Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Consolidation of powder prior to sintering
Reexamination Certificate
1999-12-08
2001-04-10
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Consolidation of powder prior to sintering
C419S054000, C148S101000, C148S103000, C148S301000, C148S302000
Reexamination Certificate
active
06214288
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for the preparation of a rare earth-based permanent magnet. More particularly, the invention relates to a method for the preparation of a neodymium/iron/boron-based permanent magnet by a powder metallurgical process involving a step of sintering of a powder compact of a magnet alloy of a specified chemical composition of the rare earth-based magnet alloy.
As is well known, the demand for rare earth-based permanent magnets is rapidly growing in recent years by virtue of their very excellent magnetic properties enabling a compact design of electric and electronic instruments with a permanent magnet built therein despite the relative expensiveness of the rare earth-based magnets as compared with ferrite-based and other conventional permanent magnets. Among the various types of rare earth-based permanent magnets, the samarium-based magnets developed in early days are under continuous replacement with neodymium-based permanent magnets or, in particular, neodymium/iron/boron-based magnets because the magnetic properties of the magnets of this latter type definitely exceed those of the former type in addition to the lower manufacturing costs owing to the relative inexpensiveness of the elements constituting the magnets.
As is also well known, the neodymium-based permanent magnets are prepared, like the rare earth-based magnets of other types, by a powder metallurgical process comprising the steps of pulverization of an alloy ingot of a specified composition of the constituent elements, e.g., neodymium, iron and boron, into a fine magnet alloy powder, compression-molding of the alloy powder, usually, in a magnetic field, into a powder compact and a sintering heat treatment of the powder compact as a green body at an elevated temperature under controlled conditions.
It is generally accepted that the magnetic properties of the thus prepared neodymium-based permanent magnets are greatly affected by the process conditions of the step of sintering heat treatment. For example, the residual magnetization of the magnet can be increased by bringing the density of the sintered magnet body as close as possible to the true density of the respective magnet alloy. Needless to say, the density of a sintered magnet body can be increased by increasing the sintering temperature and by extending the time length for the sintering treatment.
These measures to accomplish an increase in the density of the sintered magnet body can not always be applied with success to the neodymium-based permanent magnet having relatively large temperature dependence of the coercive force because an increase in the sintering temperature and/or extension of the sintering time results in undue growth of the sintered grains while coarser sintered grains have a lower coercive force than finer grains as a trend. This problem explains the residual magnetization of the neodymium-based magnets currently under use which is substantially lower than the value expected ed for an imaginary magnet having a sintering density identical to the true density of the magnet alloy.
In this regard for accomplishing a high residual magnetization, a proposal is made in Japanese Patent Publication 4-45573 for a measure of bringing the density of a sintered body of a neodymium-based magnet to a value close to the true density of the alloy with a relatively small decrement of the coercive force, according to which the density of the sintered magnet can be increased by conducting the compression molding of the magnet alloy powder by using a hot hydrostatic press under a hydrostatic pressure of 500 to 1300 atmospheres. Needless to say, a large problem involved in this method of high-pressure hydrostatic compression molding is that the hydrostatic pressure can be obtained only by using a very highly pressure-resistant vessel which is, even by setting aside the large weight and expensiveness, under strict legal regulations for safety and must be used and maintained with utmost care. In addition, this hydrostatic molding method is disadvantageous due to the low productivity taking a long time for one-shot molding resulting in an increase in the manufacturing costs of the magnet products.
Alternatively, Japanese Patent Kokai 7-335468 proposes a heat treatment under a pressure in the range from 50 to 500 atmospheres to accomplish densification of the sintered magnet body. Although the pressure can be substantially lower than in the above described method proposing a pressure of 500 to 1300 atmospheres, the disadvantage due to the requirement for a highly pressure-resistant vessel still remains unsolved.
The disadvantage caused by a low density of the sintered neodymium-based permanent magnets is not limited to a decrease in the magnetic properties such as the residual magnetization. Namely, a neodymium-based sintered magnet having an insufficient density as sintered is liable to suffer drawbacks such as low mechanical strengths of the magnet body, rusting on the surface and poor adhesive bonding of the rustproofing coating layers provided on the magnet surface.
SUMMARY OF THE INVENTION
The present invention accordingly has an object to provide a simple, convenient and inexpensive method for the preparation of a high-density rare earth-based permanent magnet having a large residual magnetization and a practically sufficient coercive force without necessitating use of an elaborate and expensive but poorly productive apparatus.
Thus, the present invention provides, in a method for the preparation of a rare earth-based permanent magnet comprising the steps of:
(a) compression-molding a powder of a rare earth-based magnet alloy having a chemical composition expressed by the composition formula giving the molar proportion of the constituent elements
R
X
(Fe
1−a
Co
a
)
Y
B
Z
T
b
, (I)
In which R is a rare earth element, T is an element selected from the group consisting of aluminum, silicon, vanadium, chromium, manganese, nickel, copper, zinc, gallium, zirconium, niobium, molybdenum, tin, hafnium, tantalum and tungsten, the subscript X is a number in the range from 11 to 16, the subscript Y is a number in the range from 70 to 85, the subscript Z is a number in the range from 4 to 9, the subscript a is 0 or a positive number not exceeding 0.2 and the subscript b is 0 or a positive number not exceeding 4, in a magnetic field into a powder compact; and
(b) subjecting the powder compact to a heat treatment to effect sintering to give a sintered magnet body, the improvement which comprises conducting the heat treatment of the powder compact in step (b) in two partial heat treatment steps consisting of:
(b1) a first partial sintering step conducted in vacuum or in an atmosphere of an inert gas under a subatmospheric pressure at a temperature in the range from 1000 to 1150° C. until the density of the powder compact under sintering reaches from 90 to 98% relative to the true density of the magnet alloy; and
(b2) a second partial sintering step conducted in an atmosphere of an inert gas under a pressure in the range from 1 to 20 atmospheres or, preferably, from 1 to 10 atmospheres at a temperature in the range from 900 to 11 50° C. for 0.1 to 5 hours.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As is summarized above, the improvement accomplished according to the invention, which has been completed as a result of the inventors' extensive investigations with an object to overcome the disadvantages accompanying the use of a high pressure of 500 to 1300 atmospheres or 50 to 500 atmospheres proposed in the prior art, is characterized by conducting the sintering heat treatment of a powder compact in two steps. Despite the unnecessity of using such a high pressure, the rare earth-based permanent magnet obtained according to the present invention has a high density close to the true density of the magnet alloy to give a large residual magnetization along with a practically sufficient coercive force.
The preparation method according to the present invention is applicable to a rare ear
Kusunoki Matou
Minowa Takehisa
Mai Ngoclan
Shin-Etsu Chemical Co. , Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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