Special-shaped rare earth permanent magnet and preparation method thereof

Abstract

The invention discloses a special-shaped rare earth permanent magnet and a preparation method thereof. The preparation method comprises the following steps: 1) forming a rare earth film on the surface of a sintered neodymium-iron-boron magnet with a set through hole to obtain a coated magnet; or forming a rare earth film on the surface of the sintered neodymium-iron-boron magnet of which at least one outer surface is a concave cambered surface to obtain a coated magnet; (2) stacking more than two coated magnets up and down along a parallel or vertical orientation direction, enabling the central axes of the coated magnets to be positioned on the same straight line, and arranging or not arranging a metal foil between adjacent coated magnets to obtain a stacked magnet; and (3) carrying out heat treatment on the stacked magnet, and enabling the stacked magnet to be in a pressed state at least in a partial stage of heat treatment, so as to obtain the special-shaped rare earth permanent magnet. By means of the preparation method, the special-shaped rare earth permanent magnet with the thickness larger than 10 mm in the orientation direction can be obtained, and the coercive force is obviously improved.

Description

Technical Field

[0001] This invention relates to an irregularly shaped rare-earth permanent magnet and its preparation method, and more particularly to a large-sized irregularly shaped rare-earth permanent magnet in the form of a tube or having deep holes and its preparation method. Background Technology

[0002] Grain boundary diffusion technology can improve the coercivity of magnets, but it suffers from slow diffusion rate and shallow diffusion depth, with heavy rare earth elements mostly concentrated in the surface region during diffusion. Therefore, magnets prepared by grain boundary diffusion are small in size, with a thickness of less than 10 mm, typically only 3–6 mm, which limits its application in large-size magnets.

[0003] Furthermore, due to the brittleness and poor toughness of sintered NdFeB magnets, conventional machining methods such as turning, milling, and planing cannot meet the processing requirements. Current mainstream machining technologies such as EDM, multi-wire cutting, and diamond wire cutting suffer from low efficiency, limited processing shapes, and low material utilization. Therefore, the "subtractive" machining techniques based on the current material properties of sintered NdFeB magnets are insufficient to meet increasingly complex processing needs, especially for machining large, irregularly shaped magnets.

[0004] CN106782977A discloses a method for preparing large-size sintered samarium cobalt permanent magnets, comprising the following steps: (1) crushing samarium cobalt alloy ingots, pulverizing, mixing, and obtaining magnetic powder; (2) oriented and pressing in a magnetic field to obtain a green blank; (3) splicing two or more green blanks together, wrapping them tightly with plastic wrap, vacuum sealing, and then performing cold isostatic pressing to obtain a shaped green blank; (4) subjecting the shaped green blank to vacuum pre-firing, sintering with inert gas, solution treatment, and air cooling to room temperature to obtain a sintered blank; (5) subjecting the sintered blank to aging treatment, cooling, heat preservation, air cooling to room temperature, and machining to obtain a large-size sintered samarium cobalt permanent magnet. Firstly, this preparation method is applicable to samarium cobalt permanent magnets. Secondly, this preparation method is for preparing large-size blank magnets and does not solve the processing problem of large-size irregularly shaped diffused magnets. Finally, the performance of the magnets obtained by this method needs improvement.

[0005] CN108615596A discloses a method for preparing an irregularly shaped permanent magnet, comprising the following steps: (1) providing magnetic powder with magnetocrystalline anisotropy; (2) sequentially hot-pressing and hot-deforming the magnetic powder to obtain a hot-deformed blank; (3) providing a shaping mold, wherein the pressure head end face of the shaping mold includes at least one arc surface, placing the hot-deformed blank into the shaping mold, and heating and applying pressure to the hot-deformed blank to cause the hot-deformed blank to undergo rheological changes and fill the mold cavity of the shaping mold, thereby obtaining an irregularly shaped permanent magnet, wherein the surface of the irregularly shaped permanent magnet includes at least one arc surface. This preparation method is a hot deformation technology, suitable for the preparation of tile-shaped magnets, but the coercivity still needs to be improved. It is not suitable for the preparation of tubular magnets or magnets with deep holes. In addition, the number of magnets processed in a single deformation in this method is limited, resulting in low production efficiency. The hot deformation of the blank requires high-precision molds and has high processing costs.

