Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2000-02-15
2002-06-11
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S038000, C241S030000
Reexamination Certificate
active
06403024
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for pulverizing rare-earth alloy magnetic materials through absorption and release of hydrogen (in this specification, such an apparatus will be called a “hydrogen pulverizer”) The present invention also relates to respective methods for preparing rare-earth alloy magnetic material powder and producing a magnet using the hydrogen pulverizer.
A rare-earth sintered magnet is produced by pulverizing a magnetic alloy into alloy powder, pressworking and sintering the alloy powder and then subjecting the sintered alloy to aging treatment. Two types of rare-earth alloy magnets, namely, samarium-cobalt (Sm—Co) magnets and neodymium-iron-boron magnets, are used widely in various applications. In this specification, a rare-earth alloy magnet of the latter type will be referred to as an “R—T—(M)—B magnet”, where R is a rare-earth element including Y, T is Fe or a compound of Fe and at least one transition metal element, M is an additive and B is boron. Part of Fe in an R—Fe—B type magnet can be replaced with a transitional metal element, e.g., cobalt. The R—T—(M)—B magnet is often applied to many kinds of electronic units, because the maximum energy product thereof is the higher than any other kind of magnet and yet the cost thereof is relatively inexpensive.
In a conventional process of pulverizing material alloy for the R—T—(M)—B magnet, a container made of stainless steel like SUS304 is loaded with the magnetic material alloy powder and then primary pulverization of the material alloy is carried out in a hydrogen furnace, where hydrogen is absorbed and released into/out of the material alloy.
Methods for preparing the rare-earth alloy are roughly classified into the following two types. The first type is an ingot mold casting technique, in which a melt of material alloy is teemed into a mold and then cooled down relatively slowly. The second type is a quenching technique, such as a strip-casting process or a centrifugal casting process, in which a melt of material alloy is rapidly quenched by a single roll, twin rolls, a rotating disk, or a rotating cylinder, thereby forming, out of the molten alloy, a solidified alloy, which is thinner than the alloy produced by the conventional ingot mold casting technique.
According to the quenching technique, the thickness of the resultant R—T—(M)—B magnet alloy is in the range from 0.03 mm to 10 mm, both inclusive. The molten alloy starts to solidify from the surface that has come into contact with the chill roll or its equivalents, and subsequently columnar crystals are growing from the surface in the thickness direction. As a result, the quenched alloy comes to have a structure including R
2
T
14
B crystal grains and R-rich phases that exist dispersively along the R
2
T
14
B crystal grain boundaries. The sizes of the R
2
T
14
B crystal grain are in the range from 0.1 &mgr;m to 100 &mgr;m, both inclusive, in the minor axis direction and in the range from 5 &mgr;m to 500 &mgr;m, both inclusive, in the major axis direction. The R-rich phases are non-magnetic phases in which the concentration of the rare-earth element R is relatively high. The thickness of the R-rich phases, which corresponds to the width of the grain boundaries, is 10 &mgr;m or less.
Compared to an ingot alloy, i.e., alloy that has been prepared by the conventional mold casting process (i.e., die casting process), the quenched alloy has been cooled down in a relatively short period of time. Thus, the crystal structure or the grain size of the quenched alloy is finer than that of the ingot alloy. That is to say, the grain boundaries of the quenched alloy are greater in area, and the R-rich phases exist in the grain boundaries. Accordingly, the quenched alloy is also superior to the ingot alloy in terms of dispersiveness of the R-rich phases.
The quenched alloy is likely to fracture at the grain boundaries during a hydrogen pulverizing process. For that reason, the R-rich phases easily appear on the surface of the alloy powder particles that are obtained by pulverizing the quenched alloy. In the R-rich phases, R easily reacts with oxygen. Accordingly, the quenched alloy powder is very likely to be oxidized, generate heat and spontaneously ignite. Thus, it is believed that the magnetic properties of the strip-cast alloy powder are deteriorative considerably.
Next, a known hydrogen pulverization process for the ingot alloy will be described.
First, a process container in the shape of a flat pack is filled with magnetic alloy blocks (each having a length of about 3 cm on each side) that have been cast in a water-cooled casting die, and then loaded into a rack. After the rack has been inserted into a hydrogen furnace, the pressure inside the furnace is reduced using a vacuum pump. Then, hydrogen gas is supplied into the hydrogen furnace, thereby getting hydrogen absorbed into the material alloy. After a predetermined time has passed, the material alloy is heated while evacuating the hydrogen furnace again, thereby getting hydrogen released from the material alloy. Once a sufficient quantity of hydrogen has been released from the material alloy and the alloy has been cooled down, the cap of the hydrogen furnace is opened and the rack, which is loaded with the process containers, is ejected to the air. At the point in time that the hydrogen pulverization process is finished, the alloy has been roughly broken up to a size of about 1 cm. Thereafter, the material, which has been pulverized roughly through this hydrogen process, is taken out of the container, ground finely to a size of about 10 &mgr;m to about 400 &mgr;m using a disk mill and then pulverized even more finely to an average particle size of about 2 &mgr;m to about 5 &mgr;m using a jet mill, for example.
A green compact (or as-pressed compact) is formed, by compaction, out of the material alloy fine powder prepared this way. Thereafter, the compact is subjected to sintering, aging treatment and so on to produce a sintered magnet.
According to the conventional process, however, resulting magnetic properties deteriorate. This is because when the material is ejected out of the hydrogen furnace to the air, the rare-earth element R contained in the hydrogen-pulverized material is oxidized due to the contact with the air.
Suppose the source material contains neodymium as the rare-earth element R, for example. In such a case, NdH
3
is formed by getting hydrogen absorbed into the material, while NdH
3
changes into NdH
2
by getting hydrogen released from the material. In an actual mass production process, however, hydrogen cannot be released completely, and NdH
3
is almost always left in part of the material. At the core of the process container, in particular, plenty of NdH
3
might be left because the core cannot always be heat-treated sufficiently. If NdH
3
remains in the material, then that NdH
3
is exposed to the air to generate heat when the material is ejected out of the process container. Accordingly, in practice, a cooling period should be provided after the material has been taken out. In other words, the fine pulverization and other subsequent process steps cannot be started immediately. What is worse, there is a risk of spontaneous ignition.
We found that the probability of heat generation and spontaneous ignition due to oxidation is remarkably high when the hydrogen pulverization process is applied to the quenched alloy produced by the quenching technique (e.g., the strip-cast process), in particular. Thus, we concluded that it is extremely difficult to realize an industrialized quenched alloy pulverization process according to the conventional technique. Hereinafter, this point will be detailed.
Compared to the ingot alloy, the quenched alloy is thinner and has a finer metal structure. Accordingly, most of the quenched alloy has already been pulverized sufficiently (e.g., with an average size of 1.0 mm or less) when the hydrogen pulverization process on the alloy is over. Thus, the total surface area of the pulverized alloy is greater. Also, si
Okayama Katsumi
Oota Akiyasu
Tsujimoto Akihito
Costellia Jeffrey L.
Mai Ngoclan
Nixon & Peabody LLP
Sumitomo Special Metals Co. Ltd.
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