Rare earth magnet and a method for manufacturing compactable powder for the rare earth magnet without jet milling

Abstract

The present invention discloses manufacturing methods of a powder for compacting rare earth magnet powder and rare earth magnet that omit jet milling process, which comprises the steps as follows: 1) casting: casting the molten alloy of rare earth magnet raw material by strip casting method to obtain a quenched alloy with average thickness in a range of 0.20.4 mm; 2) hydrogen decrepitation: decrepitating the quenched alloy and a plurality of rigid balls into a rotating hydrogen decrepitation container simultaneously, the quenched alloy is crushed under a hydrogen pressure between 0.011 MPa, cooling the alloy and the balls, then screening the mixture to remove the rigid balls and obtain the powder. As the jet milling process is omitted, the oxygenation during the process of the jet milling may be avoided, therefore the process may be non-oxide, and the mass production of magnet with super high property may be possible.

Claims

1. A method of manufacturing a compactable powder for a rare earth magnet without jet milling, the rare earth magnet comprising a R.sub.2T.sub.14B main phase, where R is at least one rare earth element including yttrium, and T is at least one transition metal element including Fe, wherein the method comprises the steps of: casting a molten alloy of a rare earth magnet raw material by strip casting and cooling to obtain a quenched alloy with an average thickness ranging from 0.20.4 mm; putting the quenched alloy and a plurality of rigid balls into a rotatable hydrogen decrepitation container; hydrogen decrepitating and simultaneously ball milling by rotating the rotatable hydrogen decrepitation container to crush the quenched alloy under a hydrogen pressure ranging between 0.01 to 1 MPa and to produce a mixture; dehydrogenating and simultaneously ball milling by rotating the rotatable hydrogen decrepitation container to crush the mixture and produce the compactable powder; screening the compactable powder from the plurality of rigid balls to remove the plurality of rigid balls; and passing the compactable powder through a 3001500 mesh screen without further pulverization of the compactable powder after dehydrogenating and simultaneously ball milling, wherein the plurality of rigid balls does not break during rotating the rotatable hydrogen decrepitation container.

2. The method according to claim 1, wherein more than 95 weight % of the quenched alloy has a thickness ranging from 0.10.7 mm.

3. The method according to claim 1, wherein the rotatable hydrogen decrepitation container has a rotation rate that ranges from 30 rpm100 rpm.

4. The method according to claim 1, wherein cooling to obtain the quenched alloy is accomplished at a cooling rate ranging between 10.sup.2 C./s10.sup.4 C./s and an average cooling rate ranging between 1*10.sup.3 C./s8*10.sup.3 C./s, wherein hydrogen decrepitating takes place for a hydrogen decrepitation period ranging from 124 hours, and wherein dehydrogenating the compactable powder takes place for a dehydrogenation period ranging from 0.510 hours.

5. The method according to claim 1, wherein the plurality of rigid balls are rigid balls selected from the group consisting of steel balls, metal Mo balls, metal W balls, stainless steel balls, tungsten carbide balls, aluminum oxide balls, zirconium oxide balls or silicon carbide balls, and have a ball size ranging from 0.5 mm60 mm.

6. The method according to claim 1, wherein the method further comprises, prior to hydrogen decrepitating, preheating the quenched alloy to a temperature ranging from 150 C.350 C.

7. The method according to claim 1, wherein the quenched alloy is expressed, in atomic percent, as:
R.sub.eT.sub.fA.sub.gJ.sub.hG.sub.iD.sub.k, where R is Nd or comprises Nd and at least one of La, Ce, Pr, Sm, Gd, Dy, Tb, Ho, Er, Eu, Tm, Lu or Y; where T is Fe or comprises Fe and at least one of Ru, Co or Ni; where A is B or comprises B and at least one of C or P; where J is at least one of Cu, Mn, Si or Cr; where G is at least one of Al, Ga, Ag, Bi or Sn; where D is at least one of Zr, Hf, V, Mo, W, Ti or Nb; and where subscripts e, f, g, h, i and k are configured as: 12e16, 5g9, 0.05h1, 0.2i2.0, k is 0k4, and f=100-e-g-h-i-k.

