CN114613587A

CN114613587A - Pretreatment method for grain boundary diffusion of Nd-system sintered magnet

Info

Publication number: CN114613587A
Application number: CN202110648596.6A
Authority: CN
Inventors: 永田浩; 武仁杰; 刘仁辉
Original assignee: Guorui Kechuang Rare Earth Functional Materials Ganzhou Co ltd; Jiangxi University of Science and Technology
Current assignee: Guorui Kechuang Rare Earth Functional Materials Ganzhou Co ltd; Jiangxi University of Science and Technology
Priority date: 2021-06-10
Filing date: 2021-06-10
Publication date: 2022-06-10

Abstract

The invention discloses a pretreatment method for grain boundary diffusion of an Nd-based sintered magnet, which comprises the steps of preparing an Nd-based sintered magnet and a medium with the average grain size of 10 mu m-5 mm, wherein the medium is a particle body/powder taking a metal X phase/X alloy system phase as a main phase; performing sand blasting treatment on the Nd system sintered magnet by using the medium, removing at least part of oxides on the surface of the Nd system sintered magnet through the sand blasting treatment, and adsorbing and forming a medium thin film layer on the surface of the Nd system sintered magnet; followed by grain boundary diffusion treatment. The method of the present invention can reduce the influence of deterioration, deformation and cracks of the magnet performance due to oxidation and carbonization caused by the RH diffusion treatment, improve the magnet performance of the final product after the RH diffusion treatment, and prevent the occurrence of undesirable phenomena such as cracks and deformation.

Description

Pretreatment method for grain boundary diffusion of Nd-system sintered magnet

Technical Field

The invention relates to the technical field of magnets, in particular to a processing method of a sintered magnet.

Background

Motors, particularly motors used in electric vehicles, air conditioning compressors, and the like, for which heat resistance is required, employ grain boundary diffusion magnets using RH (which is a heavy rare earth diffusion source added with Tb or Dy). Commonly used methods for the RH grain boundary diffusion include an RH powder coating method, an RH sputtering method, and an RH vapor diffusion method.

The problem of the RH grain boundary diffusion is that the presence of an oxide film and a carbide film on the magnet surface after the pretreatment in the past causes a reaction with the RH element in the subsequent RH diffusion step to form a rare earth carbide and a rare earth oxide, which hinders diffusion of RH into the magnet, resulting in a large number of problems such as performance degradation, performance unevenness, deformation of the magnet, and crack failure.

In particular, in the RH powder coating diffusion method which is rapidly spread, requires simple equipment and process technology, and has a large yield, in general, in order to improve the adhesion of the powder to the surface of the magnet, organic additives such as a binder and a dispersing material are added in a large amount, and the organic materials are decomposed into carbon and oxygen in the diffusion step, mainly react with RH to become RH carbide and RH oxide, and inhibit diffusion of RH into the magnet.

In the conventional step, the surface is cleaned by a known pretreatment method such as acid washing, sand blasting, alkali washing, or the like before RH grain boundary diffusion. However, since the treated product needs to be retained in the atmosphere during the RH diffusion step, oxygen, moisture, and carbon dioxide adsorbed on the surface of the magnet react with the rare earth to form an oxide, a hydroxide, a carbide, or the like, which covers the surface of the magnet and inhibits diffusion of RH into the magnet. Therefore, the magnet after RH diffusion has problems of performance degradation, performance unevenness, deformation, cracks, and the like.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provides a pretreatment method for grain boundary diffusion of an Nd-based sintered magnet.

