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Journal ArticleDOI

Study of Freckles Formation During Directional Solidification Under the Influence of Single-Phase and Multiphase Convection

01 Jun 2013-Journal of Thermal Science and Engineering Applications (American Society of Mechanical Engineers)-Vol. 5, Iss: 2, pp 021004

AboutThis article is published in Journal of Thermal Science and Engineering Applications.The article was published on 2013-06-01. It has received 8 citation(s) till now. The article focuses on the topic(s): Directional solidification.

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Citations
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Journal ArticleDOI
Abstract: A 3-D model of freckle (solute channel) formation at a microstructural level was coupled with in situ X-ray radiography to investigate the mechanisms of freckle initiation and growth. The model predictions for solute partitioning, diffusion and convection were validated via in situ X-ray radiographic measurements in Ga–25 wt.% In alloy, showing good agreement. Other key features, such as freckle channel width and critical Rayleigh number, also correlated well. The validated model was used to investigate freckle formation under a range of solidification conditions. Two distinct stages of freckle onset were observed, identified via the dendrite tip growth and solute profiles. The first stage corresponds to lower flow velocities with large fluctuations; in the second stage the velocities stabilize, with established recirculating flows forming solute channels. The influence of imperfections in dendritic morphology, such as grain boundaries and primary spacing variations, on the critical Rayleigh number was studied. It was found that that these features initiate freckles. Non-intuitively, converging grain boundaries were observed to have the greatest propensity for freckle formation. The resulting new insights on solute plume formation impact a range of phenomena from single-crystal superalloys to magma flows.

76 citations

Journal ArticleDOI
Abstract: Freckle formation during directional solidification of binary alloy is a well-researched subject area. However, the influence of shrinkage induced flow (SIF) on freckling phenomena is barely reported. The focus of this work is to investigate this effect during bottom-up solidification of binary alloys. A fixed grid-based numerical scheme involving volume averaging of conserved parameters is proposed. The solidification geometry under consideration is a two-dimensional mold cavity with a central riser allowing continuous melt flow into the cavity. Model validation is obtained against existing numerical results involving directional solidification of Al-4.1 wt. % Cu alloy. However, heavier solute (Cu) rejection in the melt during solidification renders the validation case study devoid of freckling phenomena. The postvalidation investigations involve bottom up solidification of Al-30 wt. % Mg alloy with lighter solute (Mg) rejection, leading to solutal instability and freckle formation. The effect of SIF on solutal instability, channel formation, and overall macro-segregation is investigated. The intensity of SIF hinges on both cooling condition and opening size. The penetration depth of SIF into the solidification domain gives rise to either early or late onset of solutal instability. SIF penetration depth till the melt domain adjacent to the mushy layer promotes early onset of solutal instability. However, SIF penetration into the mushy layer itself triggers redistribution of solute-rich melt inside this layer, leading to delayed onset of solutal instability. Since the macro-segregation is a direct consequence of advection of solute inside and adjacent to the mushy region, the influence of SIF is manifested by unprecedented macro-segregation pattern.

2 citations

Journal ArticleDOI
Abstract: In this study, identical experiments of bottom-cooled solidification fluidic mixtures that exhibit faceted and dendritic microstructures were performed. The strength of compositional convection, created due to the rejection of a lighter solute, was correlated with the solidifying microstructure morphology via separate Rayleigh numbers in the mushy and bulk-fluid zones. While the bulk fluid in dendritic solidification experienced a monotonic decrease in the temperature, solidification of the faceted case revealed an unconventional, anomalous temperature rise in the bulk liquid after the formation of a eutectic solid. Based on the bulk-liquid temperatures, three distinct regimes of heat transfer were observed in the liquid, namely, convection-dominated, transition and conduction-dominated. The observations were analysed and verified with the help of different initial compositions and cooling conditions, as well as other mixtures that form faceted morphology upon freezing. The observed temperature rise was further ascertained by performing an energy balance in an indicative control volume ahead of the solid–liquid interface. The plausible mechanism of permeability-driven flow causing a gain in the temperature of the liquid during freezing was generalized with the help of a semi-analytical investigation of a one-dimensional system comprising solid, porous mush and liquid regions. The analytical scaling relations for fluid velocity and vorticity, for the faceted and dentritic cases, revealed contrasting vorticity values, which are much larger in low permeability (faceted case) and cause enhanced mixing in the bulk. The study sheds new insights into the role of microstructural morphology in governing the transport phenomena in the bulk liquid.

