EP1642661B1 - Process and apparatus for producing granulation powder of rare earth alloy and process for producing sintered object of rare earth alloy - Google Patents

Process and apparatus for producing granulation powder of rare earth alloy and process for producing sintered object of rare earth alloy Download PDF

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Publication number
EP1642661B1
EP1642661B1 EP04734936A EP04734936A EP1642661B1 EP 1642661 B1 EP1642661 B1 EP 1642661B1 EP 04734936 A EP04734936 A EP 04734936A EP 04734936 A EP04734936 A EP 04734936A EP 1642661 B1 EP1642661 B1 EP 1642661B1
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Prior art keywords
powder
track
rare
granulated
alloy
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German (de)
English (en)
French (fr)
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EP1642661A1 (en
EP1642661A4 (en
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Akira Nakamura
Sumihito Nakashima
Tomoiku Otani
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Proterial Ltd
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Hitachi Metals Ltd
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    • 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
    • B22F9/00—Making metallic powder or suspensions thereof
    • B22F9/02—Making metallic powder or suspensions thereof using physical processes
    • B22F9/04—Making metallic powder or suspensions thereof using physical processes starting from solid material, e.g. by crushing, grinding or milling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C1/00—Making non-ferrous alloys
    • C22C1/11—Making amorphous alloys
    • 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
    • B22F9/00—Making metallic powder or suspensions thereof
    • B22F9/02—Making metallic powder or suspensions thereof using physical processes
    • B22F9/04—Making metallic powder or suspensions thereof using physical processes starting from solid material, e.g. by crushing, grinding or milling
    • B22F2009/045—Making metallic powder or suspensions thereof using physical processes starting from solid material, e.g. by crushing, grinding or milling by other means than ball or jet milling
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C2202/00—Physical properties
    • C22C2202/02—Magnetic
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
    • H01F1/04—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys
    • H01F1/047—Alloys characterised by their composition
    • H01F1/053—Alloys characterised by their composition containing rare earth metals
    • H01F1/055—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5
    • H01F1/057—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B
    • H01F1/0571—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes
    • H01F1/0575—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes pressed, sintered or bonded together
    • H01F1/0577—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes pressed, sintered or bonded together sintered

Definitions

  • the present invention relates to a method and apparatus for making a rare-earth alloy granulated powder and a method for making a rare-earth alloy sintered body.
  • a rare-earth alloy sintered magnet is normally produced by compacting a powder of a rare-earth alloy, sintering the resultant powder compact and then subjecting the sintered body to an aging treatment.
  • Permanent magnets currently used extensively in various applications include rare-earth-cobalt based magnets and rare-earth-iron-boron based magnets.
  • the rare-earth-iron-boron based magnets (which will be referred to herein as "R-Fe-B based magnets", where R is one of the rare-earth elements including Y, Fe is iron, and B is boron) are used more and more often in various electronic appliances. This is because an R-Fe-B based magnet exhibits a maximum energy product, which is higher than any of various other types of magnets, and yet is relatively inexpensive.
  • An R-Fe-B based sintered magnet includes a main phase consisting essentially of a tetragonal R 2 Fe 14 B compound, an R-rich phase including Nd, for example, and a B-rich phase.
  • a portion of Fe may be replaced with a transition metal such as Co or Ni and a portion of boron (B) may be replaced with carbon (C).
  • An R-Fe-B based sintered magnet, to which the present invention is applicable effectively, is described in United States Patents Nos. 4,770,723 and 4,792,368 , for example.
  • United States Patent Application Laid-open Publication Number US 5,116,434 discloses a method for producing a ferromagnetic material such as Re 2 Fe 14 B 1 by preparing a powder with remanent magnetisation, wherein said powder is exposed to a non-alternating magnetizing field. Thereafter, said powder is crushed, directly mixed with a binder and pressed into pellets.
  • an R-Fe-B based alloy has been prepared as a material for such a magnet by an ingot casting process.
  • an ingot casting process normally, rare-earth metal, electrolytic iron and ferroboron alloy as respective start materials are melted by an induction heating process, and then the melt obtained in this manner is cooled relatively slowly in a casting mold, thereby preparing an alloy ingot.
  • a rapid cooling process such as a strip casting process or a centrifugal casting process has attracted much attention in the art.
  • a molten alloy is brought into contact with, and relatively rapidly cooled by, a single chill roller, a twin chill roller, a rotating disk or the inner surface of a rotating cylindrical casting mold, thereby making a solidified alloy, which is thinner than an alloy ingot, from the molten alloy.
  • the solidified alloy prepared in this manner will be referred to herein as an "alloy flake".
  • the alloy flake produced by such a rapid cooling process usually has a thickness of about 0.03 mm to about 10 mm.
  • the molten alloy starts to be solidified from its surface that has been in contact with the surface of the chill roller. That surface of the molten alloy will be referred to herein as a "roller contact surface".
  • a roller contact surface That surface of the molten alloy will be referred to herein as a "roller contact surface”.
  • columnar crystals grow in the thickness direction from the roller contact surface.
  • the rapidly solidified alloy made by a strip casting process or any other rapid cooling process, has a structure including an R 2 Fe 14 B crystalline phase and an R-rich phase.
  • the R 2 Fe 14 B crystalline phase usually has a minor-axis size of about 0.1 ⁇ m to about 100 ⁇ m and a major-axis size of about 5 ⁇ m to about 500 ⁇ m.
  • the R-rich phase which is a non-magnetic phase including a rare-earth element R at a relatively high concentration and having a thickness (corresponding to the width of the grain boundary) of about 10 ⁇ m or less, is dispersed on the grain boundary between the R 2 Fe 14 B crystalline phases.
  • the rapidly solidified alloy has been quenched in a shorter time (i.e., at a cooling rate of 10 2 °C/s to 10 4 °C/s). Accordingly, the rapidly solidified alloy has a finer texture and a smaller crystal grain size.
