Solidification under Forced-Flow Conditions in a Shallow Cavity A.N. TURCHIN, D.G. ESKIN, and L. KATGERMAN The solidification of an Al-4.5 pct Cu alloy in a shallow cavity under conditions of forced flow was studied both by fluid-dynamics simulations with solidification included and by experiments. The variation in bulk-flow velocity and initial superheat dramatically changes the macro- and microstructure, promoting grain refinement, an equiaxed-to-columnar transition (ECT), the formation of peculiar grain and dendrite morphologies, etc. The solidification parameters during solidification in the shallow cavity under forced-flow conditions have been determined by computer simulations and partially compared with the experimental results. The interaction between flow vortices and the progressing solidification front and its effect on structure evo- lution have been analyzed. Finally, quantitative correlations between microstructure, solidifi- cation, and flow parameters have been established. DOI: 10.1007/s11661-007-9183-9 Ó The Minerals, Metals & Materials Society and ASM International 2007 I. INTRODUCTION OVER the past 50 years, many attempts have been made to design special techniques with the purpose of improving and controlling the final solidified structure by forced flow. When one considers any particular casting system, one can see that the flow is present from the early stages of the process. During the casting, flow generally occurs in the bulk liquid and in the semi-solid regions. Some of the techniques have been successfully used in academic research with the aim of studying the fundamentals of solidification under forced-flow condi- tions: gravity flow-through systems, [1] mechanical stir- ring, [2] centrifugal casting, [3] application of a magnetic or electromagnetic field creating the Lorenz force, [4,5] etc. As experimental research has developed over the last 50 years, computational modeling and simulation have been widely used in the last two decades as cost-saving tools for the prediction and interpretation of the results. Using these two approaches in combination leads to a deeper understanding of the effects of melt flow as a result of natural and forced convection on the solidifi- cation phenomenon in metallic alloys, i.e., (1) the morphology of grains and their deflection toward incoming flow, [1–5] (2) the columnar-to-equiaxed transi- tion and grain morphology, [4] and (3) the change of segregation pattern. [4,5,6] Forced flow applied to the bulk of the molten metal interacts with the growing solid producing the distortion of the solid-liquid interface, [1] altering the shape of the mushy zone [7] and affecting the solidification parame- ters. [8] Depending on the nature of the flow and the initial velocity, oscillation (vortices) of various magni- tudes may occur at the solidification front. However, many questions are still far from being understood completely. How does the forced flow and, in particular, the vorticity at the solidification front affect the macrostructural features? How does the forced flow influence the microstructure evolution? The present study is aimed at analyzing the effects of macroinstabilities at the solidification front on macro- and microstructural features. The solidification under forced-flow conditions in the chill with a shallow rectangular cavity is proposed to create the vortex structure at the solid-liquid interface while solidification progresses. The combined approach based on fluid dynamics calculations and experimental work is imple- mented to determine quantitatively the solidification parameters, i.e., the local solidification time, rate, and thermal gradient, depending on the flow and heat regimes. In addition, the correlations between the structural parameters, the associated shape of the mushy zone, and the forced flow have been established. II. EXPERIMENTAL PROCEDURE The objectives of the present study were as follows: (1) to obtain the solidification parameters during solidifi- cation in a shallow cavity under forced-flow conditions, (2) to examine the interaction of vortices with the progressing solidification front and their effect on structure evolution, and (3) to determine the quantita- tive correlations between microstructure and forced- flow parameters. To accomplish the intended goals, a controllable system that provides a constant unidirectional bulk flow along the solidification front has to be selected. The rotating chill, centrifugal or electromagnetic castings, etc. can provide the bulk flow along the solidification interface. However, a closed system such as this would cause some inconveniences associated with difficult flow velocity control, complicated flow pattern, and high velocities. Therefore, an already complex solidification A.N. TURCHIN, Ph.D. Student, D.G. ESKIN, Senior Scientist, are with the The Netherlands Institute for Metals Research, 2628CD, Delft, The Netherlands. Contact e-mail: a.turchin@nimr.nl L. KATGERMAN, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology, 2628CD, Delft, The Netherlands. Manuscript submitted December 13, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A