Method and device for preparing a sintered Nd—Fe—B permanent magnet
10274257 · 2019-04-30
Assignee
Inventors
Cpc classification
H01F1/0573
ELECTRICITY
C22C33/0207
CHEMISTRY; METALLURGY
International classification
F27D7/06
MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
Abstract
The present invention is directed to a method for preparing a permanent magnet, and more specifically, to a method for preparing a high-performance sintered NdFeB permanent magnet, in order to solve the problems of increased brittleness or high cost present in the permanent magnet prepared by the existing process. A method for preparing a sintered NdFeB permanent magnet includes the step of ingredient calculation and raw material preparation including calculating ingredients and preparing raw materials according to the ingredient formula of the resultantly sintered NdFeB permanent magnet, and dividing the raw materials into a rare earth FeB compound and rare earth metals.
Claims
1. A method for preparing a sintered NdFeB permanent magnet, including the following steps: (1) ingredient calculation comprising calculating ingredients according to the ingredient formula of the resultantly sintered NdFeB permanent magnet, the ingredient formula, in mass fraction, being (Nd.sub.A-zRE.sub.z).sub.A(Fe.sub.J-yM.sub.y).sub.JB.sub.C, wherein RE represents one or more of rare earth elements except Nd, M represents one or more among the metal elements Al, Ga, Cu, Nb, Mo, W, V, Ta, Cr, Ti, Zr, Hf, Si, Ni, Sn, Mn, 28<A33, A>z0, C ranges from 0.95 to 1.03, J>y0, and A+C+J=100; (2) weighing and preparing raw materials for a rare earth FeB compound according to the formula, in mass fraction, of (Nd.sub.28-gRE.sub.g).sub.28(Fe.sub.J-yM.sub.y).sub.JB.sub.C, wherein 28>g>0, vacuum fusing the weighed and prepared raw materials for the rare earth FeB compound, and condensing them into a casting alloy of the rare earth FeB compound, followed by hydrogen absorption to decrepitate the casting alloy into hydride powders of the rare earth FeB compound, then heating the hydride powders of the rare earth FeB compound to a temperature between 400 C. and 420 C. for thermal insulation to perform dehydrogenation until the hydrogen content of the hydride powders of the rare earth FeB compound is below 50 ppm, thereby forming dehydrogenated powders of the rare earth FeB compound; (3) separately from the weighing and preparing of the raw materials for the rare earth FeB compound, and separately from the condensing, the hydrogen absorption, and the dehydrogenation in step (2) to form dehydrogenated powders of the rare earth FeB compound, weighing and preparing raw materials consisting of rare earth metals according to the formula, in mass fraction, of (Nd.sub.A-28-hRE.sub.h).sub.A-28, wherein A-h>28 and g+h=z, performing hydrogen absorption on the weighed and prepared raw materials for the rare earth metals to decrepitate into hydride powders of the rare earth metals, then heating the hydride powders of the rare earth metals to a temperature between 830 C. and 860 C. for thermal insulation to perform dehydrogenation until the hydrogen content of the hydride powders of the rare earth metals is below 50 ppm, thereby forming dehydrogenated powders of the rare earth metals; and (4) mixing the dehydrogenated powders of both the rare earth FeB compound and the rare earth metals prepared respectively in steps (2) and (3), then airflow pulverizing them into fine powders, followed by magnetic field orienting and shaping, sintering and tempering, whereby the sintered NdFeB permanent magnet is obtained.
2. The method of claim 1, wherein, in steps (2) and (3), hydrogen absorption and decrepitation, and dehydrogenation of the rare earth FeB compound and the rare earth metals are performed in a vacuum furnace.
3. The method of claim 2, wherein, during the hydrogen absorption and decrepitation of step (2), the rare earth FeB compound is wrapped with a 1 mm-thick silica fire retardant cloth and put into an iron container in a charging amount not exceeding one seventh of a volume of the iron container.
4. The method of claim 2, wherein, during the hydrogen absorption and decrepitation of step (3), the rare earth metals are wrapped with a 1 mm-thick silica fire retardant cloth and put into an iron container in a charging amount not exceeding one seventh of a volume of the iron container.