  • the grain boundary thereof has a greater area and the R-rich phase is dispersed broadly and thinly over the grain boundary.
  • the rapidly solidified alloy also excels in the dispersiveness of the R-rich phase. Because the rapidly solidified alloy has these advantageous features, a magnet with excellent magnetic properties can be made from the rapidly solidified alloy.
  • Ca reduction process (or reduction/diffusion process)” is also known in the art.
  • This process includes the processing and manufacturing steps of: adding metal calcium (Ca) and calcium chloride (CaCl) to either the mixture of at least one rare-earth oxide, iron powder, pure boron powder and at least one of ferroboron powder and boron oxide at a predetermined ratio or a mixture including an alloy powder or mixed oxide of these constituent elements at a predetermined ratio; subjecting the resultant mixture to a reduction/diffusion treatment within an inert atmosphere; diluting the reactant obtained to make a slurry; and then treating the slurry with water. In this manner, a solid of an R-Fe-B based alloy can be obtained.
  • any small block of a solid alloy will be referred to herein as an "alloy block".
  • the "alloy block” may be any of various forms of solid alloys that include not only solidified alloys obtained by cooling a melt of a material alloy (e.g., an alloy ingot prepared by the conventional ingot casting process or an alloy flake prepared by a rapid cooling process such as a strip casting process) but also a solid alloy obtained by the Ca reduction process.
  • an alloy powder to be compacted is obtained by performing the processing steps of: coarsely pulverizing an alloy block in any of these forms by a hydrogen absorption process, for example, and/or any of various mechanical milling processes (e.g., using a disk mill); and finely pulverizing the resultant coarse powder (with a mean particle size of 10 ⁇ m to 500 ⁇ m) by a dry milling process using a jet mill, for example.
  • the R-Fe-B based alloy powder to be compacted preferably has a mean particle size of 1.5 ⁇ m to about 6 ⁇ m to achieve sufficient magnetic properties.
  • mean particle size refers to herein an FSSS particle size unless stated otherwise.
  • the resultant flowability, compactibility (including cavity fill density and compressibility) and productivity will be bad.
  • Japanese Patent Application Laid-Open Publication No. 08-111308 and United States Patent No. 5,666,635 disclose the technique of making an R-Fe-B based alloy fine powder (with a mean particle size of 1.5 ⁇ m to 5 ⁇ m) by adding 0.02 mass% to 5.0 mass% of a lubricant (including at least one liquefied fatty acid ester) to an R-Fe-B based alloy coarse powder with a mean particle size of 10 ⁇ m to 500 ⁇ m and then pulverizing the mixture by a jet mill within an inert gas.
  • a lubricant including at least one liquefied fatty acid ester
  • the lubricant not only improves the flowability and compactibility (or compressibility) of the powder but also functions as a binder for increasing the hardness (or strength) of the compact. Nevertheless, the lubricant may also remain as residual carbon in the sintered body to possibly deteriorate the magnetic properties. Accordingly, the lubricant needs to exhibit good binder removability.
  • Japanese Patent Application Laid-Open Publication No. 2000-306753 discloses, as preferred lubricants with good binder removability, depolymerized polymers, mixtures of a depolymerized polymer and a hydrocarbon solvent, and mixtures of a depolymerized polymer, a low-viscosity mineral oil and a hydrocarbon solvent.
  • a powder made by a strip casting process or any other rapid quenching process (at a cooling rate of 10 2 °C/s to 10 4 °C/s) has a smaller mean particle size and a sharper particle size distribution than a powder made by an ingot casting process, and therefore, exhibits particularly bad flowability. For that reason, the amount of the powder to fill the cavity may sometimes go beyond its allowable range or the in-cavity fill density may become non-uniform. As a result, the variations in the mass or dimensions of the compacts may exceed their allowable ranges or the compacts may crack or chip.
  • Japanese Patent Application Laid-Open Publication No. 63-237402 discloses that the compactibility should be improvable with a granulated powder to be obtained by adding 0.4 mass% to 4.0 mass% of mixture of a paraffin compound (which is liquid at room temperature) and an aliphatic carboxylate to the powder, and mulling and granulating them together.
  • a method in which polyvinyl alcohol (PVA) is used as a granulating agent is also known. It should be noted that the granulating agent, as well as a lubricant, functions as a binder for increasing the strength of the compact.
  • PVA polyvinyl alcohol
  • the granulated powder produced by applying a spray dryer method to PVA has high binding force and therefore is too hard to be broken completely even on the application of an external magnetic field. Accordingly, the primary particles thereof cannot be aligned with the magnetic field sufficiently and no magnets with excellent magnetic properties can be obtained.
  • PVA also has bad binder removability and carbon derived from PVA is likely to remain in the magnets. This problem may be overcome by performing a binder removal process within a hydrogen atmosphere. However, it is still difficult to remove that carbon sufficiently.
  • the applicant of the present application proposed a method for making a granulated powder, in which respective powder particles (i.e., primary particles) aligned with a magnetic field applied are coupled together with a granulating agent, by granulating the material powder with a static magnetic field applied thereto (see Japanese Patent Application Laid-Open Publication No. 10-140202 ). If this granulated powder is used, the magnetic properties are improvable compared with using a granulated powder in which primary particles not aligned with a magnetic field applied are coupled together with a granulating agent. However, it is difficult to align the powder particles being pressed with the magnetic field sufficiently. Consequently, the resultant magnetic properties are lower than a situation where a non-granulated rare-earth alloy powder was used.
  • a primary object of the present invention is to provide a method for making a rare-earth alloy granulated powder, which has good flowability and good compactibility and which makes it possible to produce a magnet with excellent magnetic properties, and a method for making a quality rare-earth alloy sintered body with high productivity.
