A method for preparing large circular sintered NdFeB blank

Abstract

The invention discloses a preparation method of a large circular sintered NdFeB blank. The method comprises the following steps: the method comprises: using a forming mold composed of a first pressing head, a second pressing head, a first linear moving mechanism, a second linear moving mechanism, a first side plate, a second side plate, a core rod, a bottom plate, a cover plate and a hanging ring to press and form the large circular sintered NdFeB blank. The cavity of the forming mold is approximately elliptical, and the demoulding angle is reduced to 13°-17°. The core rod adopts a truncated cone structure. When demoulding, the hanging ring is screwed into the core rod to remove the core rod. Two wooden cylindrical plugs which are 3mm-5mm smaller than the inner hole of the circular pressed blank are pressed into the inner hole of the circular pressed blank. A relatively small vacuum packaging pressure is adopted to enhance the isostatic pressing pressure to below 200MPa-230MPa. The method has the advantage that when the large circular sintered NdFeB blank is prepared by the preparation method, the product missing angle rate and cracking rate are effectively reduced, the product quality is improved, and a relatively large margin does not need to be reserved for the outer diameter of the product, so that material waste is reduced. The demoulding operation process is simple, the demoulding efficiency is high, and batch production is facilitated.

Description

Technical Field

[0001] The invention relates to a method for preparing a sintered NdFeB blank, in particular to a method for preparing a large-circular sintered NdFeB blank. Background Art

[0002] The concave cavities of the two pressing heads of the traditional forming mold for preparing large circular sintered NdFeB billets are both semicircular in shape, so that when the concave cavities of the two pressing heads are spliced, a cylindrical mold cavity is formed, that is, the outer wall of the pressed green sheet is circular. During sintering, due to the different shrinkage of the mold width and the pressing direction, the outer wall of the large circular sintered NdFeB billet obtained after sintering will become elliptical. Therefore, it is necessary to grind the outer wall of the large circular sintered NdFeB billet to make its outer wall circular. Because the grinding of the outer wall of the large circular sintered NdFeB billet needs to be based on the minimum size, when forming the green sheet, a relatively large margin will be reserved for its outer diameter, resulting in excessive waste.

[0003] At the same time, in the traditional forming mold for preparing large circular sintered NdFeB billets, since the core rod structure used for forming the inner hole is fixed on the bottom plate of the mold, when demolding after the molding is completed, it is necessary to grasp the outer wall of the large circular sintered NdFeB billet with both hands and move the large circular sintered NdFeB billet upward to slowly separate the large circular sintered NdFeB billet from the core rod, which is inefficient. During the demolding process, on the one hand, due to the large weight of the large circular sintered NdFeB billet, the product is easy to tilt and hit the core rod during the demolding process, resulting in edge loss and chipping. On the other hand, when the density of the large circular sintered NdFeB billet is too low or too high, the product is easy to break or crack.

[0004] At present, in order to overcome the problems caused by the core rod structure in the traditional forming mold for preparing large circular sintered NdFeB blanks, some technicians have proposed solutions such as demoulding with auxiliary devices and changing the core rod structure. For example, the Chinese patent with patent number CN200920103164.1 discloses a demoulding auxiliary device for large circular NdFeB, through which the large circular sintered NdFeB blank is taken out, but the demoulding operation process of the demoulding auxiliary device is cumbersome, and the demoulding efficiency is too low, which is not conducive to continuous production; the Chinese patent with patent number CN201110202977.8 discloses a method for processing a large circular NdFeB core rod and processing a large circular NdFeB core rod using the core rod, which optimizes product quality by changing the core rod structure. Although the demoulding operation process is simplified relative to the above-mentioned large circular NdFeB demoulding auxiliary device, its core rod structure is also relatively complicated, and the demoulding efficiency is still too low, which is not conducive to mass production. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing a large circular sintered NdFeB billet. When the large circular sintered NdFeB billet is prepared by the preparation method, the product's corner missing rate and cracking rate are effectively reduced, the product quality is improved, and no large margin is required for the product's outer diameter, thereby reducing material waste. The demolding operation process is simple, the demolding efficiency is high, and mass production is facilitated.

