Formation mechanism of liquid metal embrittlement in laser lap welding of zinc-coated GEN3 steels

Liquid metal embrittlement (LME) was observed in the laser lap welds of zinc-coated GEN3 steels. When line energy input ranged from 54 kJ/m to 60 kJ/m, the LME crack initiated from the weld edge at the faying interface, propagated along the growth direction of columnar grains and extended upwards along the weld centerline, exhibiting an inversed Y-shape. Multiple Fe–Zn phases such as α-Fe (Zn), Γ-Fe3Zn10 and δ-FeZn10 were created by liquid zinc in grain boundaries reacting with the austenite matrix during the cooling stage of the laser welding process. Those hard and brittle Fe–Zn phases, especially Γ-Fe3Zn10, deteriorated the grain boundary cohesion and led to crack initiation. Simulation results indicate that a weld profile being deep with partial penetration will result in a high stress concentration at the notch root during the LME active temperature range. Therefore, full penetration welding is recommended to reduce the risk of LME by enabling zinc vapor to escape from both the top and bottom of the keyhole.