RU2525867C1 - METHOD TO PRODUCE SINTERED HIGHLY ENERGY-INTENSIVE PERMANENT MAGNET FROM ALLOY BASED ON Nd-Fe-B - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method includes manufacturing of an alloy based on Nd-Fe-B, its hydrogen embrittlement, coarse grinding and subsequent fine grinding in a vibration mill using toluene as a protective medium. Lubricants are added to toluene before grinding, such as dry powders of aluminium stearates, or copper, or zinc, or ethyl ethers of homological row of carbonic acids. After grinding of the powder to the average size of particles 2.5-3.5 mcm it is loaded in dry or moist condition into a container from molybdenum and/or graphite with charging density from 3.0 to 3.5 g/cm³, and texturing is carried out with a pulse magnetic field with subsequent vacuum sintering of powders along with slow heating to temperature of not more than 500°C.

EFFECT: reduced forces of friction between particles of powder textured in magnetic field with subsequent formation of sintered magnets with high extent of texture by magnetic characteristics.

5 cl, 2 dwg, 7 tbl, 4 ex

Description

The invention relates to the field of sintered permanent magnets and can be used in the production of high-energy permanent magnets based on rare-earth alloys and, in particular, based on alloys of the neodymium-iron-boron system (Nd-Fe-B).

High-energy sintered magnets of the Nd-Fe-B system are actively used in the magnetic systems of many modern technical devices, including computer hard drives, speaker systems for cell phones, NMR tomographs, and various types of electric motors. The development of energy-saving technologies is focused on the mass introduction of Nd-Fe-B magnets in motors and generators of hybrid and electric cars, wind generators, air conditioner compressors. At the same time, the sharp increase in prices for Nd and especially for deficient Dy and Tb, which are used to replace part of Nd in order to increase the coercive force of Nd-Fe-B magnets, stimulates a systematic search for new technical approaches aimed at solving the problem of economical production of Nd- magnets Fe-B, one of which is a method characterized by the absence of a stage of powder pressing, which was called the "pressless process" or in abbreviated form (PLP), and in Russian - the "process without pressing, in abbreviated form (BSP)".

In the known method of PBP [T. Hiroki, I. Naoyaki, JP 7153612 (A); M. Sagawa, JP 2007180373 (A); M. Sagawa, JP 2007180375 (A); M. Sagawa, Y. Une, A new process for producing Nd-Fe-B sintered magnets with small grain size, Proc. 20th Int. Workshop on Rare Earth Permanent Magnets and Their Applications (Knossos-Crete, 2008), p.103-105; WF Li, T. Ohkubo, K. Hono, M. Sagawa, The origin of coercivity decrease in fine grained Nd-Fe-B sintered magnets, J. Magn. Magn. Mater., 2009, V. 321, p. 1100-1105; M. Sagawa, Development and prospect of the Nd-Fe-B sintered magnets, Proc. of 21 ^st Int. Workshop on REPM and their Applications (Bled, Slovenia, 2010), p. 183-186;

Masato Sagawa, European Patent Application EP 2 187410 A1; US Patent 20120176212 A1] Nd-Fe-B alloy powders are loaded into special containers, vibrated, textured in a pulsed magnetic field, and then sintered directly in the containers. The implementation of FSN provides a number of significant advantages:

1. The elimination of the stage of pressing the powder simplifies the technology;

2. The exclusion of large and heavy press electromagnets from the equipment chain makes it easier to seal equipment for low-oxygen technology and thereby increase the coercive force (hereinafter H _c ) of the magnets without using Dy and / or Tb;

3. The absence of the powder pressing process allows for a very uniform distribution of the density of the powders in the sintering container, which as a result contributes to the production of sintered magnets of excellent shape and final size. The procedure for final grinding of magnets after FBP is either not performed or is minimized, ensuring a reduction in the cost of the starting material and a reduction in the price of products.

Despite the simple idea of the PBP method, the study conducted on the production of magnets using this technology [A.G. Popov, A.V. Shitov, E.G. Gerasimov, D.Yu. Vasilenko, M.Yu. Govorkov, D.Yu. Bratushev, V.P. Vyatkin, K.Yu. Shunyaev, T.L. Mikhailova. Obtaining sintered Nd-Fe-B magnets without powder pressing. FMM, 2012, t.113, No. 4, p. 352-362], showed that there are a number of problems that require an adequate solution.

Firstly, a decrease in the average particle size of the powder (hereinafter, D _p ), necessary for its better sintering and increase in H _{c of} sintered magnets, is accompanied by an increase in the concentration of oxygen adsorbed by the developed surface of the powders. The formation of a large number of refractory Nd ₂ O ₃ oxides during sintering does not lead to an increase, but rather to a decrease in the density ρ and Н _{c of} sintered magnets. The critical values of D _cf were determined, below which the values of the density of the magnets sharply decrease.

