Study of Laser Welding Process Parameter on the Microstructure and Mechanical Properties of Dissimilar Joining of Dual-Phase DP780 and Cold-Rolled CR340 Steel

Laser welding was performed in dual-phase steel (DP780) and cold-rolled steel (CR340) at a series of weld parameters (series I up to series VII): power input, heat input, weld velocity, and focus offset. The study was aimed at improvement of joint performance on the laser weld-ing process parameter settings on the weld bead morphology and thereby evaluating their microstructure and mechanical properties. Optical and scanning electron microscopy and uni-axial tensile tests were performed to observe the microstructural changes, phase formation, and evaluate the mechanical properties, respectively. Tensile strength, microhardness, and strain localization using a digital image correlation technique were investigated for acquainting the local strain mapping associated with the loading. Microstructure characterization revealed that the fusion zone consisted of martensitic structure, and the heat affected contained newly formed martensite in both steels. Vickers hardness revealed that during welding, there was a softening phenomenon at the CR340 end. The DP/CR welds show a mechanical behavior that is driven by both the DP780 and CR340 steel. The joints showed strength up to similar to 410 MPa and fracture strain up to similar to 16 %, however the weld join failed from CR340 steel end weld during tensile tests. The larger heat input at series VI guarantees joints with an adequate strength but wider weld seams and a lower energy efficiency.