Modification of the microstructure by rotating magnetic field during the solidification of Al-7 wt.% Si alloy under microgravity

Abstract

Directional solidifications of Al-7 wt.% Si alloy were carried out under microgravity on board the International Space Station to investigate the impact of a rotating magnetic field (RMF) on the solidified microstructure. It has been found that the RMF significantly influences the solidified microstructure in the conditions corresponding to the lowest growth rate, whereas it has negligible influence on the microstructure under the highest growth rate, indicating that the RMF intensity and the forced liquid flow are too weak, compared to the solidification front velocity, to impact the microstructure. For the lowest growth rate applied, the RMF application results in a more uniform eutectic phase distribution and a smaller dendrite arm spacing. This is ascribed to the RMF induced forced liquid flow that makes more uniform Si concentration in the bulk liquid above the solid-liquid interface. Additionally, the RMF application possibly modifies the columnar dendritic network by changing dendrite growth directions. This observation can be attributed to the combined effect of the RMF induced forced liquid flow and of the thermoelectric magnetic force on the dendrites resulting from the application of the RMF as well.

Concurrent model for sharp and progressive columnar to equiaxed transitions validated by directional solidification experiments processed in microgravity conditions

“…Microgravity solidification experiments on CET in grain refined Al-7wt.%Si were performed on board the International Space Station. Detailed accounts of the experimental apparatus and findings have been provided elsewhere [18][19][20][21]. Nevertheless, the relevant experimental details are summarised in the following sections.…”

“…Given the lack of a consistent mechanical blocking fraction across several modelling approaches and the argument of solutal versus mechanical blocking, a model that is independent of a blocking criterion would be advantageous. Hence, the current paper reports on a new computationally-efficient, targetedphysics model that has been validated against the reported experimental findings conducted in microgravity [19][20][21]36]. One of the aims of the modelling approach is to investigate the level of physics that is commensurate to modelling the problem at hand at the macro scale of the process in order to give reliable results with low computational complexity.…”

“…Furthermore, a particular aspect of CET research that has received minor attention is the quantification and characterization of the mixed columnar-equiaxed region between fully columnar and fully equiaxed zones. Microgravity experiments conducted on board the International Space Station [18][19][20][21] showed clear evidence of both sharp (with little or no mixed region) and progressive (with a mixed or intermediate region) transitions. The analyses of the progressive transitions included characterization of the aspect ratio of the equiaxed grains that were assumed to nucleate ahead of the columnar front.…”

Influence of Solidification Parameters on the Amount of Eutectic and Secondary Arm Spacing of Al–7wt% Si Alloy Solidified under Microgravity

“…The microstructural characterization methods were described in detail in a previous study [22]. The grain structure and the columnar-to-equiaxed transition (CET) were analyzed and discussed for the four experiments in [23][24][25][26]. In the present study, we focused on the effect of solidification parameters, namely the velocity of the solid/liquid front and the temperature gradient, on the relative amount of eutectic and secondary dendrite arm spacing.…”

“…The primary dendrites and the grain structure of the samples were analyzed in detail in [23][24][25][26]. In this work, the amount of eutectic and secondary arm spacing were analyzed as a function of the processing parameters of the experiments.…”