Microstructure and Mechanical Properties of Laser Welded Dissimilar Joint of DP780/HC660 Dual-phase Steels with Different Thicknesses

1.2 mm thick DP780 dual-phase steel and 1.0 mm thick HC660 dual-phase steel were butt welded by CO₂-laser welding. The microstructure and mechanical properties of the dissimilar joint were studied. The results show that the microstructure of weld zone in the welded joint consisted of coarse lath martensite and a little ferrite. The microstructure of heat affected zone was uneven and could be divided into coarse grained zone, fine grained zone and intercritical zone. The microhardness of the welded joint was asymmetrically distributed around the weld zone center. The average microhardness of the weld zone, heat affected zone and base metal decreased successively. A softening phenomenon was observed in the heat affected zone. The tensile strength of the welded joint was close to that of the HC660 dual-phase steel, while the elongation was reduced to 7.8%. Fracture occurred in HC660 dual-phase steel near heat affected zone. The fracture mode was ductile fracture.