[0006] The technical solution adopted by the present invention to solve the above technical problems is: a method for preparing a large circular sintered NdFeB blank, comprising the following steps:

[0007] Step 1, obtain a forming mold, the forming mold includes a first pressure head, a second pressure head, a first linear motion mechanism, a second linear motion mechanism, a first side plate, a second side plate, a core rod, a bottom plate, a cover plate and a lifting ring, the bottom plate is horizontally arranged, the first side plate is detachably mounted on the front side of the bottom plate, the second side plate is detachably mounted on the rear side of the bottom plate, the core rod is in the shape of a truncated cone, the radius of the upper end face of the core rod is greater than the radius of the lower end face of the core rod, a threaded hole is provided at the center of the upper end face of the core rod extending downward, the threaded hole is coaxial with the core rod, a latch is fixedly arranged on the lower end face of the core rod, and the latch is plugged into On the bottom plate, the lower end surface of the core rod is fitted with the upper end surface of the bottom plate, the lower part of the hanging ring is provided with an external thread matching the threaded hole, the first pressure head and the second pressure head are located above the bottom plate and between the first side plate and the second side plate, the first pressure head and the second pressure head are arranged in parallel on the left and right, a first groove recessed to the left is provided on the right side surface of the first pressure head, the upper end surface of the first groove is flush with the upper end surface of the first pressure head, the lower end surface of the first groove is flush with the lower end surface of the first pressure head, a second groove recessed to the right is provided on the left side surface of the second pressure head, the upper end surface of the second groove is flush with the second The upper end surface of the pressure head is flush, the lower end surface of the second groove is flush with the lower end surface of the second pressure head, the first groove and the second groove are symmetrical, the first linear motion mechanism is used to drive the first pressure head to move linearly along the left and right directions, and the second linear motion mechanism is used to drive the second pressure head to move linearly along the left and right directions. The first pressure head and the second pressure head can be moved to their preset mold closing position and mold opening position under the drive of the first linear motion mechanism and the second linear motion mechanism. When the first pressure head and the second pressure head are located at their mold closing positions, the distance between the right end surface of the first groove and the left end surface of the second groove is 2a, a The value range of is 4mm-7mm, and the cross section of the first groove and the second groove on the same horizontal plane is located on the same elliptical surface, the major axis of the elliptical surface is along the front-to-back direction, and the minor axis is along the left-to-right direction. The distance between the major axis of the elliptical surface and the right end face of the first groove is a, and the distance between the major axis of the elliptical surface and the left end face of the second groove is also a. The length of the major axis of the elliptical surface is recorded as b1, and the length of the minor axis of the elliptical surface is recorded as b2. b1=D×a1, b2=D×a2, D represents the outer diameter of the large circular sintered NdFeB blank, a1 and a2 represent the shrinkage ratio respectively, wherein the value range of a1 is: 1.195-1.205, and the value range of a2 is 1.15-1.16. The straight line passing through the center of the elliptical surface and perpendicular to the bottom plate is called the center line. The distance between the first side plate and the center line is b1 / 2+L, and the distance between the second side plate and the center line is b1 / 2+L, where L is 0.15mm-0.2mm. The central axis of the mandrel coincides with the center line. The diameter of the upper surface of the mandrel is recorded as C, C=D'×a3, D' represents the inner diameter of the large circular sintered NdFeB billet, a3 represents the shrinkage ratio, and the value range of a3 is: 1.05-1.15. The height of the mandrel is recorded as d, d=h×a4, h is the height of the large circular sintered NdFeB billet. degree, a4 represents the shrinkage ratio, the value range of a4 is: 1.44-1.46, the upper end surface of the mandrel, the upper end surface of the first pressure head and the upper end surface of the second pressure head are located in the same plane, the upper end surface of the first side plate and the upper end surface of the second side plate are located in the same plane, and the plane is 0.25mm-0.4mm higher than the plane where the upper end surface of the mandrel is located, the cover plate is detachably mounted on the first side plate and the second side plate, the lower end surface of the cover plate is flush with the upper end surfaces of the first side plate and the second side plate, and the cover plate is located above the first pressure head and the second pressure head;.

[0008] Step 2. In an oxygen-free working atmosphere, the molding die is used to mold the green sheet and prepare a large circular sintered NdFeB blank. The specific process is as follows:

[0009] S2-1. In the initial state, the first pressing head and the second pressing head are both located at their preset mold closing positions, and the cover plate is installed on the first side plate and the second side plate; first, the cover plate is removed from the first side plate and the second side plate, and then the first linear motion mechanism drives the first pressing head to move a distance to the left to its preset mold opening position, and the second linear motion mechanism drives the second pressing head to move a distance to the right to its preset mold opening position, and the distance moved to the left by the first pressing head is equal to the distance moved to the right by the second pressing head, and at this time, a mold cavity is formed among the first side plate, the second side plate, the bottom plate, the core rod, the first pressing head and the second pressing head;

[0010] S2-2, fill the powder into the mold cavity. When the mold cavity is filled with the powder, install the cover plate onto the first side plate and the second side plate again. At this time, the cover plate covers the powder in the mold cavity. Then the first linear motion mechanism drives the first pressure head to move rightward, and the second linear motion mechanism drives the second pressure head to move leftward. The first pressure head and the second pressure head press the powder during the movement. When the first linear motion mechanism drives the first pressure head to move rightward to its preset mold closing position, and the second linear motion mechanism drives the second pressure head to move leftward to its preset mold closing position, the powder pressing is completed, and the density is 3.95g / cm 3 ~4.15g / cm 3 of compact;

