Tool materials should have basic properties
The selection of tool materials has a great impact on tool life, processing efficiency, processing quality and processing cost. When cutting, tools must withstand high pressure, high temperature, friction, impact and vibration. Therefore, tool materials should have the following basic properties:
(1) Hardness and wear resistance. The hardness of the tool material must be higher than the hardness of the workpiece material, generally required to be above 60HRC. The higher the hardness of the tool material, the better the wear resistance.
(2) Strength and toughness. The tool material should have high strength and toughness to withstand cutting force, impact and vibration, and prevent brittle fracture and chipping of the tool.
(3) Heat resistance. The tool material should have good heat resistance, be able to withstand high cutting temperatures, and have good antioxidant ability.
(4) Process performance and economy. The tool material should have good forging performance, heat treatment performance, welding performance, grinding performance, etc., and should pursue a high performance-price ratio.
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Types, properties, characteristics and applications of tool materials
1. Diamond tool materials
Diamond is an allotrope of carbon and is the hardest material found in nature. Diamond tools have high hardness, high wear resistance and high thermal conductivity and are widely used in the processing of non-ferrous metals and non-metallic materials. Especially in the high-speed cutting of aluminum and silicon aluminum alloys, diamond tools are the main cutting tool variety that is difficult to replace. Diamond tools that can achieve high efficiency, high stability and long life are an indispensable and important tool in modern CNC machining.
(1) Types of diamond tools
① Natural diamond tools: Natural diamond has been used as a cutting tool for hundreds of years. Natural single crystal diamond tools can be finely ground to an extremely sharp edge with a cutting edge radius of up to 0.002μm. They can achieve ultra-thin cutting and can produce extremely high workpiece precision and extremely low surface roughness. They are recognized, ideal and irreplaceable ultra-precision machining tools.
② PCD diamond tools: Natural diamonds are expensive, and the diamond widely used in cutting is polycrystalline diamond (PCD). Since the early 1970s, after the successful development of polycrystalline diamond (PCD blades) prepared by high temperature and high pressure synthesis technology, natural diamond tools have been replaced by artificial polycrystalline diamonds in many occasions. PCD raw materials are abundant, and its price is only a few tenths to a dozen tenths of natural diamonds. PCD tools cannot grind extremely sharp edges, and the surface quality of the processed workpieces is not as good as natural diamonds. At present, PCD blades with chip breakers cannot be easily manufactured in the industry. Therefore, PCD can only be used for precision cutting of non-ferrous metals and non-metals, and it is difficult to achieve ultra-precision mirror cutting.
③ CVD diamond tools: Since the late 1970s and early 1980s, CVD diamond technology has appeared in Japan. CVD diamond refers to diamond film synthesized on a heterogeneous substrate (such as cemented carbide, ceramic, etc.) by chemical vapor deposition (CVD). CVD diamond has the same structure and characteristics as natural diamond. The performance of CVD diamond is very close to that of natural diamond. It has the advantages of natural single crystal diamond and polycrystalline diamond (PCD), and overcomes their shortcomings to a certain extent.
(2) Performance characteristics of diamond tools
① Extremely high hardness and wear resistance: Natural diamond is the hardest substance found in nature. Diamond has extremely high wear resistance. When processing high-hardness materials, the life of diamond tools is 10 to 100 times that of cemented carbide tools, or even up to several hundred times.
② Very low friction coefficient: The friction coefficient between diamond and some non-ferrous metals is lower than that of other tools. The low friction coefficient means less deformation during processing, which can reduce cutting force.
③ Very sharp cutting edge: The cutting edge of diamond tools can be sharpened to a very sharp level. Natural single crystal diamond tools can reach 0.002 to 0.008 μm, which can perform ultra-thin cutting and ultra-precision machining.
④ High thermal conductivity: Diamond has high thermal conductivity and thermal diffusivity, cutting heat is easily dissipated, and the temperature of the cutting part of the tool is low.
