A high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet and preparation method thereof

Abstract

The invention discloses a high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet, which is composed of 2-5% by weight of a secondary phase alloy and the remainder of a main phase alloy, wherein the main phase alloy is composed of the following components by weight: 28-32% Pr 25 Nd 75 , 0.9-1.5% B, 0.1-0.5% Al, 0.05-0.2% Zr, 0.05-0.3% Ti, balance Fe; the secondary phase alloy is composed of the following components in percentage by mass: 55% Nd, 20% Cu, 25% Ga. The high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet of the present invention does not contain heavy rare earth elements, which reduces the production cost and has high remanence energy and coercivity. The present invention also provides a method for preparing a high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet, comprising the following steps: (1) preparing a main phase alloy; (2) preparing a secondary phase alloy; (3) powdering; (4) magnetic field forming; (5) cold isostatic pressing; (6) sintering. The method for preparing a NdFeB magnet of the present invention has simple process steps, strong operability, and is suitable for mass production.

Description

Technical Field

[0001] The invention relates to a sintered NdFeB magnet, in particular to a sintered NdFeB magnet with high remanence, high coercivity and no heavy rare earth and a preparation method thereof. Background Art

[0002] NdFeB permanent magnets are the rare earth permanent magnet materials with the highest comprehensive performance known today, so they are widely used in consumer electronics, energy-saving home appliances, medical equipment and other fields. Recently, the demand for NdFeB permanent magnets in environmentally friendly industries represented by hybrid electric vehicles, electric vehicles, and wind power generation has further expanded, and the requirements for the comprehensive magnetic properties of NdFeB permanent magnets are getting higher and higher. They must have both high remanence energy to provide a sufficiently strong magnetic field and high coercivity to meet the application under high temperature conditions.

[0003] At present, the main method is to introduce heavy rare earths (Dy, Tb and Ho, etc.) to increase the magnetocrystalline anisotropy field of the Nd-Fe-B main phase, thereby increasing the coercive force. However, with the addition of heavy rare earth elements, the remanence and magnetic energy product of the magnet will decrease to varying degrees, and the price of heavy rare earths is expensive, usually several times the price of light rare earths, which greatly increases the preparation cost of the product. Therefore, it is very necessary to develop and design a sintered NdFeB magnet without heavy rare earth elements. Summary of the invention

[0004] The present invention aims to solve the problem that the prior art increases the coercivity of NdFeB permanent magnets by introducing heavy rare earths, but this leads to a decrease in remanence and maximum magnetic energy product, as well as a rapid increase in preparation costs. The present invention provides a high remanence, high coercivity, heavy rare earth-free sintered NdFeB magnet, which is beneficial to reducing the production cost of sintered NdFeB magnets.

[0005] The present invention also provides a method for preparing a heavy rare earth-free sintered NdFeB magnet with high remanence and high coercivity, which has simple process steps, strong operability and is suitable for mass production.

[0006] In order to achieve the above-mentioned object, the present invention adopts the following technical scheme: a high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet of the present invention is composed of 2-4% by weight of secondary phase alloy and the remainder of main phase alloy, based on the total mass of the high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet, wherein the main phase alloy is composed of the following components by weight: 28-32% Pr 25 Nd 75, 0.9-1.5% B, 0.1-0.5% Al, 0.05-0.2% Zr, 0.05-0.3% Ti, the balance Fe; the subphase alloy is composed of the following atomic percentages: 55% Nd, 20% Cu, 25% Ga. The present invention defines the subphase alloy as having the following atomic percentages: 55% Nd, 20% Cu, 25% Ga. This atomic ratio is an optimal ratio obtained by the applicant after a large amount of theoretical deduction and experimental verification. Under this ratio, the sintered NdFeB magnet can obtain the best comprehensive magnetic properties.

