A rare-earth alloy sintered magnet is made by compacting a magnetic powder that has been obtained by pulverizing a rare-earth alloy, and then subjecting the product to a sintering step and an aging step. Currently, two types of rare-earth alloy sintered magnets are widely used in various fields: samarium-cobalt magnets and neodymium-iron-boron magnets. Particularly, neodymium-iron-boron magnets (hereinafter referred to as “R—Fe—B magnets”, wherein R denotes a rare-earth element and/or Yttrium, Fe denotes iron, and B denotes boron.) have been actively employed in various electronic devices because they exhibit the highest magnetic energy product among various magnets and are relatively inexpensive. An R—Fe—B magnet is primarily composed of a major phase of an R2Fe14B tetragonal compound, an R-rich phase of Nd, or the like, and a B-rich phase. Part of Fe may be substituted with a transitional metal such as Co or Ni, and part of B may be substituted with C.
In the prior art, such a rare-earth alloy has been made by an ingot casting method in which a material molten alloy is put in a mold and cooled at a relatively slow rate. An alloy made by the ingot casting method is crushed and pulverized through a known pulverization process. The obtained alloy powder is then compacted by any of various powder compacting apparatuses, and then transferred into a sintering chamber, where the compact (green compact) of the alloy powder undergoes a sintering step.
In recent years, rapid cooling methods such as a strip casting method and a centrifugal casting method have been attracting public attention, in which a molten alloy is contacted with a single roll, a pair of rolls, a rotating disc, a rotating cylindrical mold, or the like, so as to be cooled at a relatively high rate, thereby making a solidified alloy that is thinner than an alloy ingot. The rapidly cooled alloy thus obtained has a thickness of 0.03–10 mm. In an exemplary rapid cooling process, a chill roll in contact with a molten alloy is rotated so that the molten alloy is picked up by the roll in the form of a thin sheet on the roll surface. The solidification of the sheet of molten alloy on the chill roll starts from the plane along which the molten alloy contacts the chill roll (“roll contact plane”), wherein a columnar crystal starts growing from the roll contact plane in a direction perpendicular to the roll contact plane. As a result, a rapidly cooled alloy made by a strip casting method, or the like, has a composition containing an R2T14B crystal phase (wherein T denotes iron and/or a transition metal element substituting part of iron with Co, or the like) whose size in the short axis direction is between 0.1 μm and 100 μum and whose size in the long axis direction is between 5 μm and 500 μm, and an R-rich phase that exists dispersed along the grain boundaries of the R2T14B crystal phase. The R-rich phase is a non-magnetic phase having a relatively high concentration of rare-earth element R, and has a thickness (equivalent to the width of the grain boundary) less than or equal to 10 μm.
A rapidly cooled alloy is made at a higher cooling rate (102–104° C./sec) as compared with an alloy ingot made by a conventional ingot casting method (mold casting method), and therefore has advantageous characteristics such as a fine structure and a small crystal grain diameter. A rapidly cooled alloy is also advantageous in that it has a desirable R-rich phase dispersion as it has a large grain boundary area and the R-rich phase can exist thinly dispersed along the grain boundaries.
However, a magnetic powder of a rapidly-cooled alloy such as a strip-cast alloy is easily oxidized. It is believed that this is because the R-rich phase, which is easily oxidized, is likely to appear on the grain surface of a powder of a rapidly-cooled alloy. A powder of a rapidly-cooled alloy is very easily heated and ignited. Even if oxidization stops short of igniting the powder, the magnetic properties of the powder deteriorate significantly due to the oxidization.
While the heating and ignition of the rare-earth component due to oxidization occur also when compacting a rare-earth alloy powder that has been made by a conventional ingot casting method, the problem is more pronounced when compacting a powder of a rapidly-cooled alloy such as a strip-cast alloy.
In addition to the problem described above, the oxidization of a rare-earth alloy powder also causes a problem as follows.
It is known that the magnetic properties of an R—Fe—B magnet can be improved by increasing the content of the major phase, i.e., the R2Fe14B tetragonal compound. While a minimum amount of R-rich phase is required for a liquid phase sintering process, R also reacts with oxygen to produce an oxide, R2O3, whereby part of R is consumed for a purpose that has no contribution to sintering. Accordingly, an extra amount of R is required for the consumption by oxidization. The production of the oxide R2O3 increases as the amount of oxygen in the powder-making atmosphere increases. In view of this, attempts have been made in the prior art to reduce the amount of oxygen in the powder-making atmosphere and to reduce the relative amount of R in the final R—Fe—B magnet product, thereby improving the magnetic properties thereof.
