Microstructure and mechanical properties of fiber laser welded QP980/press-hardened 22MnB5 steel joint

Abstract

Fiber laser welding of QP980 steel and press-hardened 22MnB5 steel in butt configuration was performed in this study. The effect of heat input on microstructure evolution and mechanical properties of dissimilar joint was investigated. Laser power employed was varied from 1.6 to 2.4 kW while welding speed was kept constant at 2 m/min. The microstructure of fusion zone (FZ) was changed with the increasing laser power. Martensite and few bainite were observed when laser power ranged from 1.8 to 2.2 kW. Out of this range, the microstructure in FZ consisted of mixed structure of martensite and ferrite. Transformation from liquid phase to ferrite was promoted by Al-Si coating on the surface of press-hardened 22MnB5 steel. Heat affected zones (HAZs) of dissimilar joint were composed of martensite and bainite. Softened zone (SZ) appeared at the HAZ of 22MnB5 steel due to tempering behavior. The hardness of SZ and QP980 steel base metal declined with microstructure evolution. The fracture occurred at QP980 base metal when dissimilar joints were produced at laser power of 1.8 and 2.0 kW. When laser power reached 2.2 kW, the tensile samples failed at SZ owing to the increased softening degree. With further increasing laser power, the weakest region was FZ since ferrite declined the whole joint strength. A better Erichsen value was obtained at 2.0 kW (4.04 mm), which was balanced by more crash energy absorption and better deformation resistance.