In the manufacturing industry, new materials with lighter weight and higher strength are emerging, making processing problems more and more difficult, which also promotes the continuous development of CVD and PVD coating technology to meet the needs of new materials processing. Efficiency requirements. This paper introduces the selection of tool coating and the processing countermeasures of difficult-to-machine materials for the milling of new aerospace titanium alloys and the processing of composite materials.
Impact of tool coating on the cutting process Why are materials used in the aerospace industry more difficult to machine than cast iron, steel, etc., which are commonly used in the automotive industry? The metal removal process is a process in which the workpiece material is plastically deformed to fracture and form chips. The stress-strain diagram obtained from the standard tensile test of materials science gives the amount of plastic deformation of the material at the point of reaching the fracture or strength limit. The relevant data can be found in the standard material manual. Various steels can be roughly classified into ordinary carbon steel, unhardened alloy steel, and hardened alloy steel according to their different compositions and heat treatment conditions. Materials with low hardness and high elongation produce large plastic deformation during processing to form long chips; on the contrary, materials with high hardness and low elongation form short chips.
The unit's fracture energy is proportional to the product of the fracture stress and the strain at break. It is the energy consumed by the plastic deformation of the material and is converted into heat during the chip formation process, which is the first heat source in the cutting zone. The second heat source in the cutting zone is the heat generated by the friction between the chip and the rake face. The better the toughness of the material, the longer the contact length with the rake face, and the more frictional heat generated.
Among various workpiece materials, materials with high strength or hardness, large cutting force, good toughness, and long chips have poor workability. The metallographic properties of the material (such as precipitated hard phase, inclusions, work hardening, etc.) also affect its machinability and tool life.
Titanium alloys and nickel-based superalloys (Inconel, etc.) used in the aerospace industry require high plastic deformation energy due to high strength and toughness, and a large amount of cutting heat generated by friction with the blade, and their low heat conduction. The coefficient concentrates heat on the cutting zone and the knife/chip interface, adversely affecting tool life. Therefore, how to effectively process such materials is a great challenge to the cutting process.
Tool coating technology is a new technology developed for the high force generated by the chip formation process and the adverse effect of heat on the tool. According to different application fields, the life of the coated tool can be increased by 2 to 10 times than that of the uncoated tool. .
The correct choice of coating should match the workpiece material, chip deformation characteristics and cutting conditions. The early development of TiN and TiCN coatings is still suitable for processing carbon steel and alloy steel under general cutting conditions. However, as the cutting speed increases, a TiAlN coating is required because of its excellent stability at high temperatures.
When machining difficult-to-cut aerospace materials, the tool is required to have the best cutting edge design, including the micro-geometry associated with the sharpness of the cutting edge. For example, chamfering or passivation of the cutting edge, the degree of passivation has uniform requirements over the entire length of the cutting edge, and some requirements vary. The typical passivation arc radius is 0.025mm to reduce the concentration of the edge stress and better protect the PVD coating.
In order to better process difficult-to-machine materials, materials scientists have developed new coating grades, including the addition of Ti to TiNN coatings formed in TiN, or the addition of Al in AlTiN coatings, or the development of AlCrN using Cr instead of Ti. Coatings, as well as nanostructured coatings with higher hardness, toughness and high temperature oxidation resistance. Compared to uncoated tools, coated tools can achieve higher cutting speeds and longer tool life under otherwise identical conditions. Some research results explain the working mechanism of the coating under different cutting conditions, and develop various coating functions such as wear-resistant grain wear, inhibition of micro-cracks, reduction of friction coefficient, formation of stable AlCr oxide film and better heat resistance. .
Milling of titanium alloys In the aerospace industry, there are many high-performance materials that are difficult to process, such as synthetic materials, titanium alloys and high-temperature alloys. These materials have high strength and lighter parts to make the aircraft more fuel efficient. However, the machining of these parts requires new cutting techniques.
For example, Boeing's new medium-sized Boeing 787 aircraft, in its main structural components, the composite material accounts for about 50%, titanium alloy accounts for about 15%. Due to their light weight and high strength-to-weight ratio, the 787 is lighter and saves 20% more fuel than the same size passenger jet 777-200. In addition, in addition to the aluminum alloy, in addition to the lighter weight of the two materials, titanium is more compatible with synthetic materials than aluminum (aluminum is more likely to react with synthetic fibers). In addition, titanium is more resistant to corrosion than aluminum alloys and its maintenance is easier.
However, the workability of titanium alloys is not good, and generally only a lower cutting speed and feed rate can be used. In order to prevent heat accumulation, the radial depth of cut cannot exceed 30%, and the axial depth of cut is also small.
