Effect of an electroplated Ni interlayer on interfacial evolution and mechanical performance of resistance spot welded Al6061/St12 joints

Resistance spot welding (RSW) of aluminum to steel remains challenging due to the rapid formation of brittle Fe-Al intermetallic compounds (IMCs), which deteriorate weld integrity and mechanical performance. In the present study, an electroplated Ni interlayer (∼17 μm) was employed to control interfacial reactions and improve the joining behavior of dissimilar Al6061/St12 resistance spot joints. Microstructural characterization revealed that the Ni interlayer modified interfacial diffusion and suppressed the formation of thick and irregular Fe 2 Al 5 layers typically observed in direct Al/steel welds. In the absence of Ni, a non-uniform Fe 2 Al 5 layer with a thickness of up to ∼6 μm formed at the interface, promoting brittle interfacial fracture. In contrast, the introduction of the Ni interlayer resulted in a thinner and more uniform reaction zone (≤3 μm) consisting primarily of refined Fe-Al-Ni ternary phases. The modified interfacial structure significantly improved joint performance. Tensile-shear strength increased from 54 MPa to 93 MPa, while elongation improved from 0.80% to 3.15%. In addition, the absorbed energy increased from 0.25 J to 0.901 J. Fracture behavior transitioned from brittle interfacial failure to ductile tearing and partial plug failure in joints produced with the Ni interlayer. The findings demonstrate that electroplated Ni interlayer provides an effective approach for controlling interfacial reactions and enhancing weld performance in Al-steel joints, while maintaining compatibility with existing industrial RSW systems.