BRAZING OF SINGLE CRYSTAL NICKEL WITH NI-B-SI FILLER METAL

Abstract

Brazing of single crystal nickel components and related phenomena were studied in the present work. Pure nickel single crystals and commercially available Ni-B-Si filler foil were adopted. The brazing were conducted at various temperatures under different external pressures. Brazing below the eutectic temperature ( 1093°C) of binary nickel-boron system and that above it showed totally different microstrnctural features and associated phenomena. Lining borides were observed to be present along joint faying surfaces when brazing was conducted below the eutectic temperature of binary nickel-boron system. The separation between the lining borides increased with the square root of brazing time. At the same time, the density and size of the lining borides decreased upon prolonged holding at brazing temperature. Stray grains were also encountered in the joint. When brazing was conducted above the eutectic temperature of binary nickel-boron system, lining eutectics occurred along the joint faying surfaces indicating the occurrence of incipient melting. By prolonged holding at brazing temperature, the lining eutectics evolved from continuous normal eutectics into circular lining borides and disappeared finally, giving a uniform joint free of any second phase or stray grains. The application of external pressure during brazing was found to hasten the microstructural evolution process, but its effects saturated at very low pressure (less than 0.5 MPa). It was also noted that brazing with several layers of filler foil showed no much difference in terms of microstructure evolution. The influence of process parameters was studied and compiled in the form of process maps. Comparison was made with the prediction by conventional Transient Liquid Phase (TLP) bonding theories. Based on the observations in the present investigation, modifications on conventional TLP bonding theories were attempted which gave a better estimation of the process.