The technical scheme adopted by the invention for solving the technical problems is as follows:

a pretreatment method for grain boundary diffusion of a Nd-based sintered magnet, comprising preparing a Nd-based sintered magnet and a medium having an average particle size (average size, average diameter, average particle diameter) of 10 [ mu ] m to 5mm, wherein the medium is a particle/powder having a metal X phase/X alloy phase as a main phase; performing sand blasting treatment on the Nd system sintered magnet by using the medium, removing at least part of oxides on the surface of the Nd system sintered magnet through the sand blasting treatment, and adsorbing and forming a medium thin film layer on the surface of the Nd system sintered magnet; then carrying out grain boundary diffusion treatment; wherein:

x is Rare Earth R (R: Rare-Earth, Rare Earth R is Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy or Ho …), Ga, Al, Cu or Ni;

the X alloy system includes at least one of R-Fe alloy, R-Ni alloy, R-Ga alloy, R-Al alloy, R-Cu alloy, Ga-Al alloy, Ga-Cu alloy, Al-Cu alloy, R-Ga-Al alloy, R-Al-Cu alloy, or Ga-Al-Cu-Ni alloy, and the content of X element in the X alloy system (i.e. the sum of R (if any) + Ga (if any) + Al (if any) + Cu (if any) + Ni (if any)) is 60 at% or more.

The "particles/powder mainly composed of the metal X phase/X alloy phase" of the medium means metal/alloy particles/powder having a metal X phase/X alloy phase content of 60 at% or more; the "metal X phase/X alloy system phase" can be a metal X phase, an X alloy system phase, or a mixture of the metal X phase and the X alloy system phase, the "metal/alloy system" can be a metal, an alloy system, or a mixture of the metal and the alloy system, and the "particle/powder" can be a particle, a powder, or a mixture of the particle and the powder. Wherein the 'granule body' refers to particles with the average particle size of more than 100 mu m, namely a medium with the particle size of 100 mu m-5 mm; the term "powder" refers to particles having an average particle size of < 100. mu.m, i.e.a medium of 10 to 100. mu.m.

In the invention, the Nd-based sintered magnet is placed in a sand blasting device, and the medium is sprayed on the surface of the Nd-based sintered magnet under the action of air flow, so that on one hand, the medium cleans up at least a part of oxides on the surface of the Nd-based sintered magnet, and on the other hand, a layer of thin film of the medium is adsorbed on the surface of the Nd-based sintered magnet. After the sand blasting treatment is finished, the grain boundary diffusion treatment is carried out in vacuum or inert gas.

Further preferably, in the blast treatment of blasting a medium gas flow onto the surface of the Nd-based sintered magnet, the oxygen content in the gas flow is controlled to 3000ppm or less, and the blast treatment is performed with an inert gas. Inert gases such as Ar, N₂And He …. The blasting process of the present invention performed in a low-oxygen atmosphere can reduce oxidation accompanying the blasting process, increase the amount of X metal/X alloy adsorbed, further improve the performance of the RH diffusion magnet, and realize low-pressure blasting with less cracking in a short time. As the low oxygen content of the preferred blast treatment, 1ppm to 3000ppm is preferred. It is difficult to achieve an oxygen content of 1ppm or less economically, and if it exceeds 3000ppm, the effect of preventing oxidation becomes extremely poor. More preferably, the oxygen content in the gas stream is in the range of 100ppm to 2000 ppm.

Further preferably, the oxygen content of the Nd-series sintered magnet is 2000ppm or less. The lower the oxygen content of the magnet material is, the more the performance effect of grain boundary diffusion can be improved. If the oxygen content of the magnet material is 2000ppm or less, the coercive force after diffusion can be sufficiently increased, and the squareness Hk can also be increased to the level before diffusion.

Further preferably, the prepared magnet is heated and then subjected to the blasting treatment of the present invention. In a preferred range, the Nd-based sintered magnet is heated to 60 to 400 ℃ before the surface of the Nd-based sintered magnet is subjected to the dielectric blasting. The temperature of the magnet is higher than room temperature, which contributes to the improvement of atomic activity of the powder of the X metal or X alloy of the present invention adsorbed on the surface of the magnet, the increase of the amount of adsorption of the X metal/X alloy, and the uniform adhesion to the surface of the magnet. In view of further improvement in magnet performance after the RH diffusion treatment and mass production to achieve uniform adsorption at a low blast pressure with less cracks, 60 to 400 ℃ is a preferred range as a heating temperature of the preferred magnet. The effect of heating is not achieved below 60 ℃, and when the temperature exceeds 400 ℃, the performance of the magnet is deteriorated. Preferably in the range of 80 ℃ to 220 ℃. The magnet may be placed in the blasting apparatus after it is preheated in advance, or may be heated in the blasting apparatus.