5. The method of claim 3, further comprising, after the dehydrogenation of the hydride powders of the rare earth FeB compound: initially cooling the powders of the rare earth FeB compound after dehydrogenation to a first temperature below 80 C. under the protection of argon in the vacuum furnace; next, sealingly jointing the vacuum furnace with an anti-oxidation device and inflating the anti-oxidation device with argon until the oxygen content is below 0.1%; transferring the iron container with the dehydrogenated powders of the rare earth FeB compound from the vacuum furnace into the anti-oxidation device by using a discharging mechanism of the anti-oxidation device; cooling the powders to a second temperature less than the first temperature, the second temperature being below 20 C., through a fan of the anti-oxidation device; and unwrapping the fire retardant cloth having the dehydrogenated powders of the rare earth FeB compound to collect the dehydrogenated powders of the rare earth FeB compound into a storage tank connected with the anti-oxidation device, with an antioxidant accounting for 0.15% of the total weight of the dehydrogenated powders of the rare earth FeB compound to be prepared for use, wherein the anti-oxidation device includes a housing, with one end sealed and the other end opened and installed with a flange, in which an inflating port and an exhausting port are provided with valves, wherein a discharging port connected with the storage tank through a valve is provided at the bottom of the housing, a plurality of operating ports each of which is sealingly attached to a rubber sleeve are provided on the sidewalls of the housing, and the fan and the discharging mechanism are installed inside the housing, wherein the discharging mechanism includes a lifting mechanism installed therein, at the bottom of the housing, above which a base body is installed, and a telescope boom capable of stretching out from the opening end of the housing is slidingly connected with the base body through a track.
6. The method of claim 4, further comprising, after the dehydrogenation of the hydride powders of the rare earth metals: initially cooling the powders of the rare earth metals after dehydrogenation to a first temperature below 80 C. under the protection of argon in the vacuum furnace; next, sealingly jointing the vacuum furnace with an anti-oxidation device and inflating the anti-oxidation device with argon until the oxygen content is below 0.1%; transferring the iron container with the dehydrogenated powders of the rare earth metals from the vacuum furnace into the anti-oxidation device by using a discharging mechanism of the anti-oxidation device; cooling the powders to a second temperature less than the first temperature, the second temperature being below 20 C., through a fan of the anti-oxidation device; and unwrapping the fire retardant cloth having the dehydrogenated powders of the rare earth metals to collect the dehydrogenated powders of the rare earth metals into a storage tank connected with the anti-oxidation device, with an antioxidant accounting for 0.15% of the total weight of the dehydrogenated powders of the rare earth metals to be prepared for use, wherein the anti-oxidation device includes a housing, with one end sealed and the other end opened and installed with a flange, in which an inflating port and an exhausting port are provided with valves, wherein a discharging port connected with the storage tank through a valve is provided at the bottom of the housing, a plurality of operating ports each of which is sealingly attached to a rubber sleeve are provided on the sidewalls of the housing, and the fan and the discharging mechanism are installed inside the housing, wherein the discharging mechanism includes a lifting mechanism installed therein, at the bottom of the housing, above which a base body is installed, and a telescope boom capable of stretching out from the opening end of the housing is slidingly connected with the base body through a track.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1)
(2)
(3)
DENOTATION OF ACCOMPANYING DRAWINGS
(4) 1housing 2inflating port 3exhausting port 4storage tank 5discharging port 6operating port 7cooling device 8base body 9telescope boom 10lifting mechanism 100flange 101furnace body 102scaffold 103furnace door.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
Example 1
(5) A method for preparing a sintered NdFeB permanent magnet includes the following steps:
