EP2190612A1

EP2190612A1 - Method and device for the electromagnetic stirring of electrically conductive fluids

Info

Publication number: EP2190612A1
Application number: EP08801098A
Authority: EP
Inventors: Petr. A. Nikrityuk; Sven Eckert; Dirk RÄBIGER; Bernd Willers; Kerstin Eckert; Roger Grundmann; Gunter Gerbeth
Original assignee: Technische Universitaet Dresden; Forschungszentrum Dresden Rossendorf eV
Current assignee: Technische Universitaet Dresden; Helmholtz Zentrum Dresden Rossendorf eV
Priority date: 2007-08-03
Filing date: 2008-08-01
Publication date: 2010-06-02
Anticipated expiration: 2028-08-01
Also published as: US8944142B2; JP5124863B2; EP2190612B1; WO2009018809A1; DE102007037340B4; JP2010535105A; DE102007037340A1; US20140290433A1; US20110297239A1

Abstract

The invention relates to a method and to a device for the electromagnetic stirring of electrically conductive fluids (2, 21, 22) in the liquid state and/or in the state of onsetting solidification of the fluid (2, 21, 22), using a rotating magnetic field that is produced in the horizontal plane of a Lorentz force (FL). The aim is to achieve an intensive three-dimensional flow on the inside of the fluid for mixing in the liquid state up to the direct vicinity of solidifying fronts, and to simultaneously ensure an undisturbed, free surface of the fluid. The solution is to change the direction of rotation (15, 16) of the magnetic field rotating in the horizontal plane at regular time intervals in the form of a period duration (TP), wherein the frequency of the directional change of movement of the magnetic field vector is adjusted such that in the state of mixing the liquid fluid (2, 21, 22) a period duration (TP) is adjusted between two directional changes of the magnetic field during a time interval (ΔTPM) as a function of the adjustment time (ti.a.) with the condition (I) 0.5.ti.a < TPM < 1.5.ti.a and such that, at the beginning of the state of onsetting solidification of the fluid (2,21,22), a period duration (Tp) is set between two directional changes of the magnetic field in a time interval (ΔTPE) as a function of the adjustment time (ti.a.) with the condition (II) 0.8.ti.a < TPE < 4.ti.a, wherein the adjustment time (ti.a) is specified by the equation (III) in which after an activation of the rotating magnetic field in a fluid (2; 21, 22) in the resting state the double vortex of the meridional secondary flow (18) forms, and in which σ is defined as the electric conductivity, ρ as the density of the fluid (2, 21, 22), ω as the frequency, Bo as the amplitude of the magnetic field, and C9 as the constant for the influence of the size and shape of the volume of the fluid (2, 21, 22).

Description

Method and device for the electromagnetic stirring of electrically conductive liquids

description

The invention relates to a method and a device for the electromagnetic stirring of electrically conductive liquids in the liquid state and / or during the solidification of the liquids using a horizontal magnetic field generating a Lorentz force, rotating magnetic field. Due to their contactless interaction with electrically conductive liquids, time-dependent electromagnetic fields open up a possibility for mixing, for example, molten metal melts. The magnetic field amplitude and frequency parameters allow the electromagnetic field to be controlled directly and accurately in a simple manner.

The present invention relates to magnetic, mostly in the horizontal direction rotating traveling fields, also referred to as rotating magnetic fields.

The application of a rotating magnetic field, for example, to a liquid container filled with liquid molten metal container causes a nearly rigid rotational movement of the molten metal in a wide range, which hardly contributes to the convective exchange in the volume of the melt. For the mixing processes to be observed, essentially the so-called meridional secondary flow is responsible, which arises in the meridional plane (rz plane) due to the pressure difference between the center of the container and the surface layers at the bottom and at the free surface and their amplitude in dependence from the geometry of the considered flow about a factor of five to ten less than the rotating azimuthal flow fails. In the meridional plane, as in the document P.A. Nikrityuk, M. Ungarish, K. Eckert and R. Grundmann: Spin-up of a liquid metal flow driven by a rotating magnetic field in a finite cylinder: A numerical and analytical study, Phys. Fluids, 2005, vol. 17, 067101), a so-called double vortex structure is formed, i. in the region of the horizontal center plane of the container, the liquid molten metal is transported radially outwards, flows upwards or downwards on the side walls and flows back to the axis at the bottom and below the free surface. The direction of the secondary circulation adjusts itself in the same way for both directions of rotation of the magnetic field.

