Surface-strengthened r-t-b rare earth permanent magnet based on high melting point element grain boundary diffusion and preparation method thereof

Abstract

The application discloses a surface-strengthened R-T-B rare earth permanent magnet based on high-melting-point element grain boundary diffusion and a preparation method thereof. The application adopts heavy rare earth elements (at least one of Dy, Tb and Ho), high-melting-point elements (Zr and Ti) and low-melting-point metals (at least one of Al, Ga and Cu) to prepare a diffusion source, and performs grain boundary diffusion treatment on a sintered magnet, thereby obtaining the surface-strengthened R-T-B rare earth permanent magnet. The grain boundary diffusion high-melting-point element precipitates can inhibit the excessive growth of the main phase grains in the grain boundary diffusion process, realize the refinement of the main phase grains and improve the uniformity of the main phase grain size of the whole magnet. The grain boundary diffusion magnet has higher coercivity and squareness, reduces the reduction amount of the remanence of the magnet after the grain boundary diffusion; and can improve the crack propagation resistance of the Nd-rich phase at the grain boundary, thereby improving the fracture toughness of the magnet while improving the strength of the surface layer of the magnet, so that the qualified rate of the product during surface processing is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a surface reinforced R-T-B rare earth permanent magnet and a preparation method thereof, and belongs to the field of rare earth magnets. BACKGROUND

[0002] In recent years, with the continuous enhancement of environmental awareness, the world gradually increases the investment in green economy. In order to cope with the increasingly stringent carbon emission policy, new energy vehicles represented by electric vehicles and hybrid vehicles will become an important development direction of the future automobile industry.

[0003] Nd-Fe-B magnets have high maximum magnetic energy product, which has the advantage of realizing the miniaturization of equipment, so it is widely used in modern industry. This advantage also makes Nd-Fe-B magnets very suitable for use as magnets for electric vehicle engines. The working temperature of the automobile engine is about 200℃, and because the Curie temperature of the Nd-Fe-B magnet is relatively low, its high-temperature magnetic property is poor. At present, the main method to improve the high-temperature magnetic property of the magnet is to add heavy rare earth elements dysprosium and terbium in the magnet, which improves the high-temperature magnetic property by increasing the room temperature coercivity of the magnet. Because the heavy rare earth atoms such as dysprosium and terbium are antiferromagnetic coupled with iron atoms, it will reduce the magnetization intensity of the magnet. Adding a large amount of heavy rare earth elements during smelting can improve the coercivity of the magnet, but it will also cause the reduction of the remanence of the magnet.

[0004] The grain boundary diffusion process can achieve a large increase in coercivity with a small decrease in magnet remanence. The grain boundary diffusion process first covers the heavy rare earth elements on the surface of the magnet in the form of powder, sputtering film or suspension, and then heats to a specified temperature and keeps for a period of time. At high temperature, the heavy rare earth elements will diffuse along the grain boundary to the inside of the magnet, and form a heavy rare earth element-rich shell layer on the surface of the main phase grain. Because the shell layer has a high anisotropy field, it can improve the nucleation field of the demagnetization domain, thereby improving the coercivity of the magnet.

[0005] It is found in actual production that the main phase grains in a certain depth range inward from the diffusion source covering surface of the magnet will grow up during grain boundary diffusion. According to the viewpoint of micromagnetism, refining the grains can improve the coercivity of the magnet, while the coarse main phase grains will reduce the coercivity. The difference between the grain size near the diffusion source covering surface and the grain size in the center of the magnet will increase, which will result in the deterioration of the squareness of the magnet. By inhibiting the growth of the main phase grains in a certain depth range inward from the diffusion source covering surface of the magnet during grain boundary diffusion, the uniformity of the grain size of the whole magnet can be improved, the coercivity and squareness of the grain boundary diffusion magnet can be further improved, and it is beneficial to prepare the neodymium-iron-boron magnet with higher performance for automobile engine. In addition, there will be enrichment of the grain boundary Nd-rich phase in a certain depth range inward from the diffusion source covering surface of the grain boundary diffusion magnet, that is, the thickness of the grain boundary Nd-rich phase in the region will increase. The grain boundary Nd-rich phase has much lower hardness than the main phase, and has lower resistance to crack propagation. After the grain boundary diffusion, the surface hardness of the magnet will decrease. The surface of the grain boundary diffusion magnet is prone to crack during machining, which reduces the machining qualification rate and also easily causes the magnet to break under stress during use. SUMMARY

[0006] In view of the growth of the main phase grains in a certain depth range inward from the diffusion source covering surface of the magnet during grain boundary diffusion of the neodymium-iron-boron magnet and the low surface hardness and fracture toughness of the diffusion source covering surface of the magnet, the application provides a surface strengthened R-T-B rare earth permanent magnet based on high melting point element grain boundary diffusion and a preparation method thereof.

[0007] The technical scheme adopted by the application is as follows:

[0008] A surface strengthened R-T-B rare earth permanent magnet based on high melting point element grain boundary diffusion, characterized in that the composition of the magnet comprises the following components in mass fraction:

[0009] R: 29wt.% to 34wt.%,

[0010] B: 0.9wt.% to 1.1wt.%,

[0011] M: 0.1wt.% to 0.3wt.%,

[0012] X: 0.2wt.% to 3.0wt.%,

[0013] the balance being T and unavoidable impurities, wherein T contains Fe and Co, and more than 75wt.% of T is Fe;

[0014] R is composed of R1 and R2, wherein R1 is at least one of Nd and Pr, R2 is one or more of Tb, Dy and Ho, and R2(wt.%) / R(wt.%) is between 0.2% and 18%,

[0015] The M is a high melting point element, including at least one of Zr and Ti; preferably Zr or Ti;

[0016] The X includes one or more of Al, Ga and Cu;

[0017] The magnet is prepared by surface covering of the diffusion source of the high melting point element on the surface of the magnet base, and then performing grain boundary diffusion, the higher the content of the high melting point element in the magnet is in the area closer to the covering surface of the diffusion source of the magnet; the content of the high melting point element in the magnet gradually decreases with the increase of the distance from the covering surface of the diffusion source.

[0018] The surface strengthened R-T-B rare earth permanent magnet based on the grain boundary diffusion of the high melting point element is preferably prepared by the following method: according to the component proportion, the SC sheet is spun, the alloy powder is prepared by hydrogen crushing and airflow milling, the alloy powder is molded by die pressing in an orientation magnetic field, and a compact is prepared by isostatic pressing, after vacuum sintering, the diffusion source is covered on the surface of the magnet, the grain boundary diffusion is performed, and then the R-T-B rare earth permanent magnet is prepared by secondary aging;

[0019] The component of the diffusion source includes:

[0020] R D : 55wt.%-84wt.%, wherein R D is at least one of heavy rare earth elements Dy, Tb or Ho,

[0021] M D : 15wt.%-40wt.%, M D represents the high melting point element in the diffusion source, M D is at least one of Zr and Ti;

[0022] X D : 0.1wt.%-10wt.%, X D is at least one of Al, Ga or Cu.

[0023] The component of the diffusion source is preferably:

[0024] R D : 55wt.%-79wt.%, wherein R D is at least one of heavy rare earth elements Dy, Tb or Ho,

[0025] M D : 20wt.%-40wt.%, M D is at least one of Zr and Ti;

[0026] X D : 0.1wt.%-10wt.%.

[0027] More preferably, X DThe content is 1-10 wt.%.

[0028] The SC tablets do not contain R. D ;

[0029] Preferably, the content of high-melting-point elements in the SC wafer is less than 0.1 wt.%, and more preferably, the SC wafer does not contain any high-melting-point elements, that is, all high-melting-point elements M are derived from high-melting-point elements M in the diffusion source. D .

[0030] The X element can be entirely added to the magnet through a diffusion source, meaning that the X element originates entirely from the X element in the diffusion source. D The X element is not present in the composition of the SC sheet; the X element in the diffusion source D The mass of the element is 100% of the mass of element X in the magnet;

[0031] Alternatively, element X can be added to the magnet in two parts, with one part comprising 40-99% of the total element X, serving as the X diffusion source. D The element X is added to the diffusion source, and the remaining X element is added to the SC sheet, ultimately satisfying the requirement that the X element content in the diffused magnet is 0.2 wt.% to 3.0 wt.%. That is, the X element in the diffusion source... D The mass of the element is 40-99% of the mass of element X in the magnet;

[0032] Preferably, element X is added to the magnet in two parts, preferably from the diffusion source. D The mass of the element is 40-90% of the mass of element X in the magnet.

[0033] X in the preferred diffusion source D The element is Ga.

[0034] Preferably, the X element in the SC chip is at least one of Al, Ga, or Cu.

[0035] Furthermore, the preferred SC tablets include the following ingredients:

[0036] R1: 23 wt.% to 34 wt.%, R1 is at least one of Nd and Pr, preferably 75% to 85% of R1 is Nd;

[0037] B: 0.9 wt.% ~ 1.1 wt.%

[0038] X: 0.08 wt.% to 2.7 wt.%, where X is at least one of Al, Ga, or Cu.

[0039] The balance is T and unavoidable impurities, wherein T contains Fe and Co, and more than 75 wt.% of T is Fe.

[0040] The surface reinforced R-T-B rare earth permanent magnet based on high melting point element grain boundary diffusion provided by the application has an average grain size difference of less than 5% in a 30 μm depth range from a diffusion source covering surface, wherein the center part of the magnet refers to an internal area of the magnet with a depth of more than 40 μm from the diffusion source covering surface.

