EP1638716A1

EP1638716A1 - Method and device for the fused metal production of conducting alloys

Info

Publication number: EP1638716A1
Application number: EP04766059A
Authority: EP
Inventors: Janis Priede; Gunter Gerbeth; Regina Hermann; Octavian Filip; Ludwig Schultz
Original assignee: Leibniz Institut fuer Festkorper und Werkstofforschung Dresden eV; Forschungszentrum Dresden Rossendorf eV
Current assignee: Leibniz Institut fuer Festkorper und Werkstofforschung Dresden eV; Helmholtz Zentrum Dresden Rossendorf eV
Priority date: 2003-06-20
Filing date: 2004-06-18
Publication date: 2006-03-29
Also published as: DE10328618A1; DE10328618B4; WO2004112993A1

Abstract

The invention relates to the field of material technology and concerns a method and device for the fused metal production of conducting alloys, which find application, for example, as materials for high strength permanent magnets. The aim of the invention is disclosure of a method and device whereby the seed formation, proliferation and transformation properties of forming phases and thus the structure of the solidifying alloys can be influenced. Said aim is achieved, by means of a fused metal production of conducting alloys in which the flow within the melt is influenced by an electromagnetically-generated volume force, to give a non-flowing condition and further, by means of a device for the fused metal production of conducting alloys, whereby a secondary coil is arranged above or below the primary coil and both coils comprise a dedicated resonant circuit, by means of which a phase shift may be controlled.

Description

Method and device for the melt metallurgical production of conductive alloys

The invention relates to the field of materials science and relates to a method and a device for the melt-metallurgical production of conductive alloys, which can be used, for example, as a material for high-performance permanent magnets.

The increasing demand for high-performance permanent magnets continues (Yuji Kaneko: Proc. 16 ⁰¹ Int. Workshop on Rare-Earth Magnets and their Applications, Sendai, Japan, September 10-14, 2000).

The ferromagnetic properties of the rare earth magnets are based on the excellent hard magnetic properties of the Nd ₂ Feι ₄ B phase (φ phase) with their structural anisotropy, which was described in 1984 by Sagawa et al .: J. Appl. Phys. 55 (1984) 2083 and Croat et al .: J. Appl. Phys. 55 (1984) 2078. The ternary phase diagram Nd-Fe-B in the form used today uses the work of Schneider et al .: Z. Metallkd. 77 (1986) 755. However, the properties of real Nd-Fe-B magnets differ considerably from the intrinsic properties (magnetocrystalline anisotropy, saturation magnetization) of the Nd ₂ Fe-ι B phase, since the magnetic properties of the permanent magnetic Alloy are extremely sensitive to the morphology (extrinsic properties).

The most common manufacturing method for Nd-Fe-B permanent magnets is powder metallurgy, including in particular the sintering process. The ingot production of sintered magnets usually takes place in casting processes that take place close to the thermodynamic equilibrium.

The improvement of the extrinsic properties of sintered magnets is made possible on the one hand by modifying the structure via the process parameters. Characteristic sizes such as phase distribution, grain size distribution and shape and orientation of the grains are also positively influenced by the addition of additives.

Essential aspects in the production of the magnetic materials are the suppression of undesired phases (eg α-Fe, B-rich phases) and the reduction of grain growth during solidification and in the annealing process. There is a large number of studies that have shown that the addition of additives leads to changes in the structure morphology. It was found that additives from AI, Ga (W. Rodewald, Proc. 9th Int. Workshop on Rare-Earth-Magnets and their Applications, Bad Soden (FRG) (1987) 609 .; WC Chang, et al.: 10th Int. Workshop on Rare-Earth Magnets and their Applications, Kyoto, Japan (1989) 509) or Cu (L. Withanawasam, among others: J. Appl. Phys. 63 (1994), 6646) promote the formation of special intergranular phases and grain smoothing to lead. The elements Mo and V are added to suppress the undesired NdFe B phase while at the same time refining the grain structure (GC Hajipanayis: Rare-Earth Ion Permanent Magnets, Edited by JMD Coey, Clarendon Press, Oxford, (1996) 317). It has recently been found that additions of Ti and C enable considerable grain refinement and a reduction in the volume fraction of the α-Fe phase (MJ Kramer, inter alia: J. Appl. Phys. 81 (8) (1997) 4459). Further steps to improve the magnetic properties begin in the manufacturing process itself. Annealing processes serve to homogenize and convert the α-Fe phase into the hard magnetic Nd ₂ Fe-ι B phase via a peritectic reaction, although increased grain growth is disadvantageous. With the help of hydrogen, fine-grained coercive powder can be produced from the cast material in the HDDR process (hydrogenation, disproportionation, desorption, recombination) (IR Harris, inter alia: J. Less-Common Met., 106 (1985) L1). A very effective one The method for producing a uniaxial texture is hot pressing (T. Shimoda, inter alia: J. Appl. Phys. 64 (1988) 5290).