[0006] CN114429848B discloses an assembly method for a multipole tubular permanent magnet. The assembly method, using the assembly device described above, includes the following steps: Step 1: Fixedly connecting the inner sleeve and the base; Step 2: Successfully fitting each magnetic ring of the multipole tubular permanent magnet axially onto the outer side of the inner sleeve, wherein when fitting two adjacent magnetic rings that attract each other, a spacer assembly is used, specifically including the following steps: Step S1: Placing one of the two mutually attracting magnetic rings inside the spacer sleeve; Step S2: Simultaneously... Step S3: Gradually pull out the locking pin radially to bring the two adjacent magnetic rings close together; Step S4: Remove the spacer sleeve from the magnetic rings; Step 5: Bond the two adjacent magnetic rings of the multipole tubular permanent magnet with an adhesive. Although this method produces a tubular magnet, it requires an adhesive and a specific assembly device. Furthermore, the organic adhesive used in this method is prone to aging and failure at high temperatures, leading to magnet structure damage. Bonding the magnet does not improve its performance, and it cannot simultaneously meet the performance and specification requirements of large-size, high-performance irregularly shaped magnets. Summary of the Invention

[0007] In view of this, one object of the present invention is to provide a method for preparing irregularly shaped rare-earth permanent magnets, which can obtain large-sized irregularly shaped rare-earth permanent magnets with a thickness greater than 10 mm in the orientation direction. Furthermore, the coercivity of the obtained irregularly shaped rare-earth permanent magnets is significantly improved. Another object of the present invention is to provide an irregularly shaped rare-earth permanent magnet prepared according to the preparation method described above.

[0008] The present invention achieves the above objectives using the following technical solutions.

[0009] On one hand, the present invention provides a method for preparing an irregularly shaped rare-earth permanent magnet, comprising the following steps:

[0010] 1) A rare earth thin film is formed on the surface of a sintered NdFeB magnet with a predetermined through hole to obtain a coated magnet; wherein the central axis direction of the through hole is parallel or perpendicular to the orientation direction of the sintered NdFeB magnet; or a rare earth thin film is formed on the surface of at least one sintered NdFeB magnet with an outer concave arc surface to obtain a coated magnet.

[0011] 2) Stack two or more coated magnets vertically along a parallel or perpendicular orientation, so that their central axes are on the same straight line. Adjacent coated magnets are provided with metal foil or are in direct contact without metal foil to obtain stacked magnets.

[0012] 3) The stacked magnets are heat-treated, and at least during a portion of the heat treatment process, the stacked magnets are subjected to a state of pressure to obtain irregularly shaped rare earth permanent magnets.

[0013] According to the preparation method of the present invention, preferably, the through hole is selected from at least one of circular through holes, elliptical through holes, square through holes, rectangular through holes, parallelogram through holes, triangular through holes, and conical through holes.

[0014] According to the preparation method of the present invention, preferably:

[0015] The through hole is a circular through hole, and the sintered NdFeB magnet is in the shape of a ring; the resulting irregular rare earth permanent magnet is a tubular rare earth permanent magnet.

[0016] The through hole is circular, and the sintered NdFeB magnet is cubic with the through hole in the center; the resulting irregular rare earth permanent magnet is a rare earth permanent magnet with a deep hole.

[0017] According to the preparation method of the present invention, preferably, in step 1), the rare earth film is a film formed by rare earth elements or a film formed by rare earth elements and other metal elements;

[0018] The rare earth element is selected from at least one of Pr, Nd, La, Ce, Dy, Tb and Ho, and must contain a heavy rare earth element; the heavy rare earth element is Dy, Tb or Ho.

[0019] The other metallic elements are selected from at least one of Al, Cu, Ga, Co, Fe, Mg, Sn, Ag and Zn.

[0020] According to the preparation method of the present invention, preferably, in step 1), the rare earth film is formed by one of the following methods: spraying, screen printing, laser cladding, brazing, evaporation coating and magnetron sputtering; the thickness of the rare earth film is 0.1 to 2.5% of the thickness of the sintered NdFeB magnet.

[0021] According to the preparation method of the present invention, preferably:

[0022] In step 2), metal foil sheets are placed between adjacent coated magnets;

[0023] In step 2), the metal element in the metal foil is selected from at least one of copper, nickel, aluminum, niobium, titanium and molybdenum; the metal foil is a dense foil, a perforated foil or a foamed metal foil, and its thickness is 2 to 20 μm.

[0024] According to the preparation method of the present invention, preferably, the metal foil is selected from at least one of copper foil, nickel foil, aluminum foil, niobium foil, titanium foil, copper-molybdenum alloy foil, and titanium-niobium alloy foil.

[0025] According to the preparation method of the present invention, preferably, the stacked magnets are in a compressed state during heat treatment by any of the following methods:

[0026] (1) Before heat treatment, the stacked magnets are fixed by winding with metal wire to obtain the fixed stacked magnets. The fixed stacked magnets are placed in a vacuum sintering furnace for vacuum heat treatment. During vacuum heat treatment, the stacked magnets are under pressure.

[0027] (2) Place the stacked magnets in a matching mold and confine them in the mold, and then perform vacuum heat treatment in a vacuum sintering furnace, during which the stacked magnets are under pressure.

[0028] (3) Place the stacked magnets in the mold, and then use spark plasma sintering pressure heat treatment to put the stacked magnets under pressure.

[0029] According to the preparation method of the present invention, preferably, when using method (1) or method (2), the heat treatment in step 3) is performed using a vacuum sintering furnace, wherein the vacuum heat treatment sequentially includes a first stage heat treatment, a second stage heat treatment, and a third stage heat treatment: wherein,

[0030] The holding temperature for the first stage of heat treatment is 550–850℃, and the holding time is 0.1–5 hours.

