Although the influence of weld process variables on LME cracking was known to be significant, limited studies have been conducted on the effect of process variables with systematic approaches in the equivalent nugget growth behavior and heat input. This study aimed to identify the effect of weld process variables on LME sensitivity with the equivalent nugget diameter and underlying mechanism with induced tensile stress for cracking. Among the welds with equivalent nugget diameters in the combination of different welding current and time, higher LME sensitivity was observed with the high welding current and short welding time combination than that with the low welding current and long welding time combination for the equivalent nugget diameter. Because a high current and short time combination resulted in faster weld nugget growth than the low welding current and long welding time combination, it rapidly increased the surface temperature along with the cooling from the electrode. These combined effects induced a higher thermal gradient and thermally induced tensile stress on the weld surface, satisfying the conditions of the LME cracking. The simulation results also confirmed that the critical weld cycle time of the LME cracking (tc), which is the cross point between the nugget growth diameter and a contact diameter of the electrode, could be different with the combination of the weld process variables with the equivalent weld nugget size. Therefore, tc can be applied for the sensitivity index of LME cracking of the resistance spot weldment considering complex weld variables.

Zinc-coated advanced high-strength steels (AHSSs) are highly susceptible to surface cracking induced by liquid metal embrittlement (LME) during resistance spot welding (RSW). However, directly comparing LME sensitivity across various steel grades remains challenging due to inconsistencies in welding parameters, stress states, and quantification criteria among conventional evaluation methods. To address this limitation, this study proposes a novel pre-strained resistance spot testing (PS-RST) technique designed to evaluate LME susceptibility under precisely controlled thermomechanical conditions. The PS-RST approach utilizes a single zinc-coated AHSS sheet in which a controlled deflection, d, is introduced to impose a well-defined tensile strain during electrode squeezing and welding. LME cracking severity is quantified via a crack index, defined as the total cross-sectional crack length normalized by the sheet thickness. Comparative testing with de-coated reference specimens exhibited no cracking, confirming that the defects generated during PS-RST were exclusively driven by zinc-assisted LME. Under comparable melted area conditions, the LME susceptibility decreased in the order of TRIP 980 > CP 780 > DP 980, revealing a strong dependence on the baseline microstructure. These findings demonstrate that the proposed PS-RST method, coupled with the quantitative crack index, serves as an efficient and reliable screening tool for ranking and benchmarking the LME resistance of zinc-coated AHSSs under industrially relevant welding conditions.