When using traditional technology (TT) for Russian enterprises [RF patents 2063083, 2118007, 2136068, 2174261 and 2212075], in which the grinding of powders is carried out in vibration mills in an environment of toluene, acetone or other protective liquids, and after grinding, the powders are dried in air, the critical value of D _cf was 3.2 μm. Using low-oxygen technology (tubing), in which grinding was carried out in a jet mill, using nitrogen as the working gas, and then the powders were manipulated in airtight boxes without contact with atmospheric oxygen, and it was possible to lower D _cf to 2.5 μm. However, these powders were larger than in the technical solution [US Patent 2007245851 A1] (D _cp ~ 1 μm), the oxygen in the sintered magnets was greater (> 0.3 wt%), and their H _{c was} less (9 kOe instead of 19 kOe). This indicated that the realized tubing did not provide the necessary precision of the process and, first of all, in terms of oxygen content (<2000 ppm).

Secondly, an increase in the bulk density of ρ _n powders in containers is accompanied by an increase in the density of the obtained sintered magnets ρ, but this prevents the texture degree from approaching 100%. It was shown that the values of ρ _n , providing the optimal ratio between the density ρ and the degree of texture of sintered magnets, are 2.5-3.0 and 2.2 g / cm ³ for TT and tubing, respectively. At lower values of ρ _n , the required magnet density after sintering is not achieved, and at large values, a low level of texture and a low residual induction B _{r are formed} . The reason for the decrease in the degree of texture is an increase in the friction forces between the powder particles as ρ _n increases. The force torque acting on each particle of the powder from the side of the magnetic field is unable to overcome friction, and the particles do not orient themselves with their easy magnetization axes strictly along the direction of application of the field. Overcoming such friction forces is possible only with the application of powerful pulsed magnetic fields for texturing powders. For example, in the patent [US Patent 2007245851 A1], powders with ρ _n = 3.6 g / cm ^{3 were} textured using pulsed fields with a maximum intensity of H _m > 60 kOe. To generate such high fields in the volume necessary to accommodate large containers, very powerful capacitor units are required.

Thirdly, at not high enough bulk density of the powders (ρ _n = 2.5-3.0 g / cm), which made it possible to ensure a good texture, very large shrinkages occurred during sintering. Table 1 shows the values of the degree of shrinkage (along the height Δh / h, along the diameter ΔD / D and the total volume ΔV / V) during sintering of powders in cylindrical containers depending on the particle size and bulk density of the powder. These data show, for example, that the volume of powders is reduced by about three times! Such a strong change in the size and volume of the powder preform with small values of ρ _{n is} inevitably accompanied by surface curvature and the loss of the ideal cylindrical shape of the sintered magnet. Both to obtain the perfect shape of sintered magnets and to reduce the size of containers in the FBP technology, it is preferable to use powders with ρ _n ≥3.0 g / cm ³ .

Table 1 Ar, μm 3.1 3.2 3.3 3.9 ρ _H , g / cm ³ 2.19 2.31 2.61 2.93 1.99 2.27 2.60 2.82 2.02 2.50 3.0 3.49 2.32 2.59 2.90 3.21 ρ, g / cm ³ 6.78 7.02 7.37 7.42 7.06 7.35 7.48 7.48 6.76 7.38 7.50 7.53 6.94 7.25 7.36 7.44 Δh / h 0.36 0.40 0.40 0.35 0.46 0.46 0.42 0.40 0.42 0.41 0.37 0.29 0.44 0.43 0.42 0.36 ΔD / D 0.30 0.30 0.28 0.27 0.30 0.29 0.24 0.24 0.30 0.27 0.24 0.22 0.27 0.25 0.24 0.23 ΔV / V 0.69 0.68 0.66 0.62 0.73 0.70 0.66 0.62 0.71 0.67 0.62 0.55 0.67 0.65 0.62 0.59

Thus, the need to use powders with high bulk density is accompanied by a sharp deterioration in their texturability in a magnetic field and a decrease in the residual induction of the resulting sintered magnets.

The problem of reducing friction between particles of powder textured in a magnetic field is solved by introducing an internal lubricant. The addition of dry or liquid lubricants was previously used to increase the texture of sintered Nd-Fe-B magnets in traditional technologies using powder pressing [Yu. Kaneko, Patent RU 2112627; PC. Gaschl, RF Patent 2258345]. Since in these technologies the bulk density of the powders during texturing is small and amounts to about 2.0 g / cm ³ , the effect of the addition of internal lubricants is negligible. In addition, in the case of pressing powders in a matrix, lubricants can be used only in a small amount from 0.05 to 0.5 mass percent of the fraction of magnetic powder. Although the materials used for lubrication are highly volatile, it is difficult to remove them by heating in a vacuum before the sintering process, if they are enclosed in a powder compact, which was highly compressed by pressing in a matrix to a density of about 5 g / cm ³ . At high temperatures, the components of the lubricant can react with the components of the magnet and, thus, the magnetic characteristics are impaired.

