Rare earth permanent magnet and preparation method thereof

Abstract

The invention discloses a rare earth permanent magnet and a preparation method thereof. The preparation method comprises the following steps that (1) first fine powder formed by a first alloy raw material and second fine powder formed by a second alloy raw material are mixed according to the mass ratio of (1-1.2): 1, and mixed fine powder is obtained; wherein the average particle size of the first fine powder and the average particle size of the second fine powder are 0.5-10 microns respectively; 2) forming the mixed fine powder into a pressed blank; sintering and tempering the pressed blank to obtain a sintered neodymium-iron-boron magnet; 3) taking a third alloy target material formed by a third alloy raw material and a fourth alloy target material formed by a fourth alloy raw material as target materials, and performing magnetron sputtering on the surface of the sintered neodymium-iron-boron magnet to obtain a film-coated magnet; and carrying out heat treatment on the coated magnet to obtain the rare earth permanent magnet, wherein the film thickness of the film-coated magnet is 18 [mu] m or more. The preparation method can further reduce the weightlessness and irreversible magnetic flux loss of the obtained rare earth permanent magnet.

Description

Technical Field

[0001] The invention relates to a rare earth permanent magnet and a preparation method thereof. Background Art

[0002] RTB-based rare earth sintered magnets have a structure containing R2Fe as a tetragonal crystal compound. 14 The structure of the B-type crystalline phase (main phase) and grain boundary phase enables excellent magnetic properties. R is at least one rare earth element, primarily Nd and / or Pr, Fe is iron, and B is boron. These elements can be partially replaced with other elements. For example, replacing Nd with Ce, La, and / or Y can reduce costs.

[0003] When operating RTB rare earth sintered magnets, high operating temperatures and high humidity often lead to oxidation corrosion. Therefore, the corrosion resistance of cerium-containing NdFeB magnets needs to be considered. Furthermore, the change in magnetic flux density when the magnet operates in a high temperature environment must also be considered. Irreversible flux loss refers to the phenomenon in which the magnetic flux density cannot be restored to its original state after decreasing to a certain level when the magnet is operated in a high temperature environment. The magnitude of irreversible flux loss can indicate the thermal stability (i.e., heat resistance) of the magnet.

[0004] CN116741521A discloses a method for reducing high-temperature irreversible magnetic flux loss in sintered NdFeB magnets, including the following steps: S1: cutting the NdFeB magnet semi-finished product into black sheet magnets using a magnet wire, boiling, and chamfering; S2: testing the magnets for acceptable aging indicators; S3: baking and chamfering magnets that fail step S2, and repeating steps S2 and S3 until the indicators meet the standards; S4: pickling and electroplating. This method is suitable for solving the problems of high high-temperature irreversible magnetic flux loss and high scrap rates in conventional NdFeB magnet processes.

[0005] CN107845466A discloses a method for preparing a low-cost sintered NdFeB magnet, which is prepared in the order of NdFeB main phase → Nd-rich phase powder → NdFeB magnet, wherein the NdFeB main phase powder and the Nd-rich phase powder are prepared separately. Simultaneously, by the addition of Ho, the cost of the alloy is reduced, and the Nd-rich phase is more evenly distributed on the main phase grain boundary and its intersection, improving the microstructure of the NdFeB alloy sheet, and effectively improving the corrosion resistance of the sintered NdFeB permanent magnet, reducing weight loss. Simultaneously, Ce replaces the eutectic temperature of the Nd magnet, causing the sintering and tempering temperature to decrease, saving cost. In this method, Ce content is relatively low, and corrosion resistance is improved by the addition of Ho, reducing weight loss, but the specific weight loss situation is not mentioned.

[0006] CN107147228A discloses a method for preparing a sintered NdFeB magnet, comprising: 1) pre-treating a sintered magnet R1-Fe-BM; 2) mixing Dy or Tb fluoride or hydride powder with an organic solvent to prepare a slurry, applying the slurry to one side of the magnet R1-Fe-BM, and drying to obtain a coating; 3) placing the magnet obtained in step 2) in a vacuum furnace, and keeping it warm in the vacuum furnace to perform incomplete grain boundary diffusion; and 4) performing an aging treatment on the magnet obtained in step 3) and cooling it to room temperature to obtain the magnet. In the sintered magnet R1-Fe-BM, R1 is selected from one or more of Nd, Pr, Dy, Tb, Ho, and Gd, with a weight fraction of 26 to 33 wt%. M is selected from one or more of Ti, V, Cr, Mn, Co, Ni, Ga, Ca, Cu, Zn, Si, Al, Mg, Zr, Nb, Hf, Ta, W, and Mo, with a weight fraction of 0 to 5 wt%. B has a weight fraction of 0.9 to 1.2 wt%. The remainder is Fe and impurities. The sintered NdFeB magnet prepared by this method does not contain Ce. Summary of the Invention

[0007] In view of this, one object of the present invention is to provide a method for preparing a rare earth permanent magnet, which can obtain a rare earth permanent magnet with good heat resistance and corrosion resistance. Another object of the present invention is to provide a rare earth permanent magnet prepared according to the above preparation method.

[0008] The present invention adopts the following technical solutions to achieve the above-mentioned purpose.

[0009] In one aspect, the present invention provides a method for preparing a rare earth permanent magnet, comprising the following steps:

[0010] 1) mixing a first fine powder formed from a first alloy raw material and a second fine powder formed from a second alloy raw material in a mass ratio of 1 to 1.2:1 to obtain a mixed fine powder; wherein the average particle size of the first fine powder and the second fine powder is 0.5 to 10 μm, respectively;

[0011] 2) forming a compact from the mixed fine powder; sintering and tempering the compact to obtain a sintered NdFeB magnet;

[0012] 3) using a third alloy target formed from a third alloy raw material and a fourth alloy target formed from a fourth alloy raw material as targets, and performing magnetron sputtering on the surface of the sintered NdFeB magnet to obtain a coated magnet; and heat treating the coated magnet to obtain a rare earth permanent magnet;

[0013] The coating thickness of the coated magnet is greater than 18 μm.

