Clarify the forming mechanism and afecting factors of defects in semi-stationary shoulder bobbin tool friction stir welding

Abstract

Different defects of the upper stationary shoulder bobbin tool friction stir welding (SSUBTFSW) were identified in this study, and the Coupled Eulerian–Lagrangian model of SSUBTFSW was used to illustrate the forming mechanism of groove defect. Defects can be classified into groove, crack, and flash, which form simultaneously under excessive heat, while only the groove appears under lower heat during processing. There is a layered asymmetrical material flow from lower to upper surfaces, leading to a groove at the edge of the probe on the advancing side of the joint. Increasing rotation speed and decreasing welding speed reduce the groove defect to the point of elimination. The groove defect size goes through minimum value with the increase of the side tilt angle and the reactive forces of the upper stationary shoulder.