Macrosegregation Due to Convection in Al-19Cu Alloy Directionally Solidified Through an Abrupt Expansion in Cross-Sectio
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JMEPEG DOI: 10.1007/s11665-017-2925-y
Macrosegregation Due to Convection in Al-19Cu Alloy Directionally Solidified Through an Abrupt Expansion in Cross-Section: A Comparison with Al-7Si M. Ghods
, M. Lauer, R.N. Grugel, S.N. Tewari, and D.R. Poirier
(Submitted June 20, 2017; in revised form August 24, 2017) Hypoeutectic Al-19 wt.% Cu alloys were directionally solidified at two different growth speeds in cylindrical molds that featured an abrupt increase in cross-section, from 3.2 to 9.5 mm in diameter. The effects of thermosolutal convection and shrinkage flow induced by the cross-section change on macrosegregation were investigated. Dendrite clustering and extensive radial macrosegregation were seen, particularly in the larger cross-section after expansion. Negative longitudinal macrosegregation right after the cross-section increase was observed; the extent of macrosegregation, however, decreases with increasing growth speed. Both thermal and flow effects due to cross-section change were seen to influence the radial macrosegregation immediately before, and after the expansion. Radial macrosegregation pattern was found to be changing as the mushy zone enters the larger cross-section region above the cross-section change where the solidification is in its unsteady state. The effect of the solutal expansion coefficient on macrosegregation was studied by comparing the degree of thermosolutal convection in Al-19 wt.% Cu with a previous study in which we investigated Al-7 wt.% Si. A two-dimensional model accounting for both shrinkage and thermosolutal convection was used to simulate the resulting steepling, as well as the axial and radial macrosegregation. The experimentally observed macrosegregation associated with the expansion during directional solidification is well predicted by the numerical simulations. Keywords
aluminum, casting and solidification, convection, cross-section increase, macrosegregation, modeling and simulation
1. Introduction During the directional solidification of metallic alloys with partition coefficients k < 1, under a thermal gradient Gl and growth speed R, the interdendritic-liquid concentration increases from Ct (slightly higher than the bulk alloy solute content C0) at the array tips to the eutectic CE at its bottom. When the rejected solute increases the melt density, it is usually assumed that the mushy-zone liquid is stable with respect to natural convection. The twofold increase in the thermal diffusivity of the solid metal forming from the liquid, however, causes an inward heat flow from the mold creating a radial thermal gradient in the melt (Ref 1). This initiates ‘‘thermal convection’’ cells where the liquid flows upwards close to the mold walls and downwards at the center. This also causes the isotherms to become convex toward the all-liquid region above the mushy zone and favors the dendritic growth in the central
M. Ghods and S.N. Tewari, Chemical and Biomedical Engineering Department, Cleveland State University, Cleveland, OH 44114; M. Lauer, Department of Materials Science and Engi
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