Traveling Magnetic Field Research Articles

Comparative Study on the Grain Refinement of Al-Si Alloy Solidified under the Impact of Pulsed Electric Current and Travelling Magnetic Field

It is high of commercial importance to generate the grain refinement in alloys during solidification by means of electromagnetic fields. Two typical patterns of electromagnetic fields, pulsed electric currents (ECP) and traveling magnetic field (TMF), are frequently employed to produce the finer equiaxed grains in solidifying alloys. Various mechanisms were proposed to understand the grain refinement in alloys caused by ECP and TMF. In this paper, a comparative study is carried out in the same solidification regime to investigate the grain refinement of Al-7 wt. %Si alloy driven by ECP and TMF. Experimental results show that the application of ECP or TMF can cause the same grain refinement occurrence period, during which the refinement of primary Al continuously occurs. In addition, the related grain refinement mechanisms are reviewed and discussed, which shows the most likely one caused by ECP and TMF is the promoted dendrite fragmentation as the result of the ECP-induced or TMF-induced forced flow. It suggests that the same grain refinement process in alloys is provoked when ECP and TMF are applied in the same solidification regime, respectively.

Scale up aspects of directional solidification and Czochralski silicon growth processes in traveling magnetic fields

We performed 3D simulations of directional solidification (DS) and Czochralski (Cz) silicon growth processes in traveling magnetic fields (TMFs) and verified them with the experimental data that were available. Particularly, we studied silicon DS growth in real G1, G2 and G5 size setups and Cz growth in 6″ and 24″ crucibles in furnaces provided with KRISTMAG® heater magnet modules (HMMs). TMFs were used for a solid/liquid interface shaping and for a melt stirring. Based on our simulation findings, we discussed scale up challenges and proposed a method for safe upscaling. The method related all present driving forces using dimensionless numbers: Grashof (Gr), Stephan (Ste), Reynolds (Re), Shielding (S) and magnetic forcing number (F).

Separation of primary Si and impurity boron removal from Al-30%Si-10%Sn melt under a traveling magnetic field

Separation of primary Si phase and removal of boron in the primary Si phase during the solidification process of the Al-30%Si-10%Sn melt under a traveling magnetic field (TMF) were investigated. The results showed that the agglomeration layer of the primary Si can be formed in the periphery of the ingot while the inner microstructures mainly consist of the eutectic α-Al+Si and β-Sn phases. The intense melt flow carries the bulk liquid with higher Si content to promote the growth of the primary Si phase which is first precipitated close to the inner wall of crucible with a relatively lower temperature, resulting in the remarkable segregation of the primary Si phase. The content of impurity B in the primary Si phase can be removed effectively with an increase in magnetic flux intensity. The results of electron probe microanalysis (EPMA) clearly indicated that the average intensity of the B Ka line in the α-Al phase region of Al-Si-Sn alloy is higher in the case of solidification under TMF than that of normal solidification condition, suggesting that the electromagnetic stirring can promote the B removal from the primary Si phase.

Effect of traveling magnetic field on solute distribution and dendritic growth in unidirectionally solidifying Sn–50 wt%Pb alloy: An in situ observation

Synchrotron X-ray radiography was used to in situ study the solute distribution and the dendritic growth during the bottom-up solidification of Sn–50wt%Pb alloy under a traveling magnetic field (TMF) for the first time. The buoyance driven evolution and motion of the plumes containing Sn-rich melt are directly observed in the solidification front before the application of TMF. A forced melt flow from left to right is induced with the application of TMF, which results in the redistribution of the solute concentration (facilitate the solute transportation and reduce the local fluctuations considerably) and the change of the dendrite morphologies (promote/suppress the growth of the secondary arms, remelting and fragmentation of dendrites). Meanwhile, the concentration variations of Sn around the solidification front are quantitatively analyzed through the extraction of gray level from sequenced X-ray images.

