A sintered magnet (permanent magnet) of a rare earth alloy is generally produced by compacting powder of the rare earth alloy, sintering the resultant powder compact and subjecting the sintered body to aging. At present, two types of magnets, samarium-cobalt magnets and neodymium-iron-boron magnets, are extensively used in various fields. Among others, neodymium-iron-boron magnets (hereinafter, referred to as “R—Fe—B magnets” where R is any of the rare earth elements including Y, Fe is iron and B is boron) are higher in maximum energy product than any of other various types of magnets, and yet relatively inexpensive. Therefore, the R—Fe—B magnets find positive applications to various types of electronic appliances.
The R—Fe—B sintered magnet is essentially composed of a major phase of an R2Fe14B compound of tetragonal system, an R-rich phase made of Nd and the like, and a B-rich phase. A transition metal such as Co and Ni may substitute for part of Fe, and carbon (C) may substitute for part of boron (B). R—Fe—B sintered magnets to which the present invention is suitably applied are described in U.S. Pat. Nos. 4,770,723 and No. 4,792,368, for example.
To prepare an R—Fe—B alloy from which the magnet described above is produced, an ingot casting process has been conventionally employed. In a normal ingot casting process, rare earth metal, electrolytic iron and a ferroboron alloy as starting materials are subjected to high-frequency melting, and the resultant melt is cooled relatively slowly in a casting mold, to thereby obtain an alloy ingot.
Recently, a rapid cooling process such as a strip casting process and a centrifugal casting process has attracted much attention in the art. In the rapid cooling process, a molten alloy is brought into contact with a single roll, a twin roll, a rotating disk, the inner wall of a rotating cylindrical casting mold or the like, to permit comparatively rapid cooling of the molten alloy and in this way, prepare a solidified alloy thinner than an alloy ingot from the molten alloy (hereinafter, such a solidified alloy is called “alloy flakes”). The alloy flakes prepared by such a rapid cooling process normally have a thickness in the range of about 0.03 mm to about 10 mm. According to the rapid cooling process, the molten alloy starts to be solidified from a surface thereof brought into contact with the chill roll (roll contact surface), and crystals grow in a columnar shape in the thickness direction from the roll contact surface. The resultant rapidly solidified alloy, prepared by the strip casting process or the like, has a structure essentially composed of an R2Fe14B crystalline phase, having 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, and an R-rich phase dispersed in the grain boundary between the R2Fe14B crystalline phases. The R-rich phase is a non-magnetic phase including a rare earth element R in a relatively high concentration, and has a thickness (corresponding to the width of the grain boundary) of about 10 μm or less.
Compared to an alloy prepared by the conventional ingot casting process (die casting process) (hereinafter, such an alloy is called an “ingot alloy”), the rapidly solidified alloy has been cooled in a relatively shorter time (cooling rate: 102° C./sec to 104° C./sec). Accordingly, the rapidly solidified alloy has features that the structure is fine and the crystal grain size is small. In addition, the area of the grain boundary is large and the R-rich phase is dispersed broadly in the grain boundary. Therefore, the rapidly solidified alloy has another feature of excelling in the dispersiveness of the R-rich phase. By using the rapidly solidified alloy having these features, a magnet with excellent magnetic properties can be produced.
An alloy preparation method called a Ca reduction process (or reduction-diffusion process) is also known. This process includes the following steps. First, metal calcium (Ca) and calcium chloride (CaCl) are added to either a mixed powder including at least one kind of rare earth oxide, iron powder, pure boron powder and at least one kind of ferroboron powder and boron oxide at a predetermined ratio or a mixed powder including alloy powders or mixed oxides of these constituent elements at a predetermined ratio. The resultant mixture is subjected to reduction-diffusion treatment in an inert atmosphere. The resultant reaction product is put into a slurry state, and the slurry is then treated with water, to thereby obtain a solid of an R—Fe—B alloy.
A block of a solid alloy is herein called an “alloy block”. The “alloy block” will be any of various forms of solid alloys including not only solidified alloys obtained by cooling melts of an alloy ingot prepared by the conventional ingot casting process, alloy flakes prepared by the rapid cooling process such as the strip casting process and the like, but also a solid alloy prepared by the Ca reduction process.
An alloy powder to be compacted is obtained by coarsely pulverizing an alloy block in any form by a hydrogen pulverization process, for example, and/or any of various mechanical milling processes (for example, using a disk mill), and finely pulverizing the resultant coarse powder (mean particle size: 10 μm to 500 μm, for example) by a dry milling process using a jet mill, for example.