[0011] S2-3, first remove the cover plate from the first side plate and the second side plate, screw the lower external thread of the lifting ring into the threaded hole of the core rod, at this time, the lifting ring and the core rod are fixedly connected, then operate the lifting ring to pull out the core rod, and then remove the first side plate and the second side plate from the bottom plate, the first linear motion mechanism drives the first pressure head to move a distance to the left to its preset mold opening position, and the second linear motion mechanism drives the second pressure head to move a distance to the right to its preset mold opening position;

[0012] S2-4, first pick up the pressed green sheet and put it into a packaging bag, then insert a wooden cylindrical plug from both ends of the inner hole of the pressed green sheet through the packaging bag until the two wooden cylindrical plugs are connected, and the diameters of the two wooden cylindrical plugs are equal and 3mm-5mm smaller than the diameter of the inner hole of the pressed green sheet;

[0013] S2-5, at a pressure of 30 MPa, using a vacuum machine to vacuum-pack the packaging bag to obtain a vacuum-packed green compact;

[0014] S2-6. At a pressure of 200MPa-230MPa, the vacuum-encapsulated green compact is subjected to isostatic pressing to increase the density of the green compact to 4.4g / cm 3 -4.8g / cm 3 , to avoid cracking of the inner wall due to excessive size shrinkage;

[0015] S2-7, remove the packaging bag outside the green compact, then sinter the green compact in a vacuum sintering furnace at 998°C to 1100°C for 3-8 hours, cool to room temperature, and then perform aging treatment to obtain a large circular sintered NdFeB green compact with a circular outer wall.

[0016] The powder in step S2-2 is obtained by the following method:

[0017] S2-1, smelting: the sintered NdFeB raw materials with cleaned surface are loaded into a vacuum sheet-throwing furnace, firstly smelted at a temperature of 1455-1485°C to obtain a molten liquid, and then the molten liquid is cast to obtain a casting with a thickness of 2.5-3mm. When the casting temperature is less than or equal to 45°C, the casting is taken out from the vacuum sheet-throwing furnace;

[0018] S2-2, hydrogen crushing and powder making: Under the protection of oxygen-free atmosphere, the obtained casting flakes are firstly processed into coarse powder by hydrogen crushing process, and then lubricant is added to the coarse powder and stirred evenly, and then it is processed by air jet mill to prepare fine powder with an average particle size of 2.8μm to 3.6μm, which is the obtained powder.

[0019] The material of the first side plate and the second side plate is G60, the material of the first pressure head and the second pressure head is 70Mn, the material of the bottom plate and the cover plate is GCr15, and the hardness requirements after quenching are 45-55HRC and the roughness is Ra0.4, which ensures the wear resistance of the material, ensures that the pressed billet does not stick to the mold, and improves the forming stability. The core rod material is austenitic stainless steel with a roughness of Ra0.4 to ensure the integrity of the inner wall of the circular ring pressed billet during demolding.

[0020] The demoulding angle β of the core rod is 88° to 89°.

