Method for producing magnets

Description

[0001] This invention relates to a method for producing magnets with improved remanence.

[0002] It is conventional practice to produce magnets from powdered magnetic alloys, including rare earth cobalt magnets, by compacting as by die pressing a charge of aligned or oriented fine powder of a magnetic alloy of the desired magnet composition. Thereafter, the compacted charge is heat treated at temperatures of the order of 1093 to 1143°C (2000 to 2090°F). It is known that by increasing the density in the production of magnets of this type from particle charges of the magnetic material that remanence can be improved. Conventionally, density is increased by raising the sintering temperature after die pressing; however, this results in a corresponding lowering of coercive force.

[0003] Thus United State Patent Specification No. 3,919,003 discloses the alignment of particles of cobalt-rare earth alloy in a magnetic field prior to hydrostatic pressing to form a green bar, the green bar subsequently being sintered at about 1100°C to a density of 87% or more with substantially non-interconnecting pores.

[0004] In United States Patent Specification No. 4,322,257 a cobalt-rare earth alloy powder is magnetically aligned, isostatically compressed to about 70% of theoretical density, sintered just below the solidus temperature and finally annealed. The disclosed aligning magnetic field is a pulse magnetic field of 38000 Oe.

[0005] It is accordingly an object of the present invention to provide a method for producing from powdered magnetic alloy magnets with increased density, and thus improved remanence, without resorting to higher sintering temperatures that serve to lower coercive force.

[0006] Another object of the invention is in the production of magnets to provide for improved alignment or orientation to achieve higher remanence values.

[0007] The present invention provides a method for producing magnets with improved remanence by applying a magnetic field to a particle charge of a magnet alloy to magnetically align said particles, and thereafter consolidating said particle charge to form a magnet particle, wherein said method comprises hot isostatically pressing said particles to consolidate the same to full density.

[0008] Preferably, said magnetic field is applied as at least one pulse, and suitably a plurality of pulses, with each said pulse having a duration not exceeding one second and a power level of at least 50,000 oersted (39789 ampere turns per centimetre).

[0009] In accordance with the method of the invention the magnetically aligned particles are isostatically pressed to compact the particle charge in a hot condition. The term "hot" as used herein in this context means a temperature below the full density sintering temperature of the particles but above the temperature necessary to produce a close-pore structure.

[0010] It has been determined broadly in accordance with the invention that improved remanence is a function of both the degree of alignment of the individual magnetic dipoles (powder particles) and density (the number of dipoles that are present in a given volume of the body of the magnet material). Accordingly, in the broader aspects of the invention it has been discovered that if one subjects a particle charge of magnet alloy, which may be one or more transition elements, e.g., nickel, cobalt, iron, chromium, manganese, copper, zirconium and titanium, in combination with at least one rare earth element, e.g. samarium, to a temperature that is below the full density sintering temperature but above the temperature necessary to produce a close-pore structure and then subjects the material while at this temperature to isostatic compacting, increased density and thus improved remanence is achieved while maintaining good coercive force. Coercive force is maintained by maintaining the temperature below the full density sintering temperature. Additionally, remanence is improved by aligning or orienting the material by the use of a pulsating magnetic field within a container. The container may be a collapsible container within which the material can thereafter be isostatically compacted. The pulsating magnetic field should have a pulse duration not exceeding one second per pulse and each pulse typically will be of the order of 15 millisecond. At least one pulse and preferably two pulses at a power level of at least 50,000 Oe (39789 ampere turns per centimetre) is suitable for the purpose. After magnetic alignment of the particles, the particles may be compacted to an intermediate density by additional pulsing.

[0011] Conventionally, highly oriented SmC₀₅ magnets have been produced by the use of superconducting solenoids to generate the high-intensity magnetic fields. These superconducting solenoids must be operated at cryogenic temperatures [-268°C (-450°F)] to pass the high-density current necessary to generate these high-intensity magnetic fields. In the practice of the invention, however, the required high-intensity magnetic fields are preferably produced by discharging an assemblage of capacitors, e.g. four hundred to one thousand capacitors, thereby eliminating the need for superconducting solenoids. The container may be a rubber bag and preferably after alignment the bag is evacuated in the presence of a constant DC field which serves to maintain alignment. Alternatively, the particles of magnet material may be aligned within a preformed container, which will be collapsible and of a material such as stainless steel. The step of subjecting the aligned material to a steady DC field in an evacuated container has been found to "lock in" the alignment and thus ensure improved remanence.

