Liquid metal embrittlement cracking in dissimilar resistance spot welding of 3rd Gen – Advanced high strength steel: mitigation methods and associated mechanisms

In response to the automotive industries demands for safer, lighter, and more environmentally friendly vehicles, the third generation of advanced high-strength steels (3G-AHSS) has been developed to have both high strength and high ductility. To protect these materials from corrosion during service, these materials are typically coated with zinc. During resistance spot welding (RSW), the zinc coating can melt, allowing it to penetrate into the grain boundaries (GBs), and lead to liquid metal embrittlement (LME) cracking when combined with tensile stresses associated with the RSW process. Several possible strategies for lowering LME severity by altering welding parameters have been proposed in the literature, but these strategies have been predominantly focused on similar joints. However, as the vast majority of welds in automotive construction join materials of different thicknesses and grades, dissimilar weld performance is also of interest. Therefore, it is also of interest to understand how LME changes in dissimilar joints as compared to similar joints, as well as whether methods to reduce LME severity must take the dissimilar nature of the joint into account to improve weld quality. In this work, a 3G-AHSS galvanized (GI-coated), known to be highly susceptible to LME, with a nominal sheet thickness of 1.4 mm was welded, both to itself and to a 0.6 mm thick Interstitial Free (IF) steel. The effect of joint construction on LME cracking severity was determined by comparing the cracks resulting from welds made in both similar material and dissimilar material joints. It was found that differences in material characteristics, between the two materials, resulted in differences in temperature distribution in the similar and dissimilar joints, causing high LME severity in the dissimilar joint. An industrially viable welding schedule was developed to minimize the LME severity in dissimilar joints. The severity of the LME cracks that formed in welds made using the developed weld schedule was compared to those developed in welds made with baseline welding parameters, resulting in a 46% reduction in a severity of LME cracking. The mechanism of LME reduction was analyzed using the finite element modeling software Sysweld®. The obtained results were compared with in-situ thermography using an infrared camera. It was observed that the difference in a thermal gradient and the distance between the fusion boundary and electrode/sheet interface are responsible for LME severity. The robustness of the developed welding schedule was then tested on welds made with typical industrial disturbance factors such as pre-strained sheets (between 0 to 80% of material yield strength) and electrode misalignment (between 0° to 10° misalignment) compared to baseline parameters. When welds were made in joints affected by severe disturbance factors, the resulting optimized welding schedule decreased LME cracking and showed improved resistance to LME, lowering LME severity by 41% for the extreme pre-strain condition and 27% for the extreme misalignment angle