[0021] Compared with the prior art, the advantage of the present invention is that a forming mold is formed by a first pressure head, a second pressure head, a first linear motion mechanism, a second linear motion mechanism, a first side plate, a second side plate, a core rod, a base plate, a cover plate and a hanging ring. The base plate is arranged horizontally in the horizontal direction, the first side plate is detachably mounted on the front side of the base plate, the second side plate is detachably mounted on the rear side of the base plate, the core rod is in the shape of a truncated cone, the radius of the upper end face of the core rod is greater than the radius of the lower end face of the core rod, a threaded hole is provided at the center of the upper end face of the core rod extending downward, the threaded hole is coaxial with the core rod, a pin is fixedly provided on the lower end face of the core rod, the pin is plugged into the base plate, the lower end face of the core rod is fitted with the upper end face of the base plate, and a threaded hole matching the threaded hole is provided at the lower part of the hanging ring. The first pressure head and the second pressure head are located above the bottom plate and between the first side plate and the second side plate, the first pressure head and the second pressure head are arranged in parallel on the left and right, a first groove recessed to the left is arranged on the right side surface of the first pressure head, the upper end surface of the first groove is flush with the upper end surface of the first pressure head, the lower end surface of the first groove is flush with the lower end surface of the first pressure head, a second groove recessed to the right is arranged on the left side surface of the second pressure head, the upper end surface of the second groove is flush with the upper end surface of the second pressure head, the lower end surface of the second groove is flush with the lower end surface of the second pressure head, the first groove and the second groove are symmetrical on the left and right, the first linear moving mechanism is used to drive the first pressure head to move linearly in the left and right direction, the second linear moving mechanism is used to drive the second pressure head to move linearly in the left and right direction, The first pressing head and the second pressing head can be moved to their preset mold closing position and mold opening position under the drive of the first linear moving mechanism and the second linear moving mechanism. When the first pressing head and the second pressing head are located at their mold closing position, the distance between the right end face of the first groove and the left end face of the second groove is 2a, and the value range of a is 4mm-7mm. The cross-sections of the first groove and the second groove on the same horizontal plane are located on the same elliptical surface. The major axis of the elliptical surface is along the front-to-back direction, and the minor axis is along the left-to-right direction. The distance between the major axis of the elliptical surface and the right end face of the first groove is a, and the distance between the major axis of the elliptical surface and the left end face of the second groove is also a. The length of the major axis of the elliptical surface is recorded as b1, and the length of the minor axis of the elliptical surface is recorded as b2, b1= D×a1, b2=D×a2, D represents the outer diameter of the large circular sintered NdFeB billet, a1 and a2 represent the shrinkage ratio, wherein a1 is in the range of 1.195-1.205, a2 is in the range of 1.15-1.16, the straight line passing through the center of the elliptical surface and perpendicular to the bottom plate is called the center line, the distance between the first side plate and the center line is b1 / 2+L, the distance between the second side plate and the center line is b1 / 2+L, L is 0.15-0.2mm; the center axis of the mandrel coincides with the center line, the diameter of the upper surface of the mandrel is recorded as C, C=D'×a3, D' represents the inner diameter of the large circular sintered NdFeB billet, a3 represents the shrinkage ratio, and the value range of a3 is 1.05-1.15. The height of the core rod is recorded as d, d=h×a4, h is the height of the large circular sintered NdFeB blank, a4 represents the shrinkage ratio, and the value range of a4 is: 1.44-1.46. The upper end face of the core rod, the upper end faces of the first pressure head and the second pressure head are located in the same plane, the upper end face of the first side plate and the upper end face of the second side plate are located in the same plane, and the plane is 0.25mm-0.4mm higher than the plane where the upper end face of the core rod is located. The cover plate is detachably mounted on the first side plate and the second side plate, the lower end face of the cover plate is flush with the upper end faces of the first side plate and the second side plate, and the cover plate is located above the first pressure head and the second pressure head. In the present invention, the first groove and the second groove are approximately semi-elliptical, and the demolding angle α of the molding mold is reduced to 13°~17°, without affecting the demolding of the product Under the condition of isostatic pressing, the shrinkage consistency of the large ring sintered NdFeB blank is improved, and the waste of powder is reduced. The core rod adopts a truncated cone structure. When demoulding, the lifting ring is screwed into the core rod to remove it, avoiding collision and sticking with the core rod during the demoulding process of the product, reducing the cracking of the inner wall of the product and improving production efficiency. Before isostatic pressing, two wooden cylindrical plugs 3mm-5mm smaller than the inner hole of the ring blank are pressed into the inner hole of the ring blank to reduce the distance between the two wooden cylindrical plugs and use a smaller vacuum packaging pressure, thereby reducing the cracking caused by missing corners, falling edges and uneven force during the vacuum packaging process. During isostatic pressing, the isostatic pressing pressure is increased to 200MPa-230MPa, and the density of the compact is increased to 4.4g / cm. 3 -4.8g / cm 3 , increase its density, reduce the hidden cracks on the inner wall of the ring, so the present invention breaks the conventional mold design mode and molding process method, and reduces the processing allowance of the large ring sintered NdFeB blank by designing the pressure head to be approximately semi-elliptical, and uses the inverted core rod structure to change the molding process mode, reduce the manual force applied to the molded product, optimize the vacuum process and isostatic pressing process to improve the density of the pressed blank, effectively reduce the problem of product defects before and after molding, effectively reduce the product missing corner rate and cracking rate, and improve product quality. While effectively reducing the product missing corner rate and cracking rate and improving product quality, the product outer diameter does not need to reserve a relatively large allowance, which reduces material waste, and the demoulding operation process is simple, the demoulding efficiency is high, and it is conducive to mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic structural diagram of a forming mold in the method for preparing a large circular sintered NdFeB blank of the present invention;

[0023] Figure 2 It is a structural schematic diagram of a cover plate of a forming mold in the method for preparing a large circular sintered NdFeB blank of the present invention;

[0024] Figure 3It is a structural schematic diagram of the bottom plate of the forming mold in the method for preparing the large circular sintered NdFeB blank of the present invention;

[0025] Figure 4 It is a schematic diagram of the assembly structure of the hanging ring and the core rod of the forming mold in the method for preparing the large circular ring sintered NdFeB blank of the present invention;

[0026] Figure 5 It is a positional relationship diagram of a first pressing head and a second pressing head of a forming mold in a method for preparing a large circular sintered NdFeB blank of the present invention;

[0027] Figure 6 It is a schematic diagram of the state of the forming mold of the method for preparing the large circular sintered NdFeB blank of the present invention when the powder is filled;

[0028] Figure 7 It is a schematic diagram of the state of the forming mold of the method for preparing the large circular sintered NdFeB billet of the present invention when the powder pressing is completed. DETAILED DESCRIPTION

[0029] The present invention is further described in detail below with reference to the accompanying drawings.