[0041] The magnet has rod-shaped or short rod-shaped high melting point element precipitates in a 30 μm depth range from the diffusion source covering surface, the high melting point element precipitates are at least one of Zr compound and Ti compound, and the number of high melting point element precipitates and the content of high melting point elements in the magnet gradually decrease with the increase of the depth from the diffusion source covering surface of the magnet.

[0042] The surface reinforced R-T-B rare earth permanent magnet based on high melting point element grain boundary diffusion provided by the application has a higher content of high melting point elements in a region closer to the diffusion source covering surface of the magnet, and the content of high melting point elements in the magnet gradually decreases with the increase of the distance from the diffusion source covering surface; when the magnet matrix does not contain high melting point elements and the high melting point elements are all from the diffusion source, the content of high melting point elements in the magnet is less than 10 ppm in a region of the magnet with a distance of more than 40 μm from the diffusion source covering surface.

[0043] The diffusion source in the application can be covered on the surface of the magnet in various ways, including direct covering of powder, spraying of prepared organic suspension of the diffusion source or magnetron sputtering film coating, and the magnetron sputtering film coating is preferably used.

[0044] The diffusion source covering surface can be any one or more surfaces of the magnet, and the diffusion source is preferably covered on two surfaces perpendicular to the orientation direction of the magnet.

[0045] Preferably, the temperature of the grain boundary diffusion is 900-1000 ℃, more preferably 950-1000 ℃, and the holding time is 3-8 h, more preferably 3-6 h.

[0046] In order to prevent the volatilization of rare earth elements, 30-50 kPa of argon is preferably filled into the diffusion furnace when heated to the holding temperature. After the holding is completed, the furnace can be cooled or air cooling can be used.

[0047] The temperature of the secondary aging is 400-650 ℃, preferably 400-600 ℃, the holding time is 2-8 h, preferably 2-6 h, and the cooling speed after the secondary aging is not less than 30 ℃ / min to less than 80 ℃.

[0048] The diffusion source can be prepared by the following two methods:

[0049] Method one: a block-shaped diffusion source alloy is prepared by vacuum induction melting and casting;

[0050] Method 2: Vacuum induction melting and belt spinning are used to prepare diffusion source SC sheets, and then hydrogen crushing and air jet milling are used to prepare diffusion source powder.

[0051] Bulk diffusion sources are typically deposited onto the magnet surface using magnetron sputtering, while diffusion source powder is applied to the magnet surface either by powder coating or by preparing a suspension. When using a suspension for coating, the coated magnet needs to be dried in an oven at a temperature of 60℃ to 80℃ for 20 minutes to 2 hours.

[0052] Before grain boundary diffusion, the magnets after vacuum sintering are typically cut into thin slices with a thickness of 1mm to 20mm. Then, the magnet surface is sandblasted to expose fresh surfaces before covering them with a diffusion source to allow grain boundary diffusion to proceed.

[0053] This invention also provides a method for preparing a surface-strengthened RTB rare-earth permanent magnet based on grain boundary diffusion of high-melting-point elements. The method is as follows: SC sheets are melted and spun according to the component ratio, alloy powder is prepared by hydrogen crushing and air jet milling, the alloy powder is molded and isostatically pressed into a compact under an orientation magnetic field, and after vacuum sintering, a diffusion source is covered on the magnet surface to carry out grain boundary diffusion, and then the RTB rare-earth permanent magnet is obtained by two-stage aging.

[0054] The components of the diffusion source include:

[0055] R D 55wt.%~84wt.%, of which R D It is at least one of the heavy rare earth elements Dy, Tb, or Ho.

[0056] M D 15wt.%~40wt.%

[0057] X D : 0.1wt.%~10wt.%, X D It is at least one of Al, Ga, or Cu.

[0058] The diffused magnet composition includes the following components by mass fraction:

[0059] R: 29wt.%~34wt.%

[0060] B: 0.9 wt.% ~ 1.1 wt.%

[0061] Zr: 0.1 wt.% to 0.3 wt.%

[0062] X: 0.2 wt.% to 3.0 wt.%

[0063] a balance of T and unavoidable impurities, wherein T comprises Fe and Co, and more than 75 wt.% of T is Fe;

[0064] R consists of R1 and R2, wherein R1 is at least one of Nd and Pr, R2 is one or more of Tb, Dy and Ho, and R2(wt.%) / R(wt.%) is between 0.2% and 18%,

[0065] M is a high melting point element, comprising at least one of Zr and Ti;

[0066] X comprises one or more of Al, Ga and Cu.

[0067] The present application uses heavy rare earth elements (at least one of Dy, Tb and Ho), high melting point elements (Zr and / or Ti) and low melting point metals (at least one of Al, Ga and Cu) to prepare a diffusion source, and performs grain boundary diffusion treatment on a sintered magnet. During the grain boundary diffusion process, high melting point element atoms diffuse into the magnet along with heavy rare earth element atoms and low melting point metal atoms. In this process, the high melting point elements react with B elements and the like to form short rod-shaped or rod-shaped precipitates. These precipitates exist in the Nd-rich phase at the grain boundaries and can inhibit the excessive growth of the main phase grains during the grain boundary diffusion process.

[0068] After using the heavy rare earth element diffusion source provided by the present application for grain boundary diffusion, the main phase grains within a certain depth range of the surface layer of the magnet diffusion source will only grow slightly, with a difference of less than 5% from the average grain size in the center of the magnet, significantly improving the poor uniformity of the grain size of the near-surface grains and the center grains of the traditional grain boundary diffusion magnet. Therefore, compared with the traditional pure heavy rare earth element diffusion source, heavy rare earth element compound diffusion source or mixture of the two, the coercivity of the magnet after the grain boundary diffusion of the magnet using the diffusion source provided by the present application is higher, and the squareness of the magnet is also significantly better than that of the traditional grain boundary diffusion magnet.

[0069] In addition, the grain boundary diffusion magnet of the present application forms short rod-shaped or rod-shaped high melting point element precipitates within a certain depth range of the near-surface layer of the magnet, which can effectively improve the mechanical properties of the surface layer of the magnet and effectively improve the pass rate during machining of the surface of the magnet.

[0070] The high melting point element atoms have limited diffusion range due to the reaction of the high melting point element with B element and the limited content of the high melting point element in the diffusion source. The high melting point element precipitates can only be formed in the depth layer of 30 μm of the diffusion source coverage of the magnet, and the number of the high melting point element precipitates decreases and the content of the high melting point element in the magnet gradually decreases with the increase of the depth. When the magnet substrate does not contain the high melting point element and the high melting point element is entirely from the diffusion source, the content of the high melting point element in the internal magnet is less than 10 ppm when the internal diffusion source coverage distance of the magnet is greater than 40 μm.

[0071] The beneficial effects of the present application are:

[0072] Compared with the traditional pure heavy rare earth element diffusion source and heavy rare earth element compound diffusion source, the grain boundary diffusion magnet has higher coercivity and squareness. By adding a certain amount of high melting point element to the heavy rare earth element diffusion source, the high melting point element will diffuse along the grain boundary to the interior of the magnet during the grain boundary diffusion. And the high melting point element precipitates in the form of rod or short rod are generated by reacting with B element and other elements during the diffusion process. These high melting point element precipitates are mainly distributed in the grain boundary Nd-rich phase, which plays a role in inhibiting the excessive growth of the main phase grain during the grain boundary diffusion. By refining the main phase grain size within a certain depth range from the diffusion source coverage, the coercivity of the diffusion magnet can be further improved. At the same time, with the improvement of the uniformity of the main phase grain size of the whole magnet, the squareness of the magnet is also further improved. Adding a certain amount of low melting point metal element to the diffusion source can promote the diffusion efficiency of the high melting point element and the heavy rare earth element to a certain extent. In addition, the high melting point element precipitates in the form of rod or short rod generated in the grain boundary Nd-rich phase during high temperature grain boundary diffusion can inhibit the rotation of the main phase grain, further reducing the reduction of the remanence of the magnet after the grain boundary diffusion. The rod or short rod shaped high melting point element precipitates generated in the grain boundary Nd-rich phase within a certain area of the diffusion source coverage of the magnet can strengthen the grain boundary Nd-rich phase and improve the crack propagation resistance of the grain boundary phase, which can improve the strength and hardness of the surface layer of the magnet by combining the refinement of the main phase grain. The existence of the hardening layer can reduce the defects during the machining of the diffusion magnet surface, thereby improving the machining qualification rate of the product. BRIEF DESCRIPTION OF DRAWINGS

[0073] Figure 1 Figure 1 is a microstructure photograph of the cross section of the experimental T1-5 magnet.

[0074] Figure 2 (a) is the microstructure of the cross section of the experimental T1-5 magnet within the depth area of 10 μm-20 μm from the diffusion source coverage; Figure 2 (b) is the microstructure photograph of the cross section of the experimental T1-5 magnet within the depth area of 90 μm-100 μm from the diffusion source coverage.

[0075] Figure 3 Figure 4 is a column chart of Zr element content at different depths of the magnet surface of experiment T1-5.

[0076] Figure 4 Figure 5 is a photograph of the surface indentation of the magnet of experiment T1-15.

[0077] Figure 5 Figure 6 is a column chart of Ti element content at different depths of the magnet surface of experiment T2-5. DETAILED DESCRIPTION

[0078] The technical solutions of the present application are further described below in combination with specific examples, but the protection scope of the present application is not limited thereto.

