Manufacturing method of large boron carbide ceramic ring

Abstract

The invention discloses a large boron carbide ceramic ring manufacturing method and belongs to the field of nuclear power. The method comprises the following steps: providing first powder, second powder and third powder of which the average particle sizes are sequentially increased as boron carbide raw material powder, uniformly mixing, and sintering for 1-5 hours at the temperature of 1800-2000 DEG C and the pressure intensity of 20-40 MPa to obtain an annular blank; and grinding and polishing the plane of the annular blank, and performing ultrasonic-assisted vibration grinding and polishing on the outer arc surface to obtain a finished product of the large boron carbide ceramic ring. According to the method, the production efficiency and quality of the large boron carbide ceramic ring can be improved, and the edge breakage or cracking risk is reduced.

Description

Technical Field

[0001] The invention belongs to the field of nuclear power, and in particular relates to a method for manufacturing a large boron carbide ceramic ring. Background Art

[0002] Boron carbide ceramic materials have high strength, high hardness, high temperature resistance, and stable chemical properties. At the same time, the B element in them has a high thermal neutron capture cross section and a wide capture energy spectrum. Therefore, it has been widely used as a neutron absorption / shielding material in the nuclear industry, among which large boron carbide ceramic rings (diameter above 100mm) have a wide range of uses. However, boron carbide has a high melting point and a low self-diffusion coefficient, making it difficult to sinter and densify. At the same time, boron carbide has low fracture toughness and poor electrical conductivity. It is prone to stress cracking during the cutting and manufacturing process, resulting in low production efficiency of large boron carbide ceramic rings. Microcracks and other defects are prone to exist in the finished product, affecting its mechanical properties and neutron absorption performance. Therefore, it is of positive significance to provide a method for manufacturing large boron carbide ceramic rings that can improve the quality of the finished product. Summary of the invention

[0003] The object of the present invention is to provide a method for manufacturing a large boron carbide ceramic ring to reduce defects in finished products of the large boron carbide ceramic ring.

[0004] According to an embodiment of the present invention, a method for manufacturing a large boron carbide ceramic ring is provided, the method comprising the following steps: Step a): providing boron carbide raw material powder, wherein the boron carbide raw material powder comprises a first powder, a second powder and a third powder, wherein, in terms of average particle size, the first powder < the second powder < the third powder; uniformly mixing the first powder, the second powder and the third powder to obtain a mixed powder; Step b): the mixed powder is loaded into an annular mold, and sintered at a temperature of 1800° C. to 2000° C. and a pressure of 20 MPa to 40 MPa for 1 h to 5 h to obtain an annular blank; Step c): Grinding and polishing the plane of the annular blank, and performing ultrasonic-assisted vibration grinding and polishing on the outer arc surface of the annular blank to obtain a large boron carbide ceramic ring.

[0005] By using raw powders with different average particle sizes, the density of the green body can be effectively improved and the risk of cracking caused by stress concentration can be reduced, thereby obtaining a boron carbide ceramic body with better organizational uniformity and higher mechanical properties; further, the use of surface grinding and polishing and arc surface ultrasonic-assisted vibration grinding and polishing processing methods can further reduce residual stress and avoid defects such as cracking and edge collapse during the processing process.

[0006] Furthermore, in some embodiments, in the step a), the average particle size of the first powder is 0.7 μm-1.2 μm, the average particle size of the second powder is 4 μm-12 μm, and the average particle size of the third powder is 20 μm-30 μm.

[0007] Furthermore, in some embodiments, in the step a), by weight, the first powder accounts for 5%-15%, the second powder accounts for 68%-80%, and the third powder accounts for 5%-27%.

[0008] Furthermore, in some embodiments, in the step a), the first powder, the second powder and the third powder are mixed using a three-dimensional mixer.

[0009] Furthermore, in some embodiments, in the step b), before the mixed powder is loaded into the annular mold, graphite paper is arranged on the surface of the annular mold to separate the mixed powder from the annular mold.

[0010] Furthermore, in some embodiments, in the step c), the amount of grinding and polishing performed on the plane of the annular blank is 1 mm to 3 mm.

[0011] Furthermore, in some embodiments, in the step c), the processing depth of the ultrasonic-assisted vibration grinding and polishing of the arc surface of the annular blank is 0.1 mm-0.5 mm.

