DE102015012412A1 - Apparatus and method for producing annular permanent magnets - Google Patents

Abstract

The invention relates to a device (1) and a method for producing annular permanent magnets by electrical discharge sintering. The device comprises a plurality of tool parts (3, 4, 5, 6) defining an annular cavity (11) for receiving a magnetizable metallic powder (2) and a controllable electric pulse current generator (8, 9), at least two of the tool parts (3 , 4, 5, 6) form electrodes and are electrically connected to the pulse current generator (8, 9). The tool parts (3, 4, 5, 6) comprise at least one outer mold (3) delimiting the cavity (11) in the radially outward direction, a cavity (11) radially inwardly delimiting, coaxial with the outer mold (3) Core (6) and two the cavity (11) in the axial direction limiting, relative to each other axially movable compression means (4, 5), wherein at least one of the compression means (4, 5) in the direction of the cavity (11) is subjected to force or kraftbeaufschlagbar. Outer shape (3) and core (6) form the electrodes. The compression means (4, 5) electrically isolate the electrodes from each other so that the current flow is perpendicular to the force. In this way, homogeneously compressed ring magnets of any length can be produced in a simple manner and in a short time.

Description

The invention relates to a device for producing annular permanent magnets by electrical discharge sintering, comprising a plurality of, an annular cavity for receiving a magnetizable metallic powder defining tool parts and a controllable electric pulse generator, wherein at least two of the tool parts form electrodes and are electrically connected to the pulse generator, and the Tool parts at least one of the cavity in the radial direction outwardly limiting outer shape, as well as two the cavity in the axial direction limiting, relatively axially movable compression means comprise, wherein at least one of the compression means in the direction of the cavity is subjected to force or kraftbeaufschlagbar. Furthermore, the invention relates to a method for producing annular permanent magnets by electrical discharge sintering, in particular using the aforementioned device, in which a magnetizable metallic powder is introduced into the cavity and exposed to an electric pulse current flowing between the electrodes such that the powder is at least partially melted at the same time a force is exerted in the direction of the cavity on the powder to compress the powder in the melt state.

Ring-shaped permanent magnets are used, for example, for rotors of modern permanent-magnet electric motors. Their production is comparatively complex, long-lasting and expensive. The preparation is usually carried out by conventional sintering of magnetic powder, wherein a heating of the powder under mechanical pressure in a conventional manner, in particular by electrical heating elements, which are integrated in a powder-containing form. The heating to sintering temperature lasts comparatively long. Furthermore, the cooling of the sintered workpiece also takes time.

On the other hand, processes for powder compaction by short-term sintering are known to the person skilled in the art as EDS (Electric Discharge Sintering) or synonymously CDS (Capacitor Discharge Sintering). A publication describing these methods and corresponding devices for their execution is Eugene A. Olevsky, Elena V. Aleksandrova, Alexandra M. Ilyina, Dina V. Dudina, Alexander. "Outside Mainstream Electronic Database: Review of Studies Conducted in the USSR and Post-Soviet Countries on Electric Current-Assisted Consolidation of Powder Materials" N. Novoselov, Kirill Y. Pelve and Eugene G. Grigoryev, Materials 2013, 6, pp. 4375-4440, doi: 10.3390 / ma6104375 , Furthermore, the US patent US 3,241,956 the basic principle of the EDS. One method of adjusting the compressive force during powder compaction is in the European patent application EP 2 198 993 A1 described. This literature is hereby incorporated by reference for the explanation of the invention described herein. Detailed explanation of the technology is therefore omitted.

The EDS method is used to achieve densification of iron powder or a hard metal powder such as tungsten carbide by pulsed discharge of electrical energy into the powder. A corresponding device 1 for this according to the prior art shows 1 ,

The device comprises several tool parts 3 . 4 . 5 which is a cylindrical cavity 11 limit, as well as a controllable electrical pulse generator, a capacitor 9 and a pulse current transformer 8th having. In 1 is already in the cavity to be compacted, electrically conductive powder 2 , The cavity 11 becomes radially outward through a hollow cylindrical outer shape 3 and in the axial direction by two compression means 4 . 5 limited, which extend at least partially in the axial direction in the cavity comprising the cavity of the outer shape form fit into it. At least the upper compression means 4 axially movable and is acted upon by a mechanical force F, for example, by a hydraulic 10 generated and via a pressure pin 7a on the compression agent 4 is transmitted. This transmits the compressive force F in turn to the powder 2 and acts like a stamp. The powder 2 is thereby against the lower compression means 5 pressed, which is against another pressure pin 7b supports and absorbs the pressure force accordingly. The two force-transmitting or force-absorbing compression means 4 . 5 are electrically conductive and simultaneously represent the electrodes. The current flow I and the power transmission direction are thus parallel. In general, tools and punch arrangements in EDS systems generally work with a current flow in the pressing direction.

