Effect of rotation on fluid motion and channel formation during unidirectional solidification of a binary alloy

Abstract

When off-eutectic binary alloys are cast by means of unidirectional solidification (UDS), perturbations in density and permeability along the liquidus front may result in the nucleation and subsequent growth of vertical flow channels in the two-phase (solid/liquid) mushy region. The channels subsequently become locations of severe compositional nonhomogeneity which are commonly termed freckles. In this study UDS of hypereuteclic NH 4Cl-H 2O has been numerically simulated to determine the effects of both steady and intermittent mold rotation. For slow, steady rotation up to 120 rpm, it was found that related inerlial effects were insufficient to significantly alter buoyancy induced mechanisms responsible for channel formation. However, for intermittent rotation corresponding to successive spin-up and spin-down of the mold, channel nuclealion was confined to the centerline and outer radius of the casting. The elimination of channels from the core of the casting is attributed to the impulsive change in angular frequency associated with spin-up and its effect on establishing an Ekman layer along the liquidus front. The front is washed by flow within the layer, thereby eliminating the perturbations responsible for channel nucleation.