CN113936879B

CN113936879B - La-containing R-T-B rare earth permanent magnet

Info

Publication number: CN113936879B
Application number: CN202111065789.5A
Authority: CN
Inventors: 章兆能; 付松; 满超; 陈彪; 赵翔; 张冬冬
Original assignee: Zhejiang Innuovo Magnetics Industry Co Ltd
Current assignee: Zhejiang Innuovo Magnetics Industry Co Ltd
Priority date: 2021-09-13
Filing date: 2021-09-13
Publication date: 2024-08-06
Anticipated expiration: 2041-09-13
Also published as: WO2023035490A1; CN113936879A

Abstract

The invention discloses an R-T-B rare earth permanent magnet containing La, wherein the mass fraction of La in the components of the magnet is 0.0005wt.% to 0.01wt.%, the La content in a main phase R ₂T₁₄ B compound of the magnet is 0.0005wt.% to 0.02wt.%, and meanwhile, rare earth-rich grain boundary phases with the La content of 0.01wt.% to 0.05wt.% are contained among main phase grains. After the magnet is diffused by the heavy rare earth element grain boundary, the diffusion depth of the heavy rare earth element is improved, and the main phase crystal grains can obtain a shell structure with uniform shell thickness and equivalent HRE element concentration. The improvement of the coercive force can be obviously increased, the phenomenon that the squareness of the magnet is reduced after the traditional grain boundary diffusion is overcome, and the squareness of the magnet is not obviously deteriorated.

Description

La-containing R-T-B rare earth permanent magnet

Technical Field

The invention relates to an R-T-B rare earth permanent magnet containing La, belonging to the field of rare earth magnets.

Background

In recent years, the world has increasingly paid attention to environmental protection, and the call for reducing the use of fossil energy and reducing the emission of carbon dioxide is becoming stronger. At present, the field of automobiles belongs to the large household for discharging carbon dioxide, and it is imperative to change the energy structure of automobiles to realize emission reduction. New energy automobiles represented by electric automobiles and hybrid electric automobiles are seen as the development situation of the automobile field, and are the main direction of the development of the industry in the future.

A major difficulty in limiting the development of new energy automobiles is how to manufacture high performance automotive engine magnets. The R-T-B rare earth permanent magnet material has the highest magnetic energy product compared with other permanent magnet materials in the 80 th century, and is beneficial to the miniaturization of equipment, so that the R-T-B rare earth permanent magnet material is very suitable for being used as a magnet for an automobile engine.

The magnet for automobile engine has a service temperature of about 200 ℃, and the R-T-B type magnet has a low Curie temperature, which results in poor high-temperature magnetic performance, so that the magnet needs to work normally under high-temperature conditions by improving the coercive force of the magnet. The addition of heavy rare earth elements (HREs) is the primary method of increasing the coercivity of R-T-B magnets. Different from the traditional method of adding the HRE in the smelting stage, the method of grain boundary diffusion can effectively reduce the consumption of the HRE, and simultaneously can reduce the effect of reducing the remanence due to the addition of the HRE. This has very important economic significance in the environment where the price of HRE raw materials is continuously rising.

However, the current grain boundary diffusion technology still has problems, such as limited diffusion depth, which results in smaller (typically less than 15 mm) sizes of the magnets currently produced by grain boundary diffusion; the diffusion is non-uniform, the concentration of HRE in the outer surface and the center of the magnet are very different, and the anisotropy of HRE diffusion in the main phase grains results in non-uniform thickness of the HRE-rich shell of the main phase grains and in non-uniform distribution of HRE elements within the shell. The non-uniformity of diffusion deteriorates the squareness of the magnet after being diffused through the grain boundary, and the smaller size requirement of the magnet during the grain boundary diffusion limits the application environment of the magnet.

Disclosure of Invention

The invention aims to overcome the defects of a grain boundary diffusion technology and provides a La-containing rare earth permanent magnet. By adjusting the phase composition of the magnet matrix, the grain boundary diffusion effect of the HRE can be promoted, and the anisotropy of the diffusion of the HRE in the main phase grains can be reduced. Thus, R-T-B magnets with larger size, high coercivity and high squareness can be obtained after diffusion.

The technical scheme adopted by the invention is as follows:

An La-containing R-T-B rare earth permanent magnet, the magnet composition comprising:

r:28.0wt.% to 36.0wt.%, wherein R is at least one of rare earth elements Nd, pr,

B：0.9wt.％～1.0wt.％，

La:0.0005wt.% to 0.01wt.%, preferably 0.005 to 0.008wt.%

M:5wt.% or less, wherein M is at least one of Al, si, ga, mn, nb, ti, zr, cr, hf, cu.

The balance being T and other unavoidable impurities, wherein T is at least one of Fe and Co.

Further, 85wt.% or more of T is Fe.

Further, the La-containing R-T-B rare earth permanent magnet may further contain a heavy rare earth element (HRE) which is at least one of Dy or Tb, and the mass fraction of the heavy rare earth element is 0.15 to 2%, preferably 0.15 to 1%.

That is, it is preferable that the La-containing R-T-B rare earth permanent magnet is composed of the following components in mass fraction:

B：0.9wt.％～1.0wt.％，

La:0.0005wt.% to 0.01wt.%, preferably 0.005 to 0.008wt.%

Heavy rare earth element: 0.15-2%;

The heavy rare earth element enters the magnet through grain boundary diffusion treatment, and is prepared by adopting a grain boundary diffusion technology known in the art.

