Liquid metal embrittlement during the resistance spot welding of galvannealed steels: synergy of liquid Zn, α-Fe(Zn) and tensile stress

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Abstract
In this study, the influence of the microstructural evolution of galvannealed (GA) coating and the thermomechanical characteristics of liquid metal embrittlement (LME) cracking during resistance spot welding (RSW) of steels were investigated. The analysis of LME cracks, coating microstructure and thermomechanical characteristics at various weld regions revealed a synergistic effect of liquid Zn, α-Fe(Zn), and tensile stress on LME cracking during RSW. A continuous α-Fe(Zn) layer inhibits liquid Zn from contacting the steel substrate; hence, LME was suppressed. However, a long-duration, high-magnitude tensile stress can fragment the α-Fe(Zn) layer and enable LME cracking. Conversely, α-Fe(Zn) particles allow the easy infiltration of liquid into the steel and make the crack on tensile stress development.