[0031] The second stage of heat treatment involves holding at 830–960℃ for 0.5–8.5 hours; after the second stage of heat treatment, the temperature is cooled to below 50℃.

[0032] The holding temperature for the third stage of heat treatment is 410–670℃, and the holding time is 1–5 hours.

[0033] On the other hand, the present invention also provides an irregularly shaped rare earth permanent magnet, which is prepared according to the preparation method described above.

[0034] The preparation method of this invention can obtain irregularly shaped rare-earth permanent magnets, such as tubular rare-earth permanent magnets or rare-earth permanent magnets with deep holes. The obtained irregularly shaped rare-earth permanent magnets have large dimensions, especially in the orientation direction, and high coercivity. The thickness of the irregularly shaped rare-earth permanent magnets in the orientation direction of this invention can be greater than 10 mm, for example, reaching more than 25 mm. The specific height and thickness specifications can depend on the connection design and arrangement of the magnets. Such high-performance, large-size irregularly shaped rare-earth permanent magnets are difficult to obtain through conventional processing methods, and even if irregularly shaped sintered NdFeB magnets are obtained through special molds, it is difficult to obtain irregularly shaped rare-earth permanent magnets with significantly improved coercivity through grain boundary diffusion. In addition, the preparation method of this invention is relatively easy to operate and can be mass-produced and stably maintained. Detailed Implementation

[0035] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0036] The "remanence" mentioned in this invention refers to the magnetic flux density at the point where the magnetic field strength on the saturation hysteresis loop is zero, expressed in Tesla (T) or Gauss (Gs). 1 Gs = 0.0001 T.

[0037] The "coercivity" described in this invention, also known as intrinsic coercivity, refers to the magnetic field strength when the magnetic field is monotonically reduced to zero and then increased in the opposite direction from the saturated magnetization state of the magnet, causing its magnetization intensity to decrease to zero along the saturation hysteresis loop. The unit is Oersted (Oe) or Ampere per meter (A / m). 1 Oe = 79.6 A / m.

[0038] The irregularly shaped rare earth permanent magnets described in this invention refer to rare earth permanent magnets with irregular shapes, other than regular shapes, such as rare earth permanent magnets with internal holes or curved surfaces. Specifically, they can be tubular rare earth permanent magnets, rare earth permanent magnets with deep holes, tile-shaped rare earth permanent magnets, or arched rare earth permanent magnets, etc.

[0039] <Preparation Method of Irregularly Shaped Rare Earth Permanent Magnets>

[0040] This invention provides a method for preparing an irregularly shaped rare-earth permanent magnet, comprising the following steps: 1) a rare-earth thin film formation step; 2) a stacking step; and 3) a heat treatment step. Optionally, a fixing step is also included. The following is a detailed description.

[0041] Rare earth thin film formation steps

[0042] In some embodiments, a rare earth thin film is formed on the surface of a sintered NdFeB magnet with predetermined through-holes to obtain a coated magnet. The central axis of the through-hole is either parallel or perpendicular to the orientation direction of the sintered NdFeB magnet. This is beneficial for improving the coercivity of the resulting irregularly shaped rare earth permanent magnet.

[0043] In this invention, whether or not a rare earth film is provided in the through hole depends on the performance requirements of the original sintered NdFeB magnet and the target magnet after assembly. Preferably, a rare earth film is formed in the through hole. In this way, when the magnet is connected after stacking, the inner wall of the magnet is also undergoing grain boundary diffusion, which is beneficial to obtaining a magnet assembly with better performance, namely a shaped rare earth permanent magnet.

[0044] In this invention, the through hole refers to a through hole along the orientation direction of the sintered NdFeB magnet or a through hole perpendicular to the orientation direction. The through hole is selected from one of the following: circular through hole, elliptical through hole, square through hole, rectangular through hole, parallelogram through hole, triangular through hole, and conical through hole, preferably a circular or elliptical through hole.

[0045] According to one specific embodiment of the present invention, the sintered NdFeB magnet having a predetermined through hole is annular in shape, and the through hole is a circular through hole.

[0046] According to another specific embodiment of the present invention, the sintered NdFeB magnet having a set through hole is cubic in shape, and the through hole is a circular through hole.

[0047] In other embodiments, a rare earth thin film is formed on the surface of at least one sintered NdFeB magnet with an outer concave arc surface to obtain a coated magnet. According to a specific embodiment of the present invention, at least one sintered NdFeB magnet with an outer concave arc surface can be a tile-shaped sintered NdFeB magnet.

[0048] In this invention, the sintered NdFeB magnet is made of RE2Fe 14 Sintered permanent magnets with type B compounds as the main phase. RE represents rare earth elements, including light and heavy rare earth elements. Light rare earth elements are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), and europium (Eu). Heavy rare earth elements are gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and yttrium (Y).

