The Materials Science Laboratory - Columnar-to-Equiaxed Transition in Solidification Processing and Microstructure Formation in Casting of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions (MSL-CETSOL and MICAST) are two investigations that support research into metallurgical solidification, semiconductor crystal growth (Bridgman and zone melting), and measurement of thermo-physical properties of materials. This is a cooperative investigation with the European Space Agency and NASA for accommodation and operation aboard the International Space Station.Principal Investigator(s)
Project User Group
Transvalor S.A., , , France
Alcan CRV, Voreppe, , France
Arcelor Research S.A., Paris, , France
CorusTechnology BV, , , Netherlands
Dunaferr Zrt., , , Hungary
European Space Agency (ESA), Noordwijk, , Netherlands
Project User Group
Femalk Rt., Budapest, , Hungary
Honeywell International Technologies Ltd., , , Ireland
Hydro Aluminium GmbH, , , Germany
MAL Magyar Aluminium Rt., Budapest, , Hungary
Snecma - Safran S.A., , , France
National Aeronautics and Space Administration (NASA)Sponsoring Organization
Human Exploration and Operations Mission Directorate (HEOMD)ISS Expedition Duration:
October 2009 - May 2012
21/22,23/24,27/28,29/30Previous ISS Missions
CETSOL and MICAST hardware are delivered to the ISS during Expedition 18.
Aluminum alloys are a standard cast metal used in a number of automotive and transportation applications, allowing manufacturers to reduce vehicle weight, increase the strength of components and improve emission controls. One of the most challenging problems associated with aluminum casting is the influence of convection during all stages of solidification. The strength of fluid flow changes the "as cast" internal structure (microstructure) such that the yield, fracture and fatigue strengths of the cast ingot can vary considerably. Although the importance of fluid flow has been recognized for decades, not even a simple model has been developed to predict the effect on microstructure.
Materials Science Laboratory - Columnar-to-Equiaxed Transition in Solidification Processing (CETSOL) and Microstructure Formation in Casting of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions (MICAST) are two investigations which will examine different growth patterns and evolution of microstructures during crystallization of metallic alloys in microgravity.
The major objective CETSOL is to improve and validate the modelling of Columnar-Equiaxed Transition (CET) and of the grain microstructure in solidification processing. This aims to give industry confidence in the reliability of the numerical tools introduced in their integrated numerical models of casting, and their relationship. To achieve this goal, intensive deepening of the quantitative characterization of the basic physical phenomena that, from the microscopic to the macroscopic scales, govern microstructure formation and CET will be pursued. This endeavor will be based on the benchmark data obtained from systematic series of critical experiments under diffusive conditions (critical experiments under convective conditions), with fluid flow in the melt due to natural buoyancy-driven convection (convection controlled by applying an external field, magnetic field or vibration).
CETSOL provides science teams and industrial partners confidence in the reliability of the numerical tools introduced in their integrated numerical models of metallic alloy casting. To achieve this goal, intensive deepening of the quantitative characterisation of the basic physical phenomena that, from the microscopic to the macroscopic scales, govern microstructure formation and CET will be pursued.
Columnar-to-equiaxed transition (CET) occurs during columnar growth when new grains grow ahead of the columnar front in the undercooled liquid. Under certain conditions, these grains can stop the columnar growth and then the solidification microstructure becomes equiaxed. Experiments planned in the framework of the CETSOL experiment are expected to take place in facilities on board the International Space Station (ISS). This is justified by the long-duration required to solidify samples with the objective to study the columnar-to-equiaxed transition. Indeed, the length scale of the grain structure when columnar growth takes place is of the order of the casting scale rather than the microstructure scale. This is due to the fact that, to a first approximation, it is the heat flow that controls the transition rather than the solute flow. Experimental programs are being carried out on ground by the science team and industrial partners on aluminium-nickel and aluminium-silicon alloys.
MICAST studies microstructure formation during casting of technical alloys under diffusive and magnetically controlled convective conditions. The experimental results together with parametric studies using numerical simulations, will be used to optimize industrial casting processes.
MICAST identifies and controls experimentally the fluid-flow patterns that affect microstructure evolution during casting processes, and to develop analytical and advanced numerical models. The microgravity environment of the International Space Station (ISS) is of special importance to this project because only there are all gravity-induced convections eliminated and well-defined conditions for solidification prevail that can be disturbed by artificial fluid flow being under full control of the experimenters. Design solutions that make it possible to improve casting processes and especially aluminum alloys with well-defined properties will be provided.