For compression, electrical energy is in the capacitor 9 stored and by the pulse transformer 8th within a few milliseconds in the powder 2 discharged. A progression of the process variables current, temperature and pressure shows 2 , It shows the pulsed direct current with constant pressure. Furthermore, the dashed line shows the temperature in the powder, which increases during the current pulse to a multiple. Because of the high current of about 300 kA and the high ohmic resistance between the powder particle contact points Joule heat is generated at these contact points, which is so large that the powder particle contact points in a go over the molten phase. The pressure force exerted on the powder at the same time presses this phase into the interparticle spaces, thereby infiltrating the pore volume located in the interstices. After cooling of the processed powder is a dense structure, which was welded from the technological point of view and not sintered. Nevertheless, experts in this process speak of sintering. Depending on the design of the capacitor charging unit, a complete recharging of the capacitors is ensured within less than 6 s. The fast capacitor charging in conjunction with the rapid compaction allows the production of up to 10 workpieces per minute. For this reason, the method is particularly attractive in terms of economical production of powder-compacted workpieces.

With regard to the shape of the permanent magnets that can be produced in this way, however, one is limited to thin or flat workpieces, since the resistance of the powder quantity increases with its length. As a result, the current decreases with increasing thickness. The melting effect of the pulse current and the degree of melting at the powder particle contact points is thus the smaller, the greater the distance between the electrodes introducing the current. The fusion is incomplete with long expansions in the pressing direction. Thus, in the case of thick workpieces, the case arises that the powder particles do not melt together well in the middle of the powder quantity. The workpieces crumble apart. Thus, only permanent magnets in the form of simple round flat slices or cuboids can be produced. By contrast, the ring-shaped permanent magnets used in the construction of permanent-magnet electric motors for the rotors can not readily be produced by the EDS method according to the prior art.

It is an object of the present invention to provide a method and an apparatus which enables the production of annular permanent magnets for electric motors in shorter processing periods.

This object is achieved by a device having the features of claim 1 and by a method having the features of claim 8. Advantageous developments are specified in the subclaims and are explained below.

According to the invention, an apparatus for producing annular permanent magnets by means of electrical discharge sintering is proposed in which the magnetic powder receiving cavity in the radial direction outwardly by the outer shape, in the radial direction inwardly delimited by a coaxially extending to the outer core and in the axial direction by the compression means is, wherein the outer mold and the core form the electrodes and the compression means electrically isolate the electrodes from each other. Accordingly, a method is proposed in which the pulse flow within the cavity in the radial direction between the outer mold and the core flows, and the force of at least one of the two axially limiting the compression means in the axial direction is transmitted to the powder, wherein the Compression means electrically isolate the outer mold and the core from each other.

Experiments with this method, or this device have shown that a magnetic powder can be compressed in this way. With a suitable choice of the process parameters such that the total temperature of the powder does not exceed 400 ° C, the magnetic properties of the powder are retained. Such a device as well as such a method allow above all the use of electrical discharge sintering (EDS) for the production of annular permanent magnets of any axial length, since this length is independent of the distance of the electrodes from each other. Current flow and force flow act at right angles to each other on the magnetic powder. The cavity forms an annular space, which is flowed through homogeneously by the current, so that a homogeneous melting of the magnetic powder is achieved. Magnetic powder is understood by the person skilled in the art to mean a magnetizable metallic, d. H. electrically conductive powder, as is commonly used to make permanent magnets.

As already stated, the permanent magnets to be produced are in their axial height, and is correspondingly the height of the cavity, not fixed. Thus, the cavity may be formed flat, d. H. that their radial width is greater than or equal to their axial length, so that annular disk-shaped permanent magnets can be produced. The cavity can also be higher than wide, so that it increasingly has the shape of a hollow cylinder with increasing height.