8. The method according to claim 1, wherein the rare earth magnet raw material has a proportion of Co that is below 1 at %.

9. The method of claim 1, wherein the method further comprises, prior to hydrogen decrepitating, preheating the quenched alloy to a temperature ranging from 150 C.250 C.

10. A method of manufacturing a rare earth magnet without jet milling, the rare earth magnet comprising a R.sub.2T.sub.14B main phase, where R is at least one rare earth element including yttrium, and T is at least one transition metal element including Fe, wherein the method comprises the steps of: casting a molten alloy of a rare earth magnet raw material by strip casting to obtain a quenched alloy having an average thickness ranging from 0.20.4 mm; putting the quenched alloy and a plurality of rigid balls into a rotatable hydrogen decrepitation container; rotating the rotatable hydrogen decrepitation container to hydrogen decrepitate and simultaneously ball milling to crush the quenched alloy under a hydrogen pressure ranging between 0.01 to 1 MPa and produce a mixture; dehydrogenating and simultaneously ball milling by rotating the rotatable hydrogen decrepitation container to crush the mixture and produce compactable powder; screening the compactable powder from the plurality of rigid balls to remove the plurality of rigid balls; compacting, after screening and without further pulverization of the compactable powder after dehydrogenating and simultaneously ball milling, the compactable powder in a two-part compacting method comprising magnetic field compacting and isostatic pressing compacting to provide a green compact; and sintering the green compact to provide the rare earth magnet, wherein the rare earth magnet is a permanent magnet, wherein the plurality of rigid balls does not break during rotating the rotatable hydrogen decrepitation container.

11. The method of claim 10, wherein the method further comprises adding an organic additive to the compactable powder prior to compacting the compactable powder.

12. The method of claim 11, wherein a weight ratio of the organic additive to the compactable powder ranges from 0.01:1001.5:100.

13. The method of claim 11, wherein the organic additive is methyl caprylate.

14. The method of claim 10, wherein the two-part compacting method comprises demagnetizing the compactable powder between magnetic field compacting and isostatic pressing compacting.

15. The method of claim 14, wherein the two-part compacting method comprises sealing, so as to not expose to air, the compactable powder between magnetic field compacting and isostatic pressing compacting.

16. The method of claim 10, wherein the two-part compacting method comprises sealing, so as to not expose to air, the compactable powder between magnetic field compacting and isostatic pressing compacting.

17. The method of claim 10, wherein magnetic field compacting forms a cube in an orientation field of 2.1 T.

18. The method of claim 10, wherein the method further comprises heating the rare earth magnet in an atmosphere of Ar gas after sintering the green compact.

19. The method of claim 10, wherein an oxygen content of the rare earth magnet after the sintering is less than 1000 ppm.

20. The method of claim 10, wherein an oxygen content of the rare earth magnet after the sintering is less than 450 ppm.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) The present invention will be further described with the embodiments.

Embodiment 1

(2) In the raw material preparing process: Nd, Pr, Dy, Tb, Gd with 99.5% purity, industrial FeB, industrial pure Fe, Co with 99.99% purity and Cu, Al, Zr with 99.5% purity are prepared, counted in atomic percent, prepared in R.sub.eT.sub.fA.sub.gJ.sub.hG.sub.iD.sub.k components.

(3) The contents of the elements are shown in TABLE 1:

(4) TABLE-US-00001 TABLE 1 proportioning of each element R T A J G D Nd Pr Dy Tb Gd Fe Co C B Mn Cr Ga Sn W 8 2 1.5 1 1 79.1 0.4 0.1 6 0.2 0.2 0.2 0.2 0.1

(5) Preparing 500 Kg raw material by weighing in accordance with TABLE 1.