MICAST studies the influence of pure diffusive and convective conditions on aluminium-silicon (AlSi) and aluminium-silicon-iron (AlSiFe) cast alloys on the microstructure evolution during directional solidification with and without rotating magnetic field.
The MSL-CETSOL and MICAST investigations will provide a unique insight into microgravity solidification processes of cast alloys under well controlled conditions.Earth Applications
These linked experiments aim to improve our understanding of the solidification processes of metallic alloys. As the mechanical properties, and therefore potential Earth-based applications, are directly related to solidification conditions, it is crucial to validate the predictions of numerical models that describe solidification processes. Additionally, this research help Industry improve its knowledge of casting processes, so that later on tailored metallic alloys can be created for several applications of our daily life.
Each sample cartridge assembly (SCA) shall be fully processed in the MSRR MSL LGF furnace, including final solidification step. After return on Earth, the SCA's are destructively analyzed by the investigators. The structure of the solidified metallic alloy is then compared to predictions derived from complex numerical codes. This comparison helps to adapt and improve the numerical codes developed by scientists.Operational Protocols
The crewmember will insert one SCA into the MSRR MSL LGF. Following power on, the MSRR MSL LGF and vary the power profile of the various furnace heaters to characterize the thermal behavior of the melted metallic alloy in the SCA. Temperature sensors signals will be downlinked to Earth for in-depth assessment by science teams. Numerical codes will provide additional information about the state of the SCA under the thermal constraints on orbit. For each SCA the conditions of the Rotating Magnetic Field (RMF) of the MSL will be varied. Following cool down of the furnace, the SCA is removed from the MSL/LGF furnace and stowed passively until return to Earth. The samples for MSL CETSOL and MICAST are as follows:
When a molten metal or alloy cools and crystallizes, the resulting solid generally has two competing types of grain structures. At first, fast cooling of the melt normally forms columns of long branching grains growing inward from the side walls . Then as internal heat is shed from the remaining liquid fraction, the cooling rate decreases which often leads to seeding and growth of equiaxed (having axes of about the same length) grains. This effect is described as a columnar-to-equiaxed transition (CET) and is very important, and highly studied, in metal forming processes and metallurgy since it greatly affects the physical properties and behavior of virtually all metallic products, including high-value parts such as single crystal turbine blades in aircraft engines. CET experiments to study and control this transitional process have been successfully performed in the Materials Science Laboratory (MSL) with the Low Gradient Furnace (LGF) module onboard the ISS from November 2009 until April 2010.
Turbulent melt flow is minimized in space which enables growth of equiaxed grains free of sedimentation and buoyancy effects. The critical phases of each microgravity experiment, i.e. the homogenization and solidification phases, were performed during sleep periods of the astronauts to reduce, as well, vibrational disturbances. Gravity sensors data close to the MSL confirm that a gravity level below ▒0.0005 g was achieved during all experiments, g = 9.8m/s▓ on Earth. Aluminium-silicon (AlSi) alloys with and without grain refiners (particles added to limit crystal grain branching) were processed successfully in the LGF. First analysis shows that in the non grain refined samples columnar dendritic growth exists, whereas CET is observed in the grain refined samples. critical parameters for the temperature gradient and the cooling rate describing CET are determined from analysis of the thermal data and the grain structure. 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 (Zimmermann et al. 2011).
Preliminary results of an AlSi mixture with grain refiners show that, during solidification, the columnar crystallization front advances forward and an undercooled liquid zone develops ahead the front, thus facilitating equiaxed crystal formation. Equiaxed nucleation with grain refiners follows the free growth model in simulation. In most castings, grain refiner particles may be engulfed or pushed by the growing solid liquid interface. So, these grain refiner particles cannot initiate grains and normally end up in the grain boundaries, thus general grain refiner efficiency is very low. It was found that the efficiency of the grain refiners is at a maximum when addition level is low. Experimental CET, in this case, is at a distance of ~128 ▒ 2 mm versus the simulation distance of 127.5 mm. Hence the agreement between model simulation and experiment is reasonably strong. The columnar length is approximately equal to the distance the furnace is moving at a slower velocity and, therefore, it is possible to suggest that CET is related to the velocity jump and resulting temperature change. More studies of alloy systems without grain refiners are being conducted, and the influences of grain refiners need to be evaluated further (Mirihanage et al. 2011).
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