WO2015101682A1 - Method for producing magnets using powder metallurgy - Google Patents

Method for producing magnets using powder metallurgy Download PDF

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Publication number
WO2015101682A1
WO2015101682A1 PCT/ES2014/000223 ES2014000223W WO2015101682A1 WO 2015101682 A1 WO2015101682 A1 WO 2015101682A1 ES 2014000223 W ES2014000223 W ES 2014000223W WO 2015101682 A1 WO2015101682 A1 WO 2015101682A1
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WIPO (PCT)
Prior art keywords
consolidation
carried out
sintering
electrical
magnetic
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PCT/ES2014/000223
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Spanish (es)
French (fr)
Inventor
Juan Manuel Montes Martos
Jesús CINTAS FÍSICO
Francisco GÓMEZ CUEVAS
Fátima TERNERO FERNÁNDEZ
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Universidad De Sevilla
Universidad De Huelva
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Priority to EP14876586.0A priority Critical patent/EP3093857A4/en
Publication of WO2015101682A1 publication Critical patent/WO2015101682A1/en

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C28/00—Alloys based on a metal not provided for in groups C22C5/00 - C22C27/00
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/10—Sintering only
    • B22F3/105—Sintering only by using electric current other than for infrared radiant energy, laser radiation or plasma ; by ultrasonic bonding
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C19/00—Alloys based on nickel or cobalt
    • C22C19/007—Alloys based on nickel or cobalt with a light metal (alkali metal Li, Na, K, Rb, Cs; earth alkali metal Be, Mg, Ca, Sr, Ba, Al Ga, Ge, Ti) or B, Si, Zr, Hf, Sc, Y, lanthanides, actinides, as the next major constituent
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C19/00—Alloys based on nickel or cobalt
    • C22C19/03—Alloys based on nickel or cobalt based on nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C19/00—Alloys based on nickel or cobalt
    • C22C19/07—Alloys based on nickel or cobalt based on cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F41/00—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties
    • H01F41/02—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets
    • H01F41/0253—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets for manufacturing permanent magnets
    • H01F41/0273—Imparting anisotropy
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/10—Sintering only
    • B22F3/105—Sintering only by using electric current other than for infrared radiant energy, laser radiation or plasma ; by ultrasonic bonding
    • B22F2003/1051—Sintering only by using electric current other than for infrared radiant energy, laser radiation or plasma ; by ultrasonic bonding by electric discharge
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2999/00—Aspects linked to processes or compositions used in powder metallurgy
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/12—Both compacting and sintering
    • B22F3/14—Both compacting and sintering simultaneously
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C2200/00—Crystalline structure
    • C22C2200/02—Amorphous
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C9/00—Alloys based on copper

Definitions

  • This technical invention corresponds to the scientific-technical area of "material technology” and the sector of activity to which it would be applied is the manufacture of permanent magnets from powders.
  • the object of this patent is to show an alternative method of manufacturing magnets, especially suitable for those that include rare earths in their composition, which does not modify the magnetic processes, but those related to forming, replacing the conventional method (cold pressing followed of sintered in the oven) by a new method of consolidation consisting of any rapid modality of electrical or technical consolidation FAST (Field Assisted Sintering Technique), from which significant advantages derive: greater brevity, unification of the pressing / sintering processes into a single stage, use of low pressures, absence of controlled atmospheres and possibility of performing the processes of magnetic alignment, pressing / sintering, heat treatment and magnetization, while the powder remains in the same matrix. All this designed to achieve, mainly, a significant reduction in the cost of manufacturing the magnet.
  • FAST Field Assisted Sintering Technique
  • magnets are hard magnetic materials that do not easily demagnetize once they have been magnetized, thus providing a stable, technologically usable magnetic field, which is not affected by external magnetic fields. Magnets are characterized by a high coercive field (coercivity) and a high remaining induction (remanence), which also translates into a high energy product [JW Fiepke, Permanent Magnet Materials, "Properties and Selection: Nonferrous Alloys and Special-Purpose Materials ", Vol 2, ASM Handbooks, ASM International, 1990, p. 782-803]. Such characteristics are achieved with a meticulous manufacturing process. The process is even more elaborate if the magnet contains rare earths.
  • the conventional process for the manufacture of magnets starts from a powder of a high intrinsic crystalline anisotropy (for example, due to a crystalline structure with hexagonal cell), very specific composition and very specific granulometry (The average particle size must be comparable to the critical diameter of a monodomain particle, between 1 and 10 ⁇ , and, in addition, the size distribution must be very narrow [(KH Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications , "Powder Metal Technologies and Applications", Vol 7, ASM Handbook, ASM International, 1998, 1006-1020); (NA Spaldin, "Magnetic Materials: Fundamentals and Applications", 2nd edition, Cambridge University Press, 201 1, USA, 3-144].
  • This powder or a mixture of different types of them, is exposed to a strong magnetic field ( ⁇ 2-5 MA / m) in order to force the magnetic alignment of the constituent particles (for example, forcing preferably axis c of the hexagonal cells of the monocrystalline particles is parallel to the direction of the applied field); with this it is achieved that the magnetization of the final piece can be the highest possible [F.V. Lenel, "Powder Metallurgy: Principies and Applications", Metal Powder Industries Federation, Princeton, NJ, 1980, p. 531-548)].
  • the aligned powder is subjected to cold compression (with pressures ranging between 700 and 1500 MPa) in order to compact it.
  • This compaction process can be done in different ways: by die pressing (in English, Cold Die Pressing, CDP) or by cold isostatic pressing (Cold Isostatic Pressing, CIP).
  • the powder can remain confined in the same matrix during the alignment and compaction stages.
  • the compression system imposes limitations that force the alignment to be done prior to the introduction into the press, normally confining it in a plastic container.
  • the magnetic alignment can be performed in the same compaction matrix, the high pressures used make very large matrices necessary, which move the magnetization coil away from the dust, reducing the magnetic field applied. If the alignment is performed independently, the coil can be closer to the dust and apply larger fields that improve the degree of alignment of the particles, and with it, the magnetic properties of the final pieces. That is why, generally the alignment and pressing are developed in different matrices [(JW Fiepke, Permanent Magnet Materials, "Properties and Selection: Nonferrous Alloys and Special-Purpose Materials", Vol 2, ASM Handbooks, ASM International, 1990, p.
  • the pieces obtained can reach a relative density of the order of 80%.
  • These green compacts are then sintered in the oven, in an appropriate atmosphere (usually, argon) at temperatures around 1 100 ° C. After sintering, the relative density can increase to approximately 90%.
  • powders can also be hot formed by the hot isostatic pressing technique, Hot Isostatic Pressing, HIP.
  • Hot Isostatic Pressing HIP
  • the pieces are magnetized by applying again an intense magnetic field (about 5 MA / m). Due to the high remanence and high coercivity of some types of images thus processed, the magnetization process can be quite difficult. It is often more efficient to magnetize by applying short, but more intense, pulses, which has been revealed as an effective technique for moving and dragging the walls of the magnetic domains, thus enabling increased magnetization.
  • Coating methods can be very diverse [(JW Fiepke, Permanent Magnet Materials, “Properties and Selection: Nonferrous Alloys and Special-Purpose Materials ", Vol 2, ASM Handbooks, ASM International, 1990, p. 782-803); (KH Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications,” Powder Metal Technologies and Applications “, Vol 7, ASM Handbook, ASM International, 1998, 1006-1020)], which include, as examples, galvanizing and chrome plating.
  • the object of the present invention constitutes a method for the powder metallurgical manufacture of magnets comprising as an initial step the introduction of a powdery material containing at least one magnetic element selected from Fe, Co, Ni in a matrix; Once the powder material has been introduced into the matrix, it is subjected sequentially and always in the same matrix to magnetic alignment, electrical consolidation process, heat treatment, cooling and magnetization.
  • the powdery material additionally contains at least one element of the so-called rare earths.
