Using Simultaneous Differential Scanning Calorimetry and Thermogravimetric Analysis to Develop Braze Processes for Porous FELTMETAL™ Abradable Seal Material

The diffusional solidification of high-temperature brazing compounds interacting with open-pore metallic FELTMETAL™ (FM) materials was investigated using simultaneous differential scanning calorimetry analysis and thermogravimetric analysis (DSC-TGA). As a sintered particle material with an open-pore structure and high total pore volume, FM is an effective abradable seal material primarily used to sustain pressure differentials between stages in propulsion and power generation turbines. FM is commonly vacuum brazed into the turbine stage directly to the outer shroud or backing ring and the braze layer affects the pore structure and properties of this abradable. To facilitate the overhaul and repair of FM abradable seals while in-field and away from a particular service center, investigation into factors affecting non-vacuum brazing processes to bond FM abradables was done using simultaneous DSC-TGA and metallography. Previous work indicated that oxidation of the alloys can begin below the solidus of the brazing compound under a cover gas but that the presence of boron in the brazing alloy can increase the oxidation onset temperature, thus reducing the extent of oxidation. In this work, further investigation was done by including the factors of 1) type of cover gas, 2) heating rate, and 3) type of braze compound. This research demonstrates that simultaneous DSC-TGA can be used to determine which of these factors most facilitate non-vacuum brazing, specifically by minimizing oxidation, achieving sufficient eutectic phase transformation, and forming a diffusional bond. Further, metallography confirmed the diffusional solidification required for bonding the abradable material and extent of oxidation indicated by the DSC-TGA results.