Thermosolutal convection and macrosegregation during solidification of hypereutectic and hypoeutectic binary alloys in s
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TRODUCTION
During solidification of a binary alloy, the solute is rejected from the solidifying dendrites and accumulates in the adjoining liquid, establishing local concentration gradients. Natural convection is induced due to temperature and concentration gradients. Physical features influenced by convection include development of an irregular liquidus front, remelting of solid, development of flow channels in the mushy region, and establishment of characteristic macrosegregation patterns in the final solid. Detailed discussion of the transport mechanisms and a comprehensive review can be found in recent publications.[1–4] The knowledge of momentum, energy, and species transfer is essential to maintain satisfactory control over the casting process. Recent modeling efforts have successfully predicted various forms of macrosegregation in binary alloys. The models widely employed in the literature are based on the continuum mixture theory[5,6] or local volume averaging procedure.[7,8,9] Experimental and numerical studies on thermosolutal convection during solidification of binary alloys in a sidechilled ingot[10–14] have revealed the influence of the interdendritic flow on the macroscopic redistribution of species. ‘‘A segregates’’ have been reported to form when the interdendritic flow is in an upward direction.[10,11] During solidification of light solute-rejecting alloys in bottom-chilled ingots, double-diffusive convection is characterized by alternating fingers of compositionally lighter and heavier fluids ascending and descending to the liquidus interface.[15,16] Intermittent solute fingers fed by dendritic entrainment proM.A. RADY, formerly with the Mechanical Engineering Department, Indian Institute of Technology, is with the Mechanical Engineering Technical Department, Benha High Institute of Technology, Benha 13512, Egypt. V.V. SATYAMURTY, Professor, is with the Mechanical Engineering Department, Indian Institute of Technology, Kharagpur 721302, India. A.K. MOHANTY, formerly Professor, Mechanical Engineering Department, Indian Institute of Technology, is deceased. Manuscript submitted June 27, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
duce stable convective plumes and concomitant channels.[17–21] These channels manifest themselves in the final casting as freckles. Most of the previous studies have been performed in either rectangular or cylindrical ingots. Very little attention has been given to studying thermosolutal convection and resultant macrosegregation in irregular geometries. Solidification of a binary alloy in a trapezoidal ingot is of practical importance because of the common practice to make the ingot wall with a small slope to facilitate withdrawal of the casting. A taper in the mold is employed to reduce centerline shrinkage as well.[22] The objective of the studies reported in the present article is to examine thermosolutal convection patterns and macrosegregation during solidification of hypereutectic and hypoeutectic NH4CL-H2O binary systems in trapezoidal side-chilled ingots. Hyper
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