[0049] According to one embodiment of the present invention, RE is selected from at least one element among light rare earth elements, and must contain Nd. The sintered NdFeB magnet of the present invention may or may not contain heavy rare earth elements. In some specific embodiments, the sintered NdFeB magnet of the present invention does not contain heavy rare earth elements.

[0050] According to one specific embodiment of the present invention, RE is selected from at least one of La, Ce, Nd, Pr, and Sm. According to a preferred specific embodiment of the present invention, RE is selected from at least one element selected from Pr and Nd, and must contain Nd.

[0051] In this invention, the sintered NdFeB magnet may contain M, which is selected from one or more of Cu, Co, Al, Ga and Zr.

[0052] In certain specific embodiments, the sintered NdFeB magnet has the following specific composition: RE, Fe, B, Cu, Co, Al, Ga, Zr, and other unavoidable impurities. Based on all elements of the sintered NdFeB magnet, RE is selected from at least one of Pr, Nd, Dy, Tb, and Ce, and must contain Nd. The total content of RE is 29.0–33.0 wt%, preferably 30.0–32.0 wt%. Based on all elements of the sintered NdFeB magnet, the Cu content is 0.10–0.60 wt%, preferably 0.1–0.3 wt%. The Co content is 0.1–1.5 wt%, preferably 0.2–1.0 wt%. The Al content is 0.1–1.2 wt%, preferably 0.2–1.0%. Based on all elements of the sintered NdFeB magnet, the B content is 0.88–0.95 wt%, preferably 0.89–0.93 wt%. Based on all elements of the sintered NdFeB magnet, the Ga content is 0.05–0.6 wt%, preferably 0.1–0.5 wt%. Based on all elements of the sintered NdFeB magnet, the Zr content is 0.05–0.55 wt%, preferably 0.1–0.50 wt%. Fe is the balance.

[0053] In practical applications, the grades of sintered NdFeB magnets include, but are not limited to, N50, N52, and 40SH.

[0054] In this invention, the surface of the sintered NdFeB magnet can be cleaned first. This cleaning process may include the following steps: sandblasting to remove contaminants from the surface of the sintered NdFeB magnet, followed by ultrasonic cleaning with an organic solvent. The sandblasting pressure can be 0.2–0.5 MPa, preferably 0.25–0.4 MPa, and more preferably 0.3–0.4 MPa. The contaminants on the surface of the sintered NdFeB magnet are mainly oil stains. The organic solvent can be selected from ethanol, isopropanol, and acetone; for example, ethanol.

[0055] In this invention, the rare earth thin film can be formed by one of the following methods: spraying, screen printing, laser cladding, brazing, evaporation coating and magnetron sputtering.

[0056] In this invention, the rare earth thin film is a thin film formed of rare earth elements or a thin film formed of rare earth elements and other metallic elements. The rare earth elements are selected from at least one of Pr, Nd, La, Ce, Dy, Tb, and Ho, and must contain heavy rare earth elements; the heavy rare earth elements are Dy, Tb, or Ho. The other metallic elements are selected from at least one of Al, Cu, Ga, Co, Fe, Mg, Sn, Ag, and Zn, preferably at least one of Al, Cu, Ga, Mg, Sn, Ag, and Zn.

[0057] According to one embodiment of the present invention, the rare earth film is a film formed of rare earth elements and other metal elements, wherein, based on the total weight of the rare earth film, the content of rare earth elements is 80-96 wt%, and the content of other metal elements is 4-20 wt%. Preferably, based on the total weight of the rare earth film, the content of rare earth elements is 85-95 wt%, more preferably 90-95 wt%; and the content of other metal elements is 5-15 wt%, more preferably 5-10 wt%.

[0058] According to one specific embodiment of the present invention, the rare earth thin film is a thin film formed of Tb and other metal elements, wherein the other metal elements are selected from at least one of Al, Cu and Ga. Tb is terbium, Al is aluminum, Cu is copper, and Ga is gallium.

[0059] According to another specific embodiment of the present invention, the rare earth thin film is a thin film formed of Dy and other metal elements, wherein the other metal elements are selected from at least one of Al, Cu and Ga. Dy is dysprosium, Al is aluminum, Cu is copper, and Ga is gallium.

[0060] In this invention, the thickness of the rare earth film is 0.1–2.5% of the thickness of the sintered NdFeB magnet, preferably 0.15–2%, more preferably 0.2–1.2%, and even more preferably 0.25–1.2%. For example, when the magnet thickness is 5 mm, the thickness of the rare earth film can be 12.5–60 μm (5000 μm × (0.25–1.2%)). When the target magnet performance requirements are low, the thickness of the rare earth film is smaller, and the sintered NdFeB magnets are connected by heating metal foil. When the target magnet performance requirements are high, the thickness of the rare earth film can be larger.

[0061] Stacking steps

[0062] Two or more coated magnets are stacked vertically along parallel or perpendicular orientations, with their central axes aligned on the same straight line. Adjacent coated magnets may be in direct contact with metal foil between them, or without metal foil, to obtain stacked magnets. This method is advantageous for obtaining large-sized, irregularly shaped rare-earth permanent magnets.