A major problem with the use of a rotating magnetic field for electromagnetic stirring is that the majority of the kinetic energy of the melt is for the primary azimuthal Rotational movement is applied, but only contributes to a small extent to the mixing of the melt. An intensification of the mixing process is possible primarily by an enhancement of the secondary meridional flow. An increase in magnetic field strength or magnetic field frequency causes a widening of the secondary flow, ie an increase in the speed values in the axial and radial directions and the generation of additional turbulence, eg the occurrence of Taylor-Görtler-vortexes, as in the publications PA Nikrityuk, K. Eckert, R Grundmann: Magnetohydrodynamics, 2004, 40, pp. 127-146. and PA Nikrityuk, K. Eckert, R. Grundmann: CD Production of the Conference on Turbulence and Interactions TI2006, France, 2006, May 28-June 2, 2006, near the sidewalls. This leads to a more intensive mixing of the liquid molten metal.

One problem is that at the same time, however, the rotational movement is amplified and causes significant disturbances and deflections of the free surface of the liquid molten metal. As a result, undesirable effects such as the inclusion of slag in the melt or the absorption of oxygen from the atmosphere may occur.

Another problem arises for the electromagnetic stirring in the transition from the liquid state to the state of solidification, ie during the directional solidification of metallic alloys or semiconductor melts on. In the immediate vicinity of a progressive solidification front, the melt separates due to the different solubility of individual components in the liquid or solid phase. A flow in the immediate vicinity of the solidification front counteracts the build-up of an extended concentration boundary layer by transporting enriched melt away from the solidification front. If the melt flows exclusively in one direction, segregation may occur in other volume ranges, which noticeably worsen the mechanical properties of the resulting solid. Rotating magnetic fields are already used in metallurgical processes, such as the continuous casting of steel. In the publication DE AS 1 962 341 an arrangement of a polyphase electromagnetic winding for generating a traveling field perpendicular to the casting direction on a continuous casting plant is described.

A method for stirring the molten steel during continuous casting is also described in the publication US 2003/0106667, in which two superimposed and counter-rotating magnetic fields are used. While the lower magnetic field takes over the actual function of the stirrer, the task of the upper magnetic field is to decelerate the rotating melt in the area of the free surface to very low velocity values in order to compensate for the negative effects of stirring - a deflection and turbulence of the free surface , One problem is that it works with two magnetic stirrers - a lower magnetic stirrer and an upper magnetic stirrer. This means in comparison to the use of only one magnetic system a higher apparatus and control engineering effort. At the same time, such a method has an unfavorable energy balance. With the help of the lower magnetic stirrer mechanical energy is brought into the molten steel and the molten steel is set in rotation. However, since a much less intensive rotation of the melt is desired by the user in the upper part of the continuous casting plant, in this approach additional energy has to be expended in the upper magnetic stirrer in order to brake the flow there.

In the documents DE 2 401 145 and DE 3 730 300 each method for electromagnetic stirring in continuous casting molds are described in which a periodic change of the current is made in the coil assembly. The document DE 2 401 145 describes that the formation of secondary white bands and secondary dendrites can be avoided with this method. With the method described in the document DE 3 730 300, a calming of the free bath surface is achieved. It is assumed that the resulting magnetic field inside the melt simultaneously maintains an intense stirring motion. In both of these last-mentioned printed documents, very wide ranges, in particular between one second and 30 seconds, are specified for the cycle times in which the current direction is to be changed. The cycle time, also called period duration, or the frequency of the sign change of the current is an important parameter with a large influence on the forming flow. One problem is that in both documents no details about a predefinable period as a function of the magnetic field strength, the geometry of the arrangement of induction coils or the material properties of the liquid molten metal are described.

The invention has for its object to provide a method and a device for electromagnetic stirring of electrically conductive liquids, which are designed so suitable that an intense three-dimensional flow inside the liquid for mixing in the liquid state reaches into the immediate vicinity of solidification fronts and at the same time an undisturbed, free surface of the liquid can be ensured.

The object is solved by the features of claims 1 and 9. In the method for the electromagnetic stirring of electrically conductive liquids in the liquid state and / or in the state of the beginning of the solidification of the liquid using a in the horizontal plane a Lorentz force FL generating, rotating magnetic field are in the characterizing part according to claim 1 - the direction of rotation of in the horizontal plane rotating magnetic field at regular time intervals in the form of a period T _P changed, wherein the frequency of the change of direction of the movement of the magnetic field vector is set such - In the state of mixing of the liquid, a period T _P between two changes in direction of the magnetic field in a time interval .DELTA.T _PM depending on the response time tj _.a. with condition

(I) 0.5-tj.a. <TPM <1.5-tj.a. set, and - at the beginning of the state of solidification of the liquid, a period T _P between two changes in direction of the magnetic field in a time interval .DELTA.TPE in response to the response time tj _.a. with condition

(II) 0.8-tj.a. <TPE <4-tj.a, the setting time tj _.a. through the equation

(IM) tι. _a . = C ₈ TA ₀ &] is given,

in which forms after connection of the rotating magnetic field in a quiescent state liquid of the double vortex of the secondary meridional flow, and σ as electrical conductivity, p as density of the liquid, ω as frequency and B ₀ as the amplitude of the magnetic field and C ₉ as a constant be defined for the influence of size and shape of the volume of the liquid.

