Unidirectional solidification of a binary model alloy and the effects of induced fluid motion

Douglas Gregg Neilson, Purdue University

Abstract

To gain an improved understanding of the transport phenomena and its effects on solidification, a combined numerical and experimental study was adopted. Both phases of research employed the dendrite-forming aqueous NH$\sb4$Cl model alloy. Two-dimensional numerical simulations predicted channel growth to begin at the liquidus front and, due to a localized freezing point depression, to propagate downward. Fluid escaping the channels formed buoyant plumes that were sustained by penetration of bulk liquid across the liquidus front and subsequent advection toward the channels. The channels were precursors to freckle segregates, which were predicted along with negative macrosegregation. The numerical results were determined to be quite sensitive to the permeability of the mushy zone, with larger permeabilities yielding more dynamic response. Two-dimensional predictions for unidirectional solidification (UDS) under conditions of steady, low speed rotation revealed that channel development/evolution was essentially identical to the static case. However, slow, intermittent rotation based on the system spin-up time resulted in an Ekman boundary layer along the liquidus front during spin-up, and this layer, acting in tandem with mixing during spin-down, was effective in eliminating channels in the core region of the mushy zone. For static UDS, predictions in three-dimensions yielded behavior more representative of the actual process, but differences between the two- and three-dimensional models for macroscopic solidification parameters were minor. Observations during the static experiments revealed three stages associated with UDS: (1) the pre-channel, double-diffusive "salt-fingering" period, (2) the interval of channel nucleation and their growth to form "volcano-like" sites with buoyant plumes of interdendritic fluid, and (3) the span during which the number of channels declined. Measurements of the average axial temperature profile, liquidus/solidus advancement, and the rate of bulk liquid dilution with time agreed favorably with the predictions, but the agreement was closely tied to the assumed mushy zone permeability. However, intermittent rotation based on the system spin-up time resulted in a mushy layer free of channels in the central core of the casting but with channels along the perimeter--qualitatively consistent with the numerical predictions. (Abstract shortened with permission of author.)

Degree

Ph.D.

Advisors

Incropera, Purdue University.

Subject Area

Mechanical engineering

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