Friction stir welding is a solid phase joining method. Compared with conventional welding, the metal does not melt during the welding process, which can effectively avoid the generation of pores and reduce the incidence of cracks. In addition, under the stirring and upsetting of the stirring tool, the joint structure is fine, and the mechanical properties are obviously superior to the conventional welded joint. Figure 1 is a comparison of mechanical properties of joints obtained by different welding methods for 2219 aluminum alloy. It can be seen from Fig. 1 that the tensile strength of the friction stir welded joint reaches 350 MPa (second only to the e-book welding head, which is obviously superior to the welded joint obtained by other welding methods), and its elongation reaches 6%. It is higher than other welding method joints; therefore, its comprehensive performance is the best.
Studies have shown that in the as-welded state, the strength of the weld nugget in the weld zone is higher than that in the heat-affected zone. For annealed aluminum alloys, the fracture position of the tensile test is usually on the base metal; for the deformation strengthened and heat-treated reinforced aluminum alloy, the fracture usually occurs at the location of the welded joint. Deformation Strengthening and Heat Treatment Strengthening of aluminum alloys can improve the joint performance by controlling the welding heat cycle, especially the method of reducing the heat-return zone return and overaging; at the same time, the joint performance can be improved by post-weld heat treatment. . Table 1 shows the tensile test results of the welded and heat-treated tensile joints of different aluminum alloys.
Due to the characteristics of friction stir welding, the upper part of the weld is affected by the shoulder and the stirring needle. The lower part of the weld is only affected by the stirring needle, which causes the difference in the structure and performance of the weld in the whole thickness direction. The mechanical properties show the anisotropy of each layer. Figure 2 shows the mechanical properties of different thickness positions of 8mm thick LD10 aluminum alloy. It can be seen from Fig. 2 that the upper tensile strength of the joint is the highest, reaching 380 MPa (the data has exceeded the overall strength of the joint 360 MPa), while the tensile strength of the middle and lower portions is less than the overall strength of the joint.
Figure 2 Comparison of mechanical properties of joints at different positions
For the defect-free welded joint, the upper part of the joint is subjected to the double action of stirring needle stirring and shoulder forging. This part of the grain is very small and the structure is relatively dense, so the mechanical properties are the best. For the lower part of the joint, only the stirring action of the stirring needle is used, and the forging action of the shoulder is not carried out, and the heat affected zone near the stirring needle is heated to cause coarse grain, but since the lower part of the welded joint is in contact with the backing plate, the heat dissipation is better. Fast, effective heat input is small, so the grain growth is not obvious. For the middle of the joint, it is not subjected to the upset action of the shoulder, and can not directly dissipate heat through the pad and the air, and the effective heat input is large, so the grain growth is obvious, resulting in the worst mechanical properties.
source：未知