X-ray visualisation of melt flow effects on dendritic solidification

X-ray radiography is an effective tool for investigating flow phenomena and solidification processes in opaque metallic alloys. This work is devoted to complex interaction between dendritic growth and melt flow during solidification of Ga-In alloys under natural and forced convection. Natural convection is caused by density variations within the solidifying alloys. Forced convection was produced by electromagnetic stirring. The conventional X-ray radioscopic experiments with sufficient spatial resolution (5-10 µm) deliver simultaneous information of both the dendrite structure and the flow patterns ahead of the solidification front and especially near the mushy zone. Melt convection alters the solutal field near the solidification front leading to different microstructures or even to the formation of freckle defects. The coarsening stage of dendritic structure is characterized by transformation of the sidearm morphology present after growth. The direct investigation of dendritic sidearm evolution during coarsening appears to be rather complex and impose high requirements with respect to the spatial and temporal resolution and sensitivity of the detector. The synchrotron imaging experiments with solidifying Ga-In alloys were performed at the BM20 and ID19 beamlines (ESRF, France) at a spatial resolution of < 1 µm. The present measurements provide real-time in-situ data on three phenomena that are of major importance in coarsening of dendrites: sidearm retraction, pinch-off and coalescence of neighboring sidearms. Using an advanced image analysis of high temporal and spatial resolution experimental data allows us to verify existing microstructural models.