Closest to the claimed method is the "Method and system for producing sintered permanent rare earth magnets with magnetic anisotropy" [US Patent 2007245851 A1]. The method includes smelting and casting the Nd-Fe-B alloy onto a rotating copper drum, dry grinding the alloy into powder in a jet mill to an average particle size of 1.0-3.5 μm, loading dry powder with a loading density of about 3.5 g / cm ³ into containers of metallic material with a coating that prevents interaction with the powder during sintering, texturing of powders with a pulsed magnetic field of at least 45 kOe and, bypassing the pressing process, subsequent sintering of powders in vacuum directly in containers.

Moreover, after grinding before loading into containers (texturing and sintering), the powder is mixed with dry or liquid lubricating additives in an amount not exceeding 1% by weight of the alloy powder. Powder of zinc stearate (a salt of fatty stearic acid) is used as a dry lubricating additive, and the following methyl ethers of fatty acids of the series CH ₃ (CH ₂ ) _n COOCH ₃ are used as liquid lubricants: methyl octylate (n = 4), methylcaprylate (n = 6 ), methyl decanoate (n = 8), methyl laurate (n = 10), methyl myristate (n = 12), methyl palmylate (n = 14) and methyl stearate (n = 16). Since in the case of FSN, the density of the powder in the containers is about 3.5 g / cm ³ , the lubricant material is removed relatively easily by heating in vacuum.

The disadvantage of this method is that it is very difficult to coat dry powder particles with an even thin layer of lubricating additives, the optimum amount of which, according to the invention, does not exceed 1% by weight of the alloy powder. To overcome this obstacle, an additional process step is introduced for mixing dry powders of the Nd-Fe-B alloy with lubricants using special mixers (type Super, manufactured by KAWATA MFG Co, Ltd) or similar mixer mills. The introduction of additional technological operations and the use of special equipment (which is not produced by domestic industry) increases the cost of the sintered magnets obtained.

The basis of the present invention is the task of increasing the degree of texture and the values of B _r and (HH) _max Nd-Fe-B permanent magnets obtained by the PBP method, which is achieved by achieving a technical result, which consists in reducing the friction forces between particles of a powder textured in a magnetic field, due to the introduction of internal lubrication.

The problem is solved in that in the method for producing a sintered highly energy-intensive permanent magnet from an alloy based on Nd-Fe-B, including smelting and casting the alloy, grinding the alloy into powder in a mill with an average particle size of 2.5 - 3.5 microns, the use of lubricating additives in the form of dry stearates powder or in the form of liquid fatty acid esters, powder loading in dry or wet state from the loading density of up to 3.5 g / cm ³ in containers of a material that does not react with the powder during sintering powders momentum teksturovanie with a magnetic field of at least 45 kOe and subsequent sintering of the powders in vacuum directly in containers with the removal of residues of lubricating additives by heating the powders in vacuum, according to the invention, the powders are ground using toluene as a protective grinding medium, the lubricating additives are introduced into toluene immediately before grinding the powders , heating the powders in vacuum is carried out to a temperature of 500 ° C at a rate of not more than 10 ° C / min.

Wherein:

- as a dry powder of stearates use a powder of aluminum stearate, or copper, or zinc, the concentration of which does not exceed 0.20% by weight of the powder of the alloy based on Nd-Fe-B;

- ethyl esters of homologous series of carboxylic acids СН ₃ (СН ₃ ) _n СООС ₂ Н ₅ with low boiling points, the concentration of which does not exceed 1.5% by weight of the powder of the Nd-Fe-B alloy, are used as liquid fatty acid esters;

- grinding the alloy into powder is carried out in a vibrating mill for at least 1 hour;

- as the material of containers that do not interact with the powder during sintering, use molybdenum or graphite.

The introduction of lubricating additives in toluene because of their good solubility in toluene and the grinding time of about 1 h allows you to cover the surface of the powder particles with an even thin layer of lubricant directly in the grinding process.

The use of powder lubricating additives, in addition to zinc stearate powder, aluminum stearate or copper powder expands the range of additives used.

The optimal concentration of injected powder lubricating additives dissolved in toluene in an amount of not more than 0.20% by weight of the powder of the alloy based on Nd-Fe-B is determined by the research results (table 2).

Unlike the methyl esters used in the closest solution, ethyl esters contain less oxygen, which reduces the oxidation of the powders upon heating and helps to improve their sintering ability and increase the H _{c of} sintered magnets.