[0012] Furthermore, in some embodiments, in step c), the ultrasonic machining tool head used in ultrasonic-assisted vibration grinding and polishing is a diamond grinding wheel with a grit size of at least 200, and the outer diameter of the diamond grinding wheel is 40 mm-80 mm and the thickness is 10 mm-30 mm.

[0013] Furthermore, in some embodiments, in step c), the ultrasonic frequency used in ultrasonic-assisted vibration grinding and polishing is 16kHz-60kHz, and the grinding speed is 17000rpm-25000rpm.

[0014] Furthermore, in some embodiments, the large boron carbide ceramic ring has an outer diameter of 100 mm-350 mm, a wall thickness of 30 mm-60 mm, a height of 80 mm-180 mm, and a surface roughness of no more than 5 μm.

[0015] Furthermore, in some embodiments, the axial compressive strength of the large boron carbide ceramic ring is not less than 1 GPa, and the Vickers hardness is not less than 2000 HV1. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1This is a microscopic electron microscope photo of the structure of a large boron carbide ceramic ring in the first preferred embodiment; Figure 2 It is a schematic diagram of grinding and polishing of large boron carbide ceramic annular arc surface in one embodiment; Figure 3 This is a photo of the finished product of the large boron carbide ceramic ring in the first comparison example; Figure 4 This is a finished product photo of a large boron carbide ceramic ring in the first preferred embodiment; Figure 5 This is a cross-sectional photo of the finished product of the large boron carbide ceramic ring in the second comparative example; Figure 6 This is a cross-sectional photograph of the finished product of the large boron carbide ceramic ring in the first preferred embodiment.

[0017] Meaning of the accompanying numerals: 1-ring blank; 2-rotating chassis; 3-clamp; 4-diamond grinding wheel.

[0018] The purpose of the above drawings is to explain the present invention in detail so that those skilled in the art can understand the technical concept of the present invention, but it is not intended to limit the present invention. For the sake of simplicity, the above drawings only schematically illustrate the structures related to the technical features of the present invention, and do not strictly follow the actual proportions to draw the complete structure and all details. DETAILED DESCRIPTION

[0019] The present invention is further described in detail below through specific embodiments in conjunction with the accompanying drawings.

[0020] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present invention. The phrase appearing in various locations in the specification does not necessarily refer to the same embodiment, nor is it limited to mutually exclusive independent or alternative embodiments. Those skilled in the art should be able to understand that the embodiments herein may be combined with other embodiments without causing structural conflicts.

[0021] In the description of this article, the terms "first", "second" and the like are only used to distinguish different objects, and cannot be understood as indicating relative importance or limiting the quantity, specific order or primary and secondary relationship of the described technical features. In the description of this article, the meaning of "plurality" is at least two.

[0022] Boron carbide has the characteristics of high melting point, high vapor pressure and low self-diffusion coefficient, which makes it difficult to densify boron carbide during the sintering process. At the same time, boron carbide has high hardness, low fracture toughness, weak electrical conductivity and poor thermal conductivity, which makes it difficult to cut boron carbide using traditional wire cutting methods, making it difficult to form and having low processing efficiency. At the same time, due to its significant brittle tendency, stress cracking is prone to occur during processing, limiting the application of large boron carbide ceramic parts.

[0023] At present, the conventional sintering methods of boron carbide include pressureless sintering and hot pressing sintering. Pressureless sintering can achieve near-net-size forming of boron carbide with low production cost. However, it is difficult to densify boron carbide during pressureless sintering, and the internal uniformity is poor. The high sintering temperature easily causes abnormal growth, and micro cracks are easily generated during the sintering process, which affects the overall quality of boron carbide parts. Compared with pressureless sintering, boron carbide ceramics prepared by hot pressing sintering have better uniformity, but for boron carbide ceramic parts with certain complex structures such as large boron carbide ceramic rings, the hot pressing forming process has certain difficulties in forming. Therefore, the hot pressing sintering process is currently generally used to manufacture boron carbide ceramics with simple regular shapes, but cannot be used for ring manufacturing.

[0024] In order to solve the above problems, an embodiment of the present invention provides a method for manufacturing a large boron carbide ceramic ring, which improves the production efficiency and product quality of the boron carbide ceramic ring.

[0025] Specifically, the method comprises the following steps: Step a): Provide boron carbide raw material powder, the raw material powder includes a first powder, a second powder and a third powder. Wherein, in terms of average particle size, the first powder < the second powder < the third powder. In a preferred embodiment, the average particle size of the first powder is 0.7 μm-1.2 μm, the average particle size of the second powder is 4 μm-12 μm, and the average particle size of the third powder is 20 μm-30 μm.