[0063] In this invention, the surface on which the rare earth thin film is formed is called the coating surface. During stacking, the coating surface of the upper surface of one of the coated magnets is in contact with the coating surface of the lower surface of another coated magnet stacked above it, or a metal foil is sandwiched between the two.

[0064] In some implementations, two or more coated magnets are stacked vertically along the orientation direction, with their central axes aligned on the same straight line, and metal foils are placed between adjacent coated magnets to obtain stacked magnets.

[0065] In other embodiments, two or more coated magnets are stacked vertically along the orientation direction, with their central axes aligned on the same straight line. No metal foil is placed between adjacent coated magnets, meaning the coated magnets are in direct contact, resulting in stacked magnets.

[0066] In this invention, the metal element in the metal foil is selected from at least one of copper (Cu), nickel (Ni), aluminum (Al), niobium (Nb), titanium (Ti), and molybdenum (Mo). The metal foil can be a foil formed from an elemental metal or a foil formed from a metal alloy. The metal foil can be a foil with a metal film formed on both its upper and lower surfaces, and the metal element in the metal film is selected from at least one of copper, aluminum, niobium, titanium, and molybdenum.

[0067] According to one embodiment of the present invention, the metal foil is selected from at least one of copper foil, nickel foil, aluminum foil, niobium foil, titanium foil, copper-molybdenum foil and titanium-niobium alloy foil, preferably at least one of copper foil and titanium foil.

[0068] In this invention, the metal foil is a dense foil, a perforated foil, or a foamed metal foil. The thickness of the metal foil is 2–20 μm. The perforated foil can be a metal mesh foil or a metal grid foil.

[0069] In some specific implementations, the stacked magnets can be fixed by winding them with metal wire to obtain a fixed stacked magnet. The metal wire can be molybdenum wire, nickel alloy wire, titanium alloy wire, or stainless steel wire, preferably molybdenum wire. This allows the stacked magnets to be under pressure during vacuum heat treatment in a vacuum sintering furnace. The pressure direction is perpendicular to the surface of the stacked magnets and can be the direction of gravity.

[0070] Heat treatment steps

[0071] By heat-treating the stacked magnets, and at least during a portion of the heat treatment process, subjecting the stacked magnets to a state of pressure, an irregularly shaped rare-earth permanent magnet is obtained. This is beneficial for improving the coercivity of the irregularly shaped rare-earth permanent magnet.

[0072] In this invention, at least during a portion of the heat treatment process, the stacked magnets are subjected to pressure through external force or thermophysical pressure. Specifically, the stacked magnets can be bound together with metal wires, and the difference in the thermal expansion coefficients of the materials at high temperatures can be used to subject them to pressure during heat treatment. Alternatively, the stacked magnets can be confined within a mold or hopper, and the difference in the thermal expansion coefficients of the materials at high temperatures can be used to subject them to pressure during heat treatment. Alternatively, pressure can be applied through discharge plasma heating.

[0073] In some embodiments, when the stacked magnets are bound and fixed with metal wires or confined in a mold or a hopper, the heat treatment can be performed using a vacuum sintering furnace. The vacuum heat treatment includes a first stage heat treatment, a second stage heat treatment, and a third stage heat treatment. The first stage heat treatment is held at a temperature of 550–850°C for 0.1–5 hours; the second stage heat treatment is held at a temperature of 830–960°C for 0.5–8.5 hours; after the second stage heat treatment, the temperature is cooled to below 50°C; the third stage heat treatment is held at a temperature of 410–670°C for 1–5 hours.

[0074] In this invention, the holding temperature for the first stage heat treatment is preferably 580–800°C, more preferably 600–700°C. The holding time is preferably 0.5–4.5 h, more preferably 1–3 h. The holding temperature for the second stage heat treatment is preferably 840–950°C, more preferably 840–930°C. The holding time is preferably 0.5–7.5 h, more preferably 1–7 h. The holding temperature for the third stage heat treatment is preferably 450–660°C, more preferably 490–550°C. The holding time is preferably 1.5–4.5 h, more preferably 2–4 h.

[0075] In this invention, heat treatment can also be combined with discharge plasma heating and vacuum heat treatment in a vacuum sintering furnace, including a discharge plasma heating pressurization stage and a vacuum heat treatment stage. The heating temperature in the discharge plasma heating pressurization stage can be 500–750°C, preferably 550–700°C, more preferably 600–700°C. The applied pressure can be 3–10 MPa, preferably 5–8 MPa. The pressurization time can be 0.1–5 h.

