Effect of bonding time on microstructure and mechanical properties of diffusion brazed IN-939

The applicability of the diffusion brazing technique for joining IN-939 superalloy using a Ni-Cr-B interlayer was assessed. The phases formed during the brazing process in the isothermally solidified zone (ISZ), athermally solidified zone (ASZ) and diffusion affected zone (DAZ) were studied. An inverse relationship between the bonding time and the width of the ASZ was perceived. The analytical model predicted the holding time of ∼36 min for completion of the isothermal solidification. This was corroborated by the experimental observations. The solid-state diffusion found to be a more effective transportation phenomenon than the base metal dissolution in increasing the concentration of solute elements in the ISZ. The formation of Cr-rich boride precipitates in DAZ was determined as the main contributing factor to the high microhardness values in this region. The shear strength of the joints raised by increasing the bonding time and reached the value of 655 MPa in the bonding time of 60 min implying a remarkable joint efficiency of ∼85%. Chromium was detected as the most influential element on the shear strength of the joints. Dimple-like and semi-cleavage morphologies were observed in the fracture surfaces of the bonds with complete and partial isothermal solidification, respectively.