Investigation of the amplification of an electromagnetic field at the end wall of a conducting thin-wall body during its interaction with an alternating magnetic field

The results of theoretical and experimental studies on an electromagnetic field are presented and qualitative and quantitative estimates of the field amplification at the end of a conducting plate due to its interaction with an alternating (traveling) magnetic field are given for the development of the eddy-current testing of conducting thin-wall bodies.

Directional melting and solidification of gallium in a traveling magnetic field as a model experiment for silicon processes

Small-scale model experiments for directional solidification processes are performed using a gallium volume with a square horizontal cross-section and dimensions of 10×10×7.5cm3. A heater at the top and a cooler at the bottom generate a vertical temperature gradient while an external coil system produces a traveling magnetic field (TMF) leading to Lorentz forces in the melt. The position and shape of the phase interface as well as the melt flow during melting and solidification processes are investigated both experimentally and with a coupled 3D numerical model. Uncertainty in various experimental parameters and appropriate methods of calibration are discussed to enable precise validation of numerical simulations. A distinct influence of the melt flow is observed, which results in a concave melting interface with an upward TMF and a convex shape with a downward TMF. In both cases, the corner region demonstrates local deflections in the opposite directions, which illustrates the challenge to obtain a smooth interface shape in silicon solidification processes. These processes can be further investigated using the validated 3D model. Additionally, direct transfer of the results between model experiments and silicon processes using scaling laws is discussed.

Effect of traveling magnetic field on separation and purification of Si from Al–Si melt during solidification

Separation and purification of the Si crystal during solidification process of hypereutectic Al–30Si melt under traveling magnetic field (TMF) were investigated in the present study. The results showed that under a proper condition the Si-rich layer can be formed in the periphery of the ingot while the inner microstructure is mainly the Al–Si eutectic structure. The intense melt flow carries the bulk liquid with higher Si content to promote the growth of the primary Si phase which is first precipitated close to the inner wall of the crucible with a relatively lower temperature, which resulting in the remarkable segregation of the primary Si phase. The impurity contents of the refined Si can be reduced to a very low level. The typical metallic impurities have removal fraction higher than 99.5%. In addition, there is a significant difference in the P contents between the primary and eutectic Si phases, which might be ascribed to the formation of AlP phase that acts as the heterogeneous nucleation sites. Furthermore, a considerable amount of Fe-containing particles with a size about 100–300nm is found inside the eutectic Si phase, indicating an unintended entrapment of Fe in Si.

Экспериментальные исследования модели МГД-перемешивателя с постоянными магнитами для алюминиевых печей

A m odel experimental set-up was built aiming to ensure achievable similitude to large scale metal melting furnaces, where electromagnetic stirring is required to reduce the melting time and to achieve thermal and compositional uniformity. The widely used three-phase AC current linear travelling magnetic field inductor is substituted by a new energy-saving concept of a Permanent-Magnet (PM) inductor, consisting of a multitude of cylindrical dipoles, which form a Halbach array and are rotated synchronously. The stirrer creates a very unstable time-dependent flow pattern in the liquid metal pool. The turbulent local velocity was measured, delivering experimental data about time-averaged flow and turbulence intensity as well. Spatial components of velocity were measured by ultrasound Doppler anemometry and by a potential difference probe with an incorporated permanent magnet, delivering mutually verifying and complementary data with higher reliability. The experimental data might be used to validate the full three-dimensional numerical simulations of the stirring produced in actual large scale metal melting furnaces Normal 0 false false false RU X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:Обычная таблица; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:Calibri,sans-serif; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin;}

Flow, heat and mass transfers during solidification under traveling/rotating magnetic field

In this paper, we present the relative problem of heat and mass transfer to adopt a means of imposing an electromagnetic field to improve solutal segregation (macrosegregation) during liquid metal solidification. A well-validated, quasi-two-dimensional solidifying experi- mental benchmark was introduced, which allowed us to observe the temperature field evolution and to provide the evident clues of phase transformation. We also observed naturally formed solutal segregations in the post-mortem sample. The idea of imposing a modulated magnetic field while optimizing modulation frequency and having a cable to suppress solutal segregation was confirmed by multi- scale numerical modeling. Magnetohydrodynamics, flow driven by a modulated traveling magnetic field, was experimentally studied. Furthermore, a more practical cylindrical shape of liquid metal bulk driven by a perma- nent helical magnetic field has been achieved. The spatial flow behaviors suggested an appropriate magnetic field with optimized electromagnetic parameters for obtaining high-quality, low-defect casting products.