[0076] The vacuum heat treatment stage here can include a first-stage heat treatment, a second-stage heat treatment, and a third-stage heat treatment. The first-stage heat treatment can be carried out using a spark plasma sintering furnace or a vacuum sintering furnace. The second and third-stage heat treatments can be carried out using a vacuum sintering furnace. The holding temperature for the first-stage heat treatment is 550–850℃, and the holding time is 0.1–5 h; the holding temperature for the second-stage heat treatment is 830–960℃, and the holding time is 0.5–8.5 h; after the second-stage heat treatment, the temperature is cooled to below 50℃; the holding temperature for the third-stage heat treatment is 410–670℃, and the holding time is 1–5 h.

[0077] In this invention, the heating rate during heat treatment can be 3 to 10 °C / min, preferably 3.5 to 7.5 °C / min, and more preferably 5 to 7 °C / min.

[0078] <Irregularly Shaped Rare Earth Permanent Magnets>

[0079] This invention provides an irregularly shaped rare-earth permanent magnet prepared according to the method described above. The irregularly shaped rare-earth permanent magnet obtained by this invention has a large size, and its thickness in the orientation direction can be greater than 10 mm, for example, greater than 25 mm, and its coercivity is significantly improved. The remanence decreases less.

[0080] According to a specific embodiment of the present invention, when a sintered NdFeB magnet with a set through hole is in the shape of a ring, the resulting irregular rare earth permanent magnet is a tubular rare earth permanent magnet.

[0081] According to another specific embodiment of the present invention, when the sintered NdFeB magnet with a set through hole is cubic, the resulting irregular rare earth permanent magnet is a rare earth permanent magnet with a deep hole, wherein the deep hole is a deep through hole, and the height of the deep hole can be greater than 10 mm, and can be 25 mm.

[0082] According to another embodiment of the present invention, when the sintered NdFeB magnet is tile-shaped, the resulting irregular rare earth permanent magnet can be tile-shaped or trumpet-shaped.

[0083] The strength of the irregular rare earth permanent magnet obtained by this invention is greater than the mechanical strength of the matrix. When the interface strength test of the welded sample is conducted, the fracture area is inside the magnet rather than the welded connection area (welded joint). The stacked magnets are tightly connected through metallurgical bonding.

[0084] <Testing Methods>

[0085] Magnetic properties were measured using a BH magnetometer at room temperature, including room temperature remanence (Br) and room temperature coercivity (Hcj).

[0086] Example 1

[0087] Five identical sintered NdFeB magnets of grade N52 are provided. The length, width, and height of each magnet are 15mm, 15mm, and 5mm, respectively. Each magnet has a circular through-hole with a diameter of 6mm at its center. The central axis of the circular through-hole is aligned with the C-axis of the magnet.

[0088] Terbium-copper-aluminum alloy rare-earth thin films were formed on the surface of each sintered NdFeB magnet using magnetron sputtering (based on the total weight of the rare-earth films, the contents of terbium, copper, and aluminum were 94.4 wt%, 4.4 wt%, and 1.2 wt%, respectively), and the thickness of the formed rare-earth films was controlled to be 25 μm, resulting in coated magnets. During the coating process, rare-earth thin films were formed on all surfaces of the magnets. The surfaces on which the rare-earth thin films were sputtered are called the coated surfaces.

[0089] Five coated magnets are stacked vertically along the C-axis orientation, ensuring their central axes are aligned. A copper-titanium alloy foil is placed between the coated surfaces of adjacent magnets (this foil is obtained by magnetron sputtering 1.5 μm thick titanium films onto both sides of a 15 μm thick copper foil, resulting in a total thickness of 18 μm: 15 μm copper + 2 × 1.5 μm titanium films). The stacked magnets are then secured using molybdenum wire. To ensure strict alignment, the magnets are magnetized, utilizing their mutual attraction to fix and align them.

[0090] The fixed stacked magnets are loaded into a cassette and then placed in a vacuum sintering furnace for heat treatment. The first stage of heat treatment involves heating at a vacuum level of 3 Pa in the furnace, increasing the temperature to 625°C at a rate of 5°C / min, and holding at 625°C for 60 min. The second stage involves heating to 915°C at a rate of 5°C / min after the first stage, holding at 915°C for 325 min, and then cooling to room temperature. The third stage involves heating at a vacuum level of 3 Pa in the furnace, increasing the temperature from room temperature to 505°C at a rate of 5°C / min, holding at 505°C for 120 min, and then cooling to room temperature to obtain the irregularly shaped rare-earth permanent magnet. This irregularly shaped rare-earth permanent magnet is a cubic rare-earth permanent magnet with a deep hole (deep through-hole). The depth of the through-hole in this irregularly shaped rare-earth permanent magnet is 25 mm, meaning the magnet thickness is 25 mm.

[0091] Table 1 shows the performance results of the obtained irregular rare earth permanent magnet with a thickness of 25 mm. The coercivity of the magnet increased by 9.46 kOe, and the remanence decreased by 1.25%.

[0092] Table 1

[0093] project Remanence / kGs Coercivity / kOe Initial sintering of NdFeB magnets N52 14.38 13.36 Rare earth permanent magnets in Example 1 14.20 22.82

[0094] If a larger magnet is needed, the pre-made 25mm thick irregular rare earth permanent magnet can be welded together to obtain a magnet material with a thickness of 25×N (N is a natural number greater than or equal to 2) mm.

