PROCESS AND APPARATUS FOR PRODUCING GRANULATION POWDER OF RARE EARTH ALLOY AND PROCESS FOR PRODUCING SINTERED OBJECT OF RARE EARTH ALLOY

Description

TECHNICAL FIELD

[0001] 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.

BACKGROUND ART

[0002] A rare-earth alloy sintered magnet (permanent 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. Among other things, 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.

[0003] An R-Fe-B based sintered magnet includes a main phase consisting essentially of a tetragonal R₂Fe₁₄B compound, an R-rich phase including Nd, for example, and a B-rich phase. In the R-Fe-B based sintered magnet, 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.

[0004] United States Patent Application Laid-open Publication Number US 5,116,434 discloses a method for producing a ferromagnetic material such as Re₂Fe₁₄B₁ 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.

[0005] Further, in the prior art, an R-Fe-B based alloy has been prepared as a material for such a magnet by an ingot casting process. In 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.

[0006] Recently, a rapid cooling process such as a strip casting process or a centrifugal casting process has attracted much attention in the art. In a rapid cooling process, 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. According to the rapid cooling process, 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". Thus, in the rapid cooling process, columnar crystals grow in the thickness direction from the roller contact surface. As a result, the rapidly solidified alloy, made by a strip casting process or any other rapid cooling process, has a structure including an R₂Fe₁₄B crystalline phase and an R-rich phase. The R₂Fe₁₄B crystalline phase usually has a minor-axis size of about 0.1 µm to about 100 µm and a major-axis size of about 5 µm to about 500 µm. On the other hand, 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 µm or less, is dispersed on the grain boundary between the R₂Fe₁₄B crystalline phases.

[0007] Compared to an alloy made by the conventional ingot casting process or die casting process (such an alloy will be referred to herein as an "ingot alloy"), the rapidly solidified alloy has been quenched in a shorter time (i.e., at a cooling rate of 10² °C/s to 10⁴ °C/s). Accordingly, the rapidly solidified alloy has a finer texture and a smaller crystal grain size. In addition, in the rapidly solidified alloy, the grain boundary thereof has a greater area and the R-rich phase is dispersed broadly and thinly over the grain boundary. Thus, 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.

[0008] An alternative alloy preparation method called "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.

[0009] It should be noted that 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.

[0010] According to the prior art, 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 µm to 500 µm) by a dry milling process using a jet mill, for example.

[0011] The R-Fe-B based alloy powder to be compacted preferably has a mean particle size of 1.5 µm to about 6 µm to achieve sufficient magnetic properties. It should be noted that the "mean particle size" of a powder refers to herein an FSSS particle size unless stated otherwise. However, when a powder with such a small mean particle size is used, the resultant flowability, compactibility (including cavity fill density and compressibility) and productivity will be bad.

[0012] To overcome this problem, a method for coating the surface of alloy powder particles with a lubricant was proposed. For example, 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 µm to 5 µm) 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 µm to 500 µm and then pulverizing the mixture by a jet mill within an inert gas.

[0013] 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. For example, 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.

[0014] According to this method using a lubricant, however, a certain degree of improvement is achieved but it is still difficult to fill the cavity with the powder sufficiently uniformly or achieve a sufficient degree of compactibility. Among other things, a powder made by a strip casting process or any other rapid quenching process (at a cooling rate of 10² °C/s to 10⁴ °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.

[0015] As another method for improving the flowability and compactibility of an R-Fe-B based alloy powder, there was a proposal to make a granulated powder.

[0016] For example, 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.

[0017] If the granulating agent disclosed in Japanese Patent Application Laid-Open Publication No. 63-237402 is used, however, then the binder removability is so bad that the magnetic properties of an R-Fe-B based sintered magnet will be deteriorated by carbon remaining in the sintered body.

[0018] On the other hand, 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.

[0019] To solve the problem that the granulated powder is difficult to break even under the aligning magnetic field, 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.

[0020] Various granulating agents and granulating methods have been proposed so far as described above. However, a method for mass-producing a rare-earth alloy granulated powder, which has excellent flowability and compactibility and which can contribute to producing magnets with good magnetic properties, has not yet been developed.

