Diffusion bonding of Ti–25Al–10Nb alloy

Abstract

Titanium aluminide is a potential material for high temperature structural application. Recent research on Ti–25Al–10Nb indicates that the alloy possesses excellent superplasticity at high temperature and is suitable for diffusion bonding. In this study, the bonding process parameters, such as surface roughness, bonding temperature, bonding time, bonding pressure, and post-bond heat treatment (PBHT), affecting the properties of bonded joints were investigated. The American Welding Society constrained single lap shear test was applied to assess the quality of the joints. Experimental results showed that a smooth surface roughness is beneficial for the diffusion bonding of Ti–25Al–10Nb. Shear strength of joints increased with the bonding temperature, bonding pressure, and bonding time. However, in the bonding temperature range 850–950°C, shear strengths of joints were all less than 80% of that of the base metal in the single lap shear test. After PBHT at 1050°C for 120 min, the quality of joints could be improved significantly. As rolled Ti–25Al–10Nb can also be diffusion bonded and the bond interface and micro voids eliminated during annealing through recrystallisation and grain growth. Interlayer diffusion bonding using 18 μm thick aluminium foil as an inter layer could lower the bonding temperature to 700°C, but line defects caused by solidification were still present at the centre of the inter layer. Even if the alloy joint was homogenised at 1050°C for 24 h, voids remained in the bonded region. Solid state diffusion bonding of Ti–25Al–10Nb using a TiAl₃ interlayer of approximate thickness 50 μm, coated on one side of the joining alloy surfaces, was also investigated. No microvoids were observed in the joint of the solid state diffusion bonded alloy after PBHT at 1050°C for 12 h.