Further, preferably, the M is at least one of Al, ga, zr or Cu; preferably, the M has the composition of Ga and Cu; or Ga, cu and Al; or Ga, cu and Zr; or Al, ga, zr and Cu.

The content of M is preferably 2wt.% or less, more preferably 1wt.% or less.

Still further, the content of each of Al, ga, zr, or Cu in the M is 0wt.% to 2.0wt.%, preferably 0 to 1wt.%, more preferably 0 to 0.2wt.%. Wherein 0 represents 0.

Further, it is preferable that the mass content of Al is 0 to 0.2wt.%, the mass content of Zr is 0 to 0.2wt.%, the mass content of Cu is 0.05 to 0.2wt.%, the mass content of Ga is 0.05 to 0.2wt.%, and the like,

Among the rare earth elements R, the mass fraction of Pr is preferably 5wt.% or less, more preferably 1wt.% or less.

R:28.0wt.% to 36.0wt.%, wherein R is at least one of rare earth elements Nd, pr

B：0.9wt.％～1.0wt.％，

La：0.005～0.008wt.％

M: less than 1wt.%, M is at least one of Al, ga, zr or Cu;

heavy rare earth element: 0.15-2%.

The invention also provides a preparation method of the La-containing R-T-B rare earth permanent magnet, which comprises the following steps: and carrying out vacuum induction smelting on the raw materials of the La-containing R-T-B rare earth permanent magnet according to the component proportion, heating and melting, casting to obtain an alloy sheet, carrying out hydrogen crushing and air flow grinding treatment on the alloy sheet to obtain alloy powder, carrying out moulding forming on the alloy powder in an orientation magnetic field, and carrying out vacuum sintering and aging treatment on the formed magnet to obtain the La-containing R-T-B rare earth permanent magnet.

When the La-containing R-T-B rare earth permanent magnet contains heavy rare earth elements, the preparation method comprises the following steps: and carrying out vacuum induction smelting on the raw materials of the La-containing R-T-B rare earth permanent magnet according to the component proportion, heating and melting, casting to obtain an alloy sheet, carrying out hydrogen crushing and air flow grinding treatment on the alloy sheet to obtain alloy powder, carrying out moulding forming on the alloy powder in an orientation magnetic field, carrying out vacuum sintering on the formed magnet, and carrying out grain boundary diffusion process and aging treatment on the formed magnet to obtain the La-containing R-T-B rare earth permanent magnet.

Further, the vacuum induction smelting is preferably carried out by taking raw materials with the purity of more than 99.9 percent according to the component ratio, sequentially placing pure iron, other alloy elements and rare earth elements into a crucible, vacuumizing in a furnace until the vacuum degree reaches 10 ^-2 Pa, heating to the temperature of 1500 ℃ below zero, preserving heat for 10-20 minutes after the raw materials are completely melted, then quenching and cooling the molten liquid at the cooling speed of 100 ℃/s-10000 ℃/s (preferably 5000-6000 ℃/s), and casting to obtain the alloy sheet.

The hydrogen crushing and air stream milling treatment is preferably carried out as follows: when hydrogen is crushed, the pressure of hydrogen in the reaction kettle is generally 0.01-0.09 MPa, when hydrogen is absorbed, the pressure change in the reaction kettle is not more than 0.5% within 10 minutes, which means that the hydrogen absorption is finished, after the hydrogen absorption reaction is finished, the temperature is raised to 450-550 ℃ while vacuumizing, the heat preservation time is 3-5 hours, the hydrogen absorbed in the alloy sheet is removed, then the alloy sheet is cooled to obtain coarse powder after hydrogen crushing, the coarse powder is placed in an air flow mill, and the coarse powder is crushed by being driven by high-speed gas under the pressure of 0.6MPa, wherein the inert gas is generally inert gas such as nitrogen, argon and helium; controlling a sorting wheel and a cyclone separator of the jet mill equipment to regulate and control the particle size of powder particles; powder particles having a D50 of 2 to 5.4 microns (preferably 2.6 to 5.4 microns) are preferably obtained.

The alloy powder is molded in an orientation magnetic field preferably by the following steps: the orientation magnetic field is preferably more than 2T, the pressed compact after orientation molding is subjected to cold isostatic pressing, the density of the pressed compact after orientation molding is 3.6-4.0 g/cm ³, and the density of the pressed compact after cold isostatic pressing is more than 4.5g/cm ³.

Whether cold isostatic pressing is performed after the pressing may be chosen according to the process. The invention selects to cool isostatic pressing of the pressed compact after orientation molding, and further eliminates internal cracks of the pressed compact.

The alloy powder can be added with lubricant and/or antioxidant and then subjected to compression molding, and the lubricant or antioxidant is protected by conventional commercially available magnetic powder. The addition amount of the lubricant can be 0.01-0.1% of the mass of the alloy powder, and the addition amount of the antioxidant can be 0.01-0.1% of the mass of the alloy powder.

