EP2289082B1 - Method for producing a magnetizable metal shaped body - Google Patents

Description

The present invention relates to a method for producing a magnetizable metallic shaped article, a molded article produced by such a method and uses of such a shaped article.

Numerous magnetizable metallic bodies are known in the prior art for realizing various electromagnetic devices, such as electromagnetic actuators, transformers or the like. All these applications have in common that a material used for the production of the magnetizable components and assemblies on the one hand should have favorable magnetic properties in the form of the highest possible (saturation) flux density with low excitation and low coercive force, with pure iron (or materials of iron or made of iron-silicon alloys) in view of such magnetic properties is particularly favorable.

On the other hand, in particular in the case of magnets controlled by alternating currents (here the materials are reversed in the cycle of the alternating current frequency) losses occur, in particular in the form of eddy current losses; These are the result of induced by the magnetic alternating field voltages that cause eddy currents perpendicular to the alternating magnetic field and weaken the magnetic field (thus cause an energy loss). To reduce such eddy current losses, it is again known to increase the magnetizable material to increase resistance, for example in the form of sheets in transformers or by forming mixed crystals (eg FeNi) in the magnetic material. Such an increase in the (specific) electrical resistance reduces the described eddy current losses, but at the same time reduces the magnetic saturation flux density and also impairs mechanical properties, such as strength.

Even with DC applications, however, the negative effects of eddy currents are not completely irrelevant; For example, the magnetization associated with a switching operation leads to eddy currents which counteract magnetically and limit the dynamics or achievable speed of movement of actuators or the like with DC-operated magnet applications.

Eddy current losses are also highly frequency-dependent, so that it is also known, especially in high-frequency applications, to increase the specific electrical resistance powder composite materials from a metal powder, which with a z. B. polymeric binder is pressed. In addition to the relatively high electrical resistance relative to a sheet such a procedure also has the advantage that eddy currents can be suppressed three-dimensional. However, the magnetic properties of such powder composites are often insufficient, such as a typical saturation flux density of a metal is 1.5 to about 5 times higher than such plastic bound metal powders. Here too, a shaped article produced in this way has poor mechanical properties, for example in the form of mechanical strength.

An example of such powder composite materials shows the US 5,993,729 with metal oxide compounds between particles.

From the known state of the art it is therefore a known challenge to optimize the described, mutually potentially conflicting properties with respect to the respective application by suitable selection and design of the metallizable material, namely the best possible magnetic properties with the lowest possible eddy current losses, necessary mechanical Properties, such as acceptable strength.

Object of the present invention is therefore to provide a magnetizable metallic molded body and a method for producing such, on the one hand energetically adverse eddy currents can be effectively suppressed or minimized, on the other hand still favorable magnetic properties, especially high magnetic (saturation) Flux density and low coercive field, can be ensured, wherein such a shaped body should also have improved mechanical properties (as compared to known powder or sintered materials). Furthermore, suitable uses for such a method or molded bodies realized thereby are to be created.

The object is achieved by the method having the features of the main claim, the molded body produced by the method and uses of the molding; advantageous developments of the invention are described in the subclaims.

The invention is based first of all on the knowledge that when eddy currents are already in the micro range (ie in the range of the particle size or particle size of the pulverulent particle) ferromagnetic starting material) are limited, favorable magnetic properties of the resulting molded body can be achieved. Accordingly, the method according to the invention makes it possible, by precompression in the form of the step of first compressing the starting material, to create a (mechanically stable) body through the material bond in the form of bridges between the adjacent particles, wherein in the subsequent step of producing the electrically insulating surface coating on the particles according to the invention, the cavities (further education by the introduction of a corresponding reactive gas) are used to those surface portions of the particles that are outside the connecting portions (bridges) to a respective adjacent particles, with a (relative to the particle size) very thin partial coating to provide. The subsequent second compaction then leads to the cavities being eliminated or greatly reduced, so that the result is a highly compressed particle structure with layer sections of the isolated (surface) coating which - distributed in micro size and in the body - the effect intended according to the invention cause eddy current barriers in the micro range. In other words, the invention makes it possible to produce a magnetizable metallic material as a shaped body, in which (three-dimensionally) electrically non-conductive, thin layer layers (usually only in the nanometer range in the layer thickness) are distributed, which serve as effective eddy current barriers.

