Improvement in microstructure and mechanical properties of laser welded steel/aluminum alloy lap joints using high-entropy alloy interlayer

This study focused on the effect of a high-entropy alloy (HEA) interlayer on the improvement of microstructure and mechanical properties of steel/Al alloy lap joints by laser welding. The FeCoCrNiCu HEA improved metallurgical reaction by restricting the formation of Fe–Al intermetallic compounds (IMCs) at the interface. Results showed that in the steel on Al alloy lap joint without an interlayer, the discontinuous multi-phase IMCs layer mainly consisted of three types of IMCs, Fe-rich β-FeAl phases, and Al-rich θ-Fe 4 Al 13 + η-Fe 2 Al 5 phases. With the addition of a HEA interlayer, owing to the sluggish diffusion effect in the fusion zone (FZ), the diffusion of Fe atoms from FZ to the IMCs layer was significantly controlled, which limited the generation of IMCs at the FZ/Al alloy interface. The IMCs layer was only composed of single-phase β-FeAl with the absence of highly brittle Al-rich IMCs and the thickness of IMCs layer was reduced from 85 μm to 30 μm, resulting in the enhancement of toughness of the joint. Additionally, the improvement of mechanical properties of the joint, including microhardness and tensile shear strength was consistence with the reduction of brittleness of IMCs at the interface.