(6) (1) ingredient calculation and raw material preparation in which calculating ingredients and preparing raw materials according to the ingredient formula of the resultantly sintered NdFeB permanent magnet in mass fraction, i.e., (Nd.sub.24.51Pr.sub.5.49).sub.30(Fe.sub.68.85Ga.sub.0.2).sub.69.05B.sub.0.95, in which 24.51%+5.49%+68.85%+0.2%+0.95%=100%; then dividing the raw materials into a rare earth FeB compound and rare earth metals, the formula of the rare earth FeB compound in mass fraction being (Nd.sub.22.876Pr.sub.5.124).sub.28(Fe.sub.68.85Ga.sub.0.2).sub.69.05B.sub.0.95 and that of the rare earth metals being (Nd.sub.1.634Pr.sub.0.366).sub.2; based on 6 times of the calculation of the above formulas, weighing and preparing the raw materials for the rare earth FeB compound, that is, 168 kg of NdPr alloy (in which Pr accounts for 18.3% of the total) having 137.256 kg of Nd and 30.744 kg of Pr, 27.94 kg of FeB alloy (in which B accounts for 20.4% of the total), 1.2 kg of metal Ga, and 390.86 kg of pure iron, which together amount to 588 kg; after that, weighing and preparing the raw materials for the rare earth metals, that is, 12 kg of NdPr alloy (in which Pr accounts for 18.3% of the total); however, in practical production, in order to control the production cost and realize industrialization, 100 kg or more is usually prepared at a time;
(7) (2) according to the formula of the rare earth FeB compound in mass fraction, vacuum fusing the weighed and prepared raw materials (588 kg in total) and quickly condensing them into a casting alloy of the rare earth FeB compound; then wrapping it loosely with a 1 mm-thick high silica fire retardant cloth (which can be used in a long term in an environment of 1000 C.) to be put into an iron container, whose charging amount can not exceed one seventh of its volume; putting the iron container into a vacuum sintering furnace which is then vacuumized to below 0.1 Pa and inflated with hydrogen to absorb hydrogen; heating it after the hydrogen absorption reaches saturation, at the same time, starting a vacuum extraction unit, performing 4-hour thermal insulation as the temperature is increased to 400 C., and then performing dehydrogenation until the hydrogen content being below 50 ppm; inflating the vacuum sintering furnace with argon after the thermal insulation and starting the cooling means (such as fan) of the vacuum sintering furnace to quickly reduce the temperature to below 80 C.; jointing the vacuum sintering furnace with an anti-oxidation device and inflating the anti-oxidation device with argon until the oxygen content being below 0.1%, then supplementing the sintering furnace with argon to make the inside pressure back to the normal level; opening the furnace door of the sintering furnace under the protection of argon in the anti-oxidation device and transferring the container with hydride powders from the vacuum sintering furnace into the anti-oxidation device by a discharging mechanism of the anti-oxidation device; cooling the powders to a temperature below 20 C. through the cooling means (such as fan) of the anti-oxidation device and unwrapping the fire retardant cloth to collect the hydride powders into a storage tank connected with the anti-oxidation device, with an antioxidant (commonly used in this field) accounting for 0.15% of the total weight added therein to be prepared for use;
(8) (3) according to the formula of the rare earth metals in mass fraction, putting 100 kg of the weighed and prepared raw materials of NdPr alloy (in which Pr accounts for 18.3% of the total), wrapped loosely with a 1 mm-thick high silica fire retardant cloth (which can be used in a long term in an environment of 1000 C.), into a containing plate to be put into the vacuum sintering furnace which is then vacuumized to below 0.1 Pa and inflated with hydrogen to absorb hydrogen; heating it after the hydrogen absorption reaches saturation, at the same time, starting the vacuum extraction unit, performing 5-hour thermal insulation as the temperature is increased to 860 C., and then performing dehydrogenation until the hydrogen content being below 50 ppm; inflating the vacuum sintering furnace with argon after the thermal insulation and starting the cooling fan of the vacuum sintering furnace to quickly reduce the temperature to below 80 C.; jointing the vacuum sintering furnace with the anti-oxidation device and inflating the anti-oxidation device with argon until the oxygen content being below 0.1%, then supplementing the sintering furnace with argon to make the inside pressure back to the normal level; opening the furnace door of the sintering furnace under the protection of argon in the anti-oxidation device and transferring the container with hydride powders from the vacuum sintering furnace into the anti-oxidation device by the discharging mechanism of the anti-oxidation device; cooling the powders to a temperature below 20 C. through the cooling means (such as fan) of the anti-oxidation device and unwrapping the fire retardant cloth to collect the hydride powders into the storage tank connected with the anti-oxidation device, with the antioxidant (commonly used in this field) accounting for 0.15% of the total weight added therein to be prepared for use;
(9) (4) weighing and mixing 588 kg of the hydride powders of the rare earth FeB compound and 12 kg of the hydride powders of the rare earth metals prepared respectively in steps (2) and (3), then airflow pulverizing them into fine powders; after mixing the powders for two hours, shaping them into a compact of 56 mm40 mm36 mm by orienting the magnet fields; putting the compact into the vacuum sintering furnace to sinter and temper; and finally the sintered NdFeB permanent magnet with excellent machineability is obtained.