  • a method for making a rare-earth-iron-boron based alloy granulated powder according to the present invention includes the steps as defined in claim 1.
  • the step (c) includes the step of transporting the powder along the length of the track while making some particles of the powder, located near the side surface, climb the sloped lower surface.
  • the side surface is arranged spirally and is located on the outer side of the track.
  • step (b) is carried out after the powder has been subjected to particle sizing.
  • the powder has a mean particle size of 1.5 ⁇ m to 6 ⁇ m.
  • a granulated powder with a mean particle size of 0.05 mm to 3.0 mm is obtained.
  • a method for making a rare-earth alloy sintered body according to the present invention includes the steps as defined in claim 7.
  • An apparatus of making a granulated powder according to the present invention has the features as specified in claim 8.
  • the apparatus further includes a bowl to receive a rare-earth alloy powder with remanent magnetization, and the track is arranged spirally on an inner surface of the bowl.
  • the apparatus further includes a particle sizer between the magnetizer and the bowl.
  • a method for making an R-Fe-B based alloy sintered body includes the steps of: making an R-Fe-B based alloy powder (which will be referred to herein as a "material powder” or "primary particle powder”); generating remanent magnetization in the material powder; granulating the powder by utilizing agglomeration force produced by the remanent magnetization of the material powder; making a compact by pressing the R-Fe-B based alloy powder, including the granulated powder, with a magnetic field applied thereto; and sintering the compact.
  • an R-Fe-B based sintered magnet can be obtained.
  • the magnetizing process step may be carried out at any arbitrary point in time after the sintering process.
  • the user of the sintered magnet may perform the magnetizing process step just before he or she uses the sintered magnet. Even a non-magnetized one will also be referred to herein as a "sintered magnet”.
  • the powder is granulated by utilizing the agglomeration force produced by the remanent magnetization of the material powder. Accordingly, it is possible to either reduce the amount of a granulating agent to be added or use a binder with low binding force than a conventional one. Furthermore, even the addition of the granulating agent itself may be omitted.
  • FIGS. 1(a), 1(b) and 1(c) illustrate the features of a granulated powder making method and a resultant granulated powder according to a preferred embodiment of the present invention.
  • FIGS. 1(a), 1(b) and 1(c) illustrate the features of a granulated powder making method and a resultant granulated powder according to a preferred embodiment of the present invention.
  • FIGS. 1(a), 1(b) and 1(c) On the left-hand side of FIG. 1 , illustrated schematically are the structures of respective granulated powders.
  • FIG. 1(a) illustrates a granulated powder 12a according to a preferred embodiment of the present invention
  • FIG. 1(b) illustrates a conventional granulated powder 12b for which a granulating agent has been used
  • FIG. 1(c) illustrates a granulated powder 12c obtained by the method described in Japanese Patent Application Laid-Open Publication No. 10-140202 identified above.
  • primary particles 10a with remanent magnetization are weakly coupled together via magnetic agglomeration force.
  • no granulating agent is supposed to be used.
  • These primary particles 10a with remanent magnetization are magnetically coupled together so as to form a magnetic closed circuit, and the remanent magnetization of the granulated powder 12a is very small (e.g., more than about 0 mT and equal to or smaller than about 10 mT (millitesla)).
  • the remanent magnetization of the primary particles 10a is oriented at random unlike the granulated powder 12c shown in FIG.
  • the primary particles 10a may have a mean particle size of about 1.5 ⁇ m to about 6.0 ⁇ m and the granulated powder 12a may have a mean particle size of about 0.05 mm to about 3.0 mm, for example.
  • the remanent magnetization may be measured by inserting a probe of a gauss meter into the granulated powder.
  • This granulated powder 12a has a moderate particle size and an appropriately shape and can exhibit excellent flowability. In addition, this granulated powder 12a also has low remanent magnetization and can be loaded into a cavity easily and uniformly without causing any bridging. Furthermore, these primary particles 10a are just coupled together via the magnetic agglomeration force. Accordingly, as shown on the left-hand side of FIG. 1(a) , the granulated powder 12a can be broken down into the primary particles 10a just as intended by applying an aligning magnetic field (of about 0.1 T to about 0.8 T, for example) thereto. As a result, the primary particles 10a can be aligned with the magnetic field applied.
  • an aligning magnetic field of about 0.1 T to about 0.8 T, for example
  • the granulated powder 12a includes no granulating agent, the amount of carbon included in the sintered body never increases.
  • a magnet obtained by magnetizing a sintered body made from this granulated powder 12a has substantially the same magnetic properties as a magnet obtained without granulating the material powder (with substantially zero remanent magnetization) at all. That is to say, by using the granulated powder of the preferred embodiment of the present invention, the flowability and compactibility can be improved without deteriorating the magnetic properties.
  • it is naturally possible to add a granulating agent for the purpose of increasing the strength of the compact for example. As such a granulating agent is used just as an additional agent, the granulating agent does not have to exhibit strong binding force.
  • the amount and type of the granulating agent may be selected so as not to deteriorate the magnetic properties.
  • the granulated powder 12b obtained by binding the primary particles 10b of the material powder together with a granulating agent 14, cannot be sufficiently broken down even under an aligning magnetic field as shown in FIG. 1(b) .
  • the magnetic properties of the resultant sintered magnet deteriorate.
  • the remanent magnetization of that sintered magnet decreases by about 1% to about 10%. It should be noted that arrows are omitted from the primary particles 10b of the granulated powder 12b shown in FIG. 1(b) because the particles 10b have no remanent magnetization.
