METHOD OF MANUFACTURING MAGNETIC POWDER FOR A MAGNETICALLY ANISOTROPIC BOND MAGNET

Abstract: A magnetically anisotropic bond permanent magnet consists essentially of 15 to 40 vol-% of a resin binder, the balance being formed of R-TM-B-M system alloy powder. In this alloy system, R is at least one of the rear-earth ele­ ments including Y; TM is Te, which may be partly substitut­ ed by CO; and M is at least one of the elements Si, Al, Nb, Zr, Hf, Mo, P and C as an additive. B is boron. The alloy powder has an average crystal size of 0.01 to 0.5 µm and is magnetically anisotropic. The easy-magnetizing axes of the alloy powder are directed to a given direction. Such a per­ manent magnet has excellent thermal stability and magne­ tizing properties.

Description: This invention relates to method of manufacturing alloy powder of rare earth elements-iron-boron with crystalline magnetic anisotropy. As typical rare earth permanent magnets, there are a permanent magnet of the SmCO₅ system and a permanent magnet of the Sm₂ CO₁₇ system. These samarium cobalt magnets are produced with the following procedures: Ingot composed of samarium and cobalt is made by mixing samarium and cobalt and then melting the mixture in vacuum or inactive atmosphere. After the ingot has been crushed into fine powder, the powder is molded in a magnetic field and the green body is obtained, a permanent magnet is made by sintering the green body and then heat treating the sintered body. As mentioned above, the samarium cobalt magnet is caused with magnetic anistropy by being molded in a magnetic field. The magnetic property of the magnet is improved substantially by causing magnetic anisotropy. Resin-bonded permanent magnet of samarium cobalt magnet can obtain its magnetic anisotropy by mixing the crushed powder of sintered anisotropic samarium cobalt magnet with resin and molding the powder in a magnetic field either by injecting it into a molding die or by compressing it in a molding die. In this way, the resin-bonded samarium cobalt magnet can be produced by making the sintered magnetically anisotropic magnet and by crushing and then mixing it with resin. As compared with the samarium cobalt magnet, a rare earth magnet of new type, that is, the neodymium-iron-boron magnet has been proposed. Those proposals made with Japan Patent Laid-Open Nos. Showa 59-46008 and Showa 59-64733 proposed that, in the same way as in samarium cobalt magnet, ingot of the neodymium-iron-boron alloy is prepared and crushed into fine powder and then the powder is molded in the magnetic field and the green body is obtained. By sintering the green body and heat-treating. the sintered body, permanent magnet is prepared. This method is called the powder metallurgy method. Apart from the abovementioned powder metallurgy method, a different manufacturing method of the Nd-Fe-B system permanent magnet has been proposed by General Motors Corp. For example, the proposals have been made by Japanese Patent Laid-Opens as follows and so on: According to these publications, after neodymium, iron and boron have been mixed and melted, molten metal is rapidly quenched with such technology as spinning. The Nd₂Fe₁₄B alloy is crystallized by heat-treating the flake of the noncrystal alloy. The magnetic alloy made in this way has magnetic isotropy. Patent Laid-Open No. 60-100402 describes such technology as to furnish the isotropic magnetic alloy with magnetic anisotropy by means of making it a green body by hot press and thereafter causing plastic streaming in a part of the green body under high temperature and high pressure. This NdFeB magnet has the following problems: Firstly, although the abovementioned powder metallurgy process allows to furnish magnet with magnetic anisotropy and the obtainable magnetic property is so excellent as to reach 2.8 to 3.6 × 10⁹ A/m, its Curie point is substantially low, its crystal grain size is also large, and its thermal stability is inferior. Accordingly, it has such a shortcoming that the NdFeB magnet is not applicable to those used for motors, etc. under high temperature environment. By contrast, although the method to mix the rapidly-quenched powder with resin possibly makes such molding as compression molding comparatively easy, its obtainable magnetic property is low because it has magnetic isotropy. For example, the magnetic property obtainable by injection molding has (BH)max 2.4 to 4 × 10⁸ A/m and the one obtainable