Turbine blades must withstand severe loading conditions and damage can occur during operation due to heat, pressure, foreign objects and hot gas corrosion, despite the protective coatings applied onto the turbine blades. Instead of replacing the damaged components, maintenance, repair and overhaul are key to extend the total service life. Besides welding, the repair of turbine blades by brazing is an established repair process in the industry and involves many individual steps that often require a high degree of manual work. In the present study, a hybrid joining and coating technology was developed to shorten the state-of-the-art process chain for repairing turbine blades. With this approach, a repair coating, which consists of a filler metal, a hot gas corrosion protective layer and an aluminum top layer, is applied by atmospheric plasma spraying. The coated turbine blade then undergoes a heat-treatment so that a brazing and aluminizing process is carried out simultaneously. Due to diffusion and segregation processes, pores can occur in the heat-treated coating. In the present study, a full factorial design of experiment was performed to reduce the pores in the coating. The microstructure of the repair coating was investigated by optical- and scanning electron microscopy (SEM), and the impact of the process parameters on the resulting microstructure is discussed.

Brazing is an established repair technology for turbine blades (air foils and vanes) in the industry and includes multiple process steps that may also require a high degree of manual work. In this study, the development of a near net shape joining and coating hybrid technology for the repair of turbine blades is presented. With this technology, it is possible to shorten the current state-of-the-art process chain for repairing turbine blades. The worn turbine blade receives a repair coating applied by thermal spraying consisting of a nickel based filler metal, a hot gas corrosion protective layer, a bond coat and finally a thermal barrier coating. Subsequently the coated turbine blade is heat treated and a simultaneous brazing and aluminizing process is carried out. The technology presented here brings about technical and economic advantages and allows to shorten the state-of-the-art process chain for repairing turbine blades.

The continuous improvement of high potential repair technologies is an essential driver for cost-effective and sustainable aviation. To withstand extreme forces and temperatures, the turbine section of aircraft engines is made from Nickel-based superalloys. This class of materials routinely develops cracks and geometrical deviations in service, which require a brazing repair, thus making brazing an indispensable key technology. This study seeks to improve brazing properties on Nickel-based superalloys for aircraft applications. Facilitated by materials simulation, a novel alloying strategy is outlined to design the braze alloy chemistry. The design criteria are established on the optimization of major microstructural properties influencing the mechanical properties known for repair brazed joints. The proposed design criteria were applied to the development of two new braze alloys. The new alloy design approach is validated experimentally by using the developed alloys. Mechanical properties of brazed samples made from Nickel-based superalloys were investigated at a service equivalent temperature of 871 °C. Results indicate that the ultimate tensile strength at 871 °C is 20.5% higher than that published for legacy braze alloys. This study provides a basis for the development of repair technologies applicable to further superalloys.