Method for preparing sintered NdFeB magnets

Abstract

The present disclosure refers to a method for preparing sintered NdFeB magnets, including: a) Preparing alloy flakes from a raw material by strip casting, performing a hydrogen decrepitation to produce alloy pieces, pulverization the alloy pieces to an alloy powder, performing molding and orientation, cold isostatic pressing, and getting a green compact; b) Putting the green compact into a vacuum furnace and performing a first sintering step in 830 to 880° C. for 2 to 10 hours and 5×10.sup.−1 Pa or less; c) Performing a second sintering step while applying a pressure to the green compact achieved by step b), the pressure is 1 MPa to 5 MPa and the sintering temperature is 720 to 850° C. for 15 to 60 minutes, and the temperature of the first sintering step is at least 10° C. higher than that of the second sintering step; d) Subjecting the sintered magnet of step c) to an annealing treatment.

Claims

1. A method for preparing sintered NdFeB magnets, the method including the steps of: a) Preparing alloy flakes from a raw material of the NdFeB magnet by strip casting, then performing a hydrogen decrepitation of the alloy flakes to produce alloy pieces, then pulverization the alloy pieces to an alloy powder by jet mill, and finally performing molding and orientation, cold isostatic pressing, and then get a green compact; b) Putting the green compact into a vacuum furnace and performing a first sintering step, wherein the sintering temperature is in the range of 830° C. to 880° C. for 2 to 10 hours and the pressure in the furnace is 5×10.sup.−1 Pa or less; c) Performing a second sintering step while applying a pressure along the magnetic orientation direction of the green compact achieved by step b), wherein the pressure applied on the green compact is in the range of 1 MPa to 5 MPa and the sintering temperature is in the range of 720° C. to 850° C. for 15 to 60 minutes, and wherein the temperature of the first sintering step is at least 10° C. higher than the temperature of the second sintering step; and d) Subjecting the sintered magnet of step c) to an annealing treatment.

2. The method of claim 1, wherein in step a) a mass percentage of rare earth elements is in the range of 33.0% to 37.0% in the alloy flakes.

3. The method of claim 1, wherein in step of b) a density of the green compact is in the range of 7.08 to 7.37 g/cm.sup.3 after the first sintering step.

4. The method of claim 2, wherein in step of b) a density of the green compact is in the range of 7.08 to 7.37 g/cm.sup.3 after the first sintering step.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0016] FIG. 1 is a backscattered electron (BSE) image from a scanning electron microscopes (SEM) of implementing example 1.

[0017] FIG. 2 is a BSE image of implementing example 2.

[0018] FIG. 3 is a BSE image of implementing example 3.

[0019] FIG. 4 is a BSE image of comparative example 1.

[0020] FIG. 5 is a BSE image of comparative example 2.

[0021] FIG. 6 is a BSE image of comparative example 3.

DETAILED DESCRIPTION

[0022] Reference will now be made in detail to embodiments. The present disclosure, however, may be embodied in various different forms, and should not be construed as being limited to only the illustrated embodiments herein. Rather, these embodiments are provided as examples so that this disclosure will be thorough and complete, and will fully convey the aspects and features of the present disclosure to those skilled in the art.

[0023] A NdFeB magnet (also known as MB or Neo magnet) is the most widely used type of rare-earth magnet. It is a permanent magnet made from an alloy of neodymium, iron, and boron to form the Nd.sub.2Fe.sub.14B tetragonal crystalline structure as a main phase. Besides, the microstructure of Nd—Fe—B magnets includes usually a Nd-rich phase. The alloy may include further elements in addition to or partly substituting neodymium and iron.

[0024] The composition of the NdFeB powder may refer to the commercially available general-purpose sintered NdFeB grades. For example, its basic composition can be set to RE.sub.aT.sub.(1-abc)B.sub.bM.sub.c, where RE is a rare earth element selected from at least one of Pr, Nd, Dy, Tb, Ho, and Gd, T is at least one of Fe or Co, B is element B, M is at least one of Al, Cu, Ga, Ti, Zr, Nb, Mo, and V, and a, b, and c may be 33 wt. %<a≤wt. 37%, 0.85 wt. %<b−1.3 wt. %, and c≤5 wt. %.