The optimal concentration of introduced ethyl esters of carboxylic acids with low boiling points, the concentration of which in toluene is not more than 1.5% by weight of the powder of the alloy based on Nd-Fe-B, is determined by the research results (table 7).

Carrying out the grinding of the alloy into a powder in a vibration mill allows the direct introduction of lubricating additives into the protective environment of toluene.

The condition of slow heating of the powders in vacuum to 500 ° C is associated with the need to completely remove lubricants from the surface of the powder particles, since the melting of the eutectic component enriched in neodymium included in the phase composition of the alloy powder at temperatures above 500 ° C leads to intense interaction of neodymium with oxygen and carbon lubricants. The neodymium oxides and carbides formed in this process impede the achievement of a high density and highly coercive state of sintered magnets.

Figure 1 shows micrographs of powders crushed with the addition of zinc stearate: (a) D _cp = 3.25; (b) 3.00; (c) 2.70 and (g) 2.55 μm.

Figure 2 shows the dependences of B _r , H _c and (BH) _{max of} sintered magnets on the concentration of zinc sterate, ethyl butyrate, ethyl caprylate and methyl caprylate, added by grinding the powder.

The claimed method is as follows. A rapidly quenched plate alloy containing 31.5 wt.% Nd, 1 wt.% B, the rest Fe, and cast onto a rotating copper drum (hereinafter referred to as strip casting alloy), is embrittled by hydrogen treatment. After coarse grinding to a particle size of less than 500 μm, Nd-Fe-B alloy powders are further crushed in a vibratory mill in toluene. Along with the alloy powder, lubricant additives are added to the mill cells, which dissolves well in toluene. The concentration of powder lubricants varies in the range from 0 to 0.5%, and liquid lubricants - from 0 to 3.0% by weight of the powder loaded alloy. By varying the grinding time, powders with different average particle sizes (from 2.55 to 3.25 μm) are obtained, which is determined by the Fisher method. Wet or air-dried powders are loaded into molybdenum or graphite containers with an inner cylindrical cavity with a diameter of 16 mm and a height of 16 mm. The density of the loaded powders ρ _n is from 2.5 to 3.5 g / cm ³ . Texturing of the powders is carried out in pulsed field solenoids, which are applied in a direction parallel to the axis of the cylindrical cavity of the containers. The amplitude intensity of the pulsed field is at least 45 kOe. Excluding the pressing of powders using press equipment, they are sintered in vacuum furnaces directly in molybdenum or graphite containers. In order to remove residual lubricants from the surface of the particles, slow heating of the powders in vacuum is used at a speed of not more than 10 ° C / min. Sintering is completed by isothermal holding at 1050-1090 ° C for 1 h. After sintering, all magnets are subjected to additional heat treatment: T ₁ , 0.5 h + T ₂ , 0.5 h (T ₁ = 900 ° C and T ₂ = 550-650 ° C ) and quenched to room temperature.

Case Study 1

Strip casting alloy containing 31.5 wt.% Nd, 1 wt.% B, the rest Fe, was embrittled by hydrogen treatment, ground in a mortar to a particle size of less than 500 microns and then mixed with zinc stearate powder in a ratio of 0 to 1 wt.% in relation to the amount of alloy powder. The mixture of powders was further ground for 50 min in a vibratory mill in toluene. The powders were dried in air and then loaded into cylindrical molybdenum containers with an inner cylindrical cavity with a diameter of 16 mm and a height of 16 mm, using vibration with a frequency of 50 Hz and an amplitude of 2 mm. The powders in containers covered by lids were textured in a pulsed magnetic field of 46 kOe and a pulse duration of 6.5 ms, applying 5 pulses, and the application direction in each even pulse was reversed. The powders were sintered directly in containers in a vacuum oven at 1090 ° C for 1 h after slow heating to the indicated temperature at a rate of 10 ° C / min. After sintering, all magnets were subjected to additional heat treatment: 900 ° C, 0.5 h + 600 ° C, 0.5 h, and quenched to room temperature. The dependence of the properties of the obtained magnets on the concentration of zinc stearate C _SZn in mass percent relative to the loaded alloy powder is presented in Table 2. With an increase in C _SZn up to 0.2 mass%, the density of magnets ρ slightly increases, and the residual induction and maximum energy product (VN) _max , in contrast, decreases. This dependence is due to the fact that in the range 0≤C _SZn ≤0.20 wt.% Increases the bulk density of the powder loaded into a constant volume of the container under the same sealing conditions. Although the average powder grain size determined by the Fisher method remains constant (3.2 μm), the packing of the particles improves, apparently due to the fact that the friction forces between them decrease. At the same time, an increase in ρ _{n is} considered as the main reason for the gradual decrease in the degree of texture, despite the increasing amount of internal lubricant.