  • the powder material is mixed with at least a second powder material of different composition and comprising at least a first element selected from Fe, Co, Ni.
  • This second powder material may comprise at least a second element that is selected from the so-called rare earths.
  • Magnetic alignment is done by exposing the powder material to the action of a magnetic field.
  • the cooling stage after heat treatment is carried out by contact with the electrodes cooled by a cooling liquid.
  • the magnetization stage of the material is carried out by a magnetic field that allows the magnetic saturation of the material to be achieved.
  • the steps are performed under controlled atmosphere conditions, particularly in an inert gas atmosphere or in a vacuum.
  • Figure 1 Scheme of the proposed equipment for the manufacture of powder metallurgical magnets by electrical consolidation.
  • the dust-containing matrix is unique, and in it the powder is magnetically aligned, electrically consolidated, eventually heat treated and finally magnetized.
  • magnets especially those containing rare earth elements
  • manufacture of magnets is a complex, multi-stage task, which includes both magnetic processes (magnetic particle alignment and magnetization of the final piece) and forming processes (cold pressed, sintered and heat treatments), some of which require a rigorous control of the atmosphere used to avoid undesirable contamination. Due to the high cost of the raw material (rare earths are expensive) and its complicated manufacturing process, the final cost of these magnets is very high and prohibitive, in many cases, for certain applications.
  • the object of the invention is a process for the powder metallurgical manufacture of magnets, especially suitable for those that include in their composition rare earths, which does not modify the magnetic processes, but those related to forming, replacing the conventional procedure (cold pressed followed by sintering in the oven) by a new consolidation method consisting of any rapid modality of electrical or technical consolidation FAST ⁇ Field Assisted Sinter ⁇ ng Technique), from which there are significant advantages: greater brevity, unification of the pressing / sintering processes in a single stage, use of low pressures, absence of controlled atmospheres and the possibility of performing the processes of magnetic alignment, pressing / sintering, heat treatment and magnetization, the powder remaining in the same matrix. All this designed to achieve, mainly, a significant reduction in the cost of manufacturing the magnet.
  • the present invention aims to present a new method of manufacturing magnets (especially applicable to the manufacture of magnets containing rare earths).
  • the novelty of the method lies in the use of electrical consolidation techniques (such as the so-called electric resistance sintering, SRE, or the so-called electric shock consolidation, CDE, which are mentioned for illustrative, non-limiting purposes) instead of the conventional procedure of cold pressed + sintered, usually used.
  • electrical consolidation techniques such as the so-called electric resistance sintering, SRE, or the so-called electric shock consolidation, CDE, which are mentioned for illustrative, non-limiting purposes
  • This is achieved by: (1) combining the stages of compaction / sintering, significantly shortening its duration and reducing the magnitude of the working pressures, (2) carrying out the stages of magnetic alignment, pressing / sintering, heat treatment and magnetization in the same matrix, and (3) make unnecessary the use of protective atmospheres during the sintering process.
  • the two modalities mentioned above, the SRE and the CDE have characteristic durations around the second and the milliseg undo, respectively, and also different power sources: in the SRE, a transformer that provides low voltage, ⁇ 10 V, and high intensity, ⁇ 5 - 20 kA, and in the CDE, a capacitor bank, capable of supplying during discharge, voltages of ⁇ 50 - 300 V, and intensities of 1 - 5 kA.
  • S.R.E. You can use low frequency (50 Hz) or medium frequency electric current, around 1000 Hz.
  • the matrices can have considerably thinner walls, which makes it possible, in addition, that the coil used for magnetic alignment and, eventually, magnetization, can be more tight to the dust, allowing it to not only receive a more intense magnetic field, but it can also be magnetically aligned in the same matrix where it will later be consolidated, heat treated and, eventually, also magnetized.
  • the heat treatment prior to the final magnetization can be carried out by passing an electric current through the material, of such intensity that Joule causes the necessary heating of the piece. In general, the value of this intensity will be lower than that used during the consolidation stage.
  • the cooling will be carried out thanks to the cooling of which the machine benches in contact with the electrodes / punches must consist. Eventually, for the realization of this stage, a controlled atmosphere could be used, by means of the appropriate experimental arrangement.
  • the matrix is electrically insulating, for example, made of natural rock, refractory concrete, ceramic tube and metal strip, etc.
  • the electrodes will be of some copper alloy with high conductivity, for example, Cu-Zr alloy. To achieve greater uniformity in the indoor temperature, it may be interesting to interpose a wafer of somewhat less conductive material between the powder and the electrode, for example, a pseudo-alloy (heavy metal) of Cu-W, which will also provide resistance to EDM.
  • the power source may consist of a welding transformer that provides current intensities in the range of 2 to 12 kA, either with a grid frequency at 50 Hz or better still, with higher frequencies, in the range of the average frequencies around 1000 Hz.
  • a second possibility in the case of the CDE could be the use as a power source of a capacitor bank, large capacity and load voltages in the range of 50 at 500 V.
  • Another possibility is to operate with both types of sources, for example, in a sequential application thereof: first discharge by capacitors, then the welding transformer acts. This last possibility may have the advantage of allowing the boarding of larger pieces, whose electrical resistance is too high to be addressed only by the SRE technique
  • the mechanical device that exerts the pressure must be able to supply the force necessary to reach pressures around 100 MPa.
  • the coil that wraps the matrix will be connected to an additional power source, which can benefit from the procedures commonly used for this purpose, for example, pulse magnetization, which is mentioned for illustrative, but not limiting purposes. This coil will be responsible for applying the magnetic field of particle alignment and magnetization of the already consolidated part.
  • the powder mixture is enclosed in a plastic sheath and subjected to magnetic alignment by the action of a magnetic field of the order of 5 MA / m.
  • the mixture of magnetically aligned powders is taken to an isostatic press where it is compacted to the approximate pressure of 1400 MPa to obtain a density of 6.9 g / cm 3 , which represents approximately 80% of the absolute value of the density of the compound.
  • the green compact obtained in the previous stage is sintered in the oven, under an atmosphere of purified argon, at a temperature between 1090 and 1150 ° C, for a time of around 40 minutes. The compact densifies to a density of 7.7 g / cm 3 , which represents approximately 90% of the absolute density.
  • the piece After sintering in the oven, the piece is subjected to a heat treatment at 900 ° C for 15 minutes, in order to improve the coercive force.
  • the matrix 1 is electrically insulating (for example, made of natural rock, refractory concrete, ceramic tube and metal strip, etc.).
  • the electrodes 2 will be of some copper alloy with high conductivity (for example, Cu-Zr alloy). In order to achieve greater uniformity in the indoor temperature, it may be interesting to interpose a wafer 4 of somewhat less conductive material between the powder 3 and the electrode 2, for example, a Cu-W pseudo-alloy (heavy metal), which will also provide resistance to the EDM
  • the coil 5 is in charge of both the magnetic alignment process and the final magnetization process.
  • the power source 6 may consist of a welding transformer (in the case of the S.R.E.), or a capacitor bank (in the case of C.D. E.). Another possibility is to operate with both types of sources, for example, in a sequential application thereof: first discharge by capacitors, and then intervention of the welding transformer.
  • the mechanical device 7 that exerts the pressure must be able to supply the force necessary to reach pressures to ensure the desired level of densification.
  • the powder mixture is introduced into the matrix of the consolidation team by S.R.E., and there it is magnetically aligned by the action of a magnetic field of the order of 5 MA / m.
  • the electrical consolidation process is initiated by SRE, on air, with nominal parameters of 80 MPa pressure, a current density of -6.5 kA / cm 2 , and a passage time of 70 cycles, of 0.02 s each cycle.
  • the final density of the compact turns out to be 90% or higher.
  • the compact is magnetized without leaving the matrix by a magnetic field with a value of ⁇ 5 MA / m.