In order to form the rotating magnetic field, a three-phase alternating current current I ₀ can be applied to at least three pairs of induction coils placed on a cylindrical container containing the liquid.

In the container can be filled as electrically conductive liquids metallic or semiconductor melts.

When mixing a cooling melt is thus a period Tp according to condition (I) with 0.5-tj. _a . <T _PM <1.5 t _{i a.} chosen, as long as the melt is still completely liquid, while at the beginning of the state of solidification, the period Tp is increased so that according to condition (II) 0.8 t _ia . <T _PE <4 t _ia . is fulfilled. According to the decreasing height H _{0 of} the volume of the melt in the course of the state of directional solidification, the amplitude Bo of the magnetic field can be readjusted.

In the state of a temperature-controlled solidification, the amplitude B _{0 of} the magnetic field is to be increased so that at least the maximum of the two values

_{B <=} μ.≡μ (, v) and is reached, where v is the kinematic viscosity of the melt, V _SO | as solidification rate, H ₀ as the height of the melt volume, and Bi and B _{2 are defined} as lower limits of the amplitude B _{0 of} the magnetic field, which may vary in the course of solidification depending on the parameters v, V _SO ι and H ₀ .

The respective period durations are interrupted during mixing Tp _M and at the beginning of solidification T _P E, in which the magnetic field is applied by interrupting the pause duration Tp _au se, in which no magnetic field is applied to the melt, wherein the pause duration T _PaUse to the respective period T _{P is set} with Tpause ≤ 0.5 Tp.

When modulating the course of the electromagnetic force F _L other pulse shapes, such as sine, triangle or saw tooth, can be realized instead of the rectangular function, the course and the maximum value of the amplitude B _{0 of} the magnetic field are set so that for the different pulse shapes gives an identical energy input.

The device for the electromagnetic stirring of electrically conductive liquids in the liquid state and / or in the state of the beginning of the solidification of the liquid using a horizontal plane Lorentz force FL generating, rotating magnetic field and under control of the temperature profile of the liquid by means of the method according to the invention contains at least

a cylindrical container, a centrally symmetrical arrangement of at least three pairs of induction coils surrounding the container for forming a rotating magnetic field generating a low-force force FL,

- At least one temperature sensor for measuring the temperature of the liquid in the container, wherein according to the characterizing part of claim 9, the pairs of induction coils are in communication with a control and regulating unit, which forwards a rotary current I ₀ to the pairs of induction coils via a connected power supply unit, wherein the Phase angle of the pairs of induction coils feeding three-phase current ID at regular, time intervals and according to the predetermined period TP _M for mixing in the liquid state or T _PE for the mixing is shifted from the beginning of solidification by 180 ° and thus a reversal of the direction of rotation the magnetic field and the Lorentkraft F _L driving the flow is achieved, wherein the control unit is in communication with the temperature sensor whose temperature data at the time of solidification start the switching of the period from T _PM to T _PE .

The three-phase current I ₀ may be a three-phase alternating current.

The container with the electrically conductive liquid, which may in particular be a melt, may preferably be arranged concentrically within the induction coils.

The container may be provided with a heating device and / or cooling device, which may be in communication with a fixed metal body. The container bottom can be in direct contact with a solid metal body, which is flowed through by a cooling medium in the interior.

The side walls of the container may be thermally insulated.

The heat sink can communicate with a thermostat.

Between the heat sink and the container may be a liquid metal film to achieve a stable heat transfer with low contact resistance.

The liquid metal film may be made of a gallium alloy.

In the bottom plate and / or the side walls of the container in which the melt is located, at least one temperature sensor, e.g. be positioned in the form of a thermocouple, which provides an information signal about the time of the onset of solidification and is connected to the control unit.

The use of the device according to the invention for the electromagnetic stirring of electrically conductive liquids can according to claims 9 to 18 in the form of metallic melts in metallurgical processes or in the form of semiconductor melts in the crystal growth, for the purification of molten metal, in continuous casting or in the solidification of metallic works. Substances by means of the method according to claim 1 to 8 take place.

The direction of the rotating magnetic field is reversed at quite specific, regular time intervals. The reversal is carried out by means of the control means for shifting the phases of a three-phase alternating current, whereby the direction of rotation of the rotating phases of a three-phase alternating current and thus reverses the direction of rotation of the rotating magnetic field. In the period of reversal of the direction of flow occurs an intense meridional secondary flow at the same time mitigated pronounced azimuthal rotational movement, which is carried out by the constantly recurring directional change intensive mixing. The efficient adjustment of the duration of the period T _P between two changes of direction plays the decisive role.

