Abstract: The aluminum alloy sheet was welded by ultra-high speed friction stir welding equipment, and the microstructure and mechanical properties of the welded joint were studied. The results show that under the process parameters of rotation speed of RMIN and welding speed of mmmin, welded joints with small welding deformation, narrow heat-affected zone and good surface formation of weld can be obtainedThe microhardness of the weld nugget area reached HV, and the tensile strength of the welded joint reached MPA, and the micromorphology of the welded joint showed that there were no defects such as tunnels, cracks, and holes inside. Keywords: 3003 aluminum alloy;Sheet;Ultra-high speed friction stir welding;Welded joints;Microstructure;Tensile StrengthCLC Number: TG Literature Symbol Code: A Article Number: Aluminum alloy is the most commonly used anti-rust aluminum alloy in aluminum-manganese alloys, its strength is low, it is an aluminum alloy that cannot be strengthened by heat treatment, and it is usually strengthened by cold work hardening to improve its mechanical properties. The aluminum alloy has low density, good corrosion resistance, excellent electrical and thermal conductivity, and has good reflectivity, non-magnetism, welding process and machinability, and is widely used in decoration, heat exchange, photosensitivity, packaging and other low-load occasions. The use of traditional MIG, non-MIG, TIG, CMT, and laser welding methods to weld aluminum alloy for medium and heavy plates often have problems such as thermal cracks, porosity, poor surface forming, and high requirements for workpiece cleanliness [ When these welding methods are still used for aluminum alloy sheets with a thickness of less than mm, in addition to the above problems, there will also be serious deformation, perforation and other problems [This is because at high temperatures The lower plate has poor deformation resistance relative to the thick plate, so it is not suitable to use the traditional fusion welding method to weld the aluminum alloy sheet. The choice of friction stir welding method can effectively avoid the problems that often occur in fusion welding methods. Friction stir welding technology is a kind of solid phase connection technology [its principle is a special stirring head comprises shaft shoulder and stirring needle, insert it into the wiring place or lap joint of the welded plate in the state of rapid rotation, and then utilize the frictional heat generated by the contact part between the stirring head and the workpiece, make the surrounding metal material be heated and occur serious plasticization [ plasticizing softening layer metal fills the cavity behind the stirring head under the action of stirring head rotation, and under the action of the shaft shoulder of stirring head and stirring needle stirring together to extrude, realize material Because the aluminum alloy has the characteristics of low melting point, easy deformation, and good plasticity at high temperature, this welding method can improve its welding efficiency and process stability, but for the welding of aluminum alloy sheet less than mm, due to its excessive axial force and friction, it is easy to cause problems such as plate deformation and serious thinning. Adopting the ultra-high speed friction stir welding method can effectively reduce or even avoid the generation of defects [ In the process of ultra-high speed friction stir welding, the rotation speed of its stirring head is about twice that of the traditional friction stir welding stirring head, and the diameter of the shaft shoulder and the stirring needle of the stirring head is greatly reduced simultaneously, and this structure makes the welding reduce the requirement for stiffness and has higher welding efficiency than the traditional friction stir welding. This is due to the reduction of stiffness requirements for ultra-high-speed friction stir welding, which makes it possible to reduce the weight of friction stir welding equipment. In addition, the use of shaft shoulders and stirring needle diameter smaller stirring head is conducive to reducing the stirring area of the weld metal, greatly reducing the area affected by welding on the base metal, reducing the welding deformation [ At present, there are few reports on aluminum alloy ultra-high speed friction stir welding, so the author has conducted in-depth research on the ultra-high speed friction stir welding of aluminum alloy sheet, through the analysis of the structure and performance of the welded joint, the feasibility of ultra-high speed friction stir welding of aluminum alloy sheet is revealed, and the organizational characteristics of the welded joint are revealed and softening mechanism. Test materials and test methods Test materials: The test material is an aluminum alloy sheet with a size of mm mm mm. The shaft shoulder diameter of the stirring head is mm, the end diameter of the stirring needle is mm, the root diameter is mm, the length of the stirring needle is mm, and the material of the stirring head is cr mo w v steel, which has good strength and toughness after quenching and tempering treatment, which can meet the requirements of ultra-high speed stirring friction welding. The test method adopts ultra-high speed friction welding method to weld aluminum alloy sheet, take the welded joint sample, carry out cold mounting, water grinding, polishing, and then anodic coating with (volume fraction) of fluorobonic acid aqueous solution, the anode coating time is s, and then the microstructure observation is carried out by polarizing microscope. The tensile properties of the specimen were tested with a universal tensile testing machine at a tensile rate of mmminThe microhardness of the welded joint is tested by a microhardness tester, the loading load is n(kgf), the loading time is s, and then the microhardness distribution contour diagram of the welded joint is drawn according to the hardness distribution at different positions. Test Results and Discussion The welding process qualification table is the surface forming of aluminum alloy sheet after welding under different processes. As can be seen from the table, the optimal welding process parameters are welding speed mm min and rotation speed rminWhen the rotation speed is RMIN, the stirring speed at the edge of the shaft shoulder of the mixing head is ms, and the stirring speed at the edge of the mixing needle is ms;When the rotation speed is rmin, the stirring speed of the shaft shoulder edge of the stirring head is ms, and the stirring speed of the stirring needle edge is ms, so it can be seen that the stirring speed of the edge of the shaft shoulder of the stirring head is about twice the stirring speed of the stirring needle edge in the welding process, which leads to the formation of a fine stripe when the weld is formed, and the distance between the stripes is mm, and the distance between the stripes in the conventional friction stir welding weld is about mm, so the ultra-high speed friction stir weld weld shape is more delicate. 
