In-situ X-ray observations showing the impact of natural and forced convection on dendritic solidification

The directional solidification of Ga–25wt%In alloys was investigated using X-ray radioscopy, which offers a visual access to opaque melts and enables a basic, intuitional understanding of the complex interplay between melt flow and dendritic growth. Natural convection occurs owing to an unstable density stratification at the solid-liquid interface. Forced convection was produced by a rotating wheel. Our observations show a facilitation of the growth of primary trunks or lateral branches, suppression of side branching, dendrite remelting and fragmentation. The manifestation of all phenomena depends on the dendrite orientation, local direction and intensity of the flow. The forced flow eliminates the solutal plumes and damps the local fluctuations of solute concentration. It provokes 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 can inhibit the formation of secondary arms.