Effect of pin tool profile on mechanical and metallurgical properties in friction stir spot welding of pure copper

In this paper, the influence of thread feature on mechanical and metallurgical properties of friction stir spot weldments is studied. Two pin tools, plain cylindrical and threaded cylindrical, having the same size are used to produce lap joints by friction stir spot welding (FSSW) of commercially pure copper. All experiments have been performed under the same welding parameters. The results revealed that joints produced with threaded pin tool have more than double the tensile shear strength of those produced by plain cylindrical pin. Moreover, the samples that were FSS welded with threaded pin tool failed in a plug failure mode, which is a desirable ductile failure mode. On the other hand, the weldments that were produced using plain cylindrical pin tool failed in interfacial failure mode or shear fracture mode. Effective upper sheet thickness analysis showed upward hook formation and less effective upper sheet thickness, which indicate better material mixing in the case of threaded pin tool. Furthermore, the transition bonding region was found to be very narrow in the case of plain tool while that of the threaded pin tool showed a transition region that is almost equal to complete bonding region, indicating gradual change from unbonded to complete bond; resulting in the above observed higher strength. Scanning electron microscopy analysis of the hook formation region revealed that, contrary to the weldments performed by plain pin tool, which showed the presence of large voids around the hook region, weldments performed with threaded pins were free of defects.