Method for inhibiting growth of abnormal grains of 7xxx series aluminum alloy friction stir welding joint

Abstract

The invention provides a method for inhibiting abnormal grain growth of a 7xxx series aluminum alloy friction stir welding joint, which comprises the following steps: step 1), performing wire filling welding treatment on a to-be-welded aluminum alloy part to obtain a welding part; wherein the welding flux is an aluminum alloy containing a scandium element; (2) the welding part is subjected to friction stir welding treatment, and a friction stir welding head is obtained; and (3) post-treatment is conducted on the friction stir welding head. Welding materials used are characterized in that a certain amount of scandium is added, then differential friction stir welding treatment is carried out, the differential friction stir welding treatment is carried out by independently regulating and controlling the rotating speed of a shaft shoulder and the rotating speed of a stirring needle, and on the premise that the rotating speed of the stirring needle is greatly increased to improve material flowing, the welding efficiency is greatly improved. The low shaft shoulder rotating speed is applied to eliminate overheating, defects are avoided, pinning particles are dispersed, and the effect of completely inhibiting growth of abnormal crystal grains is achieved.

Description

Technical Field

[0001] This invention belongs to the field of friction welding technology, specifically relating to a method for suppressing abnormal grain growth in 7xxx series aluminum alloy friction stir welded joints. Background Technology

[0002] 7xxx series ultra-high strength Al-Zn-Mg-Cu aluminum alloys are important lightweight materials in the aerospace field. However, the high alloying element content significantly increases their porosity and hot cracking susceptibility, making them difficult to weld using fusion welding methods. In contrast, friction stir welding (FSW), as a solid-state joining technique, eliminates metallurgical defects associated with fusion welding, such as porosity and hot cracking. However, post-weld heat treatment is not suitable for high-strength aluminum alloys welded by FSW. This is because under the strong thermomechanical coupling of FSW, the grain structure in the weld nugget zone is significantly refined. These fine grains undergo explosive growth during solution heat treatment, even reaching millimeter scale, a phenomenon known as abnormal grain growth (AGG). The occurrence of AGG significantly reduces the tensile properties and fatigue strength of the joint, seriously jeopardizing the service safety of the material; therefore, it is necessary to suppress the occurrence of AGG.

[0003] Existing technologies incorporate a prolonged recrystallization annealing process after friction stir welding and before solution treatment to reduce the driving force for aberrant grain growth (AGG) during subsequent solution treatment. However, even though annealing reduces some of the AGG driving force, the grain boundary mobility at the weld top remains high due to temperature factors, making it prone to large-area abnormal grain growth. Furthermore, prolonged annealing significantly increases energy consumption and production costs.

[0004] In view of this, it is necessary to provide a method that can effectively suppress abnormal grain growth in friction stir welded joints of 7xxx series aluminum alloys. Summary of the Invention

[0005] Therefore, the present invention provides a method for suppressing abnormal grain growth in 7xxx series aluminum alloy friction stir welded joints, in order to solve the problem that long-term annealing treatment in the prior art cannot effectively suppress abnormal grain growth.

[0006] To address the above problems, this invention provides a method for suppressing abnormal grain growth in friction stir welded joints of 7xxx series aluminum alloys, comprising the following steps: Step 1): Perform filler wire welding on the aluminum alloy parts to be welded to obtain a welded part; wherein the solder is an aluminum alloy containing scandium. Step 2): Perform friction stir welding on the welded parts to obtain a friction stir welded head; Step 3): Post-process the friction stir welded head.

[0007] Furthermore, the filler wire welding process is performed using arc welding, resulting in no undercut defects after welding.

[0008] Furthermore, when the thickness of the aluminum alloy part to be welded is ≥4mm, a beveling treatment should be performed; Preferably, the bevel is a single V-shaped bevel with an angle of 60-80° and a gap of 0.5-1.2mm.

[0009] Furthermore, the scandium content in the solder is 0.2-0.3 wt.%.