(6) In the melting process: the 500 Kg raw material is divided into 16 copes and respectively put into an aluminum oxide made crucible, an intermediate frequency vacuum induction melting furnace is used to melt the raw material in 10.sup.2 Pa vacuum below 1550 C.

(7) In casting process: Ar gas is filled to the melting furnace so that the Ar pressure would reach 60000 Pa after the process of vacuum melting, then using following casting method respectively: the quenched alloy is obtained in a cooling rate of 10.sup.2 C./s10.sup.4 C./s with average cooling rate 1*10.sup.3 C./s8*10.sup.3 C./s, the casting manners and average strip thickness are shown in TABLE 2, therein, double-roller quenching method is used in TABLE 2, inclined surface disk casting method is used in TABLE 3.

(8) The thickness of the quenched alloy depends on the rotating rate of the roller or the rotating rate of the inclined surface disk.

(9) The strip thickness of the quenched alloy strip is measured by a micrometer and measured for 100 strips each time, and the strip thicknesses are recorded. When measuring, it has to be random sampled to measure the thickness, one strip is only once measured, the measured position is near to the geometric center of the alloy strip, and the strip can not be bended for measuring. The samples should be taken from upper layer, central layer and lower layer.

(10) To avoid impurity and pollution, the staff should wear disposable grooves when measuring.

(11) As can be seen from the measuring result, in weight ratio, the thicknesses of 95% of the quenched alloy of Embodiment 3, Embodiment 4, embodiment 5 and embodiment 11, embodiment 12, embodiment 13 are in a range of 0.10.7 mm.

(12) In the hydrogen decrepitation process: the quenched alloy and a plurality of steel balls of 10 mm40 mm are put into a container of the hydrogen decrepitation furnace, then the container is pumped to be vacuum at room temperature, then filling with hydrogen with 99.999% purity so that the hydrogen pressure is configured to reach 0.03 Mpa, absorbing hydrogen for 2 hours, during the hydrogen absorption, the container rotates at a rotating rate of 60 rpm, at the same time, the quenched alloy is ball milled, then keeping vacuum in 600 C. for 2 hours, and then cooling the container and taking the powder out.

(13) Taking the powder out, firstly the mixture is screened for separating the balls and the powder, then the powder is screened by a 500 mesh ultrasonic screen, the screened powder is then collected. The screened fine powder has a recovery rate of over 99.5%.

(14) Methyl caprylate is added to the screened powder, the additive amount is 0.4% of the weight of the screened powder, the mixture is comprehensively blended by a V-type mixer for 1 hour.

(15) In the compacting process under a magnetic field: a transversed type magnetic field molder is used, the powder with methyl caprylate is compacted in once to form a cube with sides of 40 mm in an orientation filed of 2.1 T and under a compacting pressure of 0.2 ton/cm.sup.2, then the once-forming cube is demagnetized in a 0.2 T magnetic filed. The once-forming compact (green compact) is sealed so as not to expose to air, the compact is secondary compacted by a secondary compact machine (isostatic pressing compacting machine) under a pressure of 1.2 ton/cm.sup.2.

(16) In the examination of corner-breakage of the green compact: permanent magnet material is unqualified with even a little bit corner-breakage, by visual inspection, if there are broken, corner breakage or crack with a length of more than 1 mm, it may be determined as unqualified and the defective rate is counted.

(17) In the sintering progress: the green compact is moved to a sintering furnace to sinter, in a vacuum of 10.sup.3 and respectively maintained for 2 hours in 200 C. and for 2 hours in 900 C., then in Ar gas atmosphere and under 1000 Pa pressure, sintering for 2 hours in 1080 C., after that filling Ar gas into the sintering furnace so that the Ar pressure would reach 0.1 MPa, then cooling it to room temperature.

(18) In the heating progress: the sintered magnet is heated for 1 hour in 450 C. in the atmosphere of high purity Ar gas, then cooling it to room temperature and taking it out.

(19) In magnetic property evaluation process: the sintered magnet is tested by NIM-10000H type nondestructive testing system for BH large rare earth permanent magnet of China Jiliang University.