[0026] The first powder, the second powder and the third powder are mixed evenly to obtain a mixed powder. In a preferred embodiment, by weight, the first powder accounts for 5%-15%, the second powder accounts for 68%-80%, and the third powder accounts for 5%-27%. Beyond this ratio range, the loose density and tap density of the boron carbide powder green body are reduced, and the density uniformity deteriorates during the filling of the mold, which may lead to stress concentration cracking during the sintering process. If the content of coarse-grained powder increases, in the process of obtaining a ceramic block of the same density, the hot pressing sintering process, including the sintering temperature, time or sintering pressure, will greatly increase, which will cause grain coarsening and uneven organization, reduce the mechanical properties of the ceramic, and more easily produce stress concentration to crack the highly brittle boron carbide. The lower the content of fine-grained boron carbide in theory, the lower the sintering temperature will be. However, when the content of fine-grained boron carbide increases, the powder fluidity deteriorates, which may easily cause uneven filling in the annular mold.

[0027] The powder mixing is carried out by a three-dimensional mixer to achieve the flow and diffusion between boron carbide particles of different particle sizes to obtain a uniform mixed powder. After the mixing is completed, the samples are taken by the quartering method and inspected by a scanning electron microscope to ensure the uniform dispersion of the powder particles.

[0028] Step b): The mixed powder is loaded into an annular mold and hot-pressed and sintered to obtain an annular blank.

[0029] Before the mixed powder is loaded into the mold, graphite paper is firstly arranged in the mold to separate the mixed powder from the mold, so as to avoid adhesion between the mixed powder and the mold during the hot pressing sintering process.

[0030] During sintering, the sintering temperature is controlled at 1800°C-2000°C, the pressure is 20MPa-40MPa, and the sintering time is 1h-5h.

[0031] Step c): Grinding and polishing the plane of the annular blank, and performing ultrasonic-assisted vibration grinding and polishing on the outer arc surface to obtain a large boron carbide ceramic ring finished product.

[0032] Specifically, the processing amount of the plane polishing is controlled to be 1 mm-3 mm, and the processing amount of the ultrasonic-assisted vibration grinding and polishing on the curved surface is 0.1 mm-0.5 mm.

[0033] Ultrasonic assisted vibration grinding and polishing process Figure 2 As shown, the annular blank 1 is placed on a rotating chassis 2 and fixed by inserting a fixture 3 in the axial direction, and an ultrasonic machining tool head is used to perform ultrasonic-assisted vibration grinding and polishing on the outer peripheral side of the annular blank 1.

[0034] Specifically, the ultrasonic machining tool head is configured as a diamond grinding wheel 4, the particle size of the grinding wheel is 200 mesh, and in some embodiments, a higher mesh number such as 400 mesh or 800 mesh can also be used. The outer diameter of the diamond grinding wheel is 40mm-80mm, and the thickness is 10mm-30mm. During the processing, the ultrasonic frequency used in ultrasonic-assisted vibration grinding and polishing is 16kHz-60kHz (ultrasonic tool handle frequency), the grinding speed is 17000rpm-25000rpm, and the speed of the rotating chassis 2 is 20rpm-50rpm.

[0035] The finished large boron carbide ceramic ring prepared by the above method has an outer diameter of 100mm-350mm, a wall thickness of 30mm-60mm, a height range of 80mm-180mm, and a surface roughness of no more than 5μm.

[0036] This method uses boron carbide powder of different particle sizes as raw materials, effectively improving the fluidity and formability of boron carbide powder during hot pressing and sintering. The density of the finished product reaches more than 90%, which improves the uniformity of the organization and reduces the risk of stress concentration of boron carbide ceramics after sintering. The outer arc surface of the boron carbide ring is processed by ultrasonic-assisted vibration grinding and polishing, which effectively avoids the risk of edge collapse and stress cracking of the high-hardness and brittle ring structure after sintering during the processing process, while reducing tool wear, reducing the frequency of tool changes, and improving the yield rate of finished products and production efficiency.

