Impacts of diffusion brazing time on mechanical properties of Nicrofer 5520 superalloy joint by Ni-15Cr-3.7B filler metal

The microstructure and shear strength during diffusion brazing of Nicrofer 5520 superalloy were studied. The bonding temperature and filler metal were 1120 °C and 38 μm thick Ni-15%Cr-3.7%B, respectively. Seven holding times from 5 to 240 min were used. The findings revealed that full isothermal solidification occurs after the holding time of 30 min. The presence of eutectic phases in the interlayer and non-dissolved precipitates in the diffusion-affected zone (DAZ) cause a decrease in the shear strength. With the increase in the holding time after the isothermal solidification, the volume fraction of the precipitation in the DAZ decreases, and the precipitations transform into rounder particles. Consequently, the chemical composition becomes more homogeneous, and a lower precipitation volume fraction forms in the DAZ. The failure path shifts from the athermal solidification zone (ASZ) to the base metal, and shear strength is enhanced to 593 MPa, by extending the bonding time to 240 min. Through the increase in the holding time, the percentage of interlayer elements grows increasingly close to the chemical composition of the base metal, and the mechanical characteristics in the bond area and environs areas become more uniform.