The shaped body thus produced not only has the desired high magnetic power density (which potentially comes close to pure iron material), also the Eddy current losses significantly reduced by the effect of the three-dimensionally distributed in the body layer sections. This then creates about the possibility, with improved energy efficiency (resource-saving) electromagnetic units, eg. As actuators to make, with the high flux density realized at low excitation compact devices that save space and bring other benefits.

A further advantage of the invention resides in the fact that a shaped body realized according to the invention has outstanding mechanical properties, in particular with regard to stability, tensile strength and breaking strength, in particular over traditionally known materials and material arrangements for minimizing eddy current losses. For example, it may be readily realized that, in accordance with the present invention, electromagnetic properties of a molded article made in accordance with the present invention may be achieved that correspond to a typical reference material such as FeSi3, but have significantly improved mechanical properties with respect to this material. This seems plausible against the background, for example, that in an advantageous embodiment of the invention, the production of the insulating surface coating according to the invention takes place after the adjacent particles in the first step of compacting the starting material via bridge formation or the like. have been joined together and accordingly cause a favorable basic strength of the body.

In accordance with the invention in a favorable manner in the practical implementation of the reactive gas, which in the cavities (in the manner of a coherent pore space) after the first Compressing step is introduced, a oxidizing or nitriding the particle surfaces outside of the connecting portions (bridges) causing gas, wherein such gas may also be a carbon, nitrogen, oxygen, sulfur and / or boron-containing gas. It is also within the scope of the invention not to supply such a gas separately, but to use as a reactive gas that (residual) is already present in the powdery starting material and / or formed or formed during the first compression process, in which case the Step of generating the electrically insulating surface coating is carried out with the first compression.

In addition, while in the context of preferred embodiments of the invention in the step of the first compaction, a (preferably isostatic and / or cold hydrostatic) pressing with the first pressing pressure of more than 300 bar, typically 1000 bar or more, is the second compaction after the production the insulating surface coating a process typically carried out by hot hydrostatic pressing with a significantly higher compression pressure of up to about 4000 bar. This pressing pressure at a typical temperature above 1000 ° C leads to a flow of the material, with the result that (with a significant reduction of the pores or even their disappearance), the layer portions of the insulating surface coating (each, with a thickness in the typical Nanometer range have a longitudinal extent corresponding to approximately the starting material particle sizes) in the resulting shaped body distributed and allow the intended vortex current inhibiting effect at the micro level. According to the invention, it is encompassed by the invention to subject the metallic shaped body to a mechanical forming step and / or a subsequent treatment after the second compacting in order to mold the shaped body for the intended purpose. In addition, a forming step such as rolling, drawing or the like can suitably ensure that an isotropy of the layer sections distributed in the shaped body can be specifically changed.

While it is on the one hand encompassed by the invention to use as ferromagnetic raw material uncoated ferromagnetic particles, such as pure iron particles, provides an alternative embodiment of the invention that the inventive process in powder form present particles are supplied, which themselves as coated particles, for. Example, iron particles, with (other) metal coating or semiconductor coating, present (eg., By upstream plasma coating). On the one hand, this makes it possible on the one hand to influence the mechanical connection behavior (for example the quality of the sintered bridges) after the step of the first compacting, on the other hand enables such precoating of the particles to produce favorable insulating surfaces by targeted formation of the reactive gas to be introduced into the pore space (eg B. an aluminum oxide surface coating by oxidation of an aluminum precoated iron particle by means of the coating step).

