Subproject B1 Near Net Shape Turbine Blade Repair

The repair of components in the aviation industry and power plant technology plays an increasingly important role in view of rising raw material and manufacturing costs. Besides welding, high temperature vacuum repair brazing is already established for turbine blades made of nickel-base alloys. The repair brazing of turbine blades involves complex process steps. The worn turbine blade is stripped down to the base material. The filler metal is applied manually in form of pastes, melt spun foils or tapes, which are also nickel-base alloys. After brazing in a high vacuum, the excess filler metal is removed by grinding or milling. Subsequently, the hot gas corrosion protective layer (e.g., NiCoCrAlY) is applied by thermal spraying. After the coating, the turbine blade is subjected to an aluminizing process. NiAl (β-phase) is formed which further increases the hot gas corrosion resistance. The aluminizing process is usually carried out in a pack cementation or in a special aluminizing furnace with a reactive atmosphere, respectively. The state of the art is that a turbine blade can be subjected to approximately 3 to 4 repair cycles.

In this subproject, a repair coating is applied to the turbine blade by thermal spraying, followed by a combined brazing and aluminizing process. This significantly shortens the current repair brazing process chain, resulting in mechanical and technological improvements (e.g., increasing the number of repair cycles to 6-7) and, at the same time, economic benefits of regenerated turbine blades.

Coating of a turbine blade by atmospheric plasma spraying (APS).

In the third funding period, the hybrid process developed in the first two funding periods will be further developed with the results obtained, in view of the regeneration sequence within and between the process cells of the Collaborative Research Centre. From the knowledge gained during the second funding period, the complete repair brazing process (coating and brazing/aluminizing) with regard to the functionality of the entire engine is investigated with regard to the reproducibility and the resulting process reliability. The sensitivity of the process parameters (coating and brazing/aluminizing parameters) on the microstructure after the brazing/aluminizing process by instrumental analysis (EDX and X-ray microscopy) is carried out by design of experiments (DOE). From these results, a statement can be made about the process reliability. These results will be used to extend the brazing/aluminizing process to other aviation related materials such as the single-crystal CMSX-4 alloy, where it is desired to produce a directionally solidified brazed seam. Additionally, the results obtained that the previously discontinuous brazing/aluminizing process in a vacuum furnace can be transferred into a continuous process by a continuous shielding gas furnace whereby the thermal barrier coating is integrated in the hybrid process. The results further shorten of the process chain, which are essential for the selection process with regard to the regeneration sequence within the CRC. The developed hybrid technology can also be transferred to stationary gas turbines in power plants, rolling bearings from wind turbines, rolls and cylinders from printing presses as well as cylinder liners and transmission components from diesel engines.