Using nucleators to control freckles in unidirectional solidification

Buoyancy-induced fluid flow, which is responsible for most forms of macro-segregation and channel-type segregates in castings, is not directly controllable. If left uncontrolled, natural convection will contribute to non-uniform distribution of alloy constituents and grain structure during solidification of a casting. Non-uniform distribution of chemical composition and physical structure in an alloy casting can significantly affect the reliability of mechanical components. Therefore, materials with acceptable defects can be produced only by trial-and-error and their acceptability is determined by costly inspections. We present a novel technique to control the formation of chimneys and resulting freckles in the mushy zone during the solidification of ammonium chloride that is cooled from below. This is done by placing metallic nucleators in particular arrangements on the bottom cooling plate. With this technique, freckles in a casting might be avoided and/or be forced to form where stresses are expected to be lower during use of the part.The objective of this study is to investigate the effects of the arrangement, spacing, and size of the nucleators on finger formation, plume structure, and the solidification process. Results showed that it is possible to obtain a relatively large area free of channel-segregates in a metal analog directionally solidified upward by placing nucleators in certain arrangements at the bottom of the casting. The outcomes of this study will serve as a baseline for subsequent investigations that will examine the solidification of binary alloys, and could be used to test and develop mathematical and numerical models.