Effect of Cu interlayers on element diffusion and properties of laser welded Ti6Al4V/Inconel 718 dissimilar joints

The dissimilar joints of TC4 and IN718 were successfully welded by using a fiber laser and Cu interlayer with varying thicknesses. This research focused on the interface on the TC4 side, where two transition layers were identified: a Ti-rich layer and a Ti-poor layer. The Ti-rich layer primarily consisted of Ti2Cu and Ti2Ni. Meanwhile, the Ti-poor layer contained a range of harder and more brittle intermetallic compounds (IMCs), such as TiNiCu, TiCr2 and TiFe2, with the thickness increasing as the interlayer thickness increased. The fracture of all the joints occurred at the interface on the TC4 side. Insufficient Cu introduction tended to cause the penetration of Ti into Layer B, reacting with elements such as Fe and Cr to form more brittle and hard IMCs. Conversely, excess Cu in the molten pool, resulting from the introduction of a 0.4 mm Cu interlayer could lead to the formation of much (Cu, Ni), which caused softening and a deterioration in the tensile strength of the joint. When a 0.3 mm Cu interlayer was employed, the diffusion of Ti and Cu reached equilibrium, resulting in an average tensile strength of 319 MPa for the obtained joint, which was 28 % higher than that of the joint with a 0.4 mm interlayer.