Liquid Density Inversions During the Solidification of Superalloys and Their Relationship to Freckle Formation in Castings

Auburtin, P.; Cockcroft, S. L.; Mitchell, A.

doi:10.7449/1996/superalloys_1996_443_450

Cited by 20 publications

(29 citation statements)

References 19 publications

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“…(8)). This seems to be in conflict with measurements of the freckle composition in alloy 718 implying that freckles are fed by a melt originating from a region in the mushy zone with a fraction liquid of about 0.4-0.6 [30,43]. However, it can be assumed that, even if freckles are initiated at considerably higher liquid fractions, they will erode the dendritic network, locally increase permeability, and thus penetrate downward deeper into the mushy zone.…”

Section: Discussioncontrasting

confidence: 44%

“…Similar to buoyancy driven freckles, these defects appear as dark spots in the cross-section, preferentially at mid-radius of the ingot. The freckle regions reveal a composition as observed in the strongly enriched interdendritic regions [30][31][32] and frequently consist of a chain of small equiaxed grains [31,33].…”

Section: Introductionmentioning

confidence: 99%

“…Despite of these findings, Rayleigh number based criteria are still used to describe freckle initiation under those conditions [12,23,24,27,30,31], although the downhill flow of the heavy melt can be expected to be instantaneous, not requiring any initiation.…”

Section: Introductionmentioning

confidence: 99%

“…The lack of density inversion for alloy 718 was either ignored [20,31] or rejected, pointing out that the high silicon contents of older grades of alloy 718 could have led to a decreasing density of the melt during solidification [12,27,30].…”

Section: Introductionmentioning

confidence: 99%

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Development and application of a new freckle criterion for technical remelting processes

Böttger

Schmitz

Wahlers

et al. 2014

MATEC Web of Conferences

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Abstract. In technical solidification processes like Electro Slag (ESR) or Vacuum Arc Remelting (VAR), freckles present a serious type of defects which limit the maximum ingot size for many grades of steels and superalloys. Therefore, modelling of freckle formation is an important task for optimizing industrial remelting processes. Practically all present freckle models are based on a critical Rayleigh number. They are inspired by the "classical" assumption that freckles are caused by an inversion of the liquid density in the semisolid region. Plumes of the lighter segregated liquid evolve through perturbation of the metastable layering of the melt in the mushy zone, a mechanism which motivates its description via Rayleigh criteria. But these models are not suitable for materials like Alloy 718 which do not show a liquid density inversion, but nevertheless are prone to freckle formation in technical remelting processes. In this paper, a criterion is developed which -instead of using a Rayleigh number -is based on the evaluation of the non-isothermal component of an instantaneous down-hill flow of heavy segregated melt in a melt pool with axial symmetry. With knowledge of the exact pool geometry and the shape and properties of the mushy zone, the occurrence of freckles can be predicted. The model is applied to a technical ESR casting for which temperature fields and microstructural parameters have been obtained using CFD and 3D phase-field modelling, respectively. Furthermore, the implications are discussed which the new model offers for the understanding of freckles in technical remelting processes.

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Section: Discussioncontrasting

confidence: 44%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Development and application of a new freckle criterion for technical remelting processes

Böttger

Schmitz

Wahlers

et al. 2014

MATEC Web of Conferences

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“…[19,20] The proposed mechanism for freckling in VAR ingots has been conceived mostly based on the observed morphology and characterization of freckles in solidified ingots. Auburtin et al [21] used the freckles composition determined by scanning electron microscopy/energydispersive spectroscopy and compared it with the segregation profiles to conclude that the freckles formed from a mush region between 0.4 to 0.6 solid fractions, which corresponded to the location of the isotherm 20 K (20°C) below liquidus.…”

Section: Density Inversionmentioning

confidence: 99%

On the Formulation of a Freckling Criterion for Ni-Based Superalloy Vacuum Arc Remelting Ingots

Valdés

King

Liu

2010

Metall Mater Trans A

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A criterion for freckling prediction that includes the effect of a tilted solidification front was proposed and evaluated with experimental data available in the literature. The criterion is based on the maximum local Rayleigh number in the mush layer and was developed using Flemings' criterion and assuming that the interdendritic liquid flow is governed by the Darcy law. The proposed form preserves the anisotropic nature of the permeability tensor throughout the derivation and provides improved resolution on freckle prediction. A clear separation between the freckled and nonfreckled experiments was obtained for all compositions. The effect of the tilted solidification front over the freckling potential was corroborated, and the results suggested that the directionality of permeability affects the location within the mush layer of the potential nucleation sites for the channels leading to freckles. A threshold zone was determined from the enclosing experiments data, and the range contained one of the proposed critical values for superalloys, which previously was developed by a completely different method.

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

Valdés

Liu

King

et al. 2010

Superalloy 718 and Derivatives

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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.

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Liquid Density Inversions During the Solidification of Superalloys and Their Relationship to Freckle Formation in Castings

Cited by 20 publications

References 19 publications

Development and application of a new freckle criterion for technical remelting processes

Development and application of a new freckle criterion for technical remelting processes

On the Formulation of a Freckling Criterion for Ni-Based Superalloy Vacuum Arc Remelting Ingots

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

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