[0079] The present application adopts vacuum induction melting and strip casting, and the raw materials with purity of 99.9% or above are taken according to the component proportion, and are sequentially put into the crucible according to the order from high to low melting point. The vacuum degree in the furnace is drawn until 10 -3 ~10 -4 Pa, and the dew point is lower than -50℃. Then argon is filled into the furnace to make the gas pressure reach 30-50kPa, and heating is performed to 1480-1510℃. After the raw materials are completely melted, heat preservation is performed for 3-5min. Then the alloy liquid temperature is reduced to 1430-1460℃, and heat preservation and casting are performed. The copper roll rotating speed is adjusted to 70-75r / min, then the crucible is rotated at a certain speed, so that the molten alloy liquid is transported to the cooling roll through the tundish for solidification, and then falls to the water cooling tray for cooling.

[0080] The alloy sheet is prepared into alloy powder by hydrogen breaking and airflow milling. During hydrogen breaking treatment, the hydrogen pressure in the reaction kettle is generally 0.01-0.09MPa, and the pressure change in the reaction kettle within 10min is not more than 0.5% during hydrogen absorption reaction, which represents the end of hydrogen absorption. After the hydrogen absorption reaction is completed, the hydrogen in the alloy sheet is removed by heating to 400-600℃ while vacuumizing and heat preserving for 2-6h, and then cooling to obtain the hydrogen broken coarse powder. The obtained coarse powder is placed in the airflow milling equipment, the nozzle pressure is adjusted to 0.6MPa-0.8MPa, the coarse powder is broken by high-speed gas driving and mutual collision, and the inert gas is generally helium, nitrogen or other inert gas. The powder particle size is controlled by the sorting wheel and cyclone separator of the airflow milling equipment.

[0081] After the lubricant and / or antioxidant are added to the alloy powder, the mold pressing is performed in the orientation magnetic field, and the conventional commercially available magnetic powder protection lubricant or antioxidant can be used. The addition amount of the lubricant can be 0.01%-0.1% of the mass of the alloy powder, and the antioxidant can be 0.01%-0.14% of the mass of the alloy powder.

[0082] The orienting magnetic field is preferably 3-6T, and the molding pressure is 5-7MPa. The green compact after the orienting molding is subjected to cold isostatic pressing at a pressure of 150-180MPa. The green compact after the orienting molding has a density of 3.6-4.0g / cm 3 , and the green compact after the cold isostatic pressing has a density of about 4.6g / cm 3 .

[0083] The magnet is sintered to be dense by using a vacuum sintering process. The vacuum sintering process is as follows: the vacuum degree is 10 -3 -10 -4 Pa, the sintering temperature is 1080-1120℃, and the holding time is 4-20h. In order to prevent the rare earth volatilization of the surface layer of the magnet during the high-temperature sintering process, the inert gas is filled into the sintering furnace at 30-50kPa after the sintering furnace is heated to the target sintering temperature, and the inert gas can be argon and helium.

[0084] In the present application, the diffusion source can be prepared by using the following two methods according to the covering mode of the diffusion source:

[0085] Method one: the heavy rare earth element diffusion source is prepared by using vacuum induction melting and casting. The raw materials with a purity of more than 99.9% are taken according to the component proportioning, and are sequentially put into the crucible in the order of the melting point from high to low. The vacuum degree in the furnace is extracted until the vacuum degree reaches 10 -3 -10 -4 Pa, and the dew point is lower than -50℃. Then, the argon gas is filled into the furnace to make the gas pressure reach 30-50kPa, and the temperature is heated to 1550-1580℃. After the raw materials are completely melted, the holding time is 3-5min. Then, the alloy liquid is directly cast into the mold to be cooled, and the block diffusion source alloy is prepared.

[0086] Method two: the heavy rare earth element diffusion source is prepared by using vacuum induction melting and belt casting. The raw materials with a purity of more than 99.9% are taken according to the component proportioning, and are sequentially put into the crucible in the order of the melting point from high to low. The vacuum degree in the furnace is extracted until the vacuum degree reaches 10 -3 -10 -4Pa, the dew point is below -50°C. Then, argon is filled into the furnace to reach a pressure of 30-50 kPa, and heated to 1550-1580°C, and after the raw materials are completely melted, the temperature is kept for 3-5 min. Then the temperature of the alloy liquid is reduced to 1480-1500°C, and kept and cast. The rotating speed of the copper roller is adjusted to 70-75 rpm, then the crucible is rotated at a certain speed, so that the molten alloy liquid is transported to the cooling roller through the tundish for solidification and then falls onto the water cooling plate for cooling. The alloy sheet is prepared into alloy powder by hydrogen breaking and airflow grinding. During the hydrogen breaking treatment, the hydrogen pressure in the reaction kettle is generally 0.01-0.09 MPa, and during the hydrogen absorption reaction, the pressure change in the reaction kettle within 10 min is not more than 0.5%, which represents the end of hydrogen absorption. After the hydrogen absorption reaction is completed, the temperature is raised to 400-600°C while vacuumizing, and kept for 2-6 h, so that the hydrogen in the alloy sheet is released, and then cooled to obtain the hydrogen broken coarse powder. The obtained coarse powder is placed in an airflow grinding device, the nozzle pressure is adjusted to 0.6-0.8 MPa, and the coarse powder is broken by high-speed gas collision. The inert gas is generally helium, nitrogen or other inert gas. The powder particle size is controlled by the sorting wheel and cyclone separator of the airflow grinding device.

[0087] The sintered magnet is cut into the required shape and size by wire cutting, and then the surface of the magnet is sandblasted to expose a fresh surface, and a heavy rare earth element diffusion source is covered on the surface of the magnet. The bulk diffusion source is plated on the surface of the magnet by magnetron sputtering, and the powder diffusion source is covered on the surface of the magnet in the form of powder or suspension. When the suspension is selected, the covered magnet needs to be dried in an oven, the oven temperature is 60-80°C, and the holding time is 20 min-2 h. The diffusion source covering surface can be any one or more surfaces of the magnet, and the two surfaces perpendicular to the orientation direction of the magnet are preferred.

[0088] The magnet covered with the diffusion source is placed in a vacuum diffusion furnace, and kept at 900-1000°C for 3-8 h, preferably 950-1000°C for 3-6 h. In order to prevent the volatilization of rare earth elements, 30-50 kPa of argon is filled into the diffusion furnace when heated to the holding temperature. After the holding is completed, the furnace can be cooled or air cooled.

[0089] After diffusion, the magnet is heated to 400-600°C for 2-8 h of secondary aging treatment, and then cooled to below 80°C at a speed not less than 30°C / min.

[0090] The diffused magnets are subjected to surface sand blasting treatment to expose fresh surface of the magnets. ICP is used to measure the composition of the magnets, and SEM and EDS are used to observe the microstructure of the magnets and analyze the micro composition of the magnets. After polishing the surface of the magnets, a Vickers hardness tester is used to test the hardness of the magnets, and the indentation method is used to calculate the fracture toughness of the magnets. NIM equipment is used to measure the magnetic properties of the magnets.

[0091] Example One:

[0092] For low-melting-point metals, pure metals with a purity of 99.9wt.% or higher are used as raw materials. For elements with a melting point higher than that of pure iron, alloys of the element and iron are used as raw materials. The raw materials are placed in the crucible in order of decreasing melting point. The furnace is evacuated until the vacuum degree reaches 10 -4 Pa, and the dew point is lower than -50℃. Then, argon is filled into the furnace to reach a gas pressure of 30kPa, and the temperature is raised to 1480℃. After the raw materials are completely melted, the temperature is maintained for 3min. Then, the temperature of the alloy liquid is reduced to 1430℃, and casting is performed. The rotation speed of the copper roller is adjusted to 70rpm, and then the crucible is rotated at a certain speed so that the molten alloy liquid is transported through the tundish to the cooling roller for solidification and then falls onto the water cooling tray for cooling, obtaining alloy sheets with a thickness of 0.25±0.05mm.

[0093] The alloy sheets are subjected to hydrogen absorption reaction under a hydrogen pressure of 0.09MPa. After the hydrogen absorption reaction is completed, the alloy sheets are heated to 550℃ while being evacuated, and the hydrogen in the alloy sheets is released after being maintained at 550℃ for 4h. Then, the alloy sheets are cooled to obtain hydrogen-broken coarse powder. After cooling, 0.05wt.% zinc stearate is added to the coarse powder and mixed for 3h. The coarse powder is further broken into fine powder using a nitrogen gas flow mill, and the gas pressure is 0.6MPa.

[0094] 0.03wt.% organic lubricant (magnetic powder protection lubricant 3# produced by Tianjin Yuesheng New Material Research Institute) is added to the fine powder and mixed for 3h. The mixed fine powder is oriented and formed under a magnetic field, the static magnetic field of the orientation magnetic field is 3.5T, and the pressing pressure is 5MPa. The density of the magnet after pressing is 3.9-4.0g / cm 3 . Then, cold isostatic pressing is performed, and the pressing pressure is 160MPa. The density of the magnet after pressing is greater than 4.6g / cm 3 .

[0095] The magnet is sintered to be dense by a vacuum sintering process. The vacuum sintering process is as follows: under a vacuum degree of 10 -4 Pa, the sintering magnet is heated to 7.53g / cm 3 As the minimum requirement for sintering the magnet, the sintering temperature is 1080-1120℃, and the holding time is adjusted within 4-20h. In order to prevent the rare earth elements in the surface layer of the magnet from volatilizing during high-temperature sintering, 30kPa of argon gas is filled into the sintering furnace after the temperature is raised to the target sintering temperature.

