The effect of gap size on the microstructure and mechanical properties of the transient liquid phase bonded FSX-414 superalloy

Optimization of transient liquid phase (TLP) bonding variables is essential to achieve a joint free from deleterious intermetallic constituents as well as with appropriate mechanical properties. In this research, TLP bonding of FSX-414 superalloy was performed using the MBF-80 interlayer. The effects of bonding time (1–30 min) and gap size (25–100 μm) were studied on the joint microstructure and its mechanical properties. Continuous centerline eutectic phases, characterized as nickel-rich and chromium-rich borides, were observed at the joints with incomplete isothermal solidification. The globular and acicular phases were seen at diffusion affected zone (DAZ). These phases could be nickel–chromium and cobalt–chromium borides. The time of complete isothermal solidification increased with increasing the gap size. This increase was consistent with the models based on the diffusion induced solid/liquid interface motion. A deviation of these models was observed for 75 and 100 μm gap size specimens. At complete isothermal solidification condition, the shear strength and the hardness of isothermal solidification zone decreased with increasing the gap size. Scanning electron microscopy (SEM) micrographs of shear fracture surfaces of the specimens with incomplete isothermal solidification showed secondary cracks through the brittle centerline eutectic constituents.