According to the following definition applies:

For an intensive mixing of the melt with at the same time low energy expenditure the condition applies:

(I) 0.5 t, _a <Tp <1.5 t _{ι a} or for a controlled solidification while avoiding the formation of segregation zones in the solidification structure, the condition applies: _a

The parameter t, _a represents an adjustment time (English, initial adjustment time), in which, after an abrupt connection of a rotating magnetic field in a melt, which was previously in the rest state, the double vortex typical of the meridional secondary flow has developed.

The characteristic response time t, _{a is} calculated according to a formula from the variables electrical conductivity of the melt, density of the melt and frequency and amplitude of the magnetic field. An associated constant takes into account the influence of size and shape of the melt volume and can assume numerical values between three and five. This is compared to the prior art, in particular with respect to the document DE 3 730 300 before a defined range for the period T _P , in which the direction of rotation change is adjustable.

An essential feature of the invention is that the direction of the rotating magnetic field is reversed at regular time intervals, with the period T _{P of} the change of direction an important parameter exists, which can be specified in order to intensify the stirring. Stalten. An essential criterion for the success of the process is the possibility of a targeted control of the secondary flow. Different flow patterns are advantageous for different objectives.

The present invention can be used advantageously for the efficient stirring of melts and for the directed solidification of multicomponent melts. In order to avoid an ensuing mixing effect, e.g. in the purification or degassing of melts, it is necessary to increase the intensity of the volume average secondary meridional flow compared to the primary azimuthal rotational motion. The objective in an application of the method in the directional solidification of metallic alloys is that in addition to a thermal homogenization of the melt, the direction of the flow in the immediate vicinity of the solidification front over time should be varied so that a time average for the radial velocity component close to zero results.

The present invention shows that the meridional secondary flow rate field is clearly and comprehensibly dependent on variations in the parameter T _p .

It will be apparent that for an efficient embodiment of the method of stirring, the proper adjustment of the period T _P is crucial in view of the objective of the particular application. When specifying Tp, the strength of the magnetic field, the dimensions and shape of the melt volume and the material properties of the melt must be taken into account.

The invention is described below with reference to two exemplary embodiments by means of several drawings. Show it:

Fig. 1 is a schematic representation of a device according to the invention for the electromagnetic stirring for mixing a liquid Melt in conjunction with the inventive method, wherein

1a is a schematic structure of the device in front view, Fig. 1b is a plan view of the device according to Fig. 1a,

Fig. 1c is a schematic representation of the flow modes in a rotating magnetic field in the horizontal plane, Fig. 1d a period duration (Tp) -Temperatur (T) representation of the melt in the liquid state and in the transition to solidification, wherein T _SO ι the Temperature of the container bottom too

1e denotes a Lorentz force (F _L / F _L o) time (t) representation,

Fig. 2 shows two schematic cylindrical container with liquid metal melts, wherein

FIG. 2 a shows a liquid melt of a metal and FIG. 2 b shows two superimposed melts of two different metals at rest (in the unmixed state), FIG.

3 shows the experimentally determined dependence of the intensity of the meridional secondary flow on the period Tp,

Fig. 4 Results of numerical simulations for the mixture of liquid lead (Pb) and liquid tin (Sn): mixing behavior at the same time after the beginning of the mixing (t / t _S pj _n -up = 1.92), where Fig. 4a continuous RMF, T _p = ∞ Fig. 4b T ₁ A _a . = 1.03. Fig. 4c T ₁ A _8. = 2. show.

Fig. 5 Presentation of the results of numerical simulations for mixing the tin concentration in the lower half of the container: time evolution of the volume-averaged Sn concentration in the lower container volume for different scenarios.

FIG. 6 solidification of an Al-Si alloy under magnetic field influence, FIG.

B ₀ = 6.5 mT, (macrostructure), where Fig. 6a continuous RMF, T _p = oo

Fig. 6b TpΛι. _a . = 1.67. , Fig. 6c Tp / ti.a. = 0.95 show, and

Fig. 7 solidification of an Al-Si alloy under magnetic field influence (microstructure), wherein

Fig. 7a continuous RMF, T _p = ∞ Fig. 7b T _p / t _ia. = 1.67 show.

8 shows a radial distribution of the area fraction of primary crystals in Al-7wt% Si samples (with seven parts by weight of Si) which were solidified under the influence of magnetic fields with variation of the pulse duration T _P.

In Fig. 1, 1a, 1b is a schematic representation of an inventive device 1 for stirring a liquid in the liquid state in the form of a metallic melt 2 using a horizontal plane in a Lorentz force FL generating, rotating magnetic field, wherein the device 1 comprises at least one cylindrical container 13 with the liquid melt 2 therein, as shown in Fig. 2a, or 21, 22 as shown in Fig. 2b, - A surrounding the container 13 centrally symmetrical arrangement 3 of at least three pairs 31, 32,33 of induction coils for forming a Lorentz force F _L generating, rotating magnetic field and

at least one temperature sensor 10 for measuring the temperature of the liquid 2,21, 22 in the container 13.

