cooling rate of the thick-walled area is ≥50℃ / s. This allows the high-temperature supersaturated solid solution state at the extrusion outlet to be quickly frozen, inhibiting the coarsening of Al3Sc·Zr particles during the cooling process. Step (4), double-stage aging treatment: the extruded profile is subjected to double-stage aging. The first stage is kept at 80-95℃ for 12-24h, and the second stage is kept at 155-165℃ for 8-12h, so that the aging precipitated phase and the Al3Sc·Zr dispersed phase form a composite reinforced structure. The aluminum alloy profile is thus obtained.

[0010] Furthermore, the aluminum alloy raw materials mentioned in step (1) are as follows by mass percentage: Mg 0.65-0.85%, Si 0.70-0.90%, Mn 0.15-0.25%, Cr 0.05-0.10%, Zr 0.06-0.12%, Sc 0.008-0.025%, Fe≤0.15%, Cu≤0.10%, Zn≤0.05%, Ti 0.01-0.03%, with the balance being Al.

[0011] Furthermore, the mass ratio of Zr to Sc is 4:1 to 6:1 to form core-shell Al3Sc·Zr composite particles, in which Sc is enriched in the particle core and Zr is enriched in the particle shell.

[0012] Furthermore, the heating rate of the low-temperature long-term homogenization treatment in step (2) is 20-30℃ / h, and after the heat preservation is completed, it is rapidly cooled to room temperature at a rate of ≥200℃ / h.

[0013] Furthermore, the zoned temperature control cooling device described in step (3) is divided into a thin-walled zone and a thick-walled zone according to the profile wall thickness. The thickness of the thin-walled zone is greater than 1.8 mm and less than 2.6 mm, and the thickness of the thick-walled zone is greater than or equal to 2.6 mm and less than 5.0 mm, wherein: The thin-walled area is cooled by water mist at a rate of 120-180℃ / s, which cools the profile from 520-540℃ to below 150℃ in 3-5 seconds. The thick-walled area uses high-pressure water cooling with a cooling rate of 60-100℃ / s, which cools the profile from 520-540℃ to below 150℃ in 5-8 seconds.

[0014] Furthermore, in step (4), the heating rate of the first stage of aging is 0.5-1.5℃ / min, and a high-density GP region is formed during the heat preservation period of 80-95℃, and a coherent interface is formed between the GP region and Al3Sc·Zr particles.

[0015] Furthermore, the composite reinforcement structure described in step (4) is as follows: the average particle size of the Al3Sc·Zr nano-dispersed phase is 8-25 nm, the average length of the aged precipitated phase β″ is 10-20 nm, and the Al3Sc·Zr nano-dispersed phase and the β″ phase exhibit a bimodal size distribution in the aluminum matrix.

[0016] Furthermore, the Al3Sc·Zr nano-dispersed phase is mainly distributed at grain boundaries and subgrain boundaries, while the β″ phase is mainly distributed within the grains.

[0017] Furthermore, in step (3), the extrusion temperature is controlled at 500-520℃, the extrusion speed is controlled at 2.0-3.5m / min, and the extrusion ratio is controlled at 30-45, so that the primary Al3Sc·Zr dispersed phase undergoes controlled shearing and breakage during the extrusion process, and induces the solid solution Zr and Sc atoms to precipitate secondaryly with the broken particles as the core, forming a secondary dispersed phase with an average particle size of 3-8nm.

[0018] Furthermore, the average particle size of the primary dispersed phase is 12-25 nm, the average particle size of the secondary dispersed phase is 3-8 nm, the number density ratio of the primary dispersed phase to the secondary dispersed phase is 1:3 to 1:5, and the primary dispersed phase is mainly distributed at grain boundaries and subgrain boundaries, while the secondary dispersed phase is mainly distributed within the grains.

[0019] Furthermore, during the first stage of aging in step (4), the secondary diffuse phase serves as the preferred nucleation site in the GP region, increasing the GP region number density to 5.0 × 10⁻⁶. 23 / m³ or more, which is more than 40% higher than the state without secondary diffuse phase.

[0020] Furthermore, in step (3), the cooling rate of the ultra-fast cooling at the extrusion outlet is further increased to: 150-250℃ / s for the thin-walled region and 80-150℃ / s for the thick-walled region, so as to simultaneously lock the size state of the primary and secondary dispersed phases.

[0021] Furthermore, after friction stir welding, the recrystallized grain area fraction in the heat-affected zone of the weld is ≤15%, and the weld helium leak detection rate is ≤1×10⁻⁶. -9 The hardness of the weld heat-affected zone is not less than 85% of the hardness of the base material, and the hardness of the weld heat-affected zone is not less than 85% of the hardness of the base material.

[0022] By adopting the aforementioned technical solution, the beneficial effects of the present invention are: 1. The core of this scheme lies in constructing a three-level coupled control system of "low-temperature long-term homogenization → ultra-fast gradient cooling → low-temperature long-term pre-aging". The beneficial effects of this method are reflected in the following aspects: First, in terms of composition design, by adding trace amounts of Zr and Sc, a nanoscale Al3Sc·Zr dispersed phase is formed during the homogenization stage. This dispersed phase has excellent high-temperature thermal stability and can continuously pin grain boundaries during subsequent extrusion and welding thermal cycles, effectively inhibiting recrystallization and grain growth. Second, the low-temperature long-term homogenization regime of 480-510℃ breaks the technical bias that homogenization must reach above 540℃ to allow Mg2Si to fully dissolve. At a lower temperature, the optimal size control of Al3Sc·Zr particles is prioritized, with the average particle size controlled within the range of 8-25nm, fully leveraging the pinning effect of the dispersed phase. Third, the extrusion outlet adopts ultra-fast gradient cooling, with a cooling rate of ≥100℃ / s in the thin-walled area and ≥50℃ / s in the thick-walled area, which is significantly higher than the 3-12℃ / s of the existing technology. This can quickly "freeze" the optimal size state of the dispersed phase formed during the extrusion process, avoid particle coarsening during the cooling process, and at the same time, zoned temperature control ensures the cooling uniformity of each area of ​​the complex cross-section profile. Fourth, a low-temperature long-term pre-aging process of 80-95℃ for 12-24 hours, combined with peak aging at 155-165℃, forms a high-density GP zone at extremely low temperatures. The GP zone preferentially nucleates around Al3Sc·Zr particles, forming a coherent interface composite structure. This composite structure plays a dual role in the welding thermal cycle: Al3Sc·Zr particles pin grain boundaries to inhibit recrystallization, while the GP zone acts as a preferential nucleation site for the β″ phase, promoting rapid recrystallization of the strengthening phase, thus significantly reducing weld softening. There is a close functional coupling between the above four steps: the optimal-sized dispersed phase formed by low-temperature long-term homogenization requires ultra-rapid cooling for locking; the locked dispersed phase acts as a preferential nucleation site for the GP zone during low-temperature pre-aging, ultimately forming a composite strengthening structure. This full-process microstructure control enables the prepared aluminum alloy profile to possess excellent mechanical properties, weld airtightness, and corrosion resistance. After friction stir welding, the recrystallization area fraction of the heat-affected zone is ≤15%, and the weld helium leak detection rate is ≤1×10⁻⁶. -9 With a strength of Pa·m³ / s and a weld heat-affected zone hardness of not less than 85% of the base material hardness, it can meet the stringent requirements for high airtightness and high reliability of structural components such as battery trays for new energy vehicles.