by compression molding has (BH)max 6.4 to 8 × 10⁸ A/m and, in addition, dependency on the strength of the magnetizing magnetic field. In order to obtain (BH)max 6.4 × 10⁸ A/m, the strength of the magnetizing magnetic field of about 4 × 10⁶ A/m is usually required and it is difficult in using this magnet in various kind of applications to magnetize it after it has been assembled. The hot pressing of the rapidly-quenched powder improves the weather-proof property as the result of the density increases which makes the magnet free of void holes. But since it has isotropy, it has the same problems as in the case of the permanent magnet made with the process to mix the rapidly-quenched powder with resin. Although obtainable (BH)max is increased so much as the density has been improved and about 9.6 × 10⁸ A/m is obtainable, it is still impossible to magnetize it after assembled. By causing plastic streaming of the rapidly-quenched powder after hot press, it becomes possible to furnish the magnet with magnetic anisotropy in the same way as in the case by the powder metallurgy process and obtain (BH)max of 2.8-3.2 × 10⁹ A/m. However, it is difficult to make a ring type magnet (for example, a magnet of 30 mm outside diameter × 25 mm inside diameter × 20 mm thickness) because the use of the upsetting process is required to furnish the magnet with magnetic anisotropy. As described in pages 670-672 of the Applied Physics Letters 48 (10), March 1986, it is possible to furnish the magnet with magnetic anisotropy by crushing ingot into 0.5-2 µm and making a bond magnet by solidifying the crushed powder with wax. However, on account of the grain size of the powder to be handled being fine, the possible danger of inflaming makes handling it in the air atmosphere impossible. In addition, since the squareness ratio of the demagnetization curve of the powder is comparatively low, the magnet cannot show a high magnetic property. In order to obtain a bond magnet with magnetic anisotropy, the sintered magnet with magnetic anisotropy made by the powder metallurgy process was crushed, the crushed particles were mixed with resin and the magnet body was molded in a DC magnetic field. However, it appeared that a high magnetic property was not obtainable. The object of the invention is to eliminate such shortcoming as abovementioned caused by depending on the conventional technologies. Another object of the invention is to provide a magnetic powder with crystalline magnetic anisotropy. The sole Figure shows a comparison of thermal stability among the anisotropic bond magnet and anisotropic sintered magnet both composed of Nd₁₃DyFe₇₉B₆Al as well as the Sm₂ Co₁₇ system magnet. Namely, the abovementioned objects are accomplished by the method defined in claim 1. The abovementioned alloy preferably consists essentially of 11-18 at % of R, 4-11 at % of B, 30 at % or less of Co, and the balance of Fe and unavoidable impurities and more preferably 11-18 at % or R, 4-11 at % of B, 30 at % or less of Co, 0.001-3% of the additives (the additive is at least one selected from the group of Si, Al, Nb, Zr, Hf, Mo, P and C) and the balance of Fe and unavoidable impurities. In order to obtain a magnetically anisotropic bond magnet with high properties, it is required that the residual induction of the anisotropic direction of the R-Fe-B system alloy to be crushed should be 0.8 T or more. In addition, the R-Fe-B system alloy should be the alloy furnished with magnetic anisotropy by plastic deformation after the flake of the alloy obtained by the rapidly-quenching process has been made highly dense with hot isostatic press (HIP) and hot press (HP). One of the abovementioned measures for plastically deforming the alloy is the hot upsetting process or hot die-upsetting process. The amount of the additive elements is preferable to be 0.001-3 at % and it is preferable that the average ratio of c to a is 2 or more in which (c) is the average crystal grain size in the direction perpendicular to the C axis of the grain and (a) is the average crystal grain size in the direction of the C axis. In this specification, the R-Fe-B alloy furnished with magnetic anisotropy means the R-Fe-B system alloy showing the anisotropic magnetic property in which the shape of the second quadrant of the 4n I-H demagnetization curve is different depending on the magnetizing direction. The residual induction of the consolidated body made by HIP from