2023 June The Second Week WBM Technical Knowledge on Grinding Technology Of Carbon/Carbon Composite Sealing Ring
Abstract: In response to the characteristics of carbon/carbon composite materials, through the research and application of grinding tools, abrasives, fixtures, motion trajectories, etc., the article has preliminarily summarized a practical and feasible grinding method for double sealing surface sealing rings. The surface quality of the parts has been greatly improved and relatively stable, meeting the requirements of the design drawings, and solving the problem of flat grinding for carbon/carbon composite material sealing rings with large diameter, thin wall, and double sealing surfaces.
Keywords: carbon/carbon composite materials; Sealing ring grinding; Double face seal
Carbon graphite material is the most widely used friction material and has good pairing performance with various metals, non-metals, and coatings. It is one of the only sealing dual materials suitable for most dry friction situations. Carbon/carbon composite materials are all carbon composite materials made from carbon fibers and their fabrics as reinforcing materials, carbon as the matrix, and processed and carbonized. Its characteristics are significant: the specific gravity is less than 2.0g/cm3; Excellent high-temperature mechanical properties; Good ablation resistance; Having the characteristics of other composite materials, such as high strength, high modulus, high fatigue, and creep performance; Excellent friction and wear performance, low friction coefficient, stable performance, making it the best candidate material for various wear-resistant and friction components.
The carbon/carbon composite material sealing ring (hereinafter referred to as the sealing ring) is a double face sealing component with a large diameter, high dimensional accuracy, high shape accuracy, and low surface roughness on both ends, making it very difficult to process. Flat grinding is the final processing process of sealing rings and is the key to sealing ring processing. It is important to use appropriate grinding tools, abrasives, and fixtures. At the same time, there are high requirements for grinding methods and strict requirements for the grinding environment.
Based on the characteristics of carbon/carbon composite materials, this article explores and studies the planar grinding technology of double sealing surface sealing rings from the perspectives of grinding tools, abrasives, tooling, and motion trajectory.
1 Sealing ring analysis
1.1 Analysis of Sealing Ring Structure
The sealing ring is a thin-walled circular component with a large diameter (approximately Φ 180), thin in thickness (nearly 10mm), and narrow in width (nearly 10mm), it mainly has the following structural features: end face ring groove, outer ring groove, inner and outer circular lace, end face groove, axial hole, radial hole, etc; The left and right end faces are sealing surfaces with high dimensional accuracy, roughness, and shape and position requirements, with a roughness not greater than Ra0.2 μ m. The flatness is 0.0009mm, the parallelism is not greater than 0.005mm, and the perpendicularity to the inner hole and outer circle is not greater than 0.01mm.
1.2 Material characteristics
The carbon/carbon composite material blank is shown in Figure 1, with carbon fibers interlaced and stacked in a dense mesh shape, and carbon particles filled between the gaps of the carbon fibers. Compared with ordinary graphite materials, it has good rigidity and high strength, with a density of 1.8-2.0g/cm3 and a Shore hardness of ≤ 70.
Carbon/carbon composite material is a new type of ultra-high temperature material that is focused on research and development in the field of new materials. It has the following significant characteristics:
The density is small (<2.0g/cm3), only 1/4 of that of nickel based high-temperature alloys and 1/2 of that of ceramic materials.
Excellent high-temperature mechanical properties. When the temperature rises to 2200 ℃, its strength not only does not decrease, but it is even higher than room temperature, which is incomparable to other structural materials.
Good anti ablation performance, uniform ablation, and can be used in environments with high temperature and short-term ablation above 3000 ℃.
Excellent friction and wear performance, with low friction coefficient and stable performance, making it the best candidate material for various wear-resistant and friction components.
Having the characteristics of other composite materials, such as high strength, high modulus, high fatigue, and creep performance.
1.3 Difficulties in Plane Grinding
Grinding has many advantages: high dimensional accuracy, high shape accuracy, low surface roughness, improved surface wear resistance of parts, and improved surface fatigue strength of parts. Grinding is the final process of sealing ring processing technology, and the quality of grinding directly affects the qualification of finished parts, and its importance is self-evident. The grinding of sealing rings has the following difficulties: (1) The diameter of the sealing ring is large, the wall is thin, and the grinding of the double sealing end face is difficult; (2) The surface of the sealing ring is dim, with poor luminosity, and optical flat crystal detection is difficult.
2 Research on Plane Grinding Technology
In response to the difficulties in grinding carbon/carbon composite sealing rings, preliminary exploration and research were conducted in the following seven aspects.
2.1 Research tools
A grinding tool is a carrier of abrasive used for coating and embedding abrasives, allowing free abrasive particles to embed into the grinding tool and exert cutting effects; At the same time, it is a tool for grinding and forming, which transmits its geometric shape accuracy to the workpiece in a certain way. There is a very important requirement for grinding tools: the hardness selection of grinding tools is particularly important, as being too hard can cause abrasive particles to quickly break and wear, and even some abrasive particles are squeezed into the parts, damaging the surface quality of the machining; Too soft causes the abrasive particles to be squeezed too deeply into the grinding material. Properly selecting the hardness of the grinding tool can temporarily support the abrasive particles and quickly change their position, so that each abrasive particle has new edges and corners to participate in cutting.
Our unit has always used gray cast iron, a metal material with relatively small deformation at room temperature, as the grinding tool material for grinding carbon graphite seals. It has dense organization, uniform hardness, appropriate embeddability, good wear resistance, small deformation, and moderate hardness, which is suitable for use as a grinding tool for carbon graphite materials.