[0010] Furthermore, the friction stir welding tool is made of H13 steel, with a shoulder diameter of 10-40mm; the root diameter of the stirring pin is 5-20mm; and the ratio of the root diameter of the stirring pin to the weld width of the weld joint is 0.7-0.8.

[0011] Furthermore, the differential friction stir welding process parameters are as follows: shoulder rotation speed is 100~400 rpm, stirring pin rotation speed is 600~2000 rpm, and welding speed is 100~300 mm / min.

[0012] Furthermore, the post-processing includes solution treatment and aging treatment.

[0013] Furthermore, the solution treatment process is as follows: staged solution treatment, with the first stage solution temperature at 420-460℃ and a holding time of 0.5-1h; the second stage solution treatment temperature at 470-480℃ and a holding time of 0.5-1h, and the quenching medium being room temperature water. Furthermore, the aging process is as follows: graded aging, with the first aging temperature at 80-100℃ and the holding time at 6-12h; the second aging temperature at 120-140℃ and the holding time at 6-24h, followed by air cooling.

[0014] Compared with the prior art, the present invention has at least the following beneficial effects: 1. Easy to operate. Arc welding is a welding process with high production efficiency and low cost. It can be used with automatic welding machines or manual wire filling, and the operation is simple and convenient.

[0015] 2. The Al3Sc phase precipitated in situ based on metallurgical reaction not only effectively suppresses the AGG phenomenon during post-weld heat treatment and can significantly refine the grain structure and enhance fine grain strengthening, but also exists in the form of L12 type secondary Al3Sc precipitate phase, providing precipitation strengthening effect, which is beneficial to improving the strength of the joint.

[0016] 3. Differential friction stir welding employs the optimal match of low shoulder speed and high stirring pin speed to achieve excellent weld formation and improve material flow, promoting the uniform distribution of grain boundary pinning particles. Attached Figure Description

[0017] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0018] Figure 1 Ceramic particle segregation leads to abnormal grain growth; Figure 2 Example 1: Microstructure of a novel friction stir welded head after post-weld heat treatment; Figure 3 Example 1: Grain structure of a novel friction stir welded head before and after heat treatment; Figure 4 Example 1: Tensile stress-strain curve of post-weld heat treatment of a novel friction stir welded joint; Figure 5 Comparative Example 1: Abnormal grain growth after post-weld heat treatment of conventional friction stir welded joint; Figure 6 Comparative Example 1: Tensile stress-strain curve of a conventional friction stir welded joint after post-weld heat treatment; Figure 7 Comparative Example 2: Microstructure of the novel friction stir welded head after post-weld heat treatment; Figure 8 Comparative Example 3: Residual weld structure after post-weld heat treatment of the novel friction stir welded head; Figure 9 Comparative Example 4: Grain boundary overheating phenomenon after post-weld heat treatment of novel friction stir welded joint. Detailed Implementation

[0019] To further illustrate the technical means and effects adopted by the present invention to achieve the intended purpose, the specific embodiments, structures, features, and effects according to the present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments. In the following description, different "an embodiment" or "an embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0020] The essence of AGG (Aggressive Gravel Joint) in aluminum alloys is the dissolution of second-phase particles at grain boundaries, leading to the loss of effective grain boundary protection. Even in ultra-high-strength aluminum alloy friction stir welded joints with added zirconium, AGG can easily occur in the weld nugget zone. Introducing second-phase particles by adding an intermediate layer or by grooving the base material and adding flux (such as ceramic particles) can be achieved; however, the high modulus and hardness of these particles make welding defects highly likely, even leading to tool wear and needle breakage. Furthermore, microstructure segregation and particle agglomeration occur frequently, making microstructure control of the joint even more difficult in actual production.

