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Basic performance of conventional tool materials
1) High speed steel
High-speed steel invented by American mechanical engineer FWTaylor and metallurgical engineer M. White in 1898 is still a common tool material. High-speed steel is a high-alloy tool steel with more alloying elements such as W, Mo, Cr, and V, and its carbon content is 0.7% to 1.05%. High-speed steel has high heat resistance and its cutting temperature can reach 600 °C. Compared with carbon tool steel and alloy tool steel, its cutting speed can be doubled. High-speed steel has good toughness and formability and can be used to manufacture almost all types of tools such as taps, twist drills, gear cutters, broaches, and small diameter milling cutters. However, high-speed steel also has defects such as poor wear resistance and poor heat resistance, and it has been difficult to meet the increasing requirements of modern cutting tools for tool materials; in addition, the storage of some major elements (such as tungsten) in high-speed steel materials. The resources are depleting in the world. It is estimated that the reserves are only enough to be re-exploited for 40 to 60 years, so the high-speed steel materials are facing a severe development crisis.
2) Ceramics
Ceramic materials have higher hardness, red hardness and wear resistance than cemented carbide. Therefore, when machining steel, the durability of the ceramic tool is 10 to 20 times that of the cemented carbide tool, the red hardness is 2 to 6 times higher than that of the cemented carbide, and the chemical stability and oxidation resistance are superior to those of the hard alloy. . The disadvantages of ceramic materials are high brittleness, low transverse rupture strength, and poor impact load capacity, which is the focus of people's continuous improvement in recent decades.
Ceramic tool materials can be divided into three categories: 1 alumina-based ceramics. TiC, WC, ZiC, TaC, ZrO2 and other components are usually added to the Al2O3 matrix material, and the composite ceramic cutter is hot pressed to have a hardness of 93 to 95 HRC. To improve the toughness, a small amount of Co, Ni and the like are often added. 2 silicon nitride based ceramics. The commonly used silicon nitride-based ceramics are Si3N4+TiC+Co composite ceramics, which have higher toughness than alumina-based ceramics and have comparable hardness. 3 silicon nitride-alumina composite ceramic. Also known as Sialon ceramics, its chemical composition is 77% Si3N4+13% Al2O3, hardness up to 1800HV, and bending strength up to 1.20GPa, which is most suitable for cutting superalloys and cast iron.
3) Cermet
Unlike cermets composed of WC, cermets are mainly composed of ceramic particles, TiC and TiN, binders Ni, Co, Mo, and the like. The hardness and red hardness of cermets are higher than that of cemented carbides, lower than that of ceramic materials; the transverse rupture strength is greater than that of ceramic materials, less than that of hard alloys; good chemical stability and oxidation resistance, resistance to peeling abrasion, oxidation and diffusion resistance, Lower bonding tendency and higher blade strength.
The cutting efficiency and working life of cermet cutters are higher than that of cemented carbide and coated carbide tools, and the surface roughness of the machined workpiece is small. Because of the low adhesion between cermet and steel, the cermet cutter is used instead of coating. When a layer of cemented carbide tool is used to process steel workpieces, the chip formation is relatively stable, and long chip winding is less likely to occur during automated machining, and the edges of the parts are substantially free of burrs. The disadvantage of cermet is that it is less resistant to thermal shock and is easily broken, so it has a limited range of use.
4) Superhard material
Materials with high hardness such as synthetic diamond and cubic boron nitride (CBN) are collectively referred to as superhard materials. Diamond is the hardest material known in the world, and has many excellent properties such as high thermal conductivity, high insulation, high chemical stability, high-temperature semiconductor characteristics, etc. It can be used for precision processing of non-ferrous metals such as aluminum and copper and their alloys. Particularly suitable for processing non-metallic hard and brittle materials. In 1955, GE Company of the United States successfully synthesized synthetic diamond by high temperature and high pressure method. In 1966, it also developed artificial polycrystalline diamond composite sheet (PCD). Since then, synthetic diamond has been rapidly developed as a new type of tool material. However, since the carbon in the diamond is easily dissolved by the action of iron at a high temperature, the diamond cutter is not suitable for processing the iron-based alloy, thereby greatly limiting the application of the diamond in the metal cutting process.
