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In this paper, Cu-based and Ni-based filler metals were used to obtain laser-CMT hybrid welded copper/stainless steel (Cu/SS) joints. The effect of wire compositions on the microstructure and mechanical properties of T2 Cu/304 SS dissimilar metal joint was studied. The results showed that Fe–Cu liquid phase separation occurred in the fusion zone of the Cu-based weld, and Cu penetration cracks appeared in the heat-affected zone (HAZ) on the steel side. The Cu penetration cracks formation is derived from supersaturated Cu atoms penetrating, gathering and merging at the steel side fusion line along the austenite grain boundaries. The penetration cracks can be eliminated effectively by the Ni-based filler metal, because the diffusion of Cu into the fusion zone and the HAZ on the steel side is limited significantly by the increase of Ni content. The Ni-based weld consisted of dendrite grains in the fusion zone, and Fe-rich blocks and Ni-rich islands nearby the HAZ of Cu-side. • Defect-free weld was obtained by laser-CMT hybrid welding with Ni-based wire. • Cu-based weld penetration cracks mechanism was illustrated. • The microstructures of Cu-based and Ni-based welds were compared. • The mechanism of inhibition of penetration cracks in Ni-based weld was discussed. [ABSTRACT FROM AUTHOR]