table 2 C _SZn , wt.% D _cp , μm ρ _n, g / cm ³ ρ, g / cm ³ B _r, T N _c, kA / m (BH) _max, kJ / m ³ With _O, wt.% 0 3.2 2.677 7.481 1.38 615 362 0.420 0.05 - 2.870 7.518 1.37 645 349 - 0.10 3.2 2.897 7.544 1.37 646 354 0.418 0.15 - 2.920 7.530 1.36 658 349 - 0.20 - 3.250 7.574 1.36 623 345 - 0.25 3.2 3.029 7.399 1.35 326 227 0.447 0.50 2.8 2.869 7.256 0.73 8.8 2.9 0.975 1.00 2.6 2.877 7.148 - - - -

At C _SZn > 0.20 mass%, all characteristics of sintered magnets begin to decrease quite sharply. This is due to the increasing concentration of oxygen C _O in the magnets, which reaches 0.975 mass% at C _SZn = 0.5 mass%. The increase in C _O is associated both with a beginning decrease in the particle size of the powder and intensified adsorption of oxygen by their surface, and with the introduction of additional oxygen by zinc stearate. Despite the measures taken to remove zinc stearate from the surface of the particles by slowly vacuum heating the samples in the temperature range 100-500 ° C, it was not possible to completely remove the lubricant introduced in large quantities. This leads to the appearance of many cracks in sintered magnets obtained with C _SZn > 0.20 wt.%, Which, in particular, did not allow magnetic measurements of the image, the properties of which are given in the last row of Table 3.

Thus, it was found that in the implemented scheme of a specific implementation, the critical concentration of the introduced zinc stearate is 0.2 wt.%. At large values of C _{SZn, the} properties of the magnets sharply decrease. Similar dependences were obtained for sintered magnets made with the addition of powders of aluminum and copper stearates. The properties of sintered magnets obtained by introducing 0.1 wt.% Of these powders into toluene are presented in table 3. The use of copper stearate contributed to an increase in coercive force.

Table 3 Lubricant additive Chemical formula ρ _n, g / cm ³ ρ, g / cm ³ B _r, T N _c, kA / m (BH) _max, kJ / m ³ Stearate A1 S54Nu50bA1 2.977 7.481 1.38 615 362 Stearate Cu SzbN7o04Si 3.212 7.518 1.38 781 373 Stearate Zn SzbN7o042p 2.897 7.544 1.37 646 354

Case Study 2

A powder with the addition of zinc stearate 0.15 wt.% And an average particle size of 3.2 μm was prepared under the same conditions as in Example 1. One part of the powder was dried in air, and the other was used in the wet state in order to reduce the amount of oxygen adsorbed on the surface of the particles. Dry and wet powders were loaded with different loading density ρ _n into molybdenum containers, then the powders were textured, sintered and heat treated under the same conditions as in Example 1. The dependence of the properties of sintered magnets on the density of powders loaded into containers is presented in the table 4. It should be noted that higher values of ρ _{n are} more easily achieved using wet powders. Qualitatively similar dependences were obtained for magnets prepared from both dry and wet powders. The values of B _r and (BH) _max reach a maximum at ρ _n = 2.92 and 3.25 g / cm ³ for dry and wet powders, respectively, and are higher for wet powders. The density and coercive force of magnets monotonically increase with increasing ρ _n . At the same time, the values of H _{c of} magnets prepared from wet powders turned out to be lower than the corresponding values of magnets obtained from dry powders. There are two reasons for this. First, a more rapid deterioration in the degree of texture with an increase in ρ _n for dry powders and a well-known empirical pattern of an increase in Н _c with an increase in the angle of disorientation of the axes of easy magnetization of grains in Nd-Fe-B magnets [W. Rodewald, R. Blank, W. Wall, GW Reppel, HD Zilg, Production of sintered Nd-Fe-B magnets with maximum energy density of 53 MGOe. Proc. 16th Int. Workshop on Rare Earth Permanent Magnets and Their Applications (Sendai, Japan, 2000), p.119-126]. Secondly, the increased oxidation of the neodymium-enriched intergranular phase of sintered magnets obtained from powders with ρ _n <3.1 g / cm ³ , which, in turn, can be associated with a strong deterioration in vacuum during heating of wet powders due to intense release toluene vapor. This reason is indicated by the correlation between low values of ρ and Н _c , which indicates an increased defectiveness of grain boundaries in these magnets.

