High-Temperature Oxidation Behavior of High-Aluminum (Co,Ni)-Based Superalloys for Friction Stir Welding (FSW) Tools

The present investigation focuses on the thermal oxidation of two, Nb- and Ta-alloyed (Ni,Co)-based superalloys having in their microstructures high contents of hard phases and considered potential candidates as a tool for friction stir welding process. The alloys were produced by investment casting using high-purity elements in induction furnace under vacuum atmosphere. The alloys were oxidized pseudo-isothermally up to 1000 h in muffle furnace under laboratory air, and the mass change was monitored manually using an analytical balance. External oxidation products growing on the surface of the oxidized samples were mainly Cr2O3 and Al2O3. At 800 and 900 °C, internal oxidation products were also observed and consist of Al2O3 and AlN. From a kinetic point of view, both superalloys presented the same behavior at 800 and 900 °C with kp values typical of alumina formers. However, Ta-alloyed material exhibited superior oxidation resistance at 1000 °C when compared to the alloy containing Nb.