According to the invention, the pairs 31, 32, 33 of the induction coils are connected to a control / regulation unit 12, which transmits a three-phase current b to the pairs 31, 32, 33 of induction coils via a connected power supply unit 11, the phase position of the pairs 31, 32, 33 of the induction coils feeding three-phase current ID at regular time intervals corresponding to the predetermined period T _PM for mixing in the liquid state or T _PE for mixing from the beginning of solidification is shifted by 180 ° and thus a reversal of the direction of rotation of the magnet field and the Lorentkraft F _L driving the flow is achieved, wherein the control unit 12 is in communication with the temperature sensor 10 whose temperature data at the time of solidification start the switching of the period from T _PM to TPE.

The cylindrical container 13 is filled with the liquid, electrically conductive first melt 2. The container 13 is located centrally symmetrically in the middle of the arrangement 3 of the induction coil pairs 31, 32, 33, as shown in FIG. 1 b. The induction coil pairs 31, 32, 33 are fed by a power supply unit 11 with a three-phase current I ₀ in the form of a three-phase alternating current and generate a magnetic field rotating around the axis of symmetry 14 of the container 13 and oriented horizontally with the direction of rotation 15 (arrow direction). The temporal change in the magnetic field strength generates a Lorentz force F _L with a dominant azimuthal component which causes the melt 2 in FIG. 2 a or 21, 22 in FIG. 2 b to rotate. The power supply unit 11 of the induction coil pairs 31, 32, 33 is connected to the control / regulating unit 12, which causes a displacement of the phases of the three-phase alternating current ID at predetermined time intervals. The phase shift has the consequence that the direction of rotation 15 of the horizontally oriented Magnetic field during the phase change in the direction of rotation 16 reverses, as shown in Fig. 1b.

The method can be used, for example, to homogenize the temperature distribution in a one-component melt 2, as shown in FIG. 2 a, or to balance the concentration in demixed multicomponent melts 21, 22, as shown in FIG. 2 b. cause the higher density melt 22 to be in the lower part of the container 13 prior to mixing and to be covered by the lighter melt 21.

The operation of the device 1 is explained in more detail according to FIG. 1 and FIG. 2a, 2b.

The electromagnetic stirring method is based on a periodic reversal of the direction of the Lorentz force F _L driving the flow. The character of the flow is determined by a periodic change of the direction of rotation 15-16, 16-15 of the magnetic field B ₀ . At the time of reversal of direction, the flow is slowed down and the melt 2, 21, 22 accelerates in the opposite direction. The Lorentz force FL varies in the axial direction with the associated force component and has a maximum in the center plane 17 of the container 13. In a reversal of the direction of rotation 15 of the magnetic field, the melt 2, 21, 22 braked more in the vicinity of the center plane 17 and in the Counter direction 16 accelerates as this is near the bottom 4 of the container 13 and the free surface 5 of the case. The nonuniformities in the direction reversal 15-16, 16-15 of the flow produce strong gradients of the rotational movement in the axial direction of the axis of symmetry 14. The occurrence of such gradients leads, as shown in FIG. 1 c, to an intensification of the secondary secondary flow 18. In the period of reversal of the flow direction thus occurs an intense secondary flow 18 at the same time weak rotational movement 19. The mixing of the melt 2, 21, 22 becomes more efficient the better the intensities of the primary azimuthal rotational movement 19 and the secondary meridional flow 18 can be approximated. Can be achieved this over a longer period of time by constantly recurring changes of direction of the magnetic field B ₀ . In this context, as shown in Fig. 1d, 1e, the setting of the period Tp plays a crucial role. If the period T _{P is} too long, the primary azimuthal rotational movement 19 significantly increases in intensity as compared to the secondary meridional flow 18. A shorter period T _P is advantageous because more frequent changes of direction 15-16, 16-15 increase the secondary flow 18. However, if the period Tp is too small, the melt 2, 21, 22 can not be sufficiently accelerated, both primary rotational movement 19 and secondary flow 18 lose their intensity. Thus, as shown in Fig. 1e, there exists a certain optimum value of the period T _P) which depends on the magnetic field strength B ₀ , the size and shape of the volume and the material properties of the melt 2, 21, 22.

An efficient stirring of the liquid melt 2, 21, 22, ie a maximized stirring effect with the least possible expenditure of energy, is achieved if the period T _P according to FIG.