[0021] Domestic and international research indicates that adjusting welding parameters or prolonged annealing can appropriately suppress the aberrant grain growth (AGG) phenomenon. However, adjusting welding parameters such as rotation speed, welding speed, or using multi-pass welding has very limited effect on suppressing AAGG after solution treatment. Under complex thermomechanical coupling, the microstructure of the weld nugget is extremely unstable, thus these methods lack universality and only have a certain effect on specific plate thicknesses and materials. Prolonged annealing involves setting a medium-high temperature annealing treatment before solution treatment to eliminate residual stored energy in the weld nugget. Existing technologies disclose a method for suppressing abnormal grain growth in aluminum alloy friction stir welds, which uses annealing at 300-500℃ for 48-72 hours to reduce the driving force for AAGG during subsequent solution treatment. However, on the one hand, prolonged annealing significantly increases energy consumption; on the other hand, even after annealing, severe AAGG still occurs at the top of the weld, presenting drawbacks.

[0022] Some studies have attempted to reduce particle agglomeration by increasing tool rotation speed. However, the high frictional heat generated by high rotation speeds leads to widespread overheating in 7xxx series aluminum alloy FSW joints. In some cases, the grain boundary-enriched second phase undergoes remelting under the influence of frictional heat, resulting in severe porosity defects. Conversely, at lower rotation speeds and with insufficient material flow, the uneven distribution of second-phase particles makes them prone to agglomeration, failing to effectively suppress AGG (agglomerate granulation). (See attached image) Figure 1 As shown, due to the agglomeration of the added ceramic particles, severe AGG appeared in the onion ring feature area of ​​the weld nugget.

[0023] This invention provides a universally applicable method for eliminating AGG (Aggressive Gravel) during post-weld heat treatment of 7xxx series aluminum alloy FSW joints. The method employs arc welding with filler wire, using welding materials characterized by the addition of a certain amount of scandium. Subsequently, differential friction stir welding is performed. This differential friction stir welding method independently controls the rotation speed of the shoulder and stirring pin. While significantly increasing the stirring pin speed to improve material flow, a low shoulder speed is used to eliminate overheating, preventing defects and dispersing the distribution of pinning particles, thus completely suppressing AGG. The specific solution is as follows: This invention provides a method for suppressing abnormal grain growth in friction stir welded joints of 7xxx series aluminum alloys, comprising the following steps: Step 1): Perform filler wire welding on the aluminum alloy parts to be welded to obtain a welded part; wherein the solder is an aluminum alloy containing scandium. Step 2): Perform friction stir welding on the welded parts to obtain a friction stir welded head; Step 3): Post-process the friction stir welded head.

[0024] Furthermore, the filler wire welding process employs arc welding, resulting in no undercut defects after welding. Firstly, arc welding yields primary Al3Sc phases of 500-2 μm at the grain boundaries and secondary Al3Sc phases of 50-200 nm within the grains. Changing the friction stir welding parameters promotes the uniform redistribution of these second phases, effectively pinning the grain boundaries and thus suppressing AGG (aggregate gravimetric defects).

[0025] Furthermore, when the thickness of the aluminum alloy part to be welded is ≥4mm, a beveling treatment should be performed; Preferably, the bevel is a single V-shaped bevel with an angle of 60-80° and a gap of 0.5-1.2mm.

[0026] Furthermore, the scandium content in the solder is 0.2-0.3 wt.%. Within this range, a suitable amount of Al3Sc heat-resistant phase can be formed in subsequent processes, which plays a role in pinning grain boundaries under high-temperature exposure. If the added content is too low, less Al3Sc phase will precipitate, resulting in limited suppression effect; if the added content is too high, on the one hand, it will significantly increase the production cost, and on the other hand, it will easily cause segregation, leading to uneven distribution and subsequent local abnormal grain growth.

[0027] Furthermore, the friction stir welding tool is made of H13 steel, with a shoulder diameter of 10-40mm; the root diameter of the stirring pin is 5-20mm; and the ratio of the root diameter of the stirring pin to the weld width of the weld joint is 0.7-0.8.

[0028] Furthermore, the differential friction stir welding process parameters are as follows: shoulder rotation speed is 100~400 rpm, stirring pin rotation speed is 600~2000 rpm, and welding speed is 100~300 mm / min.

[0029] Furthermore, the post-processing includes solution treatment and aging treatment.

[0030] Furthermore, the solution treatment process is as follows: staged solution treatment, with the first stage solution temperature at 420-460℃ and the holding time at 0.5-1h; the second stage solution temperature at 470-480℃ and the holding time at 0.5-1h, and the quenching medium being room temperature water.

