Production process of sintered NdFeB

In this paper, the development process, performance requirements and main types of rare earth permanent magnet materials are introduced, with emphasis on the production process of sintered NdFeB magnets. Finally, the application of sintered NdFeB magnets in production, scientific research, life and other fields is summarized, and its development direction is considered. It is pointed out that the production process of sintered NdFeB magnets should be further studied, Only by improving the product quality of NdFeB magnets in China can the competitiveness of enterprises be increased.

Since the advent of rare earth permanent magnetic materials in the late 1960s, the application of rare earth elements in magnetic materials has been increasing. Now, cerium, praseodymium, neodymium, samarium and dysprosium have been used in permanent magnetic materials, and the varieties and properties are constantly developing and improving. With the development of modern industry and technology, the demand for rare earth permanent magnet materials is increasing. According to statistics, the annual output of rare earth permanent magnet materials in the world has exceeded 30000 tons [1].

Overview of rare earth permanent magnet materials

In a broad sense, all kinds of materials which can be magnetized by magnetic field and mainly use the magnetic properties of materials in practical applications become magnetic materials. It includes hard magnetic materials, soft magnetic materials, semi hard magnetic materials, magnetostrictive materials, magneto-optical materials, magnetic bubble materials and magnetic refrigeration materials, among which hard magnetic materials and soft magnetic materials are the most used. The main differences between hard magnetic materials and soft magnetic materials are high anisotropy field, high coercivity, large hysteresis loop area, and large magnetic field required for technical magnetization to saturation. Due to the low coercivity of soft magnetic materials, it is easy to demagnetize after magnetizing to saturation and removing the external magnetic field, while hard magnetic materials still maintain strong magnetism for a long time after magnetizing to saturation and removing the magnetic field due to high coercivity. Therefore, hard magnetic materials are also called permanent magnetic materials or constant magnetic materials. In ancient times, people used the natural magnetite in the ore to grind into the required shape to guide or attract iron devices. Compass is one of the four great inventions in ancient China, which has made important contributions to human civilization and social progress [2]. In modern times, the research and application of magnetic materials began after the industrial revolution, and developed rapidly in a short period of time. Nowadays, the research and application of magnetic materials are unparalleled in both breadth and depth. The development and application of all kinds of high-performance magnetic materials, especially rare earth permanent magnetic materials, plays a huge role in promoting the development of modern industry and high-tech industry.

Performance requirements of permanent magnet materials

The main properties of permanent magnet materials are determined by the following parameters:

• Maximum magnetic energy product: the maximum magnetic energy product is the maximum product of magnetic induction intensity and magnetic field intensity on the demagnetization curve. The larger the value is, the larger the magnetic energy stored in the unit volume is, the better the performance of the material is.
• Saturation magnetization: it is a very important parameter of permanent magnet materials. The higher the saturation magnetization of permanent magnet material, the higher the maximum magnetic energy product and the upper limit of remanence.
• Coercivity: after the ferromagnet is magnetized to saturation, the reverse external magnetic field required to reduce its magnetization or magnetic induction to zero is called coercivity. It characterizes the ability of materials to resist demagnetization.
• Remanence: after the ferromagnet is magnetized to saturation and the external magnetic field is removed, the remanence or remanence induction retained in the magnetization direction is called remanence.
• Curie temperature: the critical temperature at which a strong ferromagnet changes from ferromagnetism and ferromagnetism to paramagnetism is called Curie temperature or Curie point. High Curie temperature indicates that the use temperature of permanent magnet materials is also high.

Main types of rare earth permanent magnet materials

So far, there are two kinds and three generations of rare earth permanent magnet materials
The first is rare earth cobalt alloy system (re co permanent magnet), which includes two generations of products. In 1996, K. strant discovered that the SmCo5 type alloy has a very high magnetic anisotropy constant, resulting in the first generation of rare earth permanent magnet 1:5 type SmCo alloy. Since then, the research and development of rare earth permanent magnet materials have been started, and they were put into production in 1970; The second generation of rare earth permanent magnet material is 2:17 type SmCo alloy, which was put into production in 1978. They are all permanent magnetic alloys based on cobalt.
The second type is Nd-Fe-B alloy. In 1983, neodymium iron boron alloy was discovered in Japan and the United States at the same time, which is called the third generation permanent magnetic material. When the Nd and Fe atoms are replaced by different re atoms and other metal atoms respectively, a variety of Nd-Fe-B permanent magnetic materials with different compositions and properties can be developed. Its preparation methods mainly include sintering method, reduction diffusion method, melt quenching method, bonding method, casting method, etc., among which sintering method and bonding method are most widely used in production [3]. The magnetic properties of different rare earth permanent magnetic materials are listed in the table below.