Table 4 C _SZn, wt.% Dry powders Wet powders ρ _n, g / cm ³ ρ, g / cm In _r , T N _c, kA / m (HV) _max, kJ / m ³ ρ _n , g / cm ³ ρ, g / cm ³ B _r , T H _s, kA / m (HV) _max, kJ / m ³ 0.15 2.901 7.449 1.34 586 322 2.748 7.306 1.26 546 266 0.15 2.920 7.530 1.35 658 341 3.116 7.527 1.36 579 330 0.15 3.065 7.582 1.34 677 339 3.249 7.556 1.36 592 342 0.15 3.450 7.608 1.16 754 244 3.475 7.575 1.34 622 337 0 2.677 7.481 1.38 615 362 2.579 7.313 1.30 572 302 0 2.905 7.564 1.30 666 318 2.903 7.545 1.35 593 338 0 2.974 7.564 1.25 689 292 3.063 7.523 1.29 612 307 0 3.258 7.591 1.08 738 205 3.124 7.554 1.29 595 303

Reference Example 2

A powder without the addition of zinc stearate with an average particle size of 3.2 μm was prepared and then sintered magnets were prepared from it under the same conditions as in Example 2. The dependence of the properties of sintered magnets on the density of powders loaded into containers is presented in the lower part of Table 4. Values ρ, B _r , (BH) _max magnets prepared from powders without the addition of zinc stearate exceed the corresponding magnets obtained with C _SZn = 0.15 wt.%, at a powder loading density of less than 2.9 and 3.0 g / cm ^3, respectively, in dry and wet x. For large values of ρ _{n, the} situation reverses: the ρ, B _r , (BH) _max magnets obtained with the use of a lubricant increase by 0.3, 8.0, 16.7% and 0.1, 5.8, 12.7%, respectively, in the dry and wet states. Thus, the present invention allows to obtain sintered magnets with a high degree of texture and high hysteretic properties using a lubricant of powders of zinc stearate, using their high bulk density in containers (more than 3 g / cm ³ ), but not using the pressing of powders before sintering. This is the main positive effect of using internal lubricants.

Case Study 3

Powders with different particle sizes D _cp were prepared by grinding in a vibration mill in toluene without additives and with the addition of 0.15 wt% zinc stearate. Sintered magnets were obtained from the powders in the wet state under the same conditions as in Example 2. The properties of the obtained sintered magnets are presented in Table 5.

Table 5 C _SZn, wt.% t _ism, min D _cp, μm ρ _n, g / cm ³ ρ, g / cm ³ B _r, T N _s, kA / m (BH) _max, kJ / m ³ With _O, wt.% 0 fifty 3.20 3.069 7.543 1.29 674 308 0.355 0 70 3.05 2.719 7.536 1.38 655 358 0.355 0 one hundred 2.80 2.978 7.589 1.38 719 362 0.370 0 130 2.60 2.613 7.571 1.38 639 360 0.431 0.15 fifty 3.25 3.279 7.612 1.32 760 326 0.345 0.15 70 3.00 3.105 7.552 1.40 633 369 0.367 0.15 one hundred 2.70 3.184 7.602 1.38 282 190 0.436 0.15 130 2.55 2.934 7.186 0.53 eleven 22 0.528

With the introduction of 0.15 wt.% Zinc stearate, D _cp decreases more rapidly compared with this parameter of powders prepared without zinc stearate. From this result it follows that zinc stearate acts upon grinding as a surfactant. Micrographs of powders milled with zinc stearate are shown in FIG. It is seen that, along with a decrease in D _cp , the fraction of small particles increases, and they become more rounded. Qualitatively similar micrographs were obtained for powders prepared without the introduction of zinc stearate.

The values of ρ _n gradually decrease with decreasing D _cp . For wet powders, crushed with the addition of zinc stearate, also, as was noted in Example 1 for dry powders, the ρ _n values (under the same loading conditions) are higher compared to powders prepared without zinc stearate. The tendency to a decrease in the density of sintered magnets, usually observed with a decrease in ρ _n , in the obtained dependences is compensated by an increase in the sinterability of small particles, and in the range 2.7≤D _cp ≤3.2 μm, the values of ρ remain almost constant, and make up at least 7.54 g / cm ³ . A sharp decrease in density occurred in a magnet prepared from powder with the addition of zinc stearate, the average particle size of which was 2.55 μm. The data on the oxygen content in the magnets given in the last column of Table 5 show that excessive oxidation of the neodymium-enriched intergranular phase in this magnet is the main reason for the observed decrease in ρ.