Three-dimensional instability of thermal convection in a vertical Bridgman growth configuration under traveling magnetic field

Thermal convection in a vertical Bridgman growth configuration under the effect of a traveling magnetic field (TMF) is investigated numerically. The vertical Bridgman growth configuration is approximated by convection in a cylinder with a parabolic heated sidewall and isothermal-cooled end walls. The TMF effect is introduced as a usual approximation under low frequency and a low induction assumption to use the analytical expression for the Lorentz force applied in the fluid volume. The strength of the TMF force is measured by the parameter Ft. The cylinder aspect ratio (height/radius) is fixed to , and the Prandtl number considered is fixed to Pr = 0.02. The base flow simultaneously excited by buoyancy and TMF is axisymmetric. Our primary goal is to determine how the flow loses stability to three-dimensional flow. An extremely multiple valued stability curve in the (Ft,Ra) plane is revealed by linear stability analysis. Three distinct flow regimes are classified, i.e., buoyancy dominated regime, buoyancy and TMF-counterbalanced regime, and TMF-dominated regime. Typical steady and unsteady three-dimensional flows are illustrated.

The study of turbulence in MHD flow generated by rotating and traveling magnetic fields

We consider a problem of spectral analysis of signals from electromagnetic sensors operating in a turbulent MHD flow generated by rotating and traveling magnetic fields, which create a strong electromagnetic noise. Using a wavelet-based technique for cross-correlation signal analysis and filtration, we show that at frequencies lower than the frequency of the applied magnetic field, the spectral properties of the velocity field can be clearly seen in spite of the fact that the measured fields are much weaker than the driving rotating (or traveling) magnetic field. On the basis of the proposed method, spectra of turbulent velocity fields, measured in the experiment, were studied.

Experimental study of the solidification of Sn–10 wt.%Pb alloy under different forced convection in benchmark experiment

A solidification benchmark experiment was designed and developed at SIMAP Laboratory in Grenoble in order to investigate the structure formation as well as solute macro–mesosegregation by means of a well-controlled solidification experiment. The experiment consists in solidifying a rectangular ingot of Sn–10wt.%Pb alloy by using two lateral heat exchangers, allowing extraction of the heat flux from one or two vertical sides of the sample. The domain is a quasi-two dimensional rectangular ingot (100×60×10mm). The temperature difference ΔT between the two lateral sides is 40K, and the cooling rate CR=0.03K/s. The instrumentation firstly consists in recording the instantaneous temperature maps by means of a lattice of 50 thermocouples to provide the time evolution of the isotherms. Measurements of the velocity field by ultrasonic Doppler velocimetry (UDV) method in a Ga–In–Sn pool were taken and transposed to the tin–lead alloy case before solidification. After each experiment, the segregation patterns were obtained by X-ray radiograph, and confirmed by quantitative analysis of the solutal distribution obtained by chemical method coupled with the technical ICP (Inductively Coupled Plasma).The originality of the present study is to examine the effect of various types of forced convection driven by a traveling magnetic field (TMF). Three cases of TMF were investigated: in the same direction as natural convection, reversed with respect to natural convection, and periodically reversed with a modulation frequency equal to 0.125Hz. This study allows us to evaluate the evolution due to the forced convection induced by a TMF field, as well as its influence on the initial conditions, the solidification macrostructure and the segregation behavior. While forced convection promotes equiaxed structures, the stirring pattern influences grain size according to its direction with respect to natural convection. The results show that all electromagnetic stirring modes effectively reduce macrosegregations significantly, while remaining inactive for reducing development of segregated channels.