Material type	Maximum magnetic energy product /Kj.m-3	Residual flux /T	Magnetic coercivity /kA.m-1	Intrinsic coercivity /kA.m-1	Curie temperature /0C
SmCo5 series	100	0.76	550	680	740
SmCo5 series（high Hc）	160	0.90	700	1120	740
Sm2Co17 series	240	1.10	510	530	920
Sm2Co17 series（high Hc）	280	0.95	640	800	920
Sintered Nd-Fe-B series	240-400	1.1-1.4	800-2400	－	310-510
Bonded Nd-Fe-B series	56-160	0.6-1.1	800-2100	－	310
Sm–Fe–Ｎ series	56-160	0.6-1.1	600-2000	－	310-600

It can be seen from the above table that the Nd-Fe-B permanent magnet material has the best comprehensive magnetic properties, and the sintering method is better than the bonding method. So the following mainly introduces the production process of sintered NdFeB permanent magnet materials.

Production process of sintered NdFeB

The overall process is as follows:

Smelting section

The smelting section is mainly responsible for smelting the raw materials weighed in proportion, which is divided into two parts: batching and smelting

Batching

1. The commonly used metals are as follows: metal neodymium, praseodymium, neodymium, special boron, refined boron, copper, aluminum, gallium, terbium, cobalt, iron (Taiyuan Iron, Wuhan Iron and steel, Shanghai iron), dysprosium iron, niobium iron.
2. Instruments, tools and auxiliary materials used: electronic scales of different specifications, steel bar cutting machine, derusting and polishing machine, rubber gloves and masks
3. Workflow: according to the production requirements of the day, count the consumption of various raw materials, and go to the raw material warehouse to collect materials after verification; After coming back, according to the single burden, it can be divided into two kinds, which correspond to the melting furnace in the smelting section. The large furnace is used to cast pieces and the small furnace is used to cast ingots. Relevant records shall be completed during and after batching. Next, a special person will come back and say that the material samples of the furnace should be inspected. After inspection, the lock car will be put and pushed to the melting place to prepare the material for the next day. Generally, the material of small furnace should be spot checked, about 8 barrels of each car, one barrel of each car should be sampled for inspection, and the small material and others should be weighed again to check whether it is qualified. Basically, 55 barrels of raw materials are prepared in small stoves every day. The preparation of the furnace is generally less than 10 barrels.
4. Note:

• (1) Because there is metal powder in the air when weighing metal, it is necessary to wear mask during operation. In addition, special gloves should be worn to prevent metal from hitting hands
• (2) As the iron bars in the raw material warehouse are long and easy to rust, they need to be cut off and polished during batching to facilitate weighing and reduce impurities.
• (3) Pay special attention to safety when operating the steel bar cutting machine, and be careful to hurt your fingers.
• (4) Correctly master the usage of electronic scale and strictly control the tolerance within the required range. When reweighing, strict control is needed to ensure the subsequent effective normal production. The correctness of operation process directly affects the quality of products.

Smelting

Smelting is mainly responsible for casting the prepared materials into pieces or ingots, which are completed by large furnace and small furnace respectively.
1. Slab melting
(1) Instruments, tools, auxiliary materials, etc.: fmri-i-500r vacuum melting casting furnace, crane, raw material truck, spreader, hammer, iron clip, vacuum cleaner, auxiliary lighting tools, slag bucket, stopwatch, thermocouple, nitrogen, argon, gloves, dust mask, sponge, etc
(2) The flow chart is as follows:

(3) Precautions

• ① When loading, the temperature around the crucible mouth is generally high, so you should wear big shoes, protective gloves and heat insulation pad to avoid scalding.
• ② In the process of hoisting, it is necessary to work in the hoisting area; Before hoisting, carefully check the wire rope, hook and hanger to ensure normal operation; During hoisting, it should be ensured that there is no one in the safety isolation area and no one on the traveling route of the equipment platform hoisting vehicle to prevent personal injury.
• ③ When pouring, the upper operator should stay in the pouring control position, observe the liquid flow at the pouring port, and pay attention to the information feedback of the lower operator at all times; The lower observer shall continuously observe the overflow port at the side of tundish and the joint between copper roller and tundish. When abnormal phenomena such as overflow of alloy liquid at overflow port or leakage of tundish bottom plate are found, the upper operator shall be informed in time. At this time, the pouring shall be suspended, and the pouring shall be continued after the abnormality is eliminated [4].
• ④ In order to reduce the harm of dust to human body, masks should be worn during tundish replacement.
• ⑤ During unloading and loading, operators shall wear gloves and masks and shall not work with bare hands. Prevent human body from polluting the casting piece; Prevent human body from being scratched by casting piece.