Dependencies qualitatively similar to ρ (D _cp ) were also obtained for B _r (D _cp ). A slight increase in B _r with a decrease in D _cp is due to an improvement in the degree of texture, which was confirmed by X-ray diffraction patterns that were taken from the surfaces of sintered magnets perpendicular to the direction of the texture. The reason for the improvement in the degree of texture is associated with the change in the morphology of the powders and particle size distribution shown in Fig. 1, which is accompanied by a decrease in the fraction of particles with residues of dendritic blocks of 10-15 μm in size, in which the easy magnetization axes of Nd ₂ Fe ₁₄ B grains are slightly misoriented [A .G. Popov, N.V. Kudrevatykh, V.P. Vyatkin, D.Yu. Vasilenko, D.Yu. Bratushev, T.Z. Puzanova, E.G. Gerasimov, Obtaining high-energy-intensive permanent magnets from plate alloys Nd-Fe-B, FMM, 2010, Volume 109, No. 3, p.257-266]. A sharp decrease in B _r in a magnet prepared from a powder with D _cp = 2.55 μm using an addition of zinc stearate is due not to a deterioration in texture, but to a decrease in ρ noted above and to a very low Н _с , which monotonously decreases in this series of magnets (Table 5).

A comparison of the data in Tables 3 and 5 shows that the use of wet instead of dry powders with the same value of D _cp = 3.2 μm allowed us to reduce the oxygen content in the magnets by 14% (to 0.36 wt.%). With a decrease in D _{cp of} powders to 2.6 μm, the oxygen content in magnets prepared without the introduction of zinc stearate increased to 0.43 wt.%, But their coercive force remained practically unchanged. Note that the use of dry powders with D _cp <3.2 μm in the traditional technology [A.G. Popov, A.V. Shitov, E.G. Gerasimov, D.Yu. Vasilenko, M.Yu. Govorkov, D.Yu. Bratushev, V.P. Vyatkin, K.Yu. Shunyaev, T.L. Mikhailova. Obtaining sintered Nd-Fe-B magnets without powder pressing. FMM, 2012, vol. 113, No. 4, p. 352-362] was accompanied by a sharp decrease in H _c , despite the fact that the initial alloy contained more neodymium by 1 wt.% Compared with the alloy used in this experiment.

A decrease in D _{cp of} powders by grinding with the addition of 0.15 wt% zinc stearate led to a monotonic decrease in H _from sintered magnets and, at the same time, to a significant increase in oxygen in them. Apparently, the rate of powder heating in vacuum before sintering was too high, which did not contribute to the complete removal of a large amount of zinc stearate from the highly developed surface of small particles. Magnets with fairly high properties: ρ = 7.55 g / cm ³ , B _r = 14.02 kG, H _c = 7.91 kOe and (BH) _max = 46.1 MGe were obtained only from a powder with D _cp = 3.0 μm. Reference Example 3

Powders with an average particle size of 3.2 μm were prepared by grinding in a vibration mill in toluene, acetone and hexane without an additive and with the addition of 0.15 wt.% Zinc stearate. Sintered magnets were obtained from the powders in the wet state under the same conditions as in Example 3. The properties of the obtained sintered magnets are compared in table 6.

Table 6 C _SZn, wt.% Grinding media ρ _n, g / cm ³ ρ, g / cm ³ In _r, T N _c, kA / m (BH) _max,
kJ / m ³ 0 toluene 2.713 7.536 1.38 655 358 0.15 3.105 7.552 1.40 633 369 0 acetone 3.080 7.541 1.38 706 361 0.15 3.446 7.589 1.35 723 349 0 hexane 2.937 7.563 1.39 641 365 0.15 3.184 7.592 1.37 644 358

The addition of zinc stearate promotes an increase in ρ _{n of} powders and ρ of sintered magnets for all used protective fluids, however, an increase in the values of B _r and (BH) _max occurred only if the powders were ground in toluene. The weak solubility of zinc stearate in acetone and hexane did not contribute to the appearance of a sufficient amount of lubricant on the surface of the particles and the improvement of their texturability in a magnetic field. In contrast, the values of B _r and (BH) _max as a result of the addition of zinc stearate decreased because the values of ρ _n increased and, therefore, the texturing conditions worsened.

Case Study 4

Strip casting alloy containing 31.5 wt.% Nd, 1 wt.% B, the rest Fe, as in example 1, was embrittled by hydrogen treatment, ground in a mortar to a particle size of less than 500 microns and then in a vibration mill, previously added to toluene liquid lubricant in the form of ethyl butyrate (CH ₃ (CH _{2) 2} COOC ₂ H _5), ethyl caprylate (CH ₃ (CH _{2) 2} COOC ₂ H ₅₎ and metilkaprilata (CH ₃ (CH _{2) 6} COOCH ₃₎ in an amount of from 0 up to 2.8% based on the weight of the alloy powder. The powders were crushed for 70 min, after which they were loaded into cylindrical molybdenum containers in a wet state, textured in a pulsed magnetic field, sintered, and additionally heat treated as in Example 1. The dependence of the properties of the obtained magnets on the concentration of liquid lubricants C _L in mass percent by weight relative to the loaded alloy powder is presented in table 7. A change in C _{L of} ethyl butyrate from 0 to 2% practically does not affect the properties of sintered magnets. The constancy of the values of B _r indicates that ethyl butyrate does not significantly reduce the friction forces between the particles. The decrease in the density of the magnets with the addition of 1.5-2 wt.% Ethyl butyrate is associated with the appearance of cracks in them.