[0023] 2. The combination of Mg content (0.65-0.85%) and Si content (0.70-0.90%) ensures sufficient precipitation of the Mg2Si strengthening phase, while the optimized Mg / Si ratio avoids the tendency for intergranular corrosion caused by excessive free Si. The composite addition of Mn content (0.15-0.25%) and Cr content (0.05-0.10%) forms a dispersed phase to further inhibit recrystallization and transforms coarse Fe-containing phases into finer morphologies, reducing the adverse effects of coarse phases cutting the matrix. The limitation of Zr content (0.06-0.12%) and Sc content (0.008-0.025%) is key to achieving "excellent performance at extremely low Sc content" in this invention. The Sc addition is only one-tenth to one-twentieth of that in the prior art, significantly reducing raw material costs. The impurity control of Fe≤0.15%, Cu≤0.10%, and Zn≤0.05% reduces the formation of harmful phases, while Ti (0.01-0.03%) plays a role in refining the as-cast grains. This composition design allows the profile to have a significant cost advantage while ensuring high performance.

[0024] 3. The Zr to Sc mass ratio was 4:1 to 6:1, and the formed Al3Sc·Zr nano-dispersed phase was clearly defined as a core-shell structure. This ratio range was an optimal range determined through systematic experiments. Within this range, Sc atoms preferentially form Sc-rich cores in the early stages of homogenization, while Zr atoms subsequently diffuse outwards and accumulate in the particle shell, forming a Zr-rich protective layer. This core-shell structure endows the dispersed phase with excellent high-temperature thermal stability. The Zr-rich shell can effectively suppress Ostwald ripening of particles during subsequent thermal processing, keeping the particle size within the optimal pinning range of 8-25 nm. Simultaneously, the cost of Zr is significantly lower than that of Sc. Through the core-shell structure design, the expensive Sc addition amount is greatly reduced while ensuring the dispersed phase number density, achieving a balance between technical effectiveness and economic efficiency.

[0025] 4. Heating and cooling rates for low-temperature, long-term homogenization treatment. Slow heating at a rate of 20-30℃ / h ensures uniform temperature inside and outside the ingot, preventing ingot cracking caused by thermal stress, and providing sufficient time for the full diffusion of Zr and Sc atoms and the uniform nucleation of Al3Sc·Zr particles. Rapid cooling at a rate of ≥200℃ / h after holding effectively suppresses further coarsening of the dispersed phase during cooling, locking the particle size at the optimal state formed during the homogenization stage. This combination of slow heating and rapid cooling ensures nucleation uniformity during the heating phase and dimensional stability during the cooling phase, thus guaranteeing the quality of the dispersed phase from both directions.

[0026] 5. A zoned cooling regime in gradient cooling extrusion forming. The profile wall thickness is divided into a thin-walled zone (1.8-2.5 mm) and a thick-walled zone (2.6-5.0 mm). The thin-walled zone uses water mist cooling to achieve an ultra-fast cooling rate of 120-180℃ / s, while the thick-walled zone uses high-pressure water cooling to achieve a rapid cooling rate of 60-100℃ / s. This allows each region to cool from 520-540℃ to below 150℃ within seconds. This regime solves the problem of uneven cooling caused by differences in wall thickness in complex cross-section profiles. The difference in cooling rates between the thin-walled and thick-walled zones is controlled within a reasonable range, resulting in a uniform microstructure across the cross-section. The ultra-fast cooling rate is significantly higher than the 3-12℃ / s of existing technologies, effectively "locking in" the optimal size state of Al3Sc·Zr particles and avoiding particle coarsening caused by slow cooling.

[0027] 6. Heating rate and heat preservation effect of the first stage of aging. The slow heating rate of 0.5-1.5℃ / min allows the profile to undergo a sufficient low-temperature transition before entering the pre-aging temperature, resulting in a more uniform nucleation process in the GP regions and avoiding uneven nucleation density caused by rapid heating. During the heat preservation period of 80-95℃, solute atoms diffuse at an extremely slow rate, forming high-density, small-sized GP regions, with a number density reaching 3×10⁻⁶. 23 / m³ or more. More importantly, the GP region preferentially nucleates around Al3Sc·Zr particles, forming a coherent interface between them, which lays the organizational foundation for the subsequent formation of composite strengthening structures.

[0028] 7. The composite strengthening structure is specifically quantified into detectable microstructural characteristics. The average particle size of the Al3Sc·Zr nano-dispersed phase is 8-25 nm, which is within the optimal size range for pinned grain boundaries; the average length of the aged precipitate β″ phase is 10-20 nm, which is within the optimal size range for precipitation strengthening. Both exhibit a bimodal size distribution in the aluminum matrix. The dispersed phase provides a recrystallization inhibition effect, while the precipitate phase provides a precipitation strengthening effect. The two strengthening mechanisms do not interfere with each other and work synergistically. This microstructural characteristic is a direct product of the synergistic effect of low-temperature long-term homogenization and low-temperature long-term pre-aging in this invention, and has not been disclosed in the prior art.

[0029] 8. Spatial distribution characteristics of the two types of particles in the bimodal size distribution. The Al3Sc·Zr nano-dispersed phase is mainly distributed at grain boundaries and subgrain boundaries, directly pinning grain boundary migration during welding thermal cycling, effectively inhibiting recrystallization and grain growth; the β″ phase is mainly distributed within the grains, providing matrix reinforcement. This difference in spatial distribution allows the two types of particles to perform their respective functions—the dispersed phase at the grain boundaries is responsible for structural stability, while the precipitated phase within the grains is responsible for mechanical properties, avoiding efficiency losses caused by functional overlap. During welding thermal cycling, the dispersed phase at the grain boundaries continues to exert a pinning effect, while the precipitated phase within the grains rapidly recrystallizes to restore strength. The synergy between the two ensures that the hardness of the heat-affected zone is not less than 85% of that of the base material.

[0030] 9. After friction stir welding, the recrystallized grain area fraction in the heat-affected zone of the profile is ≤15%, far lower than the 50-60% of traditional 6061-T6 and the 30-40% of conventional Sc-containing alloys. This indicates that the present invention has a significantly better effect on inhibiting recrystallization in the heat-affected zone than existing technologies. The weld helium leak detection rate is ≤1×10⁻⁶. -9 The hardness (Pa·m³ / s) meets the IP68 sealing requirements for battery boxes, providing reliable assurance for the airtight safety of new energy vehicle battery packs. The hardness of the weld heat-affected zone is not less than 85% of the base material hardness, indicating minimal weld softening and a high joint strength coefficient. This product combines high strength and toughness, excellent weldability, and high airtightness, making it suitable for structural components with stringent comprehensive performance requirements, such as battery trays and battery boxes for new energy vehicles. Attached Figure Description

[0031] Figure 1 This is a flowchart illustrating the present invention; Figure 2 This is a summary table comparing and analyzing the examples and comparative examples. Detailed Implementation

[0032] The present invention will now be further described in conjunction with the accompanying drawings and specific embodiments; Example 1, Reference Figure 1 .

[0033] I. Batching and Casting The raw materials are prepared according to the following mass percentages: Mg 0.75%, Si 0.80%, Mn 0.20%, Cr 0.08%, Zr 0.10%, Sc 0.020%, Fe 0.12%, Cu 0.06%, Zn 0.03%, Ti 0.02%, with the balance being Al. The mass ratio of Zr to Sc is 5:1.

[0034] The raw materials were fed into a gas-fired melting furnace and heated to 740℃ until completely melted. A refining agent, comprising 0.15% of the melt mass, was added. The refining agent composition was 35% NaCl, 30% KCl, 20% CaCl2, 10% NaF, and 5% Mg3N2. Refining was carried out at 730℃ for 20 minutes, followed by slag removal and standing for 25 minutes. After online degassing and ceramic filtration, semi-continuous casting was performed at 700℃. Online degassing used rotary jet Ar gas at a flow rate of 1.5 L / min, and ceramic filtration used 30 ppi filter plates. The casting speed was 70 mm / min, yielding a Φ203 mm ingot.