⑤ Low thermal expansion coefficient: The thermal expansion coefficient of diamond is several times smaller than that of cemented carbide, and the change in tool size caused by cutting heat is very small, which is particularly important for precision and ultra-precision machining with high dimensional accuracy requirements.
(3) Application of diamond tools
Diamond tools are mostly used for fine cutting and boring of non-ferrous metals and non-metallic materials at high speeds. Suitable for machining various wear-resistant non-metals, such as fiberglass powder metallurgy blanks, ceramic materials, etc.; various wear-resistant non-ferrous metals, such as various silicon aluminum alloys; various non-ferrous metal finishing.
The disadvantage of diamond tools is that they have poor thermal stability. When the cutting temperature exceeds 700℃~800℃, they will completely lose their hardness. In addition, they are not suitable for cutting ferrous metals because diamond (carbon) easily reacts with iron atoms at high temperatures, converting carbon atoms into graphite structures, making the tools extremely easy to damage.
2. Cubic Boron Nitride Tool Materials
Cubic boron nitride (CBN), a second superhard material synthesized by a method similar to the diamond manufacturing method, is second only to diamond in hardness and thermal conductivity. It has excellent thermal stability and does not oxidize even when heated to 10,000℃ in the atmosphere. CBN has extremely stable chemical properties for ferrous metals and can be widely used in the processing of steel products.
(1) Types of cubic boron nitride tools
Cubic boron nitride (CBN) is a substance that does not exist in nature. It can be divided into single crystals and polycrystalline crystals, namely CBN single crystals and polycrystalline cubic boron nitride (PCBN). CBN is one of the allotropes of boron nitride (BN) and has a structure similar to that of diamond.
PCBN (polycrystalline cubic boron nitride) is a polycrystalline material made by sintering fine CBN materials together through a bonding phase (TiC, TiN, Al, Ti, etc.) under high temperature and high pressure. It is currently the tool material with hardness second only to diamond that is artificially synthesized. It and diamond are collectively referred to as superhard tool materials. PCBN is mainly used to make tools or other tools.
PCBN tools can be divided into integral PCBN blades and PCBN composite blades sintered with cemented carbide.
PCBN composite blades are made by sintering a layer of 0.5-1.0 mm thick PCBN on a cemented carbide with good strength and toughness. Its performance combines good toughness with high hardness and wear resistance. It solves the problems of low bending strength and difficult welding of CBN blades.
(2) Main properties and characteristics of cubic boron nitride
Although the hardness of cubic boron nitride is slightly lower than that of diamond, it is much higher than other high hardness materials. The outstanding advantage of CBN is that its thermal stability is much higher than that of diamond, which can reach more than 1200℃ (diamond is 700-800℃). Another outstanding advantage is that it is chemically inert and does not react chemically with iron at 1200-1300℃. The main performance characteristics of cubic boron nitride are as follows.
① High hardness and wear resistance: The crystal structure of CBN is similar to that of diamond, and it has similar hardness and strength to diamond. PCBN is particularly suitable for processing high-hardness materials that could only be ground before, and can obtain better workpiece surface quality.
② It has high thermal stability: The heat resistance of CBN can reach 1400-1500℃, which is almost 1 times higher than the heat resistance of diamond (700-800℃). PCBN tools can cut high-temperature alloys and hardened steel at a speed 3-5 times higher than that of carbide tools.
③ Excellent chemical stability: It does not react chemically with iron materials at 1200~1300℃, and will not wear out as rapidly as diamond. At this time, it can still maintain the hardness of cemented carbide; PCBN tools are suitable for cutting hardened steel parts and chilled cast iron, and can be widely used in high-speed cutting of cast iron.
④ Good thermal conductivity: Although the thermal conductivity of CBN cannot catch up with diamond, the thermal conductivity of PCBN is second only to diamond among all types of tool materials, and is much higher than high-speed steel and cemented carbide.
⑤ Low friction coefficient: Low friction coefficient can lead to reduced cutting force, lower cutting temperature, and improved processing surface quality during cutting.
