EP2190612B1 - Method and device for the electromagnetic stirring of electrically conductive fluids - Google Patents

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 can be used to directly and accurately control the electromagnetic field 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 (English: rotating magnetic fields RMF).

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 meridionalen plane (rz plane) due to the pressure difference between the center of the container and the edge layers at the bottom and on the free surface and their amplitude as a function of Geometry of the considered flow about by a factor of five to ten less than the rotating azimuthal flow fails. In the meridional plane, as in the publication PA 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, ie in the region of the horizontal center plane of the container, the liquid molten metal is transported radially outward, flows upwards or downwards on the side walls and flows back to the bottom and below the free surface Axis. The direction of the secondary circulation adjusts itself in the same way for both directions of rotation of the magnetic field. DE 10 2004 017 443 B3 describes a method and apparatus for stirring electrically conductive liquids in containers using a rotating magnetic field in one direction of rotation. Moeinipour, K. et al. Describe the influence of the rotating magnetic field on the properties of the casting alloy EN AC-AlSi7Mg, especially on the growth of the dendrites. [ Moeinipour, K. et al.: Influence of the rotating magnetic field on the microstructure and the dendrites of the cast alloy EN AC-AlSi7Mg. Foundry Volume 91, Heft. 12 (2004), pp. 18-20, 22-23 .]

A major problem with regard to the use of a rotating magnetic field for electromagnetic stirring is that the majority of the kinetic energy of the melt for the primary azimuthal rotational movement is applied, but which contributes only slightly 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, for example the occurrence of Taylor-Görtler-vortexes, as in the documents PA Nikrityuk, K. Eckert, R. Grundmann: Magnetohydrodynamics, 2004, 40, pp. 127-146 , and PA Nikrityuk, K. Eckert, R. Grundmann: CD Proceeding of the Conference on Turbulence and Interactions TI2006, France, 2006, May 28-June 2, 2006 , described, near the side walls. 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, i. 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 For this purpose, an arrangement of a polyphase electromagnetic winding for generating a traveling field perpendicular to the casting direction is described on a continuous casting plant.

A method for stirring the molten steel in the continuous casting is also in the document US 2003/0106667 described 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 pamphlets DE 2 401 145 and DE 3 730 300 In each case, methods for electromagnetic stirring in continuous casting molds are described in which a periodic change of the current in the coil arrangement is made. In the publication DE 2 401 145 It is described that with this procedure the formation of secondary white bands and secondary dendrites can be avoided.

With the in the publication DE 3 730 300 described method, 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 last mentioned documents, very wide ranges, namely 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 for the electromagnetic stirring of electrically conductive liquids, which are designed so that an intense three-dimensional flow inside the liquid for mixing in the liquid state reaches up to the immediate vicinity of solidification fronts and at the same time undisturbed , free surface of the liquid can be ensured.

The object is solved by the features of claims 1 and 8.

• die Drehrichtung des in der horizontalen Ebene rotierenden Magnetfeldes in regelmäßigen, zeitlichen Abständen in Form einer Periodendauer T_P gewechselt, wobei die Frequenz des Richtungswechsels der Bewegung des Magnetfeldvektors derart eingestellt wird,
• im Zustand der Durchmischung der Flüssigkeit eine Periodendauer T_P zwischen zwei Richtungswechseln des Magnetfeldes in einem Zeitintervall ΔT_PM in Abhängigkeit von der Einstellzeit t_i.a. mit Bedingung $$0.5\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}.}<{\mathrm{T}}_{\mathrm{PM}}<1.5\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}.}$$
  
  eingestellt, und
• zu Beginn des Zustandes der Erstarrung der Flüssigkeit eine Periodendauer T_P zwischen zwei Richtungswechseln des Magnetfeldes in einem Zeitintervall ΔT_PE in Abhängigkeit von der Einstellzeit t_i.a. mit Bedingung $$0.8\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}.}<{\mathrm{T}}_{\mathrm{PE}}<4\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}}$$
  
  eingestellt, wobei die Einstellzeit t_i.a. durch die Gleichung $${\mathrm{t}}_{\mathrm{i}.\mathrm{a}.}=C_g\cdot {\left(B_0\sqrt{\frac{\mathit{\sigma \omega }}{\rho }}\right)}^{-1}$$
  
  gegeben wird, in der sich nach einem Zuschalten des rotierenden Magnetfeldes in einer sich im Ruhezustand befindenden Flüssigkeit der Doppelwirbel der meridionalen Sekundärströmung ausbildet, und σ als elektrische Leitfähigkeit, ρ als Dichte der Flüssigkeit, ω als Frequenz und B₀ als Amplitude des Magnetfeldes und C_g als Konstante für den Einfluss von Größe und Form des Volumens der Flüssigkeit definiert werden.