The particular strength of the device according to the invention and the corresponding method over the prior art according to 1 shows up when the ratio of length to thickness of the permanent magnets to be produced significantly exceeds the ratio 1: 1. In contrast to the state of the art, density fluctuations above the height do not lead to inhomogeneous compression. Rather, the compression due to the radial current flow is always homogeneous over the axial length. The following background is explained.

When exerting a compressive force on a pile of powder, this increases with increasing Distance from the point of application due to the friction losses between the powder particles. The result is a so-called press cone, which is the more pronounced, the higher the pile of powder. Thus, the density of the powder compact over the axial length is not constant. An axially flowing current on the way from one electrode to the other electrode then experiences a locally lower resistance in the higher-compressed region due to the larger contact surface of the powder particles and a locally correspondingly higher resistance in the lower-compressed region, so that viewed in the axial direction within the powder there is no homogeneous heat generation. Thus, the melt formation at the powder particle contact points as well as the magnetic properties of the workpiece are not homogeneous. It can happen that the powder particles in the higher-density area do not fuse completely.

This phenomenon, which occurs at high ring workpieces, is prevented in the method according to the invention due to the radial current flow, because the resistance is significantly determined by the length of the current paths and this length is smaller with radially opposite arrangement of the electrodes for producing high permanent magnets than with an axial current path after Prior art is. The inventive method is thus particularly suitable for producing high hollow cylindrical permanent magnets, preferably just those in which the axial length is equal to or greater than the radial thickness. Accordingly, for the production of these permanent magnets, the cavity may have an axial length which is greater than the distance between the core and the outer wall.

Since here the radial current paths are shorter than they would be in the axial direction in the current art, density inhomogeneities have less effect on the fusion of the powder particles and thus less on the magnetic properties of the final product.

The contour of the annular permanent magnets can basically be arbitrary. Accordingly, the geometric shape of the cavity forming the annular space is not fixed. This applies both to the outer contour of the permanent magnets or to the inner contour of the outer contour that defines the cavity outwardly, as well as to its inner contour or the outer contour of the inner cavity bounding the core.

Thus, the inner contour of the outer shape in cross section of a circular, oval, square or polygonal basic shape correspond. However, from a technical point of view and for the production of rotors for electric motors usable annular permanent magnets with circular outer contour.

Furthermore, the outer contour of the core in cross section may correspond to a circular, oval, square or polygonal basic shape. Of technically greatest importance here is also a circular outer contour of the core, but also an angular outer contour is advantageous because it can form a rotation.

Preferably, the core is pin-shaped, so that it is possible to produce permanent magnets in the form of a hollow profile.

Suitably, at least one of the compression means has a central opening into which the core is retractable due to the relative movement of the compression means.

It is particularly advantageous if the outer mold and the core are stationary tool parts. As a result, the electrodes can be contacted better and easier. Because in contrast to the prior art no moving tool parts must be electrically contacted in this case. While in the device according to 1 the compression means 4 . 5 form the electrodes, of which at least one is axially movable, the tool parts used as the electrode outer mold and core can form immovable tool parts. This simplifies electrical contacting because the electrical connection between the movable electrode and the supply current from the pulse generator does not endure the mechanical shock experienced by the movable compression means due to extrusion of the molten powder particles or at least their surface into the interparticle spaces.

It should be noted that the term "compression means" merely expresses an involvement in the compression of the powder, but does not imply that a force is applied to the powder. Thus, in one embodiment of the invention, only one of the compression means actively exert a force on the powder and the other compression means passively absorb this force, so that only one-sided pressing takes place.

According to a preferred embodiment variant, however, both compression means can also be subjected to axial force and independently of one another in the direction of the cavity or can be subjected to force, ie actively exert a force on the powder. The two-sided pressing has the advantage that a higher total pressure acts on the powder and the powder is better compressed because of the above described press cone is reduced or becomes a double cone.

In order to isolate the electrodes well from each other and at the same time to be able to absorb or transmit mechanical forces, the compression means can consist of a ceramic material. Regardless, the outer mold and / or the core may be made of copper.

To carry out the process, the powder can be introduced into the cavity as a loose powder bed or as a mechanically pre-pressed powder compact. In the former case, as a result of the application of force to the powder, the compression means cause mechanical pre-compression prior to electrical discharge sintering.

