Single-crystal turbine blades made of nickel-based superalloys, such as CMSX-4, are of great importance for gas turbines due to their excellent high-temperature properties, however, they are subject to various damage mechanisms. Repairing these costly components is desirable for both ecological and economic reasons; however, it poses a challenge as the single-crystal microstructure in the repair area must be restored. This study, conducted as part of Transfer Project 5 within Collaborative Research Center 103, investigates the thermal spray processes Cold Gas Spraying (CGS), Vacuum Plasma Spraying (VPS), and High Velocity Air Fuel Spraying (HVAF), in combination with directional recrystallization, as potential repair methods for single-crystal CMSX- 4 turbine blades. Thermal spray processes show great potential for this application due to their low oxidation and high density. Differences in the process conditions lead to varying coating properties. CGS and VPS achieve the lowest values for porosity and oxygen. HVAF coatings have the highest values for oxygen content and porosity. The residual stress state of the different coatings is also investigated. In the coatings that were produced using CGS and HVAF, compressive residual stresses can be determined, while the VPS coatings exhibit tensile stresses up to a stress-free state. The analysis of the adhesive tensile strength shows that HVAF coatings with the highest compressive stresses fail the earliest. Directional recrystallization to set a columnar microstructure of the repair coatings shows clear differences in the resulting microstructures, with the VPS coating having the most pronounced columnar structure, while the CGS coating develops predominantly equiaxed grains. The differences can mainly be attributed to the different initial microstructures, whereby the presence of the γ’ phase and the degree of work hardening are decisive influencing factors. Despite the process-related limitations of the experimental setup, the fundamental feasibility of columnar grain growth has been demonstrated, paving the way for future optimizations with improved process control.
Maike Létang (Wed,) studied this question.