(10) In addition, according to the frequently adopted process in the existing technologies, 6 times of the amount of the raw materials is calculated based on the proportion of the mass fraction with 24.51% of Nd, 5.49% of Pr, 0.95% of B, 0.2% of Ga, and 68.85% of Fe. The raw materials are weighed and prepared, and 180 kg of NdPr alloy (in which Pr accounts for 18.3% of the total), 27.94 kg of FeB alloy (in which B accounts for 20.4% of the total), 1.2 kg of metal Ga, and 390.86 kg of pure iron, which together amount to 600 kg, are put into the vacuum fusion furnace to fuse and quickly condense into a casting alloy. This casting alloy is put into a hydrogen decrepitation furnace, which is then vacuumized to below 0.1 Pa and inflated with hydrogen to absorb hydrogen. It is heated after the hydrogen absorption reaches the saturation, at the same time, the vacuum extraction unit is started, and 10-hour thermal insulation is performed as the temperature is increased to 550 C. to perform dehydrogenation. The vacuum sintering furnace is inflated with argon after the thermal insulation and the cooling mechanism of the hydrogen decrepitation furnace is started to perform cooling. After being cooled, the casting alloy is airflow pulverized and the pulverized powders are mixed for two hours, with the antioxidant accounting for 0.15% of the total weight added therein before the mixture of the powders. After that, the powders are shaped into a compact of 56 mm40 mm36 mm by orienting the magnet fields and then put into the vacuum sintering furnace to sinter and temper.
(11) The magnetic properties of the sintered NdFeB permanent magnets prepared respectively in Example 1 based on the existing technology and the method of the present invention are tested. The two square magnets with specifications of 56 mm40 mm36 mm are machined, including grinded, cut and punched, etc., to be shaped as an annulus having an outside diameter of 4.3 mm, an inner diameter of 2.2 mm and a height of 2 mm. After the annulus being chamfered, polished, plated and finished, a complete inspection on cracks is performed. The comparative data of Example 1 is listed in Table 1.
(12) TABLE-US-00001 TABLE 1 Using the method Using the method of the in the existing Item present invention technology Magnet Average 14.17 (KGs) 14.13 (KGs) properties value of Remanence Br Average 11.25 (KOe) 11.26 (KOe) value of Coercivity Hci Average of 47.58 (MGOe) 47.42 (MGOe) magnetic energy product (BH) max Hydrogen content after Rare earth FeB 773 ppm dehydrogenation compound, 36 ppm; Rare earth metals, 42 ppm Dehydrogenation 9 hours together for 10 hours time the rare earth FeB compound and the rare earth metals Fine crack ratio after 0.16% 8.7% being cut into small pieces of magnet
(13) From Table 1, it can be seen that in case of the substantially same preparation proportion, with different processes of casting and dehydrogenation and same processes of airflow pulverizing, mixing, magnet field orienting and shaping, vacuum sintering and tempering, the two sintered NdFeB magnets differ little in averages of remanence, magnetic energy product and coercivity, that is, the magnetic properties are almost same. With nearly the same dehydrogenation time, few fine cracks in the sintered NdFeB permanent magnet prepared by the method of the present invention show that, with the substantially same preparation proportion, application of the method of the present invention guarantees the magnetic properties of the sintered NdFeB permanent magnet and meanwhile, machineability of product is greatly improved, so that prominent economic effects are achieved.
