Analysis of Infiltration and Solidification of Brazing Materials Into Porous Hastelloy X Feltmetal Using Differential Scanning Calorimetry

Abstract

Abstract The diffusional solidification of high-temperature brazing compound interacting with open-pore metallic Feltmetal materials were investigated using simultaneous differential scanning calorimetry analysis with thermogravimetric analysis (DSC-TGA). Feltmetal is an abradable seal material primarily used to sustain pressure differentials between stages gas turbines. It is a sintered particle material with an open-pore structure and high total pore volume, typically 80%. Because Feltmetal is commonly brazed into the turbine stage directly to the outer shroud, the effect of the braze layer on the pore structure and on the properties of the seal material is important to the seal function. The characteristics of non-vacuum brazed Feltmetal is of particular interest in this work, as we explore processes for overhaul and repair of Feltmetal abradable seals while in-field and away from a particular service center. The ability to repair these seals in-situ, could provide a faster and less expensive route for an engine back into service. The DSC-TGA responses of a Ni-based Feltmetal material and a Fe-based Feltmetal material with one braze material AMS 4777 were examined. The DSC-TGA data to 1200°C were used to identify elemental diffusion, eutectic melting, and high temperature oxidation reactions. The target microstructure consists of a porous matrix of Feltmetal fibers where fibers and pores adjacent to the braze have reacted with the braze alloy to make a metallurgical bond. This research demonstrates that thermal analysis methods can indicate key process reactions, such as onset of oxidation and eutectic phase transformation, during atmospheric brazing process development. The results of this work are used to aid in the development of in-field seal installation where full system disassembly, and vacuum brazing, is not practical.