[0021] On the other hand, demands for smaller, thinner and performance-enhanced magnets have been escalating. Thus, the development of a method for producing small or thin high-performance magnets with high productivity is awaited. Generally speaking, if a rare-earth alloy sintered body (or a magnet obtained by magnetizing the sintered body) is machined, then its magnetic properties will deteriorate due to a strain caused by the machining process. Such deterioration in magnetic properties is non-negligible in a small magnet. Accordingly, the smaller the size of the magnet to be obtained, the more necessary it is to make a sintered body that has so high dimensional accuracy as to need almost no machining at all and also has the final shape to be obtained. Demands for a rare-earth alloy powder with excellent flowability and compactibility (e.g., an R-Fe-B based alloy powder among other things) have been further growing for these reasons, too.

DISCLOSURE OF INVENTION

[0022] In order to overcome the problems described above, 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.

[0023] 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.

[0024] Preferred embodiments of the present application are subject to the dependent claims.

[0025] In one preferred embodiment, 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.

[0026] In another preferred embodiment, the side surface is arranged spirally and is located on the outer side of the track.

[0027] In another preferred embodiment, the step (b) is carried out after the powder has been subjected to particle sizing.

[0028] In another preferred embodiment, the powder has a mean particle size of 1.5 µm to 6 µm.

[0029] In another preferred embodiment, a granulated powder with a mean particle size of 0.05 mm to 3.0 mm is obtained.

[0030] A method for making a rare-earth alloy sintered body according to the present invention includes the steps as defined in claim 7.

[0031] An apparatus of making a granulated powder according to the present invention has the features as specified in claim 8.

[0032] In one preferred embodiment, 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.

[0033] In another preferred embodiment, the apparatus further includes a particle sizer between the magnetizer and the bowl.

BRIEF DESCRIPTION OF DRAWINGS

[0034] 

FIG. 1(a) schematically illustrates the structure of a granulated powder according to a preferred embodiment of the present invention, and FIGS. 1(b) and 1(c) schematically illustrate the structures of conventional granulated powders for the purpose of comparison.

FIGS. 2(a) and 2(b) illustrate a granulating process step according to a preferred embodiment of the present invention, wherein FIG. 2(a) is a plan view of a track 22 as viewed from over it, and FIG. 2(b) is a cross-sectional view thereof as viewed on the plane B-B' shown in FIG. 2(a).

FIG. 3 schematically illustrates a granulator 120 according to a preferred embodiment of the present invention.

FIG. 4 is a plan view schematically illustrating the configuration of the bowl 120A of the granulator 120 shown in FIG. 3.

FIG. 5 is a perspective view partly in section of the bowl 120A of the granulator 120 shown in FIG. 3.

FIG. 6 schematically illustrates a granulating system 100 according to a preferred embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0035] Hereinafter, a method for making a granulated powder and a method for making a rare-earth alloy sintered body according to preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following description of preferred embodiments, the features of the present invention will be described as being applied to a method for making a sintered magnet of an R-Fe-B based alloy powder prepared by a strip casting process, which exhibits excellent magnetic properties but low flowability. However, the present invention is in no way limited to those specific preferred embodiments. Thus, a rare-earth alloy powder made by any other method may also be used instead.

[0036] A method for making an R-Fe-B based alloy sintered body according to a preferred embodiment of the present invention 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. By magnetizing the resultant sintered body by a known method, an R-Fe-B based sintered magnet can be obtained. It should be noted that the magnetizing process step may be carried out at any arbitrary point in time after the sintering process. For example, 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".

[0037] In a method for making an R-Fe-B based alloy sintered body according to a preferred embodiment of the present invention, 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.

[0038] Hereinafter, the features of a granulated powder making method and a resultant granulated powder according to a preferred embodiment of the present invention will be described with reference to 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. On the righthand side of FIG. 1, illustrated schematically are the states of respective granulated powders to which an aligning magnetic field has been applied in a cavity for the purpose of compaction. More specifically, 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, and 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.

[0039] As shown in FIG. 1(a), in the granulated powder 12a of this preferred embodiment, primary particles 10a with remanent magnetization are weakly coupled together via magnetic agglomeration force. In the illustrated example, 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)). In this granulated powder 12a, the remanent magnetization of the primary particles 10a is oriented at random unlike the granulated powder 12c shown in FIG. 1(c). The primary particles 10a may have a mean particle size of about 1.5 µm to about 6.0 µm 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.

[0040] 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. Also, since 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. Optionally, 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. Thus, the amount and type of the granulating agent may be selected so as not to deteriorate the magnetic properties.

[0041] In contrast, 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). As a result, the magnetic properties of the resultant sintered magnet deteriorate. Compared with a magnet obtained without granulating the material powder at all, 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.

