Elimination of tunnel defect in ultrasonic vibration enhanced friction stir welding

Abstract

Through observing the marker material (MM) distribution and the exit holes left by the “tool sudden stop action” technique, the formation mechanism of tunnel defect in friction stir welding (FSW) is elucidated. Based on the MM distribution on the transverse cross-sections, the weld nugget zone is divided into three subzones, i.e., shoulder affected zone (SAZ), pin affected zone (PAZ), and weld bottom zone (WBZ). There is a time-delay feature of material flow which is considered as the inducement of tunnel defect formation. When a tunnel defect occurs in FSW joints, three subzones connect with each other and there exists a converging point. The insufficient material flow from retreating side to advancing side in the PAZ and the shortage of downward material transfer in the SAZ are the primary and secondary reasons for tunnel defect occurrence, respectively. Under the same welding conditions, ultrasonic vibration enhanced friction stir welding (UVeFSW) can eliminate the tunnel defect in the weld nugget zone where the SAZ and WBZ are separated by the PAZ, because the exerted ultrasonic vibration improves the material flow in the PAZ and reduces or even suppresses the time-delay of material flow in the PAZ.