Cubic boron nitride (CBN) is a superhard material that is second only to diamond in hardness. Although CBN has a lower hardness than diamond, its oxidation temperature is as high as 1360 ° C and has a low affinity with ferromagnetic materials. Therefore, although CBN is currently prepared in the form of a sintered body, it is still an excellent tool material suitable for cutting steel materials and having high wear resistance. Because CBN has excellent properties such as high hardness, high thermal stability and high chemical stability, it is especially suitable for processing difficult-to-machine metal materials with high hardness and high toughness. If the CBN indexable insert dry-type hardened gears are used, the machining cost per gear can be reduced by 60%. The end mills equipped with spherical CBN inserts can be used to finish milling large hard tools. The grinding time can be compared with the traditional process. Reduce by 80%. The shortcoming of CBN materials is that the problem of poor toughness remains to be solved.
4) Cemented carbide
Cemented carbide was first invented by Schroter in 1926. After decades of continuous development, the hardness of cemented carbide tools has reached 98-93 HRA, and it still has good red hardness at 1000 ° C. Its durability is dozens of times that of high-speed steel tools.
Cemented carbide is prepared by powder metallurgy of refractory metal carbides such as WC, TiC, TaC, NbC, VC, and iron group metals as binders. It has higher hardness, wear resistance and red hardness than high speed steel; it has higher toughness than super hard materials. Due to its good comprehensive performance, cemented carbide has been widely used in the tool industry. At present, more than 90% of foreign turning tools and more than 55% of milling cutters are made of hard alloy materials.
Carbide grades can usually be divided into three categories: 1YG (WC-Co): This type of cemented carbide tool has good toughness, wear resistance, thermal conductivity, etc., mainly used for processing cast iron, non-ferrous metals. And non-metal. 2YT type (WC-TiC-Co type): Due to the addition of TiC to the material, the hardness and wear resistance of the material are improved, but the bending rigidity is lowered. This kind of cemented carbide has high hardness and high heat resistance, good anti-adhesion and anti-oxidation ability, suitable for processing steel, and has low tool wear and high durability during cutting. 3YW (WC-TiC-TaC-Co): TaC is added to the YT material to improve the strength, toughness and red hardness of the tool. This type of cemented carbide material has high high temperature hardness, high temperature strength and strong oxidation resistance, and is especially suitable for processing various high alloy steels, heat resistant alloys and various alloy cast irons.
Although various new tool materials have emerged in recent years, in the future, cemented carbide tools will still be widely used in cutting. Therefore, it is necessary to research and develop new material preparation technologies to further improve and improve the cemented carbide tool materials. Cutting performance.
Research Status of Cemented Carbide Tool Materials
Since the wear resistance and toughness of the cemented carbide tool material are difficult to balance, the user can only select the applicable tool material among the plurality of carbide grades according to the specific processing object and processing conditions, which gives the selection of the cemented carbide tool and Management brings a lot of inconvenience. In order to further improve the comprehensive cutting performance of cemented carbide tool materials, the current research hotspots mainly include the following aspects:
1) Refined grains
By refining the hard phase grain size, increasing the intergranular surface area of ​​the hard phase, and enhancing the intergranular bonding force, the strength and wear resistance of the cemented carbide tool material can be improved. When the WC grain size is reduced to less than submicron, the hardness, toughness, strength, wear resistance, etc. of the material can be improved, and the temperature required for complete densification can also be lowered. Ordinary cemented carbide has a grain size of 3 to 5 μm, fine grained cemented carbide has a grain size of 1 to 1.5 μm (micron), and ultrafine grained cemented carbide has a grain size of 0.5 μm or less (submicron, nanoscale). Compared with ordinary hard alloys with the same composition, the ultra-fine grained cemented carbide can increase the hardness by 2HRA or more and the bending strength can be increased by 600-800MPa.
Commonly used grain refining processes include physical vapor deposition, chemical vapor deposition, plasma deposition, and mechanical alloying. Equal-diameter lateral extrusion (ECAE) is a promising grain refining process. The method is to place the powder in a mold and extrude in a direction different from (and not opposite to) the extrusion direction, and the cross-sectional area at the time of extrusion does not change. The powder grains processed by the ECAE process can be remarkably refined.
Since the above grain refining process is still not mature enough, the nanocrystal grains tend to grow into coarse grains during the sintering process of the cemented carbide, and the grain growth generally leads to a decrease in material strength, and a single coarse WC grain is often An important factor that causes material breakage. On the other hand, the price of fine-grained hard alloy is relatively expensive, and it also plays a certain role in its promotion and application.