[0025] Commercially available or freshly produced alloy powders could be used for the inventive process of preparing the NdFeB powders, respectively sintered NdFeB magnets. Specifically, NdFeB alloy flakes may be produced by a strip casting process, then subjected to a hydrogen embrittlement process and jet milling for preparing the desired NdFeB magnet powders, which are modified by depositing a mixed metal coating. The strip casting process, the hydrogen embrittlement process, and the jet milling process are currently well-known technologies. Cold isostatic pressing of the alloy powder to a green compact while applying a magnetic field for orientation is also state of the art. In other words, preparation and composition of the NdFeB alloy flakes and the process up to the preparing of a green compact is well-known in the art.

[0026] The method of preparing sintered NdFeB magnet includes the steps of:

[0027] step a): Preparing alloy flakes from a raw material of the NdFeB magnet by strip casting, then performing a hydrogen decrepitation of the alloy flakes to produce alloy pieces, then pulverization the alloy pieces to an alloy powder by jet mill, and finally performing molding and orientation, cold isostatic pressing, and then get a green compact.

[0028] In other words, the raw materials are made into alloy flakes by strip casting method, and then hydrogen absorption and dehydrogenation are performed followed by milling powders by a jet mill process. Then the powder is molded and orientated, and being performed by cold isostatic pressing to get green compact.

[0029] In step a), a mass percentage of rare earth elements may be in the range of 33.0% to 37.0% in the alloy flakes.

[0030] step b): Putting the green compact into a vacuum furnace and performing a first sintering step, wherein the sintering temperature is in the range of 830° C. to 880° C. for 2 to 10 hours and the pressure in the furnace is 5×10.sup.−1 Pa or less.

[0031] In other words, the green compact is put into a vacuum furnace for a first-step sintering. The sintering temperature is 830° C. to 880° C. with a duration time of 2 to 10 hours. Vacuum of the furnace is under 5×10.sup.−1 Pa.

[0032] In step of b), a density of the green compact may be in the range of 7.08 to 7.37 g/cm.sup.3 after the first sintering step.

[0033] step c): Performing a second sintering step while applying a pressure along the magnetic orientation direction of the green compact achieved by step b), wherein the pressure applied on the green compact is in the range of 1 MPa to 5 MPa and the sintering temperature is in the range of 720° C. to 850° C. for 15 to 60 minutes, and wherein the temperature of the first sintering step is at least 10° C. higher than the temperature of the second sintering step.

[0034] In other words, the magnet finished after the first-step sintering is performed a second-step sintering while applying a pressure on the magnet along the orientation direction. The sintering temperature is 720° C. to 850° C. with a duration time of 15 to 60 minutes. The pressure applied on the magnet is 1 MPa to 5 MPa. This step is finished under a vacuum atmosphere. The temperature in the first-step sintering is at least 10° C. higher than it in the second-step sintering.

[0035] step d): Subjecting the sintered magnet of step c) to an annealing treatment.

[0036] To have a better understanding of the present disclosure, the examples set forth below provide illustrations of the present disclosure. The examples are only used to illustrate the present disclosure and do not limit the scope of the present disclosure.

[0037] In order to exhibit the performance, the density of the magnet was separately tested after each sintering step. The magnetic properties of the final magnet were also determined. Backscattered electron (BSE) image of the magnet was taken by scanning electron microscope.

IMPLEMENTING EXAMPLE 1

[0038] A raw material including Pr—Nd (35.0 wt. %), B (0.95 wt. %), Co (1.0 wt. %), Al (0.55 wt. %), Cu (0.10 wt. %), Ga (0.40 wt. %), Ti (0.10 wt. %), and Fe as a balance, and unavoidable impurities is made into alloy flakes by a strip casting process. The alloy flakes are put into a hydrogen treatment furnace for normal hydrogen absorption and dehydrogenation. Then after milling powders by jet mill, molding and orientation, and cold isostatic pressing, a green compact was obtained. The green compact was put into vacuum furnace for the first-step sintering, the vacuum value is under 5×10.sup.−1 Pa. The sintering temperature is 830° C. for a duration time of 10 hours and then cooled down to room temperature. The magnet obtained by the first-step sintering is then subjected a second-step sintering at a temperature of 820° C. and at the same time a pressure of 1 MPa is applied on the magnet along the orientation direction under a vacuum condition. The duration time of sintering and pressing is 30 minutes, after which the magnet is cooled to room temperature. Then the magnet is heated to 500° C. for a duration time of 2 hours for the annealing treatment.