[0007] A method for preparing a heavy rare earth-free sintered NdFeB magnet with high remanence and high coercivity comprises the following steps:

[0008] (1) Preparation of main phase alloy: The main phase alloy is prepared according to its composition. After preparation, the raw materials are added into a vacuum rapid solidification sheeting furnace. After smelting, pouring, cooling and crystallization, a rapid solidification sheet is obtained.

[0009] (2) Preparation of secondary phase alloy: The ingredients are prepared according to the composition of the secondary phase alloy, and after the ingredients are prepared, the raw materials are added into a vacuum melting furnace, and after melting and pouring, an alloy steel ingot is obtained. The present invention adopts NdCuGa alloy as the secondary phase alloy, and adds and mixes the primary phase alloy after smelting to improve the wettability of the grain boundary phase and the primary phase, and forms an amorphous layer shell structure in the epitaxial layer of the primary phase alloy, which weakens the exchange coupling between the primary phases, greatly reduces the areas of preferential reverse magnetization such as the sharp corners of the grains, weakens the ferromagnetism of the grain boundary phase, and thus realizes the effect of improving the coercive force. Different from the process of directly adding NdCuGa into the primary phase alloy with the same nominal composition and smelting at the same time, the simultaneous smelting cannot form the layer shell structure of the outer layer of the primary phase, and of course cannot play the role of improving the wettability of the grain boundary phase and the primary phase, eliminating the sharp corners, etc. Most of Ga is doped into the primary phase rather than the grain boundary phase. After the epitaxial layer of the primary phase and the grain boundary phase lose most of Ga, the excessive Cu cannot be evenly distributed around the primary phase, which not only fails to improve the coercive force, but sometimes even causes the deterioration of the squareness.

[0010] (3) Powder making: The alloy steel ingot is mixed with the quick-setting flakes, and then hydrogen crushed, and then jet milled to obtain powder.

[0011] (4) Magnetic field molding: The powder is placed in a magnetic field press and pressed under the protection of an inert gas to obtain a compact.

[0012] (5) Cold isostatic pressing: The green compact is placed in a cold isostatic pressing device in a vacuum-sealed state for re-pressing, thereby increasing the density of the green compact by 10 to 15% to obtain a cold isostatic pressed green compact.

[0013] (6) Sintering: The cold isostatically pressed blank is placed in a vacuum sintering furnace. After sintering and tempering, a sintered NdFeB magnet blank is obtained.

[0014] Preferably, in step (1), the smelting temperature is 1420-1480°C.

[0015] Preferably, in step (2), the smelting temperature is 950-1050°C.

[0016] Preferably, in step (3), the hydrogen crushing temperature is 420-500° C. The conventional hydrogen crushing process temperature is 560-600° C. The present invention adopts a low-temperature hydrogen crushing process, and the hydrogen crushing temperature is controlled at 420-500° C. The purpose is to reduce the possibility of Ga forming other difficult-to-decompose compounds in this process, such as B, N and other compounds of Ga, to ensure that it is not occupied by unexpected compounds.

[0017] In step (3), the surface area average particle size of the powder is 2.6 to 3.2 μm.

[0018] Preferably, in step (5), the re-pressing pressure is 150 to 280 MPa.

[0019] Preferably, in step (6), the sintering procedure is: sintering at 980-1030°C for 6h, cooling to below 100°C; the tempering procedure is: first-stage tempering at 880-920°C for 2-3h, cooling to below 80°C, then heating to 460-560°C for second-stage tempering for 4-6h, and cooling to room temperature. Sintering and tempering are key steps that determine factors such as whether the magnet is dense and whether the grain boundary structure is optimized, and thus greatly affect the magnetic properties of the magnet; the present invention adopts a low-temperature sintering process. After the grain boundary properties have changed, the traditional high-temperature sintering process will cause the main phase grains to grow abnormally and cause excessive burning and volatilization. Therefore, the present invention adopts a low-temperature, long-term sintering process at 980-1030°C to ensure grain consistency, and adopts a second-stage annealing process.