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 of a Lorentz force F _L generating, rotating magnetic field,
be in the characterizing part according to claim 1

• the direction of rotation of the rotating magnetic field in the horizontal plane 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 setting time t _ia with condition $$0.5\cdot {\mathrm{t}}_{\mathrm{i},\mathrm{a},}<{\mathrm{T}}_{\mathrm{PM}}<1.5\cdot {\mathrm{t}}_{\mathrm{i},\mathrm{a},}$$
  
  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.T _PE as a function of the setting time t _ia with condition $$0.8\cdot {\mathrm{t}}_{\mathrm{i},\mathrm{a},}<{\mathrm{T}}_{\mathrm{PE}}<4\cdot {\mathrm{t}}_{\mathrm{i},\mathrm{a}}$$
  
  adjusted, wherein the adjustment time t _ia by the equation $${\mathrm{t}}_{\mathrm{i},\mathrm{a},}=C_G\cdot {\left({\mathrm{<figure-callout\; id="0"\; label="B"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">B</figure-callout>}}_0\sqrt{\frac{\mathit{\sigma \omega }}{\rho }}\right)}^{-1}$$
  
  is given, in which forms after connecting the rotating magnetic field in a quiescent liquid of the double vortex of the secondary meridional flow, and σ as electrical conductivity, ρ as the density of the liquid, ω as the frequency and B ₀ as the amplitude of the magnetic field and C _g as Constant can be defined for the influence of size and shape of the volume of liquid.

For the formation of the rotating magnetic field, a three-phase current I _{D is} used, which is applied, for example in the form of a three-phase alternating current, 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.

During the mixing of a cooling melt, therefore, a period T _P according to condition (I) is chosen to be 0.5 · t _ia <T _PM <1.5 · t _ia as long as the melt is still completely liquid, while at the beginning of the state of solidification the period duration T _P is increased so that, according to condition (II), it is satisfied that 0.8 · t _ia <T _PE <4 · t _ia .

According to the decreasing height H _{0 of} the volume of the melt in the course of the state of directional solidification, the amplitude B _{0 of} the magnetic field can be readjusted.

und $$B_2=\sqrt{\frac{\rho }{\mathit{\sigma \omega }}}\cdot \frac{40\cdot V_{\mathit{sol}}^{\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}{\sqrt{H_{0}v}}$$

erreicht wird, wobei v als kinematische Viskosität der Schmelze, V_sol als Erstarrungsgeschwindigkeit, H₀ als Höhe des Schmelzenvolumens, und B₁ und B₂ als untere Grenzwerte der Amplitude B₀ des Magnetfeldes definiert werden, die sich im Verlauf der Erstarrung in Abhängigkeit der Parameter v, V_sol und H₀ verändern können.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 $$B_{}$$$${}_1=\sqrt{\frac{\rho }{\mathit{\sigma \omega }}}\cdot \frac{100\cdot {\mathrm{<figure-callout\; id="0"\; label="V\; Sol\; H"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">V\; </figure-callout>}}_{\mathit{Sol}}}{H_{}}$$

and $${\mathrm{<figure-callout\; id="2"\; label="B"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">B</figure-callout>}}_2=\sqrt{\frac{\rho }{\mathit{\sigma \omega }}}\cdot \frac{40\cdot {\mathrm{<figure-callout\; id="3"\; label="V\; Sol"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">V\; </figure-callout>}}_{\mathit{Sol}}^{\mathit{}}}{\sqrt{H_{0}v}}$$

where v is defined as the kinematic viscosity of the melt, V _sol as the solidification rate, H ₀ as the height of the melt volume, and B ₁ and B _{2 are defined} as the lower limits of the magnetic field amplitude B ₀ in the course of solidification depending on the parameters v, V _sol and H _{0 can} change.

The respective period durations with mixing T _PM and at the beginning of solidification T _PE , in which the magnetic field is applied, are interrupted by pauses in the pause duration Tpause, in which no magnetic field is applied to the melt, wherein the pause duration T _break for each period T _P with T _Pause ≤ 0.5 · T _{P is} set.

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.

