Method for preparing R-Fe-B based sintered magnet

Abstract

A method for preparing an RFeB based sintered magnet. The method includes: 1) preparing a R.sup.1FeB-M alloy, pulverizing the R.sup.1FeB-M alloy to yield a R.sup.1FeB-M alloy powder, adding a heavy rare earth powder of R.sup.2 or R.sup.2X and subsequently adding a lubricant to the R.sup.1FeB-M alloy powder and uniformly stirring to form a mixture, where R.sup.1 being Nd, Pr, Tb, Dy, La, Gd, Ho, or a mixture thereof; M being Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, Mo, or a mixture thereof; R.sup.2 being at least one from Tb, Dy, and Ho; X being at least one from O, F, and Cl; 2) pressing the mixture obtained in step 1) to form a compact, and sintering the compact in a pressure sintering device in vacuum or in an inactive gas atmosphere to obtain a magnet; and 3) aging the magnet obtained in step 2).

Claims

1. A method for preparing an RFeB based sintered magnet, the method comprising: 1) preparing an R.sup.1FeB-M alloy, pulverizing the R.sup.1FeB-M alloy to yield an R.sup.1FeB-M alloy powder, adding a powder of heavy rare earth metal R.sup.2 and subsequently adding a lubricant to the R.sup.1FeB-M alloy powder and stirring to form a uniform mixture, wherein the R.sup.1FeB-M alloy comprises between 27 wt. % and 33 wt. % (not including 27 wt. % and 33 wt. %) of R.sup.1 being at least one selected from the group consisting of Nd, Pr, Tb, Dy, La, Gd, and Ho; between 0.8 wt. % and 1.3 wt. % of B; and less than 5 wt. % of M being at least one selected from the group consisting of Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, and Mo; R.sup.2 is at least one selected from the group consisting of Tb, Dy, and Ho; and the R.sup.2 accounts for between 0.1 wt. % and 3 wt. % in total weight of the R.sup.1FeB-M alloy powder; 2) pressing the mixture obtained in step 1) to form a compact, and sintering the compact in a pressure sintering device in vacuum or in an inert gas atmosphere; the sintering of the compact comprising: degassing the compact in vacuum at a temperature of less than 970 C. for more than 45 min, and sintering the compact by applying a pressure of between 10 and 150 Megapascal at a temperature of between 930 and 970 C. to obtain a magnet having a magnet density of larger than 7.2 g/cm.sup.3, wherein the pressure applied in sintering the compact is obtained by increasing pressure at a rate less than 10 Megapascal/min; and 3) aging the magnet obtained in step 2) at a temperature between 400 and 600 C. for between 60 and 480 min in vacuum.

2. The method of claim 1, wherein the powder of heavy rare earth metal R.sup.2 in step 1) has a particle size of less than or equal to 100 m.

3. The method of claim 1, wherein the magnet obtained in step 3) comprises 1000 to 7000 ppm of oxygen, less than 1500 ppm of carbon, and less than 1200 ppm of nitrogen with respect to a weight of the magnet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is described hereinbelow with reference to the accompanying drawings, in which:

(2) FIG. 1 is a cross sectional view of a compact;

(3) FIG. 2 is a cross sectional view of a microstructure of a magnet after a whole sintering process;

(4) FIG. 3 is a structure diagram of a pressure sintering device in Example 1.

(5) In the drawings, the following reference numbers are used: 1. Upper and lower pressure head; 2. Mold cavity; 3. Heating chamber; 4. Compact.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(6) For further illustrating the invention, experiments detailing a method for preparing an RFeB based sintered magnet are described below. It should be noted that the following examples are intended to describe and not to limit the invention.

Example 1

(7) Raw materials at a certain ratio were melted in a vacuum melting furnace in an argon atmosphere to form a R.sup.1FeB-M alloy scale having a thickness of between 0.1 and 0.5 mm. The scale comprised: 5.69 wt. % of Pr, 18.22 wt. % of Nd, 6.18 wt. % of Dy, 0.98 wt. % of B, 1.51 wt. % of Co, 0.1 wt. % of Ga, 0.29 wt. % of Al, that is, the content of R.sup.1 accounted for 30.09 wt. % in total. The R.sup.1FeB-M alloy scale was pulverized by hydrogen decrepitation and jet milling to yield a powder having a particle size of 3.3 m. A heavy rare earth powder of R.sup.2 being Dy powder was added, and an average particle size of the Dy powder was 0.9 m. 10 Kg of R.sup.1FeB-M jet milled powder was added with 0.1 Kg of Dy powder for mixing for 3 hr, and added with 0.15 wt. % of a lubricant for mixing for another 3 hr. Thereafter, a resulting mixture was pressed for shaping using a 15 KOe magnetic field orientation to yield a compact having a density of 3.95 g/cm.sup.3.