2 citations

Journal ArticleDOI
Abstract: In this study, identical experiments of bottom-cooled solidification fluidic mixtures that exhibit faceted and dendritic microstructures were performed. The strength of compositional convection was correlated with the solidifying microstructure morphology, with the help of separate Rayleigh numbers in the mushy and bulk-fluid zones. While the dendritic solidification experienced a monotonic decrease in the bulk fluid temperature, solidification of the faceted case revealed an unconventional, anomalous temperature rise in the bulk liquid, at the initiation of the eutectic phase. Based on the bulk-liquid temperature profile, three distinct regimes of heat transfer were observed in the liquid over the course of solidification, namely - convection-dominated, transition, and conduction-dominated. The observations were analyzed and verified with the help of different initial compositions, as well as other mixtures that form faceted morphology upon freezing. The observed temperature rise was further ascertained by performing an energy balance in an indicative control volume ahead of the solid-liquid interface. The plausible mechanism behind the gain in temperature of the liquid during freezing was further generalized with the help of a simplified one-dimensional numerical model, and was extended to metals which are low Prandtl number mixtures. The study sheds new insights into the role of microstrostructural morphology in governing the transport phenomena in the bulk liquid.

1 citations


Cites background from "Study of Freckles Formation During ..."

  • ...Table 1: Thermo-physical properties of water-KNO3 and water-NH4Cl systems [23,24,26–28]...

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References
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Abstract: This book focuses on heat and mass transfer, fluid flow, chemical reaction, and other related processes that occur in engineering equipment, the natural environment, and living organisms. Using simple algebra and elementary calculus, the author develops numerical methods for predicting these processes mainly based on physical considerations. Through this approach, readers will develop a deeper understanding of the underlying physical aspects of heat transfer and fluid flow as well as improve their ability to analyze and interpret computed results.

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Journal ArticleDOI
Abstract: The melting of pure gallium in a rectangular cavity has been numerically investigated using the enthalpy-porosity approach for modeling combined convection-diffusion phase change. The major advantage of this technique is that it allows a fixed-grid solution of the coupled momentum and energy equations to be undertaken without resorting to variable transformations. In this work, a two-dimensional dynamic model is used and the influence of laminar natural-convection flow on the melting process is considered. Excellent agreement exists between the numerical predictions and experimental results available in the literature. The enthalpy-porosity approach has been found to converge rapidly, and is capable of producing accurate results for both the position and morphology of the melt front at different times with relatively modest computational requirements. These results may be taken to be a sound validation of this technique for modeling isothermal phase changes in metallurgical systems.

1,110 citations

Journal ArticleDOI
Abstract: Semi-empirical laws and microscopic descriptions of transport behavior have been integrated with principles of classical mixture theory to obtain a set of continuum conservation equations for binary, solid-liquid phase change systems. For a restricted, yet frequently encountered, class of phase change systems, the continuum equations have been cast into forms amenable to clear physical interpretation and solution by conventional numerical procedures.

835 citations

Journal ArticleDOI
TL;DR: Three limiting cases are identified which result in one-phase models of binary systems of binary alloy solidification and each of these models can be readily implemented in standard single phase flow numerical codes.
Abstract: The aim of this paper is to explore the range of possible one-phase models of binary alloy solidification. Starting from a general two-phase description, based on the two-fluid model, three limiting cases are identified which result in one-phase models of binary systems. Each of these models can be readily implemented in standard single phase flow numerical codes. Differences between predictions from these models are examined. In particular, the effects of the models on the predicted macro-segregation patterns are evaluated.

411 citations