[0020] Therefore, the present invention has the following beneficial effects:

[0021] (1) The composition of NdFeB magnets has been improved, and no heavy rare earth elements are contained in the composition, which reduces the production cost and has higher remanence and coercivity;

[0022] (2) A method for preparing a NdFeB magnet is provided, which has simple process steps, strong operability, is suitable for mass production, and can optimize the grain boundary structure. The magnet prepared has high remanence energy and coercive force. DETAILED DESCRIPTION

[0023] The present invention is further described below through specific implementation modes.

[0024] Example 1

[0025] (1) Preparation of main phase alloy: The main phase alloy is prepared according to its composition. The main phase alloy is composed of the following components in percentage by weight: 28.4% Pr 25 Nd 75 , 0.94% B, 0.15% Al, 0.1% Zr, 0.1% Ti, and the balance Fe; after batching, the raw materials are added into a vacuum rapid solidification sheet-throwing furnace, and after smelting, pouring, cooling and crystallization at 1450°C, a rapid solidification sheet is obtained;

[0026] (2) preparing a secondary phase alloy: preparing ingredients according to the composition of the secondary phase alloy, wherein the secondary phase alloy is composed of the following components in atomic percentage: 55% Nd, 20% Cu, and 25% Ga. After preparing the ingredients, the raw materials are added into a vacuum melting furnace, and after melting and pouring at 1000° C., an alloy steel ingot is obtained;

[0027] (3) Powder preparation: Based on the total mass of high remanence, high coercivity and heavy rare earth-free sintered NdFeB magnets, 3% of the alloy steel ingot and 97% of the quick-solidified flakes are mixed and then hydrogen crushed at a hydrogen crushing temperature of 450°C, and then jet milled to obtain a powder with an average particle size of 2.8 μm on the surface area;

[0028] (4) Magnetic field molding: The powder is placed in a magnetic field press and pressed under the protection of an inert gas to obtain a compact;

[0029] (5) Cold isostatic pressing: The green compact is placed in a cold isostatic pressing device in a vacuum-sealed state for re-pressing at a pressure of 220 MPa, increasing the density of the green compact by 13% to obtain a cold isostatic pressed green compact;

[0030] (6) Sintering: Place the cold isostatically pressed blank in a vacuum sintering furnace, and after sintering and tempering, obtain a sintered NdFeB magnet blank; the sintering procedure is: sintering at 1020℃ for 4h, cooling to below 100℃; the tempering procedure is: first-level tempering at 900℃ for 3h, cooling to below 80℃, then heating to 460℃ for second-level tempering for 5h, and cooling to room temperature.

[0031] Comparative Example 1

[0032] The difference between Comparative Example 1 and Example 1 is that in step (3), the hydrogen fragmentation temperature is 580° C., and the rest is exactly the same as Example 1.

[0033] Comparative Example 2

[0034] Compared with Example 1, Comparative Example 2 differs in that: in step (6), the sintering procedure is: sintering at 1060°C for 4 hours, cooling to below 100°C; the tempering procedure is: primary tempering at 900°C for 3 hours, cooling to below 80°C, then heating to 460°C for secondary tempering for 5 hours, and cooling to room temperature. The rest is exactly the same as Example 1.

[0035] Comparative Example 3

[0036] The difference between Comparative Example 3 and Example 1 is that in step (2), the subphase alloy is composed of the following components in atomic percentage: 75% Nd, 20% Cu, 5% Ga, and the rest is exactly the same as Example 1.

[0037] Comparative Example 4

[0038] The difference between Comparative Example 4 and Example 1 is that in step (2), Ga in the secondary phase alloy is replaced by Al, and the rest is exactly the same as Example 1.

[0039] Comparative Example 5

[0040] Compared with Example 1, Comparative Example 5 differs in that: in step (3), the mass percentage of the alloy steel ingot is 6%, and the rest is exactly the same as Example 1.

