Laszlo Sturz

Investigation of columnar-to-equiaxed transition in solidification processing of AlSi alloys in microgravity – The CETSOL project

Grain structures observed in most casting processes of metallic alloys are the result of a competition between the growth of several arrays of dendrites that develop under constrained and unconstrained conditions. Often this leads to a transition from columnar to equiaxed grain growth during solidification (CET). A microgravity environment results in suppression of buoyancy-driven melt flow and so enables growth of equiaxed grains free of sedimentation and buoyancy effects. This contribution presents first results obtained in experiments on-board the International Space Station (ISS), which were performed in the frame of the ESA-MAP programme CETSOL. Hypoeutectic aluminium-silicon alloys with and without grain refiners were processed successfully in a low gradient furnace (MSL-LGF). First analysis shows that in the non grain refined samples columnar dendritic growth exists, whereas CET is observed in the grain refined samples. From analysis of the thermal data and the grain structure the critical parameters for the temperature gradient and the cooling rate describing CET are determined. These data are used for initial numerical simulations to predict the position of the columnar-to-equiaxed transition and will form a unique database for calibration and further development of numerical CET-modeling.

Advanced Solidification Studies on Transparent Alloy Systems: A New European Solidification Insert for Material Science Glovebox on Board the International Space Station

Investigations on solidifying transparent model alloys have served frequently to gain knowledge on physical phenomena occurring during solidification of metallic alloys. However, quantitative results were obtainable in thin samples where convection can successfully be suppressed. Quantitative studies on three-dimensional phenomena not being affected by natural convection are thus only possible under microgravity conditions. Therefore, the European Space Agency (ESA) is planning to launch a new insert for the material science glovebox on board of the International Space Station for studies on solidification phenomena in thick samples. Four different classes of transparent model alloys will be used to address the following scientific topics: (I) columnar to equiaxed transition in solidification processing, (II) novel peritectic structures and in situ composites; (III) solidification along an eutectic path in binary alloys; and (IV) solidification along an eutectic path in ternary alloys. In this article, we give details on the scientific objectives and the operational features ESA’s new solidification device will offer.

Columnar-to-equiaxed transition in solidification processing of ALSi7 alloys in microgravity-the CETSOL project

This paper gives an overview of the experiments on-board the International Space Station (ISS) performed so far by the CETSOL team. Al-7 wt% Si alloys with and without grain refiners were solidified in microgravity. Detailed grain structure analysis showed columnar growth in case of non-refined alloy, but the existence of a columnar to equiaxed transition (CET) in refined alloy. One main result is a sharp CET when increasing the solidification velocity and a progressive CET for lowering the temperature gradient. Applying a front tracking model this behavior was confirmed numerically for sharp CET. Using a CAFE model both segregation and grain structures were numerically modeled and show a fair agreement with the experimental findings.

Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL): a project of the European Space Agency (ESA) - Microgravity Applications Promotion (MAP) programme

The main objective of the research project of the European Space Agency (ESA) - Microgravity Application Promotion (MAP) programme entitled Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL) is the investigation of the formation of the transition from columnar to equiaxed macrostructure that takes place in casting. Indeed, grain structures observed in most casting processes of metallic alloys are the result of a competition between the growth of several arrays of dendrites that develop under constrained and unconstrained conditions, leading to the CET. A dramatic effect of buoyancy-driven flow on the transport of equiaxed crystals on earth is acknowledged. This leads to difficulties in conducting precise investigations of the origin of the formation of the equiaxed crystals and their interaction with the development of the columnar grain structure. Consequently, critical benchmark data to test fundamental theories of grain structure formation are required, that would benefit from microgravity investigations. Accordingly, the ESA-MAP CETSOL project has gathered together European groups with complementary skills to carry out experiments and to model the processes, in particular with a view to utilization of the reduced-gravity environment that will be afforded by the International Space Station (ISS) to get benchmark data. The ultimate objective of the research program is to significantly contribute to the improvement of integrated modelling 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. Pertinent questions are attacked by well-defined model experiments on technical alloys and/or on model transparent systems, physical modelling 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.

