The Influence of Thermo-Solutal Convection on Freckle Formation and Dendritic Growth

In-situ observations of the solidification process under the influence of thermo-solutal convection were obtained by means of X-ray radioscopy within a Hele-Shaw cell filled with Ga-25wt%In alloy. The density-driven melt flow causes the formation of vertical freckles (“chimneys”) and changes the mushy zone characteristics. Experimental conditions have been identified which promote the formation of freckles in the Ga-In alloy. A strong coupling exists between the spatial and temporal properties of the flow field and the stability of segregation channels. Stable vortex flow provides a continuous upwards flow of solute-rich melt above the channel and an influx through the mushy zone towards the chimney. We also focused on the analysis of dendrite growth and microstructure features, for instance, dendrite orientation, primary and secondary arm spacing. The quantitative analysis of the flow field, temperature distribution and microstructure characteristic gives a better understanding of the complex interplay between melt flow and solidification process.