Aluminum–Steel Resistance Spot Welding with an Aluminum Interlayer: Simulation and Experimental Study of Weld Nugget Formation and Electrode Indentation

• Novel three-layer (base Al/clad Al/base steel) FEM model used for Al/steel RSW • Dome-shaped nugget formed in Al layers while steel remained solid • Thinner Al interlayer boosted nugget size despite shorter current path • Nugget microstructure shifted from columnar to equiaxed with current • Electrode indentation increased with welding current and thinner aluminum interlayer Aluminum to steel resistance spot welding (RSW) is hampered by dissimilar melting behavior and interfacial instability. This study investigates RSW of Al-Mg alloy sheets to low-carbon steel sheets using a roll-bonded aluminum interlayer, through finite element method (FEM) simulations and experimental validation. The effects of interlayer thickness (0.3 and 1 mm) and welding current (11–17 kA) were examined with respect to weld nugget formation, microstructure, and electrode indentation. Simulation results were in good agreement with experiments, with maximum deviations of ∼6% in nugget diameter and ∼11% in electrode indentation. Although the steel sheet did not melt, a dome-shaped nugget formed in the aluminum base sheet and interlayer. Increasing welding current and reducing interlayer thickness enlarged the nugget diameter, which was rationalized by current distribution, heat conduction, and latent heat considerations. Grain structure evolved from columnar to a mixed columnar–equiaxed morphology as welding current increased, consistent with predictions from solidification maps of Al-Mg alloys using the values of solidification rate (R) and temperature gradient (G) at the solid/liquid interface. Electrode indentation increased with higher current and thinner interlayer due to elevated temperature experienced in the region between the welding electrodes. Overall, the study demonstrates that finite element method can reliably capture nugget development and electrode indentation in dissimilar aluminum–steel spot welds, providing guidance for optimizing interlayer-assisted welding strategies.