In processes such as the RIP process (rubber isostatic pressing), the powder is pulsed in a rubber casting mold with the simultaneous influence of a strong one

Magnetic field pressed and aligned (M. Sagawa, i.a .: IEEE Trans. Mag., Mag-

29 (1993) 2747).

During the dependence of the structure and thus the magnetic properties of the above. The influence of hydrodynamics in the melt on the solidification processes is completely insufficiently known, although technologies such as the electromagnetic stirring of melts are used in the steel industry, in the casting of Al alloys and in semiconductor crystal growth have found. Experimental studies on the influence of strong turbulence in the production of Sn-Pb alloys showed that strong convection increases the growth rate during solidification and leads to refinement (S. Ji and Z. Fan: Metallurgical and Materials Transactions A, Volume 33, Issue 11, (2002) 3511).

The object of the present invention is to provide a method and a device for the melt metallurgical production of conductive alloys, by means of which the nucleation, growth and conversion behavior of the phases forming and thus also the structure of the solidified alloy can be influenced.

The object is achieved by the invention specified in the claims. Further training is the subject of the subclaims.

In the method according to the invention for the melt-metallurgical production of conductive alloys, the flow in the area of the melt is influenced by an electromagnetically generated volume force in the direction of a currentless state. Peritectic alloys or magnet alloys, in particular based on Nd-Fe-B, can advantageously be used as conductive alloys.

This electromagnetic volume force is generated by using a second induction coil, which is arranged above or below the primary induction coil, and a phase shift between the electrical currents in the two induction coils is thereby realized. The secondary coil can be connected to the power supply of the primary coil, but it can also advantageously have no connection to a power source.

The phase shift of the electrical currents is advantageously generated in the resonant circuits of the two induction coils, the phase shift of the electrical currents advantageously being realized by regulating the capacitive force in the secondary resonant circuit and particularly advantageously a phase shift of the electrical currents of 0 °. The latter can be achieved by connecting the two induction coils in series.

It is also advantageous if the amplitude of the currents is limited by an ohmic resistor, with the same amplitude of the currents being advantageously set in both circuits in order to ensure homogeneous melting.

It is also advantageous if the electromagnetically generated volume force is influenced by regulating the frequency of the currents and / or the current strength and / or the vertical spacing of the coils and / or the inner diameter of the coils and / or the capacitance and the ohmic resistance in the secondary resonant circuit, it being particularly advantageous to set a vertical distance between the two coils which corresponds to the radius of the starting material. Likewise advantageously, a penetration depth δ of the magnetic field into the material belonging to the frequency ω of the primary current, δ = (2 / μσω) ^"1/2 , which corresponds to the radius of the starting material, is set, where μ is the magnetic permeability and σ is the electrical conductivity of the material are at the respective temperature. The method according to the invention achieves a greatly reduced melt convection, which surprisingly has a strong effect on the nucleation and growth behavior of primary and secondary growing phases. By influencing the enamel convection in the direction of enamel calming, the kinetics of nucleation and growth processes during solidification are affected even near the liquidus temperature. For example, in the stoichiometric Nd-Fe-B alloy, the nucleation temperature of γ-Fe nuclei, which transform to α-Fe at lower temperatures in a solid state, is 1220 ° C. There is also, for example, an extended lifetime of the metastable phases that form and a delayed nucleation of the subsequent stable phases.

In the device according to the invention for the melt metallurgical production of conductive alloys, a secondary coil is arranged above or below the primary coil, both coils having their own resonant circuit, by means of which a phase shift can be regulated in a controllable manner.

It is also advantageous if the amplitude of the currents is limited by an ohmic resistor, with the same amplitude of the currents being advantageously set in both circuits.

A further advantageous embodiment of the invention is if the secondary coil is arranged above the primary coil for realizing a melt flow upward on the free surface of the melt zone or if the secondary coil is arranged under the primary coil for realizing a melt flow downward on the free surface of the melt zone ,

It is particularly advantageous if the capacitance in the secondary circuits and / or the vertical distance between the two induction coils is selected such that the double vortex structure theoretically associated with each individual coil overlaps the flow in such a way that a significant reduction in the flow is achieved inside the melt. With the device according to the invention for the inductive melting-metallurgical production of conductive alloys, the flow in the molten volume is influenced by additionally generated electromagnetic forces in such a way that the melting flow is aimed at being as calm as possible. This has resulted in the quality of the solidified structure being improved by the solution according to the invention and, in the case of the production of magnetic alloys based on Nd-Fe-B, the volume fraction of the soft magnetic α-Fe phase being able to be substantially reduced.