[0041] Example 2

[0042] (1) Preparation of main phase alloy: The main phase alloy is prepared according to its composition. The main phase alloy is composed of the following components in percentage by weight: 29.4% Pr 25 Nd 75 , 0.94% B, 0.15% Al, 0.08% Zr, 0.08% Ti, and the balance Fe; after batching, the raw materials are added into a vacuum rapid solidification sheet-throwing furnace, and after smelting, pouring, cooling and crystallization at 1420°C, a rapid solidification sheet is obtained;

[0043] (2) preparing a secondary phase alloy: preparing ingredients according to the composition of the secondary phase alloy, wherein the secondary phase alloy is composed of the following components in atomic percentage: 55% Nd, 20% Cu, and 25% Ga. After preparing the ingredients, the raw materials are added into a vacuum melting furnace, and after melting and pouring at 1000° C., an alloy steel ingot is obtained;

[0044] (3) Powder preparation: Based on the total mass of high remanence, high coercivity and heavy rare earth-free sintered NdFeB magnets, 2% of the alloy steel ingots and 98% of the quick-solidified flakes are mixed and then hydrogen crushed at a hydrogen crushing temperature of 460°C, and then jet milled to obtain a powder with an average particle size of 2.8 μm on the surface area;

[0045] (4) Magnetic field molding: The powder is placed in a magnetic field press and pressed under the protection of an inert gas to obtain a compact;

[0046] (5) Cold isostatic pressing: The green compact is placed in a cold isostatic pressing device in a vacuum-sealed state for re-pressing at a pressure of 220 MPa, increasing the density of the green compact by 13% to obtain a cold isostatic pressed green compact;

[0047] (6) Sintering: Place the cold isostatically pressed blank in a vacuum sintering furnace, and after sintering and tempering, obtain a sintered NdFeB magnet blank; the sintering procedure is: sintering at 1010℃ for 6h, cooling to below 100℃; the tempering procedure is: first-level tempering at 890℃ for 3h, cooling to below 80℃, then heating to 480℃ for second-level tempering for 5h, and cooling to room temperature.

[0048] Example 3

[0049] (1) Preparation of main phase alloy: The main phase alloy is prepared according to its composition. The main phase alloy is composed of the following components by mass percentage: 27.6% Pr 25 Nd 75 , 0.94% B, 0.1% Al, 0.12% Zr, 0.1% Ti, and the balance Fe; after batching, the raw materials are added into a vacuum rapid solidification sheet-throwing furnace, and after smelting, pouring, cooling and crystallization at 1480°C, a rapid solidification sheet is obtained;

[0050] (2) preparing a secondary phase alloy: preparing ingredients according to the composition of the secondary phase alloy, wherein the secondary phase alloy is composed of the following components in atomic percentage: 55% Nd, 20% Cu, and 25% Ga. After preparing the ingredients, the raw materials are added into a vacuum melting furnace, and after melting and pouring at 1050° C., an alloy steel ingot is obtained;

[0051] (3) Powder preparation: Based on the total mass of high remanence, high coercivity and heavy rare earth-free sintered NdFeB magnets, 4% of alloy steel ingots and 96% of quick-solidified flakes are mixed and then hydrogen crushed at a hydrogen crushing temperature of 480°C, and then jet milled to obtain a powder with an average particle size of 2.8 μm on the surface area;

[0052] (4) Magnetic field molding: The powder is placed in a magnetic field press and pressed under the protection of an inert gas to obtain a compact;

[0053] (5) Cold isostatic pressing: The green compact is placed in a cold isostatic pressing device in a vacuum plastic-sealed state for re-pressing at a pressure of 220 MPa, increasing the density of the green compact by 12% to obtain a cold isostatic pressed green compact;

[0054] (6) Sintering: Place the cold isostatically pressed blank in a vacuum sintering furnace, and after sintering and tempering, obtain a sintered NdFeB magnet blank; the sintering procedure is: sintering at 1030℃ for 4h, cooling to below 100℃; the tempering procedure is: primary tempering at 920℃ for 3h, cooling to below 80℃, then heating to 520℃ for secondary tempering for 5h, and cooling to room temperature.

