Study on microstructure and impact toughness of TC4 titanium alloy diffusion bonding joint

TC4 titanium alloy was diffusion bonded under small axial pressure of 0.5 MPa to restrain the deformation. The sample surfaces, prior to bonding, were finely polished to Ra 0.3 μm. The bonding temperatures were used as 860, 890 and 920 °C with holding time of 1 h. The joint microstructure was examined. Tensile test and Charpy impact test were conducted to evaluate the joint strength and toughness. Two parts of the 860 °C bonded sample were then held in 920 °C for 90 min and 180 min, respectively. The results show that the 920 °C joint with high bonding ratio (∼98%) fractured at the base metal during tensile test showing excellent strength of the joint. However, the impact toughness was 14.99 J/cm² that is much lower than 27.3 J/cm² of the base metal. The heat treated 860 °C samples had 100% bonding ratio and impact toughness of 15.81 and 20.02 J/cm² for the 90 and 180 min sample. The microstructure analysis indicates that the joint toughness was affected by the void defects remained and the interface grain boundary migration at the bonding interface.