Guillaume Reinhart

Fragmentation in an Al–7 wt-%Si alloy studied in real time by X-ray synchrotron techniques

Abstract One mechanism for the formation of equiaxed grains is the detachment of dendrite fragments which is believed to be at the origin of the central equiaxed core region in casting processes. Unfortunately, the dynamics of the fragmentation phenomena cannot be revealed by classical methods. Investigation of a unrefined Al–7 wt-%Si alloy using in situ and real time synchrotron X-radiography and X-ray topography at the European Synchrotron Radiation Facility, has allowed verification of the existence of dendrite fragmentation and of cascade fragmentation during directional solidification, and to study the evolution of the growth and sedimentation of the equiaxed grains formed from these fragments. An examination of the crystallographic misorientation of dendrites as fragmentation is ongoing. These results contribute to the understanding of the characteristics of the columnar to equiaxed transition and to knowledge of the origin of new equiaxed grains in unrefined alloys.

Measurement of solute profiles by means of synchrotron X-ray radiography during directional solidification of Al - 4 wt% Cu alloys

In the present study, we report on an image analysis procedure, which enables to extract from synchrotron radiographs the long range solute profiles in the whole sample and in both phases (solid and liquid). This image analysis is based on the measurement of local density differences, and is applied to study the directional solidification of Al - 4wt% Cu alloy, from planar to onset of the initial instability. Dedicated experiments were carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). In order to validate this analysis the value of a key solidification parameter, namely the partition coefficient, was experimentally determined during the planar solidification, and a very good agreement was found with value found usually in the literature. On a further step, the evolution of the microstructure and solute profile during the initial transient of solidification was analysed in detail.

Thermoelectric magnetic flows in melt during directional solidification

Thermoelectric magnetic (TEM) flows in melts, which are generated by TEM forces in liquids, were uncovered by the shape evolution of the planar solid/liquid interface during directional solidification. The solid/liquid interface developing from an initially tilted shape to a nearly flat one has been in situ and real-time observed by means of synchrotron X-ray radiography. The corresponding numerical 3D simulations and velocity measurements of flows in the melt confirm that TEM flows exist and respond to this interface shape change. This observation provides visible evidence for TEM flows in melt and their influence on the solid/liquid interface dynamics when directional solidification is conducted in a magnetic field.

Investigation of gravity effects on solidification of binary alloys with in situ X-ray radiography on earth and in microgravity environment

As most of the phenomena involved during solidification are dynamic, in situ and real-time X-ray imaging should be retained as the method of choice for investigating the solidification front evolution of metallic alloys grown from the melt. On Earth, natural convection in the melt is well known to be the major source of various disturbing effects which can significantly modify or mask important physical mechanisms. Microgravity environment is an efficient way to eliminate buoyancy and convection to provide benchmark data for the validation of models and numerical simulations. In addition, a comparative study of solidification experiments carried out on Earth and in space can also enlighten the effects of gravity. In the frame of the ESA - MAP programme entitled XRMON, an experimental set-up has been developed to perform directional solidification in microgravity conditions with in situ X-ray radiography observation. In the first part of this paper, we will present a brief review of some effects induced by gravity on the solidification process and investigated by mean of synchrotron X-ray radiography at ESRF (European Synchrotron Radiation Facility). In the second part of this paper, we will describe some results obtained with a prototype of the XRMON-Gradient Furnace set-up. These preliminary results show the large capabilities of the experimental set-up in terms of thermal behaviour, as well as X-ray observation.

Direct simulation of a directional solidification experiment observed in situ and real-time using X-ray imaging

It has been shown in the last decade that in situ and real-time observation of metallic alloy solidification is possible by using X-ray monitoring conducted at third generation synchrotron sources. A detailed analysis of a Bridgman experiment carried out at ESRF with an Al - 3.5 wt% Ni alloy was presented earlier [1]. This article proposes a direct simulation of the solidification of the entire sample for this experiment, in which all the dendritic grains are individually represented as they nucleate and grow in the experiment. This is possible by extracting from the radiographs a list of all the nucleated grains, including the positions and orientations of their main trunks. Simulation is performed using a two-dimensional (2D) Cellular Automaton (CA)-Finite Element (FE) model. As a result of the coupling between the CA and FE methods, consequences of the macroscopic transport of heat, liquid momentum and solute mass on the development of the dendritic grain structure are accounted for, and vice versa. The macroscopic deformation of the columnar front observed during the experiment is reproduced, as well as the columnar-to-equiaxed transition. The influence of flow patterns on macrosegregation is also discussed.