With this method according to the invention, it is possible to produce magnets in one process step, wherein the workpieces still have to be magnetized after electrical discharge sintering. This can be done inside the device or outside, in a separate device, in which the workpieces are exposed to a strong, oriented according to the desired direction of magnetization magnetic field. The second approach pursues the goal of initially producing a semifinished product by means of EDS, which achieves the desired mechanical and magnetic properties by hot extrusion in a subsequent process step.

Further features and advantages of the invention will be described below with reference to an embodiment and the attached 2 described.

2 shows a schematic representation of a device according to the invention 1 for producing permanent magnets with a ring shape. The device 1 makes it possible to produce ring magnets of any axial length using electrical discharge sintering.

For this purpose, the device provides 1 an annular cavity 11 ready to receive a magnetic powder 2 serves as a loose powder bed or as mechanically pre-compressed powder compact in the cavity 11 is introduced. In 2 is already magnetic powder 2 in the cavity 11 available. The cavity is made of four tool parts 3 . 4 . 5 . 6 limited, namely in the radial outward direction by an outer shape 3 radially inward through a core 6 , as well as in the axial direction by two compression means 4 . 5 ,

The outer shape 3 has the shape of a hollow cylinder, although it can have any outer contour. Within the outer shape 3 is a cylindrical cavity formed, which is the cavity 11 includes. Coaxial with the outer shape 3 The core extends 6 through this cavity and shape it respectively the cavity 11 thus to an annulus. The core 6 is formed pin-shaped. The distance between the core and outer shape defines the thickness D of the permanent magnet to be produced.

Each one of the two compression means 4 . 5 extends at least partially positively into the cavity and thus limits this in the axial direction. The distance between these compression means 4 . 5 defines the height or axial length L of the permanent magnet to be produced. The cavity thus has an axial length L that is greater than the thickness D.

The compression means 4 . 5 are cylindrical and have a coaxial opening 13 either in the form of a bore over the entire axial length as in the case of the lower compression means 5 or in the form of a blind hole as in the case of the upper compression means 4 is executed to the core 6 take. The inner diameter of the opening 13 is thus at the outer diameter of the core 6 adjusted so that core 6 and compression means 4 . 5 , can be pushed or moved in a form-fitting manner.

In the embodiment according to 2 are both compression means 4 . 5 arranged axially movable. They become independent of each other in the direction of the cavity 11 acted upon by a force F and thus form against the powder 2 pressing stamp. The force F is provided by one hydraulic system each 10 generated and over bolts 7a . 7b on the respective compression means 4 . 5 transfer. However, there is also the possibility that only one of the compression means 4 . 5 , is axially movable and kraftbeaufschlagt, as in the device 1 in 1 the case is.

The outer shape 3 and the core 6 are electrically conductive, running example consisting of copper. They form electrodes and are connected via a connecting cable with a controllable pulse current generator 8th . 9 connected here schematically by a pulse transformer 8th and a capacitor 9 is represented. Unlike the compression means 4 . 5 are outside shape 3 and core 6 fixed, ie not immovable. This way you can be contacted better and there is no risk of getting the connection line 12 solves.

The pulse transformer 8th generated from the in the capacitor 9 stored charge a current pulse I of about 300 kA and a few milliseconds in length, as in 1a is shown. The compression means 4 . 5 consist of a non-conductive material, such as a ceramic, and thus isolate outer shape 3 and core 6 from each other.

Since the magnetic powder is electrically conductive, the current pulse I flows between the outer mold 3 and core 6 through the powder 2 , ie in the radial direction and thus perpendicular to the force F. This results in a short current path and a homogeneous compression by the discharge sintering.

Since the contact points between the individual powder particles form an ohmic resistance, there arises Joule heat due to the high current, which is so large that at least the surface of the powder particles is at least partially melted. By the same time by the compression means 4 . 5 on the powder 2 exerted compressive force, this melt is pressed into the interparticle spaces and thus fill the former spaces between powder particles, ie the pore volume. At the same time, the compression means move 4 . 5 backward into the cavity and thus reduce the cavity 11 , It thus creates a dense structure. Almost as fast as the heating of the powder took place, the produced workpiece cools down.

This is then exposed to a strong magnetic field and magnetized in this way, so that the desired ring magnet is obtained.

