Effect of tool geometry and welding parameters on the microstructure and static strength of the friction-stir spot-welded DP780 dual-phase steel sheets

In this study, friction-stir spot welding is performed on DP780 dual-phase steel sheets to investigate the effect of the tool geometry and welding parameters on the static strength, the failure mode and the microstructure of the welds.First, the effects of two different parameters, the tool plunge depth and the tool holding time, were evaluated, where the tool penetration varied between 1.9-2.8mm and the dwell time between 8-16 s.The tensile shear strength was first increased with the tool penetration and then decreased, while a longer tool holding time increased the tensile shear strength of the joints.Next, welds were made to compare the effects of two tool geometries, i.e., the conventional tapered pin and the triangular pin, on the hook shape, microstructure and static strength.The difference in the hook geometry led to differences in the tensile shear strength of the welds.The static strength of the welds made with the triangular pin was higher than that obtained using the tapered pin.