Visualisation of melt flow effects on dendritic solidification

X-ray radioscopic studies have been performed to improve the understanding of the complex interrelation between melt flow and the evolution of dendritic structures during solidification of Ga-In alloys. Natural convection is caused by density variations within the solidifying alloys. Forced convection was produced by electromagnetic stirring. Within this work special interest was focused on dendrite fragmentation and segregation phenomena. Melt convection alters the solutal field near the solidification front, leading to different microstructures or even to the formation of freckle defects. Essential process parameters such as flow patterns, solute concentration, the mushy zone morphology and permeability, dendrite growth velocities were quantified by image analysis. Particular attention is paid to the development of segregation structures and to the “self-healing” process of segregation zones. The observations indicate that if the local melt flow near the solidification front is destabilized, the Indium-rich melt flows inside the channel. The consequence is the “self-healing” process, i.e. the channel is filled in a short time by new dendrites and finally disappears. Eventual mechanisms that destabilize the channel formation are discussed so that the freckle defect can be eliminated by electromagnetic stirring on early stage of solidification. Moreover, our experiments demonstrate how the melt flow contributes to grain refinement, the CET (columnar to equiaxed transition) and dendrite fragments transport, which are discussed intensively in the literature.