[0042] Furthermore, if 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.

[0043] In contrast, 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.

[0044] 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. Optionally, a granulating agent may be added if necessary.

[0045] In a method for making a granulated powder according to a preferred embodiment of the present invention, 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. However, 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. 1(c) in which a magnetic field needs to be applied continuously to align the primary particles 10c until the granulated powder 12 is definitely fixed with the granulating agent 14. As used herein, 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.

[0046] 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.

[0047] It should be noted that even after the remanent magnetization has been generated, a material powder with low coercivity might lose the magnetization and the shape of the granulated powder before the final granulated powder is obtained. For that reason, the material powder preferably has relatively high coercivity. Specifically, if 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/cm³, 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. For example, 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.

[0048] Considering the flowability and compactibility, the R-Fe-B based alloy powder to be pressed and compacted preferably consists of only the granulated powder prepared as described above. Alternatively, a mixture of the granulated powder and the material powder (i.e., powder of primary particles) may also be used. However, as the percentage of the material powder increases, the flowability decreases. Accordingly, to improve the flowability sufficiently effectively by the granulating technique, the alloy powder preferably consists essentially of the granulated powder alone. Also, when the mixture of the material powder and the granulated powder is used, the surface of the material powder particles is preferably coated with a lubricant. By coating the surface of the primary particles 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".

[0049] Hereinafter, a method for making a magnet from an R-Fe-B based alloy sintered body according to a preferred embodiment of the present invention will be described step by step.

[0050] First, 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). Specifically, 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. In a typical composition of the R-Fe-B based rare-earth alloy, Nd or Pr is usually used as R, a portion of Fe may be replaced with a transition element (e.g., Co), and a portion of B may be replaced with C.

[0051] 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. By decrepitating these alloy flakes by a hydrogen absorption process, an alloy coarse powder is obtained. Then, 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 µm to 6 µm and a specific surface area of about 0.45 m²/g to about 0.55 m²/g as measured by a BET method. This material powder has a true density of 7.5 g/cm³.

[0052] Next, remanent magnetization is generated in the material powder obtained in this manner. In this example, an alternating demagnetizing field with a peak magnetic field of 1.0 T is applied thereto by a magnetizer.

[0053] Subsequently, 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. In the preferred embodiment of the present invention, however, a granulated powder can be obtained by a shaking granulating technique more easily than by the method described in the previous applications.

[0054] A method for making a granulated powder according to this preferred embodiment of the present invention 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.

[0055] This granulating process step will be described with reference to FIGS. 2(a) and 2(b). FIG. 2(a) is plan view of a track 22 as viewed from over it, and FIG. 2(b) is a cross-sectional view thereof as viewed on the plane B-B' shown in FIG. 2(a).

[0056] The material powder with remanent magnetization is granulated while being transported along the track 22 from left to right in FIG. 2(a). As shown in FIG. 2(b), 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. In this preferred embodiment, 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. However, if a linearly extending track is used, for example, its cross-sectional structure may have the structure shown in FIG. 2(b) on the left-hand side, too, so as to be symmetric with respect to the transporting direction (i.e., the direction in which the track extends). Nevertheless, when the track 22 is arranged spirally, a relatively long track 22 can be defined within a rather small area.

[0057] As pointed by the arrows in FIG. 2(b), 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. However, the powder is preferably shaken both horizontally and vertically because the density of the powder can be increased effectively by the vertical vibrations. In addition, 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.

[0058] 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. From the standpoint of granulation efficiency, 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.

[0059] 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.

[0060] On the vibrating track 22, relatively small granulated powder particles 1a gather toward the higher-level portion of the sloped lower surface 22b, while relatively big granulated powder particles 1b gather toward the lower-level portion of the sloped lower surface 22b (i.e., near the side surface 22a). If the granulated powder is unevenly distributed in this manner according to their sizes, then the granulation efficiency will decrease. Thus, by providing a guide surface 22c, which extends from the side surface 22a toward the center of the track 22 and is tilted in the transporting direction as shown in FIG. 2(a), 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). As used herein, 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.

[0061] 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.

[0062] Next, an apparatus 120 for making a granulated powder (which will be referred to herein as a "granulator") according to a preferred embodiment of the present invention will be described with reference to FIGS. 3 through 5.

[0063] As shown in FIG. 3, 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). Thus, 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.