IMPLEMENTING EXAMPLE 2

[0039] A raw material including Pr—Nd (33.0 wt. %), B (0.95 wt. %), Co (1.0 wt. %), Al (0.55 wt. %), Cu (0.10 wt. %), Ga (0.40 wt. %), Ti (0.10 wt. %), and Fe as a balance, and unavoidable impurities is made into alloy flakes by a strip casting process. The alloy flakes are put into a hydrogen treatment furnace for normal hydrogen absorption and dehydrogenation. Then after milling powders by jet mill, molding and orientation, and cold isostatic pressing, a green compact was obtained. The green compact was put into vacuum furnace for the first-step sintering, the vacuum value is under 5×10.sup.−1 Pa. The sintering temperature is 880° C. for a duration time 2 hours and then cooled down to room temperature. The magnet obtained by the first-step sintering is then subjected a second-step sintering at a temperature of 720° C. and at the same time a pressure of 5 MPa is applied on the magnet along the orientation direction under a vacuum condition. The duration time of sintering and pressing is 60 minutes, after which the magnet is cooled to room temperature. Then the magnet is heated to 500° C. for a duration time of 2 hours for the annealing treatment.

IMPLEMENTING EXAMPLE 3

[0040] A raw material including Pr—Nd (37.0 wt. %), B (0.95 wt. %), Co (1.0 wt. %), Al (0.55 wt. %), Cu (0.10 wt. %), Ga (0.40 wt. %), Ti (0.10 wt. %), and Fe being present as a balance, and unavoidable impurities is made into alloy flakes by a strip casting process. The alloy flakes are put into a hydrogen treatment furnace for normal hydrogen absorption and dehydrogenation. Then after milling powders by jet mill, molding and orientation, and cold isostatic pressing, a green compact was obtained. The green compact was put into vacuum furnace for the first-step sintering, the vacuum value is under 5×10.sup.−1 Pa. The sintering temperature is 865° C. for a duration time 6 hours and then cooled down to room temperature. The magnet obtained by the first-step sintering is then subjected a second-step sintering with the temperature 850° C. and at the same time a pressure of 3 MPa is applied on the magnet along the orientation direction under a vacuum condition. The duration time of sintering and pressing is 15 minutes, after which the magnet is cooled to room temperature. Then the magnet is heated to 500° C. for a duration time of 2 hours for the annealing treatment.

Comparative Example 1

[0041] A raw material including Pr—Nd (35.0 wt. %), B (0.95 wt. %), Co (1.0 wt. %), Al (0.55 wt. %), Cu (0.10 wt. %), Ga (0.40 wt. %), Ti (0.10 wt. %), and Fe being present as a balance, and unavoidable impurities is made into alloy flakes by a strip casting process. The alloy flakes are put into a hydrogen treatment furnace for normal hydrogen absorption and dehydrogenation. Then after milling powders by jet mill, molding and orientation, and cold isostatic pressing, a green compact was obtained. The green compact was put into vacuum furnace for sintering, the vacuum value is under 5×10.sup.−1 Pa. Sintering temperature is 830° C. with a duration time of 10 hours. After cooled to room temperature the magnet is reheated to 500° C. for a duration time of 2 hours for the annealing treatment.

Comparative Example 2

[0042] A raw material including Pr—Nd (35.0 wt. %), B (0.95 wt. %), Co (1.0 wt. %), Al (0.55 wt. %), Cu (0.10 wt. %), Ga (0.40 wt. %), Ti (0.10 wt. %), and Fe being present as a balance, and unavoidable impurities is made into alloy flakes by a strip casting process. The alloy flakes are put into a hydrogen treatment furnace for normal hydrogen absorption and dehydrogenation. Then after milling by jet mill, molding and orientation, and cold isostatic pressing, a green compact was obtained. The green compact was put into vacuum furnace for sintering, the vacuum value is under 5×10.sup.−1 Pa. Sintering temperature is 930° C. with a duration time of 2 hours. After cooled to room temperature the magnet is reheated to 500° C. for a duration time of 2 hours for the annealing treatment.