Example 2
(14) A method for preparing a sintered NdFeB permanent magnet includes the following steps:
(15) (1) ingredient calculation and raw material preparation in which calculating ingredients and preparing raw materials according to the ingredient formula of the resultantly sintered NdFeB permanent magnet in mass fraction, i.e., (Nd.sub.23.718Pr.sub.5.782Dy.sub.2).sub.31.5(Fe.sub.64.82Al.sub.0.5Ga.sub.0.3Zr.sub.0.2Co.sub.1.5Cu.sub.0.15).sub.67.47B.sub.1.03, in which 23.718%+5.782%+2%+64.82%+0.5%+0.3%+0.2%+1.5%+0.15%+1.03% 100%; then dividing the raw materials into a rare earth FeB compound and rare earth metals, the formula of the rare earth FeB compound in mass fraction being (Nd.sub.20.904P.sub.5.096Dy.sub.2).sub.28(Fe.sub.64.82Al.sub.0.5Ga.sub.0.3Zr.sub.0.2Co.sub.1.5Cu.sub.0.15).sub.67.47B.sub.1.03 and that of the rare earth metals being (Nd.sub.2.814Pr.sub.0.686).sub.3.5; based on 6 times of the calculation of the above formulas, weighing and preparing the raw materials for the rare earth FeB compound, that is, 156 kg of NdPr alloy (in which Pr accounts for 19.6% of the total), 12 kg of Dy, 27.225 kg of FeB alloy (in which B accounts for 22.7% of the total), 3 kg of metal Al, 1.8 kg of Ga, 1.2 kg of Zr, 9 kg of Co, 0.9 kg of Cu, and 367.875 kg of pure iron, which together amount to 579 kg; after that, weighing and preparing the raw materials for the rare earth metals, that is, 21 kg of NdPr alloy (in which Pr accounts for 19.6% of the total); however, in practical production, in order to control the production cost and realize industrialization, 100 kg or more is usually prepared at a time;
(16) (2) according to the formula of the rare earth FeB compound in mass fraction, vacuum fusing the raw materials (579 kg in total) and quickly condensing them into a casting alloy of the rare earth FeB compound; then wrapping it loosely with a 1 mm-thick high silica fire retardant cloth to be put into an iron container, whose charging amount can not exceed one seventh of its volume; putting the iron container into a vacuum sintering furnace which is then vacuumized to below 0.1 Pa and inflated with hydrogen to absorb hydrogen; heating it after the hydrogen absorption reaches saturation, at the same time, starting a vacuum extraction unit, performing 6-hour thermal insulation as the temperature is increased to 420 C., and then performing dehydrogenation until the hydrogen content being below 50 ppm; inflating the vacuum sintering furnace with argon after the thermal insulation and starting the cooling fan of the vacuum sintering furnace to quickly reduce the temperature to below 80 C.; jointing the vacuum sintering furnace with an anti-oxidation device and inflating the anti-oxidation device with argon until the oxygen content being below 0.1%; then supplementing the sintering furnace with argon to make the inside pressure back to the normal level; opening the furnace door of the sintering furnace under the protection of argon in the anti-oxidation device and transferring the container with hydride powders from the vacuum sintering furnace into the anti-oxidation device by a discharging mechanism of the anti-oxidation device; cooling the powders to a temperature below 20 C. through the cooling fan of the anti-oxidation device and unwrapping the fire retardant cloth to collect the hydride powders into a storage tank connected with the anti-oxidation device, with an antioxidant accounting for 0.15% of the total weight added therein to be prepared for use;
(17) (3) according to the formula of the rare earth metals in mass fraction, putting 100 kg of the raw materials of NdPr alloy (in which Pr accounts for 19.6% of the total), wrapped loosely with a 1 mm-thick high silica fire retardant cloth, into a containing plate to be put into the vacuum sintering furnace which is then vacuumized to below 0.1 Pa and inflated with hydrogen to absorb hydrogen; heating it after the hydrogen absorption reaches saturation, at the same time, starting the vacuum extraction unit, performing 7-hour thermal insulation as the temperature is increased to 830 C., and then performing dehydrogenation until the hydrogen content being below 50 ppm; inflating the vacuum sintering furnace with argon after the thermal insulation and starting the cooling fan of the vacuum sintering furnace to quickly reduce the temperature to below 80 C.; jointing the vacuum sintering furnace with the anti-oxidation device and inflating the anti-oxidation device with argon until the oxygen content being below 0.1%, then supplementing the sintering furnace with argon to make the inside pressure back to the normal level; opening the furnace door of the sintering furnace under the protection of argon in the anti-oxidation device and transferring the container with hydride powders from the vacuum sintering furnace into the anti-oxidation device by the discharging mechanism of the anti-oxidation device; cooling the powders to a temperature below 20 C. through the cooling fan of the anti-oxidation device and unwrapping the fire retardant cloth to collect the hydride powders into the storage tank connected with the anti-oxidation device, with the antioxidant accounting for 0.15% of the total weight added therein to be prepared for use;
(18) (4) weighing and mixing 579 kg of the hydride powders of the rare earth FeB compound and 21 kg of the hydride powders of the rare earth metals prepared respectively in steps (2) and (3), then airflow pulverizing them into fine powders; after mixing the powders for two hours, shaping them into a compact of 56 mm40 mm36 mm by orienting the magnet fields; putting the compact into the vacuum sintering furnace to sinter and temper; and finally the sintered NdFeB permanent magnet with excellent machineability is obtained.