• einen zylindrischen Behälter,
• eine den Behälter umgebende zentralsymmetrische Anordnung von mindestens drei Paaren von Induktionsspulen zur Ausbildung eines eine Lorentzkraft F_L erzeugenden, rotierenden Magnetfeldes,
• mindestens einen Temperatursensor zur Temperaturmessung der Flüssigkeit im Behälter,

wobei
die Paare der Induktionsspulen mit einer Steuer- und Regeleinheit in Verbindung stehen, die über eine angeschlossene Stromversorgungseinheit einen Drehstrom I_D an die Paare von Induktionsspulen weiterleitet, wobei die Phasenlage des die Paare der Induktionsspulen speisenden Drehstromes I_D in regelmäßigen, zeitlichen Abständen und entsprechend der vorgegebenen Periodendauer T_PM für die Durchmischung im flüssigen Zustand oder T_PE für die Durchmischung ab Beginn der Erstarrung um 180° verschoben wird und damit eine Umkehrung der Drehrichtung des Magnetfeldes und der die Strömung antreibenden Lorentkraft F_L erreicht wird, wobei die Steuer-/Regeleinheit mit dem Temperatursensor in Verbindung steht, dessen Temperaturdaten zum Zeitpunkt des Erstarrungsbeginns das Umschalten der Periodendauer von T_PM zu T_PE auslösen.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 in which a Lorentz force F _L generating, rotating magnetic field and under control of the temperature profile of the liquid by means of the inventive method 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 Lorentz force F _L ,
• at least one temperature sensor for measuring the temperature of the liquid in the container,

in which
the pairs of induction coils are connected to a control and regulating unit which transmits a three-phase current I _D to the pairs of induction coils via a connected power supply unit, the phase position of the three-phase current I _D fed to the pairs of induction coils being measured at regular time intervals and corresponding to predetermined period T _PM for the 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 of the magnetic field and the flow driving Lorentkraft F _{L is} achieved, the control unit with the Temperature sensor is in communication whose temperature data at the time of solidification start the switching of the period from T _PM to T _PE trigger.

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 traversed 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 may be positioned, for example in the form of a thermocouple, which provides an information signal about the time of onset of solidification and is connected to the control unit ,

The method according to any one of claims 1 to 7 can be used for stirring electrically conductive liquids in the form of metallic melts in metallurgical processes or in the form of semiconductor melts in crystal growth, for the purification of molten metals, in continuous casting or in the solidification of metallic materials.

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.

• Für eine intensive Durchmischung des Schmelze bei gleichzeitig geringem Energieaufwand gilt die Bedingung: $$0.5\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}.}<{\mathrm{T}}_{\mathrm{P}}<1.5\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}.}$$
  
  oder
  für eine kontrollierte Erstarrung unter Vermeidung der Ausbildung von Entmischungszonen im Erstarrungsgefüge gilt die Bedingung: $$0.8\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}.}<{\mathrm{T}}_{\mathrm{P}}<4\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}\mathrm{.}}$$
  

According to the following definition applies:

• For an intensive mixing of the melt with at the same time low expenditure of energy the condition applies: $$0.5\cdot {\mathrm{t}}_{\mathrm{i},\mathrm{a},}<{\mathrm{T}}_{\mathrm{P}}<1.5\cdot {\mathrm{t}}_{\mathrm{i},\mathrm{a},}$$
  
  or
  for a controlled solidification while avoiding the formation of segregation zones in the solidification structure, the condition applies: $$0.8\cdot {\mathrm{t}}_{\mathrm{i},\mathrm{a},}<{\mathrm{T}}_{\mathrm{P}}<4\cdot {\mathrm{t}}_{\mathrm{i},\mathrm{a}\mathrm{,}}$$
  

The parameter t _ia 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 at rest, the double vortex typical of the meridional secondary flow has formed.

The characteristic response time t _{ia 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 take numbers between three and five. This is compared to the prior art, in particular with respect to the document DE 3 730 300 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 exists an important parameter that can be specified to make the stirring intensive. 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 significantly and logically 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 T _P , 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.

