A study on diffusion bonding of superplastic Ti–6Al–4V ELI grade

Ti–6Al–4V ELI (extra low interstitials) grade alloy provides improved ductility and fracture toughness comparing to grade 5 Ti–6Al–4V alloy. In order to find the optimum superplastic forming and diffusion bonding (SPF/DB) condition, a series of tensile tests was carried out at the strain rate range of 10−4 to 10−2 s−1 and temperature range of 1073–1223 K. The maximum elongation of 1898% was obtained at the strain rate of 10−3 s−1 at 850 °C. It was shown that the ELI grade alloy performs better than the grade 5 alloy in terms of the optimum superplastic condition for Ti–6Al–4V. Based on this result, diffusion bonding process of superplastic Ti–6Al–4V ELI sheet metals was developed. Bonding was completed by means of inert gas pressure applied in a bonding tool at high temperature. The microstructure of the bonding area was investigated and the bonding interface was microscopically undetectable. The evidence of nucleation of new grains and migration of grain boundaries at the interface proves the diffusion bonding process is successful. It is shown that the superplastic forming and diffusion bonding of Ti–6Al–4V ELI grade is possible at the temperature lower than those of conventional Ti–6Al–4V.