flake is usually 0.75 T or under and, by using the R-Fe-B alloy which has the residual induction of 0.8 T or more, it is possible to make the high performance bond magnet which has the residual magnetic flux density and the energy product both higher than those of isotropic bond magnet. In the invention, when the average crystal grain size becomes greater than 0.5 µm, intrinsic coercive force (IHc) is lowered and the irreversible demagnetizing coefficient at 160°C accounts for 10 at % or higher resulting in lowering thermal stability conspicuously which is inconvenient for using the magnet. In addition, when the average crystal grain size is smaller than 0.01 µm, IHc of the bond magnet after molding is low. Therefore, the average crystal grain size shall be 0.01-0.5 µm. Manufacture of the magnetic powder in the invention is carried out as follows: To begin with, the magnetic powder is manufactured by melting the alloy with the prescribed composition by high-frequency induction melting, arc melting, etc. and the molten alloy is solidified to flake by the rapidly-quenching process. For the rapid-quenching, either the single roll method or the double roll method is applicable and the material of the rolls may be Fe, Cu, etc. In case of using Cu, it is preferable to use the Cr plated rolls. In order to prevent oxidation, rapid-quenching is carried out in inert gas atmosphere of Ar, He, etc. The flake is crushed into the coarse grain size of about 100-200 µm. By molding the crushed coarse grain powder at room temperature, a green body is obtained. By carrying out HIP or hot press of the green body at 600-750°C, it is possible to manufacture a compressed block having comparatively small crystal grain size. By upsetting the block at 600-750°C, anisotropic flat plate can be obtained. The greater the deformation ratio is, the better the anisotropy is improved. IHc obtainable is improved by heat treating the flat plate at 600-800°C. By crushing the flat plate, the coarse powder for magnetically anisotropic bond magnet can be obtained. By plastic deforming the crystal grain of the R-Fe-B system alloy furnished with magnetic anisotropy shows the flat shape in the direction of the C axis. In case of the average ratio of c to a being 2 or more in which (c) is the average crystal grain size in the direction perpendicular to the C axis and (a) is the average crystal grain size in the direction of the C axis, it is desirous for the purpose of obtaining the residual induction of 0.8 T or more. By the way, the average crystal grain size in this patent application means the average value of the diameters of the balls which have same volume as those of the crystal grains concerning more than 30 pieces of crystal grains. In case of plastic deformation being hot upsetting , it is possible to obtain the particularly high magnetic property. By heat treating to the magnetically anisotropic R-Fe-B system magnet, the coercive force of the magnet can be increased. It is preferable to make heat treatment temperature from 600°C to 900°C. The reason thereof is because, with the heat treatment temperature below 600°C, the coercive force cannot be increased whereas, with the one over 900°C, the coercive force becomes lower than thai at the time before heat treatment. The time required for the temperature of the samples becoming uniform may be acceptable as the time for the coercive force. Therefore, the retention time was set to 240 minutes or less taking the industrial productivity into account. The cooling speed shall be 1°C/s or higher. With a cooling speed lower than 1°C/s the coercive force becomes lower than before heat treatment. Hereinabove, the cooling speed means the average cooling speed with which a heat treatment temperature (°C) goes down up to (the heat treatment temperature + room temperature) ÷ 2(°C). The R-Fe-B system alloy means such an alloy that contains R₂Fe₁₄B or R₂ (Fe, Co)₁₄B as the main phase. The reason for the range of compositions recommendable for a permanent magnet is as follows: In case R (a combination of at least one or rare earth elements including Y) is less than 11 at %, sufficient IHc cannot be obtained and, in case R exceeds 18 at %, it causes Br lower. The amount of R shall be 11-18 at %, accordingly. In case the amount of B is less than 4 at %, formation of the R₂Fe₁₄B phase, which is the main phase of the magnet, is insufficient and both Br and IHc are low; as well. In