Further preferably, after the blasting step of the present invention, the dielectric thin film layer adsorbed on the surface of the magnet has an average film thickness of 10nm to 1000nm, that is, the metal X phase/X alloy phase having an average film thickness of 10nm to 1000nm remains attached to the outermost layer of the magnet. The average adsorption amount was evaluated by pickling the surface of the magnet piece after the sand blasting, analyzing the amounts of the respective elements in the dissolved acid by ICP-AES, calculating the average adsorption film thickness from the magnet surface area. In addition, the average adsorbed film thickness was estimated using a GD-OES device or GDA-PSpectruma measurement in cases where acid dissolution was difficult. When the average adsorption thickness is less than 10nm, the thickness is the same as that of the oxide film or the carbide film, and the effect of recovering the processing deterioration property cannot be exhibited. When it exceeds 1000nm, Br performance of the magnet is lowered. More preferably, it is in the range of 50nm to 500 nm.

More preferably, the oxygen content of the medium is 1000ppm or less, that is, the oxygen content of the particles/powder having the metal X phase/X alloy system phase as a main phase is 1000ppm or less. If the amount exceeds 1000ppm, the adsorbed metal or alloy layer may fall off due to insufficient adhesion. Further, since diffusion inside the magnet is inhibited by the oxide, there is a problem that the increase value of Hcj and the increase value of Hk are small. More preferably 500ppm, and still more preferably 200 ppm.

More preferably, the Nd-based sintered magnet has a specific surface area (S (surface)/V (volume) and S/V value) of 0.5 or more. The specific surface area S/V value of the magnet is calculated as follows:

when the magnet shape is approximately cylindrical, the radius is recorded as r, and the height is recorded as h, then the S/V value calculation formula is as follows:

S＝2πr²+2πrh

V＝πr²h

when the magnet shape is approximately a rectangular parallelepiped, the length is denoted by a, the width is denoted by b, and the height is denoted by c, then the S/V value calculation formula is as follows:

S＝2×(ab+bc+ac)

V＝abc

since the core of the present invention is recovery from processing deterioration, the larger the specific surface area is, the larger the adverse effect of processing deterioration becomes, and as a result, the more the effect of the present invention is enhanced. If in mm²/mm³The S/V value is evaluated in units, and the effect is preferably started when the S/V value of the shape of the product magnet is 0.5 or more. More preferably, S/V.gtoreq.1, the effect is more remarkable.

Further preferably, the medium has a vickers hardness Hv of less than 300, i.e. the particles/powders having a main phase of metal X phase/X alloy system phase have a vickers hardness (Hv) of less than 300. The sand blasting is usually used as the pretreatment of the neodymium iron boron sintered magnet, and the main function is to clean the surface oxide. Since the hardness of the sintered nd-fe-b magnet is 600 to 800, it is generally treated with an abrasive having a high hardness. When the surface of the neodymium iron boron magnet is subjected to sand blasting treatment by using SiC polygonal abrasive with Hv of 2400-2500, the magnet is seriously processed and degraded due to too high hardness of the abrasive, and the performance is reduced. Therefore, the medium Vickers hardness Hv is preferably 300 or less, and more preferably Hv180 or less.

The equipment, reagents, processes, parameters and the like related to the invention are conventional equipment, reagents, processes, parameters and the like except for special description, and no embodiment is needed.

All ranges recited herein include all point values within the range.

In the invention, the room temperature, namely the normal environment temperature, can be 10-30 ℃.

The invention has the following core points and beneficial effects:

a granular/powdery medium containing an X-phase/X-alloy metal phase as a main phase is placed in a sand blast processing apparatus, and the Nd-based sintered magnet is placed in the sand blast processing apparatus, and the medium is blasted onto the surface of the Nd-based sintered magnet by an air current, whereby the medium cleans up at least a part of the oxide on the surface of the Nd-based sintered magnet, and a thin film having a layer of the medium adsorbed thereon remains on the surface of the Nd-based sintered magnet, and although the amount of adsorption is small, the granular/powdery medium has been found to have a great effect on the improvement of the magnet performance after the RH diffusion processing, the improvement of the performance unevenness, the deformation and the prevention of cracks.