(8) The compact was transferred to a pressure sintering device for vacuum sintering; a heating rate was controlled at 9 C./min during the whole heating process of the vacuum sintering. The vacuum sintering was specifically conducted as follows: the compact was firstly degassed at a temperature of 400 C. for 120 min and a temperature of 850 C. for 200 min, respectively, and was then sintered in the pressure sintering device in an inactive gas atmosphere to form a corresponding density. The pressure sintering device comprised: an upper and lower pressure head 1, a mold cavity 2, a heating chamber 3, and a compact 4 (as shown in FIG. 3). The pressure sintering temperature was controlled at 930 C., a pressure applied was 105 Megapascal, a holding time for the pressure sintering lasted for 30 min, a pressure growth was controlled at 5 Megapascal/min, and a resulting product was quenched to the room temperature after the holding time. Thereafter, the product was maintained at a temperature of 900 C. for 240 min in the absence of pressure. Finally, the product was processed with aging treatment for optimizing the microstructure at an aging temperature of 480 C. for an aging time of 300 min, and then quenched to the room temperature. Comparisons of magnetic property between magnet M.sub.1 prepared by the process of the Example and magnet M.sub.2 prepared by a common method based on the same content of heavy rare earth element was shown in Table 1.

(9) TABLE-US-00001 TABLE 1 Magnetic property of M.sub.1 and M.sub.2 Density Br Hcj (BH) max Hk/Hcj Unit Item (g/cm.sup.3) kGs kOe MGOe M.sub.2 of contrast example 7.63 12.39 26.57 37.63 0.91 M.sub.1 of Example 1 7.63 12.21 31.63 36.78 0.90

Example 2

(10) Raw materials at a certain ratio were melted in a vacuum melting furnace in an argon atmosphere to form a R.sup.1FeB-M alloy scale having an obvious metallurgical grain boundary and a thickness of between 0.1 and 0.5 mm. The scale comprised: 4.72 wt. % of Pr, 25.67 wt. % of Nd, 0.52 wt. % of Dy, 0.97 wt. % of B, 0.9 wt. % of Co, 0.1 wt. % of Ga, 0.1 wt. % of Al, that is, the content of R.sup.1 accounted for 30.91 wt. % in total. The R.sup.1FeB-M alloy scale was mechanically ground into a powder having a diameter of less than 2 mm, and was then ball-milled to form particles having an average particle size of 6 m. A heavy rare earth powder of R.sup.2 being Tb powder was added, an addition of the Tb powder accounted for 0.4 wt. % in total weight, and an average particle size of the Tb powder was 1.8 m. The above R.sup.1FeB-M powder and the Tb powder was mixed and ball-milled for 360 min for fully mixing the two. Thereafter, a resulting mixture was pressed for shaping using a 15 KOe magnetic field orientation. The compact was transferred to a pressure sintering device for degassing. The degassing of the compact was conducted at a temperature of 400 C. for 120 min and a temperature of 800 C. for 200 min, respectively. After the degassing treatment, pressure sintering was performed to produce a corresponding density, a pressure sintering temperature was controlled at 950 C., a pressure applied was 98 Megapascal, a holding time for the pressure sintering lasted for 30 min, and a resulting product was quenched to the room temperature after the holding time. Thereafter, the product was maintained at a temperature of 930 C. for 150 min. Finally, the product was process with aging treatment at an aging temperature of 500 C. for an aging time of 300 min. Comparisons of magnetic property between magnet M.sub.3 prepared by the process of the Example and magnet M.sub.4 prepared by a common method based on the same content of heavy rare earth element was shown in Table 2.

(11) TABLE-US-00002 TABLE 2 Magnetic property of M.sub.3 and M.sub.4 Density Br Hcj (BH) max Hk/Hcj Unit Item (g/cm.sup.3) kGs kOe MGOe M.sub.4 of Contrast example 7.56 14.09 13.46 47.09 0.97 M.sub.3 of Example 2 7.56 13.85 18.71 46.23 0.94

Example 3

(12) Raw materials at a certain ratio were melted in a vacuum melting furnace in vacuum or in an inactive gas atmosphere to form a R.sup.1FeB-M alloy scale having a thickness of between 0.1 and 0.5 mm. The scale comprised: 4.72 wt. % of Pr, 25.67 wt. % of Nd, 0.52 wt. % of Dy, 0.97 wt. % of B, 0.9 wt. % of Co, 0.1 wt. % of Ga, 0.1 wt. % of Al, that is, a content of R.sup.1 accounted for 30.91 wt. % in total. The R.sup.1FeB-M alloy scale was pulverized by hydrogen decrepitation and jet milling to yield a powder having a particle size of 3.2 m. A heavy rare earth powder of R.sup.2 or R.sup.2X being a mixed powder of Dy and Dy.sub.2O.sub.3 was added, and an average particle size of the mixed powder of Dy and Dy.sub.2O.sub.3 was 0.9 m. It was known from analysis that the mixed powder was composed of 93.55 wt. % of Dy and 6.45 wt. % of 0. A content of the mixed powder accounted for 1.6 wt. % of the total weight. The R.sup.1FeB-M alloy scale powder and the mixed powder of Dy and Dy.sub.2O.sub.3 were mixed for 3 hr and subsequently mixed for another 3 hr after being added with 0.15 wt. % of a lubricant. Thereafter, a resulting mixture was pressed for shaping using a 15 KOe magnetic field orientation to yield a compact having a density of 3.95 g/cm.sup.3.

