Comparative Investigation into the Liquid Metal Embrittlement Susceptibility during Resistance Spot Welding of Zn-Al-Mg and GI Coated Advanced High Strength Steels

In automotive industry, it is well known that cracks which are promoted by liquid metal embrittlement (LME) can occur during the resistance spot welding (RSW) of zinc-coated advanced high-strength steels (AHSS). The coating type is supposed to be one of the factors impacting LME susceptibility. Recently, Zn-Al-Mg coating is gaining increasing focus because of its enhanced corrosion resistance compared to traditional Galvanized (GI) coating. However, there is a lack of research on assessing the influence of this coating on LME susceptibility. In this study, the LME susceptibility of Zn-Al-Mg and GI coated advanced high strength steels with similar microstructure and strength are compared by hot tensile tests and RSW. The results show that Zn-Al-Mg coated samples present a more significant ductility loss than that of GI coated ones in hot tensile test, and also there are more LME cracks with large length occur in Zn-Al-Mg coated RSW joints, indicating that Zn-Al-Mg coatings have a higher LME sensitivity. The high temperature phase evolution analysis results show that Fe-Zn intermetallic compounds formed in two coatings are different, indicating that there are lower level of Fe-Zn alloying reactions in Zn-Al-Mg coating. The inadequate Fe-Zn reactions potentially facilitate the direct contact between liquid Zn and steel substrate, leading Zn-Al-Mg coating to a higher LME susceptibility.