[0095] Example 2

[0096] Sintered NdFeB magnets of grades N50 and 40SH are provided. These two grades of sintered NdFeB magnets have the same specifications: a cylindrical outer contour with a diameter of 18 mm and a thickness of 5 mm. Each magnet has a 4 mm diameter circular through-hole at its center. The central axis of the circular through-hole is aligned with the C-axis of the magnet.

[0097] A rare-earth thin film of dysprosium-copper-gallium alloy was formed on the surface of each sintered NdFeB magnet using magnetron sputtering (the content of dysprosium, copper, and gallium was 92 wt%, 6.5 wt%, and 1.5 wt% based on the total weight of the rare-earth film), and the thickness of the formed rare-earth film was controlled to be 30 μm, resulting in a coated magnet. The surface on which the rare-earth film was sputtered is called the coating surface. During the coating process, a rare-earth film is formed on all surfaces of the magnet.

[0098] Two types of magnets, N50 and 40SH, are stacked alternately along the C-axis orientation, with 40SH magnets at the top and bottom. They are arranged in the order of 40SH-N50-40SH-N50-40SH, so that their central axes are on the same straight line. Two layers of titanium foil (6μm thick) are sandwiched between the coating surfaces of two adjacent coated magnets. The front and back sides of the titanium foil are respectively covered with copper film with a thickness of 2μm. The total thickness of the sandwiched metal foil is 20μm.

[0099] The stacked magnets are placed into matching graphite molds, confining them within the molds, and then placed in a vacuum sintering furnace for heat treatment. The first stage of heat treatment involves heating the magnets at a vacuum level of 3 Pa, increasing the temperature to 690℃ at a rate of 5℃ / min, and holding at 690℃ for 45 min. The second stage involves further heating to 840℃ at a rate of 5℃ / min after the first stage, holding at 840℃ for 120 min, and then cooling to room temperature. The third stage involves heating the magnets in a vacuum heat treatment furnace from room temperature to 495℃ at a rate of 5℃ / min, holding at 495℃ for 150 min, and then cooling to room temperature to obtain the irregularly shaped rare-earth permanent magnet. This irregularly shaped rare-earth permanent magnet is a cylindrical rare-earth permanent magnet with a deep hole (deep through-hole). The depth of the through-hole in the irregularly shaped rare-earth permanent magnet is 25 mm.

[0100] The results of the magnetic properties test are shown in Table 2.

[0101] Table 2

[0102] project Remanence / kGs Coercivity / kOe Initial sintering of NdFeB magnets 40SH 12.45 20.52 Initial sintering of NdFeB magnets N50 14.30 13.36 Irregularly Shaped Rare Earth Permanent Magnets in Example 2 13.56 28.78

[0103] Example 3

[0104] Five identical sintered NdFeB magnets of grade N50, in a ring shape, are provided. Each magnet has an outer diameter of 20 mm, an inner diameter of 12 mm, and a thickness of 5 mm. The central axis of the ring is aligned with the C-axis of the magnet.

[0105] A rare-earth thin film of dysprosium-copper-aluminum alloy was formed on the surface of each sintered NdFeB magnet using magnetron sputtering (based on the total weight of the rare-earth film, the contents of dysprosium, copper, and aluminum alloy were 95wt%, 3.8wt%, and 1.2wt%, respectively), and the thickness of the formed rare-earth film was controlled to be 60μm, resulting in a coated magnet. The surface on which the rare-earth film was sputtered is called the coated surface. During the coating process, the inner and outer surfaces of the ring magnet were not specially shielded, so the rare-earth film was attached to both.

[0106] Five coated magnets are stacked vertically along the C-axis orientation, with no metal foil between adjacent coated magnets, so that their central axes are on the same straight line, thus obtaining stacked magnets.

[0107] The stacked magnets were placed into a graphite mold and subjected to spark plasma sintering at a temperature of 680℃. During the spark plasma sintering process, a pressure of 8MPa was maintained and the temperature and pressure were held for 10 minutes.

[0108] Then, vacuum heat treatment is performed. The first stage of heat treatment: heating begins when the vacuum level of the discharge plasma sintering furnace is 3 Pa, with a heating rate of 5 °C / min to 680 °C, and held at 680 °C for 10 min. The second stage of heat treatment: after the first stage heat treatment, heating continues at a rate of 5 °C / min to 860 °C, and held at 860 °C for 30 min, after which the temperature is cooled to room temperature. The third stage of heat treatment: using a vacuum sintering furnace, heating begins when the vacuum level of the vacuum sintering furnace is 3 Pa, with a heating rate of 5 °C / min from room temperature to 510 °C, held at 510 °C for 180 min, and after which the temperature is cooled to room temperature, thus obtaining a tubular, irregularly shaped rare-earth permanent magnet.

[0109] The results of the magnetic properties test are shown in Table 3.