(19) In addition, according to the frequently adopted process in the existing technologies, 6 times of the amount of the raw materials is calculated based on the proportion of the mass fraction with 23.718% of Nd, 5.782% of Pr, 2% of Dy, 1.03% of B, 0.5% of Al, 0.3% of Ga, 0.2% of Zr, 1.5% of Co, 0.15% of Cu, and 64.82% of Fe. The raw materials are weighed and prepared, and 177 kg of NdPr alloy (in which Pr accounts for 19.6% of the total), 12 kg of Dy, 27.225 kg of FeB alloy (in which B accounts for 22.7% of the total), 3 kg of metal Al, 1.8 kg of Ga, 1.2 kg of Zr, 9 kg of Co, 0.9 kg of Cu, and 367.875 kg of pure iron, which together amount to 600 kg, are put into the vacuum fusion furnace to fuse and quickly condense into a casting alloy. This casting alloy is put into a hydrogen decrepitation furnace, which is then vacuumized to below 0.1 Pa and inflated with hydrogen to absorb hydrogen. It is heated after the hydrogen absorption reaches the saturation, at the same time, the vacuum extraction unit is started, and 12-hour thermal insulation is performed as the temperature is increased to 590 C. to perform dehydrogenation. The vacuum sintering furnace is inflated with argon after the thermal insulation and the cooling mechanism of the hydrogen decrepitation furnace is started to perform cooling. After being cooled, the casting alloy is airflow pulverized and the pulverized powders are mixed for two hours, with the antioxidant accounting for 0.15% of the total weight added therein before the mixture of the powders. After that, the powders are shaped into a compact of 56 mm40 mm36 mm by orienting the magnet fields and then put into the vacuum sintering furnace to sinter and temper.
(20) The magnetic properties of the sintered NdFeB permanent magnets prepared respectively in Example 2 based on the existing technology and the method of the present invention are tested. The two square magnets with specifications of 56 mm40 mm36 mm are machined, including grinded, cut and punched, etc., to be shaped as an annulus having an outside diameter of 4.3 mm, an inner diameter of 2.2 mm and a height of 2 mm. After the annulus being chamfered, polished, plated and finished, a complete inspection on cracks is performed. The comparative data of Example 2 is listed in Table 2.
(21) TABLE-US-00002 TABLE 2 Using the method Using the method of the in the existing Item present invention technology Magnet Average 13.02 (KGs) 13.16 (KGs) Properties value of remanence Br Average 18.84 (KOe) 18.79 (KOe) value of Coercivity Hci Average 40.26 (MGOe) 41.13 (MGOe) value of magnetic energy product (BH) max Hydrogen content after Rare earth FeB 1325 ppm dehydrogenation compound, 43 ppm; rare earth metals, 43 ppm Dehydrogenation 13 hours together for 12 hours time the rare earth FeB compound and the rare earth metals Fine crack ratio after 0.27% 11.14% being cut into small pieces of magnet
(22) From Table 2, it can be seen that in case of the substantially same preparation proportion, with different processes of casting and dehydrogenation and same processes of airflow pulverization, mixing, magnet field orienting and shaping, vacuum sintering and tempering, the two sintered NdFeB magnets differs little in remanence, magnetic energy product and coercivity, that is, the magnetic properties are almost same. With nearly the same dehydrogenation time, few fine cracks in the sintered NdFeB permanent magnet prepared by the method of the present invention show that, with the substantially same preparation proportion, application of the method of the present invention guarantees the magnetic properties of the sintered NdFeB permanent magnet and meanwhile, machineability of product is greatly improved, so that prominent economic effects are achieved.