[0030] Embodiment: A method for preparing a large circular sintered NdFeB blank comprises the following steps:

[0031] Step 1: Get a molding die, such as Figures 1 to 5As shown, the forming mold includes a first pressure head 1, a second pressure head 2, a first linear motion mechanism 3, a second linear motion mechanism 4, a first side plate 5, a second side plate 6, a core rod 7, a bottom plate 8, a cover plate 9 and a lifting ring 10. The bottom plate 8 is arranged horizontally in the horizontal direction. The first side plate 5 is detachably mounted on the front side of the bottom plate 8, and the second side plate 6 is detachably mounted on the rear side of the bottom plate 8. The core rod 7 is in the shape of a truncated cone. The radius of the upper end face of the core rod 7 is greater than the radius of the lower end face of the core rod 7. A threaded hole 11 is provided at the center of the upper end face of the core rod 7 and extends downward. The threaded hole 11 is coaxial with the core rod 7. A latch 71 is fixedly arranged on the lower end face of the core rod 7. The latch 71 is plugged into the bottom plate 8. The lower end face of the core rod 7 is in contact with the upper end face of the bottom plate 8. The lower part of the lifting ring 10 is provided with an external thread matching the threaded hole 11, the first pressure head 1 and the second pressure head 2 are located above the bottom plate 8 and between the first side plate 5 and the second side plate 6, the first pressure head 1 and the second pressure head 2 are arranged in parallel on the left and right, a first groove 12 recessed to the left is provided on the right side surface of the first pressure head 1, the upper end surface of the first groove 12 is flush with the upper end surface of the first pressure head 1, the lower end surface of the first groove 12 is flush with the lower end surface of the first pressure head 1, a second groove 13 recessed to the right is provided on the left side surface of the second pressure head 2, the upper end surface of the second groove 13 is flush with the upper end surface of the second pressure head 2, the lower end surface of the second groove 13 is flush with the lower end surface of the second pressure head 2, the first groove 12 and the second groove 13 are symmetrical on the left and right, and the first linear moving mechanism 3 is used to drive the first ram 1 to move linearly along the left and right directions, and the second linear moving mechanism 4 is used to drive the second ram 2 to move linearly along the left and right directions. The first ram 1 and the second ram 2 can be moved to their preset mold closing position and mold opening position under the drive of the first linear moving mechanism 3 and the second linear moving mechanism 4. When the first ram 1 and the second ram 2 are located at their mold closing positions, the distance between the right end face of the first groove 12 and the left end face of the second groove 13 is 2a, and the value range of a is 4mm-7mm, and the cross-section of the first groove 12 and the second groove 13 on the same horizontal plane is located on the same elliptical surface, the major axis of the elliptical surface is along the front-to-back direction, the minor axis is along the left-to-right direction, and the distance between the major axis of the elliptical surface and the right end face of the first groove 12 is 2a. The distance between the long axis of the elliptical surface and the left end face of the second groove 13 is also a. The long axis length of the elliptical surface is recorded as b1, and the short axis length of the elliptical surface is recorded as b2. b1=D×a1, b2=D×a2, D represents the outer diameter of the large circular sintered NdFeB blank, a1 and a2 represent the shrinkage ratio respectively, wherein a1 has a value range of 1.195-1.205, a2 has a value range of 1.15-1.16, and a has a value range of 4mm-7mm. The straight line passing through the center of the elliptical surface and perpendicular to the bottom plate 8 is called the center line. The distance between the first side plate 5 and the center line is b1 / 2+L, and the distance between the second side plate 6 and the center line is b1 / 2+L. L has a value of 0.15mm-0.2mm; the central axis of the mandrel 7 coincides with the center line, and the diameter of the upper surface of the mandrel 7 is recorded as C, C = D'×a3, D' represents the inner diameter of the large circular sintered NdFeB billet, a3 represents the shrinkage ratio, and the value range of a3 is: 1.05-1.15, and the height of the mandrel 7 is recorded as d, d = h×a4, h is the height of the large circular sintered NdFeB billet, a4 represents the shrinkage ratio, and the value range of a4 is: 1.44-1.46, and the mandrel 7 The upper end surface, the upper end surface of the first pressure head 1 and the upper end surface of the second pressure head 2 are located in the same plane, the upper end surface of the first side plate 5 and the upper end surface of the second side plate 6 are located in the same plane, and the plane is 0.25mm-0.4mm higher than the plane where the upper end surface of the core rod 7 is located, and the cover plate 9 is detachably mounted on the first side plate 5 and the second side plate 6, and the lower end surface of the cover plate 9 is flush with the upper end surfaces of the first side plate 5 and the second side plate 6, and the cover plate 9 is located above the first pressure head 1 and the second pressure head 2;.

