Formation and Microstructure of Self-supporting Friction Stir Welded Joint of 7150 High-Strength Aluminum Alloy

Aiming at the problem that friction stir welding (FSW) of high-strength aluminum alloys needs rigid back supporting, a self-supporting friction stir welding tool was designed, which contains upper shoulder, stir pin and lower shoulder. The application of lower shoulder structure can realize the FSW of 7150-T77511 Al alloy without the supporting of backing plates, and eliminate the weak bond defect of conventional FSW. By optimizing the welding process parameters, the joint with excellent formation is obtained at the rotating velocity of 550 r/min and welding speed of 500 mm/min. The cross section structure of the joint present a "dumbbell" shape. Due to the influence of upper and lower shoulders, the grain sizes of upper and lower welding nugget zones are almost the same, which is smaller than that of the welding nugget area.