Example 3
(23) A method for preparing a sintered NdFeB permanent magnet includes the following steps:
(24) (1) ingredient calculation and raw material preparation in which calculating ingredients and preparing raw materials according to the ingredient formula of the resultantly sintered NdFeB permanent magnet in mass fraction, i.e., (Nd.sub.24.645Pr.sub.6.355Gd.sub.1).sub.32(Fe.sub.65.9Al.sub.0.8Nb.sub.0.3).sub.67B.sub.1, in which 24.645%+6.355%+1%+65.9%+0.8%+0.3%+1%=100%; then dividing the raw materials into a rare earth FeB compound and rare earth metals, the formula of the rare earth FeB compound in mass fraction being (Nd.sub.21.465Pr.sub.5.535Gd.sub.1).sub.28(Fe.sub.65.9Al.sub.0.8Nb.sub.0.3).sub.67B.sub.1 and that of the rare earth metals being (Nd.sub.3.18Pr.sub.0.82).sub.4; based on 6 times of the calculation of the above formulas, weighing and preparing the raw materials for the rare earth FeB compound, that is, 162 kg of NdPr alloy (in which Pr accounts for 20.5% of the total), 6 kg of Gd, 29.412 kg of FeB alloy (in which B accounts for 20.4% of the total), 4.8 kg of metal Al, 2.77 kg of NbFe alloy (in which Nb accounts for 65% of the total), and 371.018 kg of pure iron, which together amount to 576 kg; after that, weighing and preparing the raw materials for the rare earth metals, that is, 24 kg of NdPr alloy (in which Pr accounts for 20.5% of the total); however, in practical production, in order to control the production cost and realize industrialization, 100 kg or more is usually prepared at a time;
(25) (2) according to the formula of the rare earth FeB compound in mass fraction, vacuum fusing the raw materials (576 kg in total) and quickly condensing them into casting alloy of the rare earth FeB compound; then wrapping it loosely with a 1 mm-thick high silica fire retardant cloth to be put into an iron container, whose charging amount can not exceed one seventh of its volume; putting the iron container into a vacuum sintering furnace which is then vacuumized to below 0.1 Pa and inflated with hydrogen to absorb hydrogen; heating it after the hydrogen absorption reaches saturation, at the same time, starting a vacuum extraction unit, and performing 7-hour thermal insulation as the temperature is increased to 410 C., and then performing dehydrogenation until the hydrogen content being below 50 ppm; inflating the vacuum sintering furnace with argon after the thermal insulation and starting the cooling fan of the vacuum sintering furnace to quickly reduce the temperature to below 80 C.; jointing the vacuum sintering furnace with an anti-oxidation device and inflating the anti-oxidation device with argon until the oxygen content being below 0.1%; then supplementing the sintering furnace with argon to make the inside pressure back to the normal level; opening the furnace door of the sintering furnace under the protection of argon in the anti-oxidation device and transferring the container with hydride powders from the vacuum sintering furnace into the anti-oxidation device by a discharging mechanism of the anti-oxidation device; cooling the powders to a temperature below 20 C. through the cooling fan of the anti-oxidation device and unwrapping the fire retardant cloth to collect the hydride powders into a storage tank connected with the anti-oxidation device, with an antioxidant accounting for 0.15% of the total weight added therein to be prepared for use;
(26) (3) according to the formula of the rare earth metals in mass fraction, putting raw 100 kg of the raw materials of NdPr alloy (in which Pr accounts for 20.5% of the total), wrapped loosely with a 1 mm-thick high silica fire retardant cloth, into a containing plate to be put into the vacuum sintering furnace which is then vacuumized to below 0.1 Pa and inflated with hydrogen to absorb hydrogen; heating it after the hydrogen absorption reaches saturation, at the same time, starting the vacuum extraction unit, and performing 6-hour thermal insulation as the temperature is increased to 840 C., and then performing dehydrogenation until the hydrogen content being below 50 ppm; inflating the vacuum sintering furnace with argon after the thermal insulation and starting the cooling fan of the vacuum sintering furnace to quickly reduce the temperature to below 80 C.; jointing the vacuum sintering furnace with the anti-oxidation device and inflating the anti-oxidation device with argon until the oxygen content being below 0.1%, then supplementing the sintering furnace with argon to make the inside pressure back to the normal level; opening the furnace door of the sintering furnace under the protection of argon in the anti-oxidation device and transferring the container with hydride powders from the vacuum sintering furnace into the anti-oxidation device by the discharging mechanism of the anti-oxidation device; cooling the powders to a temperature below 20 C. through the cooling fan of the anti-oxidation device and unwrapping the fire retardant cloth to collect the hydride powders into the storage tank connected with the anti-oxidation device, with the antioxidant accounting for 0.15% of the total weight added therein to be prepared for use;
(27) (4) weighing and mixing 576 kg of the hydride powders of the rare earth FeB compound and 24 kg of the hydride powders of the rare earth metals prepared respectively in steps (2) and (3), then airflow pulverizing them into fine powders; after mixing the powders for two hours, shaping them into a compact of 56 mm40 mm36 mm by orienting the magnet fields; putting the compact into the vacuum sintering furnace to sinter and temper; and finally the sintered NdFeB permanent magnet with excellent machineability is obtained.
