Microstructure evolution of single crystal superalloy DD5 joints brazed using AWS BNi-2 filler alloy

Nickel based single crystal superalloy DD5 was brazed with the AWS classification filler metal BNi- 2 at brazing temperature of 1323-1423 K for holding 10-240 min. The effect of the brazing temperature and brazing time on the microstructure of the brazed joint was investigated by scanning electron microscope (SEM) and electron probe microanalyser (EPMA). The result indicates that the joints exhibit sound bonding without any defect and crack. Three distinct regions can be identified in the joint: a diffusion affect zone composed of gamma-gamma' phase and fine M3B2 precipitates, an interfacial bonding zone solidified isothermally at the brazing temperature, and braze alloy zone formed by the solidification of surplus molten filler alloy during cooling. The braze alloy zone consists of gamma+gamma' eutectic phase, chromium boride, and ternary eutectic phase of gamma-nickel, nickel boride and nickel silicide. With extending brazing time, the thickness of interfacial bonding zone and diffusion affect zone increased while brittle compounds reduced. The bonding mechanism was discussed with reference to the microstructure evolution of the joint.