Fig. 1

eine schematische Darstellung einer Einrichtung zum elektromagnetischen Rühren zur Durchmischung einer flüssigen Schmelze in Verbindung mit dem erfindungsgemäßen Verfahren, wobei

Fig. 1a

einen schematischen Aufbau der Einrichtung in Vorderansicht,

Fig. 1b

eine Draufsicht auf die Einrichtung nach Fig. 1a,

Fig. 1c

eine schematische Darstellung der Strömungsarten in einem in der horizontalen Ebene rotierenden Magnetfeld,

Fig. 1d

eine Periodendauer (Tp)-Temperatur (T)-Darstellung der Schmelze im flüssigen Zustand und im Übergang zur Erstarrung, wobei T_sol die Temperatur des Behälterbodens zu Beginn der Erstarrung bezeichnet, und

Fig. 1e

eine Lorentzkraft (F_L/F_LO)-Zeit(t)-Darstellung,

Fig. 2

zwei schematische zylindrische Behälter mit flüssigen Metallschmelzen, wobei

Fig. 2a

eine flüssige Schmelze eines Metalls und

Fig. 2b

zwei übereinander befindliche Schmelzen zweier unterschiedlicher Metalle im Ruhezustand (im entmischten Zustand) zeigen,

Fig. 3

die experimentell ermittelte Abhängigkeit der Intensität der meridionalen Sekundärströmung von der Periodendauer T_P,

Fig. 4

Ergebnisse numerischer Simulationen zur Mischung von flüssigem Blei (Pb) und flüssigem Zinn (Sn): Mischungsverhalten bei gleicher Zeit nach Beginn der Mischung (t/t_spin-up=1.92) , wobei Fig. 4a kontinuierliches RMF, T_p= ∞ Fig. 4b T_p/t_i.a. = 1.03 . Fig. 4c T_p/t_i.a. = 2. zeigen.

Fig. 5

Darstellung der Ergebnisse numerischer Simulationen zur Mischung der Zinn-Konzentration in der unteren Behälterhälfte: Zeitliche Entwicklung der volumengemittelten Sn-Konzentration im unteren Behältervolumen für verschiedene Szenarios. $$<C_{\mathit{Sn}}>=\frac{4{\displaystyle \underset{0}{\overset{{\mathrm{<figure-callout\; id="0"\; label="H"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">H</figure-callout>}}_0/2}{\int }}{\displaystyle \underset{0}{\overset{R_0}{\int }}{C}_{\mathit{Sn}}\mathrm{}\mathit{r\; dr\; dz}}}}{{\mathrm{<figure-callout\; id="0"\; label="R"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">R</figure-callout>}}_0^2{\mathrm{<figure-callout\; id="0"\; label="H"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">H</figure-callout>}}_0}$$

Fig. 6

Erstarrung einer Al-Si-Legierung unter Magnetfeldeinfluss, B₀ = 6.5 mT, (Makrogefüge), wobei Fig. 6a kontinuierliches RMF, T_p= ∞ Fig. 6b T_p/t_i.a. = 1.67 .. Fig. 6c T_p/t_i.a. = 0.95 zeigen, und

Fig. 7

Erstarrung einer Al-Si-Legierung unter Magnetfeldeinfluss (Mikrogefüge), wobei Fig. 7a kontinuierliches RMF, T_p= ∞ Fig. 7b T_p/t_i.a. = 1.67 zeigen.

Fig. 8

eine radiale Verteilung des Flächenanteils an Primärkristallen in Al7wt%Si-Proben (mit sieben Gewichtsanteilen Si), die unter Magnetfeldeinfluss mit Variation der Pulsdauer T_P erstarrt wurden.

Show it:

Fig. 1

a schematic representation of an apparatus for electromagnetic stirring for mixing a liquid melt in conjunction with the inventive method, wherein

Fig. 1a

a schematic structure of the device in front view,

Fig. 1b

a plan view of the device according to Fig. 1a .

Fig. 1c

a schematic representation of the flow types in a rotating in the horizontal plane magnetic field,

Fig. 1d

a period (Tp) -Temperatur (T) representation of the melt in the liquid state and in the transition to solidification, where T _sol denotes the temperature of the container bottom at the beginning of solidification, and

Fig. 1e

a Lorentz force (F _L / F _LO ) time (t) representation,

Fig. 2

two schematic cylindrical container with liquid molten metal, wherein

Fig. 2a

a liquid melt of a metal and

Fig. 2b

show two superimposed melting of two different metals at rest (in the unmixed state),

Fig. 3

the experimentally determined dependence of the intensity of the secondary meridional flow on the period T _P ,

