Benchmark solidification experiments were successfully performed under microgravity conditions on-board the International Space Station (ISS) within the ESAprogramme CETSOL (Columnar-to-Equiaxed Transition in SOLidification Processing). Cylindrical samples of grain-refined Al-4wt.%Cu, Al-10wt.%Cu and Al-20wt.%Cu alloys were directionally solidified in a gradient furnace to investigate columnar and equiaxed dendritic growth structures as well as the columnar to equiaxed transition under diffusive conditions. The determination of temperature gradients; interface velocities; and cooling rates at liquidus, solidus, and eutectic front positions provides well-defined thermal experimental characterization. The evaluation of the flight samples demonstrates that no significant macrosegregation along the sample axis occurred and no radial effects were observed. Therefore, purely diffusive solidification behaviour without any residual melt convection can be assumed for these microgravity experiments. The analyses of the microstructure in longitudinal cross-sections show dendritic structures without any pore formation and the averaged eutectic fraction is largely constant along the sample. The samples of refined Al-4wt.%Cu alloy show a sharp CET from columnar dendrites to a fine equiaxed steady-state grain structure whereas in the samples of refined Al-10wt.%Cu and Al-20wt.%Cu alloy, only equiaxed dendritic grain growth is observed. A quantitative analysis of the equiaxed grain morphology shows, that the shapes of the equiaxed dendrites depend on the applied temperature gradient, but the grain sizes in radial and longitudinal directions are identical. Therefore, a fully equiaxed dendritic growth structure without dendrite elongation was obtained. Compared to experiments in microgravity with non-refined Al-Cu alloys the average equiaxed grain size is about three times smaller.

During metallurgical processing, gas bubbles nucleate and interact with the microstructure, which can result in severe defects. The research presented in this article focuses on in situ observation of bubble movement in semi-solid aluminum alloys (Al-Ge and Al-Cu) during a melting cycle. X-radiography is used to monitor the migration of H-bubbles in the semi-solid mush. For both alloys, H-bubbles are observed to burrow through the mushy zone towards the colder side of a horizontal gradient furnace leaving behind solute-enriched channels. This stands in contrast to the commonly observed bubble movement towards the hot side of the material caused by Marangoni flow-fields surrounding the gas pocket induced by gradients in the bubble's surface tension. Different dependencies of the surface tension on temperature and concentration for the two alloys are presented as explanation for the observed atypical phenomenon. These results suggest different dominant effects acting on the bubbles, temperature for Al-Ge and concentration for Al-Cu, and explain the cryophile behavior observed.

Interaction of melt flow with solidification microstructures is of fundamental interest and technical relevance due to its effect on the selection of characteristic microstructural features and length scales. Here, we report on in-situ optical observation of both melt flow at the solidification front and microstructural evolution in polycrystalline cellular and dendritic microstructures during upwards directional solidification of a transparent succinonitrile-2.2wt.%(d)camphor alloy in a bulk rectangular sample. Melt flows at different flow intensity levels were applied and the microstructures were characterized in terms of morphology, spacing and growth direction.

Much research has already been focused on the solid-bubble interaction in the interdendritic space for solidifying materials. However, commonly, bubble nucleation is not limited to the mushy zone but also occurs in the liquid melt. In the present research on an Al- $$10 \, \%\mathrm {wt. \,}$$ Cu alloy, the interaction between these bubbles and the approaching solidification front becomes apparent under in situ X-radiography and allows for new insights into the influence of bubbles on the solidifying microstructure. The observed effects comprise bulging of the solidification front toward the bubble, bending of dendrites in front of the bubble, coronal outgrowths surrounding the bubbles, as well as bubble growth, bubble pushing, and bubble eruption. It is found that for the present Al–Cu alloy, the local variation in the solidification speed can be attributed to the bubbles’ insulating properties. The range of this effect was observed to be up to $$900 \,\upmu \text {m}$$ , depending on the bubble diameter, locally increasing solidification speed by up to $$350 \, \%$$ . The influences of Marangoni vortices and coronal nucleation of misoriented dendrites around bubbles on the homogeneity of the microstructure are discussed. A comparison with experiments on model alloys and simulations from various other studies highlights the similarities and differences to this metallic alloy system.

The effect of solidification velocity and electromagnetic stirring on grain refining was investigated experimentally during the directional solidification of rod-like Al-10wt%Cu alloy samples. Applying low solidification velocities leads to a dendritic microstructure consisting of elongated equiaxed crystals, which result from fragmented dendrite arms forming new grains. This grain-refining effect vanishes for higher solidification velocities, leading to a microstructure dominated by a lower number of larger columnar grains. Moderate electromagnetic stirring under laminar flow conditions does not promote grain refinement. By contrast, a sufficiently strong forced melt flow induced by a rotating magnetic field significantly increases the number of grains in the range of solidification velocities investigated within this study. It is assumed that a turbulent melt flow supports the fragmentation of dendrite arms and thus the formation of new grains, which finally leads to grain refinement.

