Spreading and solidification behavior of nickel wide-gap brazes

Abstract

Gas turbine components made of nickel-based superalloys experience cracking after service in extreme environments. As these cracks can be wide, brazing or fusion welding is typically used to repair them. Properly designed and applied, brazing filler metal will help extend the useful life of damaged turbine components. Commercially available filler metals containing boron and silicon, including BNi-2, BNi-5, and BNi-9, were mixed at varying ratios with MARM247 to create wide-gap filler alloys. Spreading was characterized using droplets placed on a stainless steel substrate. The solidification behavior of the wide-gap braze alloys was determined through metallographic analysis of butt joint specimens with a gap of 2 mm created using René 108™ as the substrate. Mechanical properties were determined through four-point bend tests and were compared to the metallographic analysis results. Boron was found to allow the spreading of the alloys at lower temperatures in comparison to the spreading of the silicon-based alloys, but the silicon-based alloys spread over a larger area. It was found that silicon promotes the growth of a solid nickel solution during isothermal solidification, as a result of the large solid solubility limit in the Ni-Si binary system. Increased presence of this isothermally solidified nickel caused a proportional increase in the joint strength. The boron-based alloys proved to promote less isothermal solidification than the silicon resulting in weaker braze joints. Braze alloys combining both silicon-containing alloys and boron proved to retain the advantages of both melting point suppressants with few disadvantages.