Effect of Type-C liquid metal embrittlement cracks on high-cycle fatigue and tensile properties in resistance spot-welded TRIP 1180 steels

Abstract

In the automotive industry, reducing the body weight is essential to meet the needs of environmental protection. Accordingly, advanced high strength steels (AHSS) utilizing transformation-induced plasticity (TRIP) have been developed. Resistance spot welding (RSW) is a dominant method for joining metal sheets in the manufacturing process. However, RSW of Zn-coated sheets often produces cracks around the welded region, leading to liquid metal embrittlement (LME). LME cracks are classified as Type-A, -B, and -C according to their location and direction of propagation. Type-A and -B are the cracks that initiate at the surface of the sheet and propagate to the nugget or heat affected zone (HAZ), while Type-C is a crack occurring in the HAZ and propagating to the surface of a sheet. This study analyzed the effect of such Type-C LME cracks in galvannealed TRIP 1180 steel on high-cycle fatigue (HCF) behavior. In particular, the fatigue test was conducted in two different loading modes: tensile shear and cross-tension. The length and width of Type-C LME cracks were measured (after fracture) by SEM. Finally, HCF life and tensile strength were found to degrade because of the Type-C LME cracks and the pre-cracks caused by the incomplete bonding of the corona bond. The tensile fracture occurred along the pre-crack. In high-cycle fatigue tests, fatigue failure occurs in both pre-crack and Type-C LME cracks.