[0035] II. Low-temperature long-term homogenization treatment The ingot was heated to 495℃ at a heating rate of 25℃ / h and held at that temperature for 14 hours. After holding, it was air-cooled to room temperature at a rate of 250℃ / h.

[0036] Transmission electron microscopy (TEM) and image analysis revealed that the particle size distribution of homogenized Al3Sc·Zr particles ranged from 5 to 35 nm, with 87% of the particles concentrated in the 8-25 nm range. The average particle size was 15 nm, and the particle number density was 4.2 × 10⁻⁶. 21 / m³. Al3Sc·Zr indicates that Sc and Zr occupy the same lattice site in the nano-dispersed phase.

[0037] III. Gradient Cooling Extrusion Molding The homogenized ingot was heated to 500℃, and the extrusion die was preheated to 475℃. A 2500T extrusion press was used, with an extrusion speed of 2.8m / min and an extrusion ratio of 32.

[0038] The extrusion outlet is equipped with a zoned temperature-controlled cooling device, divided into a thin-walled zone and a thick-walled zone according to the profile wall thickness. The wall thickness range of the thin-walled zone is 1.8-2.5mm, and the wall thickness range of the thick-walled zone is 2.6-5.0mm. The following cooling regimes are implemented respectively: For the thin-walled region with a wall thickness of 2.0 mm, water mist cooling was used, and the measured cooling rate was 152℃ / s, cooling from 532℃ to 118℃ in 3.4 seconds. For the thick-walled region with a wall thickness of 3.0 mm, high-pressure water cooling was used, and the measured cooling rate was 86℃ / s, cooling from 530℃ to 102℃ in 5.9 seconds; For the thick-walled region with a wall thickness of 4.0 mm, high-pressure water cooling was used, and the measured cooling rate was 72℃ / s, cooling from 528℃ to 95℃ in 6.8 seconds.

[0039] The outlet temperature is controlled at 530±5℃, and after cooling, it is stretched and straightened with an elongation rate of 1.0%.

[0040] IV. Two-stage aging treatment consisting of low-temperature long-term pre-aging and peak aging The first stage of aging is low-temperature long-term pre-aging: the profile is heated to 88℃ at a heating rate of 1.0℃ / min and held for 18 hours.

[0041] The second stage of aging is the peak aging: continue to heat to 160℃ and keep warm for 10 hours.

[0042] After the aging period, the material is air-cooled to room temperature to obtain the finished profile.

[0043] Transmission electron microscopy (TEM) analysis revealed that the average particle size of Al3Sc·Zr particles in the finished profile was 18 nm, and the average length of the β″ phase was 16 nm. Microstructural analysis showed that the Al3Sc·Zr particles and the β″ phase exhibited a bimodal size distribution, with Al3Sc·Zr particles mainly distributed near grain boundaries and subgrain boundaries, and the β″ phase mainly distributed within the grains. High-resolution TEM observation indicated that the Al3Sc·Zr particles maintained a coherent relationship with the aluminum matrix, and some β″ phase preferentially precipitated around the Al3Sc·Zr particles, forming a coherent interface composite structure.

[0044] V. Performance Testing 1. Room temperature tensile properties Tensile tests were conducted at room temperature according to GB / T 228.1 standard, and the results are as follows: The tensile strength is 381 MPa, the yield strength is 342 MPa, and the elongation after fracture is 13.5%.

[0045] 2. Welding performance Friction stir welding was used to butt weld the profile of Example 1. The welding parameters were: rotation speed 1200 rpm, welding speed 300 mm / min, stirring head shoulder diameter 15 mm, stirring pin diameter 5 mm, and insertion depth 4.2 mm.

[0046] After welding, helium mass spectrometry leak detection was performed according to GB / T 15823 standard, and the leak rate was 6.5 × 10⁻⁶. -10 Pa·m³ / s.

[0047] Tensile tests were conducted on the welded joint, and the tensile strength after welding was 328 MPa, with a weld strength coefficient of 0.86. The weld strength coefficient is defined as the ratio of the tensile strength after welding to the tensile strength of the base metal.

[0048] 3. Microstructure analysis of the heat-affected zone during welding Metallographic observation and image analysis of the cross-section of the welded joint were performed, and the results are as follows: The average grain size of the heat-affected zone (HAZ) is 18 μm, and the recrystallized grain area fraction in the HAZ is 12%. The microstructure is characterized by fine equiaxed grains, clear grain boundaries, no obvious grain coarsening, and slight recrystallization observed only in localized areas.

[0049] 4. Microhardness distribution of welded joints According to GB / T 4340.1 standard, microhardness tests were conducted transversely along the weld joint with a load of 100g and a holding time of 10s. The test results are as follows: the hardness at 0mm from the weld center (weld center) is 112HV; at 3mm from the weld center, the hardness is 106HV; at 6mm from the weld center, the hardness is 118HV; at 9mm from the weld center, the hardness is 124HV; at 12mm from the weld center, the hardness is 127HV; at 15mm from the weld center, the hardness is 130HV; and the base metal hardness is 132HV. The lowest hardness in the heat-affected zone is 105HV, located 5mm from the weld center. The average hardness of the heat-affected zone is 115HV, reaching 87% of the base metal hardness.

[0050] 5. Corrosion resistance A neutral salt spray test was conducted according to GB / T 10125 standard for 1000 hours. After the test, the corrosion level was rated according to GB / T 6461, and the corrosion level was 10.

[0051] 6. Statistical analysis of Al3Sc·Zr particle characteristics The finished profiles were observed using transmission electron microscopy, and particle size was statistically analyzed in 10 randomly selected fields of view. The results are as follows: The particle size distribution ranges from 5 to 40 nm, with an average particle size of 18 nm. Particles in the 8-25 nm range account for 82% of the total, and the particle number density is 3.8 × 10⁻⁶. 21 / m³. EDS energy dispersive spectroscopy analysis confirmed that the particle type is core-shell structure, with the core rich in Sc and the outer shell rich in Zr. Example 2

[0052] I. Batching and Casting The raw materials are prepared according to the following mass percentages: Mg 0.70%, Si 0.75%, Mn 0.18%, Cr 0.06%, Zr 0.08%, Sc 0.015%, Fe 0.10%, Cu 0.05%, Zn 0.02%, Ti 0.02%, with the balance being Al and unavoidable impurities. The mass ratio of Zr to Sc is 5.3:1.

[0053] The melting temperature was 735℃, the refining temperature was 725℃, the refining time was 18min, the casting temperature was 695℃, and the casting speed was 65mm / min. The remaining operations were the same as in Example 1.

[0054] II. Low-temperature long-term homogenization treatment The ingot is heated to 485℃ at a rate of 20℃ / h and held at that temperature for 16 hours. After holding, it is cooled to room temperature by air at a rate of 220℃ / h.

[0055] After homogenization, the average particle size of Al3Sc·Zr particles was 13 nm, and the number density was 4.8 × 10⁻⁶. 21 / m³.

[0056] III. Gradient Cooling Extrusion Molding The ingot is heated to 495℃, the die is preheated to 470℃, the extrusion speed is 3.2m / min, and the extrusion ratio is 28.

[0057] The zoned cooling system and measured data are as follows: For the thin-walled region with a wall thickness of 2.2 mm, water mist cooling was used with a cooling rate of 163℃ / s, a cooling time of 3.3 seconds, and a final cooling temperature of 112℃. For the thick-walled region with a wall thickness of 3.2 mm, high-pressure water cooling is used with a cooling rate of 91℃ / s, a cooling time of 5.7 seconds, and a final cooling temperature of 96℃. For the thick-walled area with a wall thickness of 4.5 mm, high-pressure water cooling is used with a cooling rate of 67℃ / s, a cooling time of 7.4 seconds, and a final cooling temperature of 88℃.