Although it is preferred to reduce the amount of oxygen in a rare-earth alloy powder that is used to produce an R—Fe—B magnet, as described above, the method of reducing the amount of oxygen in a rare-earth alloy powder to improve the magnet properties has not been realized as a mass-producing technique for the following reason. When an R—Fe—B alloy powder is made under a controlled environment with a reduced oxygen concentration so that the amount of oxygen in the alloy powder is reduced to be less than or equal to 4000 mass parts per million (ppm), for example, the powder may violently react with the oxygen in the atmosphere and may ignite within a few minutes at room temperature. Thus, although it was understood that it would be preferred to reduce the amount of oxygen in the rare-earth alloy powder in order to improve the magnetic properties thereof, it was actually difficult to handle a rare-earth alloy powder with such a reduced oxygen concentration at a manufacturing site such as a plant.
Particularly, in a pressing step for compacting a powder, the temperature of the compact increases due to the frictional heat that is generated between powder particles being compacted and/or the frictional heat that is generated between the powder and the inner wall of the cavity when the compact is taken out of the cavity, thereby increasing the risk of ignition.
It has been proposed in the art to perform a compaction process in an inert gas atmosphere in order to suppress such an oxidization as disclosed in, for example, Japanese Laid Open Patent Publication No. 6-346102,which describes providing an airtight gas chamber which accommodates at least compacting apparatus including a pressing section and a powder supply section for supplying a powder to a powder feeding device.
However, the conventional compacting apparatus is uneconomical because the gas chamber has a relatively large volume, thereby requiring a large amount of inert gas to fill the gas chamber. In the conventional compacting apparatus, the inert gas is not supplied directly to the rare-earth alloy powder, and the space around the passageway via which the rare-earth alloy powder (or the compact) is transferred (e.g., the space around the powder feeding device) is also exposed to a high concentration of inert gas, thereby failing to effectively utilize the inert gas.
Moreover, in cases where the inside of the gas chamber is frequently exposed to the air atmosphere (e.g., where die replacement is frequently needed for making various types of compacts), the use of the conventional apparatus significantly reduces the productivity as it requires a long period of time for substituting the gas in the gas chamber with an inert gas each time a die is replaced by another.
Moreover, although the pressing step with a compacting apparatus is automated, the compacting apparatus requires frequent maintenance, and such maintenance often requires a human operator. If the compacting apparatus is placed in an inert atmosphere, an operator who comes close to the compacting apparatus for trouble shooting may suffer from atmospheric hypoxia. For these and other reasons, placing the entire compacting apparatus in an inert atmosphere is not a practical approach.
In the prior art, a liquid lubricant such as a fatty acid ester is added to a fine powder prior to the pressing step in order to improve the compressibility of the powder. Although such addition of a liquid lubricant forms a thin oily coating on the surface of the powder particles, it cannot sufficiently prevent the oxidization of the powder when a powder whose oxygen concentration is less than or equal to 4000 mass ppm is exposed to the atmospheric air.
In view of this, in the prior art, a slight amount of oxygen is intentionally introduced into the atmosphere during pulverization of a rare-earth alloy so as to slightly oxidize the surface of the finely pulverized powder, thereby reducing the reactivity thereof. For example, Japanese Patent Publication for Opposition No. 6-6728 discloses a technique of using a supersonic flow of an inert gas containing a predetermined amount of oxygen to finely pulverize a rare-earth alloy while forming a thin oxidized coating on the particle surface of the fine powder produced through the pulverization. With the technique, oxygen in the atmospheric air is blocked by the oxidized coating formed on the powder particle surface, thereby preventing the heating and ignition of the powder due to oxidization. However, the presence of the oxidized coating on the powder particle surface increases the total amount of oxygen contained in the powder.
Japanese Laid-Open Patent Publication No. 10-321451 discloses a technique of mixing a low-oxygen R—Fe—B alloy powder with a mineral oil, or the like, to obtain a slurry. Since the powder particles in the slurry are not exposed to the atmospheric air, it is possible to prevent the heating and ignition of the alloy powder while reducing the amount of oxygen contained therein.
However, this conventional technique leads to a poor productivity because, after filling the cavity of the compacting apparatus with an R—Fe—B alloy powder in the form of a slurry, it is necessary to perform the pressing step while squeezing the oil component out of the alloy powder.