The vacuum sintering process comprises the following steps: 10 Sintering temperature is 950-1120 ℃ under ^-3～10^-4 Pa vacuum degree, preferably 1050-1080 ℃ for 3-24 h,

In order to prevent rare earth volatilization of the surface layer of the magnet in the high-temperature sintering process, inert gas of 0.02Mpa is filled into the sintering furnace after the temperature is raised to the target sintering temperature. The density of the R-T-B sintered body varies depending on the difference in composition, but should generally be greater than 7.50g/cm ³.

The aging treatment is that the sintered magnet is subjected to a primary aging treatment process for 3-6 hours at 800-900 ℃, then cooled to room temperature at a speed of not less than 20 ℃/min, heated to 400-600 ℃ for secondary aging treatment for 3-6 hours, and cooled to room temperature after the completion of the aging treatment. The secondary aging treatment process has no requirement on cooling speed, and can be cooled rapidly by an air cooler or along with a furnace.

The grain boundary diffusion process comprises the steps of magnet surface treatment, HRE diffusion source coating, high-temperature heat treatment and secondary aging treatment.

The magnet surface treatment is to treat the surface of the sintered magnet to expose a fresh surface suitable for coating the HRE diffusion source. Surface treatments include, but are not limited to, grit blasting, surface grinding, surface pickling, and the like.

The coating of the HRE diffusion source is to coat the HRE diffusion source onto the treated magnet surface by means of magnetron sputtering, multi-arc ion plating, dipping and the like. The diffusion source can be coated on all surfaces of the magnet or selectively selected to coat the surfaces, and the preferred coating position is the surface vertical to the orientation direction of the magnet. The thickness of the coated diffusion source is typically 5 to 25 μm.

The coating mode adopted by the embodiment of the invention is multi-arc ion coating, and the target material is pure HRE pure Dy metal with the purity of 99.99 wt.%.

And (3) after the magnet coated with the HRE diffusion source is subjected to heat preservation at 800-1100 ℃ for 3-6 hours, cooling to room temperature at a speed of not less than 20 ℃/min. And then carrying out secondary aging treatment: preserving heat for 3-6 h at 400-600 ℃, and then cooling to room temperature; the La-containing R-T-B rare earth permanent magnet is prepared.

In the R-T-B rare earth permanent magnet material, la forms a main phase La ₂Fe₁₄ B with poor intrinsic magnetic property, so La is not generally used for producing rare earth permanent magnets. Meanwhile, at present, each R-T-B magnet manufacturer can also adopt Pr/Nd with low La as raw material as much as possible. Thanks to the development of detection technology, the influence of trace elements on materials can be studied by adopting a high-precision element analysis method. According to the invention, the components of the macro-scale and micro-scale of the magnet are accurately analyzed by ICP-MS and EPMA respectively, and the fact that a proper amount of La element is added into the R-T-B magnet can promote the diffusion depth of the grain boundary and improve the diffusion uniformity of HRE in the main phase is found.

During grain boundary diffusion, HRE elements are mainly diffused toward the inside of the magnet through the grain boundary phase. The grain boundary phase belongs to a rapid path for diffusion of elements, and thus lowering the melting point of the grain boundary phase is an important method for improving the diffusion effect. At present, elements such as Al, cu and the like are added in a common method to react with the rich R phase of the grain boundary so as to reduce the melting point of the grain boundary phase. The micro-composition analysis of the La-added magnet using EPMA found that La element was easily biased in the grain boundary phase. The enrichment of La with a low melting point in the grain boundary phase as compared with elements such as Pr and Nd can further lower the melting point of the grain boundary phase to promote the element diffusion rate. Therefore, the magnet added with a proper amount of La element has a larger diffusion depth of HRE element when the grain boundary diffuses.

In the R-T-B rare earth permanent magnet, as the main phase R2T14B has obvious uniaxial anisotropy, the crystal structure causes that the lattice diffusion of the HRE element in the main phase also presents the characteristic of anisotropy in the grain boundary diffusion process, namely, the difficulty of the HRE element diffusion along different crystallographic directions of the main phase is different. Research shows that in R-T-B magnets, the lattice diffusion of heavy rare earth elements in the main phase is easy to carry out along the direction perpendicular to the c-axis (easy magnetization direction) and difficult to carry out along the direction parallel to the c-axis of the main phase. This phenomenon will result in non-uniformity of the HRE rich shell thickness around the main phase grains and HRE element concentration during grain boundary diffusion of the actual magnet. Therefore, the lifting amounts of the coercive force of the crystal grains of different main phases are different, and the phenomena of general lifting effect of the overall coercive force of the magnet and poor squareness of the magnet finally appear.