[0110] Table 3

[0111] project Remanence / kGs Coercivity / kOe Initial sintering of NdFeB magnets N50 14.30 13.36 Irregularly Shaped Rare Earth Permanent Magnets in Example 3 14.18 24.25

[0112] In summary, the preparation method of the present invention can obtain large-sized irregular rare earth permanent magnets with a thickness greater than 10 mm in the orientation direction, such as 25 mm, and the coercivity of the obtained irregular rare earth permanent magnets is greatly improved while the remanence decreases less.

[0113] This invention is not limited to the above-described embodiments. Any modifications, improvements, or substitutions that can be conceived by those skilled in the art without departing from the essential content of this invention fall within the scope of this invention.

Claims

1. A method for preparing an irregularly shaped rare-earth permanent magnet, characterized in that, Includes the following steps: 1) A rare earth thin film is formed on the surface of a sintered NdFeB magnet with a predetermined through hole to obtain a coated magnet; wherein the central axis direction of the through hole is parallel or perpendicular to the orientation direction of the sintered NdFeB magnet; or a rare earth thin film is formed on the surface of at least one sintered NdFeB magnet with an outer concave arc surface to obtain a coated magnet. 2) Stack two or more coated magnets vertically along a parallel or perpendicular orientation, so that their central axes are on the same straight line. Adjacent coated magnets are provided with metal foil or are in direct contact without metal foil to obtain stacked magnets. 3) The stacked magnets are heat-treated, and at least during a portion of the heat treatment process, the stacked magnets are subjected to a state of pressure to obtain irregularly shaped rare earth permanent magnets.

2. The preparation method according to claim 1, characterized in that, The through hole is selected from at least one of the following: circular through hole, elliptical through hole, square through hole, rectangular through hole, parallelogram through hole, triangular through hole, and conical through hole.

3. The preparation method according to claim 2, characterized in that: The through hole is a circular through hole, and the sintered NdFeB magnet is in the shape of a ring; the resulting irregular rare earth permanent magnet is a tubular rare earth permanent magnet. The through hole is circular, and the sintered NdFeB magnet is cubic with the through hole in the center; the resulting irregular rare earth permanent magnet is a rare earth permanent magnet with a deep hole.

4. The preparation method according to claim 1, characterized in that, In step 1), the rare earth film is a film formed by rare earth elements or a film formed by rare earth elements and other metal elements. The rare earth element is selected from at least one of Pr, Nd, La, Ce, Dy, Tb and Ho, and must contain a heavy rare earth element; the heavy rare earth element is Dy, Tb or Ho. The other metallic elements are selected from at least one of Al, Cu, Ga, Co, Fe, Mg, Sn, Ag and Zn.

5. The preparation method according to claim 1, characterized in that, In step 1), the rare earth thin film is formed by one of the following methods: spraying, screen printing, laser cladding, brazing, evaporation coating, and magnetron sputtering; the thickness of the rare earth thin film is 0.1 to 2.5% of the thickness of the sintered NdFeB magnet.

6. The preparation method according to claim 1, characterized in that: In step 2), metal foil sheets are placed between adjacent coated magnets; In step 2), the metal element in the metal foil is selected from at least one of copper, nickel, aluminum, niobium, titanium and molybdenum; the metal foil is a dense foil, a perforated foil or a foamed metal foil, and its thickness is 2 to 20 μm.

7. The preparation method according to claim 6, characterized in that, The metal foil is selected from at least one of copper foil, nickel foil, aluminum foil, niobium foil, titanium foil, copper-molybdenum alloy foil, and titanium-niobium alloy foil.

8. The preparation method according to claim 1, characterized in that, The stacked magnets are placed under pressure during heat treatment using any of the following methods: (1) Before heat treatment, the stacked magnets are fixed by winding with metal wire to obtain the fixed stacked magnets. The fixed stacked magnets are placed in a vacuum sintering furnace for vacuum heat treatment. During vacuum heat treatment, the stacked magnets are under pressure. (2) Place the stacked magnets in a matching mold and confine them in the mold, and then perform vacuum heat treatment in a vacuum sintering furnace, during which the stacked magnets are under pressure. (3) Place the stacked magnets in the mold, and then use spark plasma sintering pressure heat treatment to put the stacked magnets under pressure.

9. The preparation method according to claim 8, characterized in that, When using method (1) or method (2), the heat treatment in step 3) is performed using a vacuum sintering furnace, and the vacuum heat treatment sequentially includes a first stage heat treatment, a second stage heat treatment, and a third stage heat treatment: wherein, The holding temperature for the first stage of heat treatment is 550–850℃, and the holding time is 0.1–5 hours. The second stage of heat treatment involves holding at 830–960℃ for 0.5–8.5 hours; after the second stage of heat treatment, the temperature is cooled to below 50℃. The holding temperature for the third stage of heat treatment is 410–670℃, and the holding time is 1–5 hours.

10. An irregularly shaped rare-earth permanent magnet, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 9.