Influence of interface microstructure on the strength of the transition joint between Ti-6Al-4V and stainless steel

Solid-state diffusion bonding of Ti-6Al-4V and type 304 SS was investigated in the temperature range of 750 °C to 950 °C, under a uniaxial load for 5.4 ks in vacuum. The diffusion bonds were characterized using light and scanning electron microscopy. The scanning electron microscopic images in backscattered mode show the existence of different reaction layers in the diffusion zone. The composition of these layers was determined by energy-dispersive X-ray spectroscopy (EDS) to contain the α-Fe, χ, λ, FeTi, β-Ti, and Fe2Ti4O phases. The presence of these intermetallics was confirmed by X-ray diffraction. The bond strength was evaluated, and the maximum tensile strength of ∼342 MPa and the maximum shear strength of ∼237 MPa were obtained for the diffusion couple processed at 800 °C due to the finer width of the brittle intermetallic layers. With a rise in joining temperature, the bond strength drops owing to an increase in the width of the reaction layers. The activation energy and growth constant were calculated in the temperature range of 750 °C to 950 °C for the reaction products. The χ phase showed the fastest growth rate. A fracture-surface observation in a scanning electron microscope (SEM) using EDS demonstrates that failure takes place mainly through the β-Ti phase for the diffusion couples processed in the aforementioned temperature range.