[0096] The heavy rare earth element diffusion source is prepared by vacuum induction melting and casting. The raw materials with purity of 99.9% or above are put into the crucible according to the order from high to low melting point. The vacuum degree in the furnace is drawn to 10 -3 ~10 - 4 Pa, and the dew point is below -50℃. Then, the argon is filled into the furnace to make the gas pressure reach 30-50 kPa, and heated to 1550-1580℃. After the raw materials are completely melted, the alloy liquid is directly cast into the mold to cool, so as to prepare the block diffusion source alloy.

[0097] The sintered magnet is cut into a cylinder by wire cutting, and the axial direction of the cylinder is parallel to the orientation of the magnet. The surface of the magnet is sandblasted to expose the fresh surface. The diffusion source film is deposited on the two bottom surfaces of the cylindrical magnet by magnetron sputtering, and the film thickness is 10.5 μm.

[0098] The magnet covered with the diffusion source is placed in the vacuum diffusion furnace, and is kept at 950℃ for 3 h. In order to prevent the rare earth elements from volatilizing, the argon with a pressure of 30 kPa is filled into the diffusion furnace when heated to the holding temperature. After the holding is completed, the furnace can be cooled.

[0099] After the diffusion, the magnet is heated to 520℃ for 3 h of secondary aging treatment, and then cooled to below 80℃ at a speed of not less than 30℃ / min.

[0100] The diffused magnet is sandblasted to expose the fresh surface. The diffused cylindrical magnet is completely dissolved, and the composition of the magnet is measured by ICP. The microstructure and micro composition of the magnet are observed by SEM and EDS. The magnetic properties of the magnet are measured by NIM equipment.

[0101] The composition of the sintered body and the content of the diffusion source elements (mass percentage) are shown in Table 1:

[0102] Table 1

[0103]

[0104] The content of some elements of the magnet after grain boundary diffusion (mass percentage) is shown in Table 2, wherein T1-2 represents that the diffusion source of alloy 2 is used for the grain boundary diffusion of the T1 alloy, and so on.

[0105] Table 2

[0106] Experiment Dy Zr T1-2 0.28 / T1-3 0.25 0.02 T1-4 0.20 0.13 T1-5 0.19 0.24 T1-6 0.16 0.28 T1-7 0.09 0.33 T1-8 0.19 0.24 T1-9 0.19 0.24

[0107] The magnetic properties of the magnet after secondary aging are shown in Table 3:

[0108] Table 3 ​

[0109] Experiment Br (kGs) Hcj (kOe) SQ (%) T1 13.75 16.5 98 T1-2 13.63 23.2 85 T1-3 13.70 23.1 88 T1-4 13.71 23.9 94 T1-5 13.71 24.5 95 T1-6 13.70 23.3 93 T1-7 13.69 20.2 93 T1-8 13.70 24.5 96 T1-9 13.70 24.5 95

[0110] Comparing the magnetic properties of the aged magnets, it is found that the coercivity of the magnets with pure Dy as diffusion source is improved significantly, but the remanence and squareness of the magnets are decreased greatly. The analysis of the properties of the magnets T1-2 to T1-9 shows that when the Zr element in the diffusion source is added to a certain amount, the coercivity, remanence and squareness of the magnets after grain boundary diffusion are all obviously higher than those of the magnets with pure Dy as diffusion source.

[0111] The diffusioned cylindrical magnets are cut along the axial direction, and the microstructure of the cross section of the magnets is observed by SEM. Figure 1 For the microstructure photos of the cross section of the magnet T1-5, the layers at different depths from the diffusion source covering surface of the magnet are observed. Among them Figure 2 (a) is the microstructure of the magnet at the depth of 10 μm to 20 μm from the diffusion source covering surface of the magnet. It can be seen from the figure that there are rod-shaped and short rod-shaped Zr compounds in the grain boundary phase of the magnet. Figure 2 (b) is the microstructure of the magnet at the depth of 90 μm to 100 μm from the diffusion source covering surface of the magnet. It can be seen from the figure that there are no Zr compounds in any form in the grain boundary phase.

[0112] From the diffusion source covering surface of the magnet, the Zr element content in the areas of 0-10 μm, 10-20 μm, 20-30 μm, 30-40 μm and 40-50 μm from the diffusion source covering surface of the magnet is measured by EDS area scanning. The Zr element content columnar graph of different areas of the magnet T1-5 is shown in Figure 3 .

[0113] It can be known from the experimental results that the Zr element of the magnet is the highest in the area close to the diffusion source covering surface of the magnet, and the Zr content of the magnet gradually decreases with the increase of the distance from the diffusion source covering surface. When the distance from the diffusion source covering surface of the magnet is 30-40 μm, the Zr content of the magnet is very low. When the Zr is all from the diffusion source, the Zr content is less than 10 ppm when the distance from the diffusion source covering surface of the magnet is 40-50 μm.

[0114] The distribution density of Zr compounds in the range of 0-30 μm from the diffusion source covering surface of the magnet T1-2 to T1-9 is counted under the scanning electron microscope, and the results are shown in Table 4.

[0115] Table 4

[0116]

[0117] The data show that the distribution density of Zr compounds in the range of 0-30 μm from the diffusion source covering surface increases gradually with the increase of the Zr content in the diffusion source.

[0118] The average grain size of the main phase in the area 0-30 μm from the diffusion source covering surface and the magnet core (5000-5030 μm from the diffusion source covering surface) in SEM image statistical experiments T1-2 and T1-5 is shown in Table 5:

[0119] Table 5

[0120]

[0121] As can be seen from the data in the table, the average grain size of the main phase in the area close to the diffusion source covering surface of the magnet after diffusion using pure Dy as the diffusion source is obviously higher than that in the magnet core. After adding an appropriate amount of Zr in the diffusion source and using the same grain boundary diffusion process, the average grain size in the area close to the diffusion source covering surface of the magnet is basically the same as that in the magnet core.

[0122] A layer of heavy rare earth element-rich layer can be formed at the edge of the main phase grain by heavy rare earth element grain boundary diffusion, and the coercivity of the magnet can be improved by increasing the anisotropic field of the surface layer of the main phase grain. However, in the traditional grain boundary diffusion process, the main phase grain will grow. According to the viewpoint of micromagnetism, the coercivity of the magnet can be further improved by fine main phase grains. In the present application, a certain amount of Zr element is added in the grain boundary diffusion source. During diffusion, Zr will diffuse along the grain boundary into the magnet and react with B element and the like to form Zr compounds. The short rod-shaped or rod-shaped Zr compounds existing in the grain boundary Nd-rich phase can inhibit the excessive growth of the main phase during the grain boundary diffusion process. By comparing the magnetic properties of experiments T1-2-T1-9, it is found that the total Dy content of the magnet after grain boundary diffusion using pure Dy as the diffusion source is higher than that of the magnet after diffusion using the Zr-containing diffusion source, but the coercivity increment is obviously lower than that of the latter, and the squareness and remanence are also obviously lower than those of the latter. When the Zr-containing diffusion source is used for grain boundary diffusion, the growth of the outer layer main phase grain of the magnet can be inhibited due to the generation of Zr compounds, and the coercivity of the magnet can be further improved by refining the main phase grain. At the same time, the squareness of the magnet can be further improved due to the improvement of the uniformity of the main phase grain in the certain back thickness layer close to the diffusion source covering surface of the magnet and the magnet core. In addition, since the generated Zr compounds mainly exist in the grain boundary Nd-rich phase of the magnet, they can hinder the rotation of the main phase grain of the magnet during high-temperature grain boundary diffusion, and thus the remanence drop can also be reduced.

[0123] The Zr element in the diffusion source in the present application must reach a certain content to have a significant effect of improving the performance of the magnet. When the Zr content in the diffusion source is too low, the Zr content in the interior of the matrix magnet after the grain boundary diffusion is too small to form precipitates. Neither can it play a role in hindering the growth of the main phase grains during the diffusion process, nor can it reduce the content of heavy rare earth elements in the diffusion source, resulting in a decrease in the coercivity increment of the magnet after diffusion. When the Zr content in the diffusion source increases to 15wt.% to 40wt.%, a certain amount of Zr compounds can be formed in the near-surface layer of the magnet after the grain boundary diffusion, thereby achieving the purpose of improving the coercivity, remanence and squareness of the magnet. Due to the reaction of Zr element with B element and other elements during the diffusion process, combined with the limited Zr content in the diffusion source and the atomic diffusion distance, the Zr compounds formed in the grain boundary diffusion magnet in the present application are mainly concentrated in the depth range of 30 μm from the diffusion source covering surface. The Zr element in the magnet is in a gradient distribution form, which can be clearly seen from the distribution density of the Zr compounds in the magnet and the Zr element concentration in different regions. The Zr content in the magnet is the highest near the diffusion source covering surface, and gradually decreases with the increase of the distance from the diffusion source covering surface.

