Single-Crystal Superalloy Joining

A new process route of diffusion brazing has been identified for joining Ni-base single-crystal superalloys. This process was developed through an understanding of microstructure evolution observed during conventional diffusion brazing of a single-crystal superalloy Rene N5 with interlayer materials containing B as a melting point depressant. It was found that the interlayer does not fully melt during high temperature joining heat treatment due to the fast diffusion of B from the interlayer into the substrate, and that grain coarsening, instead of epitaxial growth, is responsible for producing joints free of bond line. A new two-step joining heat treatment process route was established to effectively use grain coarsening and minimize the formation of the grain defects. Diffusion calculations were applied to predict the distribution of B and thus avoid incipient melting during the heat treatments. The Rene N5 joined by the new process demonstrated high temperature tensile properties equivalent to those of the single-crystal Rene N5, and the joined specimens did not fail at the joints.