After a proper amount of La element is added into the R-T-B magnet, most of La is enriched in the grain boundary phase to reduce the melting point of the grain boundary phase. However, some of the La element is still involved in the formation of the main phase grains, and therefore, the remanence Br and the coercive force Hcj of the magnet to which La is added slightly decrease. Related researches show that the main phase La2T14B formed by La has poor stability at high temperature. The HRE element is therefore more likely to replace La atoms in the main phase upon grain boundary diffusion (temperature between 900 and 1100 ℃). Because the HRE element has stronger stability in the main phase, the HRE can replace Pr, nd and other rare earth elements in the main phase in a magnet without La, but the substitution process is greatly influenced by the grain anisotropy of the main phase. In the invention, the HRE is used for replacing unstable La element in the main phase crystal grain, so that the diffusion effect can be accelerated, and the influence of anisotropy of the main phase crystal on diffusion can be reduced. So that the magnet can form a HRE-rich shell layer with uniform thickness and approximate concentration of HRE elements around the main phase after diffusion. Therefore, compared with a magnet without La element, the magnet disclosed by the invention has better and obvious coercivity improving effect after being subjected to HRE grain boundary diffusion, and the squareness of the magnet is not obviously deteriorated. Meanwhile, by combining the characteristic that La element is enriched in the grain boundary phase, the melting point of the grain boundary phase of the magnet can be effectively reduced and the diffusion depth of the grain boundary is promoted, the invention is suitable for preparing large-size grain boundary diffusion magnets.

In the La-containing R-T-B rare earth permanent magnet provided by the invention, the La content in the R ₂T₁₄ B compound is 0.0005wt.% to 0.02wt.%, and the main phase grain boundary phase contains a rare earth-rich grain boundary phase with the La content of 0.01wt.% to 0.05 wt.%.

The La element is easier to gather at the grain boundary, so that the melting point of the grain boundary phase of the magnet can be effectively reduced, and the diffusion depth of heavy rare earth elements during grain boundary diffusion is promoted. Moreover, there is no binary compound between La and Fe, and the presence of La in the grain boundary reduces the Fe content of the grain boundary phase and the possibility of soft magnetism. Meanwhile, a small amount of La element can participate in forming a main phase, but La in the main phase is more easily replaced by heavy rare earth element when the high-temperature environment of grain boundary diffusion is adopted. Therefore, the influence of the anisotropic crystal structure of the main phase on the in-crystal diffusion of heavy rare earth elements can be obviously weakened, and the thickness of the rare earth-rich shell layer of the main phase crystal grain after the grain boundary diffusion and the uniformity of the distribution of the heavy rare earth elements in the shell layer are improved. Therefore, after the magnet disclosed by the invention is subjected to grain boundary diffusion of heavy rare earth elements, the improvement amount of coercive force can be obviously increased, and the squareness of the magnet is not obviously deteriorated.

In the present invention, the La element may be added as a simple substance raw material at the time of melting or may be added by using an alloy containing La element. It is worth noting that rare earth generally contains La element, and the La content in rare earth alloy is required to be as low as possible in the conventional magnet preparation, so that the rare earth separation cost is increased. In the invention, la element is required to be added, so that the requirement of La element content during rare earth separation can be reduced when La-containing rare earth alloy is adopted for addition, thereby reducing material cost.

The beneficial effects of the invention are mainly as follows: according to the invention, the melting point of a grain boundary phase is reduced by adding proper amount of La element into the R-T-B magnet, so that the diffusion depth of HRE element along the grain boundary is promoted; the La ₂T₁₄ B phase is unstable at high temperature, so that HRE element more easily replaces La atoms in the main phase grains at the grain boundary diffusion temperature, thereby improving the lattice diffusion effect and inhibiting the phenomenon of uneven distribution of HRE element in the main phase when lattice diffusion is caused by the uniaxial anisotropic crystal structure of the main phase. Because the main phase crystal grains of the magnet can obtain a shell structure with uniform shell thickness and equivalent HRE element concentration after the magnet is diffused through the crystal boundary. Thereby improving the coercive force improving effect of the magnet after HRE diffusion and overcoming the phenomenon of reduced squareness of the magnet after traditional grain boundary diffusion.

Drawings

FIG. 1 is a chart of the microstructure of the magnet of experiment No. 4.

FIG. 2 is a chart of the microstructure of the magnet of experiment No. 8.

FIG. 3 is a chart of the microstructure of the magnet of experiment No. 13.

Fig. 4 is a diagram showing the distribution of Dy element in the center of the magnet of experiment No. 11.

Fig. 5 is a diagram showing the distribution of Dy element in the center of the magnet of experiment No. 13.

FIG. 6 is a chart of the center microstructure of the magnet of experiment No. 16.

FIG. 7 is a chart of the center microstructure of the magnet of experiment No. 17.

FIG. 8 is a chart of the center microstructure of the magnet of experiment No. 17.

Detailed Description

The technical scheme of the present invention will be further described with reference to specific examples, but the scope of the present invention is not limited thereto.

The smelting mode adopted by the invention is vacuum induction smelting. The raw materials with proper proportion are put into a crucible and placed in sequence according to the order of pure iron, other alloy elements and rare earth elements. And vacuumizing the furnace through a vacuum system until the vacuum degree reaches 10 ^-4 Pa and the dew point is lower than-50 ℃. Heating by an intermediate frequency induction coil, and adjusting heating power for heat preservation for a plurality of minutes after the raw materials are completely melted. And tilting the crucible at a certain speed to enable the alloy liquid to be conveyed to a cooling copper roller through a tundish for solidification, and finally falling onto a water cooling disc for cooling. The thickness and the structure of the alloy sheet can be controlled by adjusting the technological parameters such as the tilting speed of the crucible, the rotating speed of the cooling copper roller, the surface roughness of the cooling copper roller and the like. According to the invention, the alloy sheet with the thickness of 0.3mm plus or minus 0.05mm and the microstructure of most columnar crystals is obtained through the working procedure.