The metal X or alloy X-based medium of the present invention is a group of metals or alloys called additive elements, which exhibit a function of recovering performance when present at grain boundaries or main phase crystal grain surfaces of Nd-based sintered magnets, and can improve Hcj and Hk of the magnets. In the present invention, since the oxide phase deteriorated in surface processing is removed after the dielectric gas stream blasting treatment, and a thin film of metal X or alloy X is left adsorbed on the surface of the magnet, the RH element is diffused into the magnet by promoting surface cleaning in the RH diffusion treatment, and the surface oxide layer, hydroxide phase, and carbide phase are removed, and the metal X/alloy X having a small oxygen content and carbon content is strongly adsorbed on the surface of the magnet, thereby preventing the deterioration of performance due to oxidation and carbonization in the heat treatment.

In recent years, with the progress of high performance of Nd-based sintered magnets, the effects of oxidation and carbonization on the magnet surface have become significant as the content of rare earth in Nd magnets and the content of oxygen inside magnets have become smaller. The method of the present invention can reduce the influence of deterioration, deformation and cracks of magnet performance due to oxidation and carbonization caused by RH diffusion treatment, improve the magnet performance, particularly Br, Hcj and Hk, of the final product after RH diffusion treatment, and prevent cracks and deformation defects of the magnet after RH diffusion heat treatment.

Drawings

FIG. 1 is a flow chart of a prior art method.

FIG. 2 is a flow chart of the method of the present invention.

Fig. 3 is a graphical representation of the size versus hardness of the blasting media.

Detailed Description

The invention is further illustrated by the following figures and examples.

Example 1

The raw material ratio is as follows: nd 13.4 at%, Dy 0.2 at%, Ho 0.05 at%, B5.9 at%, Co 1.0 at%, Si 0.05 at%, Mn 0.03 at%, Al 0.2 at%, Ga 0.2 at%, Cu 0.25 at%, Cr 0.05 at%, Nb 0.05 at%, Zr 0.05 at%, Mo 0.05 at%, and the balance Fe.

The starting magnet was produced in the following process, and 500 pieces of the magnet were prepared in a processing specification of 40X 15X 3mm (3mm is the orientation direction). The oxygen content in the sintered magnet was 950 ppm.

And smelting and throwing the sheet by an SC type smelting furnace to obtain 50kg of SC raw material alloy. Crushing SC raw material with hydrogen, jet milling with JM type, at a grain size D₅₀The fine grinding was carried out so as to have an average of 4.8 μm. The powder was placed in a 26kOe magnetic field at 0.25ton/cm²The molded body was obtained by molding, and the molded body was sintered at 1080 ℃. The sintered body was tempered in a vacuum at 500 ℃ for 2 hours. A standard sample having a diameter of 10X 10mm and a magnet sample having a diameter of 40X 15X 3mm (3mm is the orientation direction) were prepared in the above-mentioned manner, respectively.

The magnet characteristics were evaluated by a BH characteristic evaluation apparatus using a standard sample having a small specific surface area and conforming to the national standard of φ 10X 10mm, and the evaluation was recorded as comparative example 7 as a performance comparative example in which the influence of the processing deterioration layer was negligible.

10 pieces of magnet having a processing specification of 40X 15X 3mm (3mm is an orientation direction) were each put in a sand blast processing apparatus and subjected to the treatment of the present invention. The blast-treated medium (the granular body/powder of the present invention having the metal X phase/X alloy system phase as the main phase) can be suitably produced by a production method such as a gas heating method, an ingot scribing-cutting method, an ingot breaking + sieving method, an ultra-rapid cooling (MQ) method + pulverization. The nozzle of the sand blasting device is matched with the particle size of the particles/powder, and the diameter of phi 1 mm-phi 12mm is properly selected. The air blowing pressure of the blasting is appropriately selected from pressures of 0.2to 0.8MPa to prevent the surface of the magnet from being broken, and the uniform treatment can be performed, the air blowing blasting treatment being performed for 1 to 5 minutes.

