Improving mechanical properties of friction-stir-spot-welded advanced ultra-high-strength steel with additional water cooling

1.2 mm thick DP1180 advanced ultra-high-strength steel were successfully friction stir spot welded (FSSWed) with additional water cooling. No hook defect and obvious keyhole were observed in the spot joints when using a specially designed WC-Co tool. Softening of the heat-affected zone was substantially suppressed and the lap shear strength of the joints was obviously enhanced via water cooling. The enhanced strength of water-cooled joints was attributed to the grain refinement, the formation of the untempered martensite and the effective restraint of tool softening underwater. Accordingly, the maximum lap shear strength of 17.9 kN was obtained for the water-cooled joint. This study provides a novel strategy to enhance the strength of the FSSW joints for high-strength steels.