Improvement of refill friction stir spot welds by auxiliary material addition

An innovative technology to improve the bonding of refill friction stir spot welding by auxiliary material addition was proposed. The annular groove was eliminated and the energy absorption was increased. Weld bonding was enhanced because the improved technology affected the hook, retractable line and stir zone (SZ) shape significantly: (1) the effective bearing thickness of the welds increased as the hook penetration through the sheet thickness was reduced significantly, (2) the retractable line was eliminated as the dynamic recrystallisation was enhanced along the path of the sleeve moving and (3) fracture propagation was impeded by the bending interface between the SZ and the base materials.