Fig. 4

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

Fig. 5

Presentation of the results of numerical simulations for the mixing of 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. $$<C_{\mathit{sn}}>=\frac{4{\displaystyle \underset{0}{\overset{{\mathrm{<figure-callout\; id="0"\; label="H"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">H</figure-callout>}}_0/2}{\int }}{\displaystyle \underset{0}{\overset{R_0}{\int }}{C}_{\mathit{sn}}\mathrm{}\mathit{r\; dr\; dz}}}}{{\mathrm{<figure-callout\; id="0"\; label="R"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">R</figure-callout>}}_0^2{\mathrm{<figure-callout\; id="0"\; label="H"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">H</figure-callout>}}_0}$$

Fig. 6

Solidification of an Al-Si alloy under magnetic field influence, B ₀ = 6.5 mT, (macrostructure), where Fig. 6a continuous RMF, T _p = ∞ Fig. 6b T _p / t _ia = 1.67 .. Fig. 6c T _p / t _ia = 0.95, 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.

Fig. 8

a radial distribution of the area fraction of primary crystals in Al7wt% Si samples (with seven parts by weight Si), which were solidified under magnetic field influence with variation of the pulse duration T _P.

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

• einen zylindrischen Behälter 13 mit der darin befindlichen flüssigen Schmelze 2, wie in Fig. 2a gezeigt ist, oder 21,22, wie in Fig. 2b gezeigt ist,
• eine den Behälter 13 umgebende zentralsymmetrische Anordnung 3 von mindestens drei Paaren 31,32,33 von Induktionsspulen zur Ausbildung eines eine Lorentzkraft F_L erzeugenden, rotierenden Magnetfeldes und
• mindestens einen Temperatursensor 10 zur Temperaturmessung der Flüssigkeit 2,21,22 im Behälter 13.

In Fig. 1,1a, 1b is shown in a schematic representation ine 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 F _L generating, rotating magnetic field, wherein the device 1 contains at least

• a cylindrical container 13 with the liquid melt 2 therein, as in Fig. 2a is shown, or 21,22, as in Fig. 2b is shown
• a surrounding the container 13 centrally symmetrical arrangement 3 of at least three pairs 31,32,33 of induction coils to form 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 13th

The pairs 31, 32, 33 of the induction coils are connected to a control / regulating unit 12, which transmits a three-phase current I _D 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 I _D 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 magnetic field and the Lorentz force F _L driving the flow is reached, the control unit 12 being in communication with the temperature sensor 10 whose temperature data at the time of solidification start trigger the switching over of the period from T _PM to T _PE .

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 in FIG Fig. 1b is shown. The induction coil pairs 31, 32, 33 are fed by a power supply unit 11 with a three-phase current I _D in the form of a three-phase alternating current and generate a magnetic field which rotates about the symmetry axis 14 of the container 13 and is oriented horizontally with the direction of rotation 15 (arrow direction). The temporal change of the magnetic field strength generates a Lorentz force F _L with a dominant azimuthal component, which the melt 2 in Fig. 2a or 21 , 22 in Fig. 2b put in a rotary motion. The power supply unit 11 of the induction coil pairs 31, 32, 33 is connected to the control / regulation unit 12, which causes a shift of the phases of the three-phase alternating current I _D at predetermined time intervals. The result of the phase shift is that the direction of rotation 15 of the horizontally oriented magnetic field reverses during the phase change in the direction of rotation 16, as in FIG Fig. 1b is shown.

The method can be used, for example, for the temperature distribution in a one-component melt 2, as in Fig. 2a is shown to homogenize or to balance the concentration in segregated multicomponent melts 21,22, as in Fig. 2b 3, with the higher density melt 22 being in the lower part of the container 13 before the start of mixing and being covered by the lighter melt 21.

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, the flow is slowed down and the melt 2, 21, 22 is accelerated in the opposite direction. The Lorentz force F _L varies in the axial direction with the associated force component and has a maximum in the center plane 17 of the container 13. When the direction of rotation 15 of the magnetic field is reversed, the melt 2, 21, 22 is braked more strongly in the vicinity of the center plane 17 and accelerated in the opposite direction 16 than in the vicinity of the bottom 4 of the container 13 and the free surface 5 is the case. The nonuniformities in the direction reversal 15-16, 16-15 of the flow create strong gradients of rotational movement in the axial direction of the axis of symmetry 14. The occurrence of such gradients leads, as in FIG Fig. 1c In the period of reversal of the flow direction thus occurs an intense secondary flow 18 at the same time weak rotational movement 19 on. The mixing of the melt 2, 21, 22 becomes more efficient the better the intensities of the primary azimuthal rotational movement 19 and of the secondary meridional flow 18 can be approximated. This can be achieved over a longer period by constantly changing direction of the magnetic field B ₀ . In this context, as in Fig. 1d . 1e is shown, the setting of the period T _P a crucial role. If the period T _{P is} too long, the primary azimuthal rotational movement 19 increases significantly in intensity 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. If the period T _P but too small, the melt 2, 21,22 can not be sufficiently accelerated, both primary rotational movement 19 and secondary flow 18 lose intensity. Thus, as in Fig. 1e 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, is shown.

