Effect of bonding temperature on interface microstructure and properties of titanium–304 stainless steel diffusion bonded joints with Ni interlayer

Abstract

The diffusion bonding was carried out between commercially pure titanium and 304 stainless steel (SS) using nickel as an intermediate material in the temperature range of 800–950°C for 1·8 ks under a 3 MPa uniaxial load in vacuum. The microstructures of the transition joints were revealed in optical and scanning electron microscopy (SEM). TiNi₃, TiNi and Ti₂ Ni are formed at the nickel/titanium interface whereas, SS–Ni diffusion zones is free from intermetallic phases for all bonding temperatures. At 950°C, Ni/Ti interface exhibits the presence of α-β Ti discrete islands in the matrix of Ti₂Ni intermetallic. The bond strength was evaluated and maximum tensile strength of ∼281 MPa along with 7·2% ductility and shear strength of ∼202 MPa were obtained for the diffusion couple processed at 900°C owing to the better coalescence of the mating surfaces. With increasing joining temperature to 950°C, the bond strength drops owing to an increase in the width of the nickel–titanium based reaction products. At lower joining temperatures, bond strengths are also lower owing to incomplete coalescence of the mating surfaces. The activation energy and growth constant were calculated in the temperature range of 800–950°C for the reaction layers and it was found that, the values were maximum for Ti₂Ni. Observation of fracture surfaces in SEM using energy dispersion spectroscopy (EDS) demonstrates that, failure takes place mainly through nickel/titanium interface.