  • a granulated powder 12c is obtained by binding and fixing primary particles 10c together with a granulating agent 14 while aligning the primary particles 10c under a static magnetic field as shown in FIG. 1(c) , then the deterioration in magnetic properties can be minimized but the granulated powder 12c cannot be fully broken down into the primary particles 10c. Accordingly, compared with a magnet obtained without granulating the material powder at all, the remanent magnetization of the resultant sintered magnet decreases by about one to several percent. Also, as schematically illustrated in FIG. 1(c) , the granulated powder 12c is elongated in the directions of the magnetic poles, which is disadvantageous in terms of flowability. Furthermore, since the granulated powder 12c has relatively large remanent magnetization, the granulated powder 12c will produce bridging and cannot be loaded into a cavity unless demagnetized once.
  • the granulated powder 12a of the preferred embodiment of the present invention is almost spherical in shape, has too small remanent magnetization to require any demagnetization, and can fill a cavity easily and uniformly. Accordingly, a so-called “measuring and loading technique", in which a predetermined mass of granulated powder is measured in advance and then loaded into a cavity, can be adopted. As described above, the granulated powder 12a of the preferred embodiment of the present invention can exhibit excellent flowability and cavity-filling ability and can contribute to making a sintered magnet substantially without deteriorating the magnetic properties.
  • a granulated powder according to a preferred embodiment of the present invention is obtained by a granulating method including the steps of giving kinetic energy to particles of a material powder with remanent magnetization and allowing the particles to grow under a tumbling action produced by the kinetic energy given.
  • a granulating agent may be added if necessary.
  • the step of generating remanent magnetization in the material powder may be carried out at an arbitrary point in time before the material powder is fed onto the bottom of the apparatus of making the granulated powder.
  • the primary particles 10a of the granulated powder 12a of this preferred embodiment are just coupled together under magnetic agglomeration force produced by the remanent magnetization. Accordingly, the granulated powder 12a is broken down upon the application of an external magnetic field. For that reason, the particles are allowed to grow under a substantially zero magnetic field. This is contrary to the method for making the granulated powder 12c shown in FIG.
  • the "substantially zero magnetic field” refers to a magnetic field which is weak enough to obtain a granulated powder where a magnetic closed circuit has been formed by the remanent magnetization of the powder and to have no effects on the remanent magnetization of the powder while the particles grow due to the tumbling action.
  • the magnetic field to be applied to generate remanent magnetization may be any of various magnetic fields. Since the primary particles may have a little remanent magnetization, an alternating demagnetizing field is used.
  • the material powder preferably has relatively high coercivity.
  • the coercivity value of a material powder which has been loaded into a container so as to have a tap density of 2.0 g/cm 3 , is measured by a BH tracer as the apparent coercivity of the material powder
  • the material powder preferably has a coercivity of at least 60 kA/m, more preferably 70 kA/m or more.
  • an R-Fe-B based alloy preferably includes at least 1.2 mass% of Dy, at least 1 mass% of Tb or at least 1 mass% of Dy and Tb combined.
  • the R-Fe-B based alloy powder to be pressed and compacted preferably consists of only the granulated powder prepared as described above.
  • a mixture of the granulated powder and the material powder i.e., powder of primary particles
  • the alloy powder preferably consists essentially of the granulated powder alone.
  • the surface of the material powder particles is preferably coated with a lubricant.
  • the flowability of the R-Fe-B based powder can be improved and the oxidation of the R-Fe-B based alloy can be prevented as well. Furthermore, in pressing the powder under a magnetic field, the powder particles can also be aligned more easily. It should be noted that not only a powder consisting essentially of a rare-earth alloy alone (possibly with a surface oxide layer) but also a powder, including a granulating agent and/or a lubricant as well as the rare-earth alloy powder and being subjected to the compaction process, will be referred to herein as "rare-earth alloy powders".
  • flakes of an R-Fe-B based alloy are made by a strip casting process (see United States Patent No. 5,383,978 , for example).
  • an R-Fe-B based alloy prepared by a known method, is melted by an induction heating process to obtain a molten alloy.
  • the R-Fe-B based alloy may also have the composition disclosed in United States Patent No. 4,770,723 or No. 4,792,368 .
  • Nd or Pr is usually used as R
  • a portion of Fe may be replaced with a transition element (e.g., Co)
  • a portion of B may be replaced with C.
  • This molten alloy is maintained at 1,350 °C and then rapidly quenched on a single roller under the conditions including a roller peripheral velocity of about 1 m/s, a cooling rate of 500 °C/s and an undercooling of 200 °C, thereby obtaining alloy flakes with a thickness of 0.3 mm.
  • a roller peripheral velocity of about 1 m/s
  • a cooling rate of 500 °C/s
  • an undercooling of 200 °C thereby obtaining alloy flakes with a thickness of 0.3 mm.
  • this alloy coarse powder is finely pulverized by a jet mill within a nitrogen gas atmosphere, thereby obtaining an alloy powder (i.e., material powder) with a mean particle size of 1.5 ⁇ m to 6 ⁇ m and a specific surface area of about 0.45 m 2 /g to about 0.55 m 2 /g as measured by a BET method.
  • This material powder has a true density of 7.5 g/cm 3 .
  • remanent magnetization is generated in the material powder obtained in this manner.
  • an alternating demagnetizing field with a peak magnetic field of 1.0 T is applied thereto by a magnetizer.
  • the material powder with remanent magnetization is granulated.
  • the applicant of the present application described a method for granulating a material powder with remanent magnetization by a fluid-bed granulating technique in Japanese Patent Applications No. 2001-362436 and No. 2002-298621 .
  • a granulated powder can be obtained by a shaking granulating technique more easily than by the method described in the previous applications.
  • a method for making a granulated powder includes the steps of: (a) applying an alternating demagnetizing field to a rare-earth iron-boron based alloy powder to obtain said alloy powder with remanent magnetization; (b) feeding the powder onto a track, which is defined by a side surface and a lower surface that is sloped so as to decrease its height toward the side surface; and (c) setting up vibrations on the track to give the powder kinetic energy, thereby transporting the powder along the length of the track and granulating the powder under a substantially zero magnetic field by utilizing an agglomeration force produced by the remanent magnetization of the powder and a tumbling action produced by the kinetic energy.