[0031] Furthermore, the aging process is as follows: graded aging, with the first aging temperature at 80-100℃ and the holding time at 6-12h; the second aging temperature at 120-140℃ and the holding time at 6-24h, followed by air cooling.

[0032] Existing methods are ineffective in suppressing the Aggregate Gravity (AGG) phenomenon. For example, long-term annealing not only fails to completely suppress AGG but also significantly increases energy consumption and production costs. Controlling welding processes has poor universality, requiring specific plate thicknesses and materials to achieve any noticeable effect. Adding flux cannot guarantee the uniformity of the microstructure, a common problem in friction stir welding of ceramic particle-reinforced aluminum matrix composites. This is especially true when the added second-phase particles segregate under the influence of material flow, leading to AGG regions due to uneven particle distribution. Furthermore, external flux hinders material flow, easily inducing welding defects and affecting joint performance.

[0033] In the method of this application, scandium is added to the joint by forming a molten pool through the melting of welding wire. The metallurgical reaction in the molten pool causes the heat-resistant Al3Sc phase to precipitate uniformly in the form of primary and secondary phases at grain boundaries or within grains. The Al3Sc phase has strong thermal stability, especially the primary Al3Sc phase. During the high-temperature solution treatment of aluminum alloys, they are difficult to dissolve in the matrix, thus playing an excellent pinning role on grain boundaries, effectively inhibiting grain boundary migration, and thus preventing the occurrence of AGG (aggregate galvanization). Although arc welding may introduce porosity or lack of fusion defects, these melting defects are eliminated under the influence of the intense thermoplastic deformation of the friction stir welding tool. Based on the uniform precipitation of the Al3Sc phase from the in-situ metallurgical reaction, under the influence of the high stirring pin speed of differential friction stir welding, the Al3Sc phase is more uniformly distributed and smaller in size, thus effectively suppressing AGG during post-weld solution treatment.

[0034] It should be noted that, based on the molten pool metallurgical reaction, using welding wire as the solder, heat-resistant second-phase particles (Al3Sc) are uniformly added to the joint. The welding wire is selected with a composition close to that of the base material, and the Sc element content needs to be controlled. Too low a content will not produce good results, while too high a content will cause Sc element segregation. Even after friction stir welding, it is not possible to effectively suppress its segregation, resulting in local AGG (aggregate galvanization) and excessive cost.

[0035] Depending on the plate thickness, different sizes of friction stir welding tools and differential welding parameters are matched.

[0036] The mechanical properties of the 7xxx series aluminum alloy friction stir welded joints prepared by this method depend on the difference in alloy composition between the welding wire and the base material. The closer the two are, the closer the strength coefficient of the joint after post-weld heat treatment will be to 100%. Based on the plate thickness, appropriate shoulder dimensions and stirring pin root diameters need to be matched to eliminate melting defects. For the shoulder, if the shoulder size is too small, the heat generated by friction stir welding will be insufficient, resulting in welding defects. Conversely, if the shoulder size is too large, the material driven by the shoulder will contain too much base material, forming a larger weld nugget zone. This will dilute the proportion of the Al3Sc phase in the weld nugget zone, weakening the pinning effect on the grains in the weld nugget zone and resulting in insufficient suppression of AGG. The root diameter of the stirring pin has a more significant impact on the microstructure of the weld nugget zone. A larger root diameter carries more material, resulting in more complete flow, which is beneficial for particle breakage and redistribution. However, an excessively large root diameter will increase the width of the weld nugget zone and also dilute the proportion of the Al3Sc phase in the weld nugget zone. An excessively small root diameter will result in insufficient material flow and welding defects. On the other hand, it will reduce the width of the weld nugget zone, even to the width of the fusion weld zone, causing the fusion weld microstructure to remain in the friction stir weld joint, reducing mechanical properties.