(3) Application of cubic boron nitride tools
Cubic boron nitride is suitable for finishing various hard-to-cut materials such as hardened steel, hard cast iron, high-temperature alloy, cemented carbide, surface spraying materials, etc. The processing accuracy can reach IT5 (IT6 for holes), and the surface roughness value can be as small as Ra1.25~0.20μm.
Cubic boron nitride tool materials have poor toughness and bending strength. Therefore, cubic boron nitride turning tools are not suitable for low-speed, high-impact rough machining; at the same time, they are not suitable for cutting materials with high plasticity (such as aluminum alloys, copper alloys, nickel-based alloys, steels with high plasticity, etc.), because when cutting these metals, serious built-up edges will be generated, which will deteriorate the machining surface.
3. Ceramic tool materials
Ceramic tools have the characteristics of high hardness, good wear resistance, excellent heat resistance and chemical stability, and are not easy to bond with metals. Ceramic tools occupy a very important position in CNC machining. Ceramic tools have become one of the main tools for high-speed cutting and difficult-to-machine materials. Ceramic tools are widely used in high-speed cutting, dry cutting, hard cutting and cutting of difficult-to-machine materials. Ceramic cutting tools can efficiently process high-hardness materials that traditional cutting tools cannot process at all, realizing "turning instead of grinding"; the optimal cutting speed of ceramic cutting tools can be 2 to 10 times higher than that of carbide cutting tools, thereby greatly improving the cutting production efficiency; the main raw materials used in ceramic cutting tools are the most abundant elements in the earth's crust. Therefore, the promotion and application of ceramic cutting tools is of great significance to improving productivity, reducing processing costs, and saving strategic precious metals, and will also greatly promote the progress of cutting technology.
(1) Types of ceramic cutting tool materials
Ceramic cutting tool materials can generally be divided into three categories: alumina-based ceramics, silicon nitride-based ceramics, and composite silicon nitride-alumina-based ceramics. Among them, alumina-based and silicon nitride-based ceramic cutting tool materials are the most widely used. The performance of silicon nitride-based ceramics is superior to that of alumina-based ceramics.
(2) Performance and characteristics of ceramic cutting tools
① High hardness and good wear resistance: Although the hardness of ceramic cutting tools is not as high as that of PCD and PCBN, it is much higher than that of carbide and high-speed steel cutting tools, reaching 93~95HRA. Ceramic tools can process high-hardness materials that are difficult to process with traditional tools, and are suitable for high-speed cutting and hard cutting.
② High temperature resistance and good heat resistance: Ceramic tools can still cut at high temperatures above 1200℃. Ceramic tools have excellent high-temperature mechanical properties, and ceramic tools have particularly good oxidation resistance. Even if the cutting edge is in a red-hot state, it can be used continuously. Therefore, ceramic tools can achieve dry cutting, thus eliminating the need for cutting fluid.
③ Good chemical stability: Ceramic tools are not easy to bond with metals, and are corrosion-resistant and chemically stable, which can reduce the bonding wear of the tools.
④ Low friction coefficient: Ceramic tools have low affinity with metals and a low friction coefficient, which can reduce cutting force and cutting temperature.
(3) Application of ceramic tools
Ceramic is one of the tool materials mainly used for high-speed finishing and semi-finishing. Ceramic tools are suitable for cutting various cast irons (gray cast iron, ductile iron, malleable cast iron, chilled cast iron, high alloy wear-resistant cast iron) and steels (carbon structural steel, alloy structural steel, high strength steel, high manganese steel, quenched steel, etc.), and can also be used to cut copper alloys, graphite, engineering plastics and composite materials.
Ceramic tool materials have low bending strength and poor impact toughness, and are not suitable for cutting under low speed and impact load.
4. Coated tool materials
Coating the tool is one of the important ways to improve tool performance. The emergence of coated tools has made a major breakthrough in tool cutting performance. Coated tools are coated with one or more layers of refractory compounds with good wear resistance on the tough tool body. It combines the tool substrate with a hard coating, thereby greatly improving the tool performance. Coated tools can improve processing efficiency, improve processing accuracy, extend tool life, and reduce processing costs.