(20) In the oxygen content of sintered magnet evaluation process: the oxygen content of the sintered magnet is measured by EMGA-620W type oxygen and nitrogen analyzer from HORIBA company of Japan.

(21) The magnetic property evaluation results of the embodiments and the comparing samples are shown in TABLE 2 and TABLE 3:

(22) TABLE-US-00002 TABLE 2 The magnetic property and oxygen content evaluation of the embodiments and the comparing samples. Oxygen Average Defective content of strip rate of the the sintered thickness compact (BH)max magnet No. (mm) (%) Br (kGs) Hcj(k0e) SQ (%) (MG0e) (ppm) 1 Comparing 0.07 21 10.2 11.6 82.3 22.4 689 sample 2 Comparing 0.1 1 11.2 35.1 98.2 31.2 276 sample 3 embodiment 0.2 0 11.3 35.3 99.1 31.3 275 4 embodiment 0.3 0 11.2 35.2 99.1 31.2 269 5 embodiment 0.4 0 11.3 34.1 99.2 31.2 283 6 Comparing 0.5 1 11.3 34.8 98.5 31.1 265 sample 7 Comparing 0.7 24 10.6 27.6 84.2 21.2 324 sample 8 Comparing 1 67 10.2 24.3 78.6 18.5 478 sample

(23) TABLE-US-00003 TABLE 3 The magnetic property and oxygen content evaluation of the embodiments and the comparing samples. Oxygen Average Defective content of strip rate of the the sintered thickness compact (BH)max magnet No. (mm) (%) Br (kGs) Hcj(k0e) SQ (%) (MG0e) (ppm) 9 Comparing 0.05 29 12.6 26.7 77.3 25.3 923 sample 10 Comparing 0.1 1 11.2 35.6 98.1 31.2 282 sample 11 embodiment 0.2 0 11.3 35.8 99 31.2 275 12 embodiment 0.3 0 11.3 35.6 99 31.3 270 13 embodiment 0.4 0 11.3 35.6 99 31.3 275 14 Comparing 0.5 1 11.2 35.5 98.3 31 271 sample 15 Comparing 0.7 23 10.2 28.6 85.5 22.3 578 sample 16 Comparing 10 67 9.8 27.5 79.2 19.8 768 sample

(24) As can be seen from the embodiments and the comparing samples, the steel balls are put into the rotating container, the process of ball milling works along with the process of hydrogen decrepitation consequently, therefore further improving the powder crushing effect of the hydrogen decrepitation with the process of ball milling as a further process of milling is introduced.

(25) The steel balls can be generally placed in the container of the stainless steel rotating hydrogen decrepitation furnace and need not to be taken out.

(26) As can be seen from above embodiment, the quenched alloy has best condition of thickness. As a relatively thinner strip of raw material has more amorphous phase and isometric crystal, which may result in bad orientation degree, reducing of the contents of Br, (BH)max; in addition, due to the easily oxygenated ultra fine powder, the oxygen content may increase, and the properties of coercivity and squareness may be worse consequently. As a relatively thicker strip of raw material has more -Fe and R.sub.2Fe.sub.17 phase, large amount of Nd rich phase may lead to bad orientation degree and reducing of the contents of Br, (BH)max, besides, due to the easily oxygenated Nd rich phase, the oxygen content may increase, and the properties of coercivity and squareness may be worse consequently.

(27) Besides, the present invention is capable of controlling the average cooling rate of the molten alloy to obtain a strip casting with evenly crystals and reducing the number of oversize crystals and undersize crystals, so that even omitting jet milling process, it can obtain desirable powder for compacting.

Embodiment 2

(28) In the raw material preparing process: Nd, Ho, Y with 99.9% purity; industrial FeB, FeP, FeCr; industrial pure Fe; Ni, Si with 99.9% purity and Bi, V with 99.5% purity are prepared, counted in atomic percent, and prepared in R.sub.eT.sub.fA.sub.gJ.sub.hG.sub.iD.sub.k components.