[0037] As the zinc content of the materials to be welded increases, welding defects are prone to occur in friction stir welding joints at low speeds; while at high speeds, overheating can occur at the joint, inducing porosity defects caused by the melting of low-melting-point precipitates. Therefore, welding parameters need to be limited. Thus, a differential friction stir welding process is adopted, widening the welding process window by separately adjusting the speeds of the shoulder and stirring pin. This differential friction stir welding process, which is matched to fusion welding, also needs protection.

[0038] The present invention will be further described below with reference to specific embodiments and comparative examples.

[0039] Example 1 A 4mm thick 7034 ultra-high strength aluminum alloy plate with a zinc content of 11.2 wt.% (tensile strength 710 MPa) was used as the workpiece to be welded. A single V-groove was cut into the workpiece with a bevel angle of 80° and a gap of 1.2 mm. The welding wire was 7055 aluminum alloy wire with 0.26 wt.% scandium (zinc content 8.8 wt.%). The workpiece was rigidly fixed, and after cleaning water stains and oil from the surface, cold metal transfer welding was used. The wire feed rate was 8.0 m / min, and the welding speed was 6 m / min. After fusion welding, the excess weld height on both the front and back sides was cleaned. The measured weld width on the front side was 10 mm. The friction stir welding tool is made of H13 steel with a shoulder diameter of 15 mm. The stirring pin is a tapered threaded pin with a root diameter of 7 mm. The ratio of the front weld width to the weld joint is 0.7. The pin length is 3.7 mm, the pressure is 0.2 mm, and the welding angle is 3°. Differential friction stir welding is performed under the welding parameters of a shoulder rotation speed of 400 rpm, a stirring pin rotation speed of 1000 rpm, and a welding speed of 150 mm / min.

[0040] The produced welded plates underwent solution treatment at 450℃ / 1h + 475℃ / 1h, followed by water quenching; then, they were subjected to artificial aging treatment in an aging furnace at 80℃ / 8h + 120℃ / 8h, and air-cooled. Microstructural observation showed that after T6 heat treatment, the novel friction stir welded joint exhibited no abnormal grain growth and no welding defects. (See attached image) Figure 2 Electron backscatter diffraction patterns show that the average grain size in the weld nugget region was 2.6 μm before heat treatment, and the grains only grew to 3.2 μm after heat treatment. This indicates that the process effectively suppressed grain boundary migration during the high-temperature solid solution stage and refined the microstructure. (See attached image) Figure 3 Tensile testing at room temperature showed that the joint fractured in the weld nugget zone, with a tensile strength of 656 MPa and an elongation at break of 6.7% (see attached). Figure 4 ).

[0041] Comparative Example 1 A 4mm thick 7034 ultra-high strength aluminum alloy plate with a zinc content of 11.2wt.% was used as the workpiece to be welded (tensile strength 710MPa). After cleaning the surface water stains and oil stains, the workpiece to be welded was rigidly fixed. The welding tool was made of H13 steel with a shoulder diameter of 15mm. The stirring needle was a tapered threaded needle with a root diameter of 7mm and a needle length of 3.7mm. The downward pressure was 0.2mm, and the welding tilt angle was 3°. Differential speed friction stir welding was performed under the welding parameters of a shoulder rotation speed of 400 rpm, a stirring needle rotation speed of 1000 rpm, and a welding speed of 150 mm / min.

[0042] The produced welded plates were subjected to solution treatment at 450℃ / 1h + 475℃ / 1h, followed by water quenching; then artificially aged in an aging furnace at 80℃ / 8h + 120℃ / 8h, and air-cooled. Microstructural observation revealed severe abnormal grain growth in the weld nugget area (see attached image). Figure 5 Tensile testing at room temperature showed a tensile strength of only 557 MPa and an elongation at break of only 3% (see attached diagram). Figure 6 Abnormal grain growth reduces the strength and elongation of the joint. Specifically, the AGG phenomenon occurs due to the extensive dissolution of the second phase at grain boundaries during the solid solution stage.

