Electrode and process design for regulation of liquid metal embrittlement phenomenon during spot welding of dual‑phase steel: numerical modelling and experiments

Thermal and mechanical loading, combined with zinc coating, are the primary factors influencing liquid metal embrittlement (LME) in the investigated advanced high-strength dual-phase steel, with a yield strength of 1200 MPa and high ductility during resistance spot welding. LME results in a ductility loss of up to 95% and is driven by an intergranular decohesion mechanism, leading to brittle failure in otherwise ductile steel. Validated numerical models provide deeper insights into the critical conditions during welding. The presented multi-physical model enables the optimisation of welding parameters, reducing experimental efforts and enhancing manufacturing efficiency. This study introduces new methods to reduce LME while improving overall weld quality. Surface-coated refractory electrodes significantly reduce embrittlement compared to standard copper electrodes. For copper electrodes, a newly developed shape minimises stresses and misalignment-induced stresses, further reducing embrittlement and enhancing weldability. Additionally, a mandatory holding time after welding effectively lowers embrittlement, while interrupted cooling creates critical conditions. Downward ramped current improves surface conditions, rendering the weld spot less susceptible to embrittlement.