(29) The contents of the elements are shown in TABLE 4:

(30) TABLE-US-00004 TABLE 4 proportioning of each element R T A J G D Nd Ho Y Fe Ni B P Cr Si Bi V 11 2 0.5 78.7 0.3 6.55 0.05 0.2 0.1 0.3 0.3

(31) Preparing 16 copies of 100 Kg raw material by weighing in accordance with TABLE 4.

(32) In the melting process: 100 Kg of the prepared raw material is put into an aluminum oxide made crucible, an intermediate frequency vacuum induction melting furnace is used to melt the raw material in 10.sup.3 Pa vacuum in 1600 C.

(33) In casting process: Ar gas is filled to the melting furnace so that the Ar pressure would reach to 40000 Pa after vacuum melting, then on a water cooling casting plain disk, the material is casted to the quenched alloy in a cooling rate of 10.sup.2 C./s10.sup.4 C./s with average cooling rate of 1*10.sup.3 C./s8*10.sup.3 C./s.

(34) The thickness of the quenched alloy depends on the rotating rate of the water-cooling casting plain disk.

(35) The strip thickness of the quenched alloy strip is measured by a micrometer and measured for 100 strips each time, and the strip thicknesses are recorded. When measuring, it has to be random sampled to measure the thickness, one strip is only once measured, the measured position is near to the geometric center of the alloy strip, the strip can not be bended for measuring. The samples should be taken from upper layer, central layer and lower layer.

(36) To avoid impurity and pollution, the staff should wear disposable grooves when measuring.

(37) As can be seen from the measuring result, the average thickness of the quenched alloy is 0.25 mm, in weight ratio, 98% of the quenched alloy has the thickness in a range of 0.10.7 mm.

(38) In the hydrogen decrepitation process: each copy of the quenched alloy with serial numbers 17 and a plurality of tungsten carbide balls of 40 g and 5 mm60 mm are put into a container of a stainless steel rotating hydrogen decrepitation furnace, the inner diameter of the container is 1000 mm, then the container is pumped to be vacuum, then respectively filling with hydrogen of 99.99% purity and so that the hydrogen pressures are configured to respectively reach the pressures of serial numbers 17, absorbing hydrogen for 0.5 hour, pumping the furnace to be vacuum in 650 C. for 2 hours, during the hydrogen absorption and pumping processes, the stainless steel rotating container rotates at a rotating rate of 30 rpm, and the processes of hydrogen decrepitiaon and ball milling are performed simultaneously, and then cooling the container and taking the powder out. The mixture is screened by a 5 mesh screen for separating the balls and the powder, then the powder is milled by a disk miller and then screened by a 500 mesh ultrasonic screen, the screened powder is then collected. The screened fine powder has a recovery rate of over 99.7%.

(39) And in another experiment, each copy of the quenched alloy with serial numbers 816 and a plurality of tungsten carbide balls of 20 g and 3 mm20 mm are put into the stainless steel container of the hydrogen decrepitation furnace with inner diameter 600 mm, the container is pumped to be vacuum, then respectively be adjusted to reach the temperatures of No. 816, filling the hydrogen gas of 99.999% purity and so that the hydrogen pressure would reach 0.3 MPa, absorbing hydrogen absorption for 10 hours, and pumping the furnace to be vacuum in 650 C. for 2 hours, during the processes of hydrogen absorption and pumping, the stainless steel rotating container rotates at a rotating rate of 100 rpm, the processes of hydrogen decrepitiaon and ball milling are performed simultaneously, and then cooling the container and taking the powder out. The mixture is screened by a 5 mesh screen for separating the balls and the powder, then the powder is milled by a disk miller and then screened by a 800 mesh ultrasonic screen, the screened powder is then collected. The screened fine powder has a recovery rate of over 99.7%.

(40) Methyl caprylate is added to the screened powder, the additive amount is 0.2% of the weight of the screened powder, the mixture is comprehensively blended by a V-type mixer for 1 hour.

