Microstructure and mechanical properties of fiber laser welding of 950 MPa grade twinning-induced plasticity steel for automotive industry

Laser welding of 950 MPa twinning induced plasticity steel (TWIP950) for automotive industry was performed using a fiber laser in a butt-joint configuration. The microstructure and chemical compositions of the welded joint (WJ) of TWIP950 steel were characterized. The microhardness of the WJ was tested. The tensile properties and fracture behavior of the investigated materials at various strain rates were examined using tensile testing machine at room temperature. The results showed that the microstructure of fusion zone (FZ) was mainly austenitic dendrite. The softening phenomenon took place in FZ due to the evaporation loss of manganese and grain coarsening in FZ. The microhardness of the outer heat-affected zone (HAZ) was higher than that of the base metal (BM). The TWIP950 BM and WJ showed strain rate sensitivity and it changed from negative to positive with increasing strain rate. The ductility of the TWIP950 BM and WJ decreased first, then increased and decreased again with increasing strain rate. The tensile specimens of the TWIP950 WJ failed in the FZs at all strain rates, and the typical ductile fracture characteristics of the tensile fracture surface did not change with increasing strain rate.