The mechanical properties of the joint are the tensile strength test results of the ultra-high speed stirring friction welding joint of aluminum alloy under different welding processes. It can be seen from the table that the tensile strength of the joint is the highest when the rotation speed rmin and the welding speed mmmin are the highest, which is consistent with the results of the tableIn addition, the rotation speed is welded under the conditions of rmin and welding speed mmmin, and the tensile strength of the joint is only about that of the base metal. 
The fracture position of the tensile specimen is shown in the figure, and the morphology of the tensile fracture is shown in the figure, and it can be seen that the fracture of the welded joint is located in the mixing area of the weld, and there will be a certain degree of necking at the fracture, and the fracture form is a shear fracture. According to the fracture location, fracture shape and breaking strength of the welded joint, it can be preliminarily determined that the leading factor of tensile fracture of the welded joint is the softening of the welded joint during the welding process, resulting in the reduction of tensile strength. 
In order to further confirm the softening of the ultra-high speed stir friction welded joint of the aluminum alloy and to measure the softening area, a microscopic Vickers hardness tester was used to test the partition. The test area is the upper, middle and lower three rows of the welded joint section, and the length of each row must ensure that it passes through the welding mixing zone, the heat engine affected zone, the heat affected zone and the base metal. The upper layer is mm away from the upper surface of the sheet, the lower layer is mm away from the lower surface of the sheet, and the distance between adjacent test points is mm, as shown in the figure. 
According to the microhardness measured at each test point and the position of each point, the microhardness contour on the welded joint section is drawn, as shown in Fig. It can be seen that the hardness of the softened area of the welded joint is in Hv, and the hardness of the base metal is about Hv, and the hardness of the softened area is about that of the base metal, which is consistent with the tensile test resultsIn addition, the shape and size of the softened zone are quite close to that of the stirring zone. It can be seen that the aluminum alloy sheet welded by the ultra-high speed friction stir welding method is not because of the decrease of the tensile strength of the welded joint caused by the welding defect, but because of the reduction of the tensile strength caused by the metal softening of the welded joint. The softening area has a width of mm on the upper surface and mm on the lower surface, and whether the area is softened due to agitation or due to temperature is further confirmed by observing the microstructure of the area with a metallographic microscope. 
The macroscopic morphology of the weld of the aluminum alloy sheet weld is shown in the figure, which is the macroscopic morphology of the weld surface under the condition of the best welding process parameters, and it can be seen that the welding process of ultra-high speed friction stir welding is stable, the surface of the weld is well shaped, and there are no defects such as flash and cracks. The microscopic morphology of the cross-sectional section of the aluminum alloy weld is shown in the figure, and it can be seen that the weld does not have root defects and thinning phenomenon, and there is a clear flow line inside the weld, and the streamline is dense, and there are no defects such as porosity and holes. 
