Formation mechanism and improvement of liquid metal embrittlement cracks in resistance spot welding of PHS1500 galvanized steel

Abstract The third generation of advanced ultra-high-strength steel PHS1500, is widely used in automobile manufacturing due to its excellent performance. Still, severe liquid metal embrittlement (LME) cracks will occur when joined by resistance spot welding (RSW). This study aims to elucidate the formation mechanism of LME cracks and optimize welding process parameters to mitigate the severity of LME cracks in PHS1500 steel. Conventional double-pulse resistance spot welding was used to join PHS1500 steel, and the welded joints were characterized using optical microscopy and scanning electron microscopy to analyze the crack morphology, elemental distribution, and the LME formation mechanism. The results revealed that LME cracks were caused by the diffusion of liquid Zn into the grain boundaries of the base metal to form the low-melting-point Γ phase and α -Fe(Zn) phase, which fractured under tensile stress. After optimizing welding parameters, the maximum crack length at the heat-affected zone slope (HAZ slope) decreased from 117.59 μm to 32.76 μm (72.14% reduction), while that in the outward region of the HAZ (HAZ outward) decreased from 80.88 μm to 21.65 μm (73.23% reduction), with most cracks exhibiting a single dendritic morphology.