2) Coated cemented carbide
Coating a very thin layer of wear-resistant metal compound on CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), HVOF (High Velocity Oxy-Fuel Thermal Spraying), etc. on a hard alloy substrate with good toughness. The combination of the toughness of the substrate and the wear resistance of the coating improves the overall performance of the cemented carbide tool. Coated carbide tools have good wear resistance and heat resistance, especially suitable for high-speed cutting; due to their high durability and versatility, they can effectively reduce the number of tool changes during flexible automated processing in small batches and varieties. Improve processing efficiency; coated cemented carbide tools have strong anti-crater wear resistance, stable blade shape and groove shape, reliable chip breaking effect and other cutting performance, which is conducive to automatic control of machining process; coated carbide tool After passivation and refining treatment, the matrix has high dimensional accuracy, which can meet the requirements of automatic machining on the positioning accuracy of tool change. The above characteristics determine that coated carbide tools are particularly suitable for automated processing equipment such as FMS and CIMS (Computer Integrated Manufacturing Systems).
However, the coating method still fails to fundamentally solve the problem of poor toughness and impact resistance of the cemented carbide substrate.
3) Surface, overall heat treatment and cyclic heat treatment
The surface of the cemented carbide with good toughness is treated by nitriding, boronizing, etc., which can effectively improve the surface wear resistance. The overall heat treatment of the hard alloy with good wear resistance but poor toughness can change the bonding composition and structure in the material, reduce the adjacent degree of the WC hard phase, and improve the strength and toughness of the cemented carbide. By using a cyclic heat treatment process to alleviate or eliminate the stress between the grain boundaries, the overall performance of the cemented carbide material can be comprehensively improved.
4) Add rare metals
Adding rare metal carbides such as TaC and NbC to the cemented carbide material, the additive can be combined with the original hard phase WC and TiC to form a complex solid solution structure, thereby further strengthening the hard phase structure and suppressing the hard phase. The effect of phase grain growth and enhanced tissue uniformity is beneficial to improve the overall performance of cemented carbide. In the ISO standard P, K, and M type carbide grades, there are such hard alloys with Ta(Nb)C added (especially among the M grades).
5) Adding rare earth elements
Adding a small amount of rare earth elements such as cerium to the cemented carbide material can effectively improve the toughness and bending strength of the material, and the wear resistance is also improved. This is because the rare earth element strengthens the hard phase and the binder phase, purifies the grain boundary, and improves the wettability of the carbide solid solution to the binder phase. Hard alloys with rare earth elements are best suited for roughing grades and for semi-finished grades. In addition, such cemented carbides have broad application prospects in hard alloy tools such as mining tools, top hammers, and wire drawing dies. China's rare earth resources are abundant, and the research on adding rare earth elements in cemented carbides also has a high level.
Development ideas of cemented carbide tool materials
The application of whisker toughening and strengthening, nano-powder composite strengthening technology to comprehensively improve the hardness, toughness and other comprehensive properties of cemented carbide tool materials is an important direction for the future development of cemented carbide tool materials.
1) Whisker toughening and strengthening technology
a. Toughening mechanism
Due to the poor fracture toughness of cemented carbide tool materials, it is difficult to apply to some machining applications that require high tool toughness (such as micro deep hole drilling). An effective way to solve this problem is to use whisker toughening and reinforcement technology.
The whiskers added to the cemented carbide material can absorb the energy of crack propagation, and the amount of absorbed energy is determined by the bonding state of the whiskers and the matrix. The main mechanism of whisker toughening is as follows: 1 whisker is pulled out and toughened: when the whisker is pulled out from the matrix under external load, some external load energy is consumed due to interface friction, thereby achieving the purpose of toughening, which increases The toughness effect is affected by whisker and interfacial sliding resistance. There must be sufficient bonding force between the whiskers and the substrate interface so that the external load can be effectively transferred to the whiskers, but the bonding force cannot be too large in order to maintain a sufficient extraction length. 2 Crack deflection toughening: When the crack tip encounters a second phase with a modulus of elasticity greater than that of the matrix, the crack will deviate from the original direction of advancement and spread along the interface of the two phases or within the matrix. Since the non-planar fracture of the crack has a larger fracture surface than the plane fracture, it can absorb more external energy and thus toughen. The addition of high elastic modulus whiskers or particles to the matrix can cause crack deflection and toughening mechanisms. 3 Whisker bridge toughening: When the matrix breaks, the whiskers can withstand external loads and play a bridge connection between the broken crack faces. The bridged whiskers can generate a force for the substrate to close the crack, and consume external loads to work, thereby improving the toughness of the material.
b. Selection and addition of whiskers
Currently commonly used whisker materials are SiC, TiC, TiB2, Al2O3, MgO, boron nitride, mullite, and the like. However, the research focus should be on the single crystal SiC whisker material, because SiC itself has good thermal shock resistance and fibrous (needle) SiC powder is easy to obtain.