[0014] The composition of the first alloy raw material is (PrNd) a1 Al a2 Cua3 B a4 Fe bal ; a1, a2, a3, a4 represent weight fractions, a1 is 27-33wt%, a2 is 0.08-0.4wt%, a3 is 0.1-0.5wt%, and a4 is 0.8-1.5wt%;

[0015] The composition of the second alloy raw material is (PrNd) b1 Ce b2 Al b3 Cu b4 Ga b5 B b6 Fe bal ; b1, b2, b3, b4, b5, b6 represent mass fractions, b1 is 7 to 11.5wt%, b2 is 19 to 24wt%, b3 is 0.04 to 0.25%, b4 is 0.04 to 0.3wt%, b5 is 0.05 to 0.3wt%, and b6 is 0.8 to 1.25wt%;

[0016] The composition of the third alloy raw material is Pr c1 Ni c2 Nb c3 B c4 Fe bal ; c1, c2, c3, c4 represent mass fractions; c1 is 36-44wt%, c2 is 6.5-14wt%, c3 is 9-15wt%, c4 is 0.9-1.5wt%;

[0017] The composition of the fourth alloy raw material is Tb d1 Cu d2 Co bal ; d1 and d2 represent mass fractions; d1 is 58~62wt%, d2 is 18~23wt%.

[0018] According to the preparation method of the present invention, preferably, in step 1), the components of the first alloy raw material are smelted to obtain a first casting sheet; the first casting sheet is hydrogen crushed to obtain a first coarse powder; the first coarse powder is mixed with an antioxidant and a lubricant and then subjected to air flow milling to obtain a first fine powder; wherein the thickness of the first casting sheet is 0.15 to 0.5 mm.

[0019] According to the preparation method of the present invention, preferably, the amount of the antioxidant is 0.1 to 10 wt‰ of the mass of the first coarse powder; the amount of the lubricant is 0.1 to 10 wt‰ of the mass of the first coarse powder.

[0020] According to the preparation method of the present invention, preferably:

[0021] In step 1), the components of the second alloy raw material are smelted to obtain a second cast sheet; the second cast sheet is hydrogen-crushed to obtain a second coarse powder; the second coarse powder is mixed with an antioxidant and a lubricant and then jet-milled to obtain a second fine powder; wherein the thickness of the second cast sheet is 0.15 to 0.5 mm; the amount of the antioxidant is 0.1 to 10 wt‰ of the mass of the second coarse powder; the amount of the lubricant is 0.1 to 10 wt‰ of the mass of the second coarse powder;

[0022] In step 1), the first fine powder and the second fine powder are prepared in no particular order.

[0023] According to the preparation method of the present invention, preferably, the antioxidant is selected from at least one of polyethylene glycol octane, petroleum ether and isooctyl propionate; and the lubricant is selected from at least one of aviation kerosene, isopropyl alcohol and zinc stearate.

[0024] According to the preparation method of the present invention, preferably:

[0025] In step 2), the mixed fine powder is oriented, shaped, and isostatically pressed to obtain a compact;

[0026] In step 2), the sintering is vacuum sintering with a vacuum degree of less than 0.1 Pa; the sintering temperature is 950-1150° C., and the sintering time is 0.5-6 h.

[0027] According to the preparation method of the present invention, preferably, in step 2), the tempering treatment is carried out under vacuum conditions, and the vacuum degree is less than or equal to 0.01 Pa; the tempering treatment includes a first-stage tempering treatment and a second-stage tempering treatment; the temperature of the first-stage tempering treatment is 850-1090°C, and the time is 0.5-4h; after the first-stage tempering treatment is completed, the temperature is reduced to below 60°C; the temperature of the second-stage tempering treatment is 450-750°C, and the time is 0.5-4h.

[0028] According to the preparation method of the present invention, preferably:

[0029] In step 3), the components of the third alloy raw material are smelted to obtain a third alloy ingot, and the third alloy ingot is mechanically processed to obtain a third alloy target;

[0030] In step 3), the components of the fourth alloy raw material are smelted to obtain a fourth alloy ingot, and the fourth alloy ingot is mechanically processed to obtain a fourth alloy target.

[0031] According to the preparation method of the present invention, preferably, in step 3), the coating thickness of the coated magnet is 18 to 40 μm; the heat treatment temperature is 800 to 1000° C., and the time is 0.5 to 3 h.

[0032] On the other hand, the present invention also provides a rare earth permanent magnet, which is prepared according to the preparation method described above.

[0033] The preparation method of the present invention can balance the corrosion resistance and heat resistance of the resulting rare earth permanent magnet, and the weight loss and irreversible magnetic flux loss of the resulting rare earth permanent magnet are further reduced. According to the preferred technical solution of the present invention, the present invention can obtain the rare earth permanent magnet of the present invention by controlling the elemental composition, process parameters and steps. DETAILED DESCRIPTION

[0034] The present invention will be further described below with reference to specific embodiments, but the protection scope of the present invention is not limited thereto.

[0035] In the present invention, "inert atmosphere" refers to an atmosphere formed by an inert gas and does not affect the performance of the magnet. The "inert gas" includes helium, neon, argon, krypton and xenon, preferably argon.

[0036] The "vacuum degree" mentioned in the present invention refers to absolute vacuum degree; the smaller the value, the higher the vacuum degree.

[0037] The “average particle size” mentioned in the present invention refers to the equivalent diameter of the largest particle when the cumulative distribution in the particle size distribution curve is 50%.