[0043] Comparative Example 2 A 4mm thick 7034 ultra-high strength aluminum alloy plate with a zinc content of 11.2 wt.% (tensile strength 710 MPa) was used as the workpiece to be welded. A single V-groove was cut into the workpiece with a bevel angle of 80° and a gap of 1.2 mm. The welding wire was 7055 aluminum alloy wire with 0.15 wt.% scandium added (zinc content 8.8 wt.%). The workpiece was rigidly fixed, and after cleaning surface water stains and oil, cold metal transfer welding was used with a wire feed rate of 8.0 m / min and a welding speed of 6 m / min. After fusion welding, the excess weld height on both the front and back sides was cleaned. The measured weld width on the front side was 10 mm. The friction stir welding tool is made of H13 steel with a shoulder diameter of 15 mm. The stirring pin is a tapered threaded pin with a root diameter of 7 mm. The ratio of the front weld width to the weld joint is 0.7. The pin length is 3.7 mm, the pressure is 0.2 mm, and the welding angle is 3°. Differential friction stir welding is performed under the welding parameters of a shoulder rotation speed of 400 rpm, a stirring pin rotation speed of 1000 rpm, and a welding speed of 150 mm / min.

[0044] The produced welded plates underwent solution treatment at 450℃ / 1h + 475℃ / 1h, followed by water quenching; then, they were subjected to artificial aging treatment in an aging furnace at 80℃ / 8h + 120℃ / 8h, and air-cooled. Microstructural observation showed (see attached...). Figure 7 The presence of agglomerates (AGG) in the weld nugget area is due to the low Sc content. Specifically, the low Sc content in the welding wire results in limited Al3Sc phase precipitation, insufficient to completely suppress AGG by pinning grain boundaries.

[0045] Comparative Example 3 A 4mm thick 7034 ultra-high strength aluminum alloy plate with a zinc content of 11.2 wt.% (tensile strength 710 MPa) was used as the workpiece to be welded. A single V-groove was cut into the workpiece with a bevel angle of 80° and a gap of 1.2 mm. The welding wire was 7055 aluminum alloy wire with 0.26 wt.% scandium (zinc content 8.8 wt.%). The workpiece was rigidly fixed, and after cleaning surface water and oil stains, cold metal transfer welding was used with a wire feed rate of 8.0 m / min and a welding speed of 6 m / min. After fusion welding, the excess weld material on the back was cleaned. The friction stir welding tool is made of H13 steel with a shoulder diameter of 15 mm. The stirring pin is a tapered threaded pin with a root diameter of 5.5 mm. The ratio of the front weld width to the weld joint is 0.55. The pin length is 3.7 mm, the pressure is 0.2 mm, and the welding angle is 3°. Differential friction stir welding is performed under the welding parameters of a shoulder rotation speed of 400 rpm, a stirring pin rotation speed of 1000 rpm, and a welding speed of 150 mm / min.

[0046] The produced welded plates were solution treated at 450℃ / 1h + 475℃ / 1h, followed by water quenching; then artificially aged in an aging furnace at 80℃ / 8h + 120℃ / 8h, and air-cooled. Microstructural observation showed no abnormal grain growth in the weld nugget area; however, due to the small root diameter of the stirring tool, a large area of ​​residual fusion weld microstructure was observed (see attached image). Figure 8 Room temperature tensile tests showed that the joint fractured at the boundary between the weld nugget zone and the residual fusion weld structure, with a tensile strength of 602 MPa and an elongation at break of only 2.6%. The results indicate that the residual fusion weld structure has little effect on the room temperature tensile strength of the novel friction stir welded joint, but it significantly impacts the elongation at break. Specifically, the small diameter of the FSW tool root prevented the machining zone from completely covering the weld zone of the fusion welded joint, resulting in residual fusion weld structure and reduced mechanical properties. This is because the residual fusion weld structure has a coarser grain size compared to the weld nugget zone of the FSW.