The Influence of Fluid Flow on Intermetallic Phases in Al-cast Alloys

Technical Al-Si alloys always contain sufficient amounts of Fe and Mn, especially alloys made from scrap. During casting, Fe-containing intermetallics, such as Al-Fe, Al-Fe-Si and Al-Fe- Mn-Si phases, are formed between the aluminum dendrites. Fe and Mn-rich intermetallic phases are well known to be strongly influential on mechanical properties in Al-Si alloys. In the present work the influence of controlled fluid flow conditions on the morphology and spatial arrangement on intermetallic phases in cast Al-Si alloys is characterized. A binary Al-7wt.%Si and a ternary Al- 7wt.%Si-1wt.%Fe alloy was solidified under and without the influence of a rotating magnetic field (3mT at 50Hz) over a range of solidification velocities (0.015- 0.18mm/s) at a constant temperature gradient G of 3K/mm. The scientific results reached so far indicate a strong influence of the electromagnetic stirring on the primary dendrite and secondary dendrite arm spacings.

Modeling of heat and solute interactions upon grain structure solidification

Simulations of several laboratory experiments developed for the study of structure and segregation in casting are presented. Interaction between the development of dendritic grain structure and segregation due to the transport of heat and mass by diffusion and convection is modeled using a Cellular Automaton - Finite Element model. The model includes a detailed treatment of diffusion of species in both the solid and liquid phases as presented elsewhere in this volume [1]. Applications deal with prediction of columnar and equiaxed grain structures, as well as inter-dendritic and inter-granular segregations induced by diffusion and macrosegregation induced by thermosolutal buoyancy forces.

Investigations on Columnar-to-Equiaxed Transition in Binary Al Alloys with and without Grain Refiners

We have investigated the columnar to equiaxed transition experimentally in directionally solidified hypoeutectic binary AlSi alloys with and without grain refinement particles and for different processing parameters in the framework of the ESA-MAP CETSOL (Columnar-to-equiaxed transition in solidification processing). A power-down technique was used in a Bridgman-Stockbarger type gradient-furnace to simultaneously increase the solidification rate and decrease the temperature gradient in the cylindrical sample during directional solidification vertically upwards. The position of the CET was determined from the cut and polished samples and correlated to the applied cooling rate for different experiments. Critical parameters for the temperature gradient and the solidification rate at the transition were determined from cooling curves measured within the sample and from a time-of-flight analysis of ultrasonic pulses propagated in the solid part of the sample and being reflected at the solid-liquid interface. The critical values found are compared to the deterministic models of Hunt and of Martorano et al. The objective of this contribution is the presentation of preliminary results for the different alloys and processing parameters of ground-based experiments. These results will be used for the testing of different models describing the CET within the framework of the ESA-MAP CETSOL and for the preparation of comparative microgravity experiments.

Analysis of a Microgravity Solidification Experiment for Columnar to Equiaxed Transitions with Modeling Results

This paper studies the Columnar to Equiaxed Transition (CET) in an Al-7wt%Si binary alloy with and without Al-Ti-B grain refiner. A microgravity experiment was designed to produce a CET in this alloy system. The experiment was flown onboard the MAXUS-7 sounding rocket platform, which achieved twelve minutes of microgravity. Examples of CET were successfully produced during the unmanned flight. Temperature data were recorded from thermocouples in the crucible walls of the furnace. Post-mortem material characterization of the grain structure was also performed. Subsequently a model of the furnace, which used a front-tracking model of solidification and an inverse heat calculation method, was developed. In this paper, results from the model are compared to the experimental findings; agreement is found with the CET predictions. The results from the model are then used to compare findings with the CET criterion of Hunt from the literature. Agreement is found between the model predictions and the Hunt criterion.

Investigation on the Morphological Instability during Directional Solidification of a Transparent Alloy during Sounding Rocket Flights

The movement and morphological change of a solid-liquid interface in directional solidification was investigated during two sounding rocket flights. By using the transparent binary alloy Succinonitrile-Acetone the dynamic processes at the solidification front could be observed directly. Both the planar interface growth, the onset of instability and the characteristic features of the interface morphology, i.e. the evolution of the primary spacing and amplitudes of the cells and dendrites were evaluated. The comparison with a calculation of the morphological instability based on the theoretical model of Warren and Langer showed a good agreement concerning the critical time and velocity of the solidification front.

Directional solidification of cellular arrays in transparent alloys

The evolution of cellular patterns at the solid-liquid interface was investigated in directional solidification of the transparent alloy succinonitrile-acetone in bulk samples under microgravity conditions on Space-Shuttle missions and different processing parameters. Interface morphologies are observed from the top within a Bridgman-Stockbarger type furnace. A hexagonal arrangement of cells on the interface with irregularities was found in all cases. The cell spacings distribution is best described using the numerical model by Lu/Hunt. Comparisons with numerical phase-field simulations and weakly-nonlinear stability analysis also show good agreement.