Interfacial microstructure and mechanical properties of diffusion-bonded titanium–stainless steel joints using a nickel interlayer

Diffusion bonding was carried out between commercially pure titanium and 304 stainless steel using nickel interlayer in the temperature range of 800–950 °C for 3.6 ks under 3 MPa load in vacuum. The transition joints thus formed were characterized in optical and scanning electron microscopes. TiNi3, TiNi and Ti2Ni are formed at the nickel–titanium (Ni–Ti) interface; whereas, stainless steel–nickel (SS–Ni) interface is free from intermetallic compounds up to 900 °C processing temperatures. At 950 °C, Ni–Ti interface exhibits the presence of β-Ti discrete islands in the matrix of Ti2Ni and the phase mixture of λ + χ + α-Fe, λ + α-Fe, λ + FeTi + β-Ti and FeTi + β-Ti occurs at the stainless steel–nickel interface. Nickel is able to inhibit the diffusion of Ti to stainless steel side up to 900 °C temperature; however, becomes unable to restrict the migration of Ti to stainless steel at 950 °C. Bond strength was also evaluated and maximum tensile strength of ∼302 MPa and shear strength of ∼219 MPa were obtained for the diffusion couple processed at 900 °C temperature due to better contact of the mating surfaces and failure takes place at the Ni–Ti interface. At higher joining temperature, the formation of Fe–Ti bases intermetallics reduces the bond strength and failure occurs at the SS–Ni interface.