[0038] As mentioned in the background technology, the corrosion resistance of cerium-containing NdFeB magnets is an issue that needs to be considered. In addition to the influence of the working environment (for example, high-abundance rare earth elements (such as Ce and La) are easily oxidized at high temperatures to form loose and porous oxides, which provide channels for the penetration of corrosive media and accelerate the corrosion of magnets), compared with traditional rare earth elements such as Pr and Nd, the addition of high-abundance rare earth elements (such as Ce and La) often reduces the corrosion resistance of magnets. The main reasons are: (1) The electrode potential of high-abundance rare earth elements (such as Ce and La) is quite different from that of elements such as Fe and B. In a humid environment, micro-batteries are easily formed, which accelerates the electrochemical corrosion of magnets. (2) The addition of high-abundance rare earth elements will change the composition and structure of the grain boundary phase of the magnet, reduce the stability of the grain boundary phase, and make it more susceptible to erosion by corrosive media. Therefore, it is necessary to further optimize the preparation method of rare earth permanent magnets with a higher cerium content to obtain rare earth permanent magnets with a higher cerium content with further improved corrosion resistance and thermal stability, that is, the weight loss and irreversible magnetic flux loss of the obtained rare earth permanent magnets are further reduced.

[0039] <Preparation Method of Rare Earth Permanent Magnet>

[0040] The present invention provides a method for producing rare earth permanent magnets, particularly those with a high cerium content, comprising the following steps: 1) forming a mixed fine powder; 2) forming a sintered NdFeB magnet; and 3) forming a rare earth permanent magnet. Preferably, the method also includes preparing a first fine powder, a second fine powder, a third alloy target, and a fourth alloy target. This method is described in detail below.

[0041] Preparation steps of the first fine powder

[0042] The components of the first alloy raw material are smelted to obtain a first cast sheet; the first cast sheet is hydrogen crushed to obtain a first coarse powder; the first coarse powder is mixed with an antioxidant and a lubricant and then jet milled to obtain a first fine powder. This method helps to achieve both heat resistance and corrosion resistance of the resulting rare earth permanent magnet.

[0043] In the present invention, the composition of the first alloy raw material is (PrNd) a1 Al a2 Cu a3 B a4 Fe bal a1, a2, a3, and a4 represent weight fractions, with a1 being 27-33 wt%, a2 being 0.08-0.4 wt%, a3 being 0.1-0.5 wt%, and a4 being 0.8-1.5 wt%. bal represents the balance of Fe. This elemental composition improves the heat resistance and corrosion resistance of the resulting rare earth permanent magnet. The first alloy raw material contains only the elements in the above composition, excluding unavoidable impurities, and does not contain Zr, Ho, or Gd.

[0044] a1 represents the mass fraction of PrNd in the first alloy raw material, and may be 27-33 wt %, preferably 29-32 wt %, and more preferably 30-31 wt %. The mass ratio of Pr to Nd may be 1:3-4, and preferably 1:3.

[0045] a2 represents the mass fraction of the Al element in the first alloy raw material, and a2 may be 0.08 to 0.4 wt %, preferably 0.1 to 0.3 wt %, more preferably 0.15 to 0.25 wt %, and even more preferably 0.2 to 0.22 wt %.

[0046] a3 represents the mass fraction of the Cu element in the first alloy raw material, and a3 may be 0.1 to 0.5 wt %, preferably 0.15 to 0.4 wt %, more preferably 0.18 to 0.3 wt %, and even more preferably 0.2 to 0.25 wt %.

[0047] a4 represents the mass fraction of the B (boron) element in the first alloy raw material, and a4 may be 0.8 to 1.5 wt %, preferably 0.9 to 1.3 wt %, and more preferably 1 to 1.2 wt %.

[0048] In the present invention, smelting is carried out in a vacuum or inert atmosphere. The vacuum described herein is a vacuum degree of less than 10Pa, preferably less than 5Pa, and more preferably less than 1Pa. The smelting process preferably adopts an ingot casting process or a rapid solidification sheet casting process. The ingot casting process is to cool and solidify the raw materials of the sintered magnet after smelting, and to be made into an ingot. The rapid solidification sheet casting is to quickly cool and solidify the raw materials of the sintered magnet after smelting, and to be thrown into alloy sheets (casting sheets). According to one embodiment of the present invention, the smelting process for forming the first casting sheet adopts a rapid solidification sheet casting process. The rapid solidification sheet casting process of the present invention can be carried out in a vacuum rapid solidification sheet casting furnace (such as a vacuum medium frequency rapid solidification induction furnace).

[0049] According to one embodiment of the present invention, the components of the first alloy raw material are added to a vacuum rapid solidification casting furnace, evacuated to below 1 Pa, and filled with argon gas for protection. The furnace is then heated and melted, and the molten liquid is poured onto a rotating cooling copper roller to produce a first cast sheet. The thickness of the first cast sheet can be 0.15 to 0.5 mm, preferably 0.2 to 0.4 mm, and more preferably 0.25 to 0.3 mm.

[0050] In the present invention, hydrogen crushing and air jet milling can adopt those known in the art.

[0051] According to one embodiment of the present invention, the first cast slab is hydrogen-crushed to obtain a first coarse powder; the first coarse powder is mixed with an antioxidant and a lubricant and jet-milled to obtain a first fine powder. The average particle size of the first fine powder is 0.5 to 10 μm, preferably 1 to 8 μm, more preferably 2 to 6 μm, and even more preferably 3.3 to 4.5 μm, for example, 3.3 μm, 3.5 μm, 3.8 μm, 4 μm, 4.2 μm, or 4.5 μm.