[0124] The Zr content in the diffusion source in the present application is not the higher the better. With the increase of the Zr content in the diffusion source, although the number of Zr compounds in the Nd-rich grain boundaries in the magnet increases significantly, the squareness and remanence of the magnet can be improved, but the content of heavy rare earth elements in the diffusion magnet is low due to the decrease of the content of heavy rare earth elements in the diffusion source, and the coercivity increment decreases. Therefore, the suitable Zr content in the diffusion source in the present application is 15wt.% to 40wt.%. At the same time, in the present application, Zr element is mainly concentrated in the surface layer of the diffusion magnet, and the formation of Zr compounds hinders the abnormal growth of the main phase grains in the surface layer of the magnet during the grain boundary diffusion and hinders the rotation of the main phase grains, so the required Zr content is limited. Experiments show that only the Zr element content in the diffusion magnet needs to be between 0.1wt.% and 0.3wt.% to achieve the purpose. Excessive generation of Zr compounds not only cannot further improve the coercivity of the magnet, but on the contrary, the remanence of the magnet will decrease due to the increase of the non-magnetic phase in the magnet.

[0125] Example two:

[0126] For low-melting-point metals, pure metals with a purity of 99.9wt.% or more are used as raw materials, and for elements with a melting point higher than that of pure iron, alloys of the element and iron are used as raw materials. The raw materials are placed in the crucible in order from high to low melting point, and the furnace is vacuumized until the vacuum degree reaches 10 -4Pa, and the dew point is lower than -50°C. Then, argon is filled into the furnace to reach 30 kPa, and heated to 1480°C. After the raw materials are completely melted, the temperature is kept for 3 min. Then, the temperature of the alloy liquid is decreased to 1430°C, and casting is performed. The rotating speed of the copper roller is adjusted to 70 rpm, and then the crucible is rotated at a certain speed, so that the molten alloy liquid is transported to the cooling roller through the tundish for solidification, and then dropped onto the water cooling plate for cooling, to obtain alloy sheets with a thickness of 0.25±0.05 mm.

[0127] The alloy sheets are subjected to hydrogen absorption reaction under a hydrogen pressure of 0.09 MPa. After the hydrogen absorption reaction is completed, the hydrogen in the alloy sheets is removed by vacuumizing and heating to 550°C for 4 h, and then cooling to obtain hydrogen broken coarse powder. After cooling, 0.05 wt.% zinc stearate is added to the coarse powder and mixed for 3 h. The coarse powder is further broken by a nitrogen gas flow mill to obtain fine powder, and the gas pressure is 0.6 MPa.

[0128] 0.03 wt.% organic lubricant (magnetic powder protection lubricant 3# produced by Tianjin Yuesheng New Material Research Institute) is added to the fine powder and mixed for 3 h. The mixed fine powder is oriented and formed under a magnetic field, the orientation magnetic field is a static magnetic field of 3.5 T, and the pressing pressure is 5 MPa. After pressing, the density of the magnet is 3.9-4.0 g / cm 3 . Then, cold isostatic pressing is performed, and the pressing pressure is 160 MPa. After pressing, the density of the magnet is greater than 4.6 g / cm 3 .

[0129] The magnet is sintered to be dense by a vacuum sintering process. The vacuum sintering process is that, under a vacuum degree of 10 -4 Pa, 7.53 g / cm 3 is used as the sintered magnet. The sintering temperature is 1080-1120°C, and the holding time is adjusted within 4-20 h. In order to prevent the volatilization of rare earth on the surface layer of the magnet in the high-temperature sintering process, 30 kPa of argon is filled into the sintering furnace after the target sintering temperature is reached.

[0130] A heavy rare earth element diffusion source is prepared by vacuum induction melting and casting. Raw materials with a purity of more than 99.9% are taken according to the component proportioning, and are sequentially put into the crucible according to the order from high to low melting point. The vacuum degree in the furnace is extracted to 10 -3 -10 - 4 Pa, and the dew point is lower than -50°C. Then, argon is filled into the furnace to reach 30-50 kPa, and heated to 1550-1580°C. After the raw materials are completely melted, the temperature is kept for 3-5 min. Then, the alloy liquid is directly cast into a mold for cooling, to prepare a blocky diffusion source alloy.

[0131] The sintered magnet is cut by a wire cutting method A cylindrical magnet was prepared with the cylindrical axis parallel to the orientation of the magnet. The surface of the magnet was sandblasted to expose a fresh surface. Diffusion source films were deposited on the two bottom surfaces of the cylindrical magnet using magnetron sputtering, with a film thickness of 10.5 μm.

[0132] The diffusion source-coated magnet was placed in a vacuum diffusion furnace and held at 950°C for 3 h. To prevent volatilization of the rare earth elements, 30 kPa of argon was introduced into the diffusion furnace when heating to the holding temperature. After the holding period, the furnace was allowed to cool.

[0133] The diffused magnet was heated to 520°C for a secondary aging treatment of 3 h, and then cooled at a rate of no less than 30°C / min to below 80°C.

[0134] The diffused magnet was sandblasted to expose a fresh surface. The diffused cylindrical magnet was completely dissolved and the composition of the magnet was measured using ICP. The microstructure of the magnet was observed using SEM. The magnetic properties of the magnet were measured using a NIM device.

[0135] The diffusion source composition for this example was the same as alloy T1, as shown in Table 6:

[0136] Table 6

[0137]

[0138] The mass percentage of some elements in the grain boundary diffused magnet is shown in Table 7:

[0139] Table 7

[0140]

[0141]

[0142] The average grain size of the main phase in the region 0-30 μm from the diffusion source coating and in the center of the magnet (5000-5030 μm from the diffusion source coating) of the magnets of experiments T1-11 and T1-13 was determined from SEM images, and the results are shown in Table 8:

[0143] Table 8

[0144]

[0145] The magnetic properties of the secondary aged magnets are shown in Table 9:

[0146] Table 9

[0147] Experiment Br (kGs) Hcj (kOe) SQ (%) T1-10 13.70 23.0 91 T1-11 13.71 24.5 95 T1-12 13.70 23.3 90 T1-13 13.64 22.4 87

[0148] In the present application, the diffusion of heavy rare earth elements and Zr elements can be effectively promoted by adding a certain amount of low-melting-point metal to the Zr-containing diffusion source. As can be seen from the experimental results of Comparative Experiment T1-10 to Experiment T1-12, with the increase of Ga content in the diffusion source, the Zr content and Dy content in the magnet after grain boundary diffusion also increase, and the coercivity of the corresponding magnet also significantly increases. The low-melting-point metal element can significantly reduce the melting point of the grain boundary phase of the magnet during diffusion, and can promote the diffusion efficiency of Zr elements and heavy rare earth elements.

[0149] The addition amount of the low-melting-point metal element is not the higher the better, as shown in Experiment T1-13. In the present application, with the increase of Ga element content in the diffusion source, the content of heavy rare earth elements in the diffusion source decreases, resulting in a decrease in the content of heavy rare earth elements in the diffusion magnet, and finally a decrease in the coercivity increment of the magnet after grain boundary diffusion. At the same time, as can be seen from the grain size statistics, with the increase of Ga element content in the diffusion source, the average grain size of the main phase in the region 0-30 μm away from the diffusion source coverage of the magnet gradually increases, and the difference between the average grain size of the main phase in the center of the magnet and the average grain size of the main phase in the region 0-30 μm away from the diffusion source coverage of the magnet becomes larger, resulting in a decrease in the squareness of the magnet.

[0150] With the increase of Ga content in the diffusion source, a large amount of Ga will exist in the grain boundary phase in a certain depth layer of the magnet away from the diffusion source coverage, significantly reducing the melting point of the Nd-rich grain boundary phase of the magnet. The low-melting-point grain boundary phase has a greater promoting effect on the growth of the main phase grain, thus weakening the effect of Zr compounds on hindering the growth of the main phase grain and hindering the rotation of the main phase grain, thereby reducing the coercivity and squareness of the magnet and increasing the decrease in remanence. Through analysis of a large amount of experimental data, it is found that when the content of the low-melting-point metal element in the diffusion source is 0.1wt.% to 10wt.%, the comprehensive effect on the performance of the magnet is the best.

[0151] Example Three

[0152] For low-melting-point metals, pure metals with a purity of 99.9wt.% or more are used as raw materials, and for elements with a melting point higher than that of pure iron, alloys of the element and iron are used as raw materials. The raw materials are sequentially placed in the crucible in the order of decreasing melting point, the furnace is evacuated until the vacuum degree reaches 10 -4 Pa, and the dew point is lower than -50℃. Then argon gas is filled into the furnace to make the gas pressure reach 30kPa, and heated to 1480℃. After the raw materials are completely melted, the alloy liquid temperature is reduced to 1430℃, and casting is carried out. The copper roll speed is adjusted to 70 revolutions per minute, then the crucible is rotated at a certain speed, so that the molten alloy liquid is transported through the tundish to the cooling roll for solidification and then falls onto the water cooling plate for cooling, obtaining an alloy sheet with a thickness of 0.25±0.05mm.

[0153] The alloy sheet is subjected to hydrogen absorption reaction under 0.09 MPa hydrogen pressure. After the hydrogen absorption reaction, the hydrogen in the alloy sheet is removed by vacuumizing and heating to 550°C for 4 hours. Then, the hydrogen broken coarse powder is obtained after cooling. After cooling, 0.05 wt.% zinc stearate is added to the coarse powder and mixed for 3 hours. The coarse powder is further broken into fine powder by nitrogen gas flow mill, and the gas pressure is 0.6 MPa.