And (3) carrying out coarse crushing on the alloy sheet obtained by smelting through a hydrogen crushing process. Putting the alloy sheet obtained by melt throwing into a reaction kettle with hydrogen with certain pressure to make the alloy sheet perform hydrogen absorption reaction, wherein the hydrogen pressure is generally 0.01-0.09 MPa. Since the reaction is exothermic, the reaction kettle needs to be cooled in order to prevent the temperature of the reaction kettle from being too high in the reaction. The hydrogen absorption reaction takes the pressure change in the reaction kettle within 10 minutes as an ending mark, the hydrogen absorbed in the alloy sheet is removed by heating the reaction kettle after the hydrogen absorption reaction is ended, and the hydrogen is pumped out by a vacuum system.

Fine powder with the granularity meeting the requirement is prepared by an air flow grinding process. The coarse powder after hydrogen breaking is placed in an air flow mill, and the coarse powder is driven by high-speed gas to mutually impact for breaking. As the grinding gas, nitrogen, argon, helium, or a mixed gas thereof may be used. The powder particle size can be controlled by the gas pressure and by the classifying wheel and cyclone in the jet mill, preferably powder particles with a D50<5.4 microns as measured by a gas-flow dispersive laser particle sizer.

Mixing the fine powder with a certain amount of lubricant and antioxidant, molding in an orientation magnetic field, preferably >3T. The density of the pressed compact after orientation molding is 3.6-4.0 g/cm ³, and whether cold isostatic pressing is performed after the die pressing can be selected according to the process. The invention selects to perform cold isostatic pressing on the pressed compact after orientation molding, further eliminates internal cracks of the pressed compact, the pressure of the cold isostatic pressing is generally 150-200 Mpa, and the density of the pressed compact after the cold isostatic pressing is more than 4.5g/cm ³.

And sintering the magnet to be compact by adopting a vacuum sintering mode. The adsorbed gas in the compact is discharged during the temperature rise, so that several different heat-preserving sections are set according to the concentrated gas discharge stage. In order to prevent the rare earth from volatilizing at the high temperature stage, a certain amount of nitrogen or argon is filled into the sintering furnace after the temperature is raised to the target sintering temperature. The vacuum degree in the furnace is preferably 10 ^-3～10^-4 Pa when the final sintering temperature is kept, the sintering temperature is preferably 950-1120 ℃, and the keeping time is 3-24 h. The density of the R-T-B sintered body varies depending on the difference in composition, but should generally be greater than 7.50g/cm ³.

The sintered magnet requires a primary heat treatment process at least 100 ℃ below the sintering temperature. Preferably, the primary heat treatment process is carried out at 800-900 ℃, the heat preservation time is 3-6 hours, and then the primary heat treatment process is cooled to room temperature at a speed of not less than 20 ℃/min.

In order to further improve the magnetic performance of the magnet, the magnet subjected to the primary heat treatment needs to be subjected to a secondary heat treatment process of preserving heat for 3-6 hours at the temperature of 400-600 ℃, and then is cooled to room temperature after finishing. The process has no requirement on cooling speed, and can be cooled rapidly by an air cooler or along with a furnace.

For magnets that require a grain boundary diffusion process, the surface needs to be treated after sintering to expose a fresh surface suitable for coating the HRE diffusion source. Surface treatments include, but are not limited to, grit blasting, surface grinding, surface pickling, and the like. The HRE diffusion source was applied to the treated magnet surface using a coating apparatus. The diffusion source can be coated on all surfaces of the magnet or selectively selected to coat the surfaces, and the preferred coating position is the surface vertical to the orientation direction of the magnet. Coating modes include, but are not limited to, magnetron sputtering, multi-arc ion plating, dipping and the like. The coating mode adopted by the invention is multi-arc ion plating, and the target material is pure HRE pure Dy metal with the purity of 99.99 wt.%. In order to ensure the accuracy of the comparative test, the test used a magnet with a thickness of 22.+ -. 0.02mm in the orientation direction after sintering. The magnet after being coated is cooled to room temperature at a speed of not less than 20 ℃/min after being insulated for 3 to 6 hours at 800 to 1100 ℃. Then carrying out secondary heat treatment of preserving heat for 3-6 h at the temperature of 400-600 ℃, and then cooling to room temperature.

The magnetic property test sample is prepared by means of electric spark cutting, double-end surface grinding and centerless grinding, and the size of the sample is phi 10 (+ -0.01) mm-10 (+ -0.01) mm cylinder. 5 cylindrical samples were prepared for each set of experiments, and the demagnetization curve of the 20 ℃ + -3 ℃ samples was tested using NIM16000, and the average value was calculated after obtaining 5 sets of data. The magnet was crushed and sampled at the center, and the magnet composition was detected by ICP-MS. SEM and EPMA samples were prepared by means of spark cutting and surface grinding and mechanical polishing.

Embodiment one:

the low-melting point metal is a pure metal having a purity of 99.9wt.% or more, and the element having a higher melting point than pure iron is an alloy of the element and iron. And (3) after the different raw materials are reasonably proportioned, obtaining the alloy sheet with the thickness of 0.3mm plus or minus 0.05mm through a melt-throwing procedure.