[0052] The amount of antioxidant used is 0.1 to 10 wt‰ of the mass of the first coarse powder, preferably 0.5 to 5 wt‰, and more preferably 1 to 3 wt‰. The antioxidant is selected from at least one of polyethylene glycol octane, petroleum ether and isooctyl propionate, preferably polyethylene glycol octane. The amount of lubricant used is 0.1 to 10 wt‰ of the mass of the first coarse powder, preferably 0.5 to 5 wt‰, and more preferably 1 to 3 wt‰. The lubricant is selected from at least one of aviation kerosene, isopropyl alcohol and zinc stearate, preferably isopropyl alcohol. This is conducive to obtaining a first fine powder with an average particle size within a specific range, uniform dispersion, and not easily oxidized. Such a first fine powder is conducive to taking into account both the heat resistance and corrosion resistance of the obtained rare earth permanent magnet, that is, the weight loss and irreversible magnetic flux loss of the obtained rare earth permanent magnet are further reduced.

[0053] Preparation steps of the second fine powder

[0054] The components of the second alloy raw material are smelted to obtain a second cast sheet; the second cast sheet is hydrogen-crushed to obtain a second coarse powder; the second coarse powder is mixed with an antioxidant and a lubricant and then jet-milled to obtain a second fine powder. This method improves the heat resistance and corrosion resistance of the resulting rare earth permanent magnet.

[0055] In the present invention, the composition of the second alloy raw material is (PrNd) b1 Ce b2 Al b3 Cu b4 Ga b5 B b6 Fe bal b1, b2, b3, b4, b5, and b6 represent weight fractions, with b1 being 7-11.5wt%, b2 being 19-24wt%, b3 being 0.04-0.25%, b4 being 0.04-0.3wt%, b5 being 0.05-0.3wt%, and b6 being 0.8-1.25wt%. bal represents Fe as the remainder. This elemental composition is beneficial for improving the heat resistance and corrosion resistance of the resulting rare earth permanent magnet. The second alloy raw material contains only the elements in the above composition, except for unavoidable impurities, and does not contain Zr, Ho, or Gd.

[0056] b1 represents the mass fraction of PrNd in the second alloy raw material, and may be 7-11.5 wt %, preferably 8-11 wt %, and more preferably 9-10 wt %. The mass ratio of Pr to Nd may be 1:3-4, and preferably 1:3.

[0057] b2 represents the mass fraction of the Ce element in the second alloy raw material, and b2 may be 19 to 24 wt %, preferably 20 to 23 wt %, and more preferably 21 to 22 wt %.

[0058] b3 represents the mass fraction of the Al element in the second alloy raw material, and b3 may be 0.04 to 0.25 wt %, preferably 0.08 to 0.2 wt %, and more preferably 0.1 to 0.15 wt %.

[0059] b4 represents the mass fraction of the Cu element in the second alloy raw material, and b4 may be 0.04 to 0.3 wt %, preferably 0.08 to 0.2 wt %, and more preferably 0.1 to 0.15 wt %.

[0060] b5 represents the mass fraction of Ga element in the second alloy raw material, and b5 may be 0.05 to 0.3 wt %, preferably 0.08 to 0.2 wt %, and more preferably 0.1 to 0.14 wt %.

[0061] b6 represents the mass fraction of the B (boron) element in the second alloy raw material, and b6 may be 0.8 to 1.25 wt %, preferably 0.9 to 1.2 wt %, more preferably 1 to 1.2 wt %, and even more preferably 1 to 1.1 wt %.

[0062] The smelting can adopt those known in the art, and can refer to the description of the preparation step of the first casting sheet, which is not repeated here.

[0063] According to one embodiment of the present invention, the components of the second alloy raw material are added to a vacuum rapid solidification casting furnace, evacuated to below 1 Pa, filled with argon gas for protection, and then heated and melted. The molten liquid is poured onto a rotating cooling copper roller to produce a second cast sheet. The thickness of the second cast sheet can be 0.15 to 0.5 mm, preferably 0.2 to 0.4 mm, and more preferably 0.25 to 0.3 mm.

[0064] Hydrogen crushing and jet milling may employ those known in the art.

[0065] In the present invention, the average particle size of the second fine powder is 0.5-10 μm, preferably 1-8 μm, more preferably 2-6 μm, and even more preferably 3.3-4.5 μm, for example, 3.3 μm, 3.5 μm, 3.8 μm, 4 μm, 4.2 μm, or 4.5 μm.

[0066] The amount of antioxidant used is 0.1 to 10 wt‰ of the mass of the second coarse powder, preferably 0.5 to 5 wt‰, and more preferably 1 to 3 wt‰. The antioxidant is selected from at least one of polyethylene glycol octane, petroleum ether and isooctyl propionate, preferably polyethylene glycol octane. The amount of lubricant used is 0.1 to 10 wt‰ of the mass of the second coarse powder, preferably 0.5 to 5 wt‰, and more preferably 1 to 3 wt‰. The lubricant is selected from at least one of aviation kerosene, isopropyl alcohol and zinc stearate, preferably isopropyl alcohol. This is conducive to obtaining a second fine powder with an average particle size within a specific range, uniform dispersion, and not easily oxidized. Such a second fine powder is conducive to taking into account both the heat resistance and corrosion resistance of the obtained rare earth permanent magnet, that is, the weight loss and irreversible magnetic flux loss of the obtained rare earth permanent magnet are further reduced.

[0067] Preparation steps of the third alloy target

[0068] The components of the third alloy raw material are smelted to obtain a third alloy ingot, which is then machined to obtain a third alloy target. This helps to balance the heat resistance and corrosion resistance of the resulting rare earth permanent magnet.

[0069] The composition of the third alloy raw material is Pr c1 Ni c2 Nb c3 B c4 Febal c1, c2, c3, and c4 represent weight fractions; c1 is 36-44wt%, c2 is 6.5-14wt%, c3 is 9-15wt%, and c4 is 0.9-1.5wt%. bal represents the balance of Fe. The third alloy raw material contains only the above components, except for unavoidable impurities, and does not contain Gd, Ho, or Zr. This elemental composition facilitates a balanced balance of heat resistance and corrosion resistance in the resulting rare earth permanent magnet.