The R—Fe—B alloy powder to be compacted should preferably have a mean particle size of 1.5 μm to 5 μm from the standpoint of the resultant magnetic properties. “The mean particle size” of a powder herein refers to a mass median diameter (MMD) unless otherwise specified. A powder having such a small mean particle size is however poor in flowability and compactibility (including cavity loading capability and compressibility), and thus poor in productivity.
To solve the above problem, coating alloy powder particles with a lubricant has been examined. For example, Japanese Laid-Open Patent Publication No. 08-111308 and its corresponding U.S. Pat. No. 5,666,635 (Assignee: Sumitomo Special Metals Co., Ltd.) disclose the following technique. A lubricant of at least one kind of fatty ester in a liquid form is added in an amount of 0.02 mass % to 5.0 mass % to a rough powder of an R—Fe—B alloy having a mean particle size of 10 μm to 500 μm, and the resultant mixture is pulverized with a jet mill using an inert gas, to prepare a fine powder having a mean particle size of 1.5 μm to 5 μm.
A lubricant improves the flowability and compactibility of powder, and also functions as a binder for imparting rigidity (strength) to a compact. The lubricant however remains in a sintered body as residual carbon causing degradation of magnetic properties, and is therefore required to have good removability. For example, Japanese-Laid Open Patent Publication No. 2000-306753 discloses, as a lubricant having good removability, a depolymerized polymer, a mixture of a depolymerized polymer and a hydrocarbon-based solvent, and a mixture of a depolymerized polymer, a low-viscosity mineral oil and a hydrocarbon-based solvent.
Use of a lubricant described above contributes to some degree of improvement, but fails in imparting sufficient compactibility. In particular, a powder prepared by the strip casting process, which is not only small in mean particle size but also narrow in particle size distribution, is especially poor in flowability. This causes problems such that the amount of powder loaded in a cavity tends to vary beyond an acceptable range, and that the loading density in the cavity tends to lack uniformity. As a result, the mass and size of the resultant compact may vary beyond an acceptable range, and chips and fractures may be formed in the compact.
As another method for improving the flowability and compactibility of a R—Fe—B alloy powder, use of a granulated powder has been attempted.
For example, Japanese Laid-Open Patent Publication No. 63-237402 discloses that compactibility can be improved by use of a granulated powder obtained by adding a mixture of a paraffin mixture that is in the liquid state at room temperature and an aliphatic carboxylic acid in an amount of 0.4 to 4.0 mass % with respect to a powder, and kneading and then granulating the resultant mixture. A method using polyvinyl alcohol (PVA) as a granulating agent is also known. Like the lubricant, the granulating agent functions as a binder imparting strength to a compact.
The granulating agent disclosed in Japanese Laid-Open Patent Publication No. 63-237402 described above is poor in removability. Therefore, in the case of production of an R—Fe—B sintered magnet, the magnetic properties disadvantageously degrade due to carbon remaining in a sintered body.
In the method using PVA, a granulated powder prepared by a spray dryer process using PVA has strong binding force. The resultant granulated powder is too rigid to disintegrate under application of an external magnetic field. Therefore, alloy particles (crystals) fail to be sufficiently aligned in the magnetic field, and as a result, no anisotropic magnet excellent in magnetic properties is obtainable.
PVA is also poor in removability. Therefore, carbon derived from PVA tends to remain in the resultant magnet, causing degradation in magnetic properties. Debindering may be performed in a hydrogen atmosphere. Even with this treatment, however, it is difficult to sufficiently remove the carbon. Also, due to the excessively strong binding force of PVA, the granulated powder fails to disintegrate under application of a magnetic field and therefore finds difficulty in being aligned.
As described above, although various granulating agents have been examined so far, there has not yet been succeeded in development of a granulating agent that has moderate binding force and is excellent in removability. Under this circumstance, a method permitting industrial-scale production of a granulated powder suitably usable for production of an R—Fe—B sintered body has not yet been attained.
Needs for smaller/thinner magnets with higher performance have grown. In this situation, development of a method permitting production of small/thin magnets with high performance with high production efficiency is desired. In general, when an R—Fe—B alloy sintered body (or a magnet obtained by magnetizing the alloy sintered body) is machined, the magnetic properties of the machined product degrade due to machining strain. This degradation of the magnetic properties is not negligible for small magnets. In view of this, as the magnet is smaller, it is desired more strongly that a sintered body in a final use shape should be produced with a level of size accuracy high enough to substantially require no machining. Under this circumstance, also, the demand for an R—Fe—B alloy powder material excellent in flowability and compactibility has been increasingly intensified.