以7N01-T4合金作为实验材料，采用标识材料示踪法，进行了搅拌摩擦焊(FSW)和超声辅助搅拌摩擦焊(UAFSW)对比实验，重点研究了轴向超声振动与搅拌针螺纹耦合作用下搅拌区(SZ)金属的流动行为。结果表明，施加轴向超声振动没有改变SZ金属沿焊接方向的宏观流动行为(如弧纹间距保持不变)，但加剧了轴针影响区(PDZ)金属沿板厚方向的环形涡流运动，同时超声作用下轴肩与搅拌针端部的高频锻压作用促进了轴肩影响区(SDZ)和涡流区(SWZ)金属的流动。在分析轴向超声振动条件下搅拌针周围金属受力状态基础上，提出了微区“抽吸-挤压”效应模型，解释了轴向超声振动提高SZ金属流动能力的本质。当采用有螺纹的搅拌针焊接时，轴向超声振动与搅拌针螺纹的耦合作用所产生的微区“抽吸-挤压”效应导致SZ金属流动能力显著提高。当采用无螺纹的搅拌针焊接时，施加轴向超声振动显著降低搅拌针对SZ金属的剪切作用，导致SZ的金属流动能力减弱，更容易形成焊接缺陷。

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.