The influence of natural convection and electromagnetically driven flows on segregation freckle formation

Visualizations of the solidification process were obtained by means of X-ray radioscopy within a Hele-Shaw cell filled with a Ga-25wt%In alloy. Thermo-solutal convection in the solidifying melt gives rise to the development of vertical segregation channels (“chimneys”). The probability of chimney formation depends sensitively on variations of both the concentration and temperature distribution. A forced melt flow perpendicular to the growth direction accelerates the growth of the secondary dendrite arms on the upstream side and suppresses the development of secondary arms on the downstream side. The primary dendrite arm spacing is increased, whereas the secondary arm spacing remains unaffected. Flow-induced modifications of the local composition were observed within the mushy zone which may contribute to the formation of spacious segregation pattern.