Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

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 X-ray radiography was used for an in-situ study of the effect of electromagnetic stirring during the 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 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, freckle formation, changes the inclination angle of the dendrites and leads to an increasing arm spacing. These effects are primarily governed by the convective redistribution of solute. Figure 1 demonstrates an interesting effect of "repairing" of a segregation channel (see the right-hand side part of Fig. 1a) after switching off the magnetic wheel (Fig 1b). It can be seen that an area with equiaxed or fine dendrites was formed instead of a segregation channel. 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.
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