addition, in case the amount of B exceeds 11 at %, Br is lowered due to an undesirous phase in terms of the magnetic property comes into existence. The amount of B shall be 4-11 at %, accordingly. In case the amount of Co exceeds 30 at %, the Curie point is improved by the anisotropy constant of the main phase is lowered and high IHc cannot be obtained. The amount of Co shall be 30 at % or less, accordingly. Si, Al, Nb, Zr, Hf, P and C may be added to the alloy additives. Si has the effect to cause the Curie point to go up and Al, Nb and P have the effect to cause the coercive force to go up. C is an element which is apt to be mixed in at the time of electrolysis but, if the amount is small, it does not affect adversely on the magnetic property. Nb, Zr, Hf and Mo improve the anti-corrosive property. In case the amount of these additive elements is less than 0.001 at %, the effect of these added elements is insufficient but in case such amount exceeds 3 at %, Br is lowered to a great extent and this is undesirous. The amount of the additive elements shall be 0.001 at % - 3 at %, accordingly. In addition, it is permitted that impurify Al included in ferro-boron or reducing agents and impurities included at the time of reducing rare earth elements may exist in the alloys of the invention. If the average grain size is smaller than 1 µm, it is apt to cause inflammation and handling such powder in the air atmosphere is difficult. If the average grain size is greater than 1,000 µm, it is difficult to apply such powder to making a thin magnet (thickness 1-2 mm) and such powder is not suited to injection molding, as well. Such being the case, the average grain size shall be the abovementioned range. As the crushing method, the usual method using for making the magnetic powder is available as it is. Namely, disc mill, brown mill, attritor, ball mill, vibration mill, jet mill, etc. can be used. By adding a thermosetting binder to the said coarse powder and making the powder thermoset after compression molding in the magnetic field, it is possible to obtain an anisotropic bond magnet of the compression molded type. As an alternative, by adding the thermoplastic binder to the coarse powder and injection molding, it is possible to obtain an anisotropic bond magnet of the injection molded type. Among the materials which can be used as a binder aforementioned, the material easiest to use in case of compression molding is thermosetting resin. Polyamide, polyimide, polyester, polyphenol, fluorine, silicon, epoxy, etc. can be used which show all thermal stability. In addition, Al, Sn, Pb and various sorts of soldering alloys of low melting points can be used. In case of injection molding, thermoplastic resin such as EVA, nylon, etc. can be used in accordance with the usage. Example 1 An Nd₁₇Fe₇₅B₈ alloy was made by arc fusing and flake shaped filaments of the alloy were produced by rapid-quenching with the single roll method in Ar atmosphere. The peripheral speed of the roll was 30 m/ sec and the obtained filaments were about 30 µm thick and of indeterminate form and, as a result of the X-ray diffraction, it was found that these filaments were the mixtures of the amorphous substance and crystal substance. After crushing roughly these filaments so as to be 32 mesh or under, a green body was made with die compacting. The molding pressure was 6 ton/cm² and application of the magnetic field was not made. The density of the green body was 5.8 g/cm³. The green body was hot pressed at 700°C with the pressure of 2 kbar. The density of the molded body obtained by hot pressing was 7.30 g/cm³ by which high density could be furnished sufficiently. The bulk body with the high density was furthermore processed with upsetting at 700°C. The height of the sample was adjusted so as to make the deformation ratio 3 when compared before and after upsetting processing. (The deformation ratio ho/h = 3, when ho is the height before upsetting and h is the one after upsetting.) The sample processed with upsetting was heated up to 750°C in Ar atmosphere and, after having been retained, the sample was water cooled. The cooling speed was 7°C/s. The magnetic properties before and after heat treatment are shown in Table 1. It can be seen that the coercive force is improved by heat treatment. By crushing roughly the heat treated sample and adjusting the range of the grain size of the crushed sample so as to become 250-500 µm, the magnetic powder was obtained. 