The device according to the invention consists of the induction coil (primary coil) known in the devices according to the prior art, to which a second induction coil (secondary coil) is added, both coils having their own resonant circuit, by means of which a phase shift can be regulated in a controllable manner.

It is particularly advantageous if the secondary coil is not connected to a power supply and this circuit of the secondary coil has a capacitor with adjustable capacitance and an adjustable ohmic resistance. The

Current in the secondary coil is then generated solely by the primary current in the

Primary coil induced. By regulating the capacitor in the secondary circuit, a phase shift between the two resonant circuits is generated, which leads to a flow-calming volume force in the melt.

By adjusting the capacitance in the second circuit, a

Volume force can be set, which leads to a strong flow calming in the

Melt leads.

It is also advantageous to connect the secondary circuit in series with the primary circuit.

For this purpose, experimental and numerical investigations of the flow patterns were carried out with a model substance, which showed that the strongly calmed flow in a cylindrical body made of a magnetic alloy based on Nd-Fe-B leads to a drastic reduction in the volume of α-Fe.

The flow patterns in the melting zone were numerically simulated taking into account all types of convection (Marangoni, electromagnetic due to HF heating, buoyancy, mechanical rotation of the rod). The electromagnetic field equilibria in the material exactly and not calculated as usual with the approximation of an electrical surface current. This, in turn, is necessary in order to be able to influence the convection and thus the flow conditions on the solidification front in a controlled manner by modifying the electromagnetic fields.

With the method and the device according to the invention, a targeted influencing of the volume force and thus the flow in the melt is possible in the production of conductive alloys, in particular peritectic alloys and especially magnetic alloys based on Nd-Fe-B. In principle, this can be regulated from the setting of a strong convection through a quasi-currentless state to the reversal of the flow direction in front of the solidification front of the melt.

The regulation of the phase shift of the currents and thus the regulation of the flow-driving volumetric force can be achieved, for example, by regulating the frequency of the currents and / or the current strength and / or the vertical distance between the coils and / or the inside diameter of the coils and / or the capacitance and ohmic Resistance in the secondary resonant circuit. If these parameters are changed, the flow is significantly influenced.

The method according to the invention can be used for melting any large amount of material in a crucible-free process, but also for processes using crucibles.

Further examples of conductive alloys which can be produced according to the invention are peritectically solidifying alloys, such as for example Fe-Ni compounds and the large group of Fe-C steels. For the latter it is important, for example, to avoid the peritectic conversion that occurs at low carbon contents in order to improve the properties during continuous casting. Examples of conductive alloys in which a metastable mixture gap occurs which should be avoided are Cu-Co compounds and Fe-Cu compounds. Another very interesting example is Nb-Cu, which is used as a high-strength conductor material. In addition to the primary phase Nb, a long residual solidification area arises here, which leads to undesired inhomogeneities.

The invention is explained in more detail using several exemplary embodiments.

example 1

An Nd-Fe-B rod with a diameter of 6mm of the stoichiometric composition Ndn. ₈ Fe ₈ 2.3B5.6 (at.%) Is melted inductively without crucibles in a floating zone system. The floating zone system consists of the components of a water-cooled vacuum recipient with pull-turn drives and clamps for the rod-shaped material to be examined, devices for induction heating, windows for visual and camera observation, and pyrometric temperature measurement. The primary coil is operated via a 250 kHz generator. The secondary coil is located below the primary coil and has no power connection. Both coils are connected in a resonant circuit with a capacitor and an ohmic resistor (Fig. 1).

The phase shift of the currents of the primary and secondary coils, which is set to a value of 840 nF and the ohmic resistance to a value of 51.2 mΩ by setting the capacitance in the secondary resonant circuit, results in a strong, easily controllable volume force on the melt, with which Help the flow to an almost flow-free state is set (Fig. 2a).

If a value of the capacitance in the secondary resonant circuit of 330 nF is set, a strong flow in the melt is obtained (FIG. 2b). The dark areas are parts by volume of α-Fe.

