Effect of Homogenization on Metallurgical Structure of Nicrofer 5520 (IN-617) Superalloy Joints Diffusion-Brazed Using a Ni-Cr-Si-B Interlayer

This study aimed to homogenize diffusion-brazed Nicrofer 5520 superalloy joints to achieve the most similar microstructure to the base metal. In this regard, diffusion brazing treatment was carried out at a temperature of 1120 degrees C by using a 30-mu m-thick BNi-2 interlayer. After performing the diffusion brazing and completing the isothermal solidification (bonding time of 20 min), the specimens were homogenization post-bond heat treated under a vacuum atmosphere at 1180 degrees C and for different holding times of 4, 8, 10, 12, 14, 16, 18, and 20 h. Electron microscopy was used to investigate the microstructural evolutions and chemical composition of precipitated phases. The results revealed that by increasing the holding time, the distribution of elements in the width of the joint area became more uniform and the chemical composition became closer to the base metal. Simultaneously, the amount and volume fraction of precipitates in the diffusion-affected zone (DAZ) decreased. After 18 h in the homogenization treatment, the DAZ disappeared and only fine precipitates of nickel-rich silicide and carbide were retained at the boundary. After 20 h, the precipitates were dissolved, grains of the bond area began to grow, and the interlayer became significantly non-distinguishable from the adjacent base metal.

Automated summary

This study aimed to achieve a microstructure in diffusion-brazed Nicrofer 5520 superalloy joints that is most similar to the base metal. The results showed that increasing the holding time during the homogenization treatment led to a more uniform distribution of elements and a closer chemical composition to the base metal in the joint area. Additionally, the amount and volume fraction of precipitates in the diffusion-affected zone decreased with increasing holding time. After 18 hours of homogenization treatment, the diffusion-affected zone disappeared and only fine precipitates of nickel-rich silicide and carbide remained at the boundary.