Effects of Heat Input on Microstructure and Mechanical Properties of Dissimilar Laser-Welded Dual-Phase and Boron Steel Joints

Heat-affected zone (HAZ) softening occurs during the laser welding of many Advanced High-Strength Steels (AHSS) that are used for body-in-white (BIW) of automobiles, leading to degradation in the mechanical properties of the welded joints. The microstructure and mechanical properties of dissimilar laser-welded AHSS comprising of as-received 22MnB5 with dual-phase (DP) steels (DP600, DP800, and DP 1000) were investigated in this study. Welds were made at welding speeds ranging from 1 m/min to 3 m/min. Irrespective of welding speed, the DP600-22MnB5 joints fractured in the base metal (BM) of 22MnB5 during tensile tests. Likewise, welded joints of DP800-22MnB5 and DP1000-22MnB5 made at 1 m/min and 2 m/min failed in the BM; however, at 3 m/min the failure location of these joints shifted to the fusion zone (FZ). The fractured surfaces of all the welded combinations were characterized by optical and scanning electron microscopy (SEM). Based on fracture energy, joints welded at 2 m/min were optimal due to a compromise of minimizing the weld size and formation of hard phases within the weld FZ and HAZ.