2. Ingot melting
(1) Instruments, tools and auxiliary materials used: vacuum induction melting furnace, raw material truck, discharging truck, iron clip, slag bucket, auxiliary lighting lamp, set of tools, discharging bucket, nitrogen, argon, refractory, insulation gloves and dust mask.
(2) Process: preparation → charging → vacuumizing → melting → casting → cooling → discharging
(3) Operating procedures: open the pre pumping valve and vacuum gauge → pump below 0.08 → open roots pump → when the vacuum gauge is 0 → close the pre pumping valve and roots pump, close the vacuum gauge → open the charging valve to charge argon → when the pressure of the pressure gauge is 0.05 MPa (0.04-0.06 MPa) → close the charging valve to stop charging argon → turn on the main power supply and control power supply → increase the power → when the iron bar is fully dissolved in the alloy liquid, refine, Let it stand for 2 minutes → start casting → cool for 25 minutes (outlet temperature is required to be below 80 ℃) → vent gas (open gas valve, manual) → turn off power supply → outlet [5].
(4) Precautions

• ① Wear masks when loading and discharging to prevent metal dust inhalation;
• ② Attention should be paid during electrical operation and inspection to prevent electric shock;
• ③ In the smelting process, the flow rate and temperature of cooling water should be paid close attention to. When the cooling water cannot be supplied normally due to power failure or other reasons, tap water shall be sent to the cooling system; In case of water cut-off in the smelting process, the power transmission should be stopped quickly, and the tap water should be connected to the cooling system to cool the induction coil. When the circulating cooling water returns to normal, turn off the tap water, switch to circulating cooling water, and continue to heat up for smelting. In case of splashing of liquid steel, the power supply shall be stopped immediately, and the fault shall be checked and eliminated;
• ④ In the process of pouring, in case of pouring through the cold ingot mold and a large amount of water leakage, the cooling water switch of the cold ingot mold should be closed quickly, the change of argon pressure gauge should be paid attention to, and the furnace door should be opened to take out the material; In case of coil leakage during smelting, the power supply should be stopped to reduce the flow of cooling water for induction coil. Under the protection of argon, the furnace cover can be opened for treatment only after the crucible material is cooled [6].
• ⑤ After discharging, if obvious crack and fracture are found in the gate cup, it should be replaced in time to prevent the outflow of alloy liquid during the next pouring.

Milling section

The pulverizing section is responsible for making the melted products into fine powder. The main processes include hydrogen crushing (medium crushing), coarse powder mixing, jet mill and fine powder mixing. Production process of low grade products: batching → ingot → crushing → medium crushing → coarse powder mixing → jet mill → fine powder mixing. Production process of high grade products: batching → casting sheet → hydrogen crushing → coarse powder mixing → jet mill → fine powder mixing. The flow chart is as follows:

Hydrogen fragmentation

1. Hydrogen crushing principle: by using the hydrogen absorption characteristics of rare earth intermetallic compounds, the Nd-Fe-B alloy is placed in hydrogen environment. Hydrogen enters into the alloy along the thin layer of Nd rich phase, causing it to expand, burst and break, and crack along the layer of Nd rich phase, so that the sheet becomes coarse powder [7].
2. Instruments, tools and auxiliary materials used: domestic ys200 hydrogen crushing furnace, daily phggr50 / 50 / 200s hydrogen crushing furnace, charging device, loading and unloading truck, argon, hydrogen, nitrogen and fan
3. Operating procedures
(1) Preparation: observe that nitrogen pressure and water source pressure meet the requirements of hydrogen crushing process card. Set the process parameters on the control instrument of hydrogen crushing furnace and check them. Check the equipment according to the equipment check list.
(2) Charging: open the furnace door, clean the furnace with a vacuum cleaner, remove the fixed pin of the charging cylinder, put the hydrogen crushing cylinder into the furnace with a charging car, lower the height of the charging car and pull it out. Wipe the door and rubber ring with cotton cloth and close the door. Place the completed product identification card on the furnace door.
(3) Hydrogen crushing:

• ① leak detection: press “automatic operation”, nitrogen is introduced into the furnace body, and the equipment starts to conduct positive pressure leak detection. After leak detection, the furnace body is vented to atmospheric pressure for vacuum negative pressure leak detection. When the requirements of the process card are met, the equipment will give an alarm. At this time, press the H2 valve to open, and the hydrogen crusher will enter the automatic operation stage to introduce hydrogen.
• ② Hydrogen conduction: when the pressure in the furnace reaches the maximum value of hydrogen absorption in the process card, it will be closed. After hydrogen absorption, the pressure in the furnace will drop, and the system will automatically introduce hydrogen to the maximum value. Repeat this process. After confirmation by the system, the barrel will rotate and start, and the pressure will remain at the maximum value, indicating that the material will no longer absorb hydrogen. After confirmation for 8 minutes, the hydrogen conduction is completed.
• ③ Replacement: the system automatically opens the exhaust valve, closes when the exhaust gas reaches atmospheric pressure, and three argon inlet valves are opened successively. When the pressure reaches the replacement pressure, one of the argon inlet valves is closed, the other two are normally open, and the exhaust valve is automatically opened for exhaust, and this is repeated until the set time. The exhaust valve is opened to exhaust to atmospheric pressure, and the replacement is completed.
• ④ Dehydrogenation: the gas extraction system starts automatically. When the gas is pumped to below 40mba, the system will automatically power on and raise the temperature, and vacuum while raising the temperature. Generally, the temperature will rise for 40 minutes. When the temperature reaches the temperature set in the process card, keep it for 1-3 hours. When the vacuum degree meets the requirements of the process card, the dehydrogenation is completed. If the requirements are not met, the pumping will continue until the requirements are met, the dehydrogenation is completed, and the pumping system will automatically shut down.
• ⑤ Cooling: after the gas extraction system is closed, the three argon valves are automatically opened to charge argon at the replacement pressure value, the fan is automatically cooled, and when the pressure in the furnace is insufficient, it is automatically supplemented, and the air cooling is about 3-5 hours. When it reaches 35-40 degrees, the system starts to record the cooling stop time, generally about 20 minutes. After reaching the set cooling time, the argon inlet valve will close automatically, the fan will close automatically, the exhaust valve will open to exhaust, and the furnace can be discharged when the exhaust reaches atmospheric pressure.
• ⑥ Discharging: after hydrogen crushing, the “EDN” indicator light will be on, and the equipment will give an alarm sound, and the furnace can be discharged at this time. Open the furnace door, take out the charging cylinder with the charging car, place it on the charging cylinder frame in the cooling area, fill the charging cylinder with nitrogen in time, and turn on the cooling fan.
• ⑦ After the cylinder reaches the cooling temperature in the cooling zone, the cylinder is transferred to the discharge zone, the cover of the cylinder is replaced by a funnel-shaped cover, and the product is poured into the cleaned cylinder.

4. Coarse powder mixing
The coarse powder mixing is to make the coarse powder after hydrogen crushing (medium crushing) mix evenly through the rotation of the mixing tank.
Process flow:

• (1) constant pressure of mixing tank: open the stop valve of mixing tank, open the exhaust tank of mixing tank, and observe the pressure gauge until it meets the requirements of process card. Open and close the vent tank intermittently, observe the pressure gauge, close the vent valve when the pressure reaches the specified pressure, supply air when the pressure is insufficient, and close the stop valve when the pressure reaches the specified pressure.
• (2) Dosing: according to the requirements of the process card, take the dosing amount from the dosing cylinder, then pour it into the dosing device, connect the nozzle pipe with the charging valve of the mixing tank, open the charging valve, open the stop valve, set the mixing time, start the mixer, make the mixing tank swing back and forth, and open the spraying device for spraying.
• (3) Mixing: after the end of dosing, close the stop valve, close the inflation valve, close the dosing equipment, remove the dosing connection equipment, and then open the mixing tank to rotate, and the mixing will stop automatically after the end of mixing.