About 80% of the cutting tools used in new CNC machine tools use coated tools. Coated tools will be the most important tool variety in the field of CNC machining in the future.
(1) Types of coated tools
According to different coating methods, coated tools can be divided into chemical vapor deposition (CVD) coated tools and physical vapor deposition (PVD) coated tools. Coated carbide tools generally adopt chemical vapor deposition method, and the deposition temperature is about 1000℃. Coated high-speed steel tools generally adopt physical vapor deposition method, and the deposition temperature is about 500℃.
According to different coated tool substrate materials, coated tools can be divided into carbide coated tools, high-speed steel coated tools, and coated tools on ceramics and superhard materials (diamond and cubic boron nitride).
According to the properties of coating materials, coated tools can be divided into two categories, namely "hard" coated tools and "soft" coated tools. The main goal of "hard" coated tools is high hardness and wear resistance. Its main advantages are high hardness and good wear resistance. Typical examples are TiC and TiN coatings. The goal of "soft" coated tools is low friction coefficient. They are also called self-lubricating tools. The friction coefficient between them and the workpiece material is very low, only about 0.1, which can reduce adhesion, reduce friction, and reduce cutting force and cutting temperature.
Recently, nano-coated tools have been developed. This type of coated tool can use different combinations of coating materials (such as metal/metal, metal/ceramic, ceramic/ceramic, etc.) to meet different functional and performance requirements. A well-designed nano-coating can make the tool material have excellent anti-friction and anti-wear functions and self-lubricating properties, which are suitable for high-speed dry cutting.
(2) Characteristics of coated tools
① Good mechanical and cutting properties: Coated tools combine the excellent properties of the base material and the coating material, maintaining the good toughness and high strength of the base material while also having the high hardness, high wear resistance and low friction coefficient of the coating. Therefore, the cutting speed of coated tools can be increased by more than 2 times compared to uncoated tools, and a higher feed rate is allowed. The life of coated tools is also improved.
② Strong versatility: Coated tools have wide versatility and significantly expand the processing range. One coated tool can replace several uncoated tools.
③ Coating thickness: The tool life will increase with the increase of coating thickness, but when the coating thickness reaches saturation, the tool life will no longer increase significantly. When the coating is too thick, it is easy to cause peeling; when the coating is too thin, the wear resistance is poor.
④ Regrinding: The regrinding of coated blades is poor, the coating equipment is complex, the process requirements are high, and the coating time is long.
⑤ Coating material: Tools with different coating materials have different cutting performance. For example: TiC coating has an advantage in low-speed cutting; TiN is more suitable for high-speed cutting.
(2) Application of coated tools
Coated tools have great potential in the field of CNC machining and will be the most popular in the field of CNC machining in the future
Important tool varieties. Coating technology has been applied to end mills, reamers, drills, compound hole processing tools, gear hobs, gear shaping cutters, gear shaving cutters, forming broaches and various machine-clamped indexable inserts to meet the needs of high-speed cutting of various steels and cast irons, heat-resistant alloys and non-ferrous metals.
5. Cemented carbide tool materials
Cemented carbide tools, especially indexable cemented carbide tools, are the leading products of CNC machining tools. Since the 1980s, various integral and indexable cemented carbide tools or inserts have been expanded to various cutting tool fields. Among them, indexable cemented carbide tools have expanded from simple turning tools and face milling cutters to various precision, complex and forming tool fields.
(1) Types of cemented carbide tools
According to the main chemical composition, cemented carbide can be divided into tungsten carbide-based cemented carbide and titanium carbide (nitride) (TiCN)-based cemented carbide.
Tungsten carbide-based cemented carbides include tungsten cobalt (YG), tungsten cobalt titanium (YT), and rare carbide addition (YW). They each have their own advantages and disadvantages. The main components are tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC), niobium carbide (NbC), etc. The commonly used metal bonding phase is Co.