[0032] Step 2. In an oxygen-free working atmosphere, a forming mold is used to form a compact and prepare a large circular sintered NdFeB blank. The specific process is as follows:

[0033] S2-1. In the initial state, the first ram 1 and the second ram 2 are both located at their preset mold closing positions, and the cover plate 9 is installed on the first side plate 5 and the second side plate 6; first, the cover plate 9 is removed from the first side plate 5 and the second side plate 6, and then the first linear motion mechanism 3 drives the first ram 1 to move a distance to the left to its preset mold opening position, and the second linear motion mechanism 4 drives the second ram 2 to move a distance to the right to its preset mold opening position. The distance moved to the left by the first ram 1 is equal to the distance moved to the right by the second ram 2. At this time, a mold cavity is formed between the first side plate 5, the second side plate 6, the bottom plate 8, the core rod 7, the first ram 1 and the second ram 2;

[0034] S2-2, such as Figure 6 As shown, the powder 14 is filled into the mold cavity. When the mold cavity is filled with the powder 14, the cover plate 9 is installed on the first side plate 5 and the second side plate 6 again. At this time, the cover plate 9 covers the powder 14 in the mold cavity. Then the first linear motion mechanism 3 drives the first pressing head 1 to move rightward, and the second linear motion mechanism 4 drives the second pressing head 2 to move leftward. The first pressing head 1 and the second pressing head 2 press the powder during the movement. When the first linear motion mechanism 3 drives the first pressing head 1 to move rightward to its preset mold closing position, and the second linear motion mechanism 4 drives the second pressing head 2 to move leftward to its preset mold closing position, the powder 14 is pressed. The density is 3.95g / cm 3 ~4.15g / cm 3 The pressed green sheet, such as Figure 7 As shown;

[0035] S2-3, first remove the cover plate 9 from the first side plate 5 and the second side plate 6, screw the lower external thread of the lifting ring 10 into the threaded hole 11 of the core rod 7, at this time, the lifting ring 10 and the core rod 7 are fixedly connected, and then operate the lifting ring 10 to pull out the core rod 7, and then remove the first side plate 5 and the second side plate 6 from the bottom plate 8, the first linear motion mechanism 3 drives the first pressure head 1 to move a distance to the left to its preset mold opening position, and the second linear motion mechanism 4 drives the second pressure head 2 to move a distance to the right to its preset mold opening position;

[0036] S2-4, first pick up the pressed green sheet and put it into a packaging bag, then insert a wooden cylindrical plug from both ends of the inner hole of the pressed green sheet through the packaging bag until the two wooden cylindrical plugs are connected, and the diameters of the two wooden cylindrical plugs are equal and 3mm-5mm smaller than the diameter of the inner hole of the pressed green sheet;

[0037] S2-5, at a pressure of 30 MPa, using a vacuum machine to vacuum-pack the packaging bag to obtain a vacuum-packed green compact;

[0038] S2-6. At a pressure of 200MPa-230MPa, the vacuum-encapsulated green compact is subjected to isostatic pressing to increase the density of the green compact to 4.4g / cm 3 -4.8g / cm 3 , to avoid cracking of the inner wall due to excessive size shrinkage;

[0039] S2-7, remove the packaging bag outside the green compact, then sinter the green compact in a vacuum sintering furnace at 998°C to 1100°C for 3-8 hours, cool to room temperature, and then perform aging treatment to obtain a large circular sintered NdFeB green compact with a circular outer wall.

[0040] In this embodiment, the powder in step S2-2 is obtained by the following method:

[0041] S2-1, smelting: the sintered NdFeB raw materials with cleaned surface are loaded into a vacuum sheet-throwing furnace, firstly smelted at a temperature of 1455℃-1485℃ to obtain a molten liquid, and then the molten liquid is cast to obtain a casting with a thickness of 2.5mm-3mm. When the casting temperature is less than or equal to 45℃, the casting is taken out from the vacuum sheet-throwing furnace;

[0042] S2-2, hydrogen crushing and powder making: Under the protection of oxygen-free atmosphere, the obtained casting flakes are firstly processed into coarse powder by hydrogen crushing process, and then lubricant is added to the coarse powder and stirred evenly, and then it is processed by air jet mill to prepare fine powder with an average particle size of 2.8μm to 3.6μm, which is the obtained powder.

