A critical review of the microstructure characteristics of oxidation damaged nickel-based superalloy repaired by laser DED process

This paper provides a critical review of the repair of oxidation damage in nickel-based superalloys using laser direct energy deposition (L-DED) technique. The L-DED technique is characterized by its ability to produce epitaxial growth crystal structure, low dilution repair joints, and low dilution repair joints with low heat input, thereby reducing the formation of stray grains. The relationship between oxidation behaviour and microstructure is critical for ensuring oxidation resistance. Microstructural characteristics such as high volume of directional solidified crystal structure and γ’ precipitate are beneficial for oxidation resistance. However, the presence of stray grains in the repair zone including heat affected zone (HAZ) and deposition zone, reduces the oxidation resistance of a repaired nickel-based superalloy. Low heat input is the process characteristic that produces fine γ’ precipitates with high volume. High laser power and bidirectional scanning strategy are the L-DED parameters that facilitate the formation of epitaxial growth columnar structure. Low thermal gradients and element segregation lead to stray grain formation at the fusion boundary and HAZ. Future research focused on understand the relationship between microstructure features and oxidation behaviour at HAZ is crucial for the integrity of nickel-based superalloy repair joints.