Carbon (nitride) titanium-based cemented carbide is a cemented carbide with TiC as the main component (some other carbides or nitrides are added), and the commonly used metal bonding phases are Mo and Ni.
ISO (International Organization for Standardization) divides cutting cemented carbides into three categories:
K category, including Kl0~K40, is equivalent to my country's YG category (main components are WC and Co).
P category, including P01~P50, is equivalent to my country's YT category (main components are WC, TiC, Co).
M category, including M10~M40, is equivalent to my country's YW category (main components are WC-TiC-TaC, NbC=-Co).
Each brand is represented by a number between 01 and 50, which represents a series of alloys from high hardness to maximum toughness.
(2) Performance characteristics of cemented carbide tools
① High hardness: Cemented carbide tools are made of carbides with high hardness and melting point (called hard phase) and metal binder (called bonding phase) through powder metallurgy. Its hardness reaches 89~93HRA, which is much higher than high-speed steel. At 5400℃, the hardness can still reach 82~87HRA, which is the same as the hardness of high-speed steel at room temperature (83~86HRA). The hardness value of cemented carbide varies with the nature, quantity, particle size and content of metal bonding phase of carbides, and generally decreases with the increase of bonding metal phase content. When the content of bonding phase is the same, the hardness of YT alloy is higher than that of YG alloy, and the alloy with TaC (NbC) added has higher high temperature hardness.
② Flexural strength and toughness: The flexural strength of common cemented carbide is in the range of 900-1500MPa. The higher the content of metal bonding phase, the higher the flexural strength. When the content of bonding agent is the same, the strength of YG (WC-Co) alloy is higher than that of YT (WC-TiC-Co) alloy, and the strength decreases with the increase of TiC content. Cemented carbide is a brittle material, and its impact toughness at room temperature is only 1/30-1/8 of that of high-speed steel.
(3) Application of common cemented carbide tools
YG alloys are mainly used for processing cast iron, non-ferrous metals and non-metallic materials. Fine-grained carbide (such as YG3X, YG6X) has higher hardness and wear resistance than medium-grained carbide when the cobalt content is the same. It is suitable for processing some special hard cast iron, austenitic stainless steel, heat-resistant alloy, titanium alloy, hard bronze and wear-resistant insulating materials.
The outstanding advantages of YT-type carbide are high hardness, good heat resistance, higher hardness and compressive strength at high temperature than YG-type, and good oxidation resistance. Therefore, when the knife is required to have higher heat resistance and wear resistance, a grade with a higher TiC content should be selected. YT-type alloys are suitable for processing plastic materials such as steel, but not for processing titanium alloys and silicon-aluminum alloys.
YW-type alloys have the properties of YG and YT-type alloys and have good comprehensive properties. It can be used for processing steel, cast iron and non-ferrous metals. If the cobalt content of this type of alloy is appropriately increased, the strength can be very high and can be used for rough processing and intermittent cutting of various difficult-to-process materials.
6. High-speed steel tools
High-speed steel (HSS) is a high-alloy tool steel with a large amount of alloying elements such as W, Mo, Cr, and V. High-speed steel tools have excellent comprehensive performance in terms of strength, toughness, and processability. High-speed steel still occupies a major position in complex tools, especially in the manufacture of hole processing tools, milling cutters, threading tools, broaches, gear cutting tools, and other complex blade-shaped tools. High-speed steel tools are easy to sharpen.
According to different uses, high-speed steel can be divided into general-purpose high-speed steel and high-performance high-speed steel.
(1) General-purpose high-speed steel tools
General-purpose high-speed steel. Generally, it can be divided into tungsten steel and tungsten-molybdenum steel. This type of high-speed steel contains 0.7% to 0.9% (C). According to the different tungsten content in the steel, it can be divided into tungsten steel containing 12% or 18% W, tungsten-molybdenum steel containing 6% or 8% W, and molybdenum steel containing 2% or no W. General-purpose high-speed steel has a certain hardness (63~66HRC) and wear resistance, high strength and toughness, good plasticity and processing technology, so it is widely used in the manufacture of various complex tools.
