Solidification Front Tilt Angle Effect on Potential Nucleation Sites for Freckling in the Remelt of Ni‐Base Superalloys

Abstract

By considering the mushy layer as a porous media with variable permeability, a Rayleigh number based freckling criterion was developed from the Flemings ratio between the magnitude of the interdendritic flow velocity and the solidification rate. The proposed form includes the effect of the tilt angle by preserving the anisotropic nature of the permeability tensor throughout the derivation and uses Poirier's experimentally determined functional forms for the parallel and perpendicular components. The proposed form of Rayleigh number criterion was found to provide better resolution when evaluated against available experimental data in the literature.Especially, it showed that the nucleation of channels in the mushy layer leading to freckles is equally probable in the proximity of the tips of the dendrites or deeper in the mushy layer, for example at approximately 0.7 liquid fraction and 0.4 liquid fraction respectively, depending on the angle of tilt of the solidification front.

Characteristics and Formation Tendency of Freckle Segregation in Electroslag Remelted Bearing Steel

“…According to Figure 5, if the Cr concentration is used, the lower limit of the liquid fraction for freckle formation is only 0.04 (almost complete solidification), which is, in fact, impossible. However, when the Si and Mn concentrations are used, the liquid fraction range for freckle formation can be estimated to be 0.27-0.49, which agrees, to some extent, with the reported liquid fractions of 0.3-0.5 [19], 0.3-0.6 [24,28], 0.4-0.6 [20,29,30], and 0.4-0.7 [31] for freckle formation in Ni-based superalloy.…”

“…Metals 2020, 10, 246 8 of 14 extent, with the reported liquid fractions of 0.3-0.5 [19], 0.3-0.6 [24,28], 0.4-0.6 [20,29,30], and 0.4-0.7 [31] for freckle formation in Ni-based superalloy. The modified Scheil module of the Thermo-Calc software was also used to estimate the variations of interdendritic liquid density, , and the density difference, Δ (= − , representing liquid density inversion), between the density at liquidus temperature, (calculated at the liquidus temperature and solute concentrations corresponding to the nominal compositions), and local liquid density, , at a certain temperature of T during solidification.…”

Solidification and Segregation Behaviors of Superalloy IN718 at a Slow Cooling Rate

“…Furthermore, large diameter nickel-based superalloy castings and ingots are widely accepted to have a marked tendency to form macro-segregation defects, such as freckles [16,17,18,19], which appear as long trails of equiaxed grains with compositions and microstructures different from the bulk alloy and constitute crack initiation sites, which cannot be eliminated by homogenization or thermomechanical treatments [20]. Freckles are highly undesirable in critical applications because they have a considerable adverse effect on mechanical performance [21].…”

“…Freckles are highly undesirable in critical applications because they have a considerable adverse effect on mechanical performance [21]. Castings and ingots containing freckles must be scrapped [16,21]. Therefore, understanding the formation trend of freckles is a major key toward the successful manufacture of large superalloy ingots [21]; thus, it is essential to evaluate quantitative insight on the freckling conditions at slow cooling rates.…”