0.5-ti.a. <Tp <1.5 ti.a. (I)

The parameter t _{i a.} is the so-called adjustment time (English, initial adjustment time) and denotes the time scale, in which, after an abrupt connection of a rotating magnetic field in a melt 2, 21, 22, which was previously at rest, the double vortex typical of the meridional secondary flow 18 out forms. The response time tj _.a is defined by the following equation where the variables σ, p, ω and B ₀ denote the electrical conductivity and the density of the melt, the frequency and the amplitude of the magnetic field, while the constant C _{9 describes} the influence of size and shape of the melt volume and assume numbers between three and five can.

In a Plexiglas cylinder 13 with a diameter 2r and a height of 60mm each, the flow of a GalnSn melt 21, 22 was determined by means of the ultraviolet sound Doppler method measured. FIG. 3 shows the quadratic mean value of the vertical velocity U _z ² measured along an axial line at r = 18 mm as a function of the period T _P. The experimental results prove the existence of a certain period Tp at which the intensity of the secondary meridional flow 18 reaches a maximum. The position of the maximum U _zma χ ² varies with the magnetic field strength and corresponds to the respective adjustment time tj _.a ..

With the invention, various melts 21, 22, as shown in Fig. 2b, are mixed together. For example, liquid lead 22 and liquid tin 21 can each be half in the cylindrical container 13. The lead 22 is significantly heavier and stores before the start of mixing in the lower half of the container 13. At a defined time, the rotating magnetic field B _{0 is} switched on, the direction of rotation is reversed at regular time intervals. FIG. 4 and FIGS. 4a, 4b, 4c contain the results of numerical simulations at a magnetic field of 1 mT with regard to the concentration distribution of lead (black) 22 and tin (white) 21 in a half-plane after a certain time from 20s, where Fig. 4a with T _P = 0 Fig. 4b with Tp = 1.03 ti.a.

A comparison of the results of numerical simulations shown in FIG. 5 for mixing the tin concentration C _8n in the lower container half for a time evolution of the volume-average Sn concentration in the lower container volume for different scenarios with respect to the flows in FIGS. 4a, 4b. 4c

for different set values for the period T _P shows that the mixing fastest for the period T _P «tj a _. goes on.

The representation is characterized by the temporal evolution of the tin concentration 21 in the lower container half (R ₀ - radius, H ₀ - height of the container) confirmed, which is shown in Fig. 4b. Particularly noteworthy in this context is that when setting an inappropriate value of the period Tp with respect to a homogenization of the melt volume worse results are achieved than when using a continuously rotating magnetic field.

The device 1 shown in FIG. 2 of the cylindrical container 13 filled with an electrically conductive melt 2 in the arrangement 3 of induction coil pairs 31, 32, 33 as shown in FIGS. 1, 1 a, 1 b can be replaced by a cooling device 23 be supplemented for the solidification of metallic melts 2. The cooling device 23 contains a metal block 6, in the interior of which cooling channels 7 are present. The container 13 stands on the metal block 6. The cooling channels 7 located in the interior of the metal block 6 are flowed through by a coolant during the solidification process. By means of the cooling device 23, the melt 2 is removed from the heat down. A thermal insulation 8 of the container 13 prevents heat losses in the radial direction. At the bottom 4 and the side walls 20 of the container 13 is at least one temperature sensor 10, for example mounted in the form of a thermocouple. The temperature measurements make it possible to monitor the beginning and the course of the state of solidification and to enable a timely adaptation of the magnetic field parameters (eg B ₀ and Tp) by the power supply unit 11 controlled by the control unit 12 to the individual stages of the solidification process.

For further stirring of the solidifying melt 2, the periodic reversal of the direction of the Lorentz force F _L driving the flow is continued. The period T _PE is, as shown in Fig.id, set such that the melt 2 is well mixed and the direction of the secondary meridional flow 18 in the vicinity of the solidification front undergoes a constant change of direction. Al-Si alloys 21, 22 may directionally solidify directionally controlled temperature in the device 1 according to the invention shown in FIG. The microstructural properties obtained are explained in more detail with reference to FIGS. 6a, 6b, 6c, 7a, 7b and 8 with regard to the formation of columnar dendrites, grain refining and demixing.

FIG. 6 shows the macrostructure in longitudinal section of cylindrical blocks of an Al-7wt% Si alloy, eg with a diameter of 50 mm and a height of 60 mm, which were directionally solidified under the action of a rotating magnetic field at a field strength B ₀ of 6.5 mT , In the present case, the magnetic field was switched on with a time delay of 30 s after the start of solidification on the container bottom. In the period until the onset of the electromagnetically driven flow, a coarse columnar structure grows parallel to the symmetry axis of the container. In the case of a continuously acting rotating magnetic field, first of all a modified columnar structure is formed, as shown in FIG. 7a, ie the columnar grains become finer and grow inclined to the side. In the middle of the specimen a morphology transition from columnar to equiaxial grain growth can be observed. At the solidification front, the secondary flow transports Si-rich melt to the symmetry axis 14. This leads to typical segregation patterns, which have a depletion of eutectic phases in the edge zones and a concentration in the region of the axis of symmetry 14. This is equivalent to increasing the proportion of primary crystals near the sidewalls and reducing the proportion of primary crystals in the center of the sample.

