Effect of Cu interlayer on the elements diffusion and properties of laser welded Ti6Al4V/Inconel 718 dissimilar joint

The dissimilar joints of TC4/IN718 were successfully welded by using fiber laser and Cu interlayer with varying thicknesses. This research focused on the interface at TC4 side where two transition layers were identified: a Ti-rich layer and a Ti-poor layer. The Ti-rich layer primarily consists of Ti2Cu and Ti2Ni, while the Ti-poor layer contains a range of more complicated intermetallic compounds (IMCs) such as Ti-Ni-Cu and Ti-Cr-Fe-Ni. The fracture of all the joints is located at the interface on the TC4 side. When a 0.3 mm Cu is employed, along with appropriate process parameters, the average tensile strength of the obtained joint is 319 MPa, which is 28% higher than that of the joint with 0.4 mm. Insufficient Cu introduction tends to cause the penetration of Ti into layer B reacting with elements such as Fe, Cr to form more IMCs with higher brittleness and hardness. By contrast, excess Cu in molten pool caused by the introduction of 0.4 mm Cu interlayer can lead to the formation of much (Cu, Ni), which causes the softening and tensile strength deterioration of the joint. In addition, higher fluctuations of temperature distributed along the TC4-side interface speculated by numerical analysis for joints with 0.3 mm and 0.4 mm interlayer is also the reason for the penetration between the transition zones and lowering of tensile strength.