Abstract

The invention relates to a novel method for producing magnets (especially applicable to the production of magnets containing rare earths). The novelty of the method lies in its use of a type of electrical consolidation (such as so-called electrical resistance sintering, ERS, or so-called electrical discharge consolidation, EDC, but not necessarily one of the these) as a substitute for the conventional cold pressing and sintering method which is usually used. The method according to the invention achieves: (1) combining the compacting/sintering steps, significantly cutting the duration thereof and reducing the magnitude of the working pressures; (2) implementing the steps of magnetic aligning, pressing/sintering, heat treatment and magnetising in the same matrix; and (3) rendering superfluous the use of protective atmospheres during the sintering process.

Description

TÍTULO  TITLE
MÉTODO PARA LA FABRICACIÓN PULVIMETALÚRGICA DE IMANES  METHOD FOR PULVIMETALURGICAL MAGNET MANUFACTURING
OBJETO DE LA INVENCIÓN OBJECT OF THE INVENTION
Esta invención técnica corresponde al área científico-técnica de la "tecnología de materiales" y el sector de actividad al que se aplicaría es el de la fabricación de imanes permanentes a partir de polvos. This technical invention corresponds to the scientific-technical area of "material technology" and the sector of activity to which it would be applied is the manufacture of permanent magnets from powders.
El objeto de esta patente es mostrar un método alternativo de fabricación de imanes, especialmente indicada para los que incluyen en su composición tierras raras, que no modifica los procesos magnéticos, pero sí los relativos al conformado, sustituyendo el método convencional (prensado en frío seguida de sinterizado en horno) por un nuevo método de consolidación consistente en cualquier modalidad rápida de consolidación eléctrica o técnica FAST (Field Assisted Sintering Technique), del que se derivan notables ventajas: mayor brevedad, unificación de los procesos de prensado/sinterizado en una única etapa, empleo de presiones bajas, ausencia de atmósferas controladas y posibilidad de realizar los procesos de alineamiento magnético, prensado/sinterizado, tratamiento térmico y magnetización permaneciendo el polvo en una misma matriz. Todo ello concebido para lograr, principalmente, una reducción importante del coste de fabricación del imán.  The object of this patent is to show an alternative method of manufacturing magnets, especially suitable for those that include rare earths in their composition, which does not modify the magnetic processes, but those related to forming, replacing the conventional method (cold pressing followed of sintered in the oven) by a new method of consolidation consisting of any rapid modality of electrical or technical consolidation FAST (Field Assisted Sintering Technique), from which significant advantages derive: greater brevity, unification of the pressing / sintering processes into a single stage, use of low pressures, absence of controlled atmospheres and possibility of performing the processes of magnetic alignment, pressing / sintering, heat treatment and magnetization, while the powder remains in the same matrix. All this designed to achieve, mainly, a significant reduction in the cost of manufacturing the magnet.
ESTADO DE LA TÉCNICA STATE OF THE TECHNIQUE
A diferencia de los materiales magnéticos blandos, los imanes son materiales magnéticos duros que no se desmagnetizan fácilmente una vez que han sido magnetizados, proporcionando así un campo magnético estable, aprovechable tecnológicamente, que no se ve afectado por campos magnéticos externos. Los imanes se caracterizan por un elevado campo coercitivo (coercitividad) y una elevada inducción remanente (remanencia), lo que se traduce en también un elevado producto energía [J.W. Fiepke, Permanent Magnet Materials, "Properties and Selection: Nonferrous Alloys and Special- Purpose Materials", Vol 2, ASM Handbooks, ASM International, 1990, p. 782-803]. Tales características se consiguen con un meticuloso proceso de fabricación. El proceso es más elaborado aún si el imán contiene tierras raras. El desarrollo de este tipo de imanes se inició a principios de la década de 1960, cuando Nesbitt y Wernicke [(E.A. Nesbitt, H.J. Williams, J.H. Wernicke, R.C. Sherwood, J. Appl. Phys., Vol 32S, 1961 , p 342-343); (E.A. Nesbitt, H.J. Williams, J.H. Wernicke, R.C. Sherwood, J. Appl. Phys., Vol 33, 1962, p 1674-1678); (J.H. Wernicke, S. Geller, Acta Cryst., Vol 12, 1959, p 662-665)] establecieron la estructura, momento magnético y temperatura de Curie de los compuestos intermetálicos de cobalto/tierras-raras. Unlike soft magnetic materials, magnets are hard magnetic materials that do not easily demagnetize once they have been magnetized, thus providing a stable, technologically usable magnetic field, which is not affected by external magnetic fields. Magnets are characterized by a high coercive field (coercivity) and a high remaining induction (remanence), which also translates into a high energy product [JW Fiepke, Permanent Magnet Materials, "Properties and Selection: Nonferrous Alloys and Special-Purpose Materials ", Vol 2, ASM Handbooks, ASM International, 1990, p. 782-803]. Such characteristics are achieved with a meticulous manufacturing process. The process is even more elaborate if the magnet contains rare earths. The development of this type of magnets began in the early 1960s, when Nesbitt and Wernicke [(EA Nesbitt, HJ Williams, JH Wernicke, RC Sherwood, J. Appl. Phys., Vol 32S, 1961, p 342- 343); (EA Nesbitt, HJ Williams, JH Wernicke, RC Sherwood, J. Appl. Phys., Vol 33, 1962, p 1674-1678); (JH Wernicke, S. Geller, Acta Cryst., Vol 12, 1959, p 662-665)] established the structure, magnetic moment and Curie temperature of the intermetallic compounds of cobalt / rare earths.
El proceso convencional para la fabricación de imanes (por ejemplo, de aquellos que contienen tierras raras) parte de un polvo de una elevada anisotropía cristalina intrínseca (por ejemplo, debida a una estructura cristalina con celdilla hexagonal), composición muy específica y granulometría muy concreta (el tamaño medio de las partículas debe ser comparable al diámetro crítico de una partícula monodominio, entre 1 y 10 μΐη, y, además, la distribución de tamaños debe ser muy estrecha [(K.H. Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications, "Powder Metal Technologies and Applications", Vol 7, ASM Handbook, ASM International, 1998, 1006-1020); (N.A. Spaldin, "Magnetic Materials: Fundamentáis and Applications", 2nd edition, Cambridge University Press, 201 1 , USA, 3-144].  The conventional process for the manufacture of magnets (for example, those that contain rare earths) starts from a powder of a high intrinsic crystalline anisotropy (for example, due to a crystalline structure with hexagonal cell), very specific composition and very specific granulometry (The average particle size must be comparable to the critical diameter of a monodomain particle, between 1 and 10 μΐη, and, in addition, the size distribution must be very narrow [(KH Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications , "Powder Metal Technologies and Applications", Vol 7, ASM Handbook, ASM International, 1998, 1006-1020); (NA Spaldin, "Magnetic Materials: Fundamentals and Applications", 2nd edition, Cambridge University Press, 201 1, USA, 3-144].
Este polvo, o una mezcla de diferentes tipos de ellos, es expuesto a un fuerte campo magnético (~2 - 5 MA/m) con objeto de forzar el alineamiento magnético de las partículas que lo constituyen (forzando, por ejemplo, que preferentemente el eje c de las celdillas hexagonales de las partículas monocristalinas sea paralelo a la dirección del campo aplicado); con ello se consigue que la magnetización de la pieza final pueda ser la más alta posible [F.V. Lenel, "Powder Metallurgy: Principies and Applications", Metal Powder Industries Federation, Princeton, NJ, 1980, p. 531 -548)].  This powder, or a mixture of different types of them, is exposed to a strong magnetic field (~ 2-5 MA / m) in order to force the magnetic alignment of the constituent particles (for example, forcing preferably axis c of the hexagonal cells of the monocrystalline particles is parallel to the direction of the applied field); with this it is achieved that the magnetization of the final piece can be the highest possible [F.V. Lenel, "Powder Metallurgy: Principies and Applications", Metal Powder Industries Federation, Princeton, NJ, 1980, p. 531-548)].