[0012] The following constitutes specific examples with respect to the practice of the invention as described above and demonstrate its utility:

Example 1

[0013] SmC0₅ powder was oriented in a die cavity with an applied magnetic field and pressed, the applied field and the pressing direction being normal to each other. The pressed powder after sintering and post sintering had the properties as set forth in Table I.

[0014] The sintered magnet was loosely wrapped with stainless steel foil (not pressure tight; for handling convenience only) and as-hot-isostatically pressed (HIPed) at 954°C (1750°F). The as-HIPed magnet had the properties as set forth in Table I.

[0015] The HIPed magnet was reheat treated at 910°C (1670°F) for three hours and quenched. The magnetic properties after HIPing and heat treatment are set forth in Table I.

Example 2

[0016] Another magnet prepared according to the same procedures prescribed as in Example 1 had the properties set forth in Table II.

Example 3

[0017] Another magnet of SmC₀₅ from a batch other than in Examples 1 and 2 was prepared as described in Example 1. The properties are recorded in Table III.

[0018] It may be seen from the magnetic property data that remanence is improved by hot isostatic pressing after conventional aligning and cold pressing. Further improvement is achieved with respect to coercive force when after hot-isostatic pressing the magnet is subjected to post sintering heat treatment. The deterioration of the coercive force after HIPing is believed to be due to phase separation.

Example 4

[0019] Using the powder from the same batch as in Example 3, a magnet was made by sintering SmC0₅ powder that was previously oriented and cold isostatically pressed. The magnet had the properties set forth in Table IV.

[0020] With the magnet alloy of Example 4, the theoretical maximum density is 8.6 gm/Cm³. The specific magnet had a density of 8.31 g/Cm³ before hot isostatic pressing and the density increase after hot isostatic pressing was only about 2%, which accounts for the small improvement in remanence reported in the example. It is anticipated that if the theoretical maximum density had been achieved during hot isostatic pressing about a 3% increase in remanence would result.

Example 5

[0021] SmC0₅ alloy was loaded into a stainless container and hydrogen admitted into the container. The pressure was built up to 30 atmospheres; hydrogen absorption by the alloy results in a disintegration of the alloy to about -80 mesh powder. The dehydrided powder was jet milled to about 4 p particle size.

[0022] The fine powder was loaded into a rubber bag of 19.05 mm (3/4") diameter and the bag was contained in a stainless or plastics sheath. The bag was then pressurized and the powder oriented by placing the rubber bag along with the sheath inside a coil, and pulsing the coil, at least three times, with enough power to generate 60,000 Oe (47746 ampere turns per centimetre) within the coil.

[0023] The oriented powder was then placed in a steady DC field of -10 kOe and the bag evacuated to lock the alignment. The evacuated bag containing the powder was then placed in an isostatic press chamber and compressed with a pressure up to 7040 Kg/cm² (100,000 psi). The green compact was subsequently sintered between 1000-1200°C and post sinter aged between 870―930°C.

[0024] The magnets prepared from these four batches of powder in the manner described above had the properties set forth in Table V, which Table also shows magnetic properties of conventional commercial magnets.

Example 6

[0025] Powder of SmC0₅ was loaded in a rubber bag and oriented in the poles of an electromagnet in a field of 25 kOe. The oriented powder was then evacuated maintaining the steady DC field. The evacuated bag containing the oriented powder was isostatically pressed followed by sintering and heat treatment. The magnet had the following properties as shown in Table VI.

Example 7

[0026] A fourth batch of SmC0₅ was processed into magnets by procedures as described in Example 1 except for a change in the compaction method. The powder contained in the bag after alignment was initially compacted inside the bad by placing the bag towards the end of the coil and employing the field gradient present in the coil during pulsing to bring forth an initial compaction to an intermediate density by additional pulsing. The oriented compacted powder placed in a steady DC field was evacuated, isostatically pressed and sintered. The sintered sample was of uniform diameter and had a flat top and bottom contrary to the samples prepared without the field gradient packing which had a pyramidal top. The magnetic properties of the sintered magnet prepared as per this example are shown in Table VII.