The formation of the grain structure in Al-10 wt%Cu alloy during directional solidification was investigated experimentally. The alloy composition was chosen because of its special feature that both the initial melt composition and the solidifying primary Al dendrites have almost identical densities. Therefore, gravity-related effects such as buoyancy or sedimentation acting on nucleated or fragmented solid particles in the melt are expected to be minor. In 3D bulk samples at low solidification velocities, unexpected equiaxed grain growth was found instead of columnar growth. This behavior was investigated in accompanying solidification experiments with thin samples using in-situ X-ray diagnostics. It is demonstrated that fragments detach from the dendrite tip region and move slightly ahead of the solid liquid interface as they grow. As a result, a dendritic microstructure consisting of elongated equiaxed grains is developed. Accordingly, fragmentation was identified as responsible for grain refinement in the given parameter range. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Solidification experiments and numerical simulations have been performed to improve the understanding of the complex interrelation between melt flow and the formation of dendritic structures during solidification of Al-Cu and Ga-In alloys. Melt flow induces various effects on grain morphology primarily caused by convective transport of solute, such as a facilitation of the growth of primary trunks or lateral branches, dendrite remelting, fragmentation or freckle formation depending on the dendrite orientation, the flow direction and intensity. Within this project special interest was focused on fragmentation and segregation phenomena. Natural convection is caused by density variations within the solidifying alloys. Forced convection was produced by electromagnetic stirring. X-ray radioscopy was applied as a powerful tool for the visualization of dendritic growth and coarsening.

In the framework of an ESA-MAP project entitled XRMON, directional solidification experiments of Al — 20 wt% Cu with in situ and real-time X-ray radiography were carried out during Parabolic Flight campaigns. Parabolic flights offer successions of periods with different gravity levels, allowing the investigation of the impact of gravity level variations on the solidification microstructure formation. Directional solidifications of refined Al — 20 wt% Cu alloy were investigated in a dedicated apparatus for a wide range of cooling rates and a constant temperature gradient. X-ray radiography was successfully used to observe the microstructure evolution following the variations of gravity level. During the columnar growth of the refined alloy a sharp increase of gravity level provoked the sudden nucleation of numerous grains ahead of the front. The most potent explanation of this effect is the variation of the liquid undercooling ahead of solid/liquid interface due to the changes of hydrostatic pressure in the melt.

Solidification experiments on refined and non-refined Al–Cu alloys were carried out on Earth and during two parabolic flights to investigate the influence of gravity level upon solidification dynamics. In situ and real-time monitoring of the microstructure formation was characterized by using an X-ray radiography device based on a microfocus X-ray source. This paper presents for the first time direct observation of metal alloy directional solidification under varying gravity level. By a comparative study between the experiments, the influence of gravity on fragmentation and dendrite fragments transport was enlightened. In addition, a spectacular transition from a columnar to a fully equiaxed microstructure was revealed when there is a sharp increase of gravity level. It was shown that this phenomenon is a consequence of a sudden increase of the liquid undercooling ahead of the columnar front accompanying the change in hydrostatic pressure.

It is well known that the final properties of materials are strongly related to the microstructures formed during growth and to the accompanying segregation, both being very sensitive to the natural hydrodynamic movements in the melt induced by gravity. Therefore, a deeper understanding of gravity effects on the solidification microstructure is of great importance for industrial applications.In the framework of the ESA-MAP project entitled XRMON (in-situ X-Ray MONitoring of advanced metallurgical processes under microgravity and terrestrial conditions), directional solidification experiments with in situ X-ray radiography were carried out during the 60th and 61st ESA PF campaigns on-board the Airbus A300 operated by Novespace. Parabolic flights offer several successions of periods with normal gravity between two parabolas, and hyper gravity and microgravity during each parabola, which allows the impact of gravity level variations on the solidification microstructures to be investigated.For this purpose, a dedicated apparatus was designed and developed in collaboration with SSC (Swedish Space Corporation). XRMON-PFF (Parabolic Flight Facility) includes a Bridgman furnace dedicated to the solidification of Al-based alloys with an X-ray device that enables in situ characterization. Columnar and/or equiaxed growth of refined and non-refined Al-20wt.%Cu alloys were investigated and X-ray radiography was successfully used to assess the effect of periodic variations of the gravity level on the solidification microstructure formation. Preliminary results confirmed the strong influence of gravity on the solidification microstructure development.

A laboratory based high resolution x-ray radiograph was developed for the investigation of solidification dynamics in alloys. It is based on a low-power microfocus x-ray tube and is potentially appropriate for x-ray diagnostics in space. The x-ray microscope offers a high spatial resolution down to approximately 5 μm. Dynamic processes can be resolved with a frequency of up to 6 Hz. In reference experiments, the setup was optimized to yield a high contrast for AlCu-alloys. With samples of about 150 μm thickness, high quality image sequences of the solidification process were obtained with high resolution in time and space.

In the solidification of metallic alloys, columnar or equiaxed growth may occur depending on, among others, the experimental conditions, the thermophysical properties of the alloy and the presence of grain refining particles. In transient processes, typical for many casting situations, a columnar-to-equiaxed transition (CET) is often observed. We have investigated the CET experimentally in AlSi7 alloys with and without inoculants within the framework of the ESA microgravity application promotion programme (MAP) columnar-to-equiaxed transition in solidification processing (CETSOL). The effect of different cooling rates on the CET was studied in addition to the effect of grain refinement in directional solidification. From measured temperature profiles in the samples during solidification and from the analysis of longitudinal and transversal sections of the samples, critical parameters at the CET were determined and compared to predictions from numerical models following Martorano et al. and from Hunt. It was found that the results are in good agreement with the former for non-refined AlSi7 alloys and that grain refinement shifts the critical parameters significantly, concerning critical gradient and nucleation undercooling.

A method of controlling the actual growth velocity during directional solidification based on ultrasound has been developed. For this purpose a pulse echo technique is used to measure the actual solidification rate online. This quantity is used to control the furnace velocity. Solidification experiments with metallic alloys and constant furnace velocity often result in non-steady actual solidification rates. Experiments carried out with online process control demonstrate that a really steady-state solidification with a constant solidification rate is achieved.