Tras este proceso de alineamiento magnético, el polvo alineado es sometido a compresión en frío (con presiones que oscilan entre 700 y 1500 MPa) con objeto de compactarlo. Este proceso de compactacion puede hacerse de diferentes modos: por prensado en matriz (en inglés, Cold Die Pressing, CDP) o por prensado isostático en frío (Cold Isostatic Pressing, CIP). Cuando el prensado se realiza en matriz, el polvo puede permanecer confinado en la misma matriz durante las etapas de alineamiento y de compactacion. Cuando el prensado es isostático, el sistema de compresión impone limitaciones que obligan a que el alineamiento deba realizarse previamente a la introducción en la prensa, confinándolo normalmente en un envase de plástico. Aunque en teoría, si la modalidad elegida es la de prensado en matriz, el alineamiento magnético puede realizarse en la misma matriz de compactacion, las elevadas presiones empleadas hace necesarias matrices muy grandes, que alejan la bobina de magnetización del polvo, aminorando el campo magnético aplicado. Si el alineamiento es realizado de forma independiente, la bobina puede estar más próxima al polvo y aplicar campos mayores que mejoran el grado de alineamiento de las partículas, y con ello, las propiedades magnéticas de las piezas finales. Es por ello, que generalmente el alineamiento y el prensado se desarrollan en matrices diferentes [(J.W. Fiepke, Permanent Magnet Materials, "Properties and Selection: Nonferrous Alloys and Special-Purpose Materials", Vol 2, ASM Handbooks, ASM International, 1990, p. 782-803); (K.H. Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications, "Powder Metal Technologies and Applications", Vol 7, ASM Handbook, ASM International, 1998, 1006- 1020); F.V. Lenel, "Powder Metallurgy: Principies and Applications", Metal Powder Industries Federation, Princeton, NJ, 1980, p. 531-548)]. After this magnetic alignment process, the aligned powder is subjected to cold compression (with pressures ranging between 700 and 1500 MPa) in order to compact it. This compaction process can be done in different ways: by die pressing (in English, Cold Die Pressing, CDP) or by cold isostatic pressing (Cold Isostatic Pressing, CIP). When pressing is performed on matrix, the powder can remain confined in the same matrix during the alignment and compaction stages. When the pressing is isostatic, the compression system imposes limitations that force the alignment to be done prior to the introduction into the press, normally confining it in a plastic container. Although in theory, if the modality chosen is that of matrix pressing, the magnetic alignment can be performed in the same compaction matrix, the high pressures used make very large matrices necessary, which move the magnetization coil away from the dust, reducing the magnetic field applied. If the alignment is performed independently, the coil can be closer to the dust and apply larger fields that improve the degree of alignment of the particles, and with it, the magnetic properties of the final pieces. That is why, generally the alignment and pressing are developed in different matrices [(JW Fiepke, Permanent Magnet Materials, "Properties and Selection: Nonferrous Alloys and Special-Purpose Materials", Vol 2, ASM Handbooks, ASM International, 1990, p. 782-803); (KH Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications, "Powder Metal Technologies and Applications", Vol 7, ASM Handbook, ASM International, 1998, 1006-1020); FV Lenel, "Powder Metallurgy: Principies and Applications", Metal Powder Industries Federation, Princeton, NJ, 1980, p. 531-548)].
Tras el proceso de compactación las piezas obtenidas pueden alcanzar una densidad relativa del orden del 80%. Estos compactos en verde son entonces sinterizados en horno, en atmósfera apropiada (usualmente, argón) a temperaturas que rondan los 1 100 °C. Tras el sinterizado, la densidad relativa puede aumentar hasta aproximadamente el 90%.  After the compaction process the pieces obtained can reach a relative density of the order of 80%. These green compacts are then sintered in the oven, in an appropriate atmosphere (usually, argon) at temperatures around 1 100 ° C. After sintering, the relative density can increase to approximately 90%.
Aunque menos habitual, los polvos pueden también ser conformados en caliente mediante la técnica de prensado isostático en caliente, Hot Isostatic Pressing, HIP. Se aúnan con ello la etapa de prensado y sinterización, pero no el proceso de alineamiento, que debe seguir haciéndose de modo independiente [J.W. Fiepke, Permanent Magnet Materials, "Properties and Selection: Nonferrous Alloys and Special-Purpose Materials", Vol 2, ASM Handbooks, ASM International, 1990, p. 782-803]. El principal inconveniente de esta técnica, es su elevado coste y su bajo nivel de producción.  Although less common, powders can also be hot formed by the hot isostatic pressing technique, Hot Isostatic Pressing, HIP. This brings together the pressing and sintering stage, but not the alignment process, which must continue to be done independently [J.W. Fiepke, Permanent Magnet Materials, "Properties and Selection: Nonferrous Alloys and Special-Purpose Materials", Vol 2, ASM Handbooks, ASM International, 1990, p. 782-803]. The main drawback of this technique is its high cost and low production level.
Tras el sinterizado, es habitual la aplicación de algún tratamiento térmico (en torno a los 900 °C) destinado a mejorar la coercitividad del material. Dichos tratamientos térmicos persiguen, en general, la nucleación, fijación y anclaje de las paredes de los dominios magnéticos en las superficies y límites de grano del material, lo que aumenta sustancialmente la fuerza coercitiva [K.H. Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications, "Powder Metal Technologies and Applications", Vol 7, ASM Handbook, ASM International, 1998, 1006-1020].  After sintering, it is usual to apply some heat treatment (around 900 ° C) to improve the coercivity of the material. Such heat treatments generally pursue the nucleation, fixation and anchoring of the walls of the magnetic domains on the surfaces and grain boundaries of the material, which substantially increases the coercive force [K.H. Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications, "Powder Metal Technologies and Applications", Vol 7, ASM Handbook, ASM International, 1998, 1006-1020].
Finalmente, las piezas son magnetizadas aplicando nuevamente un campo magnético intenso (de unos 5 MA/m). Debido a la alta remanencia y alta coercitividad de algunos tipos de ¡manes así procesados, el proceso de magnetización puede llegar a ser bastante difícil. A menudo resulta más eficiente magnetizar mediante la aplicación de pulsos de corta duración, pero más intensos, lo que se ha revelado como una técnica eficaz para mover y arrastrar las paredes de los dominios magnéticos, posibilitando así el aumento de la magnetización.  Finally, the pieces are magnetized by applying again an intense magnetic field (about 5 MA / m). Due to the high remanence and high coercivity of some types of images thus processed, the magnetization process can be quite difficult. It is often more efficient to magnetize by applying short, but more intense, pulses, which has been revealed as an effective technique for moving and dragging the walls of the magnetic domains, thus enabling increased magnetization.
Eventualmente tras este tratamiento, algunas piezas (especialmente las que tienen al hierro entre sus componentes) deben ser recubiertas con objeto de mejorar su resistencia a la corrosión. Los métodos de recubrimiento pueden ser muy diversos [(J.W. Fiepke, Permanent Magnet Materials, "Properties and Selection: Nonferrous Alloys and Special-Purpose Materials", Vol 2, ASM Handbooks, ASM International, 1990, p. 782- 803); (K.H. Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications, "Powder Metal Technologies and Applications", Vol 7, ASM Handbook, ASM International, 1998, 1006-1020)], entre los que cabe citar, como ejemplos, el galvanizado y el cromado. Eventually after this treatment, some parts (especially those with iron in their components) must be coated in order to improve their corrosion resistance. Coating methods can be very diverse [(JW Fiepke, Permanent Magnet Materials, "Properties and Selection: Nonferrous Alloys and Special-Purpose Materials ", Vol 2, ASM Handbooks, ASM International, 1990, p. 782-803); (KH Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications," Powder Metal Technologies and Applications ", Vol 7, ASM Handbook, ASM International, 1998, 1006-1020)], which include, as examples, galvanizing and chrome plating.