2.2 Grinding Compound
Grinding agent is a mixture composed of abrasive particles, grinding fluid, and auxiliary materials. The abrasive particles used for making abrasives generally use micro powder, commonly used green silicon carbide micro powder particle size: W20, W14 for rough grinding, W14, W10 for semi precision grinding, and below W7 for precision grinding.
In the previous process of processing carbon/carbon composite sealing rings, kerosene has been used as the grinding fluid. During the grinding process, it was found that kerosene is not easily volatile, and the ground debris is sticky and difficult to clean. After grinding, it is necessary to wait for a long time for the kerosene to dry thoroughly before using optical flat crystals for testing, which wastes a lot of time; Moreover, carbon/carbon composite materials are particularly sensitive to kerosene, resulting in a decrease in luminosity after exposure to oil, blurred light band display, and difficulty in flat crystal detection.
In response to the disadvantages of using kerosene grinding fluid, after multiple experiments, it has been found that using alcohol as the grinding fluid has a significant effect on grinding. The alcohol evaporates quickly, the sealing ring is clean, and the chips are not sticky. After the alcohol evaporates quickly, the sealing ring can be used for flat crystal detection, with good luminosity and clear light band, greatly improving the grinding speed and making detection very convenient.
2.3 Grinding fixture
The initial design of the grinding fixture is an integrated structure, as shown on the left side of Figure 2. During the grinding process, it was found that after grinding one end face, the other end face was flipped and ground. The flatness of the end face after flat crystal testing was better than that of the first one, and it remained the same every time. Moreover, the problem remained unresolved when grinding the first end face again.
After careful analysis, it was found that the problem lies in the grinding fixture. The original grinding fixture is an integrated type, and the end face that comes into contact with the parts is an inner stop, which is difficult to grind. Ordinary mechanical processing cannot achieve the surface quality accuracy after grinding. The friction between the surface of the already ground sealing ring and this surface is actually wear, resulting in poor quality of the end face that comes into contact with the grinding fixture.
2.4 Grinding steps
Before grinding the sealing ring, the end face should be flattened to ensure that the roughness of the end face is not greater than Ra0.8 μ m. The parallelism between the two end faces shall not exceed 0.005mm. Specific grinding steps: (1) Grinding tool: Before grinding the sealing ring, the grinding plate needs to be embedded with abrasive and ground to achieve a roughness of Ra0.1 μ M; (2) Grinding parts: Use fixtures to grind on a grinding plate according to a certain motion trajectory, with both hands exerting steady and consistent force, and promptly clean the grinding plate to remove debris and dirt; (3) Clean the parts: After the parts are grinded, wear gloves and gently wipe off the grinding debris and dirt with clean silk cloth or Cotton wool.
2.5 Motion trajectory
Requirements for grinding motion trajectory: It should be able to ensure that all points on the machining surface have the same (or similar) cutting conditions; At the same time, it is also necessary to ensure that all points on the surface of the grinding tool have the same (or similar) cutting conditions. The requirements are as follows: (1) The workpiece moves parallel to the plane of the grinding plate, ensuring that the strokes of all points on the workpiece are the same, in order to achieve good flatness; (2) The grinding movement should strive for stability and avoid large curvature angles as much as possible; (3) The movement of the workpiece covers the entire surface of the grinding tool to facilitate uniform wear of the grinding tool; (4) Try to avoid periodic repetition of the motion trajectory of any point on the workpiece.
After experiment and exploration, it is appropriate to use Sine wave track for grinding. After grinding for a certain time, turn the sealing ring 90 °~180 ° to prevent uneven wear of parts.
2.6 Grinding environment requirements
Grinding is the final processing, and its results determine the quality of the part processing. The requirements for the sealing ring grinding environment are relatively high. The doors and windows of the operating room should be well sealed, and the operating room should be kept clean and dust-free. Temperature and humidity should be strictly controlled, and natural light should be sufficient for testing.
During the flat grinding process of carbon/carbon composite materials, they are very sensitive to small hard particles such as dust and impurities. Small hard particles such as dust and impurities can easily leave scratches on the surface of the material, affecting the grinding quality and even leading to scrapping in severe cases.
2.7 Precautions
Due to the thin wall, large diameter, and high precision requirements of this part, special attention should be paid to the following four points during the grinding process:
The grinding pressure should be low. Due to uneven control of hand force, it is easy to cause deformation of the parts, and the surface flatness produced cannot meet the requirements. When grinding, remember not to apply pressure to the grinding fixture vertically by hand, but to apply thrust parallel to the grinding plate.
During precision grinding, dry grinding method is used for grinding, without the need to add grinding agent to the grinding plate.
During dry grinding, it must be noted that after a few rounds of grinding, the flat plate will be stuck by the ground powder, causing it to lose its cutting effect, which is equivalent to the phenomenon of knife sticking. Therefore, it is necessary to clean it regularly with clean alcohol, grind repeatedly, and finally meet the size requirements.
3 Conclusion
Through grinding experiments on carbon/carbon composite material sealing rings, a practical and feasible flat grinding method has been preliminarily summarized. After practical processing verification, the surface quality of the sealing ring grinding has been greatly improved, meeting the requirements of the design drawings, enhancing our unit's trial production and processing capabilities, and laying a solid foundation for the future processing of related materials.
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