  • the step (c) preferably further includes the step of transporting the powder along the length of the track while making some particles of the powder, located near the side surface, climb the sloped lower surface.
  • FIG. 2(a) is plan view of a track 22 as viewed from over it
  • FIG. 2(b) is a cross-sectional view thereof as viewed on the plane B-B' shown in FIG. 2(a) .
  • the track 22 is defined by a side surface 22a and a sloped lower surface 22b that decreases its height toward the side surface 22a.
  • a configuration in which the track 22 is arranged spirally on the inner surface of a bowl is illustrated as an example as will be described in detail later.
  • the side surface 22a is located on one side of the track 22 (i.e., on the outer side of the spiral track) while the lower surface 22b is tilted toward one direction.
  • a linearly extending track is used, for example, its cross-sectional structure may have the structure shown in FIG.
  • the track 22 is shaken both horizontally and vertically.
  • the granulation is done by utilizing the tumbling action produced by the kinetic energy that has been given to the powder by these vibrations and the agglomeration force produced by the remanent magnetization of the powder.
  • the tumbling action is produced mainly by the horizontal vibrations.
  • the powder is preferably shaken both horizontally and vertically because the density of the powder can be increased effectively by the vertical vibrations.
  • the amplitude of the horizontal vibrations also has an effect on the transportation rate. That is to say, the transportation rate can be increased by widening the amplitude of the horizontal vibrations.
  • the amplitudes and frequencies of the horizontal and vertical vibrations, as well as the track length, are appropriately defined with the efficiency of granulation and the transportation rate taken into consideration.
  • the vertical vibrations preferably have an amplitude of at least 0.2 mm, more preferably 0.3 mm or more.
  • the horizontal vibrations preferably have an amplitude of at least 0.5 mm, more preferably 1.0 mm or more, in view of the transportation rate. However, if the amplitude exceeded 2.0 mm, sufficient granulation effects could not be achieved and the flowability might not increase so much as expected.
  • the frequencies of the horizontal and vertical vibrations may fall within the range of 70 Hz to 80 Hz but are not particularly limited.
  • the phase relationship between the horizontal and vertical vibrations is appropriately defined and may be defined so as to achieve elliptical vibrations.
  • the track length is preferably no shorter than 4,000 mm. However, if an apparatus with a short track length is used, substantially the same effects as those obtained by extending the track length can be achieved by performing the granulating process step a number of times.
  • the track length has an effect on the particle size and shape of the granulated powder. Specifically, if the track length were too short, then a sufficiently big granulated powder could not be obtained, the shape of the granulated powder might not be regular enough, and/or the percentage of sufficiently big granulated powder particles might be low.
  • the relatively big granulated powder particles 1b, gathering toward the side surface 22a of the track 22, can be moved upward (i.e., toward the higher-level portion of the lower surface 22b) against the slope of the lower surface 22b. Then, the relatively big granulated powder particles 1b and the relatively small granulated powder particles 1a will be blended together, thus achieving the granulation highly efficiently.
  • the guide surface 22c preferably defines a tilt angle of 30 degrees to 60 degrees with respect to the transporting direction (i.e., the angle defined by a normal to the guide surface 22c with respect to the transporting direction is preferably 120 degrees to 150 degrees).
  • the "transporting direction” refers to the length direction of the track or a tangential direction of the track if the track is winding.
  • the reasons are as follows. Specifically, if the tilt angle of the guide surface 22c were less than 30 degrees, then the granulation effect would be so insufficient as to increase the percentage of small granulated powder particles and the variation in particle size significantly. However, if the tilt angle exceeded 60 degrees, then the efficiency of transportation would decrease, which is an unwanted scenario.
  • the interval between adjacent guide surfaces 22c is appropriately defined according to a combination of the width and length of the track 22 and the transportation rate (i.e., a combination of vibration conditions).
  • the interval between adjacent guide surfaces 22c may be defined at about 80 mm or more. This is because if the interval were shorter than about 80 mm, then the granulation effect would decrease, which is not advantageous. However, if the interval exceeded about 200 mm, then the granulation effect produced by the guide surfaces would decline, which is not beneficial, either.
  • the granulator 120 includes a bowl 120A and a shaker 120B.
  • the shaker 120B may be substantially the same as a known bowl vibrating parts feeder (produced by Shinko Electric Co., Ltd., for example).
  • a known bowl vibrating parts feeder produced by Shinko Electric Co., Ltd., for example.
  • the description of the configuration of the shaker 120B (see Japanese Patent Application Laid-Open Publication No. 2001-114412 , for example) will be omitted herein. Instead, the structure of the bowl 120A will be described below.
  • FIG. 4 schematically illustrates the configuration of the bowl 120A as viewed from over it
  • FIG. 5 is a perspective view partly in section of the bowl 120A.
  • a track 122 which is defined by a side surface 122a and a sloped lower surface 122b that decreases its height toward the side surface 122a, is arranged spirally.
  • guide surfaces which extend from the side surface 122a toward the center of the track 122 and which are tilted in the transporting direction (i.e., corresponding to the guide surfaces 22c shown in FIG. 2 ), are defined by the side surfaces of convex portions 122d. It should be noted that each of those convex portions 122d includes not only the surface tilted in the transporting direction and functioning as the guide surface (i.e., the guide surface 22c shown in FIG. 2 ) and a surface tilted in the opposite direction.
  • baffle with the guide surface 22c such as that shown in FIG. 2 may be used instead of the convex portion 122d.
