Properties and Processing Methods of Metal Matrix Composites Composite materials are multiphase materials that are artificially synthesized from two or more chemically distinct materials. This material not only maintains the characteristics of a single component material, but also allows the components to complement each other and complement each other to form characteristics superior to those of the original materials. The composite consists of a matrix phase and a reinforcing phase. The former is the basic component and acts as a bond; the latter acts to increase hardness, strength, stiffness and wear resistance. Metal-based composite materials made of a hard, tough metal matrix (used of aluminum or aluminum alloy) with hard particle reinforcement (such as SiC, AlN, Al2O3, etc.) have low density, specific strength and specific elasticity. It has the advantages of high modulus, small thermal expansion coefficient, high temperature resistance and fatigue resistance, strong wear resistance and vibration resistance, and simple preparation process. Therefore, it has been widely used in automotive engine cylinder liners, pistons and other wear parts. Industrial sectors such as machinery, metallurgy, bearings, transportation, chemicals, construction, and aerospace. However, the metal matrix composites have poor processability. Although many new processing methods have been developed, such as EDM, laser processing and high pressure water jet processing, due to the relatively expensive equipment and low processing quality, traditional Machining (including: car, drilling, milling, punching, reaming, boring and grinding) is still the main means of current processing. Diamond is the hardest substance in the world. Its microhardness is up to 10000HV, its wear resistance is excellent, its cutting edge is very sharp, its blade roughness is small, its friction coefficient is low, its anti-adhesiveness is good, its thermal conductivity is high, cutting It is not easy to stick to the knife and produce built-up edge. The metal-based composite material has the characteristics of high work efficiency, long tool life and good processing quality, so it is the most widely used. Figure 1 shows an application example of a 20% SiCP aluminum-based composite spiral pump casing drilled with a PCD (polycrystalline synthetic diamond) tool.
Machining characteristics of metal matrix composites
Usually, metal-based composite materials are attributed to "difficult-to-machine materials". In fact, they form short chips during processing, and the matrix is ​​generally aluminum alloy. The cutting temperature is low, so the machinability is good; mainly the particle reinforcement added therein The hardness of the material is very high, such as the hardness of SiC up to 3000-3500 HV. The hard particles are distributed in the matrix, which acts like a grinding wheel in the grinding wheel to scrape and impact the cutting edge of the tool, causing the cutting edge to wear quickly. The higher the hardness of the hard particles, the larger the size of the particles, and the greater the number of particles, the faster the tool wears. Therefore, it is difficult to machine with conventional carbide tools, and the tool life is very low or not at all.
Japanese scholars used a carbide tool (grade K10) to turn A390 hypereutectic silicon aluminum alloy and cast iron with a silicon content of 16%-18%. Under the same cutting conditions, the former cutter only cuts a few minutes on the cutting edge of the tool. It produces severe abrasive wear and fails, and its life is less than 1/3 of that of cast iron. Changing the material to be cut into SiC whisker reinforced aluminum alloy results in tool wear much faster than cutting high silicon aluminum alloy. If processed with a coated carbide tool, the hard particles in the composite will quickly wear through the coating and rapidly expand into the cemented carbide matrix to disable the tool. Diamond is the hardest material known in the world, and its practical use proves that it is the best tool material for processing metal matrix composites. The metal-based composite material is processed by diamond, and the cutting speed can reach 800~1000m/min. The tool life can be several times or even several times higher than that of the hard alloy, and the surface roughness value is small, which can reach Ra0.025-0.012μm. . This is because diamond is not only high in hardness (up to 10000 HV), it has good wear resistance, it can keep sharp cutting edges for a long time, the blade roughness value is small, and the friction coefficient is low, the anti-adhesive property is good, the thermal conductivity is high, and the cutting is high. It is not easy to stick to the knife and create a built-up edge, so the quality of the machined surface is much better than other tools.
Correct selection of diamond varieties
Polycrystalline Diamond and Cemented Carbide Composite Blades (PCD/CC)
PCD/CC is one of the most widely used diamonds for processing metal matrix composites. It is formed on a cemented carbide substrate with good strength and toughness, sintered or pressed on the surface of a 0.5-1 mm thick polycrystalline diamond PCD. The bending strength of the composite blade is basically the same as that of the cemented carbide, and the hardness of the working surface is close to that of the integrated polycrystalline diamond PCD, and the weldability is good, the regrind is easy, and the cost is low. A typical polycrystalline diamond and cemented carbide composite blade is flat or sheet-shaped and then laser cut into the desired shape and size of the blade. PCD/CC is commonly used in welding or machine clamping to make various cutting tools such as car, drill, milling, punching, reaming, boring and sawing. The performance of PCD/CC is related to the size of the diamond grain. De Beers produces PCD inserts of 002, 010 and 025, with average grain sizes of 2μm, 10μm and 25μm. The larger the grain size, the greater the diamond volume concentration, the better the wear resistance and the higher the tool life, but the cutting edge quality is slightly worse, making it difficult to make high-precision tools. On the contrary, the fine-grained tool has a rounded radius of the cutting edge (1-2 of the general PCD tool) and the surface quality is good. Therefore, the crystal grains of the polycrystal are continuously refined, and there are already 1 acid and even fine crystals of 0.5 acid or less. To this end, a suitable grade of polycrystalline diamond and cemented carbide composite inserts should be selected considering the life of the tool and the quality of the machined surface.