At present, there is a new titanium alloy, Ti5553 (Ti-5Al-5V-5Mo-3Cr), which belongs to the near-ß type titanium alloy. Compared with the most commonly used Ti-6Al-4V, this material has better hardenability and fatigue strength, and is more suitable for making key parts. But it is more difficult to machine than traditional titanium alloys and has a shorter tool life.
In order to solve the processing of this new type of titanium alloy, Niagara Tool Company of the United States cooperated with the airline to jointly develop applicable tools and processing parameters. The test work is carried out in the laboratory and on the production site, including the geometry of the tool, the edge treatment and the choice of coating.
At present, the company has developed a process guide for roughing, semi-finishing and finishing of Ti5553 titanium alloy: roughing is recommended for fine-tooth end mills made of M42 cobalt high-speed steel or powder metallurgy high-speed steel. TiCN or TiAlN coating with a cutting speed of 40 to 60 ft/min and a feed rate of 3 to 5 in/min; semi-finishing with a TiAlN coated 4-tooth solid carbide end mill with a cutting speed of 70 to 100 ft / The feed rate is 5 to 7 inches per minute; the finishing is a TiAlN coated 6 to 8 tooth solid carbide end mill with a cutting speed of 400 ft/min and a feed rate of 5 to 7 inches/min.
According to the company, the new tool geometry (wave edge) combined with the coating and the correct cutting parameters improves the chip formation process and avoids chip clogging.
Cutting of composite materials Composite materials are increasingly used in the manufacture of aircraft components. Composite materials are usually synthesized from resins, reinforcing fibers and one or more filler materials. They have high specific strength and good mechanical and thermal properties and are the best alternative to metal materials.
According to the president of the American Diamond Coatings Company, CVD diamond coated tools are the best tool for machining many composite materials. The company supplies Lockheed Martin Aircraft with tool coatings for advanced composite materials. At the beginning, the tool life is shorter and the material is layered. Later, a 20μm thick CVD diamond coating was used, and the tool geometry was improved to increase the tool life by more than 6 times and reduce the delamination of the workpiece.
Improvements in tool cutting performance must rely on the optimal matching of tool materials, coatings, and geometric parameters. In recent years, the development of coatings (especially PVD coatings) has improved the cutting efficiency of difficult materials in the aerospace industry. With the advent of PVD Al2O3 coatings, the advantages of PVD coating technology will become more apparent. Figure 6 shows the proportion of various PVD coatings in the United States since 1990. The proportion of TiN coatings continues to decrease, and TiAlN coatings are used more for their excellent overall properties.
Impact of tool coating on the cutting process Why are materials used in the aerospace industry more difficult to machine than cast iron, steel, etc., which are commonly used in the automotive industry? The metal removal process is a process in which the workpiece material is plastically deformed to fracture and form chips. The stress-strain diagram obtained from the standard tensile test of materials science gives the amount of plastic deformation of the material at the point of reaching the fracture or strength limit. The relevant data can be found in the standard material manual. Various steels can be roughly classified into ordinary carbon steel, unhardened alloy steel, and hardened alloy steel according to their different compositions and heat treatment conditions. Materials with low hardness and high elongation produce large plastic deformation during processing to form long chips; on the contrary, materials with high hardness and low elongation form short chips.
The unit's fracture energy is proportional to the product of the fracture stress and the strain at break. It is the energy consumed by the plastic deformation of the material and is converted into heat during the chip formation process, which is the first heat source in the cutting zone. The second heat source in the cutting zone is the heat generated by the friction between the chip and the rake face. The better the toughness of the material, the longer the contact length with the rake face, and the more frictional heat generated.
Among various workpiece materials, materials with high strength or hardness, large cutting force, good toughness, and long chips have poor workability. The metallographic properties of the material (such as precipitated hard phase, inclusions, work hardening, etc.) also affect its machinability and tool life.
Titanium alloys and nickel-based superalloys (Inconel, etc.) used in the aerospace industry require high plastic deformation energy due to high strength and toughness, and a large amount of cutting heat generated by friction with the blade, and their low heat conduction. The coefficient concentrates heat on the cutting zone and the knife/chip interface, adversely affecting tool life. Therefore, how to effectively process such materials is a great challenge to the cutting process.
Tool coating technology is a new technology developed for the high force generated by the chip formation process and the adverse effect of heat on the tool. According to different application fields, the life of the coated tool can be increased by 2 to 10 times than that of the uncoated tool. .
The correct choice of coating should match the workpiece material, chip deformation characteristics and cutting conditions. The early development of TiN and TiCN coatings is still suitable for processing carbon steel and alloy steel under general cutting conditions. However, as the cutting speed increases, a TiAlN coating is required because of its excellent stability at high temperatures.