LIST OF REFERENCE NUMBERS

1

EDS device

2

magnetic particle

3

external form

4

upper compression means

5

lower compression means

6

core

7a, 7b

pushpin

8th

pulse transformer

9

capacitor

10

hydraulic

11

cavity

12

connection cable

13

opening

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 3241956 [0003]
• EP 2198993 A1 [0003]

Cited non-patent literature

• Eugene A. Olevsky, Elena V. Aleksandrova, Alexandra M. Ilyina, Dina V. Dudina, Alexander. "Outside Mainstream Electronic Database: Review of Studies Conducted in the USSR and Post-Soviet Countries on Electric Current-Assisted Consolidation of Powder Materials" N. Novoselov, Kirill Y. Pelve and Eugene G. Grigoryev, Materials 2013, 6, pp. 4375-4440, doi: 10.3390 / ma6104375 [0003]

Claims (10)

Contraption ( 1 ) for producing annular permanent magnets by electric discharge sintering, comprising a plurality of an annular cavity ( 11 ) for receiving a magnetizable metallic powder ( 2 ) defining tool parts ( 3 . 4 . 5 . 6 ) and a controllable electrical impulse current generator ( 8th . 9 ), wherein at least two of the tool parts ( 3 . 4 . 5 . 6 ) Electrodes and with the pulse current generator ( 8th . 9 ) are electrically connected, and the tool parts ( 3 . 4 . 5 . 6 ) at least one of the cavity ( 11 ) in the radial direction outwardly bounding outer shape ( 3 ), as well as two the cavity ( 11 ) in the axial direction limiting, relative to each other axially movable compression means ( 4 . 5 ), wherein at least one of the compression means ( 4 . 5 ) in the direction of the cavity ( 11 ) is subjected to force or kraftbeaufschlagbar, characterized in that the cavity ( 11 ) in the radial direction inwardly by a coaxial with the outer shape ( 3 ) arranged core ( 6 ), the outer shape ( 3 ) and the core ( 6 ) form the electrodes and the compression means ( 4 . 5 ) electrically isolate the electrodes from each other. Contraption ( 1 ) according to claim 1, characterized in that the cavity ( 11 ) has an axial length (L) equal to or greater than the distance between the core (L) 6 ) and the outer wall ( 3 ). Contraption ( 1 ) according to claim 1 or 2, characterized in that the core ( 6 ) is pin-shaped. Contraption ( 1 ) according to one of the preceding claims, characterized in that the cavity ( 11 ) is hollow cylindrical, in particular forms a hollow circular cylinder. Contraption ( 1 ) according to one of the preceding claims, characterized in that at least one of the compression means ( 5 . 6 ) a central opening ( 13 ) into which the core ( 6 ) due to the relative movement of the compression means ( 5 . 6 ) is retractable to each other. Contraption ( 1 ) according to one of the preceding claims, characterized in that both compression means ( 4 . 5 ) axially movable and independently in the direction of the cavity ( 11 ) are subjected to force or kraftbeaufschlagbar. Contraption ( 1 ) according to one of the preceding claims, characterized in that the compression means ( 4 . 5 ) consist of a ceramic material. Method for producing annular permanent magnets by electric discharge sintering, in which a magnetisable metallic powder ( 2 ) in an annular, by several tool parts ( 3 . 4 . 5 . 6 ) defined cavity ( 11 ) and between at least two of the tool parts ( 3 . 4 . 5 . 6 ) is subjected to a flow of electrical impulse current (I) in such a way that the powder ( 2 ) is at least partially melted, wherein at least one of the tool parts ( 5 . 6 ) a force (F) in the direction of the cavity on the powder ( 2 ) is applied to the powder ( 2 ) in the melt state, characterized in that the momentum flow (I) in the radial direction between a cavity ( 11 ) outwardly limiting outer shape ( 3 ) and one the cavity ( 11 ) bounded inwardly, coaxial with the outer shape ( 3 ) extending core ( 6 ), and the force (F) of at least one of the two cavities ( 11 ) in the axial direction limiting compression means ( 4 . 5 ) in the axial direction of the powder ( 2 ), the compression means ( 4 . 5 ) the outer shape ( 3 ) and the core ( 6 ) electrically isolate each other. Process according to claim 8, characterized in that the powder ( 2 ) mechanically pre-compressed and then into the cavity ( 11 ) is introduced. Method according to claim 8 or 9, characterized in that that of both compression means ( 4 . 5 ) a force (F) in the axial direction on the powder ( 2 ) is transmitted.

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