Table 7 With _L, wt.% ρ _n , g / cm ³ ρ, g / cm ³ In _r , T N _c, kA / m (HV) _max, kJ / m ³ ethyl butyrate 0 3.247 7.581 1.32 673 325 0.5 3.326 7.580 1.32 674 327 1.0 3.205 7.583 1.31 682 322 1.5 3.376 7.390 1.32 722 330 2.0 3.184 7.031 1.32 650 330 ethyl caprylate 0 3.247 7.581 1.32 673 325 0.47 3.313 7.564 1.37 603 354 0.94 3.319 7.592 1.37 622 351 1.41 3.403 7.584 1.37 570 348 1.88 3.307 7.528 1.33 249 148 2.81 3.317 7.430 13.25 258 173 methylcaprylate 0 3.247 7.581 1.32 673 325 0.3 3.358 7.557 1.39 583 359 0.5 3.410 7.597 1.37 650 358 1.0 3.031 7.480 1.28 493 242 1.5 3.495 7.469 1.25 305 196

With increasing concentrations of ethyl caprylate and 1.5% increases the value ρ _n powder and the values ρ, B _r, (BH) _max of the sintered magnets, since this lubrication effectively reduces the frictional force between the particles. The introduction of 0.5 wt.% Ethylcaprylate allowed us to increase the B _r and (BH) _{max of} sintered magnets by 4% and 10%, respectively.

The use of methylcaprylate, introduced in the present invention into toluene as a lubricant, before grinding, in contrast to the prototype, in which powder ready for texturing is lubricated, gives a good result at low concentrations. The introduction of methylcaprylate with concentrations of 0.3-0.5 wt.% Provides an increase in the B _r and (BH) _{max of} sintered magnets approximately the same as ethylcaprylate in the corresponding concentration range. However, the values of H _c and (BH) _{max of} magnets prepared with the addition of methylcaprylate more than 0.5 wt.% Begin to decrease sharply, which should be associated with a higher oxygen content in this ether compared to ethylcaprylate.

The maximum concentrations of lubricants for zinc stearate, ethyl butyrate, ethyl caprylate and methyl caprylate are compared in FIG. 2, representing the dependences of B _r, H _c and (BH) _max on the concentration of lubricating additives. A sharp decrease in H _c and (HH) _max occurs at concentrations exceeding 0.2, 0.5, and 1.5 wt% for zinc stearate, methylcaprylate, and ethylcaprylate, respectively. Above these concentrations, the removal of lubricants is difficult, and oxygen introduced by the residues of lubricating additives binds the intergranular neodymium during sintering into refractory oxides, thereby preventing the formation of an optimal microstructure and leading to a decrease in coercive force. The addition of ethyl butyrate in an amount of up to 2 wt.% Practically does not affect the properties of the magnets. Thus, it has been shown that liquid lubricants can be used in a wider range of concentrations compared to zinc stearate, since they are easier to remove from powders when heated in vacuum.

Claims (5)

1. A method of producing a sintered high-energy permanent magnet from an alloy based on Nd-Fe-B, including smelting and casting the alloy, grinding the alloy into powder in a mill with an average particle size of 2.5 - 3.5 microns, the use of lubricating additives in the form of a dry powder of stearate salts or as liquid fatty acid esters, powder loading in dry or wet state from the loading density of up to 3.5 g / cm ³ in containers of a material that does not react with the powder during sintering powders teksturovanie pulsed magnetic field voltage at least 45 kOe and subsequent sintering of the powders in vacuum directly in containers with the removal of residues of lubricating additives by heating the powders in vacuum, characterized in that the grinding of powders is carried out using toluene as a protective grinding medium, the lubricating additives are introduced into toluene immediately before grinding the powders, heating the powders in vacuum is carried out to 500 ° C at a rate of not more than 10 ° C / min. 2. The method according to claim 1, characterized in that the powder of stearates uses a powder of aluminum stearate, or copper, or zinc, the concentration of which does not exceed 0.20% by weight of the powder of the alloy based on Nd-Fe-B. 3. The method according to claim 1, characterized in that ethyl esters of homologous series of carboxylic acids CH ₃ (CH ₂ ) _n SOOC ₂ H ₅ with low boiling points, the concentration of which does not exceed 1.5% by weight of the powder, are used as liquid fatty acid esters Nd-Fe-B based alloy. 4. The method according to claim 1, characterized in that the grinding of the alloy into a powder is carried out in a vibration mill for at least 1 hour. 5. The method according to claim 1, characterized in that as the material of the containers that do not interact with the powder during sintering, molybdenum or graphite is used.

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