(28) In addition, according to the frequently adopted process in the existing technologies, 6 times of the amount of the raw materials is calculated based on the proportion of the mass fraction with 24.645% of Nd, 6.355% of Pr, 1% of Gd, 1% of B. 0.8% of Al, 0.3% of Nb, and 65.9% of Fe. The raw materials are weighed and prepared, and 186 kg of NdPr alloy (in which Pr accounts for 20.5% of the total), 6 kg of Gd, 29.412 kg of FeB alloy (in which B accounts for 20.4% of the total), 4.8 kg of metal Al, 2.77 kg of NbFe alloy (in which Nb accounts for 65% of the total), and 371.018 kg of pure iron, which together amount to 600 kg, are put into the vacuum fusion furnace to fuse and quickly condense into a casting alloy. This casting alloy is put into a hydrogen decrepitation furnace, which is then vacuumized to below 0.1 Pa and inflated with hydrogen to absorb hydrogen. It is heated after the hydrogen absorption reaches the saturation, at the same time, the vacuum extraction unit is started, and 14-hour thermal insulation is performed as the temperature is increased to 580 C. to perform dehydrogenation. The vacuum sintering furnace is inflated with argon after the thermal insulation and the cooling mechanism of the hydrogen decrepitation furnace is started to perform cooling. After being cooled, the casting alloy is airflow pulverized and the pulverized powders are mixed for two hours, with the antioxidant accounting for 0.15% of the total weight added therein before the mixture of powders. After that, the powders are shaped into a compact of 56 mm40 mm36 mm by orienting the magnet fields and then put into the vacuum sintering furnace to sinter and temper.
(29) The magnetic properties of the sintered NdFeB permanent magnets prepared respectively in Example 3 based on the existing technology and the method of the present invention are tested. The two square magnets with specifications of 56 mm40 mm36 mm are machined, including grinded, cut and punched, etc., to be shaped as an annulus having an outside diameter of 4.3 mm, an inner diameter of 2.2 mm and a height of 2 mm. After the annulus being chamfered, polished, plated and finished, a complete inspection on cracks is performed. The comparative data of Example 3 is listed in Table 3.
(30) TABLE-US-00003 TABLE 3 Using the method Using the method of the in existing Item present invention technology Magnet Average 13.63 (KGs) 13.61 (KGs) Properties value of remanence Br Average 15.44 (KOe) 15.52 (KOe) value of Coercivity Hci Average 44.19 (MGOe) 43.99 (MGOe) value of magnetic energy product (BH) max Hydrogen content after Rare earth FeB 2470 ppm dehydrogenation compound, 37ppm; Rare earth metals, 29 ppm Dehydrogenation 13 hours together for 14 hours time the rare earth FeB compound and the rare earth metals Fine crack ratio after 0.19% 13.2% being cut into small pieces of magnet
(31) From Table 3, it can be seen that in case of the substantially same preparation proportion, with different processes of casting and dehydrogenation and same processes of airflow pulverization, mixing, magnet field orienting and shaping, vacuum sintering and tempering, the two sintered NdFeB magnets differ little in remanence, magnetic energy product and coercivity, that is, the magnetic properties are almost same. With nearly the same dehydrogenation time, few fine cracks in the sintered NdFeB permanent magnet prepared by the method of the present invention show that, with the substantially same preparation proportion, application of the method of the present invention guarantees the magnetic properties of the sintered NdFeB permanent magnet and meanwhile, machineability of product is greatly improved, so that prominent economic effects are achieved.
Example 4
(32) As shown in