[0064] FIG. 4 schematically illustrates the configuration of the bowl 120A as viewed from over it, and FIG. 5 is a perspective view partly in section of the bowl 120A.

[0065] On the inner surface 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. Also, 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. Due to the presence of that surface tilted in the opposite direction, it is possible to prevent the powder from being collected behind the guide surface, thus improving the efficiency of transportation. Naturally, a baffle with the guide surface 22c such as that shown in FIG. 2 may be used instead of the convex portion 122d.

[0066] Meanwhile, 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.

[0067] 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. To improve the granulation efficiency, the material powder is preferably subjected to particle sizing before being fed into the bowl 120A.

[0068] 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.

[0069] In this case, if the contact resistance (frictional resistance) between the powder with remanent magnetization and the surfaces of the track 122 (i.e., the side surface 122a and the lower surface 122b) or 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. For that reason, the bowl 120A is preferably made of a stainless steel such as mirror-polished SUS and its surface is preferably further coated with urethane. Furthermore, if 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.

[0070] Exemplary specifications of the granulator 120 of this preferred embodiment may be as follows:

• Vertical vibrations: amplitude of 0.3 mm
• Horizontal vibrations: amplitude of 1.5 mm
• Bottom 124 of bowl: diameter of 350 mm
  Tilt angles to horizontal plane:

  8 degrees on bottom, 10 degrees on track

• Protrusion 125: bottom diameter of 100 mm height of 50 mm
• Track 122: width of 35 mm, side surface height of 30 mm, total length of 4,000 mm
• Diameter of outermost track: 560 mm
• Baffles 122c: angle defined by the side surface with respect to transporting direction is 45 degrees, side surface length of 22 mm
• Convex portions 122d: angle defined by the side surface with respect to transporting direction is 45 degrees, side surface length of 30 mm

[0071] According to the granulating method of the present invention, there is no need to control the pressure of the gas surrounding the powder, and therefore, the granulating process step may be carried out at the atmospheric pressure. However, since the rare-earth alloy powder is easily oxidizable, the granulating process step is preferably carried out within an inert gas (e.g., nitrogen or rare gas) atmosphere. For example, 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.

[0072] A granulated powder made from the rare-earth alloy powder described above (having a mean particle size of 1.5 µm to 6 µm) preferably has a mean particle size of 0.05 mm to 3.0 mm. Generally speaking, very few primary particles are included in a granulated powder and the number of tertiary or even higher-order particles contained there is also very small. For that reason, the mean particle size of secondary particles may be regarded as substantially representing that of the granulated powder. In this preferred embodiment, 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. If 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.

[0073] Next, the process step of pressing and compacting the resultant granulated powder is carried out. Before describing it, another advantage of the granulator 120 will be described. Unlike a conventional fluid-bed granulator, 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.

[0074] In the granulating system 100, 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.

[0075] 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.

[0076] 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. Thus, the particle size range of the resultant granulated powder will be limited and no abnormally big granulated powder particles will be created. The aperture size of the mesh may be appropriately defined according to the target particle size of the granulated powder. In this preferred embodiment, however, since the agglomeration force produced by the remanent magnetization is utilized, the particle size of the resultant granulated powder is restricted by the magnitude of the remanent magnetization. Thus, once the aperture size falls out of that range, the effect of the particle sizing would decrease. Particularly if 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.

[0077] Optionally, the mesh may be folded like a bellows, for example. By folding the mesh, the powder sorting processing can be performed more efficiently. To further increase the efficiency of the powder sorting processing, the mesh may be vibrated by connecting the mesh to a vibrating mechanism, for example.

[0078] 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.

[0079] Next, the resultant granulated powder is pressed and compacted, thereby making compacts. In this preferred embodiment, the compacts are made of only the granulated powder. This pressing/compacting process step may be carried out with a known press machine. Typically, a uniaxial press machine for pressing a powder in a die cavity (also called a "die hole") with upper and lower punches is used.

[0080] Then, 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.

[0081] Particularly, in making a small compact, the granulated powder is preferably measured with the cavity to an amount corresponding to the internal volume of the cavity. For example, 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. In this manner, 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.

[0082] After the cavity has been filled with the granulated powder, 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. However, to achieve a sufficient degree of magnetic alignment, 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. While the magnetic field is applied in this manner, the powder is pressed uniaxially between the upper and lower punches at a pressure of 98 MPa, for example. As a result, a compact with a relative density (i.e., the ratio of the compact density to the true density) of 0.5 to 0.6 can be obtained. If necessary, 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.

