Material flow in ultrasonic vibration enhanced friction stir welding

An ultrasonic vibration enhanced friction stir welding (UVeFSW) system was developed, in which the ultrasonic energy was transmitted directly into the localized area of the workpiece near and ahead of the rotating tool by a specially-designed sonotrode. The interaction mechanism of the ultrasonic vibration with the deformed material around the FSW tool was investigated experimentally by integrating three configurations of marker material and metallographic analysis. The observation and comparison of the marker material distribution on different cross-sections of butt-weld specimens revealed that there were two main material flow patterns in stir zone of 2024-T4 aluminium alloy butt welds in UVeFSW, i.e., the continuous flow (upper one third of weld) and the non-continuous flow (lower two thirds of weld). The ultrasonic energy enlarged the volume of the deformed material around the pin, enhanced the stability of the continuous material flow and the material strain of the non-continuous flow obviously, and weakened the degree of vertical material transfer. The results provided a reasonable explanation for the improvement of weld joint quality in UVeFSW.