[0070] c1 represents the mass fraction of the Pr element in the third alloy raw material, and c1 may be 36 to 44 wt %, preferably 38 to 43 wt %, and more preferably 40 to 42 wt %.

[0071] c2 The mass fraction of the Ni element in the third alloy raw material, c2 can be 6.5-14wt%, preferably 7.5-12wt%, and more preferably 8-10wt%.

[0072] c3 represents the mass fraction of the Nb element in the third alloy raw material, and c3 may be 9 to 15 wt %, preferably 10 to 14 wt %, more preferably 11 to 13 wt %, and even more preferably 12 to 13 wt %.

[0073] c4 represents the mass fraction of the B (boron) element in the third alloy raw material, and c4 may be 0.9 to 1.5 wt %, preferably 1 to 1.4 wt %, and more preferably 1.1 to 1.3 wt %.

[0074] The smelting can adopt those known in the art. During the smelting, the vacuum degree is less than or equal to 10Pa, preferably less than or equal to 5Pa, more preferably less than or equal to 1Pa.

[0075] According to one embodiment of the present invention, the components of the third alloy raw material are added to a vacuum induction melting furnace, evacuated to 1 Pa, filled with argon protection, and then heated and melted. The molten liquid is cast into a cylindrical mold equipped with a water-cooling jacket to obtain a third alloy ingot, and the third alloy ingot is machined to obtain a third alloy target.

[0076] Preparation steps of the fourth alloy target

[0077] The components of the fourth alloy raw material are smelted to obtain a fourth alloy ingot, which is then machined to obtain a fourth alloy target. This helps to balance the heat resistance and corrosion resistance of the resulting rare earth permanent magnet.

[0078] The composition of the fourth alloy raw material is Tb d1 Cu d2 Co bald1 and d2 represent mass fractions; d1 is 58-62 wt%, d2 is 18-23 wt%. bal represents the balance of Co. This elemental composition and the specific composition of the third alloy raw material, when used as a target material to form a rare earth permanent magnet, can improve the heat resistance and corrosion resistance of the resulting rare earth permanent magnet, further reducing both weight loss and irreversible magnetic flux loss.

[0079] d1 represents the mass fraction of the Tb element in the fourth alloy raw material, and d1 may be 58 to 62 wt %, preferably 59 to 61 wt %, and more preferably 60 to 61 wt %.

[0080] d2 represents the mass fraction of the Cu element in the fourth alloy raw material, and d2 may be 18 to 23 wt %, preferably 19 to 22 wt %, and more preferably 20 to 21 wt %, with Co being the balance.

[0081] The smelting can adopt those known in the art. During the smelting, the vacuum degree is less than or equal to 10Pa, preferably less than or equal to 5Pa, and more preferably less than or equal to 1Pa.

[0082] According to one embodiment of the present invention, the components of the fourth alloy raw material are added to a vacuum induction melting furnace, evacuated to 1 Pa, filled with argon protection, and then heated and melted. The molten liquid is cast into a cylindrical mold equipped with a water-cooling jacket to obtain a fourth alloy ingot, and the fourth alloy ingot is machined to obtain a fourth alloy target.

[0083] Steps for forming mixed fine powder

[0084] A first fine powder formed from a first alloy raw material and a second fine powder formed from a second alloy raw material are mixed in a mass ratio of 1 to 1.2:1 to produce a mixed fine powder. The first fine powder and the second fine powder each have an average particle size of 0.5 to 10 μm. This facilitates achieving both heat resistance and corrosion resistance in the resulting rare earth permanent magnet.

[0085] In the present invention, the preparation and detailed description of the first fine powder and the second fine powder are as described above and will not be repeated here. The mixing mass ratio of the first fine powder and the second fine powder is preferably 1 to 1.1:1, more preferably 1 to 1.05:1.

[0086] Formation steps of sintered NdFeB magnets

[0087] The mixed fine powder is formed into a compact; the compact is sintered and tempered to obtain a sintered NdFeB magnet. This is conducive to obtaining a NdFeB magnet with stable performance.

[0088] According to one embodiment of the present invention, the mixed fine powder is subjected to orientation molding and isostatic pressing to form a green compact.

[0089] In the present invention, orientation molding, isostatic pressing, sintering, and tempering can all be performed using conventional methods in the art. During orientation molding, the oxygen content can be controlled within a range of 0.1 to 100 ppm, preferably 0.1 to 10 ppm. The magnetic field strength during orientation molding can be 1 to 3 T, preferably 1.5 to 2 T. Isostatic pressing is preferably cold isostatic pressing. During sintering and tempering, the oxygen content can be controlled within a range of 0.1 to 50 ppm, preferably 0.1 to 5 ppm.

[0090] The sintering is performed under vacuum, with a vacuum degree of less than 0.1 Pa, preferably less than or equal to 0.01 Pa, and more preferably less than or equal to 0.001 Pa. The sintering temperature is 950-1150°C, preferably 980-1130°C, and more preferably 1020-1080°C. The sintering time can be 0.5-6 hours, preferably 1-5 hours, and more preferably 2-4 hours. After sintering, the temperature is lowered to below 60°C, for example, to room temperature. An inert gas can be introduced during the cooling process.