[0083] Next, 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. Thus, there is no need to separately provide any binder removal process. It should be noted that 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.

[0084] Subsequently, by subjecting the resultant sintered body to an aging treatment at a temperature of about 450 °C to about 800 °C for approximately one to eight hours, 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.

[0085] According to the present invention, a granulated powder with excellent flowability and compactibility is used as described above. Thus, 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.

[0086] In addition, 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. Thus, by applying an aligning magnetic field thereto, 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. Thus, according to the present invention, R-Fe-B based alloy sintered magnets of quality can be manufactured with high productivity.

Examples

[0087] Hereinafter, specific examples of the present invention will be described.

[0088] 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. These alloy flakes were finely pulverized by using a jet mill within an inert gas (e.g., N₂ gas with a gas pressure of 58.8 MPa), thereby making a material powder with a mean particle size of 3 µm. The powders representing Examples Nos. 1 and 2 had coercivities of 60 kA/m and 120 kA/m, respectively.

[0089] Next, 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. 1 and with 2 mass% of isoparaffin used as a granulating agent. The rest angle of that granulated powder, along with the rest angle of the material powder of Example No. 1 itself, is also shown in the following Table 1:

Table 1

Powder to press	Granulated?	Remanent Magnetization	Granulating Agent	Rest angle
Example 1	Yes	Yes	Not added	42 degrees
Example 2	Yes	Yes	Not added	41 degrees
Comp. Ex. 1	Yes	No	Added	44 degrees
Comp. Ex. 2	No	No	Not added	About 52 degrees

[0090] A powder with a large rest angle has bad flowability. Thus, the smaller the rest angle, the higher the flowability. In Comparative Example No. 2, unless the material powder was granulated, the rest angle was as large as about 52 degrees and the flowability was bad. In contrast, in each of Examples Nos. 1 and 2 and Comparative Example Nos. 1 in which the powders were granulated, the rest angle decreased to less than 45 degrees. Among other things, 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.

[0091] 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.

[0092] 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

[0093] As can be seen from Table 2, substantially no difference was sensible between Br of Example No. 1 and Br of Comparative Example No. 2. Thus, the sintered magnet exhibited excellent magnetic properties. Br of Comparative Example No. 1 to which a granulating agent was added was lower than Br of Example No. 1 or Comparative Example No. 2. This is because the granulating agent remained as carbon in the sintered magnet.

[0094] As described above, by making 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.

INDUSTRIAL APPLICABILITY

[0095] 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.

[0096] According to the present invention, the flowability and compactibility of a rare-earth alloy powder can be improved without deteriorating the magnetic properties. Thus, even 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. In addition, 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.

Claims

1. A method for making a rare-earth-iron-boron based alloy granulated powder, the method comprising 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 (22), which is defined by a side surface (22a) and a lower surface (22b) that is sloped so as to decrease its height toward the side surface (22a); and

(c) setting up vibrations on the track (22) to give the powder kinetic energy, thereby transporting the powder along the length of the track (22) 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.

2. The method of claim 1, wherein 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.

3. The method of claim 1 or 2, wherein the side surface is arranged spirally and is located on the outer side of the track.

4. The method of one of claims 1 to 3, wherein the step (b) is carried out after the powder has been subjected to particle sizing.

5. The method of one of claims 1 to 4, wherein the powder has a mean particle size of 1.5 µm to 6 µm.

6. The method of one of claims 1 to 5, wherein a granulated powder with a mean particle size of 0.05 mm to 3.0 mm is obtained.

7. A method for making a rare-earth alloy sintered body, the method comprising the steps of:

making a granulated powder by the method of one of claims 1 to 6;

filling a cavity with a rare-earth alloy powder, including the granulated powder, without applying a demagnetizing field to the granulated powder;

compacting the rare-earth alloy powder including the granulated powder with an aligning magnetic field applied to the alloy powder, thereby making a compact; and

sintering the compact.

8. An apparatus for making a granulated powder according to any of claims 1 to 7, the apparatus comprising:

a track (22), which is defined by a side surface (22a) and a lower surface (22b) that is sloped so as to decrease its height toward the side surface (22a);

a guide surface (22c) extending from the side surface (22a), defining the track (22), toward the center of the track (22), the guide surface being tilted in a transporting direction;

a shaker (120b) for setting up vibrations on the track,

a container for containing a rare-earth-iron-boron based alloy material powder;

a magnetizer including a magnetic circuit for applying a magnetic field to the material powder in the container, adapted for applying an alternating demagnetizing field to said rare-earth iron-boron based alloy powder.

