Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2000-08-29
2002-02-05
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S030000, C419S038000, C148S100000, C148S302000
Reexamination Certificate
active
06344168
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of producing an R—Fe—B type sintered magnet, a method of preparing an alloy powder material for use as a raw material in the production of the R—Fe—B type sintered magnet, and a method of preserving the same.
A sintered magnet (permanent magnet) of a rare earth alloy is typically produced by compacting a powder of a rare earth alloy, sintering a compact of the powder obtained, and performing an aging heat treatment with respect to the sintered body. At present, two types of sintered magnets of rare earth alloys, which are a samarium-cobalt type magnet and a neodium-iron-boron type magnet, are used widely in different fields. Of the two types, the neodium-iron-boron type magnet (hereinafter referred to as “R—Fe—B type magnet” where R is one of rare earth elements inclusive of Y, Fe is iron, and B is boron) has been applied positively to various electronic equipment because of its highest magnetic energy product among various magnets and relatively low cost. The R—Fe—B type rare earth alloy consists of a main phase mainly composed of an R
2
Fe
14
B tetragonal compound, an R-rich phase composed of Nd and the like, and a B-rich phase. It is to be noted that Fe may be partly replaced by a transition metal such as Co or Ni. As documents disclosing R—Fe—B type rare earth sintered magnets to which the present invention is applied appropriately, U.S. Pat. Nos. 4,770,723 and 4,792,368 are incorporated by reference in the present specification.
To produce a rare earth alloy forming such a magnet, there has conventionally been used ingot casting whereby a molten metal alloy as a raw material is placed in a mold and cooled relatively slowly. An alloy ingot produced by ingot casting is powdered by a well-known pulverizing process. The alloy powder thus produced is compacted by various powder pressers and transported into a sintering furnace, where it is subjected to a sintering process.
In recent years, attention has been focused on quenching methods represented by strip casting and centrifugal casting, whereby a solidified a alloy thinner than an ingot (hereinafter referred to as “an alloy flake”) is formed from a molten metal alloy by bringing the molten metal alloy into contact with a single roll, double roll, a rotating disk, or an inner side of a rotating cylindrical mold and thereby performing relatively rapid quenching. The thickness of an alloy piece produced by such a quenching method is normally in the range of about 0.03 mm to about 10 mm. In accordance with a quenching method, the molten metal alloy begins to solidify from a surface thereof in contact with a cooling roll (roll contact surface and a crystal grows from the roll contact surface in the direction of thickness into a columnar configuration. As consequence, a quenched alloy produced by strip casting or the like has a structure including an R
2
Fe
14
B crystal phase which has a size not less than about 0.1 &mgr;m and not more than about 100 &mgr;m in the direction of a minor axis and a size no less than about 5 &mgr;m and not more than about 500 &mgr;m in th direction of a major axis and an R-rich phase which is present dispersively in a grain boundary of the R
2
Fe
14
B crystal phase. The R-rich phase is a nonmagnetic phase containing 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 &mgr;m or less.
Since a quenched alloy has been quenched in a shorter period of time (cooling rate: not less than 10
2
° C./sec and not more than 10
4
° C./sec) than an alloy (ingot alloy) produced by the conventional ingot casting (die casting), it features a miniaturized structure and a reduced crystal particle size. The quenched alloy also has the advantage of the R-rich phase with excellent dispersion since the grain boundary occupies a large area and the R-rich phase is spread widely in the grain boundary. These features allow a magnet having superior magnetic properties to be produced by using the quenched alloy.
In the present specification, blocks of a solidified alloy obtained by quenching or cooling a molten metal will be termed “alloy blocks” which include solidified alloys in various forms such as the alloy ingot obtained by the conventional ingot casting and the alloy flake obtained by a quenching method such as strip casting. An alloy powder subjected to compacting is obtained by grinding the alloy blocks into a coarse powder (having an average particle size of, e.g., 10 &mgr;m to 500 &mgr;m) by, e.g., hydrogenation pulverizing (i.e., hydrogenation occlusion and/or various mechanical grinding methods and then fine pulverizing the coarse powder.
However, an alloy powder produced by a quenching method, which is represented by a strip cast alloy, has the problem of susceptibility to oxidation. In general, a powder of a rare earth alloy is susceptible to oxidation and has a risk of heat generation or ignition. A powder of a quenched alloy is considered to have a particularly high risk of heat generation or ignition since the R-rich phase susceptible to oxidation easily appears on a surface of a powder particle of the quenched alloy.
To circumvent the problem, e.g., Japanese Patent Publication No. 6-6728 (Applicant: Sumitomo Special Metals Co., Ltd., Filing Date: Jul. 24, 1986) discloses a method of forming a thin oxide film on a surface of a powder of a rare earth alloy. The publication also discloses that, in order to provide superior magnetic properties, the average particle size of the powder of the rare earth alloy subjected to compacting is preferably in the range of 1.5 &mgr;m to 5 &mgr;m. If the average particle size is smaller than 1.5 &mgr;m, the proportion of the oxide becomes excessively high so that the magnetic properties are degraded. If the average particle size is larger than 5 &mgr;m, magnetization inversion easily occurs to reduce a coercive force. Japanese Patent Publication No. 6-6728 is incorporated by reference in the present specification.
To improve the compressibility (compactibility) of a powder of a rare earth metal, on the other hand, the specification of U.S. Pat. No. 5,666,635 (Assignee: Sumitomo Special Metals Co., Ltd.) discloses a technique for producing a fine powder having an average particle size of 1.5 &mgr;m to 5 &mgr;m by adding and mixing a 0.02 wt % to 5.0 wt % lubricant prepared by liquidizing at least one fatty acid ester in a coarse powder of an alloy for an R—Fe—B type sintered magnet having an average particle size of 10 &mgr;m to 500 &mgr;m and milling the mixture in a jet mill by using an inert gas. U.S. Pat. No. 5,666,635 is incorporated by reference in the present specification.
As a result of conducting a study, however, the present inventor has encountered the problem that, even if the conventional technique is used, cracks or hips assumedly resulting from poor compressibility during compacting are likely to be produced in a compact of the alloy powder. The problem was particularly notable when a rare earth alloy powder having a relatively sharp particle size distribution from which the smaller and larger particle sides of the rare earth alloy powder had been removed was used.
SUMMARY OF THE INVENTION
The present invention has been achieved to solve the foregoing problem and a primary object of the present invention is to provide a method of producing an R—Fe—B type sintered magnet and a method of preparing an alloy powder material for the R—Fe—B type sintered magnet which can reduce cracks and chips in a compact by improving the compactibility, particularly compressibility, of the alloy powder material for the R—Fe—B type sintered magnet and thereby improve productivity.
In the present specification, a powder composed only of a rare earth alloy (including an oxide film formed through the oxidation of a surface of a rare earth alloy powder) will be termed “a rare earth alloy powder” and a rare earth alloy powder having a particle surface coated with a lubricant will be termed “a rare earth metal alloy powder material
Costellia Jeffrey L.
Mai Ngoclan
Nixon & Peabody LLP
Sumitomo Special Metals Co. Ltd.
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