Aun cuando la tecnología pulvimetalúrgica actual de fabricación de imanes permanentes puede considerarse satisfactoria en muchos sentidos, serían deseables ciertas mejoras encaminadas a simplificar el proceso y que redujesen significativamente los costes de producción. La técnica de fabricación propuesta en esta solicitud de patente persigue satisfacer este reto.  Even though current permanent magnet manufacturing powder technology can be considered satisfactory in many ways, certain improvements aimed at simplifying the process and significantly reducing production costs would be desirable. The manufacturing technique proposed in this patent application seeks to meet this challenge.
EXPLICACION DE LA INVENCIÓN EXPLANATION OF THE INVENTION
Constituye el objeto de la presente invención un método para la fabricación pulvi- metalúrgica de imanes que comprende como etapa inicial la introducción de un material pulverulento que contiene al menos un elemento magnético seleccionado entre Fe, Co, Ni en una matriz; una vez introducido el material pulverulento en la matriz se somete secuencialmente y siempre en la misma matriz a alineamiento magnético, proceso de consolidación eléctrica, tratamiento térmico, refrigeración y magnetización.  The object of the present invention constitutes a method for the powder metallurgical manufacture of magnets comprising as an initial step the introduction of a powdery material containing at least one magnetic element selected from Fe, Co, Ni in a matrix; Once the powder material has been introduced into the matrix, it is subjected sequentially and always in the same matrix to magnetic alignment, electrical consolidation process, heat treatment, cooling and magnetization.
En un modo de realización particular, el material pulverulento contiene adicionalmente al menos un elemento de los denominados tierras raras. In a particular embodiment, the powdery material additionally contains at least one element of the so-called rare earths.
En ulteriores modos preferentes de realización el material pulverulento se mezcla con al menos un segundo material pulverulento de composición diferente y que comprende al menos un primer elemento que se selecciona entre Fe, Co, Ni. Este segundo material pulverulento puede comprender al menos un segundo elemento que se selecciona entre los denominados tierras raras.  In further preferred embodiments the powder material is mixed with at least a second powder material of different composition and comprising at least a first element selected from Fe, Co, Ni. This second powder material may comprise at least a second element that is selected from the so-called rare earths.
Se exponen a continuación las opciones para la puesta en práctica de las etapas que constituyen el procedimiento:  The options for the implementation of the stages that constitute the procedure are set out below:
1 ) El alineamiento magnético se realiza exponiendo el material pulverulento a la acción de un campo magnético.  1) Magnetic alignment is done by exposing the powder material to the action of a magnetic field.
2) El proceso de consolidación eléctrica se puede ejecutar:  2) The electrical consolidation process can be executed:
- mediante la técnica de sinterización por resistencia eléctrica, particularmente de media frecuencia.  - by the technique of sintering by electrical resistance, particularly of medium frequency.
- mediante la técnica de consolidación por descarga eléctrica. - mediante la aplicación secuencial de las técnicas de consolidación por descarga eléctrica y sinterización por resistencia eléctrica o a la recíproca, primero sinterización y después descarga. - by the technique of consolidation by electric shock. - by sequential application of consolidation techniques by electric shock and sintering by electric or reciprocal resistance, first sintering and then discharge.
3) El tratamiento térmico del material posterior a la consolidación eléctrica se realiza mediante el paso de corriente eléctrica a través del material.  3) The heat treatment of the material after the electrical consolidation is carried out by the passage of electric current through the material.
4) La etapa de refrigeración tras el tratamiento térmico se lleva a cabo por contacto con los electrodos enfriados mediante un líquido refrigerante.  4) The cooling stage after heat treatment is carried out by contact with the electrodes cooled by a cooling liquid.
5) La etapa de magnetización del material se efectúa por acción de un campo magnético que permita alcanzar la saturación magnética del material.  5) The magnetization stage of the material is carried out by a magnetic field that allows the magnetic saturation of the material to be achieved.
Opcionalmente, las etapas se realizan en condiciones de atmósfera controlada, particularmente en atmósfera de gas inerte o en vacío.  Optionally, the steps are performed under controlled atmosphere conditions, particularly in an inert gas atmosphere or in a vacuum.
DESCRIPCIÓN DE LA FIGURA DESCRIPTION OF THE FIGURE
Figura 1 : Esquema del equipo propuesto para la fabricación de imanes pulvimetalúrgicos por consolidación eléctrica. La matriz contenedora del polvo es única, y en ella el polvo es alineado magnéticamente, consolidado eléctricamente, eventualmente tratado térmicamente y finalmente magnetizado. DESCRIPCIÓN DE LA INVENCIÓN Figure 1: Scheme of the proposed equipment for the manufacture of powder metallurgical magnets by electrical consolidation. The dust-containing matrix is unique, and in it the powder is magnetically aligned, electrically consolidated, eventually heat treated and finally magnetized. DESCRIPTION OF THE INVENTION
La fabricación de imanes (especialmente de aquellos que contienen elementos de tierras raras) es una tarea compleja, multietapa, que incluye tanto procesos magnéticos (alineamiento magnético de partículas y magnetización de la pieza final) como procesos de conformado (prensado en frío, sinterizado y tratamientos térmicos), algunos de los cuales exigen un riguroso control de la atmósfera empleada para evitar contaminaciones indeseables. Debido al alto costo de la materia prima (las tierras raras son caras) y a su complicado proceso de fabricación, el costo final de estos imanes resulta muy elevado y prohibitivo, en muchos casos, para determinadas aplicaciones.  The manufacture of magnets (especially those containing rare earth elements) is a complex, multi-stage task, which includes both magnetic processes (magnetic particle alignment and magnetization of the final piece) and forming processes (cold pressed, sintered and heat treatments), some of which require a rigorous control of the atmosphere used to avoid undesirable contamination. Due to the high cost of the raw material (rare earths are expensive) and its complicated manufacturing process, the final cost of these magnets is very high and prohibitive, in many cases, for certain applications.
El objeto de la invención, es un procedimiento para la fabricación pulvimetalúrgica de imanes, especialmente indicada para los que incluyen en su composición tierras raras, que no modifica los procesos magnéticos, pero sí los relativos al conformado, sustituyendo el procedimiento convencional (prensado en frío seguida de sinterizado en horno) por un nuevo método de consolidación consistente en cualquier modalidad rápida de consolidación eléctrica o técnica FAST {Field Assisted Sinteríng Technique), del que se derivan notables ventajas: mayor brevedad, unificación de los procesos de prensado/sinterizado en una única etapa, empleo de presiones bajas, ausencia de atmósferas controladas y posibilidad de realizar los procesos de alineamiento magnético, prensado/sinterizado, tratamiento térmico y magnetización permaneciendo el polvo en una misma matriz. Todo ello concebido para lograr, principalmente, una reducción importante del coste de fabricación del imán. The object of the invention is a process for the powder metallurgical manufacture of magnets, especially suitable for those that include in their composition rare earths, which does not modify the magnetic processes, but those related to forming, replacing the conventional procedure (cold pressed followed by sintering in the oven) by a new consolidation method consisting of any rapid modality of electrical or technical consolidation FAST {Field Assisted Sinteríng Technique), from which there are significant advantages: greater brevity, unification of the pressing / sintering processes in a single stage, use of low pressures, absence of controlled atmospheres and the possibility of performing the processes of magnetic alignment, pressing / sintering, heat treatment and magnetization, the powder remaining in the same matrix. All this designed to achieve, mainly, a significant reduction in the cost of manufacturing the magnet.