The shaped body produced according to the invention in the described manner is in principle accessible to a large number of magnetic applications, the advantages described above being suitable with regard to efficiency, magnetic behavior, mechanical compactness and stability Thus, the potential range of application of the present invention extends from magnetic actuators or drive devices (such as electromagnetic actuators and electric motors) to use in transformers and other areas of power electronics, to electromagnetic bearings and high frequency engineering tasks.

Figur 1:

Ein Flussablaufdiagramm mit Prozessschritten S1 bis S7 zum Durchführen des erfindungsgemäßen Verfahrens gemäß einer ersten Ausführungsform und

Figur 2:

Eine Ansicht mit einer Mehrzahl von schematischen Illustrationen, welche entlang der Schritte S1 bis S6 von Figur 1 die prozessgemäß veränderte Formgebung des Formkörpers bzw. der Partikel des Ausgangsmaterials illustrieren.

Further advantages, features and details of the invention will become apparent from the following description of preferred embodiments and from the drawings; these show in

FIG. 1:

A flowchart with process steps S1 to S7 for performing the method according to the invention according to a first embodiment and

FIG. 2:

A view with a plurality of schematic illustrations, which along the steps S1 to S6 of FIG. 1 illustrate the modified process shape of the molding or the particles of the starting material.

According to a first process step, powdered iron raw material of a typical average grain size in the range of about 10 μm to 500 μm is provided; the reference numeral 10 illustrate the process step S1, the presence of such powder particles in the uncoated state. Typical commercial powder materials in view of a comparatively small grain size are e.g. Pure iron powder (Fe2) with grain size <30μm, D50 (average grain size) 9μm to 11μm manufacturer ThyssenKrupp metallurgy, in the case of a larger grain size is exemplified in the product Ampersint (atomized Fe-base powder from HC Starck GmbH), here is the Grain size Fe to at least 99.5 (wt)% less than 350 microns. Alternative Fe base powders of this manufacturer are FeSi3 or FeSi6 with corresponding grain size.

Process step S2 as an optional process step provides the possibility that prior to a subsequent first compaction (Step S3), the powder particles of the raw material, such as by plasma coating or the like are provided with a metallization or semiconductor coating. This optional layer to be applied in step S2 is thin relative to the respective particle diameter and is typically in the range between 5 and 50 nm.

In the following process step S3, a first precompression of the (coated or uncoated) raw material takes place, typical is a cold hydrostatic pressing with a compression pressure of about 1000 bar. It arises in the FIG. 2 (with uncoated raw material) illustrated image of a precompressed body, in which by means of sintered bridges, the particles 10 mechanically firmly adhere to each other.

In the following process step S4, an oxidizing gas, in the present case oxygen, at a pressure of 0.01 bar and a temperature of 350 ° C is introduced into the molding such that this gas enters the cavities 14 and correspondingly the particles 10 with a (electrically insulating) thin oxide layer 14 provides in all those peripheral areas, which are not connecting portions with a respective adjacent particles. A typical resulting coating thickness on the particles after the gas treatment step S4 (duration in the example described 30 minutes) is about 10 nanometers. For example, by changing the pressure or temperature or exposure time, this layer thickness can be influenced.

A subsequent second densification step S5 (so-called consolidation) is typically performed as high temperature compression, in particular by means of hot hydrostatic Pressing performed; typical process parameters are a pressure of up to 4000 bar at 1200 ° C temperature. This leads to - compare the illustration in FIG. 2 to S5 - the pores (gaps) 12 disappear or significantly shrink, so that in the end-compacted material at the end of the process step S5 substantially only oxide layer portions 14 remain distributed in the material, which the original coating sections on the peripheral surfaces of the particles or compressed pores correspond. These very shallow oxide layer sections thus have typical lengths in the range of about 10 to 150% of the original particle size of the particles and are very thin compared to this dimension, namely again in the nanometer range (usually 5 to about 30 nanometers).

