Using short hold times to mitigate LME cracking in electrode indent of resistant spot welded third generation advanced high strength steel

Abstract

Third generation Advanced High Strength Steels (3gAHSS) crucial for the future of automotive design. When Zn-coated for corrosion protection and subjected to resistance spot welding. they can be prone to liquid metal embrittlement cracking (LME) The influence of hold time variation on LME in the electrode indent above the expulsion limit was investigated in this study. Welds were conducted with hold times of 100 ms, 167 ms, 400 ms, and 1000 ms. LME severity was measured using cracking index, revealing that LME severity peaked at a hold time of 167 ms and reduced with both longer hold times, corresponding the observations in the literature, but also decreased for shorter hold time, contradicting the literature. Finite Element (FE) simulations were performed to understand this behavior, highlighting that the timing of electrode retraction significantly affects the cooling and reheating of the weld, altering the thermal stresses that cause LME. A short hold time (100 ms) prompts radial heat dissipation into the bulk, minimizing surface thermal gradients in radial direction and thus leading to reduced LME. Conversely, at 167 ms (standard AWS hold time), heat extraction by the electrodes towards the sheet surface creates steep radial temperature gradient at the electrode-sheet interface, increasing thermal stress at the surface and promoting LME. At longer hold times (400 and 1000 ms), the prolonged electrode contact cooled and supported the sheet surface longer, resulting in lower temperatures and stress levels at electrode release, leading to less severe LME compared to moderate hold times.