La presente invención tiene por objeto presentar un nuevo método de fabricación de imanes (aplicable especialmente a la fabricación de imanes que contienen tierras raras). Lo novedoso del método reside en el empleo de técnicas de consolidación eléctrica (como la denominada sinterización por resistencia eléctrica, S.R.E., o la denominada consolidación por descarga eléctrica, C.D.E., que se mencionan a efectos ilustrativos, no limitativos) en sustitución del procedimiento convencional de prensado en frío + sinterizado, empleada habitualmente. Se logra con ello: (1) aunar las etapas de compactación/sinterizado, acortando significativamente su duración y reduciendo la magnitud de las presiones de trabajo, (2) llevar a cabo las etapas de alineamiento magnético, prensado/sinterizado, tratamiento térmico y magnetización en una misma matriz, y (3) hacer innecesario el empleo de atmósferas protectoras durante el proceso de sinterizado. The present invention aims to present a new method of manufacturing magnets (especially applicable to the manufacture of magnets containing rare earths). The novelty of the method lies in the use of electrical consolidation techniques (such as the so-called electric resistance sintering, SRE, or the so-called electric shock consolidation, CDE, which are mentioned for illustrative, non-limiting purposes) instead of the conventional procedure of cold pressed + sintered, usually used. This is achieved by: (1) combining the stages of compaction / sintering, significantly shortening its duration and reducing the magnitude of the working pressures, (2) carrying out the stages of magnetic alignment, pressing / sintering, heat treatment and magnetization in the same matrix, and (3) make unnecessary the use of protective atmospheres during the sintering process.
Las técnicas de consolidación eléctricas (las conocidas genéricamente como técnicas FAST no solo permiten aunar las etapas de prensado en frío y sinterizado en horno en una sola etapa, sino que también logran reducir su duración, de tal modo que se hace innecesario el empleo de atmósferas inertes (el tiempo en que el polvo está expuesto a las altas temperaturas es demasiado breve para que transcurran las reacciones de oxidación no deseables), y el proceso puede llevarse a cabo al aire. La reducción del tiempo puede llegar a ser muy considerable: si el proceso conjunto de prensado en frío (en matriz o isostático) y sinterizado en horno puede ocupar en torno a 30-60 minutos, la consolidación eléctrica puede llevar tan solo unos pocos segundos, o incluso menos aún, dependiendo de la modalidad concreta empleada. A modo de ejemplo, cabe decir que las dos modalidades antes mencionadas, la S.R.E. y la C.D.E., tienen duraciones características en torno al segundo y al milisegundo, respectivamente, y fuentes de potencia eléctrica también diferentes: en la S.R.E., un transformador que proporciona baja tensión, ~ 10 V, y alta intensidad, ~ 5 - 20 kA, y en la C.D.E., un banco de condensadores, capaz de suministrar durante su descarga, tensiones de ~ 50 - 300 V, e intensidades de 1 - 5 kA. La S.R.E. puede emplear corriente eléctrica de baja frecuencia (50 Hz) o media frecuencia, en torno a 1000 Hz.  Electrical consolidation techniques (known generically as FAST techniques not only allow the cold pressing and sintering stages of the oven to be combined in a single stage, but also reduce their duration, so that the use of atmospheres is unnecessary inert (the time in which the dust is exposed to high temperatures is too short for undesirable oxidation reactions to take place), and the process can be carried out in the air. The reduction in time can become very considerable: yes The whole process of cold pressing (in matrix or isostatic) and sintered in the oven can take around 30-60 minutes, the electrical consolidation can take only a few seconds, or even less, depending on the specific modality used. As an example, it can be said that the two modalities mentioned above, the SRE and the CDE, have characteristic durations around the second and the milliseg undo, respectively, and also different power sources: in the SRE, a transformer that provides low voltage, ~ 10 V, and high intensity, ~ 5 - 20 kA, and in the CDE, a capacitor bank, capable of supplying during discharge, voltages of ~ 50 - 300 V, and intensities of 1 - 5 kA. S.R.E. You can use low frequency (50 Hz) or medium frequency electric current, around 1000 Hz.
Dado que las técnicas de consolidación eléctricas son en el fondo cierto tipo de prensado en caliente, se requieren presiones de trabajo mucho menores (< 100 Pa) que las empleadas en el prensado en frío del procedimiento convencional (entre 700 y 1500 MPa). De este modo, con la consolidación eléctrica, las matrices pueden tener paredes considerablemente más finas, lo que hace posible, además, que la bobina empleada para el alineamiento magnético y, eventualmente, la magnetización, pueda ceñirse más al polvo, permitiendo que este no solo reciba un campo magnético más intenso, sino que también pueda ser alineado magnéticamente en la misma matriz donde posteriormente será consolidado, tratado térmicamente y, eventualmente, también magnetizado. Since electrical consolidation techniques are basically a certain type of hot pressing, much lower working pressures (<100 Pa) are required than those used in cold pressing of the conventional procedure (between 700 and 1500 MPa). In this way, with the electrical consolidation, the matrices can have considerably thinner walls, which makes it possible, in addition, that the coil used for magnetic alignment and, eventually, magnetization, can be more tight to the dust, allowing it to not only receive a more intense magnetic field, but it can also be magnetically aligned in the same matrix where it will later be consolidated, heat treated and, eventually, also magnetized.
El tratamiento térmico previo a la magnetización final puede llevarse a cabo por paso de una corriente eléctrica a través del material, de una intensidad tal que provoque por efecto Joule el calentamiento necesario de la pieza. En general, el valor de esta intensidad será menor que la empleada durante la etapa de consolidación. El enfriamiento se llevará a cabo merced a la refrigeración de la que deben constar las bancadas de la máquina en contacto con los electrodos/punzones. Eventualmente, para la realización de esta etapa podría emplearse atmósfera controlada, mediante el arreglo experimental adecuado.  The heat treatment prior to the final magnetization can be carried out by passing an electric current through the material, of such intensity that Joule causes the necessary heating of the piece. In general, the value of this intensity will be lower than that used during the consolidation stage. The cooling will be carried out thanks to the cooling of which the machine benches in contact with the electrodes / punches must consist. Eventually, for the realization of this stage, a controlled atmosphere could be used, by means of the appropriate experimental arrangement.
Un esquema del equipo de consolidación eléctrica, especialmente en lo concerniente a los detalles de la matriz es el mostrado en la Figura 1 , que se presenta a efectos ilustrativos, no limitativos:  An outline of the electrical consolidation equipment, especially with regard to the details of the matrix is that shown in Figure 1, which is presented for illustrative, non-limiting purposes:
• La matriz es eléctricamente aislante, por ejemplo, fabricada con roca natural, hormigón refractario, tubo cerámico y zuncho de metal, etc. · Los electrodos serán de alguna aleación de cobre con elevada conductividad por ejemplo, aleación de Cu-Zr. Para conseguir mayor uniformidad en la temperatura interior, puede ser interesante interponer entre el polvo y el electrodo una oblea de material algo menos conductor, por ejemplo, una pseudoaleación (heavy metal) de Cu-W, que además aportará resistencia a la electroerosión. • The matrix is electrically insulating, for example, made of natural rock, refractory concrete, ceramic tube and metal strip, etc. · The electrodes will be of some copper alloy with high conductivity, for example, Cu-Zr alloy. To achieve greater uniformity in the indoor temperature, it may be interesting to interpose a wafer of somewhat less conductive material between the powder and the electrode, for example, a pseudo-alloy (heavy metal) of Cu-W, which will also provide resistance to EDM.
