biology and biotechnology

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COLUMNAR-TO-EQUIAXED TRANSITION IN SOLIDIFICATION PROCESSING (CETSOL)
Research Area: Materials Science
Expedition(s): 21- 30
Principal Investigator(s): ● Charles-Andre Gandin, MINES ParisTech CEMEF, Sophia
Antipolis, France
● Bernard Billia, Aix-Marseille Universite ́, Marseille, France
● Gerhard Zimmermann, ACCESS e.V., Aachen, Germany
● Yves Fautrelle, Centre National de la Recherche Scientifique,
Grenoble, France
● David Browne, National University of Ireland, Dublin, Ireland
● David Poirier, University of Arizona, Tucson, Arizona


RESEARCH OBJECTIVES
Columnar-to-Equiaxed Transition in Solidification (CETSOL) investigates the transition from one
structure to the other in a weightlessness environment to alleviate buoyancy-driven melt flow
and the sedimentation of the equiaxed grains. This provides the benchmark samples to validate
numerical models that attempt to predict and describe this transition that is of high relevance
to industrial alloys as the properties of a casting depend on the grain structure.


RESULTS
Experimental results are obtained in space using Aluminum-Silicon alloys with, or without, a
grain refining suspension directionally solidified under different conditions in the Materials


Science Laboratory (MSL) on-board the International Space Station (ISS). The analysis of the
space samples confirms the occurrence of a columnar-to-equiaxed transition, especially in the
refined alloy. Temperature evolution and grain structure analysis provide critical values for the
position, the temperature gradient, and the solidification velocity at the columnar to equiaxed
transition. A sharp transition is detected when the rate of solidification is increased in contrast
to the progressive transition seen when lowering the temperature gradient. Triggering the
transition appears to be more difficult in the absence of grain refiner since none of the
corresponding samples showed it whereas it was found in sample processed on the ground
under similar conditions. This effect is potentially due to the detachment of fragment of
dendrites that are less dense than the melt and float up due to gravity towards regions where
they serve as nucleation sites for equiaxed grains. Such interpretation appears to be supported


A typical microstructure obtained in a sample containing grain refiner is featured in the picture below. ESA Image.
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