[0154] 0.03 wt.% organic lubricant (magnetic powder protection lubricant 3# produced by Tianjin Yuesheng New Material Research Institute) is added to the fine powder and mixed for 3 hours. The fine powder after uniform mixing is oriented and formed under magnetic field, the orientation magnetic field is 3.5 T static magnetic field, and the pressing pressure is 5 MPa. The density of the magnet after pressing is 3.9-4.0 g / cm 3 . Then, cold isostatic pressing is performed, the pressing pressure is 160 MPa, and the density of the magnet after pressing is greater than 4.6 g / cm 3 .

[0155] The magnet is sintered to be dense by vacuum sintering process. The vacuum sintering process is that under 10 -4 Pa vacuum degree, 7.53 g / cm 3 is used as the minimum requirement for sintering magnet, the sintering temperature is 1080-1120°C, and the holding time is adjusted within 4-20 hours. In order to prevent the volatilization of rare earth on the surface layer of the magnet during high-temperature sintering, 30 kPa argon gas is filled into the sintering furnace after rising to the target sintering temperature.

[0156] The heavy rare earth element diffusion source is prepared by vacuum induction melting and casting. The raw materials with purity of more than 99.9% are put into the crucible according to the order from high to low melting point, the vacuum degree in the furnace is extracted to 10 -3 -10 - 4 Pa, and the dew point is lower than -50°C. Then, the argon gas is filled into the furnace to make the gas pressure reach 30-50 kPa, and heated to 1550-1580°C. After the raw materials are completely melted, the holding time is 3-5 min. Then, the alloy liquid is directly cast into the mold to cool, and the block diffusion source alloy is prepared.

[0157] The sintered magnet is cut into a cylinder by wire cutting , and the axial direction of the cylinder is parallel to the orientation direction of the magnet. The sand blasting treatment is performed on the surface of the magnet to expose the fresh surface. The diffusion source film is deposited on the two bottom surfaces of the cylindrical magnet by magnetron sputtering method, and the film thickness is 10.5 μm.

[0158] The magnet covered with the diffusion source was placed in a vacuum diffusion furnace and heated to the target temperature for a certain time. The diffusion process is shown in Table 10. In order to prevent the rare earth elements from volatilizing, 30 kPa of argon was filled into the diffusion furnace when heated to the holding temperature. After the holding was completed, the furnace was cooled.

[0159] After diffusion, the magnet was heated to 520°C for 3 h of secondary aging treatment, and then cooled to below 80°C at a rate of not less than 30°C / min.

[0160] The diffused magnet was subjected to surface sand blasting treatment to expose a fresh surface of the magnet. After the diffused cylindrical magnet was completely dissolved, the magnet composition was measured by ICP. The microstructure of the magnet was observed by SEM. The magnetic properties of the magnet were measured by NIM equipment.

[0161] The diffusion substrate used in this example was the same as alloy T1, and the diffusion source composition was the same as alloy 5. The different experimental groups and diffusion processes are shown in Table 10:

[0162] Table 10

[0163] Experiment Diffusion temperature (°C) Soaking time (h) A 850 10 B 950 3 C 1050 3 D 950 1 E 950 10

[0164] The distribution density of Zr compounds in the range of 0-30 μm from the diffusion source covering surface of each experimental group is shown in Table 11:

[0165] Table 11

[0166]

[0167] The average grain size in the range of 0-30 μm from the magnet diffusion source covering surface and the center of the magnet (5000-5030 μm) of different experimental groups is shown in Table 12:

[0168] Table 12

[0169]

[0170] The magnetic properties of each experimental group after grain boundary diffusion are shown in Table 13:

[0171] Table 13

[0172] Experiment Br (kGs) Hcj (kOe) SQ (%) A 13.69 22.3 91 B 13.71 24.5 95 C 13.62 21.8 84 D 13.72 20.6 95 E 13.67 23.8 89

[0173] The present application adds a higher content of Zr in the diffusion source alloy, which results in a sufficient amount of diffusion into the magnet only at a higher diffusion temperature. When the diffusion temperature is lower (corresponding to Example A), the Zr element content in the magnet is still low even after a longer diffusion time, and the amount of Zr compound generated is small. Therefore, the performance improvement effect of the diffusion magnet is not obvious. With the increase of the diffusion temperature, the diffusion rate of Zr atoms increases. The Zr element diffused into the magnet and the B element generate Zr compounds, which inhibit the growth and rotation of the main phase grains in the grain boundary diffusion process, thereby improving the coercivity and squareness of the magnet and reducing the decrease of the remanence. However, when the diffusion temperature is too high, although the Zr content and the distribution density of Zr compounds in the diffusion magnet increase, the high temperature promotes the growth of the main phase grains, thereby weakening the effect of Zr compounds on inhibiting the growth of the main phase grains. Therefore, the average grain size of the magnet within a certain depth range near the diffusion source covering surface is significantly higher than that in the center of the magnet. The holding time of the grain boundary diffusion is also important for the performance of the magnet. When the holding time is too short, the diffusion of Zr elements and heavy rare earth elements is insufficient, the amount of Zr compounds generated is small, and the heavy rare earth element content of the magnet is low, resulting in low coercivity of the magnet. With the extension of the holding time, the difference between the main phase grain size within a certain depth range near the diffusion source and the grain size in the center of the magnet increases. Therefore, in the present application, the suitable diffusion temperature is 900-1000°C, and the holding time is 3-8h.

[0174] Example Four

[0175] For low-melting-point metals, pure metals with a purity of 99.9wt.% or more are used as raw materials, and for elements with a melting point higher than that of pure iron, alloys of the element and iron are used as raw materials. The raw materials are sequentially placed in the crucible in the order of decreasing melting point, the furnace is vacuumized until the vacuum degree reaches 10 -4 Pa, and the dew point is lower than -50°C. Then, argon gas is filled into the furnace to reach a gas pressure of 30kPa, and heated to 1480°C. After the raw materials are completely melted, the alloy liquid temperature is reduced to 1430°C, and casting is performed. The rotation speed of the copper roller is adjusted to 70rpm, then the crucible is rotated at a certain speed, so that the molten alloy liquid is transported through the tundish to the cooling roller for solidification and then falls onto the water cooling plate for cooling, obtaining an alloy sheet with a thickness of 0.25±0.05mm.

[0176] The alloy sheet is subjected to hydrogen absorption reaction under a hydrogen pressure of 0.09MPa. After the hydrogen absorption reaction is completed, the hydrogen in the alloy sheet is removed by vacuumizing and heating to 550°C for 4h, and then cooled to obtain a hydrogen-broken coarse powder. After cooling, 0.05wt.% zinc stearate is added to the coarse powder and mixed for 3h, and the coarse powder is further broken by a nitrogen gas stream mill to obtain a fine powder, and the gas pressure is 0.6MPa.

[0177] 0.03wt.% of organic lubricant (magnetic powder protection lubricant 3# produced by Tianjin Yuesheng New Material Research Institute) was added into the fine powder and mixed for 3h. The fine powder after uniform mixing was oriented and formed under a magnetic field, the orientation magnetic field was 3.5T static magnetic field, the pressing pressure was 5MPa, and the density of the magnet after pressing was 3.9-4.0g / cm 3 . Then cold isostatic pressing was performed, the pressing pressure was 160MPa, and the density of the magnet after pressing was greater than 4.6g / cm 3 .

[0178] The magnet was sintered and densified by vacuum sintering process. The vacuum sintering process was that under the vacuum degree of 10 -4 Pa, the sintered magnet was 7.53g / cm 3 As the minimum requirement for sintering the magnet, the sintering temperature was 1080-1120℃, and the holding time was adjusted within 4-20h. In order to prevent the rare earth volatilization of the surface layer of the magnet during high-temperature sintering, 30kPa of argon was filled into the sintering furnace after the temperature was raised to the target sintering temperature.

[0179] The heavy rare earth element diffusion source was prepared by vacuum induction melting and casting. The raw materials with the purity of more than 99.9% were taken according to the component proportioning, and were sequentially put into the crucible according to the order from high to low melting point. The vacuum degree in the furnace was extracted until the vacuum degree reached 10 -3 -10 - 4 Pa, and the dew point was lower than -50℃. Then the argon was filled into the furnace to make the gas pressure reach 30-50kPa, and was heated to 1550-1580℃. After the raw materials were completely melted, the holding time was 3-5min. Then the alloy liquid was directly cast into the mold to cool, and the block diffusion source alloy was prepared.

[0180] The sintered magnet was cut into a cylinder by wire cutting , and the axial direction of the cylinder was parallel to the orientation direction of the magnet. The sandblasting treatment was performed on the surface of the magnet to expose the fresh surface. The diffusion source film was deposited on the two bottom surfaces of the cylindrical magnet by magnetron sputtering, and the film thickness was 10.5μm.

[0181] The magnet covered with the diffusion source was placed in the vacuum diffusion furnace, and was held at 950℃ for 3h. In order to prevent the volatilization of the rare earth elements, the argon was filled into the diffusion furnace when the temperature was raised to the holding temperature. After the holding was completed, the furnace could be cooled.

[0182] After the diffusion, the magnet was heated to 520℃ for 3h of secondary aging treatment, and after the end, the cooling was performed at a speed not lower than 30℃ / min to lower than 80℃.

[0183] The diffused magnets are subjected to surface sand blasting to expose fresh surface of the magnets. The diffused cylindrical magnets are completely dissolved and the components of the magnets are measured by ICP. The mechanical property test samples are prepared by wire cutting and surface polishing, the surface hardness of the magnets is measured by Vickers hardness tester, and the fracture toughness of the magnets is calculated by indentation method.