[0058] Elongation rate: 1.2%.

[0059] IV. Two-level timeliness processing First-level aging: Increase the temperature to 85℃ at a rate of 0.8℃ / min and hold for 20 hours.

[0060] Second-stage aging: Heat to 158℃ and hold for 12 hours.

[0061] After aging, the average particle size of Al3Sc·Zr particles was 16 nm, and the average length of the β″ phase was 14 nm. Microstructure observation showed that the Al3Sc·Zr particles and the β″ phase exhibited a bimodal distribution, indicating a coherent interface between them.

[0062] V. Performance Test Results Tensile strength 376MPa, yield strength 338MPa, elongation 14.1%.

[0063] The helium leak detection rate for weld seams is 5.8 × 10⁻⁶. -10 Pa·m³ / s, weld strength coefficient is 0.85.

[0064] The average grain size of the heat-affected zone is 15 μm, and the recrystallization area fraction of the heat-affected zone is 11%.

[0065] Microhardness test results of the welded joint: 108 HV at 0 mm from the weld center, 102 HV at 3 mm from the weld center, 115 HV at 6 mm from the weld center, 121 HV at 9 mm from the weld center, 125 HV at 12 mm from the weld center, and 128 HV at 15 mm from the weld center. The base metal hardness is 130 HV. The average hardness of the heat-affected zone is 114 HV, reaching 86% of the base metal hardness.

[0066] The salt spray corrosion level is 10 after 1000 hours. Example 3

[0067] I. Batching and Casting The raw materials are prepared according to the following mass percentages: Mg 0.80%, Si 0.85%, Mn 0.22%, Cr 0.09%, Zr 0.12%, Sc 0.022%, Fe 0.13%, Cu 0.08%, Zn 0.04%, Ti 0.025%, with the balance being Al and unavoidable impurities. The mass ratio of Zr to Sc is 5.5:1.

[0068] Melting temperature 745℃, refining temperature 730℃, refining time 22min, casting temperature 705℃, casting speed 75mm / min.

[0069] II. Low-temperature long-term homogenization treatment The temperature was increased to 505℃ at a rate of 28℃ / h and held for 12 hours. After the holding period, the temperature was cooled to room temperature by air at a rate of 260℃ / h.

[0070] After homogenization, the average particle size of Al3Sc·Zr particles was 20 nm, and the number density was 3.5 × 10⁻⁶. 21 / m³.

[0071] III. Gradient Cooling Extrusion Molding The ingot is heated to 508℃, the die is preheated to 480℃, the extrusion speed is 2.5m / min, and the extrusion ratio is 36.

[0072] The zoned cooling system and measured data are as follows: For the thin-walled area with a wall thickness of 1.8 mm, water mist cooling is used with a cooling rate of 178℃ / s, a cooling time of 2.9 seconds, and a final cooling temperature of 116℃. For the thick-walled region with a wall thickness of 2.8 mm, high-pressure water cooling is used with a cooling rate of 94℃ / s, a cooling time of 5.6 seconds, and a final cooling temperature of 92℃. For the thick-walled area with a wall thickness of 3.8 mm, high-pressure water cooling is used with a cooling rate of 77℃ / s, a cooling time of 6.6 seconds, and a final cooling temperature of 86℃.

[0073] Elongation rate: 0.8%.

[0074] IV. Two-level timeliness processing First-level aging: Increase the temperature to 92℃ at a rate of 1.2℃ / min and hold for 15 hours.

[0075] Second-stage aging: Heat to 162℃ and hold for 9 hours.

[0076] After aging, the average particle size of Al3Sc·Zr particles is 22 nm, and the average length of the β″ phase is 18 nm.

[0077] V. Performance Test Results Tensile strength 386MPa, yield strength 347MPa, elongation 12.8%.

[0078] The weld helium leak detection rate was 7.2 × 10⁻⁶. -10 Pa·m³ / s, weld strength coefficient is 0.87.

[0079] The average grain size of the heat-affected zone is 21 μm, and the recrystallization area fraction of the heat-affected zone is 14%.

[0080] Microhardness test results of the welded joint: 115 HV at 0 mm from the weld center, 108 HV at 3 mm from the weld center, 120 HV at 6 mm from the weld center, 127 HV ​​at 9 mm from the weld center, 131 HV at 12 mm from the weld center, and 134 HV at 15 mm from the weld center. The base metal hardness was 136 HV. The average hardness of the heat-affected zone was 119 HV, reaching 88% of the base metal hardness.

[0081] The salt spray corrosion level is 10 after 1000 hours. Example 4

[0082] I. Batching and Casting The raw materials are prepared according to the following mass percentages: Mg 0.68%, Si 0.72%, Mn 0.16%, Cr 0.07%, Zr 0.06%, Sc 0.010%, Fe 0.11%, Cu 0.04%, Zn 0.02%, Ti 0.015%, with the balance being Al and unavoidable impurities. The mass ratio of Zr to Sc is 6:1.

[0083] Melting temperature 730℃, refining temperature 720℃, refining time 20min, casting temperature 690℃, casting speed 60mm / min.

[0084] II. Low-temperature long-term homogenization treatment The temperature was increased to 480℃ at a rate of 22℃ / h and held for 16 hours. After the holding period, the temperature was cooled to room temperature by air at a rate of 200℃ / h.

[0085] After homogenization, the average particle size of Al3Sc·Zr particles was 11 nm, and the number density was 5.0 × 10²¹ / m³. In this embodiment, a lower homogenization temperature and a longer holding time were used to obtain the smallest dispersed phase size and the highest number density.

[0086] III. Gradient Cooling Extrusion Molding The ingot is heated to 490℃, the die is preheated to 465℃, the extrusion speed is 3.5m / min, and the extrusion ratio is 30.

[0087] The zoned cooling system and measured data are as follows: For the thin-walled area with a wall thickness of 2.0 mm, water mist cooling is used with a cooling rate of 142℃ / s, a cooling time of 3.5 seconds, and a final cooling temperature of 108℃. For the thick-walled area with a wall thickness of 3.5mm, high-pressure water cooling is used with a cooling rate of 78℃ / s, a cooling time of 6.3 seconds, and a final cooling temperature of 94℃.

[0088] Elongation rate: 1.3%.

[0089] IV. Two-level timeliness processing First-stage aging: The temperature is increased to 82℃ at a rate of 0.6℃ / min and held for 24 hours. The lowest pre-aging temperature and the longest time are used to fully form the coherent composite structure of the GP zone and Al3Sc·Zr particles.

[0090] Second-stage aging: Heat to 155℃ and hold for 12 hours.

[0091] After aging, the average particle size of Al3Sc·Zr particles is 14 nm, and the average length of the β″ phase is 12 nm.

[0092] V. Performance Test Results Tensile strength 372MPa, yield strength 334MPa, elongation 14.5%.

[0093] The helium leak detection rate for weld seams is 5.2 × 10⁻⁶. -10 Pa·m³ / s, weld strength coefficient is 0.84.

[0094] The average grain size of the heat-affected zone is 13 μm, and the recrystallization area fraction of the heat-affected zone is 10%.

[0095] Microhardness test results of the welded joint: 106 HV at 0 mm from the weld center, 101 HV at 3 mm from the weld center, 112 HV at 6 mm from the weld center, 118 HV at 9 mm from the weld center, 122 HV at 12 mm from the weld center, and 125 HV at 15 mm from the weld center. The base metal hardness was 127 HV. The average hardness of the heat-affected zone was 111 HV, reaching 85% of the base metal hardness.

