Si effect on Zn-assisted liquid metal embrittlement in Zn-coated TWIP steels: Importance of Fe-Zn alloying reaction

During resistance spot welding of Zn-coated high-strength steel sheets, Zn penetration is caused by Zn-assisted liquid metal embrittlement (Zn LME), and degrades the mechanical properties of the welded parts. When electrogalvanized twinning induced plasticity (TWIP) steels with 0–1.5 wt% Si were welded, Zn LME was exacerbated by Si addition. It was found that Si in the steel delayed the dissolution of Fe into the coating from the steel, which suspended the active formation of α-Fe(Zn). Since α-Fe(Zn) fraction in the coating was insufficient to cover the steel/coating interface in the Si added steel, it could not prevent the steel from contacting the liquid Zn during welding. As a result, liquid Zn generated higher number and deeper Zn penetration into the Si containing steel than into the Si free steel. Overall, it was revealed that Si aggravated Zn LME in TWIP steel by suppressing Fe-Zn alloying reaction, which increased the possibility of steel substrate to contact liquid Zn. [Display omitted] •Si in a steel delays Fe dissolution into Zn and α-Fe(Zn) formation.•Si in a steel generates spherical α-Fe(Zn) above the steel matrix.•Suppressing Fe-Zn alloying reaction by Si aggravates Zn LME during welding.