Microstructure and fracture characteristics of spot-welded DP600 steel

Microstructural characterization, microhardness tests, tensile shear tests, and fatigue tests of spot-welded hot dipped galvanized (HDG) DP600 steel were conducted. The effects of weld expulsion on the microstructural characteristics, mechanical properties, and fracture modes were investigated using optical microscopy, image analysis and scanning electron microscopy (SEM) together with energy dispersive X-ray spectroscopy (EDS). The hardness in the weld nugget was observed to be over two times higher than that in the base metal due to the formation of lath martensite. Fatigue tests showed a slightly lower fatigue limit for the joints with the expulsion. The crack initiation was observed to occur basically at the boundary of the weld nugget and at the interface between two sheets. The causes for the interfacial fracture (IF) were discussed in relation to the microstructure and weld parameters.