[0096] The salt spray corrosion level is 10 after 1000 hours.

[0097] This embodiment achieves excellent overall performance with the lowest Sc addition of 0.010% and Zr addition of 0.06%, especially with a recrystallization area fraction of only 10% in the heat-affected zone, which is the lowest value among all embodiments of the present invention. Example 5

[0098] I. Batching and Casting The raw materials are prepared according to the following mass percentages: Mg 0.82%, Si 0.88%, Mn 0.24%, Cr 0.10%, Zr 0.11%, Sc 0.025%, Fe 0.14%, Cu 0.09%, Zn 0.04%, Ti 0.028%, with the balance being Al and unavoidable impurities. The mass ratio of Zr to Sc is 4.4:1.

[0099] Melting temperature 750℃, refining temperature 735℃, refining time 25min, casting temperature 710℃, casting speed 80mm / min.

[0100] II. Low-temperature long-term homogenization treatment The temperature was increased to 510℃ at a rate of 30℃ / h and held for 10 hours. After the holding period, the temperature was cooled to room temperature by air at a rate of 280℃ / h.

[0101] After homogenization, the average particle size of Al3Sc·Zr particles was 24 nm, and the number density was 2.8 × 10²¹ / m³. This embodiment uses the upper limit of homogenization temperature and the lower limit of holding time of the present invention, resulting in a slightly larger dispersed phase size that is still within the optimal range of 8-25 nm.

[0102] III. Gradient Cooling Extrusion Molding The ingot is heated to 510℃, the die is preheated to 485℃, the extrusion speed is 2.2m / min, and the extrusion ratio is 38.

[0103] The zoned cooling system and measured data are as follows: For the thin-walled region with a wall thickness of 2.3 mm, water mist cooling is used with a cooling rate of 182℃ / s, a cooling time of 2.8 seconds, and a final cooling temperature of 120℃. For the thick-walled area with a wall thickness of 4.8 mm, high-pressure water cooling is used with a cooling rate of 98℃ / s, a cooling time of 5.3 seconds, and a final cooling temperature of 90℃.

[0104] Elongation rate: 1.0%.

[0105] IV. Two-level timeliness processing First-level aging: Increase the temperature to 95℃ at a rate of 1.4℃ / min and hold for 12 hours.

[0106] Second-level aging: Heat to 165℃ and keep warm for 8 hours.

[0107] After aging, the average particle size of Al3Sc·Zr particles is 25 nm, and the average length of the β″ phase is 20 nm.

[0108] V. Performance Test Results Tensile strength 390MPa, yield strength 350MPa, elongation 12.1%.

[0109] The weld helium leak detection rate is 8.0 × 10⁻⁶. -10 Pa·m³ / s, weld strength coefficient is 0.88.

[0110] The average grain size of the heat-affected zone is 24 μm, and the recrystallization area fraction of the heat-affected zone is 15%.

[0111] Microhardness test results of the welded joint: 118 HV at 0 mm from the weld center, 110 HV at 3 mm from the weld center, 123 HV at 6 mm from the weld center, 130 HV at 9 mm from the weld center, 134 HV at 12 mm from the weld center, and 137 HV at 15 mm from the weld center. The base metal hardness was 140 HV. The average hardness of the heat-affected zone was 123 HV, reaching 89% of the base metal hardness.

[0112] The salt spray corrosion level is 10 after 1000 hours.

[0113] In this embodiment, even with a relatively short process time of 10 hours of homogenization, 12 hours of pre-aging, and 8 hours of peak aging, excellent mechanical and weldability were still achieved. The tensile strength reached 390 MPa, and the weld strength coefficient reached 0.88, both of which are the highest values ​​in all embodiments of the present invention.

[0114] Comparative Example 1 Comparative Example 1 uses the conventional 6061-T6 process.

[0115] The alloy used is a standard 6061 alloy, with the following chemical composition by mass percentage: Mg 0.60%, Si 0.70%, Mn 0.05%, Cr 0.08%, Fe 0.15%, Cu 0.05%, Zn 0.03%, Ti 0.02%, with the balance being Al and unavoidable impurities. It does not contain Zr or Sc.

[0116] Preparation process: single-stage homogenization treatment, holding at 560℃ for 6 hours, air cooling to room temperature and then reheating to 480℃ for extrusion; conventional extrusion, extrusion speed 4.5m / min, outlet air cooling, cooling rate about 5℃ / s; stretching and straightening, stretching rate 1.0%; single-stage aging, holding at 175℃ for 8 hours.

[0117] The test results are as follows: Tensile strength 348MPa, yield strength 302MPa, elongation 9.5%.

[0118] The helium leak detection rate for weld seams is 5.2 × 10⁻⁶. -9 Pa·m³ / s, weld strength coefficient is 0.68.

[0119] The average grain size of the heat-affected zone is 55 μm, and the recrystallization area fraction of the heat-affected zone is 58%.

[0120] Microhardness test results of the welded joint: 95 HV at 0 mm from the weld center, 82 HV at 3 mm from the weld center, 88 HV at 6 mm from the weld center, 96 HV at 9 mm from the weld center, 102 HV at 12 mm from the weld center, and 108 HV at 15 mm from the weld center. The base metal hardness was 115 HV. The lowest hardness in the heat-affected zone was 82 HV, and the average hardness of the heat-affected zone was 95 HV, reaching 71% of the base metal hardness.

[0121] The salt spray corrosion level is 9 after 1000 hours.

[0122] Comparative Example 2 Comparative Example 2 uses the composition and process disclosed in Example 2 of Chinese Patent Publication No. CN114318076A to prepare the profile.

[0123] The chemical composition by mass percentage is: Si 0.78%, Mg 0.62%, Mn 0.20%, Cr 0.17%, Sc 0.28%, without Zr, with the balance being Al and unavoidable impurities.

[0124] Preparation process: Three-stage homogenization process, first stage at 473℃ for 250 min, second stage at 563℃ for 365 min, third stage at 579℃ for 135 min; extrusion speed 6.2 m / min; water cooling at a rate of approximately 16℃ / s; single-stage aging at 175℃ for 8 h.

[0125] The test results are as follows: Tensile strength 337MPa, yield strength 321MPa, elongation 13.5%.

[0126] The weld helium leak detection rate is 1.8 × 10⁻⁶. -9 Pa·m³ / s, weld strength coefficient is 0.88.

[0127] The average grain size of the heat-affected zone is 32 μm, the recrystallization area fraction of the heat-affected zone is 35%, and the average particle size of Al3Sc particles is 65 nm.

[0128] Microhardness test results of the welded joint: 108 HV at 0 mm from the weld center, 98 HV at 3 mm from the weld center, 112 HV at 6 mm from the weld center, 118 HV at 9 mm from the weld center, 122 HV at 12 mm from the weld center, and 124 HV at 15 mm from the weld center. The base metal hardness was 126 HV. The average hardness of the heat-affected zone was 110 HV, reaching 78% of the base metal hardness.

[0129] Comparative Example 3 Comparative Example 3 uses the same alloy composition as Example 1, but is prepared using a conventional process.

[0130] Preparation process: single-stage homogenization treatment, heat treatment at 560℃ for 6 hours; conventional extrusion, outlet air cooling, cooling rate of about 5℃ / s; single-stage aging, heat treatment at 175℃ for 8 hours.

[0131] The test results are as follows: Tensile strength 365MPa, yield strength 325MPa, elongation 11.2%.

[0132] The weld helium leak detection rate is 2.5 × 10⁻⁶. -9 Pa·m³ / s, weld strength coefficient is 0.75.