The method comprises the following specific steps: sequentially placing pure iron, other alloy elements and rare earth elements into a crucible, vacuumizing in a furnace until the vacuum degree reaches 10 ^-2 Pa, heating to 1500 ℃ below zero, preserving heat for 10-20 minutes after the raw materials are completely melted, rapidly cooling the molten liquid at a cooling rate of 6000 ℃/sec, and casting to obtain the alloy sheet with the thickness of 0.3mm +/-0.05 mm.

And (3) carrying out hydrogen absorption reaction on the alloy sheet under the hydrogen pressure of 0.09MPa, vacuumizing and dehydrogenating at 550 ℃ after the hydrogen absorption is finished, and keeping the temperature for 3 hours. After cooling, 0.05wt% zinc stearate is added to the coarse powder and mixed for 3 hours, and the coarse powder is further crushed by a nitrogen air flow mill to obtain fine powder with D50 of 4.6-4.8 microns.

To the fine powder, 0.03wt.% of an organic lubricant (magnetic powder protective lubricant 3# produced by new materials institute of yue, tianjin) was added and mixed for 3 hours. The evenly mixed fine powder is oriented and molded under a magnetic field, the orientation magnetic field is a static magnetic field of 2.0T, and the density of the pressed magnet is 3.9-4.0 g/cm ³. And then carrying out cold isostatic pressing on the magnet under 180Mpa, wherein the density of the pressed magnet is more than 4.6g/cm ³.

And sintering the pressed compact in a vacuum sintering furnace to be compact, wherein the sintering temperature is 1050-1080 ℃, and the sintering time is 6h. The minimum requirement for sintered magnet density is that the density is greater than 7.53g/cm ³, while the primary phase grain size is not greater than 15 microns.

The surface of the sintered magnet is treated by adopting a sand blasting mode, so that the fresh surface is exposed, and the surface perpendicular to the orientation direction of the magnet is plated with pure Dy by adopting a multi-arc ion plating mode, wherein the thickness of the film is 8 microns.

The sintered magnet (not coated) and the coated magnet are air-cooled to room temperature after being heat-preserved for 3 hours at 850 ℃, and then are air-cooled to room temperature after being heat-preserved for 3 hours at 500 ℃. Magnet composition was measured using ICP-MS, magnet magnetic properties were measured using NIM16000 at 20 ℃ ± 3 ℃, and the magnets were subjected to microstructural analysis and micro-domain composition analysis using SEM and EPMA.

The magnet composition is expressed in mass percent and is specifically shown in the following table 1:

TABLE 1

No.	Nd	Pr	Fe	La	Dy	Co	Cu	Ga	B	Zr
											1	30.04	0.51	bal	0	0	0.1	0.105	0.15	0.96	0.15
2	30.04	0.51	bal	0.0005	0	0.1	0.105	0.15	0.96	0.15
											3	30.03	0.51	bal	0.007	0	0.1	0.105	0.15	0.96	0.15
4	30.03	0.51	bal	0.01	0	0.1	0.105	0.15	0.96	0.15
											5	29.89	0.51	bal	0.15	0	0.1	0.105	0.15	0.96	0.15
6	30.03	0.51	bal	0	0.20	0.1	0.105	0.15	0.96	0.15
											7	30.03	0.51	bal	0.0005	0.26	0.1	0.105	0.15	0.96	0.15
8	30.02	0.51	bal	0.007	0.31	0.1	0.105	0.15	0.96	0.15
											9	30.02	0.51	bal	0.01	0.31	0.1	0.105	0.15	0.96	0.15
10	29.88	0.51	bal	0.15	0.31	0.1	0.105	0.15	0.96	0.15

Experiments No.6 to No.10 are the results of experiments No.1 to No.5 after Dy grain boundary diffusion. The magnetic properties of 5 samples were measured and averaged for each set of experiments, the specific values are shown in table 2 below:

TABLE 2

The SQ reduction (%) after grain boundary diffusion refers to the difference between the average value of SQ after the same lot of non-grain boundary diffused magnets and grain boundary diffused magnets.

The microstructure of the magnet was analyzed using SEM and the magnet domain composition was measured using EPMA spectrometer. Experiment 4 the microstructure of the magnet is shown in fig. 1, and the composition of the domains is shown in table 3 below:

TABLE 3 Table 3

Point(s)	Nd	Pr	Cu	Ga	B	La	Zr	Bal
									1	27.423	1.514	0.122	0.374	1.225	0.02	0.133	69.19
2	65.385	6.753	0.637	0.361	0.749	0.017	0.000	26.10

Experiment 8 the microstructure of the magnet is shown in fig. 2 and the composition of the domains is shown in table 4 below:

TABLE 4 Table 4

Point(s)	Nd	Pr	Cu	Ga	B	La	Dy	Bal
									3	27.980	2.314	0.083	0.277	1.552	0.005	0.217	67.57
4	67.796	3.752	0.159	0.467	0.141	0.049	2.305	25.33

By combining the magnetic property data of experiment nos. 1 to 10 and the microstructure of the magnet of experiment 4 and experiment 8, it was found that when a certain amount of La was added, la was distributed in both the main phase and the grain boundary phase, but was mainly concentrated in the grain boundary phase. In fig. 2, point 3 is a main phase and point 4 is a grain boundary phase, and as can be seen from the data of table 4, la is contained in a very low amount in the main phase and is mainly concentrated in the grain boundary phase. The heavy rare earth element Dy is also enriched in a large amount in the grain boundary phase after being diffused through the grain boundary.

