During casting often a dendritic microstructure is formed, resulting in a columnar or equiaxed grain structure, or leading to a transition from columnar to equiaxed growth (CET). Especially the detailed knowledge of the critical parameters for CET is important, because the microstructure determines significantly the materials properties. To provide unique data for testing of fundamental theories of grain and microstructure formation, solidification experiments in microgravity environment were performed within the ESA MAP project CETSOL. Reduced gravity allows for pure diffusive solidification conditions, i.e., suppressing melt flow and sedimentation and floatation effects. On-board the International Space Station ISS Al-7wt%Si alloys with and without grain refiners were solidified in different temperature gradients and with different cooling conditions. Detailed analysis of the microstructure and the grain structure showed columnar growth in case of non-refined alloy. CET was detected 2 only for refined alloys, either as a sharp CET in case of a sudden increase of the solidification velocity, or as a progressive CET in case of a continuous decrease of the temperature gradient. The unique experimental data were used for numerical modelling of CET with three different approaches: (i) a front tracking model using an equiaxed growth model, (ii) a 3D CAFE model, (iii) a 3D dendrite needle network (DNN) method. Each model allows predicting the columnar dendrite tip undercooling and the growth rate with respect time. Furthermore, the positions of CET and the spatial extent of the CET, being sharp or progressive, are in reasonably good quantitative agreement with experimental measurements.

During the solidification of hypoeutectic Al–7% Si alloy, density differences develop in the melt due to variations in concentration and temperature. On Earth, melt flow can occur due to gravity, which then affects the solidification process. The microgravity environment strongly eliminates convection in the melt and allows investigation of the solidification process in purely diffusive circumstances. In this study, four solidification experiments were performed on grain-refined and non-grain-refined Al–7 wt% Si alloy on-board the International Space Station (ISS) in the Materials Science Lab (MSL) to study the effect of solidification parameters (solid/liquid front velocity (v) and temperature gradient (G)) on the grain structure and dendritic microstructure. The grain structure has been analyzed in detail in some earlier studies. The aim of this work was to carry out detailed analysis of the macrosegregation caused by the diffusion of Si from the initial mushy zone during the homogenization step and the subsequent solidification phase of the experiments as well as the correlated distribution of eutectic along the solidification direction. The secondary dendrite arm spacing (SDAS) for different process conditions was also studied. For these two issues, microgravity experimental results were compared to simulation results. The macrosegregation was calculated by the finite difference method. Because the steady-state solidification conditions were never reached, the solidification process was characterized by the average front velocity and temperature gradient. Considering the actual liquidus temperature (TL) caused by macrosegregation, the SDAS was calculated as a function of the average processing parameters and the actual liquidus temperature with the classical Kirkwood’s equation. As a result, good agreement was obtained between the calculated and measured SDAS.

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