CVD diamond
CVD diamond is a high abrasion resistant pure diamond material that does not contain a binder and is prepared at low pressure (<0.1 MPa). There are two forms of CVD diamond: CVD thin film coating (CD) and CVD thick film (TFD).
CD is a CVD (Chemical Vapor Deposition) process in which a film of diamond having a thickness of less than 50 μm (usually 10 to 30 μm) and composed of polycrystalline is deposited on a cemented carbide substrate (commonly used for K alloy). The use indicates that CD diamond coated tools are not suitable for processing metal matrix composites because the hard particles in the composite will wear through the coating on the surface of the tool in a very short time.
The TFD is a thick film without a substrate deposited to a thickness of 3-5 mm. The thick film is cut into small pieces of a certain shape as needed, and then welded to the cemented carbide to form a composite blade or a cutter.
TFD has a good comprehensive performance. It has no disadvantages of natural diamond anisotropy. Because there is no metal binder, the impurity content is low, the purity is close to 100%, so the hardness and thermal conductivity are higher than PCD, and the friction factor is smaller. The stability is better, and the performance of processing metal matrix composites is quite good.
The tool wear curve is shown for the machining of 40% SiCP A356MMC material with both PCD and TFD diamonds. The cutting conditions used in the test in Figure 2 are: cutting speed 400m/min, feed rate 0.05mm/r, back-feeding amount (cutting depth) 0.5mm, adding cutting fluid. As can be seen from Figure 2, the processing of 40% SiCP metal matrix composites is best with thick film diamond TFD; PCD025 is second, PCD002 has the lowest service life. Table 1 shows the cutting parameters recommended by De Beers for the processing of metal matrix composites with polycrystalline diamond grade SYNDITE 025 inserts. Studies have shown that as the cutting speed increases and the feed rate decreases, the machined surface roughness value Ra will decrease, while the back-feeding amount has no significant effect on Ra. Therefore, when finishing, the cutting speed should be taken as the larger value in Table 1, and the feed rate should be small.
Machining metal matrix composites with diamond can be either dry or wet. However, due to the low thermal stability of diamond, it will be carbonized (ie, graphitized) at 700-800 ° C, resulting in a sharp decrease in tool life. If the cutting fluid is wet-cut, the cutting temperature can be lowered and the tool life can be increased. However, before the tool is cut into the workpiece, the cutting fluid should be poured until the cutting is completed. The cutting fluid must be continuously supplied, and it cannot be interrupted continuously, otherwise the tool may be damaged or chipped. For the same reason, it is also necessary to avoid stopping or changing the amount of cutting during the cutting process.
in conclusion
Diamond is the best tool material for processing metal matrix composites. Processing 40% SiCP metal matrix composites, using thick film diamond TFD works best; PCD is second;
The size of the diamond grain has a direct impact on the life of the tool and the quality of the machined surface. The larger the grain size, the better the wear resistance of diamond, the higher the tool life, but the surface quality is slightly worse; on the contrary, the fine-grained knife has better surface quality;
The cutting speed and feed rate have a direct influence on the surface roughness of the machined surface. The higher the cutting speed and the smaller the feed rate, the lower the Ra of the machined surface roughness, and the effect of the amount of backing on Ra is not significant. .
Machining characteristics of metal matrix composites
Usually, metal-based composite materials are attributed to "difficult-to-machine materials". In fact, they form short chips during processing, and the matrix is ​​generally aluminum alloy. The cutting temperature is low, so the machinability is good; mainly the particle reinforcement added therein The hardness of the material is very high, such as the hardness of SiC up to 3000-3500 HV. The hard particles are distributed in the matrix, which acts like a grinding wheel in the grinding wheel to scrape and impact the cutting edge of the tool, causing the cutting edge to wear quickly. The higher the hardness of the hard particles, the larger the size of the particles, and the greater the number of particles, the faster the tool wears. Therefore, it is difficult to machine with conventional carbide tools, and the tool life is very low or not at all.
Japanese scholars used a carbide tool (grade K10) to turn A390 hypereutectic silicon aluminum alloy and cast iron with a silicon content of 16%-18%. Under the same cutting conditions, the former cutter only cuts a few minutes on the cutting edge of the tool. It produces severe abrasive wear and fails, and its life is less than 1/3 of that of cast iron. Changing the material to be cut into SiC whisker reinforced aluminum alloy results in tool wear much faster than cutting high silicon aluminum alloy. If processed with a coated carbide tool, the hard particles in the composite will quickly wear through the coating and rapidly expand into the cemented carbide matrix to disable the tool. Diamond is the hardest material known in the world, and its practical use proves that it is the best tool material for processing metal matrix composites. The metal-based composite material is processed by diamond, and the cutting speed can reach 800~1000m/min. The tool life can be several times or even several times higher than that of the hard alloy, and the surface roughness value is small, which can reach Ra0.025-0.012μm. . This is because diamond is not only high in hardness (up to 10000 HV), it has good wear resistance, it can keep sharp cutting edges for a long time, the blade roughness value is small, and the friction coefficient is low, the anti-adhesive property is good, the thermal conductivity is high, and the cutting is high. It is not easy to stick to the knife and create a built-up edge, so the quality of the machined surface is much better than other tools.