· En el caso de la S.R.E., la fuente de potencia puede consistir en un transformador de soldadura que proporcione intensidades corriente en el rango de 2 a 12 kA, ya sea con frecuencia de red a 50 Hz o mejor aún, con frecuencias mayores, en el rango de las frecuencias medias en torno a los 1000 Hz. Una segunda posibilidad en el caso de la C.D.E. podría consistir en el empleo como fuente de potencia de un banco de condensadores, de gran capacidad y tensiones de carga en el rango de los 50 a 500 V. Otra posibilidad es operar con ambos tipos de fuentes, por ejemplo, en una aplicación secuencial de las mismas: primero descarga por condensadores, luego actúa el transformador de soldadura. Esta última posibilidad puede tener la ventaja de permitir el abordaje de piezas de tamaño mayor, cuya resistencia eléctrica es demasiado elevada para ser abordadas únicamente por la técnica de S.R.E. · In the case of the SRE, the power source may consist of a welding transformer that provides current intensities in the range of 2 to 12 kA, either with a grid frequency at 50 Hz or better still, with higher frequencies, in the range of the average frequencies around 1000 Hz. A second possibility in the case of the CDE could be the use as a power source of a capacitor bank, large capacity and load voltages in the range of 50 at 500 V. Another possibility is to operate with both types of sources, for example, in a sequential application thereof: first discharge by capacitors, then the welding transformer acts. This last possibility may have the advantage of allowing the boarding of larger pieces, whose electrical resistance is too high to be addressed only by the SRE technique
• El dispositivo mecánico que ejerza la presión debe ser capaz de suministrar la fuerza necesaria para alcanzar presiones en torno a 100 MPa. · La bobina que envuelve a la matriz irá conectada a una fuente adicional de potencia, que puede beneficiarse de los procedimientos habitualmente empleados a tal efecto, por ejemplo, la magnetización por impulsos, que se menciona a efectos ilustrativos, pero no limitativos. Esta bobina será la responsable de aplicar el campo magnético de alineamiento de partículas y de magnetización de la pieza ya consolidada. • The mechanical device that exerts the pressure must be able to supply the force necessary to reach pressures around 100 MPa. · The coil that wraps the matrix will be connected to an additional power source, which can benefit from the procedures commonly used for this purpose, for example, pulse magnetization, which is mentioned for illustrative, but not limiting purposes. This coil will be responsible for applying the magnetic field of particle alignment and magnetization of the already consolidated part.
MODO DE REALIZACIÓN DE LA INVENCIÓN EMBODIMENT OF THE INVENTION
Como ejemplo ilustrativo de la novedad de la invención, se analizará a continuación la fabricación de un tipo concreto de imán, uno de los denominados imanes de cobalto/tierras-raras, de composición nominal SmCo5, (a) mediante el procedimiento convencional (seguida actualmente por la industria) y (b) mediante el método propuesto en esta solicitud de patente. As an illustrative example of the novelty of the invention, the manufacture of a specific type of magnet, one of the so-called cobalt / rare earth magnets, of nominal composition SmCo 5 , will be analyzed below (a) by the conventional procedure (followed currently by industry) and (b) by the method proposed in this patent application.
Los polvos de los que se parte, requieren las mismas características por una vía y por otra. Así, los dos primeros pasos son comunes:  The powders from which it starts, require the same characteristics in one way and another. Thus, the first two steps are common:
1. Se parte de dos polvos, con tamaño medio de partícula de 6 a 8 μιη y distribución granulométrica muy estrecha, y composiciones del 33.8%Sm-66.2%Co en peso (nominalmente, SmCo5), uno de ellos, y del 60%Sm-40%Co en peso, el otro.1. It is based on two powders, with an average particle size of 6 to 8 μιη and very narrow granulometric distribution, and compositions of 33.8% Sm-66.2% Co by weight (nominally, SmCo 5 ), one of them, and 60 % Sm-40% Co by weight, the other.
2. Se toman las cantidades pertinentes de uno y otro polvo de modo que la composición promedio sea del 62.6% Co (en peso). Estos dos polvos son mezclados por agitación, obteniéndose una composición ligeramente más pobre en cobalto que la estequiometría nominal del compuesto SmCo5. 2. The pertinent amounts of both powders are taken so that the average composition is 62.6% Co (by weight). These two powders are mixed by stirring, obtaining a slightly poorer composition in cobalt than the nominal stoichiometry of the SmCo 5 compound.
Procedimiento convencional Conventional procedure
3. La mezcla de polvos es encerrada en una funda de plástico y sometida a alineamiento magnético por acción de un campo magnético del orden de 5 MA/m. 3. The powder mixture is enclosed in a plastic sheath and subjected to magnetic alignment by the action of a magnetic field of the order of 5 MA / m.
4. La mezcla de polvos magnéticamente alineados es llevada a una prensa isostática donde es compactada a la presión aproximada de 1400 MPa para obtener una densidad de 6.9 g/cm3, que representa aproximadamente un 80% del valor absoluto de la densidad del compuesto. 5. El compacto en verde obtenido en la etapa anterior es sinterizado en horno, bajo atmósfera de argón purificado, a una temperatura entre 1090 y 1150 °C, durante un tiempo en torno a los 40 minutos. El compacto densifica hasta alcanzar una densidad de 7.7 g/cm3, lo que representa aproximadamente el 90% de la densidad absoluta. 4. The mixture of magnetically aligned powders is taken to an isostatic press where it is compacted to the approximate pressure of 1400 MPa to obtain a density of 6.9 g / cm 3 , which represents approximately 80% of the absolute value of the density of the compound. 5. The green compact obtained in the previous stage is sintered in the oven, under an atmosphere of purified argon, at a temperature between 1090 and 1150 ° C, for a time of around 40 minutes. The compact densifies to a density of 7.7 g / cm 3 , which represents approximately 90% of the absolute density.
6. Tras el sinterizado en horno, se somete a la pieza a un tratamiento térmico a 900 °C durante 15 minutos, con objeto de mejorar la fuerza coercitiva.  6. After sintering in the oven, the piece is subjected to a heat treatment at 900 ° C for 15 minutes, in order to improve the coercive force.
7. Los compactos son finalmente magnetizados en un campo de ~5 MA/m.  7. The compacts are finally magnetized in a field of ~ 5 MA / m.
8. Eventualmente, el imán es recubierto para reforzar su resistencia a la corrosión.  8. Eventually, the magnet is coated to reinforce its corrosion resistance.
Método objeto de la presente invención Method object of the present invention
Un esquema del equipo necesario podría ser el indicado en la figura 1 :  An outline of the necessary equipment could be the one indicated in Figure 1:
La matriz 1 es eléctricamente aislante (por ejemplo, fabricada con roca natural, hormigón refractario, tubo cerámico y zuncho de metal, etc.).  The matrix 1 is electrically insulating (for example, made of natural rock, refractory concrete, ceramic tube and metal strip, etc.).
Los electrodos 2 serán de alguna aleación de cobre con elevada conductividad (por ejemplo, aleación de Cu-Zr). Para conseguir mayor uniformidad en la temperatura interior, puede ser interesante interponer entre el polvo 3 y el electrodo 2 una oblea 4 de material algo menos conductor, por ejemplo, una pseudoaleación (heavy metal) de Cu-W, que además aportará resistencia a la electroerosión.  The electrodes 2 will be of some copper alloy with high conductivity (for example, Cu-Zr alloy). In order to achieve greater uniformity in the indoor temperature, it may be interesting to interpose a wafer 4 of somewhat less conductive material between the powder 3 and the electrode 2, for example, a Cu-W pseudo-alloy (heavy metal), which will also provide resistance to the EDM
La bobina 5 es la encargada tanto de llevar a cabo el proceso de alineamiento magnético, como el de magnetización final.  The coil 5 is in charge of both the magnetic alignment process and the final magnetization process.
La fuente de potencia 6 puede consistir en un transformador de soldadura (en el caso de la S.R.E.), o un banco de condensadores (en el caso de C.D. E.). Otra posibilidad es operar con ambos tipos de fuentes, por ejemplo, en una aplicación secuencial de las mismas: primero descarga por condensadores, y luego intervención del transformador de soldadura.  The power source 6 may consist of a welding transformer (in the case of the S.R.E.), or a capacitor bank (in the case of C.D. E.). Another possibility is to operate with both types of sources, for example, in a sequential application thereof: first discharge by capacitors, and then intervention of the welding transformer.
El dispositivo mecánico 7 que ejerza la presión debe ser capaz de suministrar la fuerza necesaria para alcanzar presiones para garantizar el nivel de densificación deseado.  The mechanical device 7 that exerts the pressure must be able to supply the force necessary to reach pressures to ensure the desired level of densification.
3. La mezcla de polvos es introducida en la matriz del equipo de consolidación por S.R.E., y allí mismo es magnéticamente alineada por acción de un campo magnético del orden de 5 MA/m. 3. The powder mixture is introduced into the matrix of the consolidation team by S.R.E., and there it is magnetically aligned by the action of a magnetic field of the order of 5 MA / m.
