Convection processes during directional solidification of binary alloys

Journal Article · Thu Apr 01 04:00:00 EST 1993 · Bulletin of the American Physical Society

OSTI ID:243952

Chen, C F

When an alloy melt is solidified from the bottom, the liquid melt is separated from the eutectic solid at the bottom by a mushy layer. This layer is a porous medium consisting of a dendritic structure saturated with the liquid melt. Convection occurs in the liquid, as well as in the mushy layer, due to compositional buoyancy, and the latter process leads to defects in the casting, known as freckles. Results of directional solidification experiments with an analogue casting system of NH{sub 4}Cl-H{sub 2}O solution show that finger convection appears in the liquid region as soon as crystallization of NH{sub 4}Cl occurs at the cooled bottom surface. When the mushy layer attains a certain thickness, chimneys start to appear in the mush, with upward flowing plumes of lighter fluid. These chimneys are the precursors of freckles in the casting. More recently, flow visualization studies show that, prior to the onset of plume convection, there is no convective motion in the mush, even though there is vigorous finger convection emanating from the mush-liquid interface. After plume convection is fully developed, the flow in the mush is mainly downward, with some lateral feeding into the chimneys, within which the upward flow is confined. The authors have devised a method to determine the three-dimensional porosity distribution in the mushy layer using computed tomography. This method is being implemented to scan the mush as solidification proceeds. Further experiments in a Hele Shaw cell revealed interesting behavior of the convection process both in the liquid and the mushy layer.

• OSTI ID: 243952
• Report Number(s): CONF-9304297--; CNN: Grant NAG3-723 and Grant NAG3-1268
• Journal Information: Bulletin of the American Physical Society, Journal Name: Bulletin of the American Physical Society Journal Issue: 2 Vol. 38; ISSN 0003-0503; ISSN BAPSA6
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
BINARY ALLOY SYSTEMS
CASTING
DEFECTS
NATURAL CONVECTION
SOLIDIFICATION