[0091] The tempering treatment is carried out under vacuum conditions with a vacuum degree of less than or equal to 0.01 Pa, preferably less than or equal to 0.001 Pa. The tempering treatment includes a first-stage tempering treatment and a second-stage tempering treatment. The temperature of the first-stage tempering treatment can be 850-1090°C, preferably 900-1050°C, and more preferably 900-950°C. The duration of the first-stage tempering treatment can be 0.5-4 hours, preferably 1-3 hours, and more preferably 1-1.5 hours. After the first-stage tempering treatment is completed, the temperature is lowered to below 60°C, for example, to room temperature. An inert gas can be introduced during the cooling process. The temperature of the second-stage tempering treatment can be 450-750°C, preferably 500-650°C, and more preferably 550-600°C. The duration of the second-stage tempering treatment can be 0.5-4 hours, preferably 1-3 hours, and more preferably 2-3 hours. After the second-stage tempering treatment is completed, the temperature can be naturally lowered to room temperature. An inert gas can be introduced during the cooling process.

[0092] Steps for forming rare earth permanent magnets

[0093] The third and fourth alloy targets are used as targets for magnetron sputtering on the surface of the sintered NdFeB magnet to produce a coated magnet. The coated magnet is then heat treated to produce a rare earth permanent magnet. The coating thickness of the coated magnet is at least 18 μm. The resulting rare earth permanent magnet exhibits excellent heat resistance and corrosion resistance, effectively improving both properties.

[0094] The third alloy target material and the fourth alloy target material are simultaneously used as target materials for magnetron sputtering. Specific magnetron sputtering methods can adopt those known in the art and are not described in detail here.

[0095] In the present invention, magnetron sputtering is performed on two opposing surfaces of a sintered NdFeB magnet perpendicular to the orientation direction. The resulting coated magnet preferably has a coating thickness of 18 to 40 μm on each of the two opposing surfaces, more preferably 20 to 35 μm, and even more preferably 25 to 30 μm. This helps further reduce both the weight loss and irreversible magnetic flux loss of the resulting rare earth permanent magnet.

[0096] The heat treatment temperature is 800-1000°C, preferably 850-980°C, and more preferably 900-950°C. The heat treatment time can be 0.5-12 hours, preferably 1-9 hours, more preferably 3-6 hours, and even more preferably 4-5 hours. This facilitates grain boundary diffusion, thereby obtaining a rare earth permanent magnet with both heat resistance and corrosion resistance.

[0097] In this invention, the inventors believe that by controlling the elemental composition and process, it is possible to improve both the corrosion resistance and heat resistance of rare earth permanent magnets with a high cerium content. The inventors also believe that the addition of Ni increases the corrosion potential of the grain boundary phase, and the formation of Nb alloy dispersed phases inhibits the expansion of grain boundary corrosion, thereby improving the corrosion resistance of the magnet. Trace amounts of Co replace Fe in the matrix, improving the thermal stability of the magnet.

[0098] <Rare earth permanent magnets>

[0099] The present invention also provides a rare earth permanent magnet prepared according to the above-described preparation method. The resulting rare earth permanent magnet exhibits further improved heat resistance and corrosion resistance, as evidenced by further reductions in weight loss and irreversible magnetic flux loss. The resulting rare earth permanent magnet has a cerium content greater than 8wt%, preferably greater than or equal to 8.5wt%, more preferably greater than or equal to 9wt%, even more preferably greater than or equal to 9.5wt%, and can even reach greater than 10wt%.

[0100] <Test Method>

[0101] Weight loss test: Cube samples measuring 12mm x 12mm x 8mm were cut using a wire-cut electric discharge machine. Weight loss data was collected for 96 hours using a high-temperature, high-pressure steam test apparatus at 120°C, 100% relative humidity, and an absolute vapor pressure of 0.2 MPa. Lower weight loss indicates better corrosion resistance.

[0102] Irreversible flux loss test: Use the wire-cut electric discharge machine to cut the sample to be tested into pieces of size A cylindrical test sample was saturated magnetized and placed in a constant temperature oven at 80°C for two hours. A fluxmeter was used to measure the magnetic flux density before and after the incubation period, and the irreversible flux loss under the current test conditions was calculated. The calculation formula for irreversible flux loss refers to GB / T 13560-2017. Lower irreversible flux loss indicates better heat resistance and thermal stability.

[0103] Preparation Example 1-Preparation of the first fine powder

[0104] Prepare the first alloy raw material. The composition of the first alloy raw material is (PrNd) a1 Al a2 Cu a3 B a4 Fe bal a1, a2, a3, and a4 represent weight percentages, with a1 being 30 wt%, a2 being 0.2 wt%, a3 being 0.2 wt%, and a4 being 1.0 wt%. bal represents the balance of Fe. The mass ratio of Pr to Nd is 1:3.

[0105] The components of the prepared first alloy raw material are added to a vacuum rapid solidification casting furnace, evacuated to 1 Pa, and filled with argon gas for protection (the vacuum degree after filling with argon is 50kPa). Then, the furnace is heated and melted, and the molten liquid is poured onto a rotating cooling copper roller to obtain a first cast sheet with a thickness of approximately 0.25mm. The first cast sheet is hydrogen-crushed to obtain a first coarse powder; the first coarse powder is mixed with an antioxidant (polyethylene glycol octane, the antioxidant is used in an amount of 1.5wt‰ of the mass of the first coarse powder) and a lubricant (isopropyl alcohol, the lubricant is used in an amount of 1.5wt‰ of the mass of the first coarse powder) for 1 hour, and then subjected to air flow milling to obtain a first fine powder with an average particle size of 3.5μm.

[0106] Preparation Example 2-Preparation of the Second Fine Powder

[0107] Prepare the second alloy raw material. The composition of the second alloy raw material is (PrNd) b1 Ce b2 Al b3 Cu b4 Ga b5 B b6 Fe bal b1, b2, b3, b4, b5, and b6 represent weight percentages: b1 is 9 wt%, b2 is 21 wt%, b3 is 0.1%, b4 is 0.1 wt%, b5 is 0.1 wt%, and b6 is 1.0 wt%. bal represents the balance of Fe. The mass ratio of Pr to Nd is 1:3.

