Liquid Metal Embrittlement Cracking During Resistance Spot Welding of Galvanized Q&P980 Steel

Resistance spot-welded galvanized ultrahigh-strength steels are sensitive to liquid metal embrittlement (LME), which is manifested by surface cracks on the joints. LME occurs when a solid metal contacts a liquid metal under tensile stress, and the phenomenon has not been fully understood until now, especially for resistance spot welding. In this study, the susceptibility of hot-dipped galvanized Q&P980 steel to LME cracking during resistance spot welding was systematically investigated by an orthogonal experiment. Cracks were detected by fluorescent magnetic particle testing and cross-sectional microscopic observation. Cracks were mostly located at the indentation edge and slope, and a few cracks were also located at the indentation center and slope periphery. The severity of the cracking increased with the increasing welding current and welding time, and the decreasing electrode force. The sequence of influence degree from high to low was welding current > electrode force > welding time. Holding time had no obvious effect. Microstructural analysis revealed that the content of martensite in areas with cracking increased, indicating that a high temperature was experienced at these locations. Zinc accumulated inside the cracks, and the cracks were intergranular, which coincides with the characteristics of LME. The LME cracking was provoked by the simultaneous occurrence of a tensile stress, an appropriate temperature and liquid metal, and the cracks were influenced by the welding parameters. Under suitable conditions, Zn diffused along the grain boundaries and weakened them, resulting in LME cracking.