[0133] The average grain size of the heat-affected zone is 40 μm, the recrystallization area fraction of the heat-affected zone is 42%, and the average particle size of Al3Sc·Zr particles is 48 nm.

[0134] Microhardness test results of the welded joint: 102 HV at 0 mm from the weld center, 90 HV at 3 mm from the weld center, 98 HV at 6 mm from the weld center, 106 HV at 9 mm from the weld center, 112 HV at 12 mm from the weld center, and 116 HV at 15 mm from the weld center. The base metal hardness was 120 HV. The average hardness of the heat-affected zone was 104 HV, reaching 74% of the base metal hardness.

[0135] Comparative Example 4 Comparative Example 4 used the same alloy composition and low-temperature homogenization process as Example 1, but the extrusion outlet was cooled by conventional air.

[0136] Preparation process: low-temperature long-term homogenization, holding at 495℃ for 14h; conventional extrusion, outlet air cooling, cooling rate of about 8℃ / s; two-stage aging process as in Example 1, first stage holding at 88℃ for 18h, second stage holding at 160℃ for 10h.

[0137] The test results are as follows: Tensile strength 370MPa, yield strength 335MPa, elongation 12.8%.

[0138] The weld helium leak detection rate is 1.5 × 10⁻⁶. -9 Pa·m³ / s, weld strength coefficient is 0.80.

[0139] The average grain size of the heat-affected zone is 26 μm, the recrystallization area fraction of the heat-affected zone is 28%, and the average particle size of Al3Sc·Zr particles is 35 nm.

[0140] Microhardness test results of the welded joint: 108 HV at 0 mm from the weld center, 100 HV at 3 mm from the weld center, 113 HV at 6 mm from the weld center, 119 HV at 9 mm from the weld center, 123 HV at 12 mm from the weld center, and 126 HV at 15 mm from the weld center. The base metal hardness was 128 HV. The average hardness of the heat-affected zone was 113 HV, reaching 80% of the base metal hardness.

[0141] Compared with Example 1, Comparative Example 4, due to the lack of ultra-fast cooling, the fine dispersed phase formed by homogenization coarsens during the slow cooling process at the extrusion outlet, and the average particle size coarsens from 15 nm after homogenization to 35 nm, resulting in a weakened recrystallization inhibition effect and an increase in the recrystallization area fraction of the heat-affected zone from 12% to 28%.

[0142] Comparative Example 5 Comparative Example 5 used the same alloy composition, low-temperature homogenization and ultra-fast gradient cooling regime as Example 1, but the aging was carried out using conventional single-stage aging.

[0143] Preparation process: low-temperature long-term homogenization, holding at 495℃ for 14h; ultra-rapid gradient cooling extrusion, same as in Example 1; single-stage aging, holding at 175℃ for 8h.

[0144] The test results are as follows: Tensile strength 375MPa, yield strength 338MPa, elongation 11.5%.

[0145] The weld helium leak detection rate is 1.0 × 10⁻⁶. -9 Pa·m³ / s, weld strength coefficient is 0.82.

[0146] The average grain size of the heat-affected zone is 22 μm, the recrystallization area fraction of the heat-affected zone is 18%, and the average particle size of Al3Sc·Zr particles is 20 nm.

[0147] Microhardness test results of the welded joint: 112 HV at 0 mm from the weld center, 104 HV at 3 mm from the weld center, 116 HV at 6 mm from the weld center, 121 HV at 9 mm from the weld center, 125 HV at 12 mm from the weld center, and 128 HV at 15 mm from the weld center. The base metal hardness is 130 HV. The average hardness of the heat-affected zone is 116 HV, reaching 82% of the base metal hardness.

[0148] Compared with Example 1, although the Al3Sc·Zr particle size of Comparative Example 5 was well maintained, the coherent composite structure of the GP zone and Al3Sc·Zr particles could not be fully formed due to the lack of low-temperature long-term pre-aging. The welding softening resistance was insufficient, the hardness retention rate of the heat-affected zone was only 82%, and the weld strength coefficient was 0.82, all of which were lower than those of Example 1.

[0149] refer to Figure 2 Summary table of comparative analysis of examples and comparative examples. Detailed explanation is as follows: A comparison of Example 1 and Comparative Example 1 shows that Example 1 is significantly superior to the traditional 6061-T6 process in terms of strength, plasticity, weldability, and airtightness. The recrystallization area fraction of the heat-affected zone decreased from 58% to 12%, a reduction of 79%; the hardness retention rate of the heat-affected zone increased from 71% to 87%; and the weld strength coefficient increased from 0.68 to 0.86.

[0150] A comparison of Example 1 and Comparative Example 2 shows that the amount of Sc added in Example 1 is only one-fourteenth that of Comparative Example 2, but the tensile strength is 44 MPa higher, the recrystallization area fraction in the heat-affected zone decreases from 35% to 12%, and the Al3Sc·Zr particle size is refined from 65 nm to 18 nm. This proves that the present invention achieves a better grain refinement effect with extremely low Sc content, producing unexpected technical effects.

[0151] A comparison of Example 1 and Comparative Example 3 shows that, with the same composition, the full-process coupling process in Example 1 comprehensively improves performance, reduces the recrystallization area fraction of the heat-affected zone from 42% to 12%, and refines the Al3Sc·Zr particle size from 48nm to 18nm, demonstrating the key role of process coupling.

[0152] A comparison of Example 1 and Comparative Example 4 shows that Comparative Example 4 lacks ultra-rapid cooling. The fine dispersed phase formed during homogenization coarsens during the slow cooling process at the extrusion outlet, increasing from 15 nm to 35 nm. This results in a weakened recrystallization inhibition effect, with the recrystallization area fraction in the heat-affected zone reaching 28%. This demonstrates that ultra-rapid cooling is a necessary condition for maintaining the optimal size of the dispersed phase.

[0153] A comparison of Example 1 and Comparative Example 5 shows that Comparative Example 5 lacks low-temperature long-term pre-aging. Although the Al3Sc·Zr particle size is well maintained at 20nm, the coherent composite structure of the GP region and Al3Sc·Zr particles is not fully formed, resulting in insufficient resistance to welding softening and a heat-affected zone hardness retention rate of only 82%. This demonstrates that low-temperature long-term pre-aging is a key step in obtaining excellent welding performance.

[0154] The above comparison fully demonstrates that there is a close functional coupling relationship between the low-temperature long-term homogenization, ultra-rapid gradient cooling, and low-temperature long-term pre-aging of this invention, and none of them can be omitted. Only through the synergistic effect of the three can effective suppression of recrystallization in the weld heat-affected zone be achieved with extremely low Sc addition, resulting in excellent airtightness and welding performance. Example 6

[0155] Based on Example 1, this embodiment further optimizes the extrusion process parameters to induce secondary precipitation of the dispersed phase, forming a two-stage dispersed structure of "primary dispersed phase + secondary dispersed phase".

[0156] I. Batching and Casting The raw materials are prepared according to the following mass percentages: Mg 0.75%, Si 0.80%, Mn 0.20%, Cr 0.08%, Zr 0.10%, Sc 0.020%, Fe 0.12%, Cu 0.06%, Zn 0.03%, Ti 0.02%, with the balance being Al and unavoidable impurities. The mass ratio of Zr to Sc is 5:1.

[0157] The raw materials were fed into a gas-fired melting furnace and heated to 740℃ until completely melted. A refining agent, comprising 0.15% of the melt mass, was added. The refining agent composition was 35% NaCl, 30% KCl, 20% CaCl2, 10% NaF, and 5% Mg3N2. Refining was carried out at 730℃ for 20 minutes, followed by slag removal and standing for 25 minutes. After online degassing and ceramic filtration, semi-continuous casting was performed at 700℃. Online degassing used rotary jet Ar gas at a flow rate of 1.5 L / min, and ceramic filtration used 30 ppi filter plates. The casting speed was 70 mm / min, yielding a Φ203 mm ingot.

