Effect of the thickness of Cu interlayer on dissimilar laser welding of 304 stainless steel to tantalum

304 stainless steel (304SS) tube and tantalum (Ta) cylinder were successfully joined by laser welding with copper (Cu) interlayer. The influence of Cu interlayer thickness on the microstructure and mechanical properties of joints was clarified. The intermetallic compounds (IMCs) generated between the 304SS and Ta were determined as Fe₂Ta and FeTa in the joint without Cu interlayer. The addition of Cu interlayer could effectively inhibit the atomic diffusion between the base metal and the generation of IMCs. In addition to other laser welding parameters, the thickness of the Cu interlayer also has an extremely important effect on the amount of IMC at the fusion zone (FZ)/Ta interface and the heat input on Ta side. As a result, the formation of Fe₂Ta and FeTa was reduced and the weld consisted of Fe-based solid solution and Cu-rich phases. Joints produced with a 0.4 mm thick Cu interlayer exhibited the highest tensile strength of 288.7 MPa, which was 54.1% higher than that without Cu interlayer, due to the excellent metallurgical bond and few IMCs at the FZ/Ta interface. A large amount of IMCs was found at the FZ/Ta interface of the joint with a 0.2-mm Cu interlayer, which reduced the strength of the joint. A weak metallurgical bond was formed due to insufficient heat input, as the thickness of the Cu interlayer reached 0.6 mm. Thermodynamic calculations of the Fe-Ta-Cu ternary system explain the diffusion behavior of Cu atoms at the FZ/Ta interface and its effect on the formation of Fe-Ta reaction layer.