Solid-State Diffusion Bonding of Pseudo-α-Ti Alloy to Ti-Stabilized Stainless Steel: With and Without Interlayer

Solid-state diffusion bonding of pseudo-α-Ti alloy and Ti-stabilized stainless steel (SS321), with and without Ni interlayer, was investigated in the temperature range of 800-940 °C for different times. Microstructural investigation of the bond interfaces in the directly bonded diffusion couple showed a distinct diffusion zone which was composed of layers of (1) Fe-Ti intermetallic phases (Fe₂Ti and FeTi), (2) transformed β-Ti and (3) (α+β) Ti. Shear strength of the joint was seen to be dependent on the thickness of diffusion zone and intermetallic phase layer. For direct diffusion bonding, maximum shear strength of 223.6 ± 17 MPa was observed for the joints processed at a temperature of 920 °C with a holding time of 8 min under a load corresponding to ~0.8 times yield strength (YS) of Ti alloy. Shear strength results showed that the optimum thickness of the Fe-Ti intermetallic phase layer is approximately 2 ± 0.5 µm. For the diffusion bonding joints in the presence of a Ni interlayer, formation of layers of Ni-Ti intermetallic compounds (Ni₃Ti, NiTi and NiTi₂) was observed at the interface between Ni and Ti alloys. Joint bonded at 940 °C with a holding time of 1 min under a load of 0.8 × YS showed the maximum shear strength of 180 ± 23 MPa. The optimum thickness of the Ni-Ti intermetallic layers for maximum shear strength was found to be approximately 5 ± 2 µm.