[0047] Comparative Example 4 A 4mm thick 7034 ultra-high strength aluminum alloy plate with a zinc content of 11.2 wt.% (tensile strength 710 MPa) was used as the workpiece to be welded. A single V-groove was cut into the workpiece with a bevel angle of 80° and a gap of 1.2 mm. The welding wire was 7055 aluminum alloy wire with 0.26 wt.% scandium (zinc content 8.8 wt.%). The workpiece was rigidly fixed, and after cleaning water stains and oil from the surface, cold metal transfer welding was used. The wire feed rate was 8.0 m / min, and the welding speed was 6 m / min. After fusion welding, the excess weld height on both the front and back sides was cleaned. The measured weld width on the front side was 10 mm. The friction stir welding tool is made of H13 steel with a shoulder diameter of 15 mm. The stirring pin is a tapered threaded pin with a root diameter of 7 mm. The ratio of the front weld width to the weld joint is 0.7. The pin length is 3.7 mm, the pressure is 0.2 mm, and the welding angle is 3°. Traditional friction stir welding is performed under the welding parameters of a shoulder rotation speed of 1000 rpm, a stirring pin rotation speed of 1000 rpm, and a welding speed of 150 mm / min.

[0048] The produced welded plates underwent solution treatment at 450℃ / 1h + 475℃ / 1h, followed by water quenching; then, they were subjected to artificial aging treatment in an aging furnace at 80℃ / 8h + 120℃ / 8h, and air-cooled. After post-weld heat treatment, no abnormal grain growth was observed in the new friction stir welded joint. However, grain boundary overburning was observed in the weld nugget area (see attached). Figure 9This is because the high rotational speed of the shoulder increases frictional heat generation, leading to "overburning" of the low-melting-point precipitates at the grain boundaries. This phenomenon is exacerbated after high-temperature solution treatment. Room temperature tensile tests showed that the joint fractured in the weld nugget region, with a tensile strength of 535 MPa and an elongation at break of only 1.7%. The results indicate that overburning reduces the elongation of the joint. Specifically, using the high rotational speed of a traditional FSW, the excessively fast shoulder rotation and high heat generation caused grain boundary overburning in the weld nugget region, resulting in deterioration of the joint's mechanical properties.

[0049] Example 2 A 6mm thick 7055 ultra-high strength aluminum alloy plate with a zinc content of 8.6wt.% (tensile strength 615MPa) was used as the workpiece to be welded. A single V-groove was cut on the workpiece with a 60° bevel angle and a 0.8mm gap. The welding wire was 7055 aluminum alloy wire with 0.28wt.% scandium added (zinc content 8.9wt.%). The workpiece was rigidly fixed, and after cleaning surface water and oil stains, manual arc welding was performed with a welding current of 100-130A and a welding voltage of 20-24V. After fusion welding, the excess weld material on both the front and back sides was cleaned. The friction stir welding tool is made of H13 steel with a shoulder diameter of 24 mm. The stirring pin is a tapered threaded pin with a root diameter of 8 mm. The ratio of the front weld width to the weld joint width is 0.72. The pin length is 5.7 mm, the pressure is 0.2 mm, and the welding angle is 3°. Differential friction stir welding is performed under the welding parameters of a shoulder rotation speed of 400 rpm, a stirring pin rotation speed of 1200 rpm, and a welding speed of 100 mm / min.

[0050] The produced welded plates were solution treated at 450℃ / 1h + 475℃ / 1h, followed by water quenching; then artificially aged in an aging furnace at 80℃ / 8h + 120℃ / 8h, and air-cooled. No abnormal grain growth was observed in the novel friction stir welded joint. Room temperature tensile testing showed that the joint fractured in the weld nugget zone, with a tensile strength of 621 MPa and an elongation at break of 7.0%.

[0051] Example 3 A 16mm thick 7075 ultra-high strength aluminum alloy plate with a zinc content of 5.8 wt.% (tensile strength 556 MPa) was used as the workpiece to be welded. The workpiece was beveled with a double V-shaped groove, a 60° bevel angle, a 2mm blunt edge, and a 0.8mm gap. The welding wire was 7075 aluminum alloy wire with 0.25 wt.% scandium added (zinc content 6.2 wt.%). The workpiece was rigidly fixed, and after cleaning surface water and oil, manual arc welding was performed at a current of 190-220A and a voltage of 24-26V. After fusion welding, the excess weld material on both the front and back sides was cleaned. The friction stir welding tool is made of H13 steel with a shoulder diameter of 24 mm. The stirring pin is a tapered threaded pin with a root diameter of 8 mm. The ratio of the stirring pin's diameter to the weld width of the weld head is 0.72. The pin length is 15.7 mm, the pressure is 0.2 mm, and the welding angle is 3°. Differential friction stir welding is performed under the welding parameters of a shoulder rotation speed of 100 rpm, a stirring pin rotation speed of 300 rpm, and a welding speed of 100 mm / min.

