Comparative study of directional solidification of Al-7 wt% Si alloys in Space and on Earth: Effects of gravity on dendrite growth and Columnar-to-equiaxed transition

Abstract

Directional solidification experiments of grain refined Al -7 wt.% Si alloy were carried out on Earth under normal gravity conditions (1g) and in the Material Science Laboratory on board the International Space Station in microgravity environment (µg), to investigate the impact of the gravity on the solidification microstructure and the columnar-to-equiaxed transition (CET). The increase of the dendrite growth velocity imposed by the processing conditions during the experiments leads to a size decrease of the dendrite microstructure and to a more homogeneous eutectic distribution under both 1g and μg conditions. A progressive CET is obtained in both samples implying the existence of an intermediate region after the inception position of CET defined as the end of growth of the columnar dendrites. However, a more progressive CET and longer dendrites aligned with the applied temperature gradient are observed in presence of gravity. This difference is attributed to the convective flow on Earth.On the one hand, it carries the grains that nucleate ahead of the columnar front away into the bulk liquid phase. On the other hand, it sweeps the solute away from the dendrite tip zone.Consequently, the blocking effect is diminished, allowing extended continuous growth of the elongated dendrites.

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

“…images. As shown in Fig.6a1 to 6d1, columnar structure is observed from the initial region to the entire stage I for all samples as expected under these growth conditions [43,46].…”

“…Therefore, more homogeneous eutectic distribution and lower E% are obtained, accompanied with the decrease of the microstructure size (DAS), due to the stronger dendritic branching triggered by the higher dendrite tip velocity in stage II of B2F5 and B2F6 [2,46]. As expected,…”

Impact of growth velocity on grain structure formation during directional solidification of a refined Al-20 wt.%Cu alloy

“…7b). Such long columnar grains crossing through the other grains were also observed during the solidification of Al-7wt%Si in cylindrical samples under microgravity conditions on board of the ISS (International Space Station) [43]. This behavior was attributed to the better alignment of the columnar dendrite axis with the temperature gradient, that favored its development with respect to the less well oriented equiaxed grains.…”

Impact of gravity on directional solidification of refined Al-20wt.%Cu alloy investigated by in situ X-radiography

“…Diffusive mass transport without any buoyancy effect can be achieved during microgravity experiments [17,[19][20][21][22][23], independently of alloy composition and container shape. However, such type of experiments is rare and very expensive, because of the limited number of microgravity opportunities.…”