In FIG. 8, a radial distribution of the area fraction of primary crystals in Al-7wt% Si samples (with seven parts by weight of Si), which were solidified under magnetic field influence with variation of the pulse duration T _p .

FIGS. 6 to 8 show that in the case of electromagnetic stirring with a change of direction of the magnetic field when the magnetic field is switched on, a direct transition to equiaxial grain growth can be achieved. The periodic change of the direction of rotation of the magnetic field leads in each case to an can, as shown in Fig. 7b ner reduction of segregation, which is also completely avoided with a suitable choice of the pulse duration T _P almost be.

The advantages of the invention are as follows: formation of an intensive, three-dimensional flow in the interior of the molten metal 2, 21, 22,

very good mixing of the molten metal 2, 21, 22 by intensive meridional secondary flow 18,

- Less energy compared to the continuously rotating magnetic field, since not the vast majority of the energy used in the

Maintenance of the azimuthal rotational flow must be applied, and a higher proportion of energy is applied to the more effective for mixing meridional secondary flow 18,

the frequency of the periodic reversal of the direction of the secondary meridional flow 18 determined according to the invention makes it possible to determine definable values for mixing or directional solidification,

Disturbances and deflections of the free surface 5 of the melt 2, 21, 22 shown in FIGS. 1, 2 a, 2 b, with undesirable effects, such as slag inclusions, are avoided, - in directional solidification, the formation of demixing zones in the solidification structure, which causes the deteriorate mechanical properties, be avoided

- Only one magnet system and thus lower equipment and control effort over stacked, counter-rotating systems are required.

The application of the invention may be used for the mixing of molten metals 2, 21, 22, for continuous casting, for the directed solidification of mixed metallic alloys and for the directed solidification of semiconductor melts, among others. be used.

List of Reference Numbers 1 device 2 first melt

3 arrangement of induction coils

31 first pair of induction coils

32 second pair of induction coils 33 third pair of induction coils

4 base plate

5 surface

6 metal block

7 cooling channels 8 insulation

9 heatsinks

10 temperature sensor

11 power supply unit

12 control unit 13 container

14 symmetry axis

15 First direction of rotation

16 second direction of rotation

17 midplane 18 meridional secondary flow

19 azimuthal rotational flow

20 side walls

21 second melt

22 third melt 23 cooling device

Tp period

T _P M Period during mixing

TPE period at the beginning of the solidification Tpause pause duration tj _.a response time

Claims

claims

A method of electromagnetically stirring electrically conductive liquids (2,21,22) in the liquid state and / or in the state of solidification of the liquid (2,21,22) using a Lorentz force (F _L ) generating, rotating magnetic field, characterized in that the direction of rotation (15,16) of the rotating magnetic field in the horizontal plane at regular time intervals in the form of a period (Tp) is changed, wherein the frequency of the change of direction of the movement of the magnetic field vector set is that in the state of mixing of the liquid liquid (2,21, 22) has a period (T _P ) between two changes of direction of the magnetic field in a time interval (.DELTA.T _PM ) depending on the set time (tj _.a. ) With the condition

(I) 0.5-tj.a. <TPM <1.5-tj.a. is provided, and that at the beginning of the state of solidification of the liquid (2,21, 22) has a period (T _P ) between two changes of direction of the magnetic field in a time interval (.DELTA.T _PE ) in dependence on the setting time tj _.a. under the condition

(II) 0.8-ti.a. <T _PE <4-ti.a, wherein the set time (tj _a ) is given by the equation

(III) tj.a. = C _g ^{• B} oJ- is given,

in which the double vortex of the secondary meridional flow (18) is formed after a connection of the rotating magnetic field in a liquid (2; 21, 22) at rest, and σ is the electrical conductivity, p is the density of the liquid (2, 21, 22 ), ω as the frequency and B ₀ as the amplitude of the magnetic field and C ₉ as the constant for the influence of the size and shape of the volume of the liquid (2, 21, 22).

2. The method according to claim 1, characterized in that for the formation of the rotating magnetic field, a three-phase alternating current (ID) in the form of a three-phase alternating current to at least three on a cylindric, the liquid (2,21, 22) containing container (13) placed

Coupling (31, 32, 33) of induction coils is applied.

3. The method according to claim 1 or 2, characterized in that in the container (13) as electrically conductive liquids metallic or semiconductor melts (2,21, 22) are filled.

