Abstract: Insufficient metal flow in the stirred zone(SZ) may easily lead to loose defects in the transition zone between the shoulder-affected zone and the pin-affected zone， and deteriorate the mechanical properties of the joint. 7075-T6 aluminum alloy was subjected to conventional friction stir welding (FSW) and ultrasonic assisted friction stir welding (UAFSW) under different welding parameters， and the effects of ultrasonic vibration on the metal flow behavior， microstructure characteristics and mechanical properties of the SZ were investigated. The results show that the mechanical properties of the SZ of UAFSW joints are better than that of FSW joints processed using the same welding parameters. Tensile strength and elongation of the SZ of UAFSW joint with the welding parameters of 1000r/min-110mm/min reach the maximum values， i.e. 515 MPa and 17.3%， respectively. The application of axial ultrasonic vibration during FSW can significantly reduce yield stress and flow stress of the metal in the SZ， promoting the flow of plasticized metal along the normal direction of the plate. Eliminating the loose defects in the transition zone of the SZ and refining the microstructure are the main reasons for improving the mechanical properties of the joint.

Key words: ultrasonic-assisted friction stir welding, aluminum alloy, stirred zone, microstructure, mechanical property