The magnet after the sand blasting was continuously subjected to RH diffusion heat treatment. The RH diffusion treatment was performed by using 6 μm powder of an alloy of Tb 70 at%, Ga 5 at%, Al 5 at%, and Cu 20 at% suspended in alcohol, and spraying about 30 μm with 1% PVA and 0.2% of a dispersant as a binder. After coating, the coating was dried and subjected to diffusion treatment at 900 ℃ for 18 hours in vacuum. And cooling the magnet after the diffusion treatment to room temperature and then tempering. The tempering treatment was carried out under vacuum at 500 ℃ for 2 hours. Thereafter, 5 test pieces of Φ 10 × 3mm were cut from the central portion of each magnet, and BH magnetic properties were measured by a PFM evaluation apparatus to evaluate the average value.

As a comparative example, a magnet subjected to grain boundary diffusion treatment without performing the blasting treatment of the present invention was recorded as comparative example 1. Further, a grain boundary diffusion magnet was similarly produced by blasting with a commercially available abrasive medium (abrasive sand) without performing blasting with the granular body/powder or the like having the metal X phase/X alloy system phase as the main phase of the present invention as a medium, and the results were recorded as comparative examples 2 and 3. In addition, in alloy compositions outside the specified content range of the present invention, grain boundary diffusion magnets were produced in the same manner by blast treatment, and the results were recorded as comparative examples 4, 5, and 6.

In addition, when the oxygen content of the processed 10 magnets is compared with the oxygen content of the processed surfaces of the 10 magnets processed by the invention, the oxygen content of the magnet processed by the invention is reduced by 3-16%. It may be that the oxide layer on the surface of the magnet is at least partly removed when the particles/powder of the invention are blown onto the surface of the magnet and polished.

Table 1 treatment of example 1 and comparative examples

Table 2 evaluation results of magnetic properties of example 1 and each comparative example

According to this example, it can be seen that the Hcj and Hk characteristics of the magnet of the pretreatment + grain boundary diffusion treatment (example 1) of the present invention are further improved than those of the magnet of the ordinary pretreatment + grain boundary diffusion treatment (comparative example 1).

Also, it can be seen that the magnets of comparative examples 2, 3, which were pretreated with a commercially available grinding medium (grinding sand) plus grain boundary diffusion, had an increased work-degraded layer and produced an additional oxidized layer by intensive grinding with the grinding sand, and the performance after grain boundary diffusion was worse than that of comparative example 1.

In addition, the magnets in comparative examples 4, 5, and 6 were powders/powders of components close to the medium of the present invention, but since the component ranges were out of the component content range of the present invention, the magnet performance as good as that of example 1 of the present invention was not obtained after the diffusion treatment.

Example 2

The raw material ratio is as follows: 10.43 at% Nd, 3.45 at% Pr, 0.2 at% Gd, 5.65 at% B, 2.8 at% Co, 0.02 at% Si, 0.02 at% Mn, 0.1 at% Al, 0.3 at% Ga, 0.25 at% Cu, 0.02 at% Cr, 0.15 at% Nb, 0.03 at% Zr, 0.02 at% W, and the balance Fe.

The raw magnet was produced by preparing green magnets having the following different oxygen contents, and processing them to a gauge of 44 × 22 × 5.5mm (5.5mm is the orientation direction).

And smelting and throwing the sheet by an SC type smelting furnace to obtain 50kg of SC raw material alloy. Crushing SC raw material with hydrogen, jet milling with JM type, and milling with D₅₀The fine grinding was carried out so as to have an average of 4.8 μm. The powder was divided into 7 equal portions, 1 portion in a 26kOe magnetic field at 0.25ton/cm²The molded body was obtained by molding, and the molded body was sintered at 1040 ℃. Tempering the blank at 540 deg.C for 2 hr, cutting into phi 10 × 10mm, and making into unoxidized magnetComparative example 9 of magnet properties before diffusion treatment.