4. The method according to claim 1 to 3, characterized in that during the mixing of a cooling melt (2,21, 22) has a period (T _P ) with a. 0.5-tj.a. <TPM <1.5-tj.a. is selected as long as the melt (2,21, 22) is still completely liquid, while the beginning of the state of solidification, the period (T _P ) is increased so that

(II) 0.8-ti.a. <TPE <4-tj.a. is fulfilled.

5. The method according to at least one preceding claim, characterized in that in accordance with the decreasing in the course of the state of directional solidification height (H ₀ ) of the volume of the melt (2; 21, 22), the amplitude (B ₀ ) of the magnetic field is readjusted ,

6. The method according to claim 5, characterized in that in the state of a temperature-controlled solidification, the amplitude (B ₀ ) of the magnetic field according to the course of the process so is increased, that the amplitude (B ₀ ) the respective maximum of the two values

corresponds, where v as the kinematic viscosity of the melt (2,21, 22), V _SO ι as solidification rate and H ₀ as the height of the melt volume and B ₁ and B _{2 are defined} as lower limits of the amplitude of the magnetic field B ₀ .

7. The method according to claim 1 to 5, characterized in that the respective period durations with mixing (T _PM ) and solidification start (Tp _E ), in which the magnetic field is applied, by pauses of the pause duration (Tp _au se), in which no magnetic field at the melt (2,21, 22), are interrupted, the pause duration

(Tpause) to the respective period (Tp) with T _PaU se ≤ 0.5 Tp is set.

8. The method according to claim 1 to 6, characterized in that in the modulation of the course of the Lorentz force (F _L ) instead of the rectangular function other pulse shapes, such as sine, triangle or saw tooth, realized, the course and the maximum value of the amplitude ( B ₀ ) of the magnetic field are set so that results in an identical energy input for the different pulse shapes.

9. Device (1) for the electromagnetic stirring of electrically conductive liquids (2,21, 22) in the liquid state and / or in the state of the beginning of the solidification of the liquid (2,21, 22) using a in the horizontal plane a Lorentz force (F _L ) generating, rotating Magnetic field by means of the method according to claim 1 to 8, at least comprising

a cylindrical container (13),

- A the container (13) surrounding centrally symmetrical arrangement (3) of at least three pairs (31, 32, 33) of induction coils for forming a Lorentz force (FL) generating, rotating magnetic field and

- At least one temperature sensor (10) for measuring the temperature of the liquid (2,21, 22) in the container (13), characterized in that the pairs (31, 32,33) of the induction coil with a control and regulating unit (12) in connection are connected via a connected power supply unit (11) a three-phase current (ID) to the pairs (31, 32,33) of induction coils, wherein the phase position of the pairs (31, 32,33) of the induction coils feeding three-phase current (I ₀ ) at regular time intervals according to the predetermined period (TPM) for the mixing in the liquid state or (T _PE ) for mixing from the beginning of solidification is shifted by 180 ° and thus a reversal of the direction of rotation of the magnetic field and the Lorentraft driving force ( FL), whereby the tax

/ Control unit (12) with the temperature sensor (10) is in communication whose temperature data at the time of solidification start the switching of the period from T _PM to TPE trigger.

10. Device according to claim 9, characterized in that the three-phase current (I ₀ ) is designed as a three-phase alternating current.

11. Device according to claim 9, characterized in that the container (13) with the liquid in the form of a melt (2, 21, 22) concentrically within the induction coils (31, 32,33) is arranged.

12. Device according to claim 9, characterized in that the container (13) is provided with a heating device and / or cooling device (23).

13. Device according to claim 9 to 12, characterized in that the container (13) associated bottom plate (4) is in direct contact with a solid metal body (9), which in the interior of a

Coolant is flowed through.

14. Device according to claim 9 to 13, characterized in that the side walls (20) of the container (13) are thermally insulated.

15. Device according to claim 13, characterized in that the cooling body (9) is in communication with a thermostat.

16. Device according to claim 12 to 15, characterized in that between the heat sink (9) and container (13) is a liquid metal film to achieve a stable heat transfer with low transition resistance.

17. Device according to claim 16, characterized in that the liquid metal film consists of a gallium alloy.

18. Device according to claim 9 to 17, characterized in that at least one temperature sensor (10), preferably in the form of a thermocouple, is positioned in the bottom plate (4) and / or the side walls (20) of the container (13) in which the melt (2; 21, 22) is located, which provides an information signal about the time of commencement of solidification and is connected to the control unit (12).

19. Use of the device (1) for electromagnetic stirring of electrically conductive liquids (2,21, 22) according to claims 9 to 18 in the form of metallic melts in metallurgical processes or in the form of semiconductor melts in the crystal growth, for the purification of

Metal melts, in continuous casting or in the solidification of metallic materials by means of the method according to claim 1 to 8.