Production method of anisotropic rare earth magnet powder

Details Production method of anisotropic rare earth magnet powder Production method of anisotropic rare earth magnet powder

1999-10-13

2002-09-03

Sheehan, John (Department: 1742)

Metal treatment

Process of modifying or maintaining internal physical...

Magnetic materials

C148S101000

Reexamination Certificate

active

06444052

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a production method of anisotropic rare earth magnet powder.
BACKGROUND OF THE INVENTION
A rare earth magnet, which is mainly composed of a rare earth element, boron and iron is widely used due to its excellent magnetic properties, such as coercivity and residual induction.
Rare earth magnet powder having good magnetic property can be produced by an elevated hydrogenation at the temperature of 750° C.-950° C. in which phase transformation in the rare earth magnet as raw material is induced by hydrogen absorption and subsequent hydrogen desorption in which reverse phase transformation is induced by hydrogen desorption.
Generally speaking, magnetic properties are estimated based upon the coercivity, residual induction and maximum energy product. The coercivity depends on the grain size in the microstructure of a magnet alloy. The fine grain size can improve the coercivity. On the other hand, the residual induction depends on the alignment of the crystallographic orientation of grains. High alignment increases the residual induction. Improvement of both the coercivity and the residual induction gives high maximum energy product.
Here, the inventors define the anisotropy as anisotropic ratio Br/Bs of more than 0.8, where Bs means the saturation induction which is equal to 16 kG and Br means residual induction. Br/Bs ratio of unity shows perfect anisotropy. The ratio of 0.5 shows ideal isotropy. An actual magnet takes only a medium ratio value from 0.5 to 1.0. If more than 0.8, the magnet is defined as an anisotropic magnet. If less than 0.6, it is defined as an isotropic magnet. If 0.6 to 0.8, it is a poor anisotropic magnet. By the way, practical applications of magnets require a coercivity of more than 9 kOe.
The production methods to improve magnetic property of magnets have been disclosed in the following patents.
Japanese Examined Patent Application Publication (Kokoku) No. 7-110965 discloses a production method characterized by hydrogen heat treatment which comprise hydrogenation and subsequent desorption. In this patent, the raw material is prepared through the process that RFeB based alloy is melted, cast into a ingot, crushed to powder and sintered or pressed into a block. Then, a lot of hydrogen is stored in the block under high hydrogen pressure. After that, heated at the temperature of 600° C. to 1000° C., hydrogenation reaction is carried out accompanied by the phase transformation from single R
2
Fe
14
B phase to a mixture of RH
2
, Fe and Fe
2
B. Subsequently desorption reaction accompanied by the reverse transformation is carried out to make a recombination phase.
However, there is a drawback that an inhomogeneous phase which is mixtured with fine grains and coarse grains appears because phase transformation takes place only in a partial area. The inhomogeneous phase causes too large a decrease in the coercivity to put the magnet in practical use. In addition, it is not good that this production method offers at most the anisotropic ratio of 0.7.
Japanese Examined Patent Publication (Kokoku) No. 7-68561 discloses an improved hydrogen heat treatment, in which, at first an ingot of NdFeB alloy is made, next a hydrogenation process accompanied by phase transformation is carried out in a manner to be heated at the temperature of 500° C. to 1000° C. under hydrogen pressure of more than 10 torr and then a desorption process accompanied by reverse phase transformation is carried out in the manner to be heated at the same temperature under vacuums of less than 10
−1
torr.
This production method makes a fine recrystallized microstructure that gives high coercivity through phase transformation and subsequent reverse phase transformation. However, magnet powder that at most has a poor anisotropic ratio of 0.67 is obtained. This fact means that the hydrogen heat treatment accompanied by phase transformation and subsequent reverse phase transformation cannot produce anisotropic magnet powder having a high anisotropic ratio of more than 0.80.
The inventors of No. 7-68561 have been proceeding with their work up to the present to get excellent anisotropic magnet powder having a higher anisotropic ratio and have succeeded in inventing many advanced production methods.
At a beginning stage, Japanese Patent Application Laid-Open No. 3-129703 (1991) and No. 4-133407 (1992) were invented. These patents disclosed that when NdFeB based alloy including a large amount of Cobalt (Co) element and minor additive elements of Gallium (Ca), Zirconium (Zr), Titanium (Ti), Vanadium (V) and so on are subjected to the above mentioned hydrogen heat treatment, an anisotropic ratio of 0.75 at most can be obtained. These inventions give improvement in anisotropy ratio but have a big drawback that a large amount of Co element has to bring a high cost to magnet powder because the Co element is very expensive.
To solve the cost problem of the above inventions, Japanese Patent Application Laid-open No. 3-129702 (1991) and No. 4-133406 (1992) were invented. These patents disclosed that when NdFeB based alloys including minor additive elements of Ga, Zr, Ti, V without Co element are subjected to the above mentioned hydrogen heat treatment, an anisotropic ratio shows little improvement. But the improvement in anisotropy is insufficient since it gives only at most an anisotropic ratio of 0.68.
In addition, if the above mentioned hydrogen heat treatment is applied to mass production, there is a crucial barrier on controlling the temperature of hydrogen reaction, because the heat amount generated by its exothermic or endothermic reaction is proportional to the production volume. The deviation of the heat temperature from the optimum deteriorates the anisotropy of magnet powders considerably. To prevent the deterioration of anisotropy attributed to its exothermic or endothermic reaction in the mass production, the same inventors have produced five inventions. At first Japanese Patent Application Laid-open No. 3-146608 (1991) and No. 4-17604 (1992) were invented to disclose the mass production method where RFeB based alloy or RFeCoB based alloy are installed with heat storage material in the vessel. But this method gives only at most an anisotropic ratio of 0.69 which is far below the desirable anisotropic ratio of more than 0.80. So this method is not satisfied with the requirement to improve anisotropy of RFeB alloy.
Next, Japanese Patent Application Laid-open No. 5-163509 (1993) was invented to disclose a further advanced method where RFeB or RFeCoB based type ingots are homogenized and crushed into powder with uniform particle size. But this method also gives only at most an anisotropic ratio of 0.74, which means to give only a little improvement in anisotropy.
Furthermore, Japanese Patent Application Laid-open No. 5-163510 (1993) was invented to disclose a further advanced method where RFeB or RFeCoB based type ingots were subjected to the hydrogen heat treatment in the tubular vacuum furnace. But this method also gives only at most an anisotropic ratio of 0.74, so it is not satisfied.
Japanese Patent Application Laid-open No. 6-302412 (1994) was invented to disclose another technique where hydrogen pressure goes up and down during the hydrogen heat treatment of RFeB or RFeCoB type ingots. But this method also gives only at most an anisotropic ratio of 0.76. This method also is not sufficient.
It is clear that the above mentioned inventions cannot disclose production methods to get high anisotropy. So the inventors invented a more complicated technique that is disclosed in Japanese Patent Application Laid-open No. 8-288113 (1996), where the above mentioned hydrogen heat treatment of RFeB or RFeCoB type ingots are carried out, and subsequently a similar hydrogen heat treatment is repeated which comprises hydrogenation under the hydrogen pressure of 1 torr to 760 torr at low temperature of less than 500° C. and subsequent desorption under vacuum at the temperature of 500° C. to 1000° C. This technique improves the anisotropy due to the decrease of

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