Role of the internal oxidation layer in the liquid metal embrittlement during the resistance spot welding of the Zn-coated advanced high strength steel

Advanced high strength steel (AHSS) with the Zn coating can produce high susceptibility of liquid metal embrittlement (LME) particularly when the resistance spot welding (RSW) is involved. The internal oxidation layer formed in the subsurface of AHSS is believed to be one of the most important factors to control the Zn-assisted LME susceptibility. The role of the internal oxidation layer in the formation of the LME for the resistance spot welded joint of the Zn-coated AHSS was comprehensively investigated in the present study. The results showed that increasing the dew point of the annealing atmosphere from −40 to −10 °C can successfully produce the internal oxidation layer in the subsurface of the steel. The Zn-coated AHSS without the internal oxidation layer revealed higher LME susceptibility during the RSW compared with the one with internal oxidation layer. In particular, the elements Si and O exhibit a synchronous segregation tendency, forming an internal oxidation layer in the subsurface of the steel. The internal oxidation layer together with the internal oxides distributed at the grain boundaries can decrease the penetration motivation of the liquid Zn, reducing the LME susceptibility.