  • baffles 122c which extend from the inner side of the track 122 (i.e., the higher-level portion of the lower surface 122b) toward the center of the track 122 and which are tilted in the transporting direction, are further provided. These baffles 122c work so as to push the granulated powder, which has been moved by the guide surface of the convex portion 122d upward against the slope of the lower surface 122b, back to the side surface 122a (i.e., toward the lower-level portion of the lower surface 122b) again. By providing these baffles 122c, the granulation effects can be increased and the granulated powder can be flushed more efficiently.
  • the baffles 122c are preferably provided for the inside portion of the spiral track 122. In the arrangement shown in FIG. 4 , the baffles 122c are provided for just 1.5 inner rounds of approximately 3 rounds of track 122.
  • the bowl 120A as a whole has a mortar shape and a rare-earth alloy powder is fed into the bottom 124 at the center of the bowl 120A.
  • a conical protrusion 125 is provided at the center of the bottom 124 and the circular bottom thereof is surrounded with ridge-shaped small protrusions 126, which extend in the tangential directions of the circular bottom, thereby supplying the fed powder onto the track 122 on the inner surface efficiently.
  • the material powder is preferably subjected to particle sizing before being fed into the bowl 120A.
  • the powder that has been fed into the bowl 120A is granulated as already described with reference to FIG. 2 , while climbing the inner surface of the mortar bowl 120A from the bottom thereof and along the spiral track 122, and then transported to an outlet port 128 at the top of the bowl 120A.
  • the outlet port 128 may be connected to a feeder (not shown) for use in the next compacting process step.
  • the powder with remanent magnetization and the surfaces of the track 122 i.e., the side surface 122a and the lower surface 122b
  • the surfaces of the baffles 122c and convex portions 122d were too high, then the powder would adhere to those surfaces to possibly decrease the granulation efficiency. Accordingly, those surfaces to contact with the powder are preferably smooth.
  • the bowl 120A is preferably made of a stainless steel such as mirror-polished SUS and its surface is preferably further coated with urethane.
  • a granulating agent were added to the powder, then the powder would adhere to the surfaces of the bowl 120A easily. Thus, the granulating agent would rather not be used in many cases. It should be noted that even if a granulating agent was added to a powder with no remanent magnetization, it was difficult to obtain a granulated powder by this method.
  • Exemplary specifications of the granulator 120 of this preferred embodiment may be as follows:
  • the granulating process step may be carried out at the atmospheric pressure.
  • the granulating process step is preferably carried out within an inert gas (e.g., nitrogen or rare gas) atmosphere.
  • the overall granulator 120 may be covered with a shield that is filled with a nitrogen gas. The shield does not have to have an airtight structure but may be ventilated with the nitrogen gas, for example.
  • a granulated powder made from the rare-earth alloy powder described above (having a mean particle size of 1.5 ⁇ m to 6 ⁇ m) preferably has a mean particle size of 0.05 mm to 3.0 mm.
  • the mean particle size of secondary particles may be regarded as substantially representing that of the granulated powder.
  • the mean particle size of secondary particles obtained by observing the powder with a microscope, is used as the mean particle size of the granulated powder.
  • the mean particle size of the granulated powder were less than 0.05 mm, then the flowability could not be improved significantly and it would be difficult to obtain a uniform compact with a sufficient density. However, if the mean particle size of the granulated powder exceeded 3 mm, then the cavity fill density would decrease and it should be difficult to obtain a uniform compact with a sufficient density, too. More preferably, the mean particle size of the granulated powder falls within the range of 0.1 mm to 1.5 mm. By using the granulator 120 illustrated herein, a granulated powder with a mean particle size falling within that range of 0.1 mm to 1.5 mm can be obtained efficiently.
  • this granulator 120 can simplify and/or automate the production line of rare-earth alloy compacts for sintered magnets. That is to say, a granulating system 100, including the granulator 120 described above, can be fabricated as schematically shown in FIG. 6 .
  • the granulator 120 described above and a number of machines for performing preprocessing for the granulator 120 are connected together vertically. More specifically, in this granulating system 100, a hopper 130 to receive the material powder, a meter (scale) 140, a magnetizer 150, and a particle sizer 160 are connected together with coupling pipes.
  • the rare-earth alloy material powder is fed into the hopper 130, is measured by the meter 140 to a predetermined mass, and then supplied to the container (with the predetermined mass) of the magnetizer 150.
  • the magnetizer 150 includes a magnetic circuit (not shown). When a predetermined pulse current is supplied to the coil, the magnetizer 150 generates an alternating demagnetizing field, thereby applying remanent magnetization to the material powder in the container.
  • the powder with remanent magnetization is subjected to particle sizing by the particle sizer 160 so as to consist of blocks of a predetermined size, which are then supplied to the bowl 120A of the granulator 120.
  • the particle sizer 160 may be implemented as a mesh (of wires) with an aperture size of 0.5 mm to 1.5 mm.
  • the powder having been sorted into blocks of the predetermined size by the particle sizer 160 and then fed to the granulator 120, is granulated with those blocks used as nuclei.
  • the aperture size of the mesh may be appropriately defined according to the target particle size of the granulated powder.
  • the particle size of the resultant granulated powder is restricted by the magnitude of the remanent magnetization.
  • the effect of the particle sizing would decrease.
  • the aperture size were less than 0.5 mm, then it would be hard for those blocks to function as the nuclei of granulation, and therefore, the granulation efficiency would decrease.
  • the mesh may be folded like a bellows, for example. By folding the mesh, the powder sorting processing can be performed more efficiently.
  • the mesh may be vibrated by connecting the mesh to a vibrating mechanism, for example.
  • the granulated powder is flushed through the outlet port 128 (see FIG. 4 ) of the granulator 120. If this granulated powder is supplied by a feeder, for example, to a feeder box for use in the next compacting process step, then the manufacturing process can be fully automated from the material powder feeding process step through the pressing/compacting process step.