[0108] The components of the prepared second alloy raw material were added to a vacuum rapid solidification casting furnace, evacuated to 1 Pa, and filled with argon gas for protection (the vacuum degree after filling with argon was 50 kPa). Then, they were heated and melted. The molten liquid was poured onto a rotating cooling copper roller to obtain a second cast sheet with a thickness of approximately 0.25 mm. The second cast sheet was hydrogen crushed to obtain a second coarse powder. The second coarse powder was mixed with an antioxidant (the antioxidant was polyethylene glycol octane, the amount was 1.5 wt‰ of the mass of the second coarse powder) and a lubricant (the lubricant was isopropyl alcohol, the amount was 1.5 wt‰ of the mass of the second coarse powder) for 1 hour, and then jet milled to obtain a second fine powder with an average particle size of 3.5 μm.

[0109] Preparation Example 3-Preparation of the third alloy target

[0110] Prepare the third alloy raw material. The composition of the third alloy raw material is Pr c1 Ni c2 Nb c3 B c4 Fe bal ; c1, c2, c3, c4 represent mass fractions; c1 is 40wt%, c2 is 8wt%, c3 is 12wt%, c4 is 1.1wt%, and bal represents Fe as the balance.

[0111] The components of the third alloy raw material are added into a vacuum induction melting furnace, evacuated to 1Pa, filled with argon for protection (the vacuum degree is 50kPa after filling with argon), and then heated and melted. The molten liquid is cast into a cylindrical mold equipped with a water-cooling jacket to obtain a third alloy ingot, and the third alloy ingot is machined to obtain a third alloy target.

[0112] Preparation Example 4-Preparation of the Fourth Alloy Target

[0113] Prepare the fourth alloy raw material. The composition of the fourth alloy raw material is Tb d1 Cu d2 Co bal ; d1 and d2 represent mass fractions; d1 is 60wt%, d2 is 20wt%, and bal represents Co as the balance.

[0114] The components of the fourth alloy raw material are added to a vacuum induction melting furnace, evacuated to 1Pa, filled with argon for protection (the vacuum degree is 50kPa after filling with argon), and then heated and melted. The molten liquid is cast into a cylindrical mold equipped with a water-cooling jacket to obtain a fourth alloy ingot, and the fourth alloy ingot is machined to obtain a fourth alloy target.

[0115] Example 1

[0116] 1) The first fine powder prepared in Preparation Example 1 and the second fine powder prepared in Preparation Example 2 were mixed in a mixer at a mass ratio of 1:1 for 1.5 h to obtain a mixed fine powder.

[0117] 2) The mixed fine powder is oriented and isostatically pressed in a 2T magnetic field to obtain a green compact. The green compact is vacuum sintered at 1080°C for 3 hours, cooled to room temperature in an argon atmosphere, then heated to 900°C and subjected to a first-stage tempering treatment at 900°C for 1.5 hours. The green compact is then cooled to room temperature in an argon atmosphere, then heated to 550°C and subjected to a second-stage tempering treatment at 550°C for 2 hours to obtain a sintered NdFeB magnet.

[0118] 3) Using the third alloy target prepared in Preparation Example 3 and the fourth alloy target prepared in Preparation Example 4 as targets, magnetron sputtering was performed on two opposing surfaces of the resulting sintered NdFeB magnet perpendicular to the orientation direction to produce a coated magnet. The coating thickness of each of the two opposing surfaces of the coated magnet was 25 μm. The coated magnet was heat treated at 940°C for 5 hours to obtain a rare earth permanent magnet.

[0119] Comparative Example 1

[0120] 1) The first fine powder prepared in Preparation Example 1 and the second fine powder prepared in Preparation Example 2 were mixed in a mixer at a mass ratio of 1:1 for 1.5 h to obtain a mixed fine powder.

[0121] 2) The mixed fine powder is oriented and isostatically pressed in a 2T magnetic field to obtain a green compact. The green compact is vacuum sintered at 1080°C for 3 hours, cooled to room temperature in an argon atmosphere, then heated to 900°C and subjected to a first-stage tempering treatment at 900°C for 1.5 hours. The green compact is then cooled to room temperature in an argon atmosphere, then heated to 550°C and subjected to a second-stage tempering treatment at 550°C for 2 hours to obtain a sintered NdFeB magnet.

[0122] 3) Using the third alloy target prepared in Preparation Example 3 and the fourth alloy target prepared in Preparation Example 4 as targets, magnetron sputtering was performed on two opposing surfaces of the resulting sintered NdFeB magnet perpendicular to the orientation direction to produce a coated magnet. The coating thickness on each of the two opposing surfaces of the coated magnet was 15 μm. The coated magnet was heat treated at 940°C for 5 hours to obtain a rare earth permanent magnet.

[0123] Comparative Example 2

[0124] 1) The first fine powder prepared in Preparation Example 1 and the second fine powder prepared in Preparation Example 2 were mixed in a mixer at a mass ratio of 1:1 for 1.5 h to obtain a mixed fine powder.

[0125] 2) The mixed fine powder is oriented and isostatically pressed in a 2T magnetic field to obtain a green compact. The green compact is vacuum sintered at 1080°C for 3 hours, cooled to room temperature in an argon atmosphere, then heated to 900°C and subjected to a first-stage tempering treatment at 900°C for 1.5 hours. The green compact is then cooled to room temperature in an argon atmosphere, then heated to 550°C and subjected to a second-stage tempering treatment at 550°C for 2 hours to obtain a sintered NdFeB magnet.

[0126] The rare earth permanent magnets obtained in Example 1 and Comparative Example 1, and the sintered NdFeB magnets obtained in Comparative Example 2 were tested, respectively. The results are shown in Table 1.