There are two main ways of adding SiC whiskers: 1 Adding whisker method: a certain amount of SiC powder is added to a powder material based on oxide, nitride or the like, and a whisker toughened product is obtained through manufacturing. This method is currently used more widely. 2 Synthetic whisker method: After mixing the powder matrix with SiO2, carbon black, sintering aid, etc., the SiCw whisker is synthesized under a certain temperature and pressure, and then the whisker toughening product is obtained through manufacturing. This method is currently under further research and development. Generally, SiCw whiskers have a diameter ranging from 0.01 to 3 μm, a length ranging from 0.1 to 300 μm, a whisker length to diameter ratio of 10, and a SiCw whisker addition amount of 5% to 40%. The SiCw whisker properties currently used in China are shown in Table 1. .jpg)
c. Orientation and content of whiskers
After hot pressing of whisker toughened cemented carbide materials, the distribution of whiskers exhibits obvious directionality, and different toughening effects are exhibited in different directions due to different whisker orientations. Therefore, the influence of whisker orientation on the cutting performance of the tool should be considered when manufacturing the cemented carbide insert. In addition, the whisker content in the WC-Co-SiCw material is different, and the toughening effect is also greatly different. If the whisker content is too much, it will be difficult to obtain the material structure with high density due to the difficulty of sintering, thus affecting the strength of the cemented carbide material; if the whisker content is too small, the toughening effect of the whisker is not obvious, and the fracture toughness of the material is limited. The whiskers may not only be toughened, but become redundant inclusions or even sources of defects. Therefore, there is an optimum whisker ratio, and the addition of whiskers according to the ratio can not only obtain a material with high density, but also the external load can be transmitted to the whiskers through the interface, thereby effectively achieving the toughening effect of the whiskers. In order to achieve this goal, WC-Co-SiCw tools with different whisker content and different whisker orientation should be selected for cutting according to different tool damage methods, so as to fully realize the toughening and reinforcing effect of the tool material. .
2) Nanocomposite strengthening technology
a. Strengthening mechanism
Nanotechnology is an emerging technology that has developed rapidly in recent years. When the grain size of the material reaches the nanometer level, many specific energies are generated. Due to the large interface of the nanomaterials, the arrangement of atoms on the interface is quite confusing, and it is easy to migrate under the condition of external force deformation, so that the material exhibits good toughness and ductility. The microstructure phase of the nano-tool material has a nanometer scale. Due to the size effect, the grain boundary area increases, and the crack propagation resistance increases, so that excellent mechanical properties (such as fracture toughness, flexural strength, hardness, etc.) can be obtained. ), showing good cutting performance.
Due to the immature production process, high price, and the susceptibility of nano-grains during the sintering process, no company in the world has achieved industrial scale production of 100 nm particle size cemented carbide materials. Therefore, the industrial application of nano-hard alloy materials will take time. However, it has been found that the addition of nanoparticles to the fine-grained cemented carbide matrix can also improve the overall properties of the hardness and toughness of the cemented carbide substrate. Therefore, the use of nanocomposite strengthening is an effective way to improve the performance of fine-grained cemented carbide materials.
The nanocomposite strengthening mechanism is mainly because the nanoparticles dispersed in the cemented carbide matrix material have a dispersion toughening effect. When a second phase particle (nanoparticle) of high elastic modulus is added to the matrix material, these particles will prevent the transverse cross-section from shrinking when the matrix material is subjected to stretching, and the same lateral shrinkage as the matrix is ​​increased. Large longitudinal tensile stress, which allows the material to consume more energy and toughen. At the same time, the high elastic modulus particles can “pin†the crack, causing the crack to deflect and bypass, thereby dissipating the power of the crack advancement and playing a toughening effect. In addition, the nanoparticles dispersed in the cemented carbide matrix material can inhibit the growth of the cemented carbide grains during the sintering process, and comprehensively improve the mechanical properties of the cemented carbide material.