(41) In the compacting process under a magnetic field: a transversed type magnetic field molder is used, the powder with methyl caprylate is compacted in once to form a cube with sides of 25 mm in an orientation filed of 1.8 T and under a compacting pressure of 0.2 ton/cm.sup.2, then the once-forming cube is demagnetized in a 0.2 T magnetic filed. The once-forming compact (green compact) is sealed so as not to expose to air, the compact is secondary compacted by a secondary compact machine (isostatic pressing compacting machine) under a pressure of 1.2 ton/cm.sup.2.

(42) In the examination of corner-breakage of the green compact: permanent magnet material is unqualified with even a little bit corner-breakage, by visual inspection, if there are broken, corner breakage or crack with a length of more than 1 mm, it may be determined as unqualified and the defective rate is counted.

(43) In the sintering progress: the green compact is moved to the sintering furnace to sinter, in a vacuum of 10.sup.1 Pa and respectively maintained for 2 hours in 200 C. and for 2 hours in 900 C., then sintering for 4 hours in 980 C., after that filling Ar gas into the sintering furnace so that the Ar pressure would reach 0.1 MPa, then cooling it to room temperature.

(44) In the heating progress: the sintered magnet is heated for 1 hour in 650 C. in the atmosphere of high purity Ar gas, then cooling it to room temperature and taking it out.

(45) In magnetic property evaluation process: the sintered magnet is tested by NIM-10000H type nondestructive testing system for BH large rare earth permanent magnet from China Jiliang University.

(46) In the oxygen content of sintered magnet evaluation process: the oxygen content of the sintered magnet is measured by EMGA-620W type oxygen and nitrogen analyzer from HORIBA company of Japan.

(47) The magnetic property and oxygen content evaluation of the embodiments and the comparing samples in different pressures are shown in TABLE 5, the magnetic property and oxygen content evaluation of the embodiments in different preheating temperature of the quenched alloy are shown in TABLE 6.

(48) TABLE-US-00005 TABLE 5 The magnetic property and oxygen content evaluation of the embodiments and the comparing samples in different pressures. Oxygen Defective content of Hydrogen rate of the the sintered pressure compact (BH)max magnet No. (atm) (%) Br (kGs) Hcj(k0e) SQ(%) (MG0e) (ppm) 1 comparing 0.08 56 12.3 19.2 86.6 32.5 421 sample 2 embodiment 0.1 1 13 26.4 98.4 41.2 278 3 embodiment 0.6 0 13.1 26.5 99.2 41.3 276 4 embodiment 1.5 0 13.2 26.7 99.1 41.2 289 5 embodiment 6 0 13.1 26.3 99.1 41.1 282 6 embodiment 10 1 13.1 26.4 98.3 40.8 267 7 comparing 15 23 12.2 19.8 75.1 23.8 398 sample

(49) TABLE-US-00006 TABLE 6 The magnetic property and oxygen content evaluation of the embodiments in different preheating temperature of the quenched alloy. Oxygen Defective content of Preheat rate of the the sintered temperature compact (BH)max magnet No. ( ) (%) Br(kGs) Hcj(k0e) SQ(%) (MG0e) (ppm) 8 embodiment 25 2 13 26.1 96.7 41.4 324 9 embodiment 100 1 13.1 26.3 98.2 41.6 356 10 embodiment 150 0 13.2 27.2 99.1 42.2 253 11 embodiment 200 0 13.3 27.1 99.1 42.3 243 12 embodiment 250 0 13.3 27.4 99.1 42.3 212 13 embodiment 350 0 13.3 27.3 99 42.1 209 14 embodiment 450 0 13.3 27.1 98.2 42.1 162 15 embodiment 600 1 13.2 26.7 95.5 41.7 329 16 embodiment 650 2 13.1 26.3 94.5 41.6 397

(50) As can be seen from above, the present invention has the most appropriate decrepitation pressure in the hydrogen decrepitation process. In low pressure, the alloy can not fully absorb hydrogen, so that it can not be fully crushed. And if the hydrogen pressure is too high, there are safety risks, there may not only has safety risks, but also can not be fully crushed, the reason is that if the main phase and Nd rich absorb hydrogen at the same time, the decrepitation may be difficult, and also results in high defective rate.