[0158] II. Low-temperature long-term homogenization treatment The ingot was heated to 495℃ at a heating rate of 25℃ / h and held at that temperature for 14 hours. After holding, it was air-cooled to room temperature at a rate of 250℃ / h.

[0159] Transmission electron microscopy (TEM) and image analysis revealed that the particle size distribution of the primary dispersed phase Al3Sc·Zr after homogenization ranged from 5 to 35 nm, with 87% of the particles concentrated in the 8-25 nm range. The average particle size was 15 nm, and the particle number density was 4.2 × 10⁻⁶. 21 / m³.

[0160] III. Extrusion molding with deformation-induced secondary precipitation The homogenized ingot was heated to 510℃, and the extrusion die was preheated to 480℃. A 2500T extrusion press was used, with an extrusion speed of 2.5 m / min and an extrusion ratio of 42. It should be noted that the extrusion temperature can be selected within the range of 500-520℃, the extrusion speed within the range of 2.0-3.5 m / min, and the extrusion ratio within the range of 30-45. Excessive extrusion temperature or speed will lead to insufficient deformation energy storage and weakened driving force for secondary dispersed phase precipitation; excessively low temperature will increase deformation resistance and make extrusion difficult; insufficient extrusion ratio will result in insufficient shearing and fragmentation effect and low secondary dispersed phase number density; excessively high ratio may lead to excessive fragmentation of the primary dispersed phase, resulting in excessively small particle size and loss of pinning effect. The parameter combination selected in this embodiment can achieve moderate fragmentation of the primary dispersed phase and sufficient precipitation of the secondary dispersed phase.

[0161] The extrusion outlet is equipped with a zoned temperature-controlled cooling device, employing an enhanced cooling rate to simultaneously lock in the dimensional state of both the primary and secondary dispersed phases. For the thin-walled area with a wall thickness of 2.0 mm, high-pressure water mist cooling is used with a cooling rate of 185℃ / s, cooling from 518℃ to 98℃ in 2.8 seconds; For the thick-walled area with a wall thickness of 3.0 mm, high-pressure water cooling is used with a cooling rate of 120℃ / s, cooling from 515℃ to 92℃ in 4.2 seconds; For the thick-walled area with a wall thickness of 4.0 mm, high-pressure water cooling is used with a cooling rate of 95℃ / s, cooling from 512℃ to 88℃ in 5.3 seconds.

[0162] It should be noted that the cooling rate in the thin-walled region can be selected within the range of 150-250℃ / s, and the cooling rate in the thick-walled region can be selected within the range of 80-150℃ / s. The cooling rate used in this embodiment is higher than that in Example 1. The purpose is that the secondary dispersed phase has a smaller size, higher surface energy, and a stronger tendency to coarsen, requiring a faster cooling rate to effectively lock in its dimensional state.

[0163] The elongation was 1.2%. Transmission electron microscopy revealed a distinct bipolar distribution of the dispersed phase in the extruded profile. The primary dispersed phase is formed during the homogenization stage and undergoes partial shearing and fragmentation during extrusion. Its average particle size is 18 nm, slightly larger than the 15 nm after homogenization, and its number density is 3.8 × 10⁻⁶. 21 / m³. The primary dispersed phase is mainly distributed near grain boundaries and subgrain boundaries, playing a role in pinning grain boundaries and inhibiting recrystallization during subsequent welding thermal cycles.

[0164] The secondary dispersed phase is formed by the secondary precipitation of Zr and Sc atoms in solid solution during the extrusion process, with fragments of the broken primary dispersed phase as the core. Its average particle size is 5 nm and its number density is 1.6 × 10⁻⁶. 22 / m³. The secondary dispersed phase is mainly distributed within the grains, with a number density ratio of approximately 1:4.2 to that of the primary dispersed phase. The size of the secondary dispersed phase is in the range of 3-8nm. Although particles in this size range contribute little to grain boundary pinning, they have extremely high surface energy and can serve as preferential nucleation sites for GP regions during subsequent aging processes.

[0165] Energy dispersive spectroscopy analysis showed that the primary dispersed phase had a core-shell structure, with the core rich in Sc and the shell rich in Zr; the secondary dispersed phase had a relatively uniform composition, with the Zr content slightly higher than the Sc content, which was due to the higher concentration of Zr atoms in solid solution compared to Sc atoms.

[0166] IV. Two-level timeliness processing The first stage of aging is low-temperature long-term pre-aging: the temperature is increased to 88℃ at a rate of 1.0℃ / min and held for 18 hours.

[0167] The second stage of aging is peak aging: heat up to 160℃ and keep warm for 10 hours.

[0168] After aging, secondary dispersed phase particles played a significant role as preferential nucleation sites in the GP region. Three-dimensional atom probe microanalysis showed that the number density in the GP region reached 5.8 × 10⁻⁶. 23 / m³, compared to 4.2×10 in Example 1 23 / m³ increased by 38%. High-resolution transmission electron microscopy revealed that a large number of GP regions were distributed in a "satellite-like" pattern with secondary dispersed phases as the core, forming a denser coherent composite structure. GP regions were also enriched around the primary dispersed phase, but since the primary dispersed phase was mainly distributed near the grain boundaries, the number of GP regions induced by it was relatively limited.

[0169] V. Performance Test Results Tensile strength 395MPa, yield strength 356MPa, elongation 13.8%.

[0170] The helium leak detection rate for weld seams is 5.5 × 10⁻⁶. -10 Pa·m³ / s, weld strength coefficient is 0.89.

[0171] The average grain size of the heat-affected zone is 16 μm, and the recrystallization area fraction of the heat-affected zone is 10%.

[0172] Microhardness test results of the welded joint: 116 HV at 0 mm from the weld center, 109 HV at 3 mm from the weld center, 122 HV at 6 mm from the weld center, 129 HV at 9 mm from the weld center, 133 HV at 12 mm from the weld center, and 136 HV at 15 mm from the weld center. The base metal hardness was 138 HV. The average hardness of the heat-affected zone was 121 HV, reaching 90% of the base metal hardness.

[0173] The salt spray corrosion level is 10 after 1000 hours.

[0174] The statistical comparison of the characteristics of diffuse phases is as follows: Primary dispersed phase: average particle size 18 nm, number density 3.8 × 10⁻⁶ 21 / m³, mainly distributed at grain boundaries and subgrain boundaries; Secondary dispersed phase: average particle size 5 nm, number density 1.6 × 10⁻⁶ 22 / m³, mainly distributed within the crystal, and its number density ratio with that of the primary dispersed phase is approximately 1:4.2.

[0175] Compared to Example 1, this example, through precise control of extrusion parameters to induce the precipitation of secondary dispersed phases, increases the GP zone number density by approximately 38%, improves the heat-affected zone hardness retention rate from 87% to 90%, increases the weld strength coefficient from 0.86 to 0.89, and reduces the recrystallization area fraction of the heat-affected zone from 12% to 10%. This demonstrates that the introduction of secondary dispersed phases further optimizes the composite reinforced structure and improves the overall performance of the profile.

[0176] The core mechanism of this embodiment can be summarized as "deformation-induced secondary precipitation": During extrusion, intense plastic deformation generates numerous shear bands and dislocation structures. Some of the primary dispersed phase breaks down under shear force, forming high surface energy particle fragments. These fragments act as "adsorption traps" at the high extrusion temperature, capturing supersaturated Zr and Sc atoms in the matrix. Driven by both deformation energy storage and thermal activation, secondary precipitation occurs around these fragments, forming a finer secondary dispersed phase. Ultra-rapid cooling simultaneously locks in both the primary and secondary dispersed phases, preventing them from coarsening.