Effect of Alternating Traveling Magnetic Field on the Removal of Inclusions from Aluminum Melt

Alternating traveling magnetic field (TMF) was introduced to agglomerate the inclusions with a density smaller than surrounding melt. Primary silicon particles precipitating from the solidification process of hypereutectic Al-Si alloy was regarded as inclusions need removing. Results indicated that alternating TMF was more effective to promote the inclusions to agglomerate into clusters than downward TMF. The effect of alternating TMF to agglomerate the inclusions increases with the increase of current and frequency. There exists the best alternating time to get the best agglomeration effect. In this study, 10s is the best alternating time.

GaAs Vertical Gradient Freeze Process Intensification

The GaAs vertical gradient freeze (VGF) crystal growth process can be intensified in various ways, e.g., utilizing external fields, scaling up, numbering up the crucibles, etc. Successful application of traveling magnetic fields (TMFs) in 4 in. VGF GaAs growth for the flow control and process acceleration encouraged us to search for synergistic conceptions. Pros and cons of process scale up and numbering up under TMFs were addressed using three-dimensional numerical simulations. The comparison of concepts was focused on the control of solid/liquid interface morphology and energy balance. A novel multicrucible furnace design was proposed and compared with a single-crucible design, both based on KRISTMAG technology. The simulation results showed the clear superiority of the numbering-up concept; e.g., the total energy consumption per run with equal yield was reduced to 32% of the value for the standard process. Moreover, a beneficial interface morphology was achievable without a trade-off with growth rates.

A three-dimensional phase field model coupled with a lattice kinetics solver for modeling crystal growth in furnaces with accelerated crucible rotation and traveling magnetic field

In this study, we present a new three-dimensional numerical model for crystal growth in a vertical solidification system. This model accounts for buoyancy, accelerated crucible rotation technique (ACRT), and traveling magnetic field (TMF) induced convective flow and their effect on crystal growth and the chemical component’s transport process. The evolution of the crystal growth interface is simulated using the phase-field method. A semi-implicit lattice kinetics solver based on the Boltzmann equation is employed to model the unsteady incompressible flow. A one-way coupled concentration transport model is used to simulate the component fraction variation in both the liquid and solid phases, which can be used to check the quality of the crystal growth. Numerical results indicate that ACRT can slightly increase the quality of grown crystal, but the effect of TMF on quality of grown crystal depends on the temperature profile of the ampoule wall. Finally, excellent scalability of our developed parallel methods is demonstrated on the three-dimensional cases.

Modeling and Experiments on Electromagnetic Separation of Inclusions from Aluminum Melt under Combined Magnetic Field

Abstract Numerical simulation and experiment studies have been conducted to investigate the effect of inclusions separation under combined magnetic field (CMF) generated by synchronous application of a rotating magnetic field (RMF) and a downward traveling magnetic field (TMF). The experimental setup is constituted by two sets of equidistant coils systems fed by out-of-phase alternating currents. The electromagnetic force field and flow field were calculated via finite-element analysis. CMF combines the rotational effect of RMF and the transport effect of TMF. Volume fraction and particle trajectories were simulated to examine the mechanism of particle separating. Good agreement between experimental and numerical results has been achieved. The flow field and the Archimedes electromagnetic force are the two main factors responsible for particle motion. This type of CMF provides a highly efficient approach at eliminating inclusions.