When machining difficult-to-cut aerospace materials, the tool is required to have the best cutting edge design, including the micro-geometry associated with the sharpness of the cutting edge. For example, chamfering or passivation of the cutting edge, the degree of passivation has uniform requirements over the entire length of the cutting edge, and some requirements vary. The typical passivation arc radius is 0.025mm to reduce the concentration of the edge stress and better protect the PVD coating.
In order to better process difficult-to-machine materials, materials scientists have developed new coating grades, including the addition of Ti to TiNN coatings formed in TiN, or the addition of Al in AlTiN coatings, or the development of AlCrN using Cr instead of Ti. Coatings, as well as nanostructured coatings with higher hardness, toughness and high temperature oxidation resistance. Compared to uncoated tools, coated tools can achieve higher cutting speeds and longer tool life under otherwise identical conditions. Some research results explain the working mechanism of the coating under different cutting conditions, and develop various coating functions such as wear-resistant grain wear, inhibition of micro-cracks, reduction of friction coefficient, formation of stable AlCr oxide film and better heat resistance. .
Milling of titanium alloys In the aerospace industry, there are many high-performance materials that are difficult to process, such as synthetic materials, titanium alloys and high-temperature alloys. These materials have high strength and lighter parts to make the aircraft more fuel efficient. However, the machining of these parts requires new cutting techniques.
For example, Boeing's new medium-sized Boeing 787 aircraft, in its main structural components, the composite material accounts for about 50%, titanium alloy accounts for about 15%. Due to their light weight and high strength-to-weight ratio, the 787 is lighter and saves 20% more fuel than the same size passenger jet 777-200. In addition, in addition to the aluminum alloy, in addition to the lighter weight of the two materials, titanium is more compatible with synthetic materials than aluminum (aluminum is more likely to react with synthetic fibers). In addition, titanium is more resistant to corrosion than aluminum alloys and its maintenance is easier.
However, the workability of titanium alloys is not good, and generally only a lower cutting speed and feed rate can be used. In order to prevent heat accumulation, the radial depth of cut cannot exceed 30%, and the axial depth of cut is also small.
At present, there is a new titanium alloy, Ti5553 (Ti-5Al-5V-5Mo-3Cr), which belongs to the near-ß type titanium alloy. Compared with the most commonly used Ti-6Al-4V, this material has better hardenability and fatigue strength, and is more suitable for making key parts. But it is more difficult to machine than traditional titanium alloys and has a shorter tool life.
In order to solve the processing of this new type of titanium alloy, Niagara Tool Company of the United States cooperated with the airline to jointly develop applicable tools and processing parameters. The test work is carried out in the laboratory and on the production site, including the geometry of the tool, the edge treatment and the choice of coating.
At present, the company has developed a process guide for roughing, semi-finishing and finishing of Ti5553 titanium alloy: roughing is recommended for fine-tooth end mills made of M42 cobalt high-speed steel or powder metallurgy high-speed steel. TiCN or TiAlN coating with a cutting speed of 40 to 60 ft/min and a feed rate of 3 to 5 in/min; semi-finishing with a TiAlN coated 4-tooth solid carbide end mill with a cutting speed of 70 to 100 ft / The feed rate is 5 to 7 inches per minute; the finishing is a TiAlN coated 6 to 8 tooth solid carbide end mill with a cutting speed of 400 ft/min and a feed rate of 5 to 7 inches/min.
According to the company, the new tool geometry (wave edge) combined with the coating and the correct cutting parameters improves the chip formation process and avoids chip clogging.
Cutting of composite materials Composite materials are increasingly used in the manufacture of aircraft components. Composite materials are usually synthesized from resins, reinforcing fibers and one or more filler materials. They have high specific strength and good mechanical and thermal properties and are the best alternative to metal materials.
According to the president of the American Diamond Coatings Company, CVD diamond coated tools are the best tool for machining many composite materials. The company supplies Lockheed Martin Aircraft with tool coatings for advanced composite materials. At the beginning, the tool life is shorter and the material is layered. Later, a 20μm thick CVD diamond coating was used, and the tool geometry was improved to increase the tool life by more than 6 times and reduce the delamination of the workpiece.
Improvements in tool cutting performance must rely on the optimal matching of tool materials, coatings, and geometric parameters. In recent years, the development of coatings (especially PVD coatings) has improved the cutting efficiency of difficult materials in the aerospace industry. With the advent of PVD Al2O3 coatings, the advantages of PVD coating technology will become more apparent. Figure 6 shows the proportion of various PVD coatings in the United States since 1990. The proportion of TiN coatings continues to decrease, and TiAlN coatings are used more for their excellent overall properties.
Torch Body,Collet Body,Ceramic Nozzle
MAG/MIG Welding Accessories MAG/MIG Welding Torches Co.,Ltd , http://www.china-welding-torch.com