Evolution of interface microstructure and strength properties in titanium – stainless steel diffusion bonded transition joints

Abstract

Diffusion bonding was carried out to produce transition joints between commercially pure titanium and 304 stainless steel at a temperature of 800°C for different times ranging from 30 to 180 min. in steps of 30 min under load in vacuum. The diffusion couples thus produced were studied using optical microscopy, scanning electron microscopy, and electron probe microanalysis to characterise the reaction layers formed in the diffusion zone. The chemical compositions of these layers indicate that intermetallics like σ phase, Fe₂ Ti, Cr₂ Ti, χ phase, FeTi, β-Ti, and Fe₂ Ti₄ O are formed in the reaction zone. The presence of these intermetallic compounds was also confirmed by the X-ray diffraction technique. Maximum bond strength of ∼242 MPa was obtained for diffusion welded joints processed for 120 min. At this joining time, the plastic collapse of the surface asperities reaches near completion, favouring the interdiffusion of chemical species. Reduction in the bond strength of the transition joint processed for 180 min is due to the formation of a large volume fraction of voids in the reaction zone. Under tensile loading, failure takes place through α-Fe + χ phase mixture for transition joints processed in the time range of 30 – 90 min and through β titanium for joining times greater than 120 min.