Freckle formation and freckle criterion in superalloy castings

Abstract

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, RENÉ 88, 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 Ϫ1 (5 Ͻ G Ͻ 40 ЊC cm Ϫ1 ) and solidification rate between 1.67 ϫ 10 Ϫ5 and 1.0 ϫ 10 Ϫ4 m s Ϫ1 (1 Ͻ R Ͻ 6mm min Ϫ1 )), 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 RENÉ 88, 0.90, for NIM80A, 0.85, for WASPALOY, 0.95, for MAR-M247, 0.86, and for IN718-Si, 0.65.

Freckle Formation in Superalloys

“…Using the modified Rayliegh Number as developed by Auburtin et al (8) H I Table 2. It is seen from this analysis that whilst the threshold value can be used to predict areas in a casting which would be sensitive to freckle formation, there remains an alloy dependence which is not covered by parameters contained in the Rayleigh Number as formulated above.…”

“…Estimations of the flow velocity in a freckle channel (8) lead to values at which it is feasible to envisage solid particles persisting against the dissolution process until they reach a position at the liquidus where they would melt. If the undercooling at that position is sufticient, such particles could, therefore nucleate fresh grains.…”

“…The hypothesis is supported by the presence of both grain fragments and random grams in experimental castings which contain freckles. The second (8) links the two phenomena by the presence of isotherm curvature in the solidification region of the casting. The curvature, as shown below, provides au additional factor to the bouyancy driving force for freckle formation.…”

Effect of Cooling Rate on Microstructural Development in Alloy 718

“…It should be noted that the the square of the secondary dendrite arm spacing. The effectiveness of a homogenization treatment is expressed in terms of a residual segregation index, 6 1171. In the as cast condition, 6 is unity, and after homogenization, 6 approaches 0.…”

Microstructure Development during Controlled Directional Solidification in Alloy 718

“…In all cases, the exponent of the solidification time is greater than one third, which is typically considered as the nominal value for such correlations. These relationships are very valuable in predicting the likelihood of defects in VAR and ESR using the Rayleigh number based criteria, which relies on the both the secondary and primary dendrite arm spacing to determine the permeability in the mushy zone [4]. These days, process models for VIM, ESR and VAR are mature enough to predict trends during ingot solidification.…”