[0184] The diffused substrate used in the embodiment is the same as alloy T1, and the diffused source components are shown in Table 14:

[0185] Table 14

[0186]

[0187] The hardness of the diffused source covering surface of the magnets is measured by Vickers hardness tester, and the fracture toughness is calculated by measuring the crack length of the indentation tip. The surface indentation of the experimental T1-15 magnet is shown in Figure 4 , and the crack length is obtained by measuring the total crack length of the indentation tip and averaging. The hardness and fracture toughness are measured for ten groups of values, and then the average values are calculated, which are shown in Table 15. The experimental T1-17 is a comparative example, i.e. the surface of the magnet is not covered with any diffused source, but the magnet is subjected to the same heat treatment process as the other experimental groups.

[0188] Table 15

[0189] Experiment Vickers hardness Hv Fracture toughness K IC (MPa·m 1 / 2 )]]> T1-14 631 4.12 T1-15 553 3.74 T1-16 515 3.53 T1-17 620 3.86

[0190] It can be seen from the experimental results that when pure Dy is used as the diffused source, the surface hardness and fracture toughness of the magnet are lower than those of the magnet of the present application, and when the Ga content in the diffused source is too high, the surface hardness and fracture toughness of the magnet will also be reduced.

[0191] The present application adds a certain amount of Zr element to the diffused source of the magnet. When the grain boundary diffuses, Zr will diffuse to the surface layer region of the magnet and form Zr compounds with B elements and the like. These Zr compounds are mainly distributed in the grain boundary Nd-rich phase of the magnet, which can inhibit the growth of the main phase grains and achieve the refinement of the main phase grains. At the same time, these Zr compounds can effectively strengthen the grain boundary Nd-rich phase and improve the crack propagation resistance of the grain boundary Nd-rich phase. The Zr compounds can improve the hardness and fracture toughness of the diffused source covering surface of the magnet by affecting the main phase grains and Nd-rich phase in the surface layer of the magnet, thereby improving the yield of the product during the machining of the magnet surface.

[0192] Example Five:

[0193] For low-melting-point metals, pure metals with a purity of 99.9wt.% or more are used as raw materials, and for elements with a melting point higher than that of pure iron, alloys of the element and iron are used as raw materials. The raw materials are sequentially placed in the crucible in order of decreasing melting point, and the furnace is vacuumized until the vacuum degree reaches 10 -4Pa, and the dew point is lower than -50°C. Then, argon is filled into the furnace to reach 30 kPa, and heated to 1480°C. After the raw materials are completely melted, the temperature is kept for 3 min. Then, the temperature of the alloy liquid is decreased to 1430°C, and casting is performed. The rotating speed of the copper roller is adjusted to 70 rpm, and then the crucible is rotated at a certain speed, so that the molten alloy liquid is transported to the cooling roller through the tundish for solidification, and then dropped onto the water cooling plate for cooling, to obtain alloy sheets with a thickness of 0.25±0.05 mm.

[0194] The alloy sheets are subjected to hydrogen absorption reaction under a hydrogen pressure of 0.09 MPa. After the hydrogen absorption reaction is completed, the hydrogen in the alloy sheets is removed by vacuumizing and heating to 550°C for 4 h, and then cooling to obtain hydrogen broken coarse powder. After cooling, 0.05 wt.% zinc stearate is added to the coarse powder and mixed for 3 h. The coarse powder is further broken by a nitrogen gas flow mill to obtain fine powder, and the gas pressure is 0.6 MPa.

[0195] 0.03 wt.% organic lubricant (magnetic powder protection lubricant 3# produced by Tianjin Yuesheng New Material Research Institute) is added to the fine powder and mixed for 3 h. The mixed fine powder is oriented and formed under a magnetic field, the orientation magnetic field is a static magnetic field of 3.5 T, and the pressing pressure is 5 MPa. After pressing, the density of the magnet is 3.9-4.0 g / cm 3 . Then, cold isostatic pressing is performed, and the pressing pressure is 160 MPa. After pressing, the density of the magnet is greater than 4.6 g / cm 3 .

[0196] The magnet is sintered to be dense by a vacuum sintering process. The vacuum sintering process is that, under a vacuum degree of 10 -4 Pa, 7.53 g / cm 3 is used as the sintered magnet. The sintering temperature is 1080-1120°C, and the holding time is adjusted within 4-20 h. In order to prevent the volatilization of rare earth on the surface layer of the magnet in the high-temperature sintering process, 30 kPa argon is filled into the sintering furnace after the target sintering temperature is reached.

[0197] A heavy rare earth element diffusion source is prepared by vacuum induction melting and casting. Raw materials with a purity of more than 99.9% are taken according to the component proportioning, and are sequentially put into the crucible according to the order from high to low melting point. The vacuum degree in the furnace is extracted to 10 -3 -10 - 4 Pa, and the dew point is lower than -50°C. Then, argon is filled into the furnace to reach 30-50 kPa, and heated to 1550-1580°C. After the raw materials are completely melted, the temperature is kept for 3-5 min. Then, the alloy liquid is directly cast into a mold for cooling, to prepare a blocky diffusion source alloy.

[0198] The sintered magnet is cut by a wire cutting method A cylindrical magnet is constructed with its axial direction parallel to the orientation of the magnet. The magnet surface is sandblasted to expose a fresh surface. A diffusion source film with a thickness of 10.5 μm is deposited on the two bottom surfaces of the cylindrical magnet using magnetron sputtering.

[0199] The magnet covered with the diffusion source was placed in a vacuum diffusion furnace and held at 950℃ for 3 hours. To prevent the volatilization of rare earth elements, argon gas at 30 kPa was introduced into the diffusion furnace when the holding temperature was reached. After the holding period, the furnace could be cooled.

[0200] After diffusion, the magnet is heated to 520℃ for a two-stage aging treatment of 3 hours. After the treatment, it is cooled to below 80℃ at a rate of not less than 30℃ / min.

[0201] The diffused magnets underwent surface sandblasting to expose a fresh surface. After complete dissolution, the diffused cylindrical magnets were analyzed using ICP-MS to determine their composition. The magnet microstructure was observed and its microstructure analyzed using SEM and EDS. Finally, the magnetic properties of the magnets were measured using NIM equipment.

[0202] The composition of the sintered body and the content (mass percentage) of diffusion source elements are shown in Table 16:

[0203] Table 16

[0204]

[0205] The partial elemental contents (mass percentage) of the magnet after grain boundary diffusion are shown in Table 17. Among them, Experiment T2-2 indicates that the diffusion source of Alloy 2 was used when the matrix alloy T2 was diffused at the grain boundary, and so on.

[0206] Table 17

[0207]

[0208]

[0209] The magnetic properties of the magnet after second-order aging are shown in Table 18:

[0210] Table 18

[0211] Experiment Br (kGs) Hcj (kOe) SQ (%) T2 14.26 14.45 98 T2-2 14.07 20.95 85 T2-3 14.16 20.75 88 T2-4 14.17 21.65 94 T2-5 14.17 22.04 95 T2-6 14.16 21.43 93 T2-7 14.14 18.28 93 T2-8 14.15 22.08 96 T2-9 14.15 22.09 95

[0212] Comparing the magnetic properties after secondary aging, it was found that using pure Dy as the diffusion source significantly improved the coercivity of the magnet, but the remanence and squareness of the magnet decreased considerably. Analysis of the performance of magnets T2-2 to T2-9 in experiments revealed that when a certain amount of Ti element was added to the diffusion source, the coercivity, remanence, and squareness of the magnets after grain boundary diffusion were significantly higher than those using pure Dy as the diffusion source.

[0213] From the diffusion source coverage of the magnet, the Ti element content in the range of 0-10 μm, 10-20 μm, 20-30 μm, 30-40 μm and 40-50 μm from the diffusion source coverage of the magnet was measured by EDS area scanning. The Ti element content columnar graph of the magnet in different areas of experiment T2-5 is shown in FIG. 8. Figure 5

[0214] From the experimental results, it can be seen that the Ti element of the magnet is the highest in the area close to the diffusion source coverage of the magnet, and the Ti content of the magnet gradually decreases with the increase of the distance from the diffusion source coverage. When the distance from the diffusion source coverage of the magnet is 30-40 μm, the Ti content of the magnet is very low. When the Ti is all from the diffusion source, the Ti content is less than 10 ppm when the distance from the diffusion source coverage of the magnet is increased to 40-50 μm.

[0215] The distribution density of Ti compound of the magnet in the range of 0-30 μm from the diffusion source coverage was counted under the scanning electron microscope in experiments T2-2 to T2-9, and the results are shown in Table 19.

[0216] Table 19

[0217]

[0218] The data shows that the distribution density of Ti compound in the range of 0-30 μm from the diffusion source coverage gradually increases with the increase of the Ti content in the diffusion source.

[0219] The average grain size of the main phase in the range of 0-30 μm from the diffusion source coverage and the heart of the magnet (5000-5030 μm from the diffusion source coverage) of experiment T2-2 and experiment T2-5 was counted in the SEM image, and the results are shown in Table 20.

[0220] Table 20

[0221]

[0222] From the data in the table, it can be seen that for the magnet diffused by using pure Dy as the diffusion source, the average grain size of the main phase in the area close to the diffusion source coverage of the magnet is obviously higher than that in the heart of the magnet. After adding an appropriate amount of Ti in the diffusion source and using the same grain boundary diffusion process, the average grain size in the area close to the diffusion source coverage of the magnet is basically the same as that in the heart of the magnet.