[0043] In this embodiment, the material of the first side plate 5 and the second side plate 6 is G60, the material of the first pressure head 1 and the second pressure head 2 is 70Mn, the material of the bottom plate 8 and the cover plate 9 is GCr15, and the hardness requirements after quenching are 45-55HRC and the roughness is Ra0.4, which ensures the wear resistance of the material, ensures that the blank does not stick to the mold, and improves the forming stability. The material of the core rod 7 is austenitic stainless steel with a roughness of Ra0.4 to ensure the integrity of the inner wall of the circular ring blank during demolding.

[0044] In this embodiment, the demoulding angle β of the core rod 7 (ie, the angle between the lower base and the waist of the isosceles trapezoidal cross section of the core rod 7 along the vertical direction) is 88° to 89°.

Claims

1. A method for preparing a large circular sintered NdFeB blank, characterized by comprising the following steps: Step 1, obtain a forming mold, the forming mold includes a first pressure head, a second pressure head, a first linear motion mechanism, a second linear motion mechanism, a first side plate, a second side plate, a core rod, a bottom plate, a cover plate and a lifting ring, the bottom plate is horizontally arranged, the first side plate is detachably mounted on the front side of the bottom plate, the second side plate is detachably mounted on the rear side of the bottom plate, the core rod is in the shape of a truncated cone, the radius of the upper end face of the core rod is greater than the radius of the lower end face of the core rod, a threaded hole is provided at the center of the upper end face of the core rod extending downward, the threaded hole is coaxial with the core rod, a latch is fixedly arranged on the lower end face of the core rod, and the latch is plugged into On the bottom plate, the lower end surface of the core rod is fitted with the upper end surface of the bottom plate, the lower part of the hanging ring is provided with an external thread matching the threaded hole, the first pressure head and the second pressure head are located above the bottom plate and between the first side plate and the second side plate, the first pressure head and the second pressure head are arranged in parallel on the left and right, a first groove recessed to the left is provided on the right side surface of the first pressure head, the upper end surface of the first groove is flush with the upper end surface of the first pressure head, the lower end surface of the first groove is flush with the lower end surface of the first pressure head, a second groove recessed to the right is provided on the left side surface of the second pressure head, the upper end surface of the second groove is flush with the second The upper end surface of the pressure head is flush, the lower end surface of the second groove is flush with the lower end surface of the second pressure head, the first groove and the second groove are symmetrical, the first linear motion mechanism is used to drive the first pressure head to move linearly along the left and right directions, and the second linear motion mechanism is used to drive the second pressure head to move linearly along the left and right directions. The first pressure head and the second pressure head can be moved to their preset mold closing position and mold opening position under the drive of the first linear motion mechanism and the second linear motion mechanism. When the first pressure head and the second pressure head are located at their mold closing positions, the distance between the right end surface of the first groove and the left end surface of the second groove is 2a, a The value range of is 4mm-7mm, and the cross section of the first groove and the second groove on the same horizontal plane is located on the same elliptical surface, the major axis of the elliptical surface is along the front-to-back direction, and the minor axis is along the left-to-right direction. The distance between the major axis of the elliptical surface and the right end face of the first groove is a, and the distance between the major axis of the elliptical surface and the left end face of the second groove is also a. The length of the major axis of the elliptical surface is recorded as b1, and the length of the minor axis of the elliptical surface is recorded as b2. b1=D×a1, b2=D×a2, D represents the outer diameter of the large circular sintered NdFeB blank, a1 and a2 represent the shrinkage ratio respectively, wherein the value range of a1 is: 1.195-1.205, and the value range of a2 is 1.15-1.