(51) As can be seen from this embodiment, the present invention also discloses a proper preheating temperature range for the quenched alloy at the beginning of the hydrogen decrepitation, however, with the increasing of the initial temperature, the hydrogen amount mixed to the main phase may decrease consequently, and crack may happen along the Nd rich phase, furthermore, if the temperature reaches 600 C., the hydrogen absorbed by the Nd rich phase may decrease, thus may not acquire a comprehensive decrepitation.

(52) Same as the Embodiment 1, this embodiment is capable of controlling the average cooling rate of the molten alloy to obtain strips with evenly crystals and less oversize crystals and undersize crystals, so that even omitting jet milling process, it can make required powder for compacting.

Embodiment 3

(53) In the raw material preparing process: Nd, Pr, Dy with 99.9% purity; industrial FeB, C; industrial pure Fe; Cu, Sn, Hf, Co with 99.9% purity are prepared, in atomic percent, prepared in R.sub.eT.sub.fA.sub.gJ.sub.hG.sub.iD.sub.k components.

(54) The contents of the elements are shown in TABLE 7:

(55) TABLE-US-00007 TABLE 7 proportioning of each element R T A J G D No. Nd Pr Dy Fe Co B C Cu Sn Hf 1 12 3 0.6 75.9 0 6 0.25 0.05 0.2 2 2 12 3 0.6 75.5 0.4 6 0.25 0.05 0.2 2 3 12 3 0.6 74.9 1 6 0.25 0.05 0.2 2 4 12 3 0.6 74.5 1.4 6 0.25 0.05 0.2 2 5 12 3 0.6 73.9 2 6 0.25 0.05 0.2 2

(56) According to above 5 serial numbers, each serial number is prepared with 100 Kg raw material by respectively weighing.

(57) In the melting process: 100 Kg of the prepared raw material according to the serial number is put into an magnesium oxide made crucible respectively, an intermediate frequency vacuum induction melting furnace is used to melt the raw materials in 1 Pa vacuum below 1600 C.

(58) In casting process: Ar gas is filled to the melting furnace to 65000 Pa after vacuum melting, then a centrifugal casting device is used, the material is casted to the quenched alloy in a cooling rate of 10.sup.2 C./s10.sup.4 C./s with average cooling rate of 1*10.sup.3 C./s8*10.sup.3 C./s.

(59) The thickness of the quenched alloy depends on the rotating rate of the centrifugal casting device.

(60) The strip thickness of the quenched alloy strip is measured by a micrometer and for measured for 100 strips each time, and the strip thicknesses are recorded. When measuring, it has to be random sampled to measure the thickness, one strip is only once measured, the measured position is near to the geometric center of the alloy strip, the strip can not be bended for measuring. The samples should be taken from upper layer, central layer and lower layer.

(61) To avoid impurity and pollution, the staff should wear disposable grooves when measuring.

(62) As can be seen from the measuring result, the average thickness of the quenched alloy is 0.4 mm, in weight ratio, 95% of the quenched alloy has the thickness in a range of 0.10.7 mm.

(63) In the hydrogen decrepitation process: the quenched alloy with average thickness of 0.4 mm and a plurality of stainless steel balls of 10 g and 20 mm40 mm are put into a container of the hydrogen decrepitation furnace with inner diameter of 1000 mm, then the container is pumped to be vacuum and heated to 200 C. under a pressure of 10.sup.2 Pa, then filling hydrogen with 99.999% purity into the container so that the pressure would reach 0.1 Mpa, absorbing hydrogen for 0.2 hour, and pumping to be vacuum for 0.5 hour in 550 C., during the processes of the hydrogen absorption and vacuum pumping, the container rotates at a rotating rate of 100 rpm, at the same time, the quenched alloy is ball milled and cooled afterward, then taking the powder out. After taking the powder out, firstly the mixture is screened by a 3 mesh screen for separating the balls and the powder, then the powder is screened by a 300 mesh ultrasonic screen after passing through a continuous mortar type grinder, the screened powder is then collected. The screened fine powder has a recovery rate of over 99.95%.

