Freckle Formation in Superalloys

Abstract

The analysis of the formation mechanisms for freckling and for random gram formation in single crystal castings has resulted in a general understanding of the principles involved, but has not been tested extensively against results in actual superalloy castings. In this work, we present the results of experimental work on the production of freckling in superalloy castings, made directionally in a Bridgemau furnace capable of producing DS castings in which the directional axis can be rotated with respect to gravity.The results indicate that we may account quantitatively for freckle formation through an analysis based on fluid-flow as described by a modified Rayleigh criterion. The theoretical critical condition for freckle initiation is found to be a Rayleigh Number of unity; the experimental determination of this critical value is found to be in the region of 0.7 -0.9 depending on the alloy examined. We ascribe the difference in these values to the secondary features of the dendrite morphology. The relation to random grain nucleation is found to lie in the role of isotherm curvature in the solidifying region. This aspect is examined by the use of ProCAST computations in both castings and remelt ingots. morphology may have a large influence on the numerical value of Ra, and therefore on the development of freckling in a casting. Thirdly, the common industrial solution to freckling in directional castings, that of increasing the solidification temperature gradient, is shown to be reasonable in that its primary influence is on the dendrite spacing, but it is also seen to have clear limitations in the case of large thermal sections as would, for example, exist in large castings. Finally, the role of casting geometry and imperfect isotherm control during solidification is not at present part of the analysis and should be included for a full description of the phenomenon.

A castability model based on elemental solid-liquid partitioning in advanced nickel-base single-crystal superalloys

“…[4][5][6][7][8][9] Several models have been proposed to predict freckle formation during single-crystal solidification. [10,11,12] These have typically been based on the Rayleigh number (Eq. [1]), which gives a ratio of the buoyancy forces acting on a liquid (numerator) to the frictional forces that oppose it (denominator).…”

Nickel-Based Superalloys for Advanced Turbine Engines: Chemistry, Microstructure and Properties

“…Highly enriched with solute, freckle chains are compositionally different from the bulk alloy and form as a result of thermosolutal convection. 43 In a process common to many multicomponent superalloys, as soluteaccumulates within the mushy zone during dendritic solidification, the subsequent density imbalance between the solute and bulk liquid serves as a driving force for the onset of convective fluid flow. Upon cooling, the solute-enriched convective instabilities solidify as isolated regions of equiaxed grains.…”

“…36,38,43,44 The Rayleigh number Ra s is a measure of the ratio of the buoyancy force to the retarding frictional force in the mushy zone:…”

Modeling of Vacuum Arc Remelting of Alloy 718 Ingots

“…Over the years, different criteria for freckle formation in the VAR process have been proposed [27,28]. These criteria are essentially based on a critical mushy zone size and thus a maximum local solidification time beyond which freckling occurs in the ingot.…”