X-ray radioscopic visualisation of solidifying metallic melts

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Objectives

• Experimental assessment of the solidification process in opaque metal alloys
• Investigation of the fundamental mechanisms how melt convection affects the solidification
• Providing qualified knowledge on how tailored melt flow control techniques can modify the properties of the solidified material

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X-ray radioscopy Non-invasive, contactless imaging method enabling real-time and in-situ observation of the solidification front and with a spatial resolution of a few microns Two-dimensional X-ray shadow image provides information on the density and composition variations in the sample

Fig. 1: Radioscopic setup

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Materials, methods and results

Solidification experiment Ga-30wt%In, a binary metal alloy with low melting point, provides sufficient X-ray attenuation contrast between the separated phases The alloy was contained in a flat solidification cell (150 µm gap width), which was aligned in parallel with gravitation By means of a heating/cooling device the alloy was directionally solidified from below

Fig. 2: Schematic depiction of the solidification experiment

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Image processing Pre-processing involves noise-filtering and brightness-to-composition mapping Optical flow (motion field of brightness patterns between consecutive image frames) computation delivers inform-ation on the flow velocities in the melt

Fig. 3: Growth of in-crystals accompanied by the rejection of solute in the formation of large plumes of Ga-enriched melt

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Experimental results Visualisation of - Dendritic growth of In-2wt%Ga crystals - Solute rejection and local accumulation Formation of unstable density stratification Strong correspondence between melt convection and crystal growth Modification of the diffusion layer ahead of the solidification front Transport of fresh melt towards the mushy zone accelerates growth Accumulation of solute-rich melt decelerates crystal growth Local temporal re-melting triggered by solutal undercooling Convection affects the selection between primary and secondary arm growth

Fig. 4: Detailed view on the crystal formation during the directionaly solidification of the Ga-30wt%In alloy. Vector arrows represent the computed optical flow field

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Perspectives

• Validation of results from computational micro-structure simulations of the solidification process
• Elucidation of the effects of electromagnetically forced melt-convection on the crystal growth
• Clarification of the dendritic fragmentation process

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Radioscopic visualisation of bubble motion in liquid metals

Objectives Background from metallurgy and process engineering: Melt refining and melt stirring by gas bubbling Radioscopy provides information on bubble formation, shape evolution and motion trajectories Results will be useful for the evaluation of analytical and computational liquid metal two-phase flow models

Fig. 5: Bubble motion in liquid metal GaInSn, channel depth: 12 mm

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References

Peer-reviewed journals

• Boden, S.; Eckert, S.; Willers, B.; Gerbeth, G.: X-ray radioscopic visualization of the solutal convection during solidification of a Ga-30 wt pct In alloy, Metall. Mater. Trans. A 39 (2008) 613-623, doi:10.1007/s11661-007-9462-5
• Boden, S.; Willers, B.; Eckert, S.; Gerbeth, G.: Observation of dendritic growth and fragmentation in Ga-In alloys by X-ray radioscopy, Int. J. Cast. Metal. Res. 22 (2009) 30-33, doi:10.1179/136404609X367263
• Boden, S.; Eckert, S.; Gerbeth, G.: Visualization of freckle formation induced by forced melt convection in solidifying GaIn alloys, Mater. Lett. 64 (2010) 1340-1343, doi:10.1016/j.matlet.2010.03.044
• Shevchenko, N.; Boden, S.; Eckert, S.; Gerbeth, G.: Observation of segregation freckle formation under the influence of melt convection, IOP Conf. Series: Mater. Sci. Eng. 27 (2011) 012085, doi:10.1088/1757-899X/27/1/012085
• Shevchenko, N.; Eckert, S.; Boden, S.; Gerbeth, G.: In situ X-ray monitoring of convection effects on segregation freckle formation, IOP Conf. Series: Mater. Sci. Eng. 33 (2012) 012035, doi:10.1088/1757-899X/33/1/012035
• Fröhlich, J.; Schwarz, S.; Heitkam, S.; Santarelli, C.; Zhang, C.; Vogt, T.; Boden, S.; Andruszkiewicz, A.; Eckert, K.; Odenbach, S.; Eckert, S.: Influence of magnetic fields on the behavior of bubbles in liquid metals, Eur. Phys. J. Special Topics 220 (2013) 167-183, doi:10.1140/epjst/e2013-01805-4
• Shevchenko, N.; Boden, S.; Eckert, S.; Borin, D.; Heinze, M.; Odenbach, S.: Application of X-ray radioscopic methods for characterisation of two-phase phenomena and solidfication processes in metallic melts, Eur. Phys. J. Special Topics 220 (2013) 63-77, doi:10.1140/epjst/e2013-01797-y

Conference proceedings

• Boden, S.; Willers, B.; Eckert, S.; Gerbeth, G.: Visualisation of the concentration distribution and the flow field in solidifying metallic melts by means of X-ray radioscopy, Proceedings of the 5th Decennial International Conference on Solidification Processing, Sheffield, UK, 2007, 311-315
• Boden, S.; Willers, B.; Eckert, S.; Gerbeth, G.: X-ray visualisation of solidifying GaIn-alloys in the presence of melt convection, Proceedings on the 6th International Conference on Electromagnetic Processing of Materials (EPM 2009), 19.-23.10.2009, Dresden, Germany, 261-264
• Boden, S.; Eckert, S.; Gerbeth, G.; Simonnet, M.; Anderhuber, M.; Gardin, P.: X-ray visualisation of bubble formation and bubble motion in liquid metals, Proceedings on the 6th International Conference on Electromagnetic Processing of Materials (EPM 2009), 19.-23.10.2009, Dresden, Germany, 387-390