The microstructure and morphology of the aluminum alloy sheet weld are shown in Fig. As can be seen from Figure a), the microstructure of the aluminum alloy sheet base metal presents a ribbon-like deformation structure, which is the microstructure formed during the stamping process of the aluminum alloy before weldingAs can be seen from Fig. b), the grain changes significantly in the nugget area as fine equiaxed grains, which is due to the strong stirring effect of the mixing head on the material, and the strong plastic deformation occurs under the action of thermal cycling at higher temperatures, resulting in the transformation of the strip rolling deformation structure and elongated recrystallized grains of the base metal into fine equiaxed recrystallized grains. As can be seen from Fig. c) and Fig. d), the dividing line between the heat-affected zone and the weld zone on the forward side is relatively obvious, indicating that the range of the heat-affected zone on the forward side is larger, while the dividing line on the backward side is relatively blurred, and the range of the heat-affected zone is relatively small. This situation occurs because there is a big difference in the plastic flow of metal on both sides of the heat-affected zone in the welding process, and on the forward side, the velocity gradient between the plasticizers is relatively large, and the deformation degree and fluidity of the plasticizer close to the outside of the stirring head are far less than that of the plasticity metal in the weld nugget area, causing them to form a relatively obvious dividing line. Thus, the arrangement of the grain of the metal in the center area of the weld has undergone a great change under the action of intense heat and stirring, and the grain dislocation relationship produced by the stamping process is completely lost or partially lost, so the cold work strengthening of the metal in the center area of the weld is eliminated in the process of friction stir welding, and the hardness and tensile strength of the joint are not as good as that of the base metal. 
Conclusion ( ) The ultra-high speed friction stir welding method is used to weld aluminum alloy sheets, and under the condition of reasonable welding parameters, the welding joint can be guaranteed to have no defects, no thinning of the workpiece, and the deformation is small, but the softening of the joint is inevitable. (Under the process parameters of rotation speed of RMIN and welding speed of mmmin, the weld joint with the best surface quality can be obtained, the microhardness of the weld core area reaches HV, and the tensile strength of the welded joint reaches MPa.)The main reason for the softening of the heat-affected zone of the aluminum alloy sheet welded joint is that under the action of heat and stirring, the welded joint has undergone a similar annealing process, so that the cold work hardening in the local area has been weakened to varying degrees, and the metal softening in the heat-affected zone closer to the center of the weld is more serious. References:[ Chen Guiqing, Fu Gaosheng, Yan Wenxuan, et al Experimental study on dynamic recrystallization of aluminum alloys[J] Materials Engineering, D**iesg M,Goodyer B g aluminum in automotive applications[J] Metals and materials Burystedmunds, Yang Zonghui, Sun Xiaochun Modern aluminum alloy welding technology[J] Aluminum processing, Silvacl M, Scottia TheInfluenceofDouble PulseOnporosityFormationinaluminumgmaw [J] Journal of Materials Processing Technology, LI Jinmei, SHI Wei, WANG Shunhua, et al Analysis of the causes of cracking of aluminum alloy containers and tanks[J] Physical and chemical testing (physical fascicle), Shi Huaijiang Aluminum alloy sheet pulse MIG welding technology[J] automotive practical technology, LI Yanjun, KANG Ju, WU Aiping, et al. Effect of welding process on porosity of aluminum alloy joints[J] Journal of Welding Science, From Baoqiang, Ouyang Ruijie, Qiao Liuping Formation and porosity analysis of different CMT processes T[J] Journal of Welding Society, Wang Zhonglin, Wu Xiaohong, et al Aluminum alloy sheet laser welding process and laser multi-optical path system design[J] Welding equipment and materials, Xu Fei, Li Xiaoyan, Yang Jing, et al. A Research on aluminum alloy laser wire filling welding technology[J] welding, Sanchez Amayajm, Delgado T, Gonzalez Rovira L, etal laser welding of aluminium alloys and under condition regime[J] appliedsurfacescience, Li Bing, Xie Liyang, Wang Lei, et al Research on friction stir welding process and mechanism[J] modern manufacturing technology and equipment, Xiong Jiankun, Tong Yangang Research progress and application of friction stir welding technology[J] electric welding machine, mukherjees,ghosh a k simulationofanew solidstatejoiningprocessusingsingle shouldertwo pintool[j] journalof manufacturingscienceand engineering, wiliamssw WeldingoFairFrameSusingfriction Stir[J] AirandSpaceEurope, Zhao Huihui, Feng Xiaosong, Xiong Yanyan, et al. Structural performance of non-inclination micro friction stir welding joints of aluminum alloy ultra-thin plates[J] Welding Journal, He Diqiu, Li Shengpeng, Li Jian, et al Friction stir welding of aluminum alloy plates [J] hot working technology, Zhou Yang, Dai Zhixin, Fang Fang Wei, et al. Welding deformation and residual stress of ultra-high speed friction stir aluminum alloy plates[J]. ] Physical and Chemical Testing (Physics Volume), Zhang Xian, Mu Danning Evaluation of the Measurement Uncertainty of Vickers Hardness Test for Metal Materials[J] Physical and Chemical Testing (Physical Volume), Article**—Materials and Testing Network.