Effect of Axial Ultrasonic Vibration on Metal Flow Behavior During Friction Stir Welding

Metal flow behavior in the stir zone (SZ) is important in friction stir welding (FSW) because it determines the formation of defects, and evolution of microstructure, and affects the mechanical properties of the joint. Applying axial ultrasonic vibration (ultrasonic energy is applied to the stirring tool along the axial direction) during FSW can improve the flowability of SZ metal; however, the reason is unclear. In this study, 6-mm-thick 7N01-T4 alloy plates were welded using FSW and ultrasonic-assisted FSW (UAFSW), using a thin foil of pure aluminum as a marker placed at the butt interface before welding to highlight the actual metal flow during welding. Alongwith the FSW experimental results, the influence of the coupling effect of axial ultrasonic vibration and thread of tool pin on the flow behavior of SZ metal was studied. The results revealed that the macroscopic flow behavior of SZ metal along the welding direction was not affected by axial ultrasonic vibration (e.g., the distance between the arc lines remains unchanged); however, the axial ultrasonic vibration intensified the ring vortex movement of the pin-driven zone (PDZ) metal along the plate-thickness direction. Moreover, the high-frequency forging effect of the shoulder and pin end under the action of ultrasound promoted the flow of metal in the shoulder-driven zone (SDZ) and swirl zone (SWZ). Based on the analysis of the force condition of the plastic metal around the pin, under axial ultrasonic vibration, a microscale sucking-extruding effect model was proposed, and the flowability improvement of SZ metal by axial ultrasonic vibration was explained. The stress superposition and acoustic softening effects induced by ultrasonic vibration are not the only factors affecting the flowability of SZ metal; tool pin geometric features also determine the flow behavior of SZ metal under the action of axial ultrasonic vibration. When a tool with a threaded pin is used for welding, the microscale sucking-extruding effect caused by the coupling of axial ultrasonic vibration and the pin thread improves the SZ metal flowability. When welding using the tool with a smooth pin, axial ultrasonic vibration reduces the shearing effect of the pin on SZ metal, resulting in the weakening of the metal flowability of the SZ, and a high tendency for welding defect formation.