Due to their distribution in the finely compacted material, these oxide layer sections act as eddy current inhibitors in the micro range according to the invention; at the same time, the final compacted material (which in the exemplary embodiment shown in a subsequent step S6 by rolling still allows a transformation into an intended final shape and also a subsequent cutting step in step S7 Post-treatment) has very favorable magnetic properties in terms of high saturation flux density and low coercive force, and even at the scale of a known free-cutting steel (eg, 1.0715), which is often used for DC applications, favorable behavior is realized. A material thus produced is also significantly superior to a typical reference material for AC applications (such as FeSi3).

Claims (15)

1. A method for producing a magnetisable metal shaped body formed of a ferromagnetic raw material (10) present in powdery and in particle form, comprising the following steps:

   - preliminary compression, as first compression, of the raw material (S3), such that adjacent particles are interconnected by integral bonding in portions at their peripheral surface by means of sinter bridges, thus forming cavities (12),

   - production of an electrically insulating surface coating (14) on the peripheral surfaces of the particles in regions outside the connection portions (S4), and

   - second compression of the particles (S5) provided with the surface coating, such that the cavities are reduced in size or eliminated.
2. The method according to Claim 1, characterised in that the electrically insulating surface coating (S4) is produced by introducing into the cavities a gas that produces the surface coating by reaction with the peripheral surfaces.
3. The method according to Claim 1 or 2, characterised in that the electrically insulating surface coating is produced by a gas that is already present in or with the raw material during the step of first compression of the raw material or that is created during the first compression.
4. The method according to Claim 2 or 3, characterised in that the gas is a carbon-containing, nitrogen-containing, oxygen-containing, sulphur-containing and/or boron-containing gas and/or causes a chemical reaction, such that the peripheral surface outside the connection portions experiences the electrically insulating surface coating.
5. The method according to one of Claims 1 to 4, characterised in that the electrically insulating surface coating has a layer thickness in the range between 2 nm and 50 nm.
6. The method according to one of Claims 1 to 5, characterised in that the first compression is carried out by sintering and/or preliminary sintering of a powder, which is compressed by shaking, as ferromagnetic raw material.
7. The method according to Claim 6, characterised in that the sintering or preliminary sintering is carried out by thermal treatment and without pressing.
8. The method according to one of Claims 1 to 5, characterised in that the second compression (S5) involves pressing the particles which have been compressed by the first compression and provided with the electrically insulating surface coating at a second pressing pressure, which is higher than the first pressing pressure, in particular at least 10 % higher, preferably at least 200 % higher, the first pressing pressure being the pressure at which the raw material is pressed during the first compression (S3).
9. The method according to Claim 8, characterised in that the first and/or the second compression is carried out by means of hot hydrostatic or isostatic pressing.
10. The method according to Claim 9, characterised in that the hot hydrostatic or isostatic pressing during the second compression (S5) is carried out at a temperature and a pressing pressure which result in a flowing of the particles and/or the layer portions of the insulating surface coating.
11. The method according to one of Claims 1 to 10, characterised in that a reforming of the shaped body after the second compression results in a change to and/or elimination of an isotropy of layer portions of the insulating surface coating that are present in the shaped body after the second compression.
12. The method according to one of Claims 1 to 11, characterised in that the ferromagnetic raw material has iron particles coated by a metal material or semiconductor material, the coating of the iron particles in the raw material preferably having a thickness of < 1000 nm, preferably < 100 nm, more preferably < 10 nm.
13. The method according to one of Claims 1 to 12, characterised in that a mean particle size of the particles, present as powder, of the ferromagnetic raw material lies in the range between 5 µm and 1000 µm.
14. The method according to one of Claims 1 to 13, characterised in that the metal shaped body is used to produce magnetisable components of electromagnetic actuator and/or drive apparatuses, in particular of an electromagnetic actuating element or of an electric motor, of a magnetic bearing or of a transformer.
15. The method according to one of Claims 1 to 14, characterised in that the shaped body is used to produce a high-frequency component or a high-frequency module.

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