[0027] It may be seen from the data reported in Examples 5 to 7 that aligning by the use of a pulsating magnetic field in accordance with the practice of the invention, as opposed to the conventional practice of aligning by the use of a steady-state magnetic field, resulted in improvement in remanence and energy product.

Claims

1. A method for producing magnets with improved remanence by applying a magnetic field to a particle charge of a magnet alloy to magnetically align said particles, and thereafter consolidating said particle charge to form a magnet article, characterised in that said method comprises hot isostatically pressing said particles to consolidate the same to full density.

2. A method according to claim 1, wherein prior to hot isostatic pressing said particle charge is heated to a temperature below the full density sintering temperature thereof but above the temperature necessary to render the particle charge substantially gas-impervious.

3. A method according to claim 1 or claim 2, wherein the aligned charge is given preliminary room temperature densification by die pressing or cold isostatic compaction before heating or final densification by hot isostatical pressing.

4. A method according to any one of the preceding claims, wherein said magnetic field is applied as at least one pulse with each said pulse having a duration not exceeding one second and a power level of at least 50,000 oersted (39789 ampere turns per centimetre).

5. A method according to claim 4, wherein said magnetic field is applied as a plurality of pulses with each said pulse having a duration not exceeding one second and a power level of at least 50,000 oersted (39789 ampere turns per centimetre).

6. A method according to any one of the preceding claims wherein said magneticfield is applied to said particle charge within a container.

7. A method according to claim 6, wherein said particles are loaded into a collapsible container for magnetic alignment and subsequent consolidation.

8. A method according to claim 7, wherein said container is a rubber bag.

9. A method according to claim 6, 7 or 8, wherein said container is preformed to the desired shape of the particle charge after consolidation.

10. A method according to any one of claims 6 to 9, wherein said container is evacuated in the presence of a DC electric field after alignment of said particles.

11. A method according to any one of the preceding claims, wherein said magnet alloy comprises at least one transition element and at least one rare earth element.

12. A method according to claim 11, wherein at least one of said transition elements is cobalt.

13. A method according to claim 11 or 12, wherein at least one of the said rare earth elements is samarium.

14. A method according to any one of the preceding claims, wherein after magnetic alignment of said particles, said particles are compacted to an intermediate density by additional pulsing.

Ansprüche

1. Verfahren zur Herstellen von Magneten mit erhöhter Remanenz durch Anlegen eines magnetischen Feldes an eine Teilchenfüllung aus Magnetlegierung um die Teilchen magnetisch auszurichten, wonach die Teilchenfüllung verfestigt wird um einen Magnetkörper zu bilden, dadurch gekennzeichnet, daß das Verfahren den Schritt des heißen isostatischen Pressens der Teilchen aufweist um diese zur vollen Dichte zu verfestigen.

2. Verfahren nach Anspruch 1, bei dem vor dem heißen isostatischen Pressen die Teilchenfüllung auf eine Temperatur aufgeheizt wird, die unterhalb der zur vollen Dichte erforderlichen Sintertemperatur ist, aber oberhalb derjenigen Temperatur ist, bei der die Teilchenfüllung im wesentlichen gasdicht ist.

3. Verfahren nach Ansprüchen 1 oder 2, bei dem die ausgerichtete Füllung zuerst bei Raumtemperatur durch Druckpressen oder kalte isostatische Komprimierung verdichtet wird, bevor sie erwärmt oder durch heißes isostatisches Pressen endverdichtet wird.

4. Verfahren nach einem der vorhergehenden Ansprüche, bei dem das magnetische Feld in Form von wenigstens einem Impuls angelegt wird, wobei jeder Impuls eine Dauer aufweist, die eine Sekunde nicht überschreitet und eine Stärke von wenigstens 50.000 Oersted (39789 Ampere-Windungen pro Zentimeter) aufweist.

5. Verfahren nach Anspruch 4, bei dem das magnetische Feld in Form einer Vielzahl von Impulsen angelegt wird, wobei jeder Impuls eine dauer aufweist, die eine Sekunde nicht überschreitet und eine Stärke von wenigstens 50.000 Oersted (39789 Ampere-Windungen pro Zentimeter) aufweist.

6. Verfahren nach einem der vorhergehenden Ansprüche, bei dem das magnetische Feld an die Teilchenfüllung innerhalb eines Behälters angelegt wird.