When the La addition amount was small (experiment No. 3), the magnetic properties of the magnet were not lowered, and when the addition amount was large, both the coercive force and the remanence of the magnet were lowered (experiment No. 5). This is because when the La element is added too much, excessive La participates in forming the main phase, thereby deteriorating the magnetic properties of the magnet.

From the magnetic performance data of the magnet after HRE diffusion, it can be seen that the magnet with a proper amount of La added in the magnet matrix has obvious coercivity improvement and reduced squareness reduction compared with the magnet without La after HRE grain boundary diffusion by the same process, which indicates that proper La addition can promote grain boundary diffusion effect.

The La element added in proper amount is mainly concentrated in the grain boundary phase of the magnet, and the melting point of the grain boundary phase can be effectively reduced due to the fact that the melting point of La is lower. The low-melting-point grain boundary phase can accelerate movement of diffusion source atoms along the grain boundary phase during grain boundary diffusion, and thus can deepen diffusion depth to improve coercive force of the magnet.

Embodiment two:

And (3) carrying out hydrogen absorption reaction on the alloy sheet under the hydrogen pressure of 0.09MPa, and dehydrogenating at 550 ℃ after the hydrogen absorption is finished, wherein the heat preservation time is 3h. After cooling, 0.05wt% zinc stearate is added to the coarse powder and mixed for 3 hours, and the coarse powder is further crushed by a nitrogen air flow mill to obtain fine powder with D50 of 4.6-4.8 microns.

To the fine powder, 0.03wt.% of an organic lubricant (magnetic powder protective lubricant 3# produced by new materials institute of yue, tianjin) was added and mixed for 3 hours. The evenly mixed fine powder is oriented and molded under a magnetic field, the orientation magnetic field is a static magnetic field of 3.5T, and the density of the pressed magnet is 3.9-4.0 g/cm ³. And then carrying out cold isostatic pressing on the magnet, wherein the density of the pressed magnet is more than 4.6g/cm ³.

The surface of the sintered magnet is treated by adopting a sand blasting mode, so that the fresh surface is exposed, and the surface perpendicular to the orientation direction of the magnet is plated with pure Dy by adopting a multi-arc ion plating mode, wherein the thickness of the film is 5 microns.

And (3) carrying out air cooling to room temperature after the coated magnet is subjected to heat preservation at 850 ℃ for 3 hours, and then carrying out air cooling to room temperature after the coated magnet is subjected to heat preservation at 500 ℃ for 3 hours.

The magnet composition was measured by ICP-MS, expressed in mass ratio, as shown in table 5 below:

TABLE 5

No	Nd	Pr	B	Al	Co	Cu	Ga	La	Dy	Bal
											11	28.53	0.34	0.93	0.06	0.95	0.18	0.12	0	0.258	68.63
12	28.53	0.34	0.93	0.06	0.95	0.18	0.12	0.0008	0.272	68.62
											13	28.53	0.34	0.93	0.06	0.95	0.18	0.12	0.0090	0.273	68.61
14	28.52	0.34	0.93	0.06	0.95	0.18	0.12	0.015	0.273	68.61
											15	28.40	0.34	0.93	0.06	0.95	0.18	0.12	0.16	0.273	68.59

Magnet magnetic properties at 20 ℃ ± 3 ℃ were measured using NIM16000, as shown in table 6 below:

TABLE 6

No	Br(kGs)	Hcj(kOe)	SQ(％)
				11	14.34	15.74	86.5
12	14.37	15.89	91.3
				13	14.45	16.26	95.2
14	14.39	16.17	95.3
				15	14.23	15.78	93.8

The microstructure of the magnet was analyzed using SEM and the magnet domain composition was measured using EPMA spectrometer. Experiment 13 the microstructure of the magnet is shown in fig. 3 and the composition of the domains is shown in table 7 below:

TABLE 7

Point(s)	Nd	Pr	Cu	Ga	Al	B	La	Dy	Bal
										5	25.23	3.295	0.021	0.014	0.365	1.217	0.003	0.264	69.59
6	49.092	10.633	2.115	1.671	0.381	0.447	0.01	1.161	35.49

The EPMA point scanning result shows that La element is mainly distributed in the crystal boundary, and the main phase only has trace La content. La is enriched in the grain boundary phase, so that the melting point of the grain boundary phase is effectively reduced, and the diffusion depth of Dy element during grain boundary diffusion can be promoted.

EPMA was used to scan the surface of the magnet center and analyze Dy element distribution. Fig. 4 and 5 show the results of the face scan of the magnets of experiments 11 and 13, respectively. From the distribution of Dy element in the center of the magnets of experiments 11 and 13, it can be seen that, for the magnets added with a proper amount of La content, the diffusion depth of Dy element increases after Dy grain boundary diffusion treatment, and thicker Dy-rich shell layers can still be formed around the main phase grains in the center portion of the bulk magnet. Meanwhile, the Dy-rich shell layer around the La-containing magnet main phase crystal grain has uniform and continuous thickness, and is more beneficial to the improvement of coercive force. The combined magnet performance can find that the coercivity is improved more obviously after the La-containing magnet grain boundary is diffused.

