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Emms, P. W.

doi:10.1023/a:1004335317650

Cited by 15 publications

(3 citation statements)

References 30 publications

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“…The first mode called boundary layer mode corresponds to the double diffusive fingers on the interface mush-bulk liquid, which is understood to leave the fluid in the mush stagnant, while the second convection mode denominated mush layer mode happens within the mush and causes perturbations in the solid fraction leading to formation of chimneys. 12,13 This hypothesis of nucleation and growth of channels inside the mush layer has also been supported by theoretical and experimental results. 14,15,16 When segregation of the interdendritic liquid builds a negative density gradient in the mush layer, it will be gravitationally stabilizing and the interdendritic liquid will be stratified with no density inversion observed.…”

Section: Nucleation Sites and Freckle Formation Mechanismmentioning

confidence: 58%

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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Section: Nucleation Sites and Freckle Formation Mechanismmentioning

confidence: 58%

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

View full text Add to dashboard Cite

show abstract

“…The first mode, called ''boundary layer mode,'' corresponds to the mentioned double diffusive fingers on the interface mush-bulk liquid, that leaves the fluid in the mush stagnant, whereas the second convection mode denominated ''mush layer mode,'' happens within the mush and causes perturbations in the solid fraction that lead to the formation of chimneys. [8,14,15] The hypothesis of nucleation of the channels inside the mush layer also is supported by theoretical and experimental results. [16][17][18] 2.…”

Section: Density Inversionmentioning

confidence: 67%

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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“…The first mode called boundary layer mode corresponds to the mentioned double diffusive fingers on the interface mush-bulk liquid, which leaves the fluid in the mush stagnant, whereas the second convection mode denominated mush layer mode happens within the mush and causes perturbations in the solid fraction that lead to the formation of chimneys. 59,64,65 The hypothesis of nucleation of the channels inside the mush layer also is supported by theoretical and experimental results. 66,67,68 Experiments supporting the mechanism of freckles nucleation at the tips of the dendrites region are based on induced shear movement of the bulk liquid at the boundary layer vicinity.…”

Section: Density Inversionmentioning

confidence: 67%

Preventing solidification defects in large superalloy castings used in advanced electric power systems

Ortiz¹,

Antonio²

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The need for clean and affordable power has resulted in great demand for high-efficiency gas turbine power systems. To improve energy generation efficiency, and lower CO 2 emissions, the new generation of land based gas turbines needs to be larger and operated at a higher temperature. Consequently, it is required the use of Nb-containing superalloys such as Inconel 718 for the rotor disk, to avoid overaging of the microstructure during the forging process, but its applicability is being limited by difficulties in the reliable production of large Vacuum Arc Remelting (VAR) ingots (33 to ~35 in. diameter) without macrosegregation defects such as freckles and center segregation. These defects impair the mechanical properties of the material and are believed to be caused by convection in the mush layer. Because they cannot be corrected by subsequent treatment, the affected ingots should be scrapped highly increasing the production costs.

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Untitled

Cited by 15 publications

References 30 publications

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

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

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

Preventing solidification defects in large superalloy castings used in advanced electric power systems

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