b. Selection of inhibitors
An important issue in the preparation of nanocomposite fine-grained cemented carbides is how to suppress grain growth during sintering. Fine-grained cemented carbides grow very easily and rapidly during sintering. The grain length leads to a decrease in material strength. A single coarse WC grain is often an important cause of fracture of cemented carbide. By adding an inhibitor, the growth of WC grains during sintering can be effectively prevented, and the key to eliminating the local growth of WC grains is the uniform distribution of the inhibitor. The grain growth phenomenon mainly occurs in the dissolution and precipitation process of WC, that is, WC dissolves in the liquid phase and precipitates on the larger WC crystal to cause grain growth. An important mechanism for inhibitors to inhibit grain growth is that the addition of inhibitors can reduce the solubility of WC in the binder phase, hinder the dissolution-precipitation mechanism of WC grains, and thereby destroy the conditions for grain growth; The added inhibitor can be deposited on the activated grown grains of the WC grains, thereby preventing the grains from growing further.
Generally, inhibitors for controlling WC grain growth include VC, Cr3C2, and the like, and further, insoluble carbides are added, such as TiC, ZrC, NbC, Mo2, HfC, TaC, and the like. Figure 1 shows the average grain size of WC between WC-X-20%Co (X is an added carbide) cemented carbide (sintered at 1400 °C for 1 hour) and the amount of each carbide added separately. relationship. It can be seen from the figure that the order of effect of various carbide inhibitors controlling WC grain growth is: VC>Mo2C>Cr3C2>NbC>TaC>TiC>ZrC>HfC, wherein VC has the most obvious inhibitory effect, while adding trace Mo2C and Cr3C2 has almost no effect of suppressing the growth of WC grains.
Figure 1 Relationship between carbide addition and WC grain size .jpg)
The way in which the inhibitor is added has a great influence on the performance of the ultrafine cemented carbide. In the same amount of addition, the addition of the inhibitor in the form of a simple substance generally results in a higher porosity and a finer grain of the cemented carbide material, and when the inhibitor is added as a solid solution, the porosity of the cemented carbide material is relatively less. The grain is thicker. The performance indexes of WC-8%Co cemented carbide with different inhibitors added in different ways are shown in Table 2. It can be seen that the performance index of the cemented carbide in which the inhibitor is added in the form of a solid solution is good, and the bending strength of the material is greatly improved. Taking VC as an example, if it is added in a simple form, VC is more soluble in the Co phase, thereby reducing the amount of W dissolved; VC is arranged at the WC/Co interface to prevent grain growth and grain growth. Complete; during the cooling process, #0å‘!0 grain diffusion, forming (W,V)C solid solution, due to the short time to form a solid solution, causing large microscopic strain in the grain, thus affecting the mechanical and physical properties of the cemented carbide . If VC inhibitor is added in the form of solid solution, WC and VC diffuse into the Co phase at the same time, the dissolved amount of V decreases, and the dissolved amount of W increases, and the pore filling is easier, but also the inhibition of VC is decreased. During the cooling process, since part of the VC has existed in the form of (W, V) C, the strain inside the grain is reduced, and the grain growth is more complete, thereby improving the mechanical and physical properties of the cemented carbide.
Table 2 Properties of WC-8% Co cemented carbide with different inhibitors .jpg)
Conclusion
Based on the comprehensive review of the research status of cemented carbide tool materials, this paper proposes the research and development ideas of using the synergistic toughening effect of whisker toughening and nanocomposite strengthening to improve and improve the comprehensive performance of cemented carbide. It can be expected that due to the significant advantages of whisker toughening and nanocomposite strengthening processes in improving the performance of cemented carbides, they will be widely used in the research and development of new high performance cemented carbide tool materials.
According to China Steel News Network on November 8, 2007, materials, structures and geometries are the three factors that determine the cutting performance of a tool. The performance of the tool material plays a key role. The International Society of Production Engineering (CIRP) pointed out in a research report: "As the tool material improves, the allowable cutting speed almost doubles every 10 years." Tool materials have been developed from high-speed steel and hard alloys in the early 20th century to high-performance ceramics and super-hard materials. The heat-resistant temperature has been increased from 500-600 °C to over 1200 °C, allowing the cutting speed to exceed 1000 m/min. This has improved the machining productivity by more than 100 times in less than 100 years. Therefore, it can be said that the development of tool materials actually reflects the development history of cutting technology.