Microstructural analysis of rapidly solidified aluminium–nickel alloys

Abstract Powders of Al–50 wt-%Ni and Al–36 wt-%Ni were produced using the impulse atomisation technique, a rapid solidification technique. The molten droplets were cooled in flight by the stagnant helium or nitrogen in the atomising chamber. The resulting powders were sieved into different size ranges. X-ray diffraction and neutron diffraction were used in order to quantify the phase fractions in the samples. Profile refinement, using the computer software GSAS, was used to calculate the weight fraction of the existing phases, namely Al3Ni2, Al3Ni and Al, as a result of different processing parameters. The Scheil–Gulliver model was applied to investigate the extent to which it can predict phase fractions in the Al–Ni system. In Al–50 wt-%Ni, by increasing cooling rate, the ratio of Al3Ni to Al3Ni2 approaches that of Scheil–Gulliver’s prediction. Opposite behaviour was observed in Al–36 wt-%Ni. In addition, from the profile refinement, the effect of composition and cooling rate on the lattice parameter of Al3Ni2 was investigated. In Al–36 wt-%Ni, the c/a ratio is significantly smaller than the stoichiometric c/a ratio of Al3Ni2, and it decreases with increasing cooling rate. On the other hand, for Al–50 wt-%Ni, the c/a ratio is much closer to the stoichiometric value and it increases with increasing cooling rate. On a produit des poudres d’Al-50% en poids de Ni et d’Al-36% en poids de Ni en utilisant la technique d’atomisation par impulsion, une technique de solidification rapide. Les gouttelettes fondues étaient refroidies en vol par l’hélium ou l’azote stagnants dans la chambre d’atomisation. Les poudres résultantes étaient tamisées en différentes gammes de taille. On a utilisé la diffraction des rayons X et la diffraction des neutrons pour quantifier les fractions de phase dans les échantillons. Le raffinement du profil, obtenu grâce au logiciel d’ordinateur GSAS, était utilisé pour calculer la fraction de poids des phases existantes, soit Al3Ni2, Al3Ni et Al, comme résultat des différents paramètres de traitement. On a appliqué le modèle de Scheil-Gulliver pour vérifier à quel point il peut prédire les fractions de phase du système Al-Ni. Pour l’Al-50% en poids de Ni, en augmentant la vitesse de refroidissement, le rapport d’Al3Ni à Al3Ni2 s’approche de celui de la prédiction de Scheil-Gulliver. On a observé le comportement opposé pour l’Al-36% en poids de Ni. De plus, à partir du raffinement du profil, on a étudié l’effet de la composition et de la vitesse de refroidissement sur le paramètre de réseau de l’Al3Ni2. Pour l’Al-36% en poids de Ni, le rapport c/a est significativement plus petit que le rapport c/a stoechiométrique de l’Al3Ni2, et il diminue avec une augmentation de la vitesse de refroidissement. D’un autre côté, pour l’Al-50% en poids de Ni, le rapport c/a est beaucoup plus près de la valeur stoechiométrique et il augmente avec une augmentation de la vitesse de refroidissement.

Characterization of Motion of Dendrite Fragment by X-Ray Radiography on Earth and under Microgravity Environment

In the frame of ESA-MAP (Microgravity Application Promotion) project entitled XRMON (In situ X-Ray MONitoring of advanced metallurgical processes under microgravity and terrestrial conditions), a microgravity (μg) experiment in the XRMON-GF (Gradient Furnace) setup was successfully launched in 2012 on board MASER 12 sounding rocket. During this experiment, in situ and real time observations of the formation of the solidification microstructures in diffusive conditions were carried out for the first time by using X-ray radiography. In addition, two reference experiments with the same control parameters but in ground-based conditions were performed to enable us a direct comparison with the μg experiment and therefore to enlighten the effects of gravity upon microstructure formation. This communication reports on fragmentation phenomenon observed during those experiments. For 1g upward solidification, fragmentations mainly take place in the upper part of the mushy zone. After their detachments, dendrite fragments are carried away by buoyancy force in the bulk liquid where they are gradually remelted. For μg experiment and horizontal solidification, this type of fragmentation is not observed. However, a great number of fragmentations are surprisingly revealed by in situ observation in the deep part of the mushy zone, when the liquid fraction is very small. Moreover, as soon as they are detached, the dendrite fragments move toward the cold part of the mushy zone, even in the case of μg experiment. The observations suggest that sample shrinkage may be at the origin of this fragment motion.

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.

A method to determine the active particle nucleation undercooling distribution in a refined alloy

We propose a method to determine the active particle distribution of nucleation undercooling in a refined alloy. The experimental data used in this work are inferred from solidification experiments on a refined Al-3.5 wt% Ni alloy performed with X-ray radiography at the European Synchrotron Radiation Facility. These in situ and real time observations allow the accurate and direct determination of the grain origin (heterogeneous nucleation on particles or fragmentation), of the density and of the equiaxed front growth rate. The LGK classical dendrite growth model is used to evaluate the front undercooling (ΔTC) corresponding to the measured equiaxed front growth rate. Then, the corresponding cumulative distribution of active refining particles is determined. From this cumulative distribution, we derive the corresponding Gaussian and log-normal laws to obtain the nucleation undercooling distribution of active particles. Results are discussed and compared to available measurements in the literature. The standard particle distribution parameters (density of nuclei, mean nucleation undercooling and standard deviation) are determined. We plan to use the determined nucleation undercooling particle distribution in a stochastic CAFE model for the grain structure without preliminary adjustment of the nucleation undercooling.

CET during the solidification of refined Al-3.5wt%Ni alloys and characterization of the subsequent grain structure

The mechanical properties of a cast product, and therefore its application, depend strongly on its inner microstructure. During the solidification step a change from columnar to equiaxed grain structure can occur. It is thus critical to understand the physical mechanisms of this transition in order to accurately predict and control its occurrence and the final grain structure morphology. This article reports on observations of the CET (Columnar to Equiaxed Transition) induced by a sudden increase of the pulling velocity during the directional solidification on a refined Al-3.5wt%Ni alloy by using synchrotron X-ray radiography. The influence of the pulling velocity on the blocking of the columnar structure is described. Next, the distribution of surface area and longitudinal asymmetry of the grains after CET are quantitatively characterized and discussed.