Method of pressing rare earth alloy magnetic powder

Details Method of pressing rare earth alloy magnetic powder Method of pressing rare earth alloy magnetic powder

2001-03-08

2002-10-08

Jenkins, Daniel J. (Department: 1742)

Powder metallurgy processes

Powder metallurgy processes with heating or sintering

Consolidation of powder prior to sintering

C419S028000, C419S033000, C419S066000

Reexamination Certificate

active

06461565

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of making a green compact of a rare earth alloy magnetic powder and a method of producing a rare earth permanent magnet.
2. Description of the Related Art
A rare earth alloy sintered magnet is produced by pulverizing a rare earth alloy into a magnetic alloy powder, pressing and compacting the powder into a green compact in a desired shape and then subjecting green compact to sintering and aging processes. Currently, rare earth alloy sintered magnets have found a broad variety of applications and are typically made of either a samarium-cobalt compound or a neodymium-iron-boron compound. A neodymium-iron-boron magnet (which will be herein called an “R—T—B magnet”), in particular, has a higher maximum energy product than a magnet of any other type, and yet is available at a reasonable price. Accordingly, R—T—B magnets have been used for various kinds of electronic appliances with increasing frequency. In an R—T—B magnet, R is a rare earth element including Y, T is either iron or a compound of iron and a transition metal (e.g., Co) in which iron is partially replaced with the metal, and B is boron. Part of boron can be replaced with carbon.
To prepare such a rare earth alloy, an ingot casting process has been used. In an ingot casting process, a molten material alloy is poured (or teemed) into ingot casting molds and then cooled down relatively slowly. The alloy ingot, once formed by this ingot casting process, is pulverized into an alloy powder by a known technique. Next, the resultant alloy powder is pressed and compacted by various types of powder presses, forming a green compact. Finally, the green compact is loaded into a furnace chamber for sintering.
Recently, however, a rapid quenching process, like strip casting or centrifugal casting, has been preferred. In a rapid quenching process, a solidified alloy strip or flake, thinner than an alloy ingot, can be made from a molten alloy by contacting the melt with single or twin roller, rotating disk or rotating cylindrical mold, for example, so that the alloy is quenched relatively rapidly. An alloy strip prepared by a process like this generally has a thickness of 0.03 mm to 10 mm. According to the rapid quenching process, the molten alloy starts to be solidified at the surface being in contact with the chill roller (which will be herein called a “roller-alloy contact surface”). Then, columnar crystals grow from the roller-alloy contact surface in the thickness direction, or outward. Accordingly, when prepared by a strip casting method, for example, a rapidly solidified alloy has a structure including a combination of R
2
T
14
B crystal phases and R-rich phases. Normally, the sizes of each of the R
2
T
14
B crystal phases are from 0.1 &mgr;m through 100 &mgr;m in the minor axis direction and from 5 &mgr;m through 500 &mgr;m in the major axis direction. The R-rich phases exist dispersively around the grain boundaries of the R
2
T
14
B crystal phases. Also, each of the R-rich phases is a non-magnetic phase in which the concentration of the rare earth element R is relatively high, and has a thickness of 10 &mgr;m or less, corresponding to the width of the associated grain boundary.
In a rapid quenching process, an alloy is quenched and solidified in a shorter time (at a cooling rate between 10
2
° C./sec. and 10
4
° C./sec.) compared to the conventional ingot casting process. Thus, the rapidly solidified alloy can have a finer micro-structure and a smaller crystal grain size. In addition, the grain boundary (or intergranular phases) of the alloy of this type has a broader area and includes a thin layer of R-rich phases. As a result, the rapidly solidified alloy advantageously exhibits a wider dispersion of R-rich phases.
However, the present inventors found that if a magnetic powder of a rapidly solidified alloy (e.g., a strip cast alloy, typically) is compacted by a known pressing technique, the as-pressed, green compact has a potential to generate sufficient heat for combustion, depending on the particular state of the environment. This is probably because easily oxidizable R-rich phases are often exposed on the surface of powder particles of the rapidly solidified alloy, thus making the powder of the rapidly solidified alloy subject to oxidation and the resultant heat therefrom. Also, even if the heat from the oxidation of the powder is insufficient to cause combustion, the oxidization may deteriorate the magnetic properties of resultant magnets.
The heat generation resulting from the oxidization of rare earth elements is also observable when the powder of a rare earth alloy, prepared by a known ingot casting process, is pressed and compacted. However, the heat generation is markedly increased when the pressed and compacted powder is made from a rapidly solidified alloy (e.g., a strip cast alloy, in particular). Accordingly, even though a rapidly solidified alloy powder has a finer structure and potentially contributes to better magnetic properties, the rapid quenching process is still unqualified for mass production so long as there is any risk of heat generation or combustion left during the pressing.
It is possible to suppress oxidation of the rare earth alloy powder by carrying out the pressing and compacting process within an inert gas environment. However, pressing within an inert gas environment is far from a practical approach to the oxidation problem. This is because even though a pressing process can be performed fully automatically using a compacting machine, the process itself still requires frequent maintenance. That is to say, workers often have to check the presses. For example, in the event that a press placed within an inert gas (e.g., N
2
) environment fails, a worker must tend to the machine. However, the worker must either bring his own supply of oxygen, or he must replace the inert gas environment with a breathable environment. Moreover, placing the press entirely within such an inert gas environment requires an large amount of inert gas. Accordingly, this approach is neither cost-effective nor practical.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method of making a green compact of a rare earth alloy magnetic powder in such a manner as to avoid the combustion accidents and to attain superior magnetic properties even when the powder is easily oxidizable.
It is another object of the invention to provide a method of producing a rare earth permanent magnet by utilizing the inventive powder compacting method.
According to an embodiment of the powder compacting method of the present invention, a green compact of a rare earth alloy magnetic powder is made by pressing the powder within an air environment that has a temperature controlled at 30° C. or less and a relative humidity controlled at 65% or less.
According to another embodiment of the compacting method of the present invention, a green compact of a rare earth alloy magnetic powder is pressed in an air environment that also has a temperature controlled at 30° C. or less. The temperature minus a dew point is controlled at 6° C. or more. As used herein, the “dew point” is the temperature at which a given parcel of air is saturated with water vapor.
In one embodiment of the compacting method of the present invention, the powder may be prepared by pulverizing a rapidly solidified alloy that has been obtained by quenching a molten alloy at a rate from 10
2
° C./sec. through 10
4
° C./sec.
In this particular embodiment, the rapidly solidified alloy is a rare earth alloy with a thickness between 0.03 mm and 10 mm, and preferably includes R
2
T
14
B crystal grains (where R is a rare earth element, T is either iron or a compound of iron and a transition metal element in which iron is partially replaced with the metal, and B is boron) and R-rich phases. The sizes of the R
2
T
14
B crystal grains are preferably from 0.1 &mgr;m to 100 &mgr;m in a minor axis direction, and from 5 &mgr;m to 500 &mgr;m a major axis direction. The

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