16. The straight line passing through the center of the elliptical surface and perpendicular to the bottom plate is called the center line. The distance between the first side plate and the center line is b1 / 2+L, and the distance between the second side plate and the center line is b1 / 2+L, where L is 0.15-0.2mm. The center axis of the mandrel coincides with the center line. The diameter of the upper surface of the mandrel is C, C=D'×a3, where D' represents the inner diameter of the large circular sintered NdFeB blank, and a3 represents the shrinkage ratio, and the value range of a3 is:. 1.05-1.15, the height of the core rod is recorded as d, d = h × a4, h is the height of the large ring sintered NdFeB billet, a4 represents the shrinkage ratio, and the value range of a4 is: 1.44-1.46, the upper end face of the core rod, the upper end faces of the first pressure head and the second pressure head are located in the same plane, the upper end face of the first side plate and the upper end face of the second side plate are located in the same plane, and the plane is 0.25-0.4mm higher than the plane where the upper end face of the core rod is located, the cover plate is detachably mounted on the first side plate and the second side plate, the lower end face of the cover plate is flush with the upper end faces of the first side plate and the second side plate, and the cover plate is located above the first pressure head and the second pressure head; Step 2. In an oxygen-free working atmosphere, the molding die is used to mold the green sheet and prepare a large circular sintered NdFeB blank. The specific process is as follows: S2-1. In the initial state, the first pressing head and the second pressing head are both located at their preset mold closing positions, and the cover plate is installed on the first side plate and the second side plate; first, the cover plate is removed from the first side plate and the second side plate, and then the first linear motion mechanism drives the first pressing head to move a distance to the left to its preset mold opening position, and the second linear motion mechanism drives the second pressing head to move a distance to the right to its preset mold opening position, and the distance moved to the left by the first pressing head is equal to the distance moved to the right by the second pressing head, and at this time, a mold cavity is formed among the first side plate, the second side plate, the bottom plate, the core rod, the first pressing head and the second pressing head; S2-2, fill the powder into the mold cavity. When the mold cavity is filled with the powder, install the cover plate onto the first side plate and the second side plate again. At this time, the cover plate covers the powder in the mold cavity. Then the first linear motion mechanism drives the first pressure head to move rightward, and the second linear motion mechanism drives the second pressure head to move leftward. The first pressure head and the second pressure head press the powder during the movement. When the first linear motion mechanism drives the first pressure head to move rightward to its preset mold closing position, and the second linear motion mechanism drives the second pressure head to move leftward to its preset mold closing position, the powder pressing is completed, and the density is 3.95g / cm 3 ~4.15g / cm 3 of compact; S2-3, first remove the cover plate from the first side plate and the second side plate, screw the lower external thread of the lifting ring into the threaded hole of the core rod, at this time, the lifting ring and the core rod are fixedly connected, then operate the lifting ring to pull out the core rod, and then remove the first side plate and the second side plate from the bottom plate, the first linear motion mechanism drives the first pressure head to move a distance to the left to its preset mold opening position, and the second linear motion mechanism drives the second pressure head to move a distance to the right to its preset mold opening position; S2-4, first pick up the pressed green sheet and put it into a packaging bag, then insert a wooden cylindrical plug from both ends of the inner hole of the pressed green sheet through the packaging bag until the two wooden cylindrical plugs are connected, and the diameters of the two wooden cylindrical plugs are equal and 3mm-5mm smaller than the diameter of the inner hole of the pressed green sheet; S2-5, at a pressure of 30 MPa, using a vacuum machine to vacuum-pack the packaging bag to obtain a vacuum-packed green compact; S2-6. At a pressure of 200MPa-230MPa, the vacuum-encapsulated green compact is subjected to isostatic pressing to increase the density of the green compact to 4.4g / cm 3 -4.8g / cm 3 , to avoid cracking of the inner wall due to excessive size shrinkage; S2-7, remove the packaging bag outside the green compact, then sinter the green compact in a vacuum sintering furnace at 998°C to 1100°C for 3-8 hours, cool to room temperature, and then perform aging treatment to obtain a large circular sintered NdFeB green compact with a circular outer wall.

2. The method for preparing a large circular sintered NdFeB blank according to claim 1, characterized in that The powder in step S2-2 is obtained by the following method: S2-1, smelting: the sintered NdFeB raw materials with cleaned surface are loaded into a vacuum sheet-throwing furnace, firstly smelted at a temperature of 1455℃-1485℃ to obtain a molten liquid, and then the molten liquid is cast to obtain a casting with a thickness of 2.5mm-3mm. When the casting temperature is less than or equal to 45℃, the casting is taken out from the vacuum sheet-throwing furnace; S2-2, hydrogen crushing and powder making: Under the protection of oxygen-free atmosphere, the obtained casting flakes are firstly processed into coarse powder by hydrogen crushing process, and then lubricant is added to the coarse powder and stirred evenly, and then it is processed by air jet mill to prepare fine powder with an average particle size of 2.8μm to 3.6μm, which is the obtained powder.

3. The method for preparing a large circular sintered NdFeB blank according to claim 1, characterized in that The material of the first side plate and the second side plate is G60, the material of the first pressure head and the second pressure head is 70Mn, the material of the bottom plate and the cover plate is GCr15, and the hardness requirements after quenching are 45-55HRC and the roughness is Ra0.4, which ensures the wear resistance of the material, ensures that the pressed billet does not stick to the mold, and improves the forming stability. The core rod material is austenitic stainless steel with a roughness of Ra0.4 to ensure the integrity of the inner wall of the circular ring pressed billet during demolding.

4. The method for preparing a large circular sintered NdFeB blank according to claim 1, characterized in that The demoulding angle β of the core rod is 88° to 89°.

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