① Tungsten steel: The typical grade of general-purpose high-speed steel tungsten steel is W18Cr4V (abbreviated as W18), which has good comprehensive performance. The high temperature hardness at 6000℃ is 48.5HRC, which can be used to manufacture various complex tools. It has the advantages of good grindability and low decarburization sensitivity, but due to the high carbide content, uneven distribution, large particles, low strength and toughness.
② Tungsten-molybdenum steel: It refers to a high-speed steel obtained by replacing part of the tungsten in tungsten steel with molybdenum. The typical grade of tungsten-molybdenum steel is W6Mo5Cr4V2 (abbreviated as M2). The carbide particles of M2 are fine and uniform, and the strength, toughness and high-temperature plasticity are better than W18Cr4V. Another type of tungsten-molybdenum steel is W9Mo3Cr4V (abbreviated as W9), which has slightly higher thermal stability than M2 steel, better bending strength and toughness than W6M05Cr4V2, and has good machinability.
(2) High-performance high-speed steel tools
High-performance high-speed steel refers to a new type of steel that adds some carbon content, vanadium content, and alloying elements such as Co and Al to the general high-speed steel composition, thereby improving its heat resistance and wear resistance. There are mainly the following categories:
① High-carbon high-speed steel. High-carbon high-speed steel (such as 95W18Cr4V) has high hardness at room temperature and high temperature. It is suitable for manufacturing tools for processing ordinary steel and cast iron, drills, reamers, taps and milling cutters with high wear resistance requirements, or processing harder materials. It is not suitable for large impact.
② High-vanadium high-speed steel. Typical grades, such as W12Cr4V4Mo (abbreviated as EV4), contain V to 3%~5%, good wear resistance, suitable for cutting materials with great tool wear, such as fiber, hard rubber, plastic, etc., can also be used to process stainless steel, high-strength steel and high-temperature alloys.
③ Cobalt high-speed steel. It is a cobalt-containing super-hard high-speed steel. Typical grades such as W2Mo9Cr4VCo8 (abbreviated as M42) have high hardness, which can reach 69~70HRC. It is suitable for processing high-strength heat-resistant steel, high-temperature alloy, titanium alloy and other difficult-to-process materials. M42 has good grindability and is suitable for making precision and complex tools, but it is not suitable for working under impact cutting conditions.
④ Aluminum high-speed steel. It is an aluminum-containing super-hard high-speed steel. Typical grades include W6Mo5Cr4V2Al (abbreviated as 501). Its high temperature hardness at 6000℃ also reaches 54HRC. Its cutting performance is equivalent to M42. It is suitable for manufacturing milling cutters, drills, reamers, gear cutters, broaches, etc., and is used to process alloy steel, stainless steel, high-strength steel and high-temperature alloys.
⑤ Nitrogen super-hard high-speed steel. Typical grades include W12M03Cr4V3N (abbreviated as V3N). It is a nitrogen-containing super-hard high-speed steel. Its hardness, strength and toughness are equivalent to M42. It can be used as a substitute for cobalt-containing high-speed steel and is used for low-speed cutting of difficult-to-process materials and low-speed high-precision processing.
(3) Melting high-speed steel and powder metallurgy high-speed steel
According to different manufacturing processes, high-speed steel can be divided into melting high-speed steel and powder metallurgy high-speed steel.
① Melting high-speed steel: Ordinary high-speed steel and high-performance high-speed steel are both manufactured by melting methods. They are made into cutting tools through processes such as smelting, ingot casting and plating and rolling. The serious problem that is easy to occur in the melting of high-speed steel is carbide segregation. The hard and brittle carbides are unevenly distributed in the high-speed steel, and the grains are coarse (up to tens of microns), which has an adverse effect on the wear resistance, toughness and cutting performance of the high-speed steel tools.