(64) Methyl caprylate is added to the screened powder, the additive amount is 0.2% of the weight of the screened powder, the mixture is comprehensively blended by a V-type mixer for 1 hour.

(65) In pressing under magnetic field process: a traversed type magnetic field molder is used, the powder with methyl caprylate is compacted in once to form a cube with sides of 25 mm in an orientation filed of 2.2 T and under a compacting pressure of 0.3 ton/cm.sup.2, then the once-forming cube is demagnetized in a magnetic filed of 0.15 T. The once-forming compact (green compact) is sealed so as not to expose to air, the compact is secondary compacted by a secondary compact machine (isostatic pressing compacting machine) under a pressure of 1.0 ton/cm.sup.2.

(66) In the examination of corner-breakage of the green compact: permanent magnet material is unqualified with even a little bit corner-breakage, by visual inspection, if there are broken, corner breakage or crack with a length of more than 1 mm, it may be determined as unqualified and the defective rate is counted.

(67) In the sintering progress: the green compact is moved to a sintering furnace to sinter, in a vacuum of 10.sup.2 Pa and respectively maintained for 2 hours in 150 C., for 2 hours in 650 C. and for 2 hours in 800 C., then sintering for 4 hours in 1080 C., after that filling Ar gas into the sintering furnace so that the Ar pressure would reach 10000 Pa, then cooling it to room temperature.

(68) In the heating progress: the sintered magnet is heated for 1 hour in 540 C. in the atmosphere of high purity Ar gas, then taking it out after cooling it to room temperature.

(69) In magnetic property evaluation process: the sintered magnet is tested by NIM-10000H type nondestructive testing system for BH large rare earth permanent magnet of China Jiliang University.

(70) In the oxygen content of sintered magnet evaluation process: the oxygen content of the sintered magnet is measured by EMGA-620W type oxygen and nitrogen analyzer from HORIBA company of Japan.

(71) The magnetic property evaluation results of the embodiments are shown in TABLE 8:

(72) TABLE-US-00008 TABLE 8 The magnetic property and oxygen content evaluation of the embodiments. Oxygen Additive Defective content of amount rate of the the sintered of Co compact (BH)max magnet No. (at %) (%) Br(kGs) Hcj(k0e) SQ(%) (MG0e) (ppm) 1 Embodiment 0 0 13.1 18.3 99.4 42.2 245 2 Embodiment 0.4 0 13 18.1 98.4 42.1 258 3 Embodiment 1 1 12.9 18.2 98.1 42 265 4 Embodiment 1.4 2 12.7 17.3 95.7 40.9 276 5 Embodiment 2 4 12.5 17.1 94.3 36.8 285

(73) As can be seen from above embodiments and comparing samples, the crushing method of the present invention has most appropriate additive amount of Co, if the additive amount of Co is too much, it may result in bad crushing effect and increasing of defective rate. Based on investigation of the powder by X-ray diffraction, with the increasing of the additive amount of Co, R.sub.2Co.sub.2 and R.sub.2Co.sub.3 crystal can be observed, it can be noted that, metallic compound with Co doesn't absorb hydrogen, thus resulting in bad crushing and formability effects.

(74) Same as the Embodiment 1, this embodiment is capable of controlling the average cooling rate of the molten alloy to obtain a strip casting with evenly crystals and reducing the number of oversize crystals and undersize crystals, so that even omitting jet milling process, it can obtain desirable powder for compacting.

(75) Although the present invention has been described with reference to the preferred embodiments thereof for carrying out the patent for invention, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the patent for invention which is intended to be defined by the appended claims.