Influence of microstructural morphologies on liquid metal embrittlement of Zn-coated dual-phase steels

Abstract

Dual-phase (DP, ferrite+martensite) steels are crucial for automotive components, predominantly spot-welded using thin Zn-coated cold-rolled sheets. However, the effect of martensite morphologies, namely fine-lamellar (LD) or coarse-globular (GD) shapes, on Zn-assisted liquid metal embrittlement (LME) remains underexplored. This study investigates LME mechanisms during resistance spot welding of Zn-coated DP steel sheets with LD or GD microstructures. Analysis focuses on cracks within the weld shoulder zone, categorized into sub-critical HAZ (SCHAZ), inter-critical HAZ (ICHAZ), and upper-critical HAZ (UCHAZ). The LME susceptibility index, calculated as total crack length divided by total contact length, increased in the order of LD-SCHAZ, LD-ICHAZ, GD-ICHAZ, and GD-SCHAZ. These results indicated that the microstructure of the GD sheet itself was very vulnerable to the cracking because the initial microstructure was almost retained in the SCHAZ. Cracking analysis in the LD-SCHAZ and GD-SCHAZ suggested that martensite grains near ferrite/ferrite boundaries impeded crack growth by deflecting propagation, and that the LD sheet showed higher crack resistance due to unfavorably oriented ferrite/martensite boundaries.