Resistance spot welding of steel and aluminum with Cu/Zn bimetal interlayers: mechanical properties, microstructure and mechanisms

To mitigate the formation of brittle intermetallic compounds during aluminum/steel welding, resistance spot welding (RSW) experiments were conducted using 6082-T6 aluminum alloy and DP590 steel with the incorporation of Cu/Zn bimetal interlayers. The microstructure and mechanical properties of the weld joints were analyzed using scanning electron microscopy, electron probe microanalysis, electron backscatter diffraction, and transmission electron microscopy. The results revealed that the Cu/Zn bimetal interlayers substantially increased the shear strength of the weld joints. As the welding current increased from 13 to 16 kA, the shear strength initially increased before declining, which was accompanied by a fracture-mode transition from interfacial fracture to button fracture. A maximum shear strength of 7.199 kN was achieved at a welding current of 15 kA, reflecting a 52% improvement compared with joints without the interlayer. TEM analysis indicated the formation of novel phases, such as Fe 2 Al 5 Zn 0.4 and Al 2 Cu, with significant Cu and Zn enrichment on the Al side. Additionally, the intermetallic compound (IMC) layer thickness was reduced to 1.42 μm, representing an 81.44% reduction relative to joints without the interlayer. These results indicated that the Cu/Zn bimetal interlayers effectively suppressed the growth of the IMC layer, improved the joint microstructure, and enhanced the mechanical performance of the welds. • Shear strength of aluminum/steel joints improved by 52% with a Cu/Zn bimetal interlayers. • Cu/Zn bimetal interlayer inhibits Fe and Al atoms diffusion, and participates in the interfacial metallurgical reactions. • Cu/Zn bimetal interlayers reduces IMC layer thickness by 81.44% during spot welding. • Novel phases Fe 2 Al 5 Zn 0.4 and Al 2 Cu were identified at the aluminum/steel interface. • Novel insights into interlayer effects on resistance spot welding processes.