[0055] The sintered NdFeB magnet blanks obtained in Examples 1 to 3 and Comparative Examples 1 to 5 were subjected to conventional mechanical processing and surface treatment, and then subjected to magnetic property testing. The test results are shown in Table 1.

[0056] Table 1 Magnetic property test results of sintered NdFeB magnet blanks obtained in Examples 1 to 3 and Comparative Examples 1 to 5

[0057]

[0058] It can be seen from Table 1 that the magnetic properties of the sintered NdFeB magnet blank obtained in Example 1 are better than those of the sintered NdFeB magnet blanks obtained in Comparative Examples 1 to 5; the remanence of the sintered NdFeB magnets obtained in Examples 1 to 3 reaches more than 14KGs, the intrinsic coercive force reaches more than 17KOe, and the maximum magnetic energy product reaches more than 50MGoe, indicating that the sintered NdFeB magnets prepared by the present invention can obtain better comprehensive magnetic properties without adding heavy rare earth.

[0059] The above-described embodiment is only a preferred solution of the present invention and does not limit the present invention in any form. There are other variations and modifications without exceeding the technical solution described in the claims.

Claims

1. A high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet, characterized in that: The high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet is composed of 2-5% by weight of secondary phase alloy and the remainder of main phase alloy, wherein the main phase alloy is composed of the following components by weight: 28-32% Pr 25 Nd 75 , 0.9-1.5% B, 0.1-0.5% Al, 0.05-0.2% Zr, 0.05-0.3% Ti, and the balance Fe; the secondary phase alloy is composed of the following components in percentage by mass: 55% Nd, 20% Cu, and 25% Ga; the high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet is prepared by the following method: (1) Preparing the main phase alloy: the ingredients are prepared according to the composition of the main phase alloy, and after the ingredients are prepared, the raw materials are added into a vacuum rapid solidification sheet-spinning furnace, and after melting, pouring, cooling and crystallization at 1420-1480° C., a rapid solidification sheet is obtained; (2) preparing the secondary phase alloy: preparing the ingredients according to the composition of the secondary phase alloy, adding the raw materials into a vacuum melting furnace, melting and pouring at 950-1050° C., and obtaining an alloy steel ingot; (3) Powder making: According to the ratio of the secondary phase alloy to the primary phase alloy, the alloy steel ingot and the quick-solidified flakes are mixed and then hydrogen crushed at a hydrogen crushing temperature of 420-500°C, and then jet milled to obtain powder; (4) Magnetic field molding: The powder is placed in a magnetic field press and pressed under the protection of an inert gas to obtain a compact; (5) Cold isostatic pressing: The green compact is placed in a cold isostatic pressing device in a vacuum-sealed state for re-pressing, thereby increasing the density of the green compact by 10 to 15% to obtain a cold isostatic pressed green compact; (6) Sintering: Place the cold isostatically pressed blank in a vacuum sintering furnace. After sintering and tempering, a sintered NdFeB magnet blank is obtained. The sintering procedure is: sintering at 1010-1090°C for 4 hours, cooling to below 100°C; the tempering procedure is: primary tempering at 880-920°C for 2-3 hours, cooling to below 80°C, then heating to 460-560°C for secondary tempering for 4-6 hours, and cooling to room temperature.

2. The high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet according to claim 1, characterized in that: In step (3), the surface area average particle size of the powder is 2.6 to 3.2 μm.

3. The high remanence and high coercivity heavy rare earth-free sintered NdFeB magnet according to claim 1, characterized in that: In step (5), the pressing pressure is 150 to 280 MPa.

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