Effect of temperature on formation of reaction products and strength properties of titanium and stainless steel joints using copper interlayer

Diffusion bonded assemblies of titanium |Cu| stainless steel were carried out in the temperature range of 900–1000°C for 30 min under 3 MPa load in vacuum. The microstructures of the bonded assemblies were observed using optical and scanning electron microscopes (SEMs). The present study exhibited the presence of different reaction layers in the diffusion zone and their chemical compositions were determined by energy dispersive spectroscopy (EDS). The CuTi₂, CuTi, Cu₃Ti₂, Cu₄Ti₃, FeTi, Fe₂Ti, Cr₂Ti, T₁ (Ti₃₃Cu_67−xFe_x; 1<x<2·5), T₂ (Ti₄₀Cu_60−xFe_x; 5<x<17), T₃ (Ti₄₃Cu_57−xFe_x; 21<x<24), T₄ (Ti₃₇Cu_63−xFe_x; 6<x<7) intermetallic phases are formed in the diffusion interfaces. These reaction products were confirmed by X-ray diffraction technique. The maximum tensile strength of ∼318 MPa (∼99% of Ti) and shear strength of ∼239 MPa (∼81% of Ti) along with 7·2% ductility were obtained for the couple bonded at 950°C owing to better coalescence of mating surface. At 1000°C processing temperature, bond strength decreases owing to the increase in the width of the brittle Fe–Ti base intermetallic phases. At 900°C, the bond strength is also poor presumably owing to the incomplete coalescence of the mating surfaces. Observation of fracture surfaces in SEM using EDS demonstrates that failure takes place through stainless steel/copper interface.