Effect of diffusion brazing time on microstructure, isothermal solidification completion and microhardness distribution during joining of Nicrofer 5520 superalloy using a liquated Ni–Cr–B interlayer

Abstract

In the present study, the effect of time on the microstructural evolution, distribution of elements, type of precipitated phases, and microhardness of diffusion brazed Nicrofer 5520 superalloy using a BNi-9 (Ni–15%Cr–3.7%B) interlayer was investigated. The diffusion brazing was carried out at 1120 °C for seven different holding times by using a 38 μm thick interlayer. The results indicated that the holding time of 30 min was sufficient for complete isothermal solidification. The eutectic gamma solid solution, Ni-rich borides, and Cr–Mo-rich borides were identified near the bond center before the completion of isothermal solidification, (Ni, Mo, Cr)-rich borides and Cr–Ni–Mo–Fe carbides with different morphologies were found in the diffusion affected zone (DAZ). In the holding time longer than 30 min and, the volume fraction of the precipitated phases in the DAZ decreased, and they transformed into rounder particles. Accordingly, the chemical composition of the interlayer became more homogeneous, resulting in a lower DAZ phases’ density that is happened over the higher brazing times. As the holding time was increased, the chemical composition in the interlayer became more similar to the base metal, and the microhardness in the bond zone and its surrounding became more uniform.