Freckle Formation and Freckle Criterion in Superalloy Castings

The evaluation of a numerical criterion to provide quantitative insight on freckling conditions is critical to the successful manufacture of large superalloy castings. Of the criteria reported in the literature, those based on the Rayleigh number seem best suited to predict the onset of freckle formation. However, in their current form, these criteria cannot explain why freckles develop predominantly at the surface of single crystal (SX) castings and at midradius in VAR/ESR ingots. An experimental Bridgman-type furnace has been built to directionally solidify freckle-prone superalloys, CMSX-11B, RENE88, NIM80A, WASPALOY, MAR-M247, and a variation of IN718 with high silicon content, at various angles to the vertical. Under typical industrial solidification conditions (thermal gradient between 500 and 4000 K m-1 (5 < G < 40°C cm-1) and solidification rate between 1.67 × 10-5 and 1.0 × 10-4 m s-1 (1 < R < 6mm min-1)), the results indicate a dependency of freckling on growth front angle likely related to the anisotropy in permeability. A modified Rayleigh criterion has been developed which accounts for directional permeability and orientation of the growth front relative to the gravity vector. Application to the experimental data shows good correlation with the onset of freckling for the range of solidification conditions examined in the study. The approximate threshold value for the modified Rayleigh number was estimated to be for CMSX-11B, 0.88, for RENE88, 0.90 for NIM80A, 0.85, for WASPALOY, 0.95, for MAR-M247, 0.86, and for IN718-Si, 0.65.