  • the resultant granulated powder is pressed and compacted, thereby making compacts.
  • the compacts are made of only the granulated powder.
  • This pressing/compacting process step may be carried out with a known press machine.
  • a uniaxial press machine for pressing a powder in a die cavity (also called a "die hole") with upper and lower punches is used.
  • the die cavity of the uniaxial press machine is filled with the granulated powder.
  • This process step of filling the cavity with the granulated powder may be carried out by either a filling method using a sieve or a filling method using a feeder box as disclosed in Japanese Patent Gazette for Opposition No. 59-40560 , Japanese Patent Application Laid-Open Publication No. 10-58198 , Japanese Utility Model Application Laid-Open Publication No. 63-110521 and Japanese Patent Application Laid-Open Publication No. 2000-248301 . These two methods are sometimes called "dropping methods" collectively.
  • the granulated powder is preferably measured with the cavity to an amount corresponding to the internal volume of the cavity.
  • a feeder box having an opening at the bottom may be shifted to over the cavity to let the granulated powder drop due to its own gravity (i.e., by itself), and then the excess of the granulated powder loaded into the cavity is sliced off.
  • a predetermined amount of granulated powder can be loaded relatively uniformly. It is naturally possible to fill the cavity with a separately measured granulated powder using a funnel, for example.
  • the upper punch of the uniaxial press machine is lowered. With the cavity closed in this manner, an aligning magnetic field is applied to the powder, thereby breaking down the granulated powder into primary particles and also aligning those primary particles with the magnetic field applied.
  • the granulated powder of this preferred embodiment of the present invention can be broken down into primary particles just as intended with the application of a relatively weak magnetic field of 0.1 T to 0.8 T.
  • the magnetic field applied preferably has a strength of about 0.5 T to about 1.5 T.
  • the magnetic field direction may be perpendicular to the pressing direction, for example.
  • the powder is pressed uniaxially between the upper and lower punches at a pressure of 98 MPa, for example.
  • a compact with a relative density i.e., the ratio of the compact density to the true density
  • the magnetic field direction may be parallel to the pressing direction.
  • the granulated powder obtained by the process of the present invention has an adequate strength, i.e., too strong to be broken in the filling process step but weak enough to be broken down into primary particles with the application of the aligning magnetic field.
  • the resultant compact is sintered at a temperature of about 1,000 °C to about 1,180 °C for approximately one to six hours within either a vacuum or an inert gas atmosphere.
  • the granulated powder of this preferred embodiment includes either no granulating agent at all or just a small amount of granulating agent, if any, which is small enough to be substantially eliminated by the sintering process.
  • a typical conventional binder removal process is carried out at a temperature of about 200 °C to about 800 °C for approximately three to six hours within an inert gas atmosphere at a pressure of about 2 Pa.
  • an R-Fe-B based sintered magnet can be obtained. Thereafter, the magnet is magnetized at an arbitrary stage, thereby completing an R-Fe-B based sintered magnet.
  • a granulated powder with excellent flowability and compactibility is used as described above.
  • the cavities can be filled with such a granulated powder uniformly with the variation in fill density reduced. Accordingly, compacts obtained by the compaction process have a reduced variation in mass or size. Furthermore, those compacts rarely crack or chip.
  • the primary particles of the granulated powder of this preferred embodiment are just coupled together substantially due to the magnetic agglomeration force produced by the remanent magnetization.
  • the powder can be broken down into the primary particles just as intended. Accordingly, the degree of magnetic alignment of the primary particles never drops. Furthermore, the deterioration in magnetic properties, which would otherwise be caused if the carbon atoms of a granulating agent remained in the sintered body, can be minimized, too. Consequently, a sintered magnet with excellent magnetic properties can be obtained.
  • R-Fe-B based alloy sintered magnets of quality can be manufactured with high productivity.
  • An R-Fe-B based alloy powder was made in the following manner.
  • a molten alloy was prepared by using ferroboron alloy including electrolytic iron with a purity of 99.9% and 19.8 mass% of B, and Nd and Dy with purity of 99.7% or more as respective start materials. Flakes of an R-Fe-B based alloy, having a composition including 34.0 mass% of Nd, 1.0 mass% of Dy, 1.0 mass% of B and Fe as the balance, and flakes of another R-Fe-B based alloy, having a composition including 30.0 mass% of Nd, 5.0 mass% of Dy, 1.0 mass% of B and Fe as the balance, were obtained as Examples Nos. 1 and 2, respectively, from this molten alloy by a strip casting process.
  • alloy flakes were finely pulverized by using a jet mill within an inert gas (e.g., N 2 gas with a gas pressure of 58.8 MPa), thereby making a material powder with a mean particle size of 3 ⁇ m.
  • an inert gas e.g., N 2 gas with a gas pressure of 58.8 MPa
  • the powders representing Examples Nos. 1 and 2 had coercivities of 60 kA/m and 120 kA/m, respectively.
  • remanent magnetization was generated in the material powders of these specific examples by applying an alternating demagnetizing field (with a peak magnetic field of 1.0 T) thereto. Thereafter, granulated powders with a mean particle size of 0.3 mm were obtained by using the granulator 120 described above without adding any granulating agent. Each of the granulated powders thus obtained had a remanent magnetization of about 0.2 mT. The rest angles measured for the respective granulated powders are shown in the following Table 1. Another granulated powder was prepared as Comparative Example No. 1 by a tumbling granulating technique with no remanent magnetization generated in the material powder of Example No.
  • a powder with a large rest angle has bad flowability.
  • the smaller the rest angle the higher the flowability.
  • the rest angle was as large as about 52 degrees and the flowability was bad.
  • the rest angle decreased to less than 45 degrees.
  • the granulated powders representing Examples Nos. 1 and 2 had smaller rest angles and exhibited better flowability than the powder to be pressed representing Comparative Example No. 1. That is to say, it can be seen that by taking advantage of remanent magnetization, the flowability can be improved even without using any granulating agent.