The remaining 6 portions were oxidized by changing the oxidation time of the powder to produce powders having different average oxygen contents at 0.25ton/cm in a 26kOe magnetic field²The molded article was obtained by molding, and the molded article was processed into a thickness of 44X 22X 5.5mm by sintering at 1040 ℃. After the magnet pieces having different oxygen contents were washed, 50 pieces of each magnet were put into a blast treatment apparatus and subjected to the blast treatment of the present invention. The medium (granules/powder of the present invention having a metal X phase/X alloy phase as the main phase) prepared by sandblasting was made of alloy powder having a composition of 70 at% Dy, 10 at% Al, 5 at% Ga, 10 at% Cu and 5 at% Ni and having a particle size of 0.5mm or less prepared by ingot grinding/grinding + sieving. The nozzle of the sand blasting device is phi 3mm, the working pressure of the sand blasting device is properly selected from the pressure of 0.2MPa to 0.8MPa, so that the surface of the magnet is not broken and uniform treatment can be carried out, and the surface of the magnet is respectively subjected to 3 to 5 minutes of sand blasting treatment. Followed by grain boundary diffusion using Dy vapor treatment. Dy vapor treatment test pieces were put into a heat treatment furnace together with a Dy evaporation source and subjected to diffusion treatment at 900 ℃ for 24 hours in an inert gas. Then, the 11 samples were tempered in an inert gas atmosphere at 440 ℃ to 640 ℃ (20 ℃ intervals) for 4 hours to test the optimum heat treatment process, and the remaining pieces were tempered under the optimum heat treatment conditions to obtain the highest magnet performance. 5 pieces of residual magnet were selected from among the magnets subjected to Dy vapor diffusion treatment and optimum tempering treatment, and the magnet was processed to Φ 10 × 5.5mm, and BH magnetic properties were measured, and the average value of the magnetic properties thereof was evaluated as example 2.

The amount of oxygen in the sintered magnet after processing was 470, 820, 1280, 1820, 2000, 2380, and 3020ppm as shown in table 4 below.

Table 3 treatment method of example 2 and each comparative example

Table 4 evaluation results of magnetic properties of example 2 and each comparative example

According to example 2, it can be seen that the lower the oxygen content in the magnet material, the more the performance effect of grain boundary diffusion can be improved. If the oxygen content of the magnet material is 2000ppm or less, the coercive force after diffusion can be sufficiently increased, and the squareness Hk can also be increased to the level before diffusion.

Example 3

The raw material ratio is as follows: 10.4 at% of Nd, 3.4 at% of Pr, 0.05 at% of Tb, 2.2 at% of Co, 0.02 at% of Si, 0.02 at% of Mn, 0.2 at% of Al, 0.4 at% of Ga, 0.3 at% of Cu, 0.02 at% of Cr, 0.05 at% of Nb, 0.05 at% of Zr, 0.05 at% of Ta, and the balance of Fe.

The magnet was produced in the following steps, processed into 30X 10X 2.0mm (2.0mm is the orientation direction), and 1000 pieces were prepared. The oxygen content in the sintered magnet was 300 ppm.

Smelting and melt spinning by an SC type smelting furnace to obtain 50kg of SC raw material alloy. Pulverizing SC raw material with hydrogen, and pulverizing with JM type jet mill to obtain D₅₀The average value reached 4.5. mu.m. The powder was heated at 0.2ton/cm in a 23kOe magnetic field²The resulting mixture was molded to obtain a molded article, and the molded article was sintered at 1060 ℃. The sintered body was heat-treated at 480 ℃ in vacuum for 2 hours, and the magnet characteristics were evaluated by a BH characteristic evaluation apparatus in a standard sample of the national standard φ 10mm × 10mm, and recorded as comparative example 11 as a comparative example of the magnet performance before grain boundary diffusion.

