Identification and Evaluation of Freckles in Directionally Solidified Casting Made of PWA 1426 Nickel-Based Superalloy
Manufacturing of modern aero engine turbine blades made of nickel-based superalloys is very complex and expensive. The thrust and performance of new engines must address constantly more demanding requirements. Therefore, turbine blades must be characterised by very good mechanical properties, which is possible only if the blades are free of casting defects. An important innovation has been the launching of directionally solidified (DS) and single crystal (SX) turbine blades. But, manufacturing procedures and the chemical composition of many superalloys promote the formation of casting defects that are characteristic only for directional solidification. One of these defects is freckles. Freckles are small equiaxed grains in the form of long chains parallel to the solidification direction and are located on the surface of the casting. Freckles decrease the mechanical properties of DS and SX blades; therefore, they should be always unambiguously identified to improve the manufacturing process. This work presents the possibilities of identifying and evaluating freckles in DS casting made of PWA 1426 superalloy by combining the scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and electron backscatter diffraction (EBSD) techniques.