Chimney Formation in Solidifying Ga-25wt pct In Alloys Under the Influence of Thermosolutal Melt Convection
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LIDIFICATION processes are affected by natural convection as soon as instable density stratifications arise in the melt from local variations of the temperature and/or the concentration field. In particular, the rejection of solute at the solidification front which is lighter as the initial melt composition results in the formation of rising plumes in the liquid driven by the buoyancy force. Such convective plumes have been identified as the main reason for the development of solute-rich channels in solidifying metal alloys.[1–6] These channels may lead to freckle formation in fully solidified castings which is considered as a serious material defect deteriorating the mechanical properties of the final product. Hellawell et al.[3] distinguished between two convective regimes. Immediately after the initiation of the solidification process, the so-called finger regime develops which consists of ascending, solute-rich fluid fingers with diameters comparable with the primary dendritic spacing. In the course of solidification when larger volumes of solute are rejected at the growth front, the fine-scale finger regime will be superseded by the occurrence of solute plumes showing lateral extensions up to a few interdendritic spacings. It is widely assumed NATALIA SHEVCHENKO and STEPHAN BODEN, Research Fellows, GUNTER GERBETH, Institute Director, and SVEN ECKERT, Head of Magnetohydrodynamics Department, are with the Helmholtz-Zentrum Dresden-Rossendorf, Institute of Fluid Dynamics, P.O. Box 510119, 01314 Dresden, Germany. Contact e-mail: [email protected] Manuscript submitted November 16, 2012. Article published online April 3, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
that these large-scale plumes cause the channel segregation defects frequently observed in metal castings. The fundamental description of the linear instability with respect to the melt convection in the situation of directional solidification was given by Worster.[7,8] He revealed the existence of two different modes of convective instability: a boundary layer mode, where the occurrence of convective cells is restricted to the liquid phase ahead of the solidification front, and a mushy-layer mode appearing at longer wave lengths, where the convection penetrates the entire mushy zone. The formation of the chimneys was attributed to the impact of the long-wavelength mushy-layer mode, which in turn will be triggered by a sufficiently strong convection in the liquid zone through the development of significant corrugations in the upper mushy layer. For reasons of continuity, rising plumes of solute-rich fluid in the liquid zone initiate an ascending flow also in the mushy layer beneath. Fluid in the lower mushy zone, which is enriched in the lighter solute, is mobilized and begins to rise into increasingly warmer zones, where it becomes undersaturated. Therefore, these interdendritic flows can dissolve the dendrites carving out chimneys into the porous dendritic network. A strong coupling exists between melt flow and the solidification process. In the situation of an upward soli
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