Repair of Ni-based single-crystal superalloys using vacuum plasma spray
T. Kalfhaus, M. Schneider, B. Ruttert, D. Sebold, T. Hammerschmidt, J. Frenzel, R. Drautz, W. Theisen, G. Eggeler, O. Guillon, R. Vaßen.
Materials and Design, 168, 107656, (2019)
Turbine blades in aviation engines and land based gas-turbines are exposed to extreme environments. They suffer damage accumulation associated with creep, oxidation and fatigue loading. Therefore, advanced repair methods are of special interest for the gas-turbine industry. In this study, CMSX-4 powder is sprayed by Vacuum Plasma Spray (VPS) on single-crystalline substrates with similar compositions. The influence of the substrate temperature is investigated altering the temperature of the heating stage between 850 °C to 1000 °C. Different spray parameters were explored to identify their influence on the microstructure. Hot isostatic pressing (HIP) featuring fast quenching rates was used to minimize porosity and to allow for well-defined heat-treatments of the coatings. The microstructure was analysed by orientation imaging scanning electron microscopy (SEM), using electron backscatter diffraction (EBSD). The effects of different processing parameters were analysed regarding their influence on porosity and grain size. The results show that optimized HIP heat-treatments can lead to dense coatings with optimum γ/γ′ microstructure. The interface between the coating and the substrate is oxide free and shows good mechanical integrity. The formation of fine crystalline regions as a result of fast cooling was observed at the single-crystal surface, which resulted in grain growth during heat-treatment in orientations determined by the crystallography of the substrate.
Keyword(s): superalloys; vacuum plasma spray; TCP phases; structure maps