Liquid Metal Embrittlement of Superalloys by Braze Materials

Fabrication of many advanced aerospace propulsion systems will require arc brazing as a secondary manufacturing process to seal interfaces between components such as nozzle tubes and manifolds and, when required, repairs to these assemblies to correct inadvertent damage. Five potential braze alloys were identified for use in arc braze operations. Experience demonstrates that liquid metal embrittlement can occur with some of these alloys during arc brazing. A thermo-mechanical study was conducted to screen both A-286 and JBK-75 for LME with these brazing alloys. Three different methods were used to determine LME susceptibility of these alloys. All employed standard 0.250 inch diameter Gleeble samples. In one approach the machined samples were nickel plated prior to coating with braze material. Half of the samples were maintained in the nickel plated condition to act as control samples. On the other half of the samples, braze alloy was pre-placed on the sample and the alloy melted in a tube furnace. In another approach, samples were wrapped with braze foil and the foil was melted in situ during testing. In the final test configuration, a cavity in the sample was filled with brazing alloy and heated to testing temperatures. Samples were tested at a variety of strain rates, and cooling rates. In parallel, Varestraint tests were conducted on several alloy /base metal pairings. Testing was performed in a hard tank evacuated prior to test and back filled with argon. The peak test temperature for both control and alloyed samples was approximately 20 degrees C above the braze alloy liquidus temperature. Only bars alloyed with Nioro and Palniro7 retained their original ductility. Samples alloyed with Nicoro and RI-49 showed almost complete embrittlement. The samples alloyed with RI-46 showed evidence of reduced ductility but were not fully embrittled in the range studied. Varestraint results were consistent.