Transient Liquid Phase Bonding of Cobalt‐Based Single Crystal Superalloys: Study on the Precipitation Behavior of Borides in the Diffusion‐Affected Zone

This study investigates the precipitation behavior of needle‐like borides in the diffusion‐affected zone (DAZ) during transient liquid phase (TLP) bonding of DD688 single‐crystal cobalt‐based superalloy. The effects of matrix orientation and bonding temperature on boride precipitation are systematically analyzed. The results indicate that M₂B‐type borides preferentially grow along the [001] direction at a bonding temperature of 1220 °C. Increasing the matrix orientation deviation from 0° to 45° reduces the number of preferentially grown borides and leads to a dense distribution of small borides near the isothermal solidification zone/DAZ interface. Higher bonding temperatures enhance the preferential growth of borides and reduce the density of small borides near the interface. At 1280 °C, significant morphological changes occur in the DAZ borides, including partial liquation of the matrix. These findings provide insights for optimizing TLP bonding and postweld heat treatment processes to improve the microstructure and mechanical properties of brazed joints in cobalt‐based superalloys.