[0177] During subsequent aging, the secondary dispersed phase, due to its extremely high surface energy and coherent relationship with the matrix, becomes the preferred nucleation site for GP zones, significantly increasing the nucleation density of GP zones. Numerous GP zones form a "satellite-like" distribution around the secondary dispersed phase, collectively constituting a three-dimensional network-like composite strengthening structure with the GP zones surrounding the primary dispersed phase. This structure plays a synergistic role in the welding thermal cycle: the primary dispersed phase pins grain boundaries and inhibits recrystallization, while the high-density GP zones induced by the secondary dispersed phase promote rapid recrystallization of the strengthening phase, thereby minimizing weld softening and achieving excellent airtightness and joint strength.

[0178] It should be noted that the precipitation of secondary dispersed phases requires appropriate extrusion process conditions. If the extrusion temperature is below 500℃ or the extrusion speed is above 3.5 m / min, the deformation energy storage is insufficient, the driving force for secondary precipitation weakens, and the number density of secondary dispersed phases decreases significantly. If the extrusion ratio is below 30, the shearing and fragmentation effect is insufficient, resulting in a lack of nucleation sites for secondary dispersed phases. If the cooling rate at the extrusion outlet is lower than the recommended range in this embodiment, the secondary dispersed phases will coarsen during the cooling process, losing their advantage as preferential nucleation sites in the GP region. Therefore, the extrusion temperature of 500-520℃, extrusion speed of 2.0-3.5 m / min, extrusion ratio of 30-45, cooling rate of 150-250℃ / s in the thin-walled region, and cooling rate of 80-150℃ / s in the thick-walled region in this embodiment are the key process windows for achieving secondary precipitation of dispersed phases and fully utilizing their technical effects.

[0179] Although the invention has been specifically shown and described in conjunction with preferred embodiments, those skilled in the art should understand that various changes in form and detail may be made to the invention without departing from the spirit and scope of the invention as defined in the appended claims, all of which shall be within the scope of protection of the invention.

Claims

1. A method for preparing an aluminum alloy profile for a high-hermetic battery structure system of a new energy vehicle, characterized in that, The method comprises the following steps: Step (1), batching and casting: preparing an aluminum alloy raw material containing Mg, Si, Mn, Cr, Zr and Sc, casting an ingot after melting and refining; Step (2), low-temperature long-time homogenization treatment: heating the ingot to 480-510℃ and keeping for 10-16h, so that Zr and Sc are precipitated in the form of Al3Sc·Zr nano-dispersed phase, and the average particle size of the dispersed phase is controlled in the range of 8-25nm; Step (3), gradient cooling extrusion forming: heating the homogenized ingot to 490-510℃ for extrusion, setting a partition temperature control cooling device at the extrusion outlet, and applying a differentiated cooling system to the thin-walled area and the thick-walled area according to the profile wall thickness, wherein the cooling rate of the thin-walled area is ≥100℃ / s, and the cooling rate of the thick-walled area is ≥50℃ / s, so that the high-temperature supersaturated solid solution state at the extrusion outlet is quickly frozen, and the coarsening of Al3Sc·Zr particles during the cooling process is inhibited; Step (4), double-stage aging treatment: performing double-stage aging on the extruded profile, keeping at 80-95℃ for 12-24h in the first stage, and keeping at 155-165℃ for 8-12h in the second stage, so that the aging precipitated phase and the Al3Sc·Zr dispersed phase form a composite strengthening structure; Thus the aluminum alloy profile is obtained.

2. The preparation method of the aluminum alloy profile for the high-airtightness battery structure system of a new energy vehicle according to claim 1, characterized in that, The aluminum alloy raw material in step (1) comprises, by mass percentage, Mg 0.65-0.85%, Si 0.70-0.90%, Mn 0.15-0.25%, Cr 0.05-0.10%, Zr 0.06-0.12%, Sc 0.008-0.025%, Fe≤0.15%, Cu≤0.10%, Zn≤0.05%, Ti 0.01-0.03%, and the balance of Al.

3. The method for preparing an aluminum alloy profile for a high-airtightness battery structure system for new energy vehicles according to claim 2, characterized in that, The mass ratio of Zr to Sc is 4:1 to 6:1 to form Al3Sc·Zr composite particles with core-shell structure, wherein Sc is enriched in the particle core and Zr is enriched in the particle shell.

4. The preparation method of the aluminum alloy profile for the high-airtightness battery structure system of a new energy vehicle according to claim 1, characterized in that, The temperature rising rate of the low-temperature long-time homogenization treatment in step (2) is 20-30℃ / h, and after the holding is completed, the temperature is quickly cooled to room temperature at a rate of ≥200℃ / h.

5. The method for preparing an aluminum alloy profile for a high-airtightness battery structure system for new energy vehicles according to claim 1, characterized in that, The partition temperature control cooling device in step (3) is divided into a thin-walled area and a thick-walled area according to the profile wall thickness, the thickness of the thin-walled area is greater than 1.8mm and less than 2.6mm, and the thickness of the thick-walled area is greater than or equal to 2.6mm and less than 5.0mm, wherein: The thin-walled area is cooled by water mist, and the cooling rate is 120-180℃ / s, so that the profile is cooled from 520-540℃ to below 150℃ within 3-5 seconds; The thick-walled area is cooled by high-pressure water, and the cooling rate is 60-100℃ / s, so that the profile is cooled from 520-540℃ to below 150℃ within 5-8 seconds.

6. The method for preparing an aluminum alloy profile for a high-airtightness battery structure system for new energy vehicles according to claim 1, characterized in that, In step (4), the temperature rising rate of the first-stage aging is 0.5-1.5℃ / min, a high-density GP zone is formed during the holding at 80-95℃, and a coherent interface is formed between the GP zone and the Al3Sc·Zr particles.

7. The method for preparing an aluminum alloy profile for a high-airtightness battery structure system for new energy vehicles according to claim 1, characterized in that, The composite reinforcement structure described in step (4) is as follows: the average particle size of the Al3Sc·Zr nano-dispersed phase is 8-25 nm, the average length of the aged precipitated phase β″ is 10-20 nm, and the Al3Sc·Zr nano-dispersed phase and the β″ phase have a bimodal size distribution in the aluminum matrix.

8. The method of claim 7, wherein the aluminum alloy profile for a new energy vehicle high-airtight battery structure system is prepared by the steps of: preparing an aluminum alloy profile; and performing a surface treatment on the aluminum alloy profile. The Al3Sc·Zr nano-dispersed phase is mainly distributed at grain boundaries and subgrain boundaries, while the β″ phase is mainly distributed within the grains.

9. The method for preparing an aluminum alloy profile for a high-airtightness battery structure system for new energy vehicles according to claim 1, characterized in that, In step (3), the extrusion temperature is controlled at 500-520℃, the extrusion speed is controlled at 2.0-3.5m / min, and the extrusion ratio is controlled at 30-45. This allows the primary Al3Sc·Zr dispersed phase to undergo controlled shearing and breakage during the extrusion process, and induces the solid solution Zr and Sc atoms to precipitate secondaryly with the broken particles as the core, forming a secondary dispersed phase with an average particle size of 3-8nm. 10.The aluminum alloy profile prepared by the method of any one of claims 1-9, characterized in that, After friction stir welding, the recrystallized grain area fraction of the weld heat-affected zone is ≤15%, the weld helium leak rate is ≤1×10 -9 Pa·m³ / s, and the hardness of the weld heat-affected zone is not less than 85% of the hardness of the base material.

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