[0037] In the first preferred embodiment, the manufacturing process of the large boron carbide ceramic ring is as follows: Nuclear-grade boron carbide powders were weighed according to the ratio, wherein the first powder had an average particle size of 0.8 μm, the second powder had an average particle size of 6 μm, and the third powder had an average particle size of 23 μm. The weight ratio of the first powder, the second powder, and the third powder was 1:8:1, and the three powders were mixed and evenly mixed using a three-dimensional mixer to obtain a mixed powder.

[0038] The graphite paper is evenly spread on the surface of the annular mold for molding, the mixed powder is filled into the mold, the mold is placed in a vacuum hot pressing sintering furnace, and sintered at 1850°C for 4 hours to obtain a near-net-size annular blank.

[0039] The plane surfaces at both ends of the ring blank were ground and polished using a surface grinder, with a grinding and polishing amount of 0.5 mm. Figure 2 The outer ring surface of the ring blank is subjected to ultrasonic assisted vibration grinding in the manner shown, the grinding and polishing speed is 19000rpm, the speed of the rotating chassis 2 is 35rpm, the grinding amount is 0.1mm, and a large boron carbide ceramic ring that meets the design requirements is obtained. The boron carbide ceramic ring has an outer diameter of 260mm, a wall thickness of 30mm, a height of 120mm, a complete structure, and no defects such as cracks and edge collapse.

[0040] The boron carbide ceramic ring was sampled and analyzed by scanning electron microscope. The microstructure morphology is as follows: Figure 1 As shown, it can be seen that its organizational structure is uniform, without obvious defects such as cracks.

[0041] The mechanical properties of the large boron carbide were tested, and the maximum axial compressive load could reach 250368N. The axial compressive strength of the sample reached 1364MPa, and the hardness reached 2093HV1, showing good mechanical properties.

[0042] In the second preferred embodiment, the manufacturing process of the large boron carbide ceramic ring is as follows: Nuclear-grade boron carbide powders were weighed according to the ratio, wherein the first powder had an average particle size of 1.1 μm, the second powder had an average particle size of 12 μm, and the third powder had an average particle size of 28 μm. The weight ratio of the first powder, the second powder, and the third powder was 1:7.5:1.5, and the three powders were mixed and evenly mixed using a three-dimensional mixer to obtain a mixed powder.

[0043] The graphite paper is evenly spread on the surface of the annular mold for molding, the mixed powder is filled into the mold, the mold is placed in a vacuum hot pressing sintering furnace, and sintered at 2000°C for 1 hour to obtain a nearly net-size annular blank.

[0044] The plane surfaces at both ends of the ring blank were ground and polished using a surface grinder, with a grinding and polishing amount of 0.3 mm. Figure 2The outer ring surface of the ring blank is subjected to ultrasonic assisted vibration grinding in the manner shown, the grinding and polishing speed is 23000rpm, the speed of the rotating chassis 2 is 25rpm, the grinding amount is 0.2mm, and a large boron carbide ceramic ring that meets the design requirements is obtained. The boron carbide ceramic ring has an outer diameter of 260.04mm, an inner diameter of 200.01mm, and a height of 120.02mm.

[0045] The mechanical properties of the large boron carbide were tested, and the maximum axial compressive load could reach 250665N. The axial compressive strength of the sample reached 1740MPa, and the hardness reached 2135HV1, showing good mechanical properties.

[0046] In the first comparative example, the large boron carbide ceramic ring product obtained after sintering was processed by conventional diamond wire cutting process. Figure 3 As shown in FIG. 1 , due to the significant brittleness of the boron carbide ceramic material, the edge of the sample is broken and the corner is chipped, resulting in damage and scrapping of the product. In the first preferred embodiment, the finished product of the large boron carbide ceramic ring treated by ultrasonic assisted vibration grinding and polishing is as follows: Figure 4 As shown, it can be seen that its appearance is complete, without edge collapse or corner chipping defects, and has good integrity and dimensional accuracy.

[0047] In the second comparative example, a large boron carbide ceramic ring is prepared by sintering boron carbide powder of a single particle size (particle size is 18 μm), and the cross section of the obtained finished product is as follows: Figure 5 As shown in the A area, the fracture surface has obvious macroscopic fluctuations and shows obvious non-uniformity, indicating that there is obvious stress concentration in the structure of the large boron carbide ceramic ring. In the first preferred embodiment, the large boron carbide ceramic ring is obtained by mixing and sintering the first powder (particle size 0.8μm), the second powder (particle size 6μm) and the third powder (particle size 23μm) in a weight ratio of 1:8:1. The cross-sectional morphology is as follows Figure 6 As shown in the B area, it can be seen that the fracture morphology is uniform, and the cross section appears grayish white due to the fine and uniform brittle fracture structure, without obvious macroscopic fluctuations, indicating that there is no obvious stress concentration phenomenon inside the large boron carbide ceramic provided in the embodiment.