9. The apparatus of claim 8, further comprising a bowl (120a) to receive a rare-earth alloy powder with remanent magnetization,
wherein the track is arranged spirally on an inner surface of the bowl.

10. The apparatus of claim 9, further comprising a particle sizer between the magnetizer and the bowl.

Ansprüche

1. Verfahren zur Herstellung eines granulierten Legierungspulvers auf Seltene-Erden-Eisen-Bor-Basis, wobei das Verfahren die nachfolgenden Schritte umfasst:

(a) Anlegen eines wechselnden entmagnetisierenden Feldes an einem Legierungspulver auf Seltene-Erden-Eisen-Bor-Basis, wodurch man das Legierungspulver mit remanenter Magnetisierung erhält;

(b) Aufbringen des Pulvers auf eine Bahn (22), die durch eine Seitenoberfläche (22a) und eine untere Oberfläche (22b), die derart geneigt ist, dass ihre Höhe hin zu der Seitenoberfläche (22a) abnimmt, definiert ist; und

(c) Bewirken von Vibrationen an der Bahn (22), um dem Pulver kinetische Energie zu verleihen, wodurch das Pulver entlang der Länge der Bahn (22) gefördert wird und das Pulver bei einem magnetischen Feld, das im Wesentlichen gleich 0 ist, granuliert wird, indem eine durch die remanente Magnetisierung des Pulvers bewirkte Agglomerationskraft und ein durch die kinetische Energie bewirkter Durchrüttelvorgang einwirken.

2. Verfahren nach Anspruch 1, wobei Schritt (c) den Schritt enthält, dass das Pulver entlang der Länge der Bahn gefördert wird, während bewirkt wird, dass einige nahe der Seitenoberfläche befindliche Teilchen des Pulvers an der geneigten unteren Oberfläche hinaufsteigen.

3. Verfahren nach Anspruch 1 oder 2, wobei die Seitenoberfläche spiralförmig angeordnet und an der Außenseite der Bahn befindlich ist.

4. Verfahren nach einem der Ansprüche 1 bis 3, wobei Schritt (b) ausgeführt wird, nachdem das Pulver einer Größensortierung unterzogen worden ist.

5. Verfahren nach einem der Ansprüche 1 bis 4, wobei das Pulver eine mittlere Teilchengröße von 1,5 µm bis 6 µm aufweist.

6. Verfahren nach einem der Ansprüche 1 bis 5, wobei man ein granuliertes Pulver mit einer mittleren Teilchengröße von 0,05 mm bis 3,0 mm erhält.

7. Verfahren zur Herstellung eines gesinterten Seltene-Erden-Legierungskörpers, wobei das Verfahren die nachfolgenden Schritte umfasst:

Herstellen eines granulierten Pulvers durch das Verfahren nach einem der Ansprüche 1 bis 6;

Füllen eines Hohlraumes mit einem das granulierte Pulver enthaltenden Seltene-Erden-Legierungspulver ohne Anlegen eines entmagnetisierenden Feldes an dem granulierten Pulver;

Pressen bzw. Verdichten des das granulierte Pulver enthaltenden Seltene-Erden-Legierungspulvers, wobei ein ausrichtendes magnetisches Feld an dem Legierungspulver angelegt ist, wodurch ein Pressling hergestellt wird; und

Sintern des Presslings.

8. Vorrichtung zur Herstellung eines granulierten Pulvers nach einem der Ansprüche 1 bis 7, wobei die Vorrichtung umfasst:

eine Bahn (22), die durch eine Seitenoberfläche (22a) und eine untere Oberfläche (22b), die derart geneigt ist, dass ihre Höhe hin zu der Seitenoberfläche (22a) abnimmt, definiert ist;

eine Führungsoberfläche (22c), die sich von der die Bahn (22) definierenden Seitenoberfläche (22a) hin zur Mitte der Bahn (22) erstreckt, wobei die Führungsoberfläche in einer Förderrichtung gekippt ist;

einen Rüttler (120b) zum Bewirken von Vibrationen an der Bahn,

einen Behälter zum Vorhalten eines Legierungsmaterialpulvers auf Seltene-Erden-Eisen-Bor-Basis;

einen Magnetisierer, der eine Magnetschaltung zum Anlegen eines magnetischen Feldes an dem Materialpulver in dem Behälter enthält und dafür ausgelegt ist, ein wechselndes entmagnetisierendes Feld an dem Legierungspulver auf Seltene-Erden-Eisen-Bor-Basis anzulegen.

