Insight into the sensitivities of freckles in the directional solidification of single-crystal turbine blades

Freckle, a thermal-solutal-fluid flow induced metallurgical defect, can degrade the mechanical performance of the investment cast nickel-based single-crystal turbine aerofoil. Channel segregation, appearing as channel-liked solute enrichment pattern, is generally considered to be a representative phenomenon in the formation of freckles. The geometrical effect of the intricate curve turbine blade with relevant processing conditions into the freckle mechanism is still unprecedented. In this work, we proposed the Eulerian multiphase thermal-fluid flow model for predicting the freckles (channel segregation) in the representative single crystal turbine blades. The prediction which captures the complex transport phenomena of thermal-solutal convection, solute plumes and segregation profile is in good agreement with the available experimentation. Based upon the thermal-solutal-fluid flow rationalisation of the solidification process, mechanisms proposed provide better understanding of the channel segregation sensitivities to the cooling conditions and geometrical factors. Sensitivity studies verify the mechanisms and give further implication on preventing and even eliminating the freckles.