5. Precautions
(1) When the furnace door needs to be opened under the abnormal state of the equipment, argon must be used for replacement first, and then vacuum
(2) In the process of hydrogen crushing, attention should be paid to check whether there is leakage in the hydrogen pipeline system. In case of leakage, the pipeline should be closed and overhauled in time to prevent open fire in this process.
(3) When loading and unloading the barrel, the action should be gentle to prevent the barrel from sliding out

Jet mill

1. Air flow mill principle: high pressure air flow will be used to blow up the coarse powder after mixing, and the force will be reduced by mutual collision to become fine powder [8].
2. Instruments, tools and auxiliary materials used: 400afm-r air flow mill, electronic scale, particle size meter, steel cylinder, trolley, traveling crane, rubber ring, clamp, rubber hammer and protective mask
3. Process flow

• (1) Preparation: arrange grinding operation, check equipment, set parameters, and arrange material according to production plan.
• (2) Start up: switch to the automatic mode before start-up. If the start-up status is met, press the “S11” key to return to the compressor diagram, press the “S3” key to start the air mill automatically, record the start-up time, and start the air compressor.
• (3) Connect the fine powder cylinder: connect the discharged fine powder cylinder with the outlet of the powder screening machine.
• (4) Feeding: hoist the cylinder to the feeding port with crane for feeding, and cover the feeding bucket after feeding.
• (5) Oxygen discharge: open the vibrating screen and discharge valve for oxygen discharge.
• (6) Abrasive: press the feeding button after the speed of the separating wheel reaches the requirements, and the coarse powder at the feeding port of the air flow mill will automatically enter the grinding chamber for grinding; When the abrasive falls into the first cylinder, record the starting time and relevant parameters.
• (7) Oxygenation passivation: when the automatic feeding reaches the set value, adjust the oxygenation flow rate for oxygenation.
• (8) First cylinder receiving and cylinder replacement: when the powder in the first cylinder meets the requirements of the process card, replace the cylinder, and prepare for the first cylinder particle size sampling.
• (9) The first bottle of particle size sampling: take the plastic bag filled with nitrogen in advance, tie the mouth by hand and tightly wrap the sampling valve, open the nitrogen pipe and insert it into the plastic bag together; Open the sampling ball valve instantly and then close it. The powder sample is filled into the plastic bag and the sampling is completed. After passivation preparation measurement, used for particle size inspection.
• (10) Replacement of cylinder and continuous feeding.
• (11) Sampling and application for inspection of particle size sample: according to the requirements of the process card of the pneumatic mill, the cylinder sequence number is sampled, and the particle size sample sequence number is the sequence number of the previous cylinder at the time of sampling.
• (12) First inspection and disposal: the operator shall conduct self inspection on the average particle size of the first bottle of powder, and continue if it is qualified, and stop feeding if it is unqualified for three consecutive bottles.
• (13) Weighing and transfer: the loading cylinder is weighed and moved to the fine powder warehouse, and the nitrogen gas pipe is connected to the cylinder.
• (14) Treatment of slab tailings: the tailings are placed in the fine powder tailings area.
• (15) Shut down: when there is no material in the feeding hopper, it should be converted to idling, and then press “S4” to shut down the equipment automatically and turn off the power supply of the air compressor.
• (16) Site clearing: when the ingot is transferred to sheet metal, the bottom material shall be cleaned.

4. Fine powder mixing
Fine powder mixing is to mix different brands of fine powder after air flow grinding according to the proportion. The operation process is the same as that of coarse powder mixing.
5. Precautions

• (1) In the process of operation, if air leakage is found in the pipeline, it should be stuck with adhesive tape in time. If the air leakage is serious, stop feeding and fill the powder in the grinding chamber with nitrogen for protection.
• (2) During the operation, if the superfine powder in the grinding chamber or filter catches fire, it should be shut down and filled with nitrogen for protection. If the situation is serious, press “emergency stop”.
• (3) In case of powder leakage, the powder shall be covered with yellow sand or asbestos cloth as soon as possible and cleaned up after cooling.
• (4) Special attention should be paid to the safety of personnel in the process of cylinder hoisting, and personnel are strictly prohibited to enter under the hanger
• (5) When the cylinder is turned over, special attention should be paid to prevent hand clamping. In the process of discharging and sampling, protective masks must be worn to prevent powder from spraying out and damaging human body.

Forming section

The molding section is responsible for pressing the powder, including molding and isostatic pressing.

Forming

1. Molding principle: select the appropriate mold according to the requirements, and press the fine powder into the required shape.
2. Instruments, tools and auxiliary materials used: automatic magnetic field forming press, cylindrical forming press, square forming press, vacuum packaging machine, automatic powder weighing machine, transfer car, electronic scale, caliper, small copper shovel, brush, magnetic powder sanitary ware, magnetic column, pressure handle, protective pad, vacuum packaging, inner packaging film, transfer basket and transfer box.
3. Flow chart:

4. Specific operating procedures

(1) Mold installation and disassembly: preparation → mold installation → press preparation → lower head installation → mold frame installation → upper head installation → verticality inspection → demoulding.
(2) Magnetic field measurement and adjustment of molding press: calibration → magnetic field measurement and adjustment → site clearing.
(3) Weighing: preparation before weighing → reclaiming → loading → blanking → weighing → replacing barrel → cleaning and clearing.
(4) Press operation
Cylinder type: upper cylinder lower → pause → orientation → pressing → holding pressure → demagnetization → upper cylinder upper → lower cylinder upper.
Block type: upper cylinder down → press die → side cylinder forward → orientation → pressing → pressure maintaining → demagnetization → side cylinder backward → upper cylinder up [9].
(5) Packaging: preparation → inspection and record → inner membrane → bagging → vacuum → packing → marking and sequence transfer.
(6) Cleaning: press cleaning → automatic weighing machine cleaning → packing box cleaning → vacuum packing machine cleaning → floor cleaning of working area.
(7) Cleaning of molding press: disassembly of empty drum → cleaning of automatic weighing machine → cleaning of press → cleaning of packing box → cleaning of working area floor.
5. Precautions
(1) In the process of magnetization, the probe of Tesla meter should be placed in the middle of the magnetic pole, not in contact with any object, and the manual magnetization time should not be too long to avoid burning the equipment.
(2) When the oxygen content is more than 0.05%, the operation should be suspended and the nitrogen content should be adjusted to ensure that the oxygen content is qualified.
(3) The manual butterfly valve at the feed port must be closed before disassembling the material cylinder. The nitrogen in the material cylinder must be discharged before replacement, so as to avoid the magnetic powder spraying to the human body during blanking.

Isostatic pressing

1. Isostatic pressure principle: the working principle of isostatic pressure is Pascal’s Law: “the pressure of medium (liquid or gas) in a closed container can be equally transmitted to all directions.” Specifically, after the pressed product is loaded into the equipment, the product is subjected to the action of ultra-high pressure medium in all directions, which increases the density of the product. The shrinkage of the briquette depends on the compressibility of the material and the pressure during pressing [10].
2. Instruments, tools and auxiliary materials used: ldj320 / 1500-300ys cold isostatic press (water medium), crane, hanging cage, circulation vehicle, antiskid boots, plastic gloves, apron and sleeve.
3. Operating procedures

• (1) Preparation: before the operation, the operator is responsible for checking the operation documents, equipment, instruments, tooling and tools to make them complete. Then carry out equipment spot check.
• (2) Parameter setting: according to the requirements of isostatic pressing process card, the parameters are set on the input panel.
• (3) Loading: place the cage on the loading platform; Open the cage door; Stack the products in the hanging cage; Close the cage door.
• (4) Cylinder inlet: turn the working cylinder (on / off) knob on the control panel to the “on” position, and then the working cylinder head will be opened. Hoist the cage into the working cylinder with a crane. After the cage is put into the working cylinder, observe whether all the cages intrude into the liquid level. If not, replenish the working fluid into the cylinder to completely submerge the product; Turn the working status knob to the “off” position to close the cylinder head.
• (5) Isostatic pressing: start the isostatic pressing machine, the equipment automatically pressurizes, and the pressurization light is on; When the pressure reaches the set value, the device will stop pressurizing automatically and the pressurizing light will be off; The pressure maintaining lamp is on, and the system maintains the pressure; When the holding time is up, the holding light goes out and the pressure relief begins; When the pressure relief reaches the preset value, the equipment will stop the pressure relief automatically.
• (6) Cylinder outlet: open the cylinder head; Lift out the cage with crane; After 40-60 seconds in the air, place it on the discharging table. Take the product out of the cage and place it on the transfer car.

4. Precautions

• (1) It is strictly forbidden to pressurize the machine frame before it has fully entered the working position;
• (2) During the operation of the press, closely monitor the operation panel on the electric control cabinet and the ultra-high pressure gauge on the pressure gauge bracket to prevent overpressure;
• (3) No one is allowed to walk around the high-pressure site during pressurization, pressure maintaining and pressure relief.

Sintering section

The sintering section is responsible for sintering the products after oil stripping, including oil stripping and sintering.