An efficient stirring of the liquid melt 2, 21, 22, ie a maximized stirring effect with the lowest possible expenditure of energy, is achieved if the period T _P according to FIG Fig. 1d is set as follows: $$0.5\cdot {\mathrm{t}}_{\mathrm{i},\mathrm{a},}<{\mathrm{T}}_{\mathrm{P}}<1.5\cdot {\mathrm{t}}_{\mathrm{i},\mathrm{a},}$$

wobei die Variablen σ, ρ, ω und B₀ die elektrische Leitfähigkeit und die Dichte der Schmelze, die Frequenz und die Amplitude des Magnetfeldes bezeichnen, während die Konstante C_g den Einfluss von Größe und Form des Schmelzenvolumens beschreibt und Zahlenwerte zwischen Drei und Fünf annehmen kann.The parameter t _ia 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 in the resting state, that for the meridional Secondary flow 18 typical double vortex forms. The set time t _ia is defined by the following equation $${\mathrm{t}}_{\mathrm{i},\mathrm{a},}=C_G\cdot {\left({\mathrm{<figure-callout\; id="0"\; label="B"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">B</figure-callout>}}_0\sqrt{\frac{\mathit{\sigma \omega }}{\rho }}\right)}^{-1}.$$

where the variables σ, ρ, 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 _{g 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 GaInSn melt 21,22 was measured by means of the ultrasonic Doppler method. Fig. 3 shows the along a axial line at r = 18mm measured root mean square value of the vertical velocity U _z ² as a function of the period T _P. The experimental results prove the existence of a certain period T _P at which the intensity of the secondary meridional flow 18 reaches a maximum. The position of the maximum U _zmax ² varies with the magnetic field strength and corresponds to the respective adjustment time t _ia .

With the invention, various melts 21, 22, as in FIG Fig. 2b is shown 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. The Figure 4 and the 4a, 4b, 4c contain the results of numerical simulations at a magnetic field of 1 mT with respect to the concentration distribution of lead (black) 22 and tin (white) 21 in a rz half-plane after a certain time of 20s, where
Fig. 4a with T _P = 0
Fig. 4b with T _P = 1.03 t _ia
Fig. 4c with T _P = 2 t _ia

für verschieden eingestellte Werte für die Periodendauer T_P zeigt, dass die Durchmischung am schnellsten für die Periodendauer T_P ≈ t_i.a. vonstatten geht. Die Darstellung wird durch die zeitliche Entwicklung der Zinn-Konzentration 21 in der unteren Behälterhälfte (R₀ - Radius, H₀ - Höhe des Behälters) bestätigt, welche in Fig. 4b dargestellt ist. Besonders festzuhalten ist in diesem Zusammenhang, dass bei der Einstellung eines ungeeigneten Wertes der Periodendauer T_P im Hinblick auf eine Homogenisierung des Schmelzenvolumens schlechtere Ergebnisse erzielt werden als bei der Anwendung eines kontinuierlich rotierenden Magnetfeldes.A comparison of in Fig. 5 presented results of numerical simulations for mixing the tin concentration C _Sn in the lower half of the container for a time evolution of the volume-average Sn concentration in the lower container volume for different scenarios in relation to the flows in Fig. 4a, 4b, 4c $$<C_{\mathit{sn}}>=\frac{4{\displaystyle \underset{0}{\overset{{\mathrm{<figure-callout\; id="0"\; label="H"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">H</figure-callout>}}_0/2}{\int }}{\displaystyle \underset{0}{\overset{R_0}{\int }}{C}_{\mathit{sn}}\mathrm{}\mathit{r\; dr\; dz}}}}{{\mathrm{<figure-callout\; id="0"\; label="R"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">R</figure-callout>}}_0^2{\mathrm{<figure-callout\; id="0"\; label="H"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">H</figure-callout>}}_0}$$

for various set values for the period T _P shows that the mixing is the fastest for the period T _P ≈ t _ia takes place. The representation is confirmed by the time evolution of the tin concentration 21 in the lower half of the container (R ₀ - radius, H ₀ - height of the container), which in Fig. 4b is shown. It should be particularly noted in this context that when setting an inappropriate value of the period T _P with respect to a homogenization of the melt volume worse results are achieved than in the application of a continuously rotating magnetic field.