7. Verfahren nach Anspruch 6, bei dem die Teilchen in einem zusammenklappbaren Behälter zum magnetsichen Ausrichten und anschließender Verdichtung eingegeben werden.

8. Verfahren nach Anspruch 7, bei dem der Behälter ein Gummibehälter ist.

9. Verfahren nach Ansprüchen 6, 7 und 8, bei dem der Behälter so vorgeformt ist, daß er der gewünschten Gestalt der Teilchenfüllung nach der Verfestigung entspricht.

10. Verfahren nach einem der Ansprüche 6 bis 9, bei dem der Behälter in Gegenwart eines elektrischen Gleichstromfeldes nach dem Ausrichten der Teilchen evakuiert wird.

11. Verfahren nach einem der vorhergehenden Ansprüche, bei dem die Magnetlegierung wenigstens ein Übergangselement und wenigstens ein Seltene Erde-Element aufweist.

12. Verfahren nach Anspruch 11, bei dem wenigstens eines der Übergangselemente Kobalt ist.

13. Verfahren nach Ansprüche 11 oder 12, bei dem wenigstens eines der Seltene Erde-Elemente Samarium ist.

14. Verfahren nach einem der vorhergehenden Ansprüche, bei dem nach dem magnetischen Ausrichten der Teilchen diese Teilchen zu einer mittleren Dichte durch zusätzliche Impulse verdichtet werden.

Revendications

1. Procédé pour la fabrication d'aimants avec une rémanence améliorée en appliquant un champ magnétique sur une charge de particules d'un alliage magnétique pour aligner magnétiquement ces particules et consolider ensuite cette charge de particules pour en faire un aimant, caractérisé en ce qu'il consiste à comprimer isostatiquement à chaud ces particules pour les consolider à pleine densité.

2. Procédé selon la revendication 1, dans lequel, avant la compression isostatique à chaud, cette charge de particules est chauffée à une température inférieure à sa température de frittage à pleine densité, mais supérieure à la température nécessaire pour rendre la charge de particules pratiquement imperméable au gaz.

3. Procédé selon la revendication 1 ou la revendication 2, dans lequel la charge alignée est d'abord densifiée à température ambiante par compression en matrice ou compression isostatique à froid avant chauffage ou densification finale par compression isostatique à chaud.

4. Procédé selon l'une quelconque des revendications précédentes, dans lequel le champ magnétique est appliqué sous forme d'au moins une impulsion, avec chaque impulsion ayant une durée ne dépassant pas une seconde et un niveau de puissance d'au moins 50000 Oersted. (39 789 ampères-tours par centimètre).

5. Procédé selon la revendication 4, dans lequel le champ magnétique est appliqué sous forme d'une multiplicité d'impulsions avec chaque impulsion ayant une durée ne dépassant pas une seconde et un niveau de puissance d'au moins 50 000 Oersted (39 789 ampères-tours par centimètre).

6. Procédé selon l'une quelconque des revendications précédentes, dans lequel ce champ magnétique est appliqué à cette charge de particules à l'intérieur d'un récipient.

7. Procédé selon la revendication 6, dans lequel ces particules sont chargées dans un récipient écrasable pour l'alignement magnétique et la consolidation ultérieure.

8. Procédé selon la revendication 7, dans lequel ce récipient est un sac de caoutchouc.

9. Procédé selon la revendication 6, 7 ou 8, dans lequel ce récipient est préformé à la forme désirée de la charge de particules après consolidation.

10. Procédé selon l'une quelconque des revendications 6 à 9, dans lequel on fait le vide dans ce récipient en la présence d'un champ électrique courant continu, après alignment des particules.

11. Procédé selon l'une quelconque des revendications précédentes, dans lequel l'alliage magnétique comprend au moins un élément de transition et au moins un élément de terre rare.

12. Procédé selon la revendication 11, dans lequel au moins l'un des éléments de transition est le cobalt.

13. Procédé selon la revendication 11 ou 12, dans lequel au moins l'un des éléments de terre rare est le samarium.

14. Procédé selon l'une quelconque des revendications précédentes, dans lequel, après alignement magnétique des particules, celles-ci sont compactées à une densité intermédiaire par des impulsions additionnelles.