Effect of bonding temperature on microstructure and mechanical properties of Inconel 718 alloy transient liquid phase joints

Transient liquid phase (TLP) bonding was used to join Inconel 718 alloy using a Ni-based amorphous foil at the temperature range of 1050–1150 °C and a pressure of 10 MPa applied for 60 min. Ultrasonic non-destructive testing revealed no lack-of-bonding porosities at the TLP bonding interface, indicating an excellent bonding quality. Details on microstructure and mechanical properties with increasing bonding temperature on the TLP-bonded joints were investigated. Microstructure analysis showed that joints consisted of three regions: isothermal solidified zone (ISZ), diffusion-affected zone (DAZ), and base material (BM). The width of ISZ and DAZ increased with the increase of bonding temperature from 1050 to 1150 °C. As the bonding temperature increased, the sizes of grains and Cr–Mo–Nb-enriched boride precipitates increased correspondingly. Mechanical tests confirmed that the mechanical properties of the TLP bonding samples are lower than those of the BM. For the TLP bonding specimens, the tensile strength of the joint obtained at 1050 °C was higher than those obtained at 1100 and 1150 °C. The fracture sites of the joints were located at the grain boundary decorated with Cr–Mo–Nb-enriched borides in the DAZ, exhibiting the characteristics of intergranular fracture. The deterioration of tensile properties with increasing bonding temperature was attributed to the coarsening of grains and Cr–Mo–Nb-enriched borides in the DAZ.