Simultaneous Observation of Melt Flow and Motion of Equiaxed Crystals During Solidification Using a Dual Phase Particle
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MODELING of metallurgical processes is a rapidly expanding field and the research activities in the last decades cover a wide range of areas including melt pretreatment, solidification, and subsequent manufacturing routes. Among those activities, solidification stands in the central position because the primary structure of the materials, and even many defects such as porosity, (macro or micro) segregation, and hot tears, forms during solidification. Those primarily formed structures or defects once existing are difficult to remove or modify by the subsequent material processing. The more realistic models for describing industryrelevant manufacturing processes, with involvement of solidification, are based on the solution of the macroscopic transport equations (mass, momentum, enthalpy, and species) at the system length scale. Microscopic phenomena such as nucleation, crystal growth kinetics, and thermodynamic equilibria at liquid-solid interfaces are considered with simplified models, which are coupled with the macroscopic transport equations.[1,2] An important phenomenon in solidification is the simultaneous occurrence of melt flow and crystal motion. Solidification takes place either by a growing columnar front and/or by the growth of equiaxed crystals. Columnar growth happens in the form of cells or dendrites from the mold walls into the bulk melt due to heat extraction from outside. In equiaxed solidification, globular or ABDELLAH KHARICHA, Research Team Leader, MIHAELA STEFAN-KHARICHA, Ph.D. Student, ANDREAS LUDWIG, Professor, and MENGHUAI WU, Associate Professor, are with the Department of Metallurgy University of Leoben, Leoben, Austria. Contact e-mail: [email protected] Manuscript submitted March 23, 2012. Article published online September 19, 2012 650—VOLUME 44A, FEBRUARY 2013
dendritic crystals form by nucleation or fragmentation and subsequent growth into the surrounding melt. Convection in the melt might have different reasons. Most often, thermal and/or solutal buoyancy together with external body forces occurs. In most of the cases, the flow is unstable and even turbulent. In the case when columnar growth is present, there exists a mushy region where both solid and liquid phases are present. When equiaxed solidification is present, the phases (liquid and equiaxed crystals) interact with each other through momentum and energy exchange. The resulting solid-liquid multiphase flow pattern strongly depends on the microstructure of the equiaxed crystals, which in turn is governed by grain nucleation and growth mechanisms. Because the coupled liquid-solid flow causes structural and chemical inhomogeneities in the final solidified products, a fundamental understanding of the multiphase transport phenomena coupled with the grain nucleation and growth mechanisms is required. The occurrence of dispersed two-phase flows in nature and industrial applications is abundant. The current paper presents some similarity with processes in which dispersed solid particles interact with a carrier flow. However, there
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