In the Fig. 2 illustrated device 1 of the filled with an electrically conductive melt 2 cylindrical container 13 in the arrangement 3 of induction coil pairs 31,32,33, as in Fig. 1,1a, 1b can be supplemented by a cooling device 23 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 allow monitoring of the beginning and the course of the state of solidification and allow a timely adjustment of the magnetic field parameters (eg B ₀ and T _P ) by the control unit 12 controlled by the power supply unit 11 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 in Fig.1d is shown, adjusted so 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 be used in the device 1 according to FIG Fig. 1 . 2 B directionally controlled solidify. The structural properties obtained are based on the Fig. 6a, 6b, 6c . 7a, 7b and 8th concerning the formation of columnar dendrites, grain refining and segregation:

Fig. 6 shows the macrostructure in longitudinal section of cylindrical blocks of an Al-7wt% Si alloy, for example, with a diameter of 50mm and a height of 60mm, which were directionally solidified under the action of a rotating magnetic field at a field strength B ₀ of 6.5mT. 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, a modified columnar structure is first formed, as in Fig. 7a shown, ie the columnar grains are finer and grow tilted 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 demixing patterns, the depletion of eutectic phases in the peripheral zones and a Concentration in the region of the axis of symmetry 14 have. 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 is a radial distribution of the area fraction of primary crystals in Al-7wt% Si samples (with seven parts by weight Si), which were solidified under magnetic field influence with variation of the pulse duration T _p .

The Fig. 6 to 8 show that in the case of electromagnetic stirring with a change of direction of the magnetic field with switching on the magnetic field, 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 a reduction of segregation, which with a suitable choice of the pulse duration T _{P is} also almost completely avoided, as in Fig. 7b can be shown.

• Ausbildung einer intensiven, dreidimensionalen Strömung im Innern der Metallschmelze 2;21,22,
• sehr gute Durchmischung der Metallschmelze 2;21,22 durch intensive meridionale Sekundärströmung 18,
• geringerer Energieaufwand im Vergleich zum kontinuierlich rotierenden Magnetfeld, da nicht der überwiegende Teil der aufgewendeten Energie für in die Aufrechterhaltung der azimutalen Rotationsströmung aufgebracht werden muss, und ein höherer Energieanteil in die für die Durchmischung effektivere meridionale Sekundärströmung 18 aufgebracht wird,
• die erfindungsgemäß festgelegte Frequenz der periodischen Richtungsumkehr der meridionalen Sekundärströmung 18 ermöglicht bestimmbare Werte für die Durchmischung oder bei gerichteter Erstarrung,
• Störungen und Auslenkungen der freien, in Fig. 1,2a,2b dargestellten Oberfläche 5 der Schmelze 2;21,22 mit unerwünschten Effekten, wie Schlackeneinschlüsse, werden vermieden,
• bei der gerichteten Erstarrung kann die Ausbildung von Entmischungszonen im Erstarrungsgefüge, die die mechanischen Eigenschaften verschlechtern, vermieden werden,
• nur ein Magnetsystem und damit geringerer apparativer und regelungstechnischer Aufwand gegenüber übereinander angeordneten, gegenläufig rotierenden Systemen sind erforderlich.

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 secondary meridional flow 18,
• less energy input compared to the continuously rotating magnetic field, since not the majority of the energy expended in the maintenance of the azimuthal rotational flow must be applied, and a higher proportion of energy is applied in the more effective for mixing meridional secondary flow 18,
• the frequency of the periodic reversal of 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, in Fig. 1 . 2a, 2b shown surface 5 of the melt 2, 21,22 with undesirable effects, such as slag inclusions are avoided,
• in directional solidification, the formation of segregation zones in the solidification structure, which worsen the mechanical properties, can be avoided,
• only a magnetic system and thus less 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

Facility

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

baseplate

5

surface

6

metal block

7

cooling channels

8th

insulation

9

heatsink

10

temperature sensor

11

Power supply unit

12

Control / regulation unit

13

container

14

axis of symmetry

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

T _P

period

T _pm

Period duration with mixing

T _PE

Period duration at the beginning of solidification

T _break

break time

t _ia

Response time

Claims (8)

