Abstract
Cast shops have found that reduced grain size can be obtained in a finished casting if stirring velocity and melt temperature are carefully controlled. Forced convection in the melt causes fragmentation of dendrites in the mushy zone. A model system has been developed that allows independent control of concentration, temperature, and fluid flow velocity while viewing the mushy zone with an optical microscope. Initial results indicate an underlying mechanism associated with a decreasing interfacial area per unit volume of mushy zone combined with recalescence. This mechanism causes the dendrite side branches to pinch off at their bases, where they connect to the primary dendrite stem. Crystal fragments are then transported by convection in the melt and can either develop into independent equiaxed grains or remelt. Preliminary results indicate that the velocity of the flow dramatically alters the mushy zone causing both a change in the amount of fragmentation and the fragmentation rate.
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