Microstructural evolution and deformation behavior of fiber laser welded QP980 steel joint

Advanced high strength steels for the auto industry are in high demand regarding the development of new type of steels, reliable and economic welding techniques, etc. In this study, the third generation advanced high strength steels, cold-rolled quenching and partitioning steels (QP980), were butt-welded using fiber laser to evaluate the microstructure, mechanical properties and formability of joint. The fusion zone composed of lath martensite with high microhardness (499 HV). Martensite tempering and carbide precipitation were found at the sub-critical heat affected zone, resulting in a slight decrease of microhardness (~ 37 HV drop). The presence of joint had no effect on tensile strength and all welded samples failed at the base metal with 98.8% joint efficiency. The formability of welded blanks in Erichsen cupping test was dependent on the weld line position. The formability ratio was only 68.08% of base metal when weld line offset was 0 mm, and increased as the weld line moving away from the blank center because more deformation was transferred to base metal. Stretch-flange-formability of the joint in hole expanding test was inferior to that of base metal due to the high volume fraction of coarse martensite in fusion zone.