Research on cutting status of nickel base superall

Research on the cutting status of Nickel Base Superalloys for aviation

with the continuous update and improvement of aeroengine technology, more and more difficult to machine materials and composite materials are widely used in new engines, which puts forward higher requirements for the experimental process methods of parts to be widely used in rubber, canvas, aviation, wires, plastics and other related data and processing capabilities. Nickel base superalloys play an important role in the difficult machining materials of engines, such as compressor disks, turbine disks, bearing rings, casings, fasteners, blades and other engine parts that work at high temperatures for a long time

nickel base superalloys have good mechanical, oxidation resistance and high temperature deformation resistance, but low thermal conductivity, large material plasticity and work hardening often restrict the wide use of nickel base superalloys. Therefore, analyzing and studying the machinability of part materials, tool making materials and cutting parameters are of great help to solve the difficult machining problem of nickel base superalloys

nickel base superalloys and their machinability

superalloys can be divided into iron base, nickel base and cobalt base superalloys according to matrix elements. According to the manufacturing process, it is divided into: deformation (GH4169, GH4133, Inconel718, etc.), casting (K477, k421, rene77, etc.), directional crystallization (DZ4, etc.) and powder metallurgy (fgh97, fgh98, etc.) superalloys. The heat-resistant temperature of nickel base superalloy is above 950 ℃, which has good mechanical properties and structural characteristics, and has the characteristics of oxidation resistance, corrosion resistance and high-temperature alternating stress resistance. Nickel base superalloys have been widely used in engines (see Table 1)

due to the high hardness, strength and plasticity of nickel base superalloy, its machinability is poor. The machinability of cast materials in nickel base superalloys is worse than that of forged materials, and the machinability of single crystal and powder metallurgy superalloys is even worse. Other difficult machining characteristics of nickel base superalloy are as follows: the cutting force is generally 1.5~2 times that of steel, and the cutting temperature is about 2 times that of steel; The material has low thermal conductivity and poor thermal conductivity, and the cutting heat is concentrated on the tool tip, which is not easy to dissipate. The high temperature produced by cutting can cause serious diffusion wear, oxidation wear and bonding wear of cutting tools; The surface hardening of machined parts is very serious, and the hardness of the work hardened surface is about 2 times higher than that of the normal surface; The chip has high hardness, good toughness and is not easy to break, which makes it difficult to interrupt the cutting process and difficult to deal with the chip; There are many metal compounds and hard points in the material, and the tool is easy to break off, so it is not easy to ensure the size and accuracy requirements

the disk shaft, gearbox and blade are the key parts of the engine. Nickel base superalloy is used as the material in the high-temperature working area. These parts have high requirements for technical indicators such as mating surface size, surface integrity and position accuracy, and these parts are typical difficult to machine parts with complex structure, thin wall and easy deformation. Disc and shaft parts involve more turning processes, and gearbox and blade parts involve more milling processes. The wall thickness of the disc is small and uneven, the dimensional accuracy is required to be high, and the external surface is complex, so it cannot be cut continuously along the circumference. During the processing, the deformation of the part is obvious, and it needs to be repaired many times

for example, the dimensional accuracy of the outer diameter of a certain disc is it6~it7, and the fitting accuracy of the weld surface is as high as it4~it5. The perpendicularity is 0.01~0.03mm, and the surface roughness is Ra0.8 μ m。 The parts of the gearbox have poor rigidity and complex shape, and the parts are easy to deform during processing. The dimensional accuracy, surface roughness and position accuracy of parts are strictly required. For example, the surface roughness of a gearbox part is 1.6 μ m. The thickness of thin-walled end is 2.5mm, and the dimensional tolerance of positioning hole is 0.015. The machining allowance of a gearbox blank is large, and the unilateral allowance is 20~30mm. Most of the allowance should be removed by milling, and the tool consumption is large. The shape and structure of blade parts are complex and irregular, with many sizes and long processing time, and the processing benchmark needs to be switched repeatedly. The thinnest part of the blade of a certain machine is 0.18mm, and the surface roughness of the part is 0.4 μ m。 In the process of machining, the system vibrates greatly, causing serious knife cutting, and the surface integrity of parts is poor

selection of tools

1 selection of tool materials

in the processing of Nickel Base Superalloys for aviation, the dimensional accuracy and surface effect of parts largely depend on the material of tools. According to the characteristics of nickel base superalloy, the tool materials processed should generally meet the following requirements

· good stability, oxidation resistance, high temperature resistance and strong impact resistance

· good hardness and wear resistance. The hardness of the tool material must be higher than that of the part material, generally above HRC60

· have sufficient bending strength and impact toughness

· good heat resistance, maintain certain strength and toughness at high temperature, and resist adhesion and diffusion

· good heat treatment performance, grindability, forging performance and high temperature plastic deformation performance

(1) carbide

cemented carbides are sintered from insoluble metal carbides and bonding metals, and have high strength and hardness. The hardness reaches hrc69~81, and the hardness can maintain HRC60 at 900~1000 ℃. The development of fine and ultra-fine cemented carbide materials has significantly improved the strength and toughness of cemented carbide tools. Figure 1 shows an example of turning with cemented carbide inserts. The coated cemented carbide blade sintered under pressure has good plastic deformation resistance and gradient cemented carbide with tough surface, which improves the cutting performance and application range of the coated cemented carbide blade and makes the cemented carbide tool enter the era of high-speed cutting. Cemented carbide tools have the above advantages, but because of their brittleness, low bending strength and poor seismic resistance, they are mostly used for turning, semi finishing and finishing with less impact. The commonly used material grades of rough machined nickel base superalloys are YG8, yd15, yf06, etc., while finish machined ones are YD05, yg643m, and ys2t alloys are suitable for intermittent cutting