[0127] Table 1

[0128] serial number <![CDATA[Weight loss (mg / cm 2 )]]> Irreversible flux loss (%) Example 1 3.3 3.8 Comparative Example 1 5.1 4.9 Comparative Example 2 18.2 21.3

[0129] As can be seen from the table, the preparation method of the present invention can obtain rare earth permanent magnets with further reduced weight loss and irreversible magnetic flux loss. In other words, the preparation method of the present invention can improve both the corrosion resistance and heat resistance of the obtained rare earth permanent magnets.

[0130] The present invention is not limited to the above-mentioned embodiments. Any modification, improvement, or substitution that can be conceived by those skilled in the art without departing from the essential content of the present invention shall fall within the scope of the present invention.

Claims

1. A method for preparing a rare earth permanent magnet, characterized in that: The following steps are involved: 1) mixing a first fine powder formed from a first alloy raw material and a second fine powder formed from a second alloy raw material in a mass ratio of 1 to 1.2:1 to obtain a mixed fine powder; wherein the average particle size of the first fine powder and the second fine powder is 0.5 to 10 μm, respectively; 2) forming a compact from the mixed fine powder; sintering and tempering the compact to obtain a sintered NdFeB magnet; 3) using a third alloy target formed from a third alloy raw material and a fourth alloy target formed from a fourth alloy raw material as targets, and performing magnetron sputtering on the surface of the sintered NdFeB magnet to obtain a coated magnet; and heat treating the coated magnet to obtain a rare earth permanent magnet; The coating thickness of the coated magnet is greater than 18 μm. The composition of the first alloy raw material is (PrNd) a1 Al a2 Cu a3 B a4 Fe bal ; a1, a2, a3, a4 represent weight fractions, a1 is 27-33wt%, a2 is 0.08-0.4wt%, a3 is 0.1-0.5wt%, and a4 is 0.8-1.5wt%; The composition of the second alloy raw material is (PrNd) b1 Ce b2 Al b3 Cu b4 Ga b5 B b6 Fe bal ; b1, b2, b3, b4, b5, b6 represent mass fractions, b1 is 7 to 11.5wt%, b2 is 19 to 24wt%, b3 is 0.04 to 0.25%, b4 is 0.04 to 0.3wt%, b5 is 0.05 to 0.3wt%, and b6 is 0.8 to 1.25wt%; The composition of the third alloy raw material is Pr c1 Ni c2 Nb c3 B c4 Fe bal ; c1, c2, c3, c4 represent mass fractions; c1 is 36-44wt%, c2 is 6.5-14wt%, c3 is 9-15wt%, c4 is 0.9-1.5wt%; The composition of the fourth alloy raw material is Tb d1 Cu d2 Co bal ; d1 and d2 represent mass fractions; d1 is 58~62wt%, d2 is 18~23wt%.

2. The preparation method according to claim 1, characterized in that In step 1), the components of the first alloy raw material are smelted to obtain a first casting sheet; the first casting sheet is hydrogen-crushed to obtain a first coarse powder; the first coarse powder is mixed with an antioxidant and a lubricant and then jet-milled to obtain a first fine powder; wherein the thickness of the first casting sheet is 0.15 to 0.5 mm.

3. The preparation method according to claim 2, characterized in that The amount of the antioxidant is 0.1 to 10 wt‰ of the mass of the first coarse powder; the amount of the lubricant is 0.1 to 10 wt‰ of the mass of the first coarse powder.

4. The preparation method according to claim 2, wherein: In step 1), the components of the second alloy raw material are smelted to obtain a second cast sheet; the second cast sheet is hydrogen-crushed to obtain a second coarse powder; the second coarse powder is mixed with an antioxidant and a lubricant and then jet-milled to obtain a second fine powder; wherein the thickness of the second cast sheet is 0.15 to 0.5 mm; the amount of the antioxidant is 0.1 to 10 wt‰ of the mass of the second coarse powder; the amount of the lubricant is 0.1 to 10 wt‰ of the mass of the second coarse powder; In step 1), the first fine powder and the second fine powder are prepared in no particular order.

5. The preparation method according to claim 4, characterized in that The antioxidant is selected from at least one of polyethylene glycol octane, petroleum ether and isooctyl propionate; and the lubricant is selected from at least one of aviation kerosene, isopropyl alcohol and zinc stearate.

6. The preparation method according to claim 1, wherein: In step 2), the mixed fine powder is oriented, shaped, and isostatically pressed to obtain a compact; In step 2), the sintering is vacuum sintering with a vacuum degree of less than 0.1 Pa; the sintering temperature is 950-1150° C., and the sintering time is 0.5-6 h.

7. The preparation method according to claim 1, characterized in that In step 2), the tempering treatment is carried out under vacuum conditions with a vacuum degree of less than or equal to 0.01 Pa; the tempering treatment includes a first-stage tempering treatment and a second-stage tempering treatment; the temperature of the first-stage tempering treatment is 850-1090° C. and the time is 0.5-4 hours; after the first-stage tempering treatment is completed, the temperature is lowered to below 60° C.; the temperature of the second-stage tempering treatment is 450-750° C. and the time is 0.5-4 hours.

8. The preparation method according to claim 1, wherein: In step 3), the components of the third alloy raw material are smelted to obtain a third alloy ingot, and the third alloy ingot is mechanically processed to obtain a third alloy target; In step 3), the components of the fourth alloy raw material are smelted to obtain a fourth alloy ingot, and the fourth alloy ingot is mechanically processed to obtain a fourth alloy target.

9. The preparation method according to claim 1, characterized in that In step 3), the coating thickness of the coated magnet is 18 to 40 μm; the heat treatment temperature is 800 to 1000° C., and the time is 0.5 to 3 hours.

10. A rare earth permanent magnet, characterized in that: The invention can be prepared according to the preparation method according to any one of claims 1 to 9.