Influence of Cu/Ni coating on microstructure and mechanical properties in steel/aluminum single-sided resistance spot welding joint

Due to significant differences in physical and chemical properties between aluminum and steel, directly joining them using traditional resistance spot welding (RSW) process often cannot achieve satisfactory high-quality joints. In this work, some key factors influencing the quality of steel/aluminum welded joints were analyzed and a method based on fracture mechanism analysis was proposed aiming to improve the strength of steel/aluminum joints. The proposed method utilized a copper/nickel bimetallic coating on the steel surfaces, which was joined with aluminum alloy through single-sided RSW. This approach effectively addressed severe deformation of aluminum alloy and reduced stress cracks caused by residual stress in joints. Furthermore, the influence of coating on the microstructure and mechanical properties of steel/aluminum interface was further investigated. Experimental results showed the coating participated in interfacial metallurgical reactions, mainly forming intermetallic compounds (IMCs) such as Ni‍‍-Al, Fe-Cu, Cu-Al, and Fe-Ni, which mixed with Fe-Al compounds. Additionally, the coating refined the IMC grain size, resulting in smaller grains compared to Fe₂Al₅, the primary IMC in uncoated joints. Moreover, the coating reduced IMC layer thickness to within 2.0‍ μm at the weld center, and increased thinner IMC layer thickness by 0.4 μm at the weld periphery. This implied that the coating inhibited the mutual diffusion of Fe and Al atoms, preventing the formation of Fe-Al compounds and promoting a more uniform IMC layer thickness. Microhardness and tensile tests indicated that the coating reduced the hardness gradient of steel/aluminum interface, and increased IMC layer toughness and strength, improving the overall mechanical properties of the joints. The coated steel/aluminum joints with button fracture achieved a peak load of 6.7 kN and an average tensile-shear strength of 127.5 MPa, representing a 49.3 % increase in strength compared to uncoated steel/aluminum joints. This work will provide theoretical insights for steel/aluminum RSW and promote academic and practical engineering applications.