[0223] ​A layer of heavy rare earth element enrichment layer can be formed at the edge of the main phase grain by heavy rare earth element grain boundary diffusion, and the coercivity of the magnet can be improved by increasing the anisotropy field of the surface layer of the main phase grain. However, in the traditional grain boundary diffusion process, the main phase grain will grow. According to the viewpoint of micromagnetism, the coercivity of the magnet can be further improved by fine main phase grain. In the present application, a certain amount of Ti element is added to the grain boundary diffusion source. During diffusion, Ti will diffuse along the grain boundary into the magnet and react with B element to form Ti compound. The short rod or rod-shaped Ti compound existing in the grain boundary Nd-rich phase can inhibit the excessive growth of the main phase during the grain boundary diffusion process. The magnetic properties of comparative experiments T2-2 to T2-9 show that the total Dy content of the magnet after grain boundary diffusion using pure Dy as the diffusion source is higher than that of the magnet after diffusion using Ti-containing diffusion source, but the coercivity increment is obviously lower than that of the latter, and the squareness and remanence are also significantly lower than those of the latter. When Ti-containing diffusion source is used for grain boundary diffusion, the growth of the outer layer main phase grain of the magnet can be inhibited due to the generation of Ti compound, and the coercivity of the magnet can be further improved by refining the main phase grain. At the same time, the squareness of the magnet can be further improved due to the improvement of the uniformity of the main phase grain in the certain back thickness layer near the diffusion source covering surface and the center of the magnet. In addition, since the generated Ti compound mainly exists in the grain boundary Nd-rich phase of the magnet, it can hinder the rotation of the main phase grain of the magnet during high-temperature grain boundary diffusion, and thus the remanence drop can be reduced.

[0224] In the present application, the Ti element in the diffusion source must reach a certain content to have a significant effect on improving the performance of the magnet. When the Ti content of the diffusion source is too low, the Ti content in the internal magnet after grain boundary diffusion is too low to form precipitates. It cannot play a role in inhibiting the growth of the main phase grain during the diffusion process, and the coercivity increment of the magnet after diffusion is reduced due to the reduction of the heavy rare earth element content of the diffusion source. When the Ti content in the diffusion source increases to 15wt.%-40wt.%, a certain amount of Ti compound can be formed in the near-surface layer of the magnet after grain boundary diffusion, thereby achieving the purpose of improving the coercivity, remanence and squareness of the magnet. Due to the reaction of Ti element with B element and other elements during diffusion, combined with the limited Ti content of the diffusion source and the atomic diffusion distance, the Ti compound formed in the grain boundary diffusion magnet in the present application is mainly concentrated in the depth range of 30μm from the diffusion source covering surface. The Ti element in the magnet is in a gradient distribution form, which can be clearly seen from the Ti compound distribution density and Ti element concentration in different regions of the magnet. The Ti content of the magnet near the diffusion source covering surface is the highest, and the Ti content of the magnet gradually decreases with the increase of the distance from the diffusion source covering surface.

[0225] The Ti content in the diffusion source is not higher the better in the present application. With the increase of Ti content in the diffusion source, although the number of Ti compounds in the Nd-rich grain boundary of the magnet is obviously increased, which can improve the squareness and remanence of the magnet, but due to the decrease of heavy rare earth content in the diffusion source, the heavy rare earth element content of the diffusion magnet is low, and the coercivity increment decreases. Therefore, the suitable Ti content in the diffusion source in the present application is 15wt.% to 40wt.%. At the same time, in the present application, since Ti element is mainly concentrated in the surface layer of the diffusion magnet, by forming Ti compounds to hinder the abnormal growth of the main phase grain in the surface layer of the magnet and hinder the rotation of the main phase grain when the grain boundary diffuses, therefore the required Ti content is limited. Experiments show that only the Ti element content in the diffusion magnet between 0.1wt.% to 0.3wt.% is needed to achieve the purpose. Too much Ti compound generation not only cannot further improve the coercivity of the magnet, but on the contrary, will decrease the remanence of the magnet due to the increase of the non-magnetic phase in the magnet.

Claims

1. A surface strengthened R-T-B rare earth permanent magnet based on grain boundary diffusion of high melting point elements, characterized by, The magnet comprises the following mass fractions of components: R: 29wt.%-34wt.%, B: 0.9wt.%-1.1wt.%, M: 0.1wt.%-0.3wt.%, X: 0.2wt.%-3.0wt.%, the balance being T and inevitable impurities, wherein T comprises Fe and Co, and more than 75wt.% of T is Fe; R is composed of R1 and R2, wherein R1 is at least one of Nd and Pr, R2 is one or more of Tb, Dy and Ho, and the mass ratio of R2 to R is 0.2%-18%; M is a high-melting-point element, comprising at least one of Zr and Ti; X comprises one or more of Al, Ga and Cu; The magnet is prepared by performing grain boundary diffusion after covering the diffusion source containing high-melting-point elements on the surface of a magnet substrate, and the magnet is prepared by the following method: according to the component proportion, SC sheets are spun, hydrogen crushing and air flow milling are used to prepare alloy powder, the alloy powder is subjected to die forming in an orientation magnetic field and isostatic pressing to prepare a compact, after vacuum sintering, the diffusion source is covered on the surface of the magnet, and then grain boundary diffusion and secondary aging are performed to obtain the R-T-B rare earth permanent magnet; The components of the diffusion source comprise the following mass fractions of components: R D : 55 wt.% - 84 wt.% wherein R D is at least one of the heavy rare earth elements Dy, Tb or Ho, M D : 15 wt. % - 40 wt. %, M D is at least one of Zr, Ti; X D : 0.1 wt. % - 10 wt. %, X D is at least one of Al, Ga or Cu; The SC sheet does not contain R in the composition thereof D The SC sheet does not contain high melting point elements The temperature of the grain boundary diffusion is 900-1000℃, and the holding time is 3-8h; The closer the area of the magnet to the diffusion source covering surface, the higher the content of the high-melting-point element; with the increase of the distance from the diffusion source covering surface, the content of the high-melting-point element in the magnet gradually decreases.

2. The high-melting-point element grain-boundary diffusion-based surface- strengthened R-T-B rare-earth permanent magnet according to claim 1, characterized by The mass of the XD element in the diffusion source is 100% of the mass of the X element in the magnet; or the X element is divided into two parts and added into the magnet, the mass of the XD element in the diffusion source is 40-99% of the mass of the X element in the magnet, and the balance of the X element is added into the SC sheet, finally satisfying that the content of the X element in the magnet after diffusion is 0.2wt.%-3.0wt.%.

3. The high-melting-point element grain-boundary diffusion-based surface- strengthened R-T-B rare-earth permanent magnet according to any one of claims 1 to 2, characterized by The difference between the average grain size in the range of 30μm depth from the diffusion source covering surface and the average grain size of the central part of the magnet is less than 5%; the central part of the magnet refers to the internal area of the magnet with a depth of more than 40μm from the diffusion source covering surface.

4. The high-melting-point element grain-boundary diffusion-based surface- strengthened R-T-B rare-earth permanent magnet as claimed in any one of claims 1 to 2, characterized by The magnet has rod-shaped or short rod-shaped high-melting-point element precipitates in the range of 30μm depth from the diffusion source covering surface, and with the increase of the depth from the diffusion source covering surface, the number of high-melting-point element precipitates decreases, and the content of the high-melting-point element in the magnet gradually decreases.

5. The high-melting-point element grain-boundary diffusion-based surface- strengthened R-T-B rare earth permanent magnet as claimed in any one of claims 1 to 2, wherein When the magnet substrate does not contain high-melting-point elements, and the high-melting-point elements all come from the diffusion source, the content of the high-melting-point element in the magnet is less than 10ppm in the area of the magnet with a distance of more than 40μm from the diffusion source covering surface.

6. A method of producing the surface- reinforced R-T-B based rare earth magnet as claimed in claim 1, characterized by The method is as follows: according to the component proportion, SC sheets are spun, hydrogen crushing and air flow milling are used to prepare alloy powder, the alloy powder is subjected to die forming in an orientation magnetic field and isostatic pressing to prepare a compact, after vacuum sintering, the diffusion source is covered on the surface of the magnet, and then grain boundary diffusion and secondary aging are performed to obtain the R-T-B rare earth permanent magnet; The components of the diffusion source comprise: R D : 55 wt.% - 84 wt.% wherein R D is at least one of the heavy rare earth elements Dy, Tb or Ho, M D : 15 wt. % - 40 wt. %, M D is at least one of Zr, Ti; X D : 0.1 wt. % - 10 wt. %, X D is at least one of Al, Ga or Cu; The components of the diffusion source comprise the following mass fractions of components: The components of the diffusion source comprise the following mass fractions of components: R: 29 wt.% - 34 wt.%, B: 0.9 wt.% - 1.1 wt.%, M: 0.1 wt.% - 0.3 wt.%, X: 0.2 wt.% - 3.0 wt.%, the balance being T and unavoidable impurities, wherein T comprises Fe and Co, and more than 75 wt.% of T is Fe; said R consists of R1 and R2, wherein R1 is at least one of Nd and Pr, R2 is one or more of Tb, Dy and Ho, and R2 (wt.%) / R (wt.%) is between 0.2% and 18%, said M is a high melting point element comprising at least one of Zr, Ti; said X comprises one or more of Al, Ga, Cu.

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