Examination of Ti2Ni formation during dissimilar friction stir welding of thin sheet NiTi and Ti-6Al-4V

Solid-state dissimilar joining of NiTi and Ti-6Al-4V is a currently underexplored field with high potential to expand applications for each alloy in the aerospace industry. Friction stir welding of butted 1 mm thick sheets of NiTi and Ti-6Al-4V was tested under a matrix of welding parameters. Six of eight tested parameter conditions joined but each showed degraded mechanical properties. Higher traverse speed conditions joined more successfully into testable samples. Upper and lower quartiles for tensile strength varied between 140 to 60 MPa with the lower rotation speed showing higher median values. Weld degradation is attributed to the formation of an up to 10 µm wide Ti2Ni intermetallic compound layer at the weld interface. Higher rotation speeds showed a thicker intermetallic layer. The Ti2Ni layer showed equiaxed grains on the order of 2-3µm in diameter. It is theorized that this layer grew from pre-existing Ti-6Al-4V via nickel diffusion from the NiTi due to in-process heating. Accelerated property mapping nanoindentation shows that the Ti2Ni layer has a greater microhardness and reduced elastic modulus (12.42 GPa and 150.06 GPa) than the stir zone of the NiTi (5.10 GPa and 97.65 GPa) and Ti-6Al-4V (5.93 GPa and 137.2 GPa). Crack propagation along this brittle, high stiffness intermetallic layer is proposed as the cause of failure in the welded samples.