Linear instability analysis of Rayleigh–Bénard convection in a cylinder with traveling magnetic field

Rayleigh–Bénard convection under a low frequency traveling magnetic field (TMF) in a vertical cylinder is investigated by linear stability analysis. The cylinder with a adiabatic vertical wall is heated from below and cooled from above. The effects of traveling magnetic field on flow instability are studied for four aspect ratios (height/radius) A=1, 2, 3, 4 and three Prandtl numbers Pr=0.001, 0.02, 0.1. The critical Rayleigh number and corresponding critical frequency are evaluated as a function of TMF force Ft, and diversity profiles of stability curves are obtained under different control parameters. Moreover, stability properties for different Prandtl numbers are compared at fixed aspect ratios. The results obtained show that the magnetic force is able to either stabilize or destabilize the flow, which is determined by the effects of combination of the strength of the force, the aspect ratio and the Prandtl number.

Characterization of 4 in VGF-GaAs single crystals grown in a heater-magnet module

Silicon-doped 4in VGF-GaAs single crystals were grown under the influence of traveling magnetic fields (TMF). A KRISTMAG˜® heater-magnet module (HMM) was used for the simultaneous generation of heat and TMF through a combination of DC and AC control. In this study, we investigated changing structural and electronical properties as well as micro- and macrosegregation of VGF-GaAs single crystals grown with applied TMF.Striations were observed in crystals grown without or too strong TMF. Almost no micro-inhomogeneities were detected when the magnetic flux densities of the TMF were matched to progression of solidification. With utilized TMF, induced melt flow opposed natural convection driven by buoyancy forces. Axial dopant incorporation was enhanced through a reduction of flow velocities and converging melt flow towards the center of the solid–liquid interface. The radial segregation profiles were flattened through a reduction of the concave deflection. While low frequency TMF bended the interface center more convex, the effect of high frequency TMF was more limited to the melt periphery. Furthermore, with large high frequency TMF current shares the impact of the minor three-dimensional asymmetric magnetic field distribution in the HMM became more relevant. Consequently, the application of double-frequency TMF led to a reduction of etch pit density through reduction of interface concavity.

VGF growth of GaAs utilizing heater-magnet module

The recent development of the vertical gradient freeze (VGF) growth of GaAs focuses on the increase of process efficiency, a reduction of production costs and a simultaneous improvement of the crystal quality. To meet this technological and scientific challenge, different strategies are proposed, e.g. an increase of crystal size, simultaneous crystallization in multi-crucible furnaces or an increase of growth rate. To achieve these goals, an exact and permanent control of the melt flow is of crucial importance that is easily provided by traveling magnetic fields (TMF).The key aspect of the presented study is the well-defined control of the solid/liquid interface bending during VGF GaAs growth. Therefore, global simulations of the whole furnace are inevitable. Results of numerical simulations enable to properly adjust downward-directed Lorentz forces to achieve a significant decrease of the concavity of the solid–liquid interface. Compared to a reference crystal grown without TMF, a reduction of the interface deflection by about 30% was obtained. Gained experience from experiments in a single-crucible heater-magnet module (HMM) and numerical simulations led to the design of a novel multi-crucible HMM.

Global simulations of heat transfer in directional solidification of multi-crystalline silicon ingots under a traveling magnetic field

We have performed global simulations of heat transfer during the complete directional solidification (DS) process of multi-crystalline silicon (mc-Si) ingots under a traveling magnetic field (TMF) to investigate the thermal and flow fields in the silicon melt. The melt convection pattern, thermal field, and melt–crystal (m–c) interface shape at different DS stages were compared without a TMF, with an upward TMF, and with a downward TMF. We found that the distribution and the magnitude of the Lorentz force are similar for different melt heights. The melt is mainly occupied by a large vortex, which flows in opposite directions for an upward TMF and a downward TMF. The TMF has a beneficial effect on the melt mixing. The upward TMF makes the m–c interface more concave with respect to the melt, while the downward TMF makes it less concave or convex at the same solidification stage. The interface becomes less concave or convex with increasing solidification fraction under the downward TMF. The amplitude of imposed electric current was adjusted to successfully obtain a slightly convex interface. These results provide important information for optimizing the complete DS process for mc–Si ingots with a TMF.