16 vol% of epoxy resin was mixed with the magnetic powder with the dry mixer and lateral magnetic field molding of the powder was carried out in the magnetic field of 8 × 10⁵ A/m. Next, by thermosetting at 120°C for 3 hrs, the molded body was made into an anisotropic bond magnet. When measured with the strength of the magnetizing magnet field of 2 × 10⁶ A/m, the anisotropic bond magnet showed such magnetic properties as Br = 0.68 T, BHc = 5.04 × 10⁵ A/m, IHc = 9.84 × 10⁵ A/m, (BH)max = 8.48 × 10⁸ A/m. With the purpose of comparison, the rapidly-quenched filaments of an alloy composed of Nd₁₇Fe₇₅B₈ were heat treated in a vacuum at 600°C for 1 hr, crushed roughly into 250-500 µm, and made into a bond magnet with the same method as the one used for the example. However, application of the magnetic field was not made in compression molding of the reference bond magnet because the magnet was isotropic. The magnetic properties obtained by the strength of the magnetizing magnetic field of 2 × 10⁶ A/m were Br = 4.72 × 10⁵ A/m, BHc = 3,92 × 10⁵ A/m, IHc = 1.02 × 10⁶ A/m, (BH)max = 5.28 × 10⁸ A/m. When compared with the isotropic bond magnet, it is found that the anisotropic bond magnet made by the invention has the better magnetizing properties and can obtain the higher magnetic properties. In addition, with the purpose of comparing these properties, a piece of ingot of an alloy composed of Nd₁₇Fe₇₅B₈ was crushed roughly, mixed with the binder, molded in a magnetic field and treated with thermosetting with the same method as the one used for the example. The magnetic properties obtained by the strength of the magnetizing magnetic field of 2 × 10⁶ A/m were Br = 4 × 10⁵ A/m, BHc = 6.4 × 10⁴ A/m, IHc = 9.6 × 10⁴ A/m, (BH)max = 6 × 10⁷ A/m. In such a way as this, it can be seen that the anisotropic bond magnet prepared from ingot as raw material cannot obtain high IHC and cannot be utilized as material for practical use. The results obtained from example 1 in the above is shown in Table 2 together with the references. Example 2 It is shown in the next how the deformation ratio at the time of upsetting process affects the anisotropic bond magnet which can be obtained ultimately. The conditions of the composition, rapidly-quenching, hot press, lateral magnetic field molding, heat treatment, thermosetting etc. are same as those in example 1. The results are shown in Table 3. The magnetic properties shown in Table 3 are the value obtained by the magnetizing strength of 2 × 10⁶ A/m. As shown in Table 3, by increasing the deformation ratio, the magnetic properties of the anisotropic bond magnet are improved. In addition, when the deformation ratio was ho/h ≧ 5.6, cracks generated in the periphery of the sample after upsetting process but no impact comes to appearance in the anisotropic bond magnet of compression molded type which is the ultimate product. Example 3 The Nd₁₄Fe₈₀B₆ alloy was treated into magnetic powder with the same method as for the example 1. The magnetic powder was kneaded with 33 vol% of EVA and pellets were made. Using the pellets, injection molding was done at 150°C. The form of the test piece obtained from injection molding was 20 mm dia. × 10 mm t and the magnetic field applied at the time of injection molding was 6.4 × 10⁵ A/m. The magnetic properties were Br = 0.56 T, BHc = 3.92 × 10⁵ A/m, IHc = 10.4 × 10⁵ A/m, (BH)max = 5.12 × 10⁸ A/m. The magnetic property is the value with the magnetizing strength of 2 × 10⁹ A/m. Example 4 The anisotropic bond magnets having the composition shown in Table 4 were prepared with the method same as for example 1. The making method was compression molding. The obtained magnetic properties are shown in Table 5. Example 5 The magnetic powder was made by treating the Nd₁₆Fe₇₅B₇AlSi alloy with the same method as for example 1. Using the magnetic powder, pellets were made by kneading the magnetic powder with binder EVA and a magnet with the inner diameter 12 mm, outer diameter 16 mm and height 25 mm was obtained by injection molding. The anisotropy of the said magnet is radial direction and, in order to evaluate the magnetic property, a sample of 1.5 mm × 1.5 mm × 1.5 mm was cut and magnetic measurement was conducted with the sample. The magnetic properties were Br = 0.55 T, BHc = 3.76 × 10⁵ A/m, IHc = 10⁵ A/m, (BH)max = 5.04 × 10⁸ A/m. Example 6 The anisotropic bond magnet of compression molded type composed of Nd₁₃DyFe₇₉B₈Al