Vacuum diffusion bonding of α-titanium alloy to stainless steel for aerospace applications: Interfacial microstructure and mechanical characteristics

In present study dissimilar metal joints between austenitic stainless steel (modified SS321) and alpha-Ti alloy (Ti-5Al-2.5Sn ELI) were produced by solid-state diffusion bonding process performed at different temperatures (750-950 degrees C) in vacuum. The effects of bonding temperature, time and pressure on interface microchemistry and microstructure were analyzed and subsequently the local hardness of interface layer(s) and the bond (shear) strength of the joints were evaluated. The results indicate the formation of four layers comprised of intermetallic phases viz. sigma, chi, lambda + Fe2Ti + FeTi and beta-Ti at the joint interface, and their width increased with increase in the bonding temperature and time. The maximum shear strength of similar to 348 MPa, which is similar to 65% of the shear strength of SS321, was noted for the joint formed at the bonding temperature of 850 degrees C. This was achieved primarily due to complete collapse of surface asperities making intimate contact at the joint surface with formation of optimal width of interface layers having minimum embrittlement effect. The fractography of the specimens failed under shear loading revealed the transition of failure of the joints from featureless brittle fracture to mixed mode of ductile-brittle failure to quasi-cleavage brittle fracture with increasing the bonding temperature. Furthermore, it was noticed that the bonded joint specimens failed either through chi (layer 2) or lambda + Fe2Ti + FeTi (layer 3). Interestingly, the optimized diffusion bonding parameters of the dissimilar metal joints of SS321/Ti-5Al-2.5Sn ELI have yielded different shear strength levels for the similar metal joints of SS321/SS321 (similar to 35% of base alloy) and Ti-5Al-2.5Sn ELI/Ti-5Al-2.5Sn ELI (75% of base alloy).