Quantifying LME Susceptibility of Zinc-Coated Advanced High-Strength Steels via Pre-Strained Resistance Spot Testing and Crack Index

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.