Figure 1 example of turning cemented carbide blades

coated cemented carbide tools: the coating is mainly divided into chemical coating (CVD) and physical coating (PVD). More than 50% of the tools used in modern China are coated tools, and the coating proportion of cemented carbide indexable blades abroad has reached more than 70%. Coating has become a key technology to improve the performance of cutting tools. The use of coating technology can enable cutting tools to obtain very excellent comprehensive mechanical properties, greatly improve cutting efficiency and improve the service life of cutting tools. In addition, ordinary computers can also be equipped. The new coating is suitable for high-speed cutting, dry cutting and hard cutting. The development of nano ultra-thin and ultra-multilayer coating and new coating materials has greatly improved the control accuracy and quality of the coating, and greatly improved the hardness and toughness. The application of the new coating will become the main way to improve the tool performance

(2) ceramic tools

ceramic tools are suitable for high-speed cutting, which can increase the cutting speed by 3-5 times. The hardness of ceramic cutting tools can reach hra93~95, and can process high hardness materials of HRC65. It can still maintain good adhesion resistance and chemical stability at 1200 ℃, and the friction coefficient is lower than that of cemented carbide. The ceramic blade is shown in Figure 2. Ceramic cutting tools have good wear resistance and high temperature stability, but due to poor impact toughness, it is required that the material to be processed should be uniform, the cutting angle should be correct, and the cutting process should be stable. It is best not to use intermittent cutting. Ceramic tools include alumina based and silicon nitride based. When cutting tools of nickel based superalloys, silicon nitride blades should be selected. Although the hardness of alumina series ceramics is high, the toughness and strength are relatively poor. Taking advantage of the high temperature stability of ceramic cutting tools, the cutting heat generated by dry cutting such as air cooling is used to soften the processed material without phase transformation, making the cutting process easier. When machining high nickel base superalloy with ceramic tools, its performance is much better than that of cemented carbide tools, which can not only greatly improve the cutting speed, but also better solve the problem of difficult removal of cutting heat. For example, when the cutting speed is more than 420m/min, the chip is segmented and the surface is oxidized to golden yellow; When the linear speed is above 700mm/min, most of the cutting heat is taken away by the chips, which are oxidized, discolored, loose and brittle [1]

Figure 2 ceramic blade

(3) superhard tool

cubic boron nitride (CBN) has high hardness, thermal stability and chemical stability, and can be used as the first choice for high-speed cutting of difficult to machine materials, as shown in Figure 3. Cubic boron nitride cutting tool belongs to negative rake angle cutting, which is suitable for high-speed cutting of nickel base and cobalt base superalloys, especially powder superalloys. However, CBN is prone to bond at high temperature, so the bonding wear can be reduced by using high-pressure cutting fluid, but water-soluble coolant cannot be used. The efficiency of CBN tool is better than that of cemented carbide, and its service life is longer than that of ceramic tool. Polycrystalline cubic boron nitride (PCBN) has high hardness, wear resistance, good thermal stability, high chemical stability and thermal conductivity at 1400~1500 ℃. Its friction coefficient is 0.1~0.3, which is about 1/2~1/4 of cemented carbide, so it has high-quality anti adhesion and reduces the generation of accumulated and chipped tumors. For example, the best speed for high-speed machining GH4169 is 110m/min, f=1mm/r, ap=1mm. However, because of its high price, it is not widely used in production

Figure 3 CBN blade

2 selection of tool spatial geometry

the factors that determine the cutting performance of the tool are not only the tool material, but also the spatial geometry of the cutting part. These factors directly affect the size of the cutting heat, the flow direction and shape of the chips, the quality of the machined surface, and the size of the cutting force. The rake angle of 0 ° ~10 ° is usually selected for machining nickel base superalloy. When the cutting depth and feed are relatively large during rough machining, a relatively small rake angle should be selected. Increasing the rake angle during finish machining can reduce the deformation, which is about 5 ° ~10 °. When turning deformed superalloy, the rake angle is 5 ° ~10 °. When turning cast superalloy, the rake angle is 0 ° ~5 °. Increasing the back angle can reduce the friction between the back surface and the machined surface, and can also reduce the radius of the blade, but too large back angle will reduce the strength of the tool. The back angle of rough machining nickel base superalloy is generally 6 ° ~8 °, and finish machining is 10 ° ~12 °. The main deflection angle will affect the tool durability, cutting force and chip thickness. Among all kinds of tool angles, the main deflection angle has the greatest impact on the machining accuracy and surface shape of parts. Therefore, when choosing the main deflection angle, the shape of the part should be considered first, followed by the rigidity of the system. The main declination angle of processing nickel and superalloy is generally 30 ° ~60 °, and the secondary declination angle is generally 0.5 ° ~3 °, and the blade inclination angle is generally about -10 ° ~-20 °

selection of reasonable cutting parameters

on the basis of having selected the tool material and spatial geometric angle, reasonable selection of cutting parameters can achieve the purpose of improving efficiency and optimizing machining effect. At present, the indicators used to measure the processing effect are: single piece processing cost, processing time, surface roughness, part size accuracy, etc. The influence direction of cutting parameters on these indicators is not consistent. The standard usually used is to obtain higher efficiency at the lowest cost in order to maximize benefits

1 selection of cutting depth

on the premise of ensuring tool durability, system rigidity and tool strength, the cutting depth is determined according to the machining allowance. Metal removal rate by turning