Influence of copper interlayer on composition and mechanical properties of Ti6Al4V-nitinol dissimilar joint

Abstract

This study investigated the Laser Beam Welding (LBW) of Ti6Al4V and Nitinol alloys, with and without a copper interlayer, using a pulse current of 110 A, a pulse duration of 8 ms, a pulse frequency of 12 Hz, and a welding speed of 3.4 mm/s. The microstructure and mechanical properties of the welds were characterized using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), tensile testing and hardness profiling. Direct joints were prone to cracks, high concentration of Ti₂Ni intermetallic, and low tensile strength (∼110 MPa). The insertion of a copper interlayer altered the interfacial reactions by promoting the formation of Ti₂Cu intermetallic phases while reducing the concentration of brittle Ti₂Ni, which resulted in crack-free joints with significantly improved tensile strength (∼250 MPa). This represents approximately 40% of the strength of the Nitinol-Nitinol similar joint (630 MPa). Fractographic analysis revealed brittle fracture in direct joints, while interlayered joints showed mixed fracture modes with ductile features. Hardness profiling revealed a reduction in peak hardness from 600 HV in direct welds to 430 HV in interlayered welds, confirming the beneficial role of the copper interlayer in enhancing overall mechanical performance.