[0048] It can be seen that compared with the comparative example, the large boron carbide ceramic ring manufacturing method provided by the embodiment of the present invention effectively improves the molding quality of the boron carbide ceramic ring, reduces the risk of edge collapse or cracking caused by stress concentration, improves the production efficiency of large boron carbide ceramic rings, and optimizes the product economy and reliability.

[0049] The purpose of the above embodiments is to further explain the present invention in detail in conjunction with the accompanying drawings so that those skilled in the art can understand the technical concept of the present invention. Within the scope of the present invention, the optimization or equivalent replacement of the method steps involved, and the combination of the implementation methods in different embodiments without conflict of structure and principle, all fall within the protection scope of the present invention.

Claims

1. A method for manufacturing a large boron carbide ceramic ring, characterized in that: The following steps are involved: Step a): providing boron carbide raw material powder, wherein the boron carbide raw material powder comprises a first powder, a second powder and a third powder, wherein, in terms of average particle size, the first powder < the second powder < the third powder; uniformly mixing the first powder, the second powder and the third powder to obtain a mixed powder; Step b): the mixed powder is loaded into an annular mold, and sintered at a temperature of 1800° C. to 2000° C. and a pressure of 20 MPa to 40 MPa for 1 h to 5 h to obtain an annular blank; Step c): Grinding and polishing the plane of the annular blank, and performing ultrasonic-assisted vibration grinding and polishing on the outer arc surface of the annular blank to obtain a large boron carbide ceramic ring.

2. The method for manufacturing a large boron carbide ceramic ring according to claim 1, characterized in that: In the step a), the average particle size of the first powder is 0.7 μm-1.2 μm, the average particle size of the second powder is 4 μm-12 μm, and the average particle size of the third powder is 20 μm-30 μm.

3. The method for manufacturing a large boron carbide ceramic ring according to claim 2, characterized in that: In the step a), by weight, the first powder accounts for 5%-15%, the second powder accounts for 68%-80%, and the third powder accounts for 5%-27%.

4. The method for manufacturing a large boron carbide ceramic ring according to claim 1, 2 or 3, characterized in that: In the step a), the first powder, the second powder and the third powder are mixed by a three-dimensional mixer.

5. The method for manufacturing a large boron carbide ceramic ring according to claim 1, characterized in that: In the step b), before the mixed powder is loaded into the annular mold, graphite paper is arranged on the surface of the annular mold to separate the mixed powder from the annular mold.

6. The method for manufacturing a large boron carbide ceramic ring according to claim 1, characterized in that: In the step c), the surface of the annular blank is ground and polished to a depth of 1 mm to 3 mm.

7. The method for manufacturing a large boron carbide ceramic ring according to claim 1, characterized in that: In the step c), the processing depth of the ultrasonic-assisted vibration grinding and polishing on the arc surface of the annular blank is 0.1 mm-0.5 mm.

8. The method for manufacturing a large boron carbide ceramic ring according to claim 7, characterized in that: In the step c), the ultrasonic machining tool head used in the ultrasonic-assisted vibration grinding and polishing is a diamond grinding wheel with a particle size of at least 200 meshes, and the outer diameter of the diamond grinding wheel is 40 mm-80 mm and the thickness is 10 mm-30 mm.

9. The method for manufacturing a large boron carbide ceramic ring according to claim 7 or 8, characterized in that: In the step c), the ultrasonic frequency used in the ultrasonic-assisted vibration grinding and polishing is 16kHz-60kHz, and the grinding speed is 17000rpm-25000rpm.

10. The method for manufacturing a large boron carbide ceramic ring according to claim 1, characterized in that: The large boron carbide ceramic ring has an outer diameter of 100 mm to 350 mm, a wall thickness of 30 mm to 60 mm, a height of 80 mm to 180 mm, and a surface roughness of no more than 5 μm.

11. The method for manufacturing a large boron carbide ceramic ring according to claim 10, characterized in that: The large boron carbide ceramic ring has an axial compressive strength of not less than 1 GPa and a Vickers hardness of not less than 2000 HV1.

Images