(13) The compact was transferred to a pressure sintering device for vacuum sintering; a heating rate was controlled at 9 C./min during the whole heating process of the vacuum sintering. The vacuum sintering was specifically conducted as follows: the compact was firstly degassed at a temperature of 400 C. for 120 min and a temperature of 850 C. for 200 min, respectively. After that, between 5 and 10 KPa of argon was charged, and the compact was then sintered in the pressure sintering device in the argon atmosphere. A pressure sintering temperature was controlled at 910 C., a pressure applied was 115 Megapascal, a holding time for the pressure sintering lasted for 30 min, a pressure growth was controlled at 5 Megapascal/min, and a resulting product was quenched to the room temperature after the holding time, and a magnetic density was 7.42 g/cm.sup.3. Thereafter, the product was maintained at a temperature of 900 C. for 120 min in the absence of pressure for further optimizing the particle size. Finally, the product was processed with aging treatment for optimizing the microstructure at an aging temperature of 500 C. for an aging time of 300 min, and then quenched to the room temperature. Comparisons of magnetic property between magnet M.sub.5 prepared by the process of the Example and magnet M.sub.6 prepared by a common method based on the same content of heavy rare earth element was shown in Table 3.

(14) TABLE-US-00003 TABLE 3 Magnetic property of M.sub.5 and M.sub.6 Density Br Hcj (BH) max Hk/Hcj Unit Item (g/cm.sup.3) kGs kOe MGOe M.sub.6 of Contrast example 7.56 14.09 13.46 47.09 0.97 M.sub.5 of Example 3 7.57 13.88 19.13 46.17 0.93

Example 4

(15) A first step: raw materials at a certain ratio were melted in a vacuum melting furnace in vacuum or in an inactive gas atmosphere to form a R.sup.1FeB-M alloy scale having a thickness of between 0.1 and 0.5 mm. The scale comprised: 5.88 wt. % of Pr, 22.4 wt. % of Nd, 0.7 wt. % of Dy, 0.5 wt. % of Tb, 0.99 wt. % of B, 0.6 wt. % of Co, 0.15 wt. % of Ga, 0.1 wt. % of Al, that is, a content of R.sup.1 accounted for 29.48 wt. % in total. The R.sup.1FeB-M alloy scale was pulverized by hydrogen decrepitation and jet milling to yield a powder having a particle size of 3.1 m. A heavy rare earth powder of R.sup.2 and R.sup.2X being a mixed powder of DyF.sub.3 and Dy.sub.2O.sub.3 at a ratio of 1:1 were added, and an average particle size of the mixed powder of DyF.sub.3 and Dy.sub.2O.sub.3 was 0.8 m. A content of the mixed powder accounted for 0.5 wt. % of the total weight. The R.sup.1FeB-M alloy scale powder and the mixed powder of DyF.sub.3 and Dy.sub.2O.sub.3 were mixed for 3 hr and subsequently mixed for another 3 hr after being added with 0.15 wt. % of a lubricant. Thereafter, a resulting mixture was pressed for shaping using a 15 KOe magnetic field orientation to yield a compact having a density of 3.95 g/cm.sup.3.

(16) A second step: the compact was transferred to a pressure sintering device for vacuum sintering, the compact was degassed at a temperature of 400 C. for 120 min and a temperature of 800 C. for 240 min, respectively. After that, the compact was sintered in the pressure sintering device to yield a corresponding density. A pressure sintering temperature was controlled at 920 C., a pressure applied was 110 Megapascal, a holding time for the pressure sintering lasted for 30 min, and a resulting product was quenched to the room temperature after the holding time.

(17) A third step: the product was maintained at a temperature of 900 C. for 150 min in the absence of pressure for further optimizing the particle size. Finally, the product was aged at a temperature of 490 C. for 300 min. Comparisons of magnetic property between magnet M.sub.7 prepared by the process of the Example and magnet Mg prepared by a common method based on the same content of heavy rare earth element was shown in Table 4.

(18) TABLE-US-00004 TABLE 4 Magnetic property of M.sub.8 and M.sub.7 Density Br Hcj (BH) max Hk/Hcj Unit Item (g/cm.sup.3) kGs kOe MGOe M.sub.8 of Contrast example 7.57 14.31 15.42 48.73 0.99 M.sub.7 of Example 4 7.58 14.04 20.92 47.61 0.94

(19) Unless otherwise indicated, the numerical ranges involved in the invention include the end values.

(20) While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.