Abstract
Automotive advanced high-strength steels and ultra-high-strength steels, such as Q&P980 steel with a Zn coating, are prone to liquid metal embrittlement (LME) surface cracking during resistance spot welding, while traditional low carbon steels (LCSs) do not appear to undergo LME. In this work, galvanized Q&P980 steel sheets were welded with LCS sheets. LME cracks were found on both the Q&P980 steel and LCS sides. However, the severity of LME cracking on the Q&P980 side was much higher than that on the LCS side, indicating that Q&P980 steel has a higher LME susceptibility than LCS, which was also proven by a hot tensile test. The higher LME susceptibility of Q&P980 was attributed to its lower reactivity with liquid Zn, higher achievable stress and increased number of LME-susceptible grain boundaries. Micro-analysis revealed that Zn most likely entered the steel matrix via diffusion and not via liquid penetration, and stress-assisted diffusion is the likely mechanism for LME.
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