Detection of melt flow in solidifying metal alloys by X-ray radioscopy

X-ray radioscopic methods enable the in-situ real-time observation of solidification processes in metal alloys with a spatial resolution of a few microns. Visualization of bottom up directional solidification of a Ga-25wt%In binary metal alloy contained in a capillary slit container was conducted. The solidification is obviously affected by natural thermo-solutal melt flow patterns. Externally forced melt convection was superimposed by means of a magnetic stirrer in form of a rotating wheel equipped with permanent magnets. The electromagnetic flow provokes a considerable redistribution of the solutal boundary layers around the solidifying dendrites, and influences therewith the formation of the microstructure.
The X-ray radioscopy delivers a two-dimensional projection of the local density in the slit container corresponding to the distribution of the relative brightness P in the acquired images. The relative brightness P allows for an assessment of the local constitution inside the liquid phase. Our analysis of the flow field in the present article follows the Optical Flow approach proposed by Horn and Schunck. [1] The applied algorithm to determine the optical flow from the X-ray images delivers reliable information concerning the velocity field in regions where sufficiently large brightness gradients occurs. The solidification process creates differences of the local composition within the melt leading to characteristic pattern of transmitted light intensity. The analysis of the flow field relies on the monitoring of the temporal variations of characteristic brightness patterns in the liquid phase.
Our results show that melt flow induces various effects on the grain morphology primarily caused by the convective transport of solute. 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 branches completely. Moreover, the flow changes the inclination angle of the dendrites and the angle between primary trunks and secondary arms.