Oil stripping

Oil stripping is to remove the product package after isostatic pressing to facilitate sintering
1. Instruments, tools and auxiliary materials used: Mobile glove box, oxygen detector, circulation vehicle, turnover box, oil stripping box, auxiliary feeding box, sintering basin, sintering bracket, scissors, mask, rubber gloves, finger cover, garbage bag, sponge, identification iron and isolation strip
2. Operation process

• (1) Preparation: before the operation, the operator is responsible for checking the operation documents, equipment, instruments, tooling and tools to make them complete. Then pick up the material, and pick up the whole batch according to the production batch. Prepare for oil stripping.
• (2) Oxygen exhaust: open the glove box charging valve and auxiliary feeding box charging valve, and charge nitrogen; Observe the oxygen meter and cut the bag when the oxygen content is less than 0.05%.
• (3) Oil stripping and basin loading: bag cutting → packing bag stripping → inner membrane stripping → basin setting
• (4) Cleaning: after oil stripping and basin filling, the glove box should be cleaned carefully to ensure that there is no material left; Put the vacuum packaging bag and inner packaging film into the garbage bag; Transfer the garbage bag, transfer plastic box and material box to the auxiliary feeding box; Check and make sure that there are no sundries (packaging bags, inner membrane debris, etc.) in the sintering basin and bracket to prevent the sintering process from polluting the products; Inform the operator of sintering section to carry out furnace feeding operation.

3. Precautions
(1) During the operation, the oxygen meter should be observed at all times to ensure that the oxygen content is less than or equal to 0.05%, so as to avoid the powder oxidation and ignition, causing human injury;
(2) In the process of operation, attention should be paid to observe the fastening of the window and whether the sleeve is in good condition, so as to avoid human injury under the action of gas pressure;
(3) When the sintering basin moves, the cooperative operation action should be coordinated to prevent the sintering basin from toppling, sliding and hurting fingers.

Sintering

1. The sintering process flow chart is as follows:

2. Instruments, tools and auxiliary materials used: VS-200RPA sintering furnace, glove box, sintering basin, hydraulic truck, circulation truck and insulating gloves
3. Operation process

• (1) Preparation: before operation, the operator shall check the equipment, instruments, tooling and tools to make them complete.
• (2) Vacuum inspection: check and confirm that the furnace door is closed, open the sintering furnace for vacuum operation. Observe the vacuum degree, calculate the vacuum time, and confirm that the product meets the requirements before entering the furnace.
• (3) Entering the furnace: confirm the end of product oil stripping and shaking basin, fill the sintering furnace with nitrogen to atmospheric pressure according to the operation procedures, and then stop charging. Open the gate valve, open the screen valve, open the feeding mechanism, and slowly send the sintering basin into the sintering furnace to make it in place. Reset the feeding mechanism. Close the shield valve and close the gate valve. Close the glove box inflation valve and the oxygen meter. Then set the parameters.
• (4) Sintering: start the sintering furnace according to the operation regulations, and inspect the vacuum meter, voltmeter, ammeter and cooling water of vacuum equipment during the process of heating, insulation and cooling.
• (5) Discharging: after sintering, confirm that the product temperature in the furnace meets the process requirements, and open the furnace door; Take out the sintering basin in the furnace with hydraulic forklift to prevent it from being on the transfer car. Close the furnace door and open the vacuum device. Put the transfer car in the waiting area.

4. Precautions

• (1) In case of water leakage, gas leakage and other abnormal conditions in the sintering process, vacuum should be pumped first, and then argon gas should be filled into the furnace for replacement. The furnace door can only be opened under the condition of ensuring safety.
• (2) When the product is out of the oven, you should wear insulating gloves to prevent high temperature from scalding the human body.
• (3) When using the hydraulic truck to load and unload materials, it should be handled with care to prevent the sintering basin from toppling and damaging the human body.
• (4) When moving the transfer material car, it should be ensured that the line is barrier free to prevent human body from being injured.

Application of rare earth permanent magnetic materials

Micro and special motors

In the United States, Japan and Western Europe and other developed countries, the application of rare earth permanent magnet materials in motor has accounted for more than 60% of the total sales of rare earth. The application of rare earth permanent magnet in motor is different in different countries. Japan uses about 50% of the rare earth permanent magnets in VCM, while the United States uses the most in aviation, aerospace, military industry, automobile and machine tools, and Europe uses the most in CNC machine tools. In 2006, China produced more than 37 million rare earth permanent magnet motors, including 50000 large and medium-sized energy-saving motors [11].

Automotive industry

As the automobile industry has become the fifth pillar industry of China’s national economic development, its development will drive a series of industries, including magnetic materials industry. One of the biggest application markets of rare earth permanent magnet motor is automobile industry. Automobile industry is one of the most widely used fields of NdFeB permanent magnet. In each car, there are generally dozens of parts to use permanent magnet motors, such as electric seats, electric rear-view mirrors, electric skylights, electric wipers, electric doors and windows, air conditioners, etc. with the continuous improvement of automotive electronic technology requirements, the number of motors used will be more and more [11].

