The effect of melt flow on dendritic solidification in Ga–In alloys

Simultaneous study of solidification phenomena on different length scales (dendrite networks - microscale and flow structure - mesoscale) is a main advantage of a conventional X-ray radiography. This work is focused on new benchmark experiments with different modes of melt convection and their effect on the dendritic growth in metallic alloys. The directional solidification of Ga-In alloy within a Hele-Shaw cell was investigated in situ. The visualization experiments with sufficient spatial resolution (5-10 µm) deliver simultaneous information of both the dendrite structure and the flow patterns especially in the mushy zone and ahead of the solidification front. Our results show that convection alters the solutal field near the solidification front, leading to different microstructures or even the formation of freckle defects. Flow patterns, solute concentration, the mushy zone morphology and permeability, dendrite growth velocities were quantified by image analysis. Particular attention was paid to the temporal dynamics of equiaxed grain growth, which plays an important role in grain structure formation in solidifying alloys. Small equiaxed grains frequently appeared in the undercooled melt near a continuous strong plume. Two mechanisms are under discussion: (i) crystal fragments transported by the ascending flow from the mushy zone; (ii) the equiaxed dendrites nucleated in the vicinity of the ascending plume due to modification of the the temperature/ solutal fields. We explored experimentally the size, the velocity and the conditions of formation of these equiaxed dendrites.