1. Method for the electromagnetic stirring of electrically conductive fluids (2, 21, 22) in the liquid state and/or in the state at the beginning of the solidification of the fluid (2, 21, 22) by using a rotating magnetic field which produces a Lorentz force (F_L) in the horizontal plane, characterized in that the direction of rotation (15, 16) of the magnetic field rotating in the horizontal plane is changed in regular time intervals in the form of a period (T_P), the frequency of the change in direction of the movement of the magnetic field vector being set in such a way that in the state of the mixing of the liquid fluid (2, 21, 22) a period (T_P) between two changes in direction of the magnetic field in a time interval (ΔT_PM) is provided as a function of the initial adjustment time (t_i.a.) with the condition that $$0.5\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}.}<{\mathrm{T}}_{\mathrm{PM}}<1.5\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}.},$$

   

   and that at the beginning of the state of solidification of the fluid (2, 21, 22) a period (T_P) is adjusted between two changes in direction of the magnetic field in a time interval (ΔT_PE) as a function of the initial adjustment time t_i.a. with the condition that $$0.8\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}.}<{\mathrm{T}}_{\mathrm{PE}}<4\cdot {\mathrm{t}}_{\mathrm{i}.\mathrm{a}},$$

   

   the initial adjustment time (t_i.a.) being given by the equation $${\mathrm{t}}_{\mathrm{i}.\mathrm{a}.}=C_g\cdot {\left(B_0\sqrt{\frac{\mathit{\sigma \omega }}{\rho }}\right)}^{-1}$$

   

   in which after the rotating magnetic field is switched on in a fluid (2; 21, 22) in a state of rest the double vortex of the meridional secondary flow (18) is formed, and σ is defined as the electrical conductivity, ρ as the density of the fluid (2, 21, 22), ω as a frequency and B₀ as the amplitude of the magnetic field, and C_g is defined as a constant for the influence of the size and shape of the volume of the fluid (2, 21, 22).
2. Method according to Claim 1, characterized in that in order to form the rotating magnetic field a rotary current (I_D) in the form of a three-phase alternating current is applied to at least three pairs (31, 32, 33) of induction coils placed on a cylindrical container (13) containing the fluid (2, 21, 22).
3. Method according to Claim 1 or 2, characterized in that metal or semiconductor melts (2, 21, 22) are poured as electrically conductive fluids into the container (13).
4. Method according to at least one preceding claim, characterized in that the amplitude (B₀) of the magnetic field is corrected in accordance with the height (H₀) of the volume of the melt (2; 21, 22), which decreases in the course of the state of the directional solidification.
5. Method according to Claim 4, characterized in that in the state of a directional solidification under temperature control the amplitude (B₀) of the magnetic field is increased in accordance with the course of the process such that the amplitude (B₀) corresponds to the respective maximum of the two values $$B_1=\sqrt{\frac{\rho }{\mathit{\sigma \omega }}}\cdot \frac{100\cdot V_{\mathit{sol}}}{H_0}$$

   

   and $$B_2=\sqrt{\frac{\rho }{\mathit{\sigma \omega }}}\cdot \frac{40\cdot V_{\mathit{sol}}^{\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}{\sqrt{H_{0}v}}$$

   

   v being defined as the kinematic viscosity of the melt (2, 21, 22), V_sol being defined as the rate of solidification, and H₀ being defined as the height of the melt volume and B₁ and B₂ as lower limit values of the amplitude of the magnetic field B₀.
6. Method according to Claims 1 to 4, characterized in that the respective periods during mixing (T_PM) and the beginning of solidification (T_PE) in which the magnetic field is present and switched on are interrupted by pauses of pause duration (T_Pause) in which no magnetic field is present at the melt (2, 21, 22), the pause duration (T_Pause) being adjusted relative to the respective period (T_P) with T_Pause ≤ 0.5 · T_P.
7. Method according to Claims 1 to 6, characterized in that pulsed functions, for example a sine, triangle or sawtooth function, are implemented instead of the rectangular function when modulating the profile of the Lorentz force (F_L), the profile and the maximum value of the amplitude (B₀) of the magnetic field being defined such that an identical energy input results for the pulsed functions.
8. Use of the method according to one of Claims 1 to 7 for the electromagnetic stirring of electrically conductive fluids (2, 21, 22) in the form of metallic melts in metallurgical processes, or in the form of semiconductor melts in crystal growth for the purpose of cleaning metal melts, during continuous casting or during the solidification of metallic materials.

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