Forced convection effects on dendritic growth and freckle formation

A directional solidification of Ga–25wt%In alloy within a Hele-Shaw cell under the influence of electromagnetically forced melt convection was studied by X-ray radioscopy. The forced convection was produced by a rotating wheel with two parallel disks containing at their inner sides a set of permanent NdFeB magnets with alternating polarisation [1, 2]. The top part of the solidification cell was positioned in the gap between the disks. Rotation speeds of the magnetic wheel were chosen in the range of 30 – 80 revolutions per minute. The melt flows are almost perpendicular with respect to the growth direction of the dendrites (local flow velocities 0.5 - 1.5 mm/s).
The forced convection provoked a preferential growth of the secondary arms at the upstream side of the primary dendrite arms, whereas the high solute concentration at the downstream side of the dendrites inhibited the formation of secondary branches completely. The changes of secondary arm spacing depended on the melt flow conditions. The accelerated growth of the secondary arms towards the incoming flow blocked the growth of neighboring primary dendrites leading to a considerable increase in the spacing between the primary trunks. Moreover, the primary trunks show a slight inclination towards the inflow. The flow-induced redistribution of the solute concentration leads to the formation of spacious segregation pattern.