was prepared with the same method as for example 1. The magnetic properties were Br = 0.66 T, BHc = 4.96 × 10⁵ A/m, IHc = 16.8 × 10⁵ A/m, (BH)max = 8.16 × 10⁸ A/m. The crystal grain size of the magnet was 0.11 µm. The magnet was machined so as to be 10 mm dia. × 7 mm t and thermal stability was tested. The results are shown in Fig. 1. As the materials to be compared with the sample, the anisotropic sintered Sm2C017 magnet and the R-Fe-B anisotropic sintered magnet with same composition as that of the sample were used. It can be seen that the anisotropic bond magnet has thermal stability superior to the anisotropic sintered magnet when compared. Example 7 The Nd₁₄Fe₈₀B₆ anisotropic bond magnets were made with the same method as for the example 1 except the crushed grain size of the magnetic powder. By using the Nd₁₃Dy₂Fe₇₈B₇ anisotropic sintered magnet for reference, the change in the coercive force depending on the change in the crushed grain size was checked. The results are shown in Table 6. Although, when the sintered body is crushed, the coercive force is lowered and becomes unable to be used as raw material for making bond magnets, it is seen that the material made by the invention shows almost no lowering of the coercive force. Example 8 The anisotropic bond magnets were made with the same method as for example 1 except that the crystal grain size was changed by changing the temperature for upsetting. The results are shown in Table 7. It can be seen that, when the average crystal size is from 0.001 µm to 0.5 µm, the magnet has superior magnetic properties. Example 9 The R-Fe-B system permanent magnets were made with the same method as for example 1 except the retention time in heat treatment. The results are shown in Table 8. It can be seen that there is no change in the magnetic properties provided that the retention time at 750°C is within 240 minutes. Example 10 The R-Fe-B system permanent magnets were made with the same method as for example 1 except that the heat treatment temperature was varied and the retention time was set to 10 minutes. The results are shown in Table 9. It can be seen that the superior magnetic properties are shown when the heat treatment temperature is 600-900°C. Example 11 The R-Fe-B permanent magnets were made with the same method as for example 1 except that the retention time was set to 10 minutes and the cooling method was varied. The results are shown in Table 10. The superior results can be obtained when the cooling speed is 1°C/sec or over. As described in the above, the magnetic powder for the anisotropic bond magnet is excellent in the magnetizing property and its irreversible demagnetizing factor is small even under the environment of relatively high temperature and, therefore, it is useful as the raw material used for an anisotropic bond magnet which can be magnetized after the magnet has been assembled.

Claims: A method of manufacturing magnetic powder for a magnetically anisotropic bond magnet, comprising the steps of rapidly-quenching a molten metal of the R-TM-B-M alloy, wherein R is at least one of rare earth elements including Y; TM is Fe or Fe a part of which has been substituted by Co; B is boron; and M is at least one selected from the group of Si, Al, Nb, Zr, Hf, Mo, P and C as additives, to make flakes of the alloy, compacting the flakes to form a high density body, plastically deforming the body to produce an alloy having an average crystal grain size of 0.01 to 0.5 µm and magnetic anisotropy, heat-treating the alloy to enhance the coercive force of the alloy and crushing it into particles of 1 to 1000 µm. The method of Claim 1, wherein the anisotropic R-TM-B-M system alloy is heated to a temperature of from 600°C to 900°C, retained at the temperature for no longer than 240 min, and then cooled at a cooling rate of 1°C/s or higher. The method of Claim 2, wherein the R-TM-B-M system alloy consists essentially of 11 to 18 atom-% of rare earth elements, 4 to 11 atom-% of boron, 30 atom-% or less of Co, 3 atom-% or less of the additives and the balance of Fe and unavoidable impurities. The method of any of the preceding Claims, wherein M is preferably Si, Al, Nb, Zr, P or C.

Inventors: IWASAKI KATSUNORI, NOZAWA YASUTO, TOKUNAGA MASATOKI, IWASAKI, KATSUNORI, NOZAWA, YASUTO, TOKUNAGA, MASATOKI
Applicants: HITACHI METALS LTD, HITACHI METALS, LTD.
Application year: 1987
Main classification: H01F1
IPC codes: C22C38, H01F1, B22F9