It can be clearly seen that the melt flow has a great influence on the microstructure. The comparison between the microstructure of FIGS. 2a and 2b shows a larger α-Fe volume fraction in the microstructure that was produced with a strong melt flow. A significantly lower α-Fe volume fraction is formed under the manufacturing conditions according to the invention with a calm melt flow. The result was verified by measuring the volume fraction of the α-Fe phase using a vibration magnetometer. It is 12.6 mass% for the sample in Fig. 2a and 26.4 mass% for the sample in Fig. 2b.

For comparison, the volume fraction of the α-Fe phase in a sample which was produced according to the prior art without influencing the melt flow was determined. Here a value of 22.5 mass% was determined.

Example 2

Rods from a (Nd ₂ Feι ₄ B) ιoo- ₂ χTi _x Cχ connection with x = 0; 0.5; 1 ; 3, and a diameter of 6 mm are melted inductively without crucibles in a floating zone system. The floating zone system consists of the components of a water-cooled vacuum recipient with pull-turn drives and clamps for the rod-shaped material to be examined, devices for induction heating, windows for visual and camera observation, and pyrometric temperature measurement. The primary coil is operated via a 250 kHz generator. The secondary coil is located below the primary coil and has no power connection. Both coils are connected in a resonant circuit with a capacitor and an ohmic resistor according to Example 1.

The phase shift of the currents of the primary and secondary coils, which is set to a value of 800 nF and the ohmic resistance to a value of 51.2 mΩ by setting the capacitance in the secondary resonant circuit, results in a strong, easily controllable volume force on the melt, with which Help the flow to be set to an almost flow-free state.

If a value of the capacitance in the secondary resonant circuit of 405 nF is set, a strong flow in the melt is obtained.

It was found that the melting of the melt also leads to a reduction in the volume fraction of the α-Fe phase

Claims

claims

1. A process for the melt-metallurgical production of conductive alloys in which the flow in the area of the melt is influenced by an electromagnetically generated volume force in the direction of a currentless state.

2. The method according to claim 1, in which peritectic alloys are produced as conductive alloys.

3. The method according to claim 1, in which magnetic alloys based on Nd-Fe-B are produced as conductive alloys.

4. The method according to claim 1, wherein the volume force is generated by a phase shift of two electrical currents flowing through two induction coils.

5. The method of claim 1, wherein the volume force is generated by a second induction coil which is connected in series with the primary induction coil.

6. The method according to claim 4, wherein the phase shift of the electrical currents is generated in resonant circuits of the two induction coils.

7. The method according to claim 6, wherein the phase shift of the electrical currents is realized by regulating the capacitive force in the secondary resonant circuit.

8. The method according to claim 6, wherein a phase shift of the electrical currents of 0 ° is set.

9. The method according to claim 1, wherein the amplitude of the electrical currents is limited by an ohmic resistor.

10.A method according to claim 9, in which an equal amplitude of the currents is set in both circuits.

11. The method according to claim 1, wherein the electromagnetically generated volume force by regulating the frequency of the currents and / or the current strength and / or the vertical distance of the coils and / or the inner diameter of the coils and / or the capacitance and ohmic resistance in the secondary resonant circuit being affected.

12. The method according to claim 11, wherein a vertical distance between the two coils is set, which corresponds to the radius of the starting material to be melted.

13. The method according to claim 11, wherein a penetration depth δ of the magnetic field into the Nd-Fe-B material, δ = (2 / μσω) ^"1/2 , which corresponds to the radius of the starting material, is set, which corresponds to the radius of the starting material, where μ is the magnetic permeability and σ is the electrical conductivity of the Nd-Fe-B material at the respective temperature.

14. Device for the melt-metallurgical production of conductive alloys, in which a secondary coil is arranged above or below the primary coil, both coils having their own resonant circuit, by means of which a phase shift can be adjusted in a controllable manner.

15. The apparatus of claim 14, wherein the secondary coil has no connection to a power source.

16. The apparatus of claim 14, wherein the secondary coil is connected in series to the primary coil.

17. The apparatus of claim 14, wherein the primary coil is integrated in a resonant circuit with adjustable capacitance and controllable ohmic resistance.

18. The apparatus of claim 14, wherein the secondary coil is integrated in a resonant circuit with adjustable capacitance and controllable ohmic resistance.

19. The apparatus of claim 14, wherein the secondary coil is arranged above the primary coil to realize a melt flow upward on the free surface of the melting zone.

20. The apparatus of claim 14, wherein the secondary coil is arranged below the primary coil to realize a melt flow down on the free surface of the melting zone.

2 I .Device according to claim 14, wherein the regulation of the phase shift via the regulation of the frequency of the currents and / or the current intensity and / or the vertical distance of the coils and / or the inner diameter of the coils and / or the capacitance and ohmic resistance are realized in the secondary resonant circuit.