Reduced Gravity Environment Of Space Research Articles

Columnar-EquiaxedTransition inSolidification processing: The ESA-MAP CETSOL project

Many castings are the result of a competition between the growth of columnar and equiaxed grains. Indeed, microstructures are at the center of materials science and engineering, and solidification is the most important processing route for structural materials, especially metals and alloys. Presently, microstructure models remain mostly based on diffusive transport mechanisms so that there is a need of critical benchmark data to test fundamental theories of microstructure formation, which often necessitates to have recourse to solidification experiments in the reduced-gravity environment of space. Accordingly, the CETSOL (Columnar-Equiaxed Transition in SOLidification processing)-MAP project of ESA is gathering together European groups with complementary skills to carry out experiments and model the processes, in particular in view of the utilization of reduced-gravity environment that will be afforded by the International Space Station (ISS) to get benchmark data. The ultimate objective of the CETSOL research program is to significantly contribute to the improvement of integrated modeling of grain structure in industrially important castings. To reach this goal, the approach is devised to deepen the quantitative understanding of the basic physical principles that, from the microscopic to the macroscopic scales, govern microstructure formation in solidification processing under diffusive conditions and with fluid flow in the melt. Pending questions are attacked by well-defined model experiments on technical alloys and/or on model transparent systems, physical modeling at microstructure and mesoscopic scales (e.g. large columnar front or equiaxed crystals) and numerical simulation at all scales, up to the macroscopic scales of casting with integrated numerical models.

Preparation of the initial solid–liquid interface and melt in directional solidification

The preparation of the initial conditions (solid–liquid interface morphology and solute segregation in the liquid phase) on which growth is started is a very critical step in directional-solidification experiments. Dedicated experiments on Al–1.5 wt% Ni consisting in directional melting followed by thermal stabilisation with different lengths, show that precise control is in practice not straightforward. Indeed, in the mushy zone created by melting the original solid sample, temperature gradient zone melting (TGZM) causes migration of solute-rich liquid droplets and channels. A model is proposed to describe this process and validate the physical interpretation of the experiments through numerical simulation. Knowing the status of the preparation, the intriguing observations in the partially melted region of the Al–1.5 wt% Ni alloys solidified in the Advanced Gradient Heating Facility of European Space Agency during the LMS and STS-95 space missions can now be explained. Finally, the influence of initial interface morphology and melt segregation on directional-solidification transient is discussed, based on a comparison of Al–Ni alloys with hypoeutectic Al–Li alloys previously grown on Earth and in space. It follows that for experiments achieved on original rods with equiaxed microstructure, the efficiency of the preparatory melting and stabilisation phases can be evaluated from the solute macrosegregation profile in the region in between the non-melted solid and directional solidification. The major conclusion is that when the melt is mixed by fluid flow, the initial conditions are near to their asymptotic state at the end of TGZM whereas, when solute diffusion is the mode of transport into the bulk liquid, the condition of homogeneous melt becomes limiting and too much time-consuming to be fulfilled, which in particular holds for the 3D-experiments carried out in the reduced-gravity environment of space.

Materials Processing in the Reduced-Gravity Environment of Space

▪ Abstract The phase-field method has recently emerged as a powerful computational approach to modeling and predicting mesoscale morphological and microstructure evolution in materials. It describes a microstructure using a set of conserved and ...Read More