  • Each of the powders to be pressed shown in Table 1 was loaded into a cavity with a length of 20 mm, a width of 15 mm and a depth of 10 mm by the method using a feeder box as described above and then pressed and compacted uniaxially (under a pressure of 98 MPa and with an aligning magnetic field of 1.3 T applied perpendicularly to the pressing direction). These loading and compacting process steps were carried out under the same conditions for all of the examples of the present invention and comparative examples. It should be noted that compacts with various compact densities (i.e., green densities) were obtained with the pressing conditions changed.
  • various compact densities i.e., green densities
  • Each of the resultant compacts was sintered at 1,060 °C for approximately four hours within an Ar atmosphere, and then subjected to an aging treatment at 500 °C for one hour, thereby obtaining a sintered body. Thereafter, this sintered body was further magnetized at 2,387 kA/m to obtain a sintered magnet. 50 samples were obtained for each of the examples of the present invention and comparative examples.
  • the remanences B r (T) of the resultant sintered magnets are shown in the following Table 2: Table 2 Br (T) Example 1 1.36 Example 2 1.27 Comparative example 1 1.33 Comparative example 2 1.36
  • a granulated powder by utilizing the magnetic agglomeration force produced by the remanent magnetization of primary particles, even if no granulating agent is used, at least the same degree of flowability is achieved compared with a conventional granulated powder to which a granulating agent is added. Accordingly, a sintered magnet exhibiting better magnetic properties can be produced with at least similar productivity compared with the conventional one. Furthermore, if a granulated powder is produced by utilizing only the remanent magnetization of primary particles, deterioration in magnetic properties can be substantially eliminated.
  • the present invention provides a method for making a rare-earth alloy granulated powder, which has good flowability and good compactibility and which makes it possible to produce a magnet with excellent magnetic properties.
  • a method for making a high quality rare-earth alloy sintered body with high productivity by using such a granulated powder is provided.
  • the flowability and compactibility of a rare-earth alloy powder can be improved without deteriorating the magnetic properties.
  • a sintered magnet which should have too intricate a shape to be pressed and compacted easily and which should have sacrificed its magnetic properties to a certain degree in the prior art, can also have improved magnetic properties.
  • the granulating time can be shortened and the binder removal process can be omitted. As a result, the productivity of rare-earth sintered magnets can be increased.

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Abstract

本発明の造粒粉の製造方法は、残留磁化を有する希土類合金の粉末を用意する工程と、側面２２ａと側面に向かって低くなるように傾斜した底面２２ｂとによって規定されるトラック２２に粉末を供給する工程と、トラックを振動させることによって粉末に運動エネルギーを与え、粉末をトラックの長さ方向に移送しながら、粉末の残留磁化による凝集力と、運動エネルギーによる転動作用とを利用して、実質的にゼロ磁界下で造粒する工程とを包含する。その結果、流動性やプレス成形性に優れ、且つ、優れた磁気特性を有する磁石を製造することが可能な希土類合金の造粒粉が得られる。
EP04734936A 2003-05-27 2004-05-26 Process and apparatus for producing granulation powder of rare earth alloy and process for producing sintered object of rare earth alloy Expired - Lifetime EP1642661B1 (en)

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PCT/JP2004/007586 WO2004105982A1 (ja) 2003-05-27 2004-05-26 希土類合金の造粒粉の製造方法および製造装置ならびに希土類合金焼結体の製造方法

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US5116434A (en) 1987-06-19 1992-05-26 Ovonic Synthetic Materials Company, Inc. Method of manufacturing, concentrating, and separating enhanced magnetic parameter material from other magnetic co-products
JPH01114005A (ja) * 1987-10-28 1989-05-02 Fuji Elelctrochem Co Ltd 永久磁石粉体の造粒方法
JPH06262055A (ja) * 1993-03-09 1994-09-20 Daiwa Kasei Kogyo Kk 合着団塊粒の製造法及び装置
EP0775363A1 (de) * 1995-06-14 1997-05-28 Institut für Festkörper- und Werkstofforschung Dresden e.V. Verfahren zur herstellung hartmagnetischer teile
JPH10120148A (ja) * 1996-10-22 1998-05-12 Shinko Electric Co Ltd 部品整送装置
US6478890B2 (en) * 1997-12-30 2002-11-12 Magnequench, Inc. Isotropic rare earth material of high intrinsic induction
JP4081846B2 (ja) * 1998-03-24 2008-04-30 神鋼電機株式会社 部品整送装置
US6302939B1 (en) * 1999-02-01 2001-10-16 Magnequench International, Inc. Rare earth permanent magnet and method for making same
JP2002298621A (ja) 2001-01-11 2002-10-11 Denki Kogyo Kk 回転表示装置
JP4089212B2 (ja) * 2001-11-28 2008-05-28 日立金属株式会社 希土類合金の造粒粉の製造方法および希土類合金焼結体の製造方法
US7622010B2 (en) * 2001-11-28 2009-11-24 Hitachi Metals, Ltd. Method and apparatus for producing granulated powder of rare earth alloy and method for producing rare earth alloy sintered compact
JP4240988B2 (ja) * 2002-10-11 2009-03-18 日立金属株式会社 希土類合金の造粒粉の製造方法、希土類合金の造粒粉の製造装置および希土類合金焼結体の製造方法

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CN101811191B (zh) 2011-09-21
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WO2004105982A1 (ja) 2004-12-09
JP4910393B2 (ja) 2012-04-04
EP1642661A4 (en) 2007-10-10
CN1717290A (zh) 2006-01-04
DE602004022051D1 (de) 2009-08-27
CN101811191A (zh) 2010-08-25
CN1717290B (zh) 2010-08-11

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