Mechanism of flash nickel interlayer in mitigating LME in resistance spot welding of high-strength steel with Zn–Al–Mg coating

Liquid metal assisted cracking (LME) are a critical limitation in the performance of Zn-Al-Mg coated High Strength Steel (HSS) during resistance spot welding (RSW). To mitigate this issue, a flash nickel interlayer has been implemented on the steel surface. The mechanism by which the flash nickel interlayer mitigated LME is investigated, focusing on the combined effects of stabilizing grain boundaries and influencing crack plasticity. The findings demonstrate that the flash nickel interlayer induces grain refinement and optimizes grain orientation, enhancing dislocation accumulation near the crack tip to dissipate strain energy. This process increases the proportion of grains with Schmid factors between 0.4 and 0.5, dispersing local stress and altering the crack propagation path. The flash nickel interlayer reduces crack depth by 60.42% and crack quantity by 90.47%. Furthermore, it significantly enhances the strength and ductility of the welded joints, transitioning the fracture morphology from brittle-dominated to a brittle-ductile combination. Ni and Al additions strengthen Fe-Fe bonding and stabilize grain boundaries at the electronic level, whereas Mg addition weakens interfacial cohesion and thus promotes embrittlement. This study elucidates the mechanism of the flash nickel interlayer in mitigating LME across atomic, mesoscopic, and macroscopic scales, offering theoretical and experimental insights for the optimizing HSS during RSW processes and developing anti-embrittlement coatings.