Fracture mode characteristics and microstructure analysis ofresistance spot welded joint of cold rolled DP980

Resistance spot welding test was applied to DP980 steel with 1. 2 mm thickness by a medium frequency inverter welding machine,through which fracture mode of welding points under the spherical electrode was determined. Moreover,parameters of diameter of nugget,shear force,failure energy and fracture displacement under different fracture modes were characterized. In addition,typical microstructures and microhardness of welding points were analyzed. The results indicated that the fracture mode of welding points under the spherical electrode exhibited with interface fracture and nugget draw. Both in the two fracture modes,microstructures of nugget were composed of lath martensite and twinning martensite. The soften zone could be observed in HAZ,in which microstructures consisted of tempered martensite and ferrite. Under the mode of interface fracture,values of diameter of nugget,shear force and fracture displacement exhibited low,and failure energy fast increased with the increasing of welding electricity. Under the mode of nugget draw,values of diameter of nugget,shear force and fracture displacement exhibited high,and failure energy waved little with the increasing of the welding electricity.