Hot isostatic diffusion bonding of titanium alloy Ti-6Al-4V to gamma titanium aluminide IHI alloy 01A

Abstract

Diffusion bonding of the Ti-alloy Ti-6Al-4V to the γ-TiAl IHI Alloy 01A using hot isostatic pressing at three temperatures, 900°C, 940°C and 980°C with a pressure of 200 MPa held for 1 hour has been studied. Tensile testing of the diffusion bonded material in combination with microstructural and fractographic evaluation showed that: • Sound joints without any pores or cracks can be produced through diffusion bonding. The thickness of the bond-line is related to the HIP-temperature with a bond width of around 9 μm when HIPed at 900°C to a bond width of 11 μm when HIPed at 980°C. • The EDS chemical composition profiles show a diffusion flux of Al-atoms from IHI Alloy 01A towards Ti-6Al-4V and of Ti-atoms in the opposite direction. Fe is rejected from the bond-line whereas the content of V is at the same level as in the IHI Alloy 01A through the diffusion bond. • The diffusion bond-line shows two zones. Zone 1 adjacent to the γ-TiAl alloy is wider and contains 5-6 μm large γ- and α2-grains. The content of the latter phase is increased in comparison with the γ-TiAl material due to enrichment of Ti and depletion of Al. In zone 2, the applied temperature and pressure are enough to cause recrystallisation leading to a finer grain structure of 1-2 μm. Diffusion of Al and depletion in Ti leads to nucleation of α2 grains in the original α-Ti grain structure. • The tensile strengths and fracture ductilities of the joints at ambient temperature are similar to the 7-TiAl IHI Alloy 01A base material. A variation in thickness of the bond-line between 9 μm and 11 μm does not affect the tensile properties. • The fracture at tensile testing occurs mostly in the joint. The interface between the γ-TiAl base material and the bond-line seems to be more pronounced than between the Ti-alloy and the bond-line creating a larger discontinuity in mechanical properties concentrating the fracture to this interface. The fracture surfaces becomes very flat and featureless with some minor pull-out of the γ-TiAl material.