4. Se inicia el proceso de consolidación eléctrica por S.R.E., al aire, con unos parámetros nominales de 80 MPa de presión, una densidad de corriente de -6.5 kA/cm2, y un tiempo de paso de 70 ciclos, de 0.02 s cada ciclo. La densidad final del compacto resulta ser del 90% o superior. 4. The electrical consolidation process is initiated by SRE, on air, with nominal parameters of 80 MPa pressure, a current density of -6.5 kA / cm 2 , and a passage time of 70 cycles, of 0.02 s each cycle. The final density of the compact turns out to be 90% or higher.
5. Concluida la sinterización, y sin retirar el compacto del equipo de S.R.E., se hace pasar una densidad de corriente en torno a 1 - 2 kA/cm2, durante tiempos que oscilan entre 400 ciclos (8 segundos). Se consigue con ello efectuar un tratamiento térmico suave. El compacto es enfriado in situ, por efecto de los electrodos que se hallan refrigerados por agua. 5. After sintering is completed, and without removing the compact from the SRE equipment, a current density of about 1-2 kA / cm 2 is passed, during times ranging from 400 cycles (8 seconds). With this, a gentle heat treatment is achieved. The compact is cooled in situ, due to the effect of electrodes that are cooled by water.
6. Terminado el tratamiento térmico, el compacto es magnetizado sin abandonar la matriz por acción de un campo magnético de valor ~5 MA/m.  6. Once the heat treatment is finished, the compact is magnetized without leaving the matrix by a magnetic field with a value of ~ 5 MA / m.
7. Eventualmente, el imán es recubierto para reforzar su resistencia a la corrosión.  7. Eventually, the magnet is coated to reinforce its corrosion resistance.
Las remanencias, coercitividades y productos energía máximos alcanzados por ambas vías son de 1.0 T, 3024 kA/m y 605 kT A/m, respectivamente. Los datos convencionales han sido tomados de Lenel [F.V. Lenel, "Powder Metallurgy: Principies and Applications", Metal Powder Industries Federation, Princeton, NJ, 1980, p. 531-548]. The maximum retentions, coercivity and energy products reached by both routes are 1.0 T, 3024 kA / m and 605 kT A / m, respectively. Conventional data has been taken from Lenel [F.V. Lenel, "Powder Metallurgy: Principies and Applications", Metal Powder Industries Federation, Princeton, NJ, 1980, p. 531-548].

Claims

Método para la fabricación pulvimetalúrgica de imanes que comprende como etapa inicial la introducción de un material pulverulento que contiene al menos un elemento magnético seleccionado entre Fe, Co, Ni en una matriz, caracterizado porque una vez introducido el material pulverulento en la matriz se somete secuencialmente y siempre en la misma matriz a: Method for powder metallurgical manufacturing of magnets comprising as an initial stage the introduction of a powdery material containing at least one magnetic element selected from Fe, Co, Ni in a matrix, characterized in that once the powdery material has been introduced into the matrix, it is sequentially subjected and always in the same matrix to:
- alineamiento magnético  - magnetic alignment
- proceso de consolidación eléctrica.  - electrical consolidation process.
- tratamiento térmico  - heat treatment
- refrigeración y  - cooling and
- magnetización  - magnetization
Método según la reivindicación 1 , caracterizado porque el material pulverulento contiene adicionalmente al menos un elemento de los denominados tierras raras. Method according to claim 1, characterized in that the powdery material additionally contains at least one element of the so-called rare earths.
Método según una cualquiera de las reivindicaciones 1 a 2, caracterizado porque antes de la introducción en la matriz, el material pulverulento se mezcla con al menos un segundo material pulverulento de composición diferente y que comprende al menos un primer elemento que se selecciona entre Fe, Co, Ni. Method according to any one of claims 1 to 2, characterized in that before the introduction into the matrix, the pulverulent material is mixed with at least a second pulverulent material of different composition and comprising at least a first element selected from Fe, Co, Ni.
Método según la reivindicación 3, caracterizado porque el segundo material pulverulento comprende al menos un segundo elemento que se selecciona entre los denominados tierras raras. Method according to claim 3, characterized in that the second powder material comprises at least a second element that is selected from the so-called rare earths.
Método según una cualquiera de las reivindicaciones 1 a 4, caracterizado porque la etapa de alineamiento magnético se realiza exponiendo el material pulverulento a la acción de un campo magnético. Method according to any one of claims 1 to 4, characterized in that the magnetic alignment step is carried out by exposing the pulverulent material to the action of a magnetic field.
Método según una cualquiera de las reivindicaciones 1 a 5, caracterizado porque el proceso de consolidación eléctrica se realiza mediante la técnica de sinterización por resistencia eléctrica. Method according to any one of claims 1 to 5, characterized in that the electrical consolidation process is carried out by means of the sintering technique by electrical resistance.
7. Método según la reivindicación 6, caracterizado porque el proceso de consolidación eléctrica se realiza mediante la técnica de sinterización por resistencia eléctrica de media frecuencia. 7. Method according to claim 6, characterized in that the electrical consolidation process is carried out by means of the medium frequency electrical resistance sintering technique.
8. Método según una cualquiera de las reivindicaciones 1 a 5, caracterizado porque el proceso de consolidación eléctrica se realiza mediante la técnica de consolidación por descarga eléctrica. 9. Método según una cualquiera de las reivindicaciones 1 a 5, caracterizado porque el proceso de consolidación eléctrica se realiza mediante la aplicación secuencial de las técnicas de consolidación por descarga eléctrica y sinterización por resistencia eléctrica. 10. Método según una cualquiera de las reivindicaciones 1 a 5, caracterizado porque el proceso de consolidación eléctrica se realiza mediante la aplicación secuencial de las técnicas de sinterización por resistencia eléctrica y consolidación por descarga eléctrica. 11. Método según una cualquiera de las reivindicaciones 1 a 10, caracterizado porque el tratamiento térmico del material posterior a la consolidación eléctrica se realiza mediante el paso de corriente eléctrica a través del material. 8. Method according to any one of claims 1 to 5, characterized in that the electrical consolidation process is carried out by the electric shock consolidation technique. 9. Method according to any one of claims 1 to 5, characterized in that the process of electrical consolidation is carried out by sequential application of the techniques of consolidation by electric shock and sintering by electrical resistance. Method according to any one of claims 1 to 5, characterized in that the process of electrical consolidation is carried out by means of the sequential application of the techniques of sintering by electrical resistance and consolidation by electric discharge. Method according to any one of claims 1 to 10, characterized in that the heat treatment of the material after the electrical consolidation is carried out by the passage of electric current through the material.
12. Método según una cualquiera de las reivindicaciones 1 a 1 1 , caracterizado porque la refrigeración del material tras la etapa de tratamiento térmico se lleva a cabo por contacto con los electrodos enfriados mediante un líquido refrigerante. 12. Method according to any one of claims 1 to 1, characterized in that the cooling of the material after the heat treatment stage is carried out by contact with the electrodes cooled by a cooling liquid.
13. Método según cualquiera de las reivindicaciones 1 a 12, caracterizado porque la etapa de magnetización del material se efectúa por acción de un campo magnético que permita alcanzar la saturación magnética del material. 13. Method according to any of claims 1 to 12, characterized in that the magnetization stage of the material is carried out by a magnetic field that allows the magnetic saturation of the material to be achieved.
14. Método según una cualquiera de las reivindicaciones 1 a 13, caracterizado porque las etapas se realizan en condiciones de atmósfera controlada. 15. Método según la reivindicación 14, caracterizado porque las etapas se realizan en atmósfera de gas inerte o en vacío. 14. Method according to any one of claims 1 to 13, characterized in that the steps are carried out under controlled atmosphere conditions. 15. Method according to claim 14, characterized in that the steps are carried out under an inert gas atmosphere or under vacuum.
PCT/ES2014/000223 2013-12-30 2014-12-29 Method for producing magnets using powder metallurgy WO2015101682A1 (en)

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