Yves Fautrelle

Thermoelectric magnetic force acting on the solid during directional solidification under a static magnetic field

Thermoelectric magnetic force (TEMF), which is induced by the interaction between the thermoelectric current and the applied magnetic field, acting on the solid during directional solidification under a static magnetic field was derived. Equipping the derived equation, an analytical calculation of the velocity of a solid spherical particle submitted to the TEMF was carried out. The experiment with corresponding phenomenon was performed and recorded by the in situ synchrotron X-ray imaging, which permitted a direct measurement of the velocity of the TEMF-driven motion of detached fragments. The measurement of the velocities showed a reasonable agreement with the calculation results.

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.

Modification of liquid/solid interface shape in directionally solidifying Al–Cu alloys by a transverse magnetic field

Al-0.85wt%Cu and Al-2.5wt%Cu alloys were directionally solidified under different transverse magnetic field (TMF) intensities to investigate the influence of TMF on the liquid/solid interface shape with respect to the various length scales appearing (planar, cellular, and dendritic interfaces). Results show that planar and cellular interfaces tilt to one side and then level off with increasing TMF although the dendritic interface appears not to behave in this manner. In situ synchrotron X-ray imaging was applied during directional solidification of the Al-4wt%Cu alloy under a 0.08T TMF, revealing leveling of the initially sloped interface. Solute redistribution, caused by thermoelectric magnetic convection (TEMC), responds to the changes in the interface shape. Because different typical length scales should be used in estimating the velocity of TEMC for planar, cellular, and dendritic interfaces, the maximum velocity of the convection ahead of the interface is obtained under different TMF intensities; correspondingly, leveling of the interface’s degree of slop varies with TMF.

Control of the Macrosegregations during Solidification of a Binary Alloy by Means of a AC Magnetic Field

A numerical model aimed at simulating the segregations during the columnar solidification of a binary alloy is used to investigate the effects of a forced convection. Our objective is to study how the segregation characteristics in the mushy zone are influenced by laminar flows driven both by buoyancy and by AC fields of moderate intensity. Various types of magnetic fields have been tested, namely travelling, rotating magnetic field and slowly modulated electromagnetic forces. The calculations have been achieved on two types of alloys, namely tin-lead and aluminiumsilicon. It is shown that the flow configuration changes the segregation pattern. The change comes from the coupling between the liquid flow and the top of the mushy zone via the pressure distribution along the solidification front. The pressure difference along the front drives a mush flow, which transports the solute in the mushy region. Another interesting type of travelling magnetic field has been tested. It consists of a slowly modulated travelling magnetic field. It is shown that in a certain range of values of the modulation period, the channels are almost suppressed. The normal macrosegregation remains, but the averaged segregation in the mushy zone is weaker than in the natural convection case. The optimal period depends on the electromagnetic force strength as well as the cooling rate. The latter phenomenon cannot occur in the case of rotating magnetic fields, since in that configuration the sign of the pressure gradient along the solidification front remains unchanged. Recent solidification experiments with electromagnetic stirring confirm the predicted macrosegregation patterns.

Refinement and growth enhancement of Al2Cu phase during magnetic field assisting directional solidification of hypereutectic Al-Cu alloy

Understanding how the magnetic fields affect the formation of reinforced phase during solidification is crucial to tailor the structure and therefor the performance of metal matrix in situ composites. In this study, a hypereutectic Al-40 wt.%Cu alloy has been directionally solidified under various axial magnetic fields and the morphology of Al2Cu phase was quantified in 3D by means of high resolution synchrotron X-ray tomography. With rising magnetic fields, both increase of Al2Cu phase’s total volume and decrease of each column’s transverse section area were found. These results respectively indicate the growth enhancement and refinement of the primary Al2Cu phase in the magnetic field assisting directional solidification. The thermoelectric magnetic forces (TEMF) causing torque and dislocation multiplication in the faceted primary phases were thought dedicate to respectively the refinement and growth enhancement. To verify this, a real structure based 3D simulation of TEMF in Al2Cu column was carried out, and the dislocations in the Al2Cu phase obtained without and with a 10T high magnetic field were analysed by the transmission electron microscope.

Combined Analytical and Numerical Front Tracking Approach to Modeling Directional Solidification of a TiAl-Based Intermetallic Alloy for Design of Microgravity Experiments

A three-step combined analytical and numerical approach to thermal modelling of a two-heater power-down furnace for controlled directional solidification of an intermetallic alloy is proposed. An analytical sensitivity analysis of the thermal model is carried out to show the effect of adiabatic zone length, and both hot-zone and cold-zone heater temperatures, on the initial thermal gradient in the sample and on the length of melt in the adiabatic zone. The subsequent axisymmetric front tracking method (FTM) simulations of directional solidification of a binary intermetallic Ti-46at.%Al alloy show that temperature gradient in the melt declines and velocity of the solid-liquid front increases with time, thus promoting good conditions for a columnar to equiaxed transition. The proposed analytical calculations combined with full-scale numerical FTM simulations provide a convenient and predictive optimization tool for the two-heater power-down furnace design and growth conditions for the future microgravity experiments.

The development of a microgravity experiment involving columnar to equiaxed transition for solidification of a Ti-Al based alloy

The authors are members of the integrated project Intermetallic Materials Processing in Relation to Earth and Space Solidification (IMPRESS), funded within the European Framework (FP6). One of the aims of IMPRESS is to develop new alloys and processes for the casting of TiAl-based turbine blades for the next generation of aero and industrial gas turbine engines. Within IMPRESS, two related issues have been identified during the primary solidification stage, namely, segregation and the columnar-to-equiaxed transition (CET). The authors have set out to isolate the effects of thermo-solutal convection, by designing a microgravity experiment to be performed on a European Space Agency platform. This experiment will investigate the CET formation during solidification. It is planned to use a sounding rocket providing a microgravity time of approximately twelve minutes. The results of this microgravity solidification experiment will be used as unique benchmark data for development and validation of new computational models of TiAl solidification. This in turn will produce accurate models and ultimately new robust industrial processes by project partners in the aerospace industry. The evolution of the design of the microgravity experiment is discussed and the results of preliminary ground reference experiments are presented. Future plans and objectives for the project are also highlighted.

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.

Flow Driven by an Archimedean Helical Permanent Magnetic Field. Part I: Flow Patterns and Their Transitions

In this study, an Archimedean helical permanent magnetic field was constructed and its driving effects on liquid metal were examined. A magnetic stirrer was constructed using a series of arc-like magnets. The helical distribution of its magnetic field, which was confirmed via Gauss probe measurements and numerical simulations, can be considered a combination of rotating and traveling magnetic fields. The characteristics of the flow patterns, particularly the transitions between the meridian secondary flow (two vortices) and the global axial flow (one vortex), driven by this magnetic field were quantitatively measured using ultrasonic Doppler velocimetry. The transient and modulated flow behaviors will be presented in a companion article. The D/H dimension ratio was used to characterize the transitions of these two flow patterns. The results demonstrated that the flow patterns depend on not only the intrinsic structure of the magnetic field, e.g., the helix lead angle, but also the performance parameters, e.g., the dimensional ratio of the liquid bulk. The notable opposing roles of these two flow patterns in the improvement of macrosegregations when imposing such magnetic fields near the solidifying front were qualitatively addressed.

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.