Each of 100 magnet pieces was put in a sand blast processing apparatus and subjected to sand blast processing according to the present invention. The blasting medium (the particles/powder mainly composed of the metal X phase/X alloy system phase of the present invention) may be produced by a method such as a gas blasting method, an ingot wire cutting method, an ingot grinding + sieving method, an ultra-quenching (MQ) method + grinding, and if necessary, a heat treatment may be appropriately performed to obtain an appropriate hardness. The nozzle of the sand blasting device is matched with the particle size of the granules/powder, and the diameter of phi 2 mm-8 mm is properly selected. The working pressure of the sand blasting is properly selected within the range of 0.2MPa to 0.6MPa, so that the surface of the magnet is not cracked and can be uniformly treated, and the sand blasting is carried out on each surface of the magnet for 1 minute to 5 minutes.

The magnet after the sand blasting treatment was subjected to grain boundary diffusion. The grain boundary diffusion adopts a Dy metal coating method, so that a Dy film is attached to the surface of the magnet and has the thickness of 8 mu m. Then, Dy diffusion treatment was performed at 900 ℃ in an Ar atmosphere for 16 hours. The magnet was cooled to room temperature, and then annealed at 480 ℃ for 2 hours under Ar gas. A test piece of phi 10X 2mm was machined from the center of the 5 magnets after the diffusion treatment, and the magnet measurement was performed by a PFM apparatus, and the average value thereof was evaluated as example 3.

In comparative example 10, the performance of the magnet was evaluated by performing grain boundary diffusion and tempering treatment by Dy sputtering in the same manner as in example 3, except that the blast treatment of the present invention was not performed after the processing and cleaning.

Table 5 treatment method of example 3 and each comparative example

Table 6 evaluation results of magnetic properties of example 3 and each comparative example

According to example 3, it can be seen that the hardness of the blasting medium (the particles/powder having the metal X phase/X alloy system phase as the main phase in the present invention) is as low as possible. Since the medium has low hardness, the resulting crystal lattice is less deteriorated, and the amount of adsorption of the X metal/X alloy element is increased, further enhancing the effect of the present invention. The hardness Hv is preferably 300 or less, and more preferably 180 or less.

The above description is only a preferred embodiment of the present invention, and therefore should not be taken as limiting the scope of the invention, which is defined by the appended claims and their equivalents.

Claims

1. A pretreatment method for grain boundary diffusion of Nd-based sintered magnets is characterized by comprising the following steps: preparing an Nd-based sintered magnet and a medium having an average particle size of 10 [ mu ] m to 5mm, the medium being a particle/powder having a metal X phase/X alloy phase as a main phase; performing sand blasting treatment on the Nd system sintered magnet by using the medium, removing at least part of oxides on the surface of the Nd system sintered magnet through the sand blasting treatment, and adsorbing and forming a medium thin film layer on the surface of the Nd system sintered magnet; then carrying out grain boundary diffusion treatment;

wherein X ═ rare earth R, Ga, Al, Cu, or Ni; the X alloy system comprises at least one of R-Fe alloy, R-Ni alloy, R-Ga alloy, R-Al alloy, R-Cu alloy, Ga-Al alloy, Ga-Cu alloy, Al-Cu alloy, R-Ga-Al alloy, R-Al-Cu alloy or Ga-Al-Cu-Ni alloy, and the content of the X element in the X alloy system is more than 60 at%.

2. The method of claim 1, wherein: the medium is a metal/alloy particle/powder containing a metal X phase/X alloy phase at 60 at% or more.

3. The method of claim 1, wherein: the Nd-based sintered magnet has an oxygen content of 2000ppm or less.

4. The method of claim 1, wherein: the medium has a Vickers hardness Hv of less than 300.

5. The method of claim 1, wherein: the medium is sprayed onto the surface of the Nd-based sintered magnet under the action of a gas flow including an inert gas, and the oxygen content in the gas flow is 3000ppm or less.

6. The method of claim 1, wherein: heating the Nd-based sintered magnet to 60 to 400 ℃ before performing a dielectric blasting process on the surface of the Nd-based sintered magnet.

7. The method of claim 1, wherein: the average film thickness of the dielectric thin film layer is 10 nm-1000 nm.

8. The method of claim 1, wherein: the oxygen content of the medium is below 1000 ppm.

9. The method of claim 1, wherein: the Nd-based sintered magnet has a specific surface area S/V value of 0.5 or more.