Resistance spot welding process and liquid metal embrittlement crack distribution of Q&P980 galvanized high-strength steel

By designing the orthogonal experiment of resistance spot welding process, the range of spot welding process parameters of Q&P980 galvanized high-strength steel was determined, meanwhile the microstructure characterization and the mechanical properties analysis of the welding joint were carried out. The results show that the microstructure of nugget zone is mainly staggered lath martensite, whereas the microstructure of HAZ is composed of lath martensite, residual austenite and ferrite. The maximum average width of martensite lath in incompletely quenched zone is 4.86 mu m.The microhardness test exhibits that the hardness value of the welding joint shows a "W"-shaped symmetrical distribution. The peak hardness appears in the fine-grain zone, which is 559HV, while the minimum hardness appears in the incompletely quenched zone, which is 338HV. The incompletely quenched zone shows obvious softening phenomenon. The room temperature tensile tests of the welding joints are carried out, and the peak value of maximum tension-shear load is 27.92 kN. The fracture morphology shows typical dimples, which belongs to ductile fracture.Since the melting point of zinc is lower than that of the steel matrix, the zinc layer is prior to melting and penetrates downward along the grain boundary of the matrix at the welding joints, and then the liquid metal embrittlement cracks can be observed obviously.