Correct selection of diamond varieties
Polycrystalline Diamond and Cemented Carbide Composite Blades (PCD/CC)
PCD/CC is one of the most widely used diamonds for processing metal matrix composites. It is formed on a cemented carbide substrate with good strength and toughness, sintered or pressed on the surface of a 0.5-1 mm thick polycrystalline diamond PCD. The bending strength of the composite blade is basically the same as that of the cemented carbide, and the hardness of the working surface is close to that of the integrated polycrystalline diamond PCD, and the weldability is good, the regrind is easy, and the cost is low. A typical polycrystalline diamond and cemented carbide composite blade is flat or sheet-shaped and then laser cut into the desired shape and size of the blade. PCD/CC is commonly used in welding or machine clamping to make various cutting tools such as car, drill, milling, punching, reaming, boring and sawing. The performance of PCD/CC is related to the size of the diamond grain. De Beers produces PCD inserts of 002, 010 and 025, with average grain sizes of 2μm, 10μm and 25μm. The larger the grain size, the greater the diamond volume concentration, the better the wear resistance and the higher the tool life, but the cutting edge quality is slightly worse, making it difficult to make high-precision tools. On the contrary, the fine-grained tool has a rounded radius of the cutting edge (1-2 of the general PCD tool) and the surface quality is good. Therefore, the crystal grains of the polycrystal are continuously refined, and there are already 1 acid and even fine crystals of 0.5 acid or less. To this end, a suitable grade of polycrystalline diamond and cemented carbide composite inserts should be selected considering the life of the tool and the quality of the machined surface.
CVD diamond
CVD diamond is a high abrasion resistant pure diamond material that does not contain a binder and is prepared at low pressure (<0.1 MPa). There are two forms of CVD diamond: CVD thin film coating (CD) and CVD thick film (TFD).
CD is a CVD (Chemical Vapor Deposition) process in which a film of diamond having a thickness of less than 50 μm (usually 10 to 30 μm) and composed of polycrystalline is deposited on a cemented carbide substrate (commonly used for K alloy). The use indicates that CD diamond coated tools are not suitable for processing metal matrix composites because the hard particles in the composite will wear through the coating on the surface of the tool in a very short time.
The TFD is a thick film without a substrate deposited to a thickness of 3-5 mm. The thick film is cut into small pieces of a certain shape as needed, and then welded to the cemented carbide to form a composite blade or a cutter.
TFD has a good comprehensive performance. It has no disadvantages of natural diamond anisotropy. Because there is no metal binder, the impurity content is low, the purity is close to 100%, so the hardness and thermal conductivity are higher than PCD, and the friction factor is smaller. The stability is better, and the performance of processing metal matrix composites is quite good.
The tool wear curve is shown for the machining of 40% SiCP A356MMC material with both PCD and TFD diamonds. The cutting conditions used in the test in Figure 2 are: cutting speed 400m/min, feed rate 0.05mm/r, back-feeding amount (cutting depth) 0.5mm, adding cutting fluid. As can be seen from Figure 2, the processing of 40% SiCP metal matrix composites is best with thick film diamond TFD; PCD025 is second, PCD002 has the lowest service life. Table 1 shows the cutting parameters recommended by De Beers for the processing of metal matrix composites with polycrystalline diamond grade SYNDITE 025 inserts. Studies have shown that as the cutting speed increases and the feed rate decreases, the machined surface roughness value Ra will decrease, while the back-feeding amount has no significant effect on Ra. Therefore, when finishing, the cutting speed should be taken as the larger value in Table 1, and the feed rate should be small.
Machining metal matrix composites with diamond can be either dry or wet. However, due to the low thermal stability of diamond, it will be carbonized (ie, graphitized) at 700-800 ° C, resulting in a sharp decrease in tool life. If the cutting fluid is wet-cut, the cutting temperature can be lowered and the tool life can be increased. However, before the tool is cut into the workpiece, the cutting fluid should be poured until the cutting is completed. The cutting fluid must be continuously supplied, and it cannot be interrupted continuously, otherwise the tool may be damaged or chipped. For the same reason, it is also necessary to avoid stopping or changing the amount of cutting during the cutting process.
in conclusion
Diamond is the best tool material for processing metal matrix composites. Processing 40% SiCP metal matrix composites, using thick film diamond TFD works best; PCD is second;
The size of the diamond grain has a direct impact on the life of the tool and the quality of the machined surface. The larger the grain size, the better the wear resistance of diamond, the higher the tool life, but the surface quality is slightly worse; on the contrary, the fine-grained knife has better surface quality;
The cutting speed and feed rate have a direct influence on the surface roughness of the machined surface. The higher the cutting speed and the smaller the feed rate, the lower the Ra of the machined surface roughness, and the effect of the amount of backing on Ra is not significant. .
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