Investigations of forced flow effects on dendritic solidification

Many studies have demonstrated that the application of electromagnetic stirring enhances the area of equiaxed grains and reduces the mean grain size (see e.g. [1-2]). It is widely accepted that flow-induced grain refinement and the CET (columnar to equiaxed transition) in metallic alloys is triggered by the appearance of additional dendrite fragments originating from the columnar front. The mechanism for grain multiplication by melt convection is supposed to be complex and is not fully understood until now. The idea to apply electromagnetic stirring to control the defects arising from the action of natural convection is not straightforward too.
The X-ray radiography was used for an in-situ study of the effect of electromagnetic stirring during the directional bottom-up solidification of a Ga-25wt%In alloy in a Hele-Shaw cell [3]. The experimental setup was extended by a magnetic wheel, which allowed for controlled excitation of a melt flow in the liquid phase. The forced flow eliminates the solutal plumes and damps the local fluctuations of solute concentration. The induced redistribution of solute induces different effects on dendrite morphology, such as the uneven growth of primary trunks or lateral branches, remelting of single dendrites and also of lager dendrite ensembles, changes the inclination angle of the dendrites and leads to an increasing arm spacing. The uneven growth of primary dendrites at the beginning of the solidification experiment leads to the formation of Ga-rich zones near the solidification front which develop into distinct segregation freckles.
Another interesting effect can be observed during solidification experiment: the switching off the magnetic wheel leads to "repairing" of a segregation channel due to growth of equiaxed or fine dendrites in areas of Ga-rich pools. It has been demonstrated that the appearance of small equiaxed grains in the undercooled melt in the segregation pools is triggered by quick redistribution of solute after stopping the magnetic pump. A more detailed study of the "repairing" mechanisms of channels is the subject of ongoing work.

References
1. B. Willers et al, Materials Science and Engineering A 402 (2005) 55-65
2. T. Campanella et al, Metallurgical and Materials Transactions A 35 (2004) 3201-3210
3. N. Shevchenko et al, Journal of Crystal Growth 417 (2015) 1-8