9. Vorrichtung nach Anspruch 8, des Weiteren umfassend eine Schale (120a) zum Aufnehmen eines Seltene-Erden-Legierungspulvers mit remanenter Magnetisierung,
wobei die Bahn spiralförmig an einer inneren Oberfläche der Schale angeordnet ist.

10. Vorrichtung nach Anspruch 9, des Weiteren umfassend einen Teilchengrößensortierer zwischen dem Magnetisierer und der Schale.

Revendications

1. Procédé pour réaliser une poudre granulée d'alliage à base de terres rares - fer - bore, le procédé comprenant les étapes consistant à :

(a) appliquer un champ alternatif de démagnétisation à une poudre d'alliage à base de terres rares - fer - bore pour obtenir ladite poudre d'alliage avec une magnétisation rémanente ;

(b) amener la poudre sur une piste (22), qui est définie par une surface latérale (22a) et une surface inférieure (22b) qui est inclinée de façon à diminuer sa hauteur vers la surface latérale (22a) ; et

(c) établir des vibrations sur la piste (22) pour donner de l'énergie cinétique à la poudre, transportant ainsi la poudre le long de la piste (22) et granulant la poudre sous un champ magnétique sensiblement nul en utilisant une force d'agglomération produite par la magnétisation rémanente de la poudre et une action de tonnelage produite par l'énergie cinétique.

2. Procédé selon la revendication 1, dans lequel l'étape (c) comprend l'étape consistant à transporter la poudre le long de la piste tout en faisant en sorte que quelques particules de la poudre, situées près de la surface latérale, remontent la surface inférieure inclinée.

3. Procédé selon la revendication 1 ou 2, dans lequel la surface latérale est agencée en spirale et est située sur le côté extérieur de la piste.

4. Procédé selon l'une des revendications 1 à 3, dans lequel l'étape (b) est effectuée après que la poudre a été soumise à un classement par taille de particule.

5. Procédé selon l'une des revendications 1 à 4, dans lequel la poudre a une taille de particule moyenne de 1,5 µm à 6 µm.

6. Procédé selon l'une des revendications 1 à 5, dans lequel on obtient une poudre granulée ayant une taille de particule moyenne de 0,05 mm à 3,0 mm.

7. Procédé pour réaliser un corps fritté en alliage de terres rares, le procédé comprenant les étapes consistant à :

réaliser une poudre granulée par le procédé de l'une des revendications 1 à 6 ;

remplir une cavité avec une poudre d'alliage de terres rares, incluant la poudre granulée, sans appliquer de champ de démagnétisation à la poudre granulée ;

compacter la poudre d'alliage de terres rares incluant la poudre granulée, un champ magnétique d'alignement étant appliqué à la poudre d'alliage, réalisant ainsi un comprimé ; et

fritter le comprimé.

8. Appareil pour réaliser une poudre granulée selon l'une quelconque des revendications 1 à 7, l'appareil comprenant :

une piste (22), qui est définie par une surface latérale (22a) et une surface inférieure (22b) qui est inclinée de façon à diminuer sa hauteur vers la surface latérale (22a) ;

une surface de guidage (22c) s'étendant depuis la surface latérale (22a), définissant la piste (22), vers le centre de la piste (22), la surface de guidage étant inclinée dans une direction de transport ;

un dispositif d'agitation (120b) pour établir des vibrations sur la piste,

un récipient pour contenir une poudre de matière d'alliage à base de terres rares - fer - bore ;

un dispositif de magnétisation comprenant un circuit magnétique pour appliquer un champ magnétique à la poudre formant matière dans le récipient, conçu pour appliquer un champ alternatif de démagnétisation à ladite poudre d'alliage à base de terres rares - fer - bore.

9. Appareil selon la revendication 8, comprenant en outre un bol (120a) pour recevoir une poudre d'alliage de terres rares ayant une magnétisation rémanente,
dans lequel la piste est agencée en spirale sur une surface intérieure du bol.

10. Appareil selon la revendication 9, comprenant en outre un dispositif de classement par taille de particule entre le dispositif de magnétisation et le bol.

Drawing