[0052] The produced welded plates were solution treated at 470℃ for 2 hours and then water-quenched; subsequently, they were artificially aged at 120℃ for 24 hours in an aging furnace and then air-cooled. No abnormal grain growth was observed in the novel friction stir welded joint. Room temperature tensile testing showed that the joint fractured at the base material, with a tensile strength of 558 MPa and an elongation at break of 10.7%.

[0053] It will be readily understood by those skilled in the art that, without conflict, the advantageous technical features of the above-mentioned methods can be freely combined and superimposed.

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention. The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the protection scope of the present invention.

Claims

1. A method of inhibiting abnormal grain growth in a 7xxx-series aluminum alloy friction stir welded joint, comprising: The method comprises the following steps: Step 1): performing a filler welding treatment on the aluminum alloy piece to be welded to obtain a welded piece; wherein the filler metal is an aluminum alloy containing scandium; Step 2): performing a friction stir welding treatment on the welded piece to obtain a friction stir welded joint; Step 3): performing a post-treatment on the friction stir welded joint.

2. The method of suppressing abnormal grain growth of a 7xxx-series aluminum alloy friction stir welded joint according to claim 1, wherein, The filler welding treatment adopts an arc welding mode.

3. The method of suppressing abnormal grain growth of a 7xxx-series aluminum alloy friction stir welded joint according to claim 1, wherein, When the plate thickness of the aluminum alloy piece to be welded is greater than or equal to 4 mm, the aluminum alloy piece to be welded is subjected to a groove opening treatment; Preferably, the groove is a single V-shaped groove, the angle is 60-80°, and the gap is 0.5-1.2 mm.

4. The method of suppressing abnormal grain growth of a 7xxx-series aluminum alloy friction stir welded joint according to claim 1, wherein The content of scandium in the filler metal is 0.2-0.3 wt.%.

5. The method of suppressing abnormal grain growth of a 7xxx-series aluminum alloy friction stir welded joint according to claim 1, wherein The material of the friction stir welding tool is H13 steel, the diameter of the shoulder is 10-40 mm, the diameter of the root of the stirring pin is 5-20 mm, and the ratio of the diameter of the root of the stirring pin to the size of the front fusion width of the fusion welded joint is 0.7-0.

8.

6. The method of inhibiting abnormal grain growth in a 7xxx-series aluminum alloy friction stir welded joint according to claim 5, wherein, The differential friction stir welding process parameters are as follows: the shoulder rotation speed is 100-400 rpm, the stirring pin rotation speed is 600-2000 rpm, and the welding speed is 100-300 mm / min.

7. The method of suppressing abnormal grain growth of a 7xxx-series aluminum alloy friction stir welded joint according to claim 1, wherein The post-treatment comprises a solid solution treatment and an aging treatment.

8. The method of inhibiting abnormal grain growth in a 7xxx-series aluminum alloy friction stir welded joint according to claim 7, wherein, The solid solution treatment system is as follows: graded solid solution, the first-stage solid solution temperature is 420-460℃, the holding time is 0.5-1 h, the second-stage solid solution temperature is 470-480℃, the holding time is 0.5-1 h, and the quenching medium is normal temperature water.

9. The method of suppressing abnormal grain growth of a 7xxx-series aluminum alloy friction stir welded joint according to claim 7 or 8, characterized in that, The aging treatment system is as follows: graded aging, the first-stage aging temperature is 80-100℃, the holding time is 6-12 h, the second-stage aging temperature is 120-140℃, the holding time is 6-24 h, and the aging is followed by air cooling.

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