Comparative Example 3

[0043] A raw material including Pr—Nd (35.0 wt. %), B (0.95 wt. %), Co (1.0 wt. %), Al (0.55 wt. %), Cu (0.10 wt. %), Ga (0.40 wt. %), Ti (0.10 wt. %), and Fe being present as a balance, and unavoidable impurities is made into alloy flakes by a strip casting process. The alloy flakes are put into a hydrogen treatment furnace for normal hydrogen absorption and dehydrogenation. Then after milling powders by jet mill, molding and orientation, and cold isostatic pressing, a green compact was obtained. The green compact was put into vacuum furnace for the first-step sintering, the vacuum value is under 5×10.sup.−1 Pa. The sintering temperature is 830° C. for a duration time 10 hours and then cool to room temperature. The magnet obtained by the first-step sintering is then subjected a second-step sintering with the temperature 700° C. and at the same time a pressure of 0.5 MPa is applied on the magnet along the orientation direction under a vacuum condition. The duration time of sintering and pressing is 30 minutes, after which the magnet is cooled to room temperature. Then the magnet is heated to 500° C. for a duration time of 2 hours for the annealing treatment.

[0044] Process parameters of implementing examples and comparative examples are listed in table 1.

TABLE-US-00001 TABLE 1 Process parameters of the examples Conditions of Content of Conditions of second-step sintering rare earth first-step sintering pres- Pr—Nd Temp. time Temp. time sure (wt. %) (° C.) (hours) (° C.) (minutes) (MPa) Implementing 35.0 830 10 820 30 1.0 example 1 Implementing 33.0 880 2 720 60 5.0 example 2 Implementing 37.0 865 6 850 15 3.0 example 3 Comparative 35.0 830 10 — — — example 1 Comparative 35.0 930 2 — — — example 2 Comparative 35.0 830 10 700 30 0.5 example 3

[0045] Density and magnetic properties of magnets in implementing and comparative examples are listed in table 2.

TABLE-US-00002 TABLE 2 Density and magnetic properties of magnets Magnetic density (g/cm.sup.3) properties after after Hcj first-step second-step Br (T) (kA/m) sintering sintering Implementing 1.250 1711 7.28 7.43 example 1 Implementing 1.305 1640 7.08 7.46 example 2 Implementing 1.213 1783 7.37 7.43 example 3 Comparative 1.226 1489 7.28 — example 1 Comparative 1.241 1656 7.41 — example 2 Comparative 1.236 1616 7.28 7.37 example 3

[0046] It can be seen from the test results of implementing examples 1, 2, and 3 that using the method of the present disclosure, the density of the magnet after the first step of sintering is low. And after the second step sintering with pressure at lower temperature, the density of the magnet can be significantly improved to 7.43 g/cm.sup.3 or higher. It also enables the magnet to have a higher remanence.

[0047] Microstructure of the magnets in implementing examples seems more densified than magnets in comparative examples according to the BSE images. And also there are more uniform grain boundary phase, which makes the magnets have higher coercivity. In comparative example 1, only the first step of sintering was carried out. The density of the magnet was low and seems less densified which makes both remanence and coercivity lower. Magnet in comparative example 2 was sintered by the traditional process at 930° C. It is easy to lead abnormal grain growth, which can be seen from FIG. 5 due to high amount of rare earth in the composition. The two-step sintering process proposed was also used in comparative example 3, but the pressure and temperature in the second sintering process were lower than the requirements of the present disclosure, resulting in a low magnet density after the second sintering process. It can be seen from FIG. 6 that the grain boundary phase distributes not as uniform as it in magnet of the implementing examples. This is one of the main reasons of poor coercivity in the comparative examples.

[0048] In summary, using the method of the present can significantly improve microstructure and magnetic properties of the NdFeB sintered magnet.