This is because the main phase La2T14B formed by La has poor stability at high temperature, and thus Dy element more easily replaces La atoms in the main phase at the time of grain boundary diffusion. In La-free magnets, dy element has stronger stability in the main phase, so that the La-free magnets can replace Pr, nd and other rare earth elements in the main phase, but the substitution process is greatly influenced by the grain anisotropy of the main phase. In the invention, the diffusion effect can be accelerated and the influence of anisotropy of the main phase crystal on diffusion can be reduced by substituting unstable La element in the main phase crystal grains. So that the magnet can form a Dy-rich shell layer having a uniform thickness around the main phase after diffusion.

Embodiment III:

And (3) carrying out hydrogen absorption reaction on the alloy sheet under the hydrogen pressure of 0.09MPa, and dehydrogenating at 550 ℃ after the hydrogen absorption is finished, wherein the heat preservation time is 3h. After cooling, 0.05 wt% zinc stearate is added to the coarse powder and mixed for 3 hours, and the coarse powder is further crushed by a nitrogen air flow mill to obtain fine powder with D50 of 4.6-4.8 microns.

The magnet composition after Dy grain boundary diffusion was measured by ICP-MS and expressed in terms of mass ratio as shown in table 8 below:

TABLE 8

No	Nd	Pr	B	Co	Cu	Ga	La	Dy	Bal
										16	30.14	0.81	0.96	0.50	0.2	0.18	0	0.28	66.93
17	30.14	0.81	0.96	0.50	0.2	0.18	0.01	0.31	66.89

EPMA point scanning is adopted to analyze Dy content of the edge shell layer of the main phase crystal grain of the center of the magnet of experiments No16 and No17 after Dy grain boundary diffusion, and the point scanning acquisition schematic diagrams are shown in FIG. 6 and FIG. 7. 10 main phase grains are randomly selected for each sample, dy content at a shell layer around the main phase grains is detected, and an average value and a standard deviation are calculated, and the results are shown in the following table 9:

TABLE 9

No	Average Dy content (wt.%)	Standard deviation of
			16	0.168	±0.017
17	0.243	±0.006

The La content at the center and at the edge of the main phase grain at the center of the experimental No. 17 magnet was analyzed using EPMA spot scanning, and a schematic dotted diagram is shown in FIG. 8. 10 main phase grains were selected from the samples for analysis, and the average value of the La content was calculated, and the results are shown in Table 10 below:

Table 10

Point scan position	Grain center	Grain edges
			Average La content (ppm)	715	-

Analysis of the Dy content outside the main phase grains in the center portion of the grain boundary diffusion magnet revealed that the Dy content outside the main phase grains was higher and the Dy concentration distribution difference was small in the magnet to which appropriate La was added as compared with the magnet not containing La. It is shown that the shell layer formed after the diffusion of the main phase in the grain boundary in the La-added magnet is more uniform. Meanwhile, la content analysis at the center and the edge of the main phase crystal grains found that the average La content of the center of the main phase was 715ppm. And the La content outside the main phase crystal grains is lower and exceeds the detection limit of EPMA equipment, which shows that La outside the main phase crystal grains is basically replaced by Dy. Because the Dy is easier to replace La in the main phase at the diffusion temperature, the thickness of a Dy-rich shell layer formed by adding a magnet with proper La content after being diffused through Dy grain boundaries and the Dy element distribution are more uniform. The squareness of the magnet can be guaranteed not to be obviously deteriorated in the process of effectively improving the coercive force.

The specific embodiments described herein are merely illustrative of the principles of the present invention. Those skilled in the art may make various modifications or additions to the described embodiments or substitutions thereof without departing from the spirit of the invention or exceeding the scope of the invention as defined in the accompanying claims.

Claims

1. The La-containing R-T-B rare earth permanent magnet is characterized by comprising the following components in percentage by mass:

Heavy rare earth element: 0.15-2%,

B：0.9wt.%~1.0wt.%，

La：0.0005wt.%~0.01wt.%

M:5wt.% or less, wherein M is at least one of Al, si, ga, mn, nb, ti, zr, cr, hf, cu;

The balance of T and other unavoidable impurities, wherein T is at least one of Fe and Co;

The heavy rare earth element is at least one of Dy and Tb, and the mass fraction of the heavy rare earth element is 0.15-2%;

The heavy rare earth element enters the magnet through a grain boundary diffusion process;

R2T14B compound with La content of 0.005-0.02 wt.% exists in the main phase of the magnet;

A grain boundary phase with La content of 0.01-0.05 wt.% exists among main phase grains of the magnet.

2. The La-containing R-T-B rare earth permanent magnet according to claim 1, wherein the La content in the magnet is 0.005 to 0.008wt.%.

3. The La-containing R-T-B rare earth permanent magnet according to claim 1, wherein M is at least one of Al, ga, zr, or Cu.

4. The La-containing R-T-B rare earth permanent magnet according to claim 1, characterized in that it consists of the following components in mass fraction:

B：0.9wt.%~1.0wt.%，

La：0.005~0.008wt.%

M: less than 1wt.%, M is at least one of Al, ga, zr or Cu;

heavy rare earth element: 0.15-2%.