Electric vehicles

At present, countries all over the world pay more and more attention to electric vehicles. Some countries invest a lot of money in research and development. Among them, motor and transmission system are the heart of electric vehicles. Rare earth permanent magnet motor has become the first choice for countries to develop a new generation of electric vehicles because of its small size, high efficiency and excellent performance. China First Automobile Corporation and Toyota Motor Corporation have cooperated to promote the development and popularization of hybrid electric vehicles in the Chinese market. In 2005, Toyota’s most advanced “green environmental protection and energy saving vehicle” – Pioneer hybrid electric vehicle was produced in China, and now it has been put into the market in small quantities. Its output is 10000 sets per month.
It can be predicted that in the near future, a large number of hybrid electric vehicles will enter ordinary families and become a kind of mass consumption, which is considerable for rare earth permanent magnet motors and NdFeB permanent magnet materials.

Electric bicycle

Bicycle is one of the traditional pillar industries in China. With the improvement of people’s living standards and the enhancement of environmental awareness, the demand for electric bicycle is increasing in recent years. According to the incomplete statistics of China electric bicycle professional committee, the production and sales of electric bicycles in China have been increasing year by year from R & D to small batch launch in 1997. The output of electric bicycles in 1998 was 5.45 million, and it has exceeded 10 million in 2009. Based on the average demand of 0.3KG sintered NdFeB for each electric bicycle, the annual demand for magnets is about 3000t (equivalent to nearly 6000t [12]).

Modern medical equipment

In the past, the permanent magnet rmi-ct MRI equipment used ferrite permanent magnet, which weighs 50 tons. Now it uses the latest Nd-Fe-B permanent magnet material, which has doubled the magnetic field strength, greatly improved the image clarity, and saved a lot of raw materials. Each MRI instrument needs 0.5-3 tons of NdFeB permanent magnets. According to the annual demand of 1000 sets in the world market, the annual demand for magnets is 500-3000 tons. At present, GM of the United States and Siemens of Germany have MRI equipment production bases in China.

Loudspeaker, earphone and other electroacoustic components

Loudspeakers and earphones are traditional applications of permanent magnets. After the appearance of rare earth permanent magnet, the size of Nd-Fe-B permanent magnet can be reduced and the performance can be improved under the same output power and sound quality. At present, rare earth permanent magnet loudspeakers and earphones have been used in advanced Walkman and other fields. With the development of high fidelity and miniaturization of electroacoustic equipment, magnets with higher performance are required. In this field, NdFeB permanent magnet materials will be more widely used.

Maglev train

Zhonghua No.01 maglev technology verification vehicle, which can ride 32 people and has independent intellectual property rights, appeared in Dalian on October 22, 2004. The successful “Zhonghua 01” adopts the permanent magnet compensation suspension technology developed by China. Its principle is to use the repulsive force and attractive force generated between the vehicle magnet and the track magnet to produce upward levitation force and make the train run off the track. The magnets used are rare earth permanent magnets“ The suspension energy consumption of “Zhonghua 01” is almost zero, and its net suspension force can reach 4 tons per meter. Its transportation capacity is equivalent to that of the current train, and its safety is greatly improved by adopting the structural design of vehicle road integration. China’s first maglev verification vehicle “Zhonghua 06” was launched in Dalian in May 2005. The permanent magnetic levitation train line using NdFeB magnets has been included in the construction schedule.

Outlook

At present, the output of sintered NdFeB permanent magnet materials in China has ranked the first in the world, and it is a real big producer [13]. However, there is still a big gap between domestic Nd-Fe-B permanent magnet materials and foreign ones in terms of product performance, mainly because the thermal stability and oxidation resistance of the materials have not been completely solved; The magnetic property of the product is not stable and the dispersion is large; Permanent magnet processing and magnetization technology are still not up to standard, which are in urgent need of further exploration. We should make full use of China’s rare earth resources and human resources. We will intensify technological research and development.
With the continuous progress of science and technology, the application fields of sintered NdFeB permanent magnet materials are also expanding. For the production enterprises, on the one hand, they should improve the magnet production process, strive to improve the performance of products and make them more stable; On the other hand, we should also strive to develop a larger application market [14]. This requires not only the production enterprises to provide good products, but also the corresponding technical support to increase the competitiveness of enterprises, so as to better serve the application enterprises of sintered NdFeB permanent magnet materials, so as to promote the sustainable development of NdFeB and related industries.

Source: China Permanent Magnet Manufacturer www.rizinia.com

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