② Powder metallurgy high-speed steel (PM HSS): Powder metallurgy high-speed steel (PM HSS) is a steel liquid melted in a high-frequency induction furnace, which is atomized with high-pressure argon or pure nitrogen, and then rapidly cooled to obtain a fine and uniform crystalline structure (high-speed steel powder). The resulting powder is then pressed into a knife blank under high temperature and high pressure, or first made into a steel billet and then forged and rolled into a tool shape. Compared with high-speed steel manufactured by the melting method, PM HSS has the advantages of fine and uniform carbide grains, and much higher strength, toughness and wear resistance than smelting high-speed steel. In the field of complex CNC tools, PM HSS tools will further develop and occupy an important position. Typical grades, such as F15, FR71, GFl, GF2, GF3, PT1, PVN, etc., can be used to manufacture large-sized, heavy-loaded, and high-impact tools, and can also be used to manufacture precision tools.
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Principles for the selection of CNC tool materials
Currently, the widely used CNC tool materials mainly include diamond tools, cubic boron nitride tools, ceramic tools, coated tools, carbide tools, and high-speed steel tools. There are many grades of tool materials, and their performance varies greatly. The main performance indicators of various tool materials are shown in the following table.
Main performance indicators of various tool materials
Tool materials for CNC machining must be selected according to the workpiece being machined and the processing properties. The selection of tool materials should be reasonably matched with the processing object. The matching of cutting tool materials and processing objects mainly refers to the matching of the mechanical properties, physical properties, and chemical properties of the two to obtain the longest tool life and the maximum cutting productivity.
1. Mechanical properties matching between cutting tool materials and processing objects
The mechanical properties matching problem between cutting tools and processing objects mainly refers to the matching of mechanical properties parameters such as strength, toughness and hardness between tools and workpiece materials. Tool materials with different mechanical properties are suitable for processing different workpiece materials.
① The order of tool material hardness is: diamond tool> cubic boron nitride tool> ceramic tool> cemented carbide> high-speed steel.
② The order of bending strength of tool materials is: high-speed steel> cemented carbide> ceramic tool> diamond and cubic boron nitride tools.
③ The order of toughness of tool materials is: high-speed steel> cemented carbide> cubic boron nitride, diamond and ceramic tools.
High-hardness workpiece materials must be processed with tools of higher hardness. The hardness of tool materials must be higher than that of workpiece materials, generally required to be above 60HRC. The higher the hardness of tool materials, the better its wear resistance. For example, when the cobalt content in cemented carbide increases, its strength and toughness increase, and its hardness decreases, which is suitable for rough machining; when the cobalt content decreases, its hardness and wear resistance increase, which is suitable for fine machining.
Tools with excellent high-temperature mechanical properties are particularly suitable for high-speed cutting. The excellent high-temperature performance of ceramic tools enables them to cut at high speeds, and the allowable cutting speed can be increased by 2 to 10 times compared with cemented carbide.
2. Matching the physical properties of cutting tool materials and processing objects
Tools with different physical properties, such as high-speed steel tools with high thermal conductivity and low melting point, ceramic tools with high melting point and low thermal expansion, diamond tools with high thermal conductivity and low thermal expansion, etc., are suitable for processing different workpiece materials. When processing workpieces with poor thermal conductivity, tool materials with good thermal conductivity should be used to allow the cutting heat to be quickly transferred and reduce the cutting temperature. Diamond has high thermal conductivity and thermal diffusivity, so cutting heat is easy to dissipate and will not produce large thermal deformation, which is especially important for precision machining tools with high dimensional accuracy requirements.
① Heat-resistant temperature of various tool materials: 700-8000℃ for diamond tools, 13000-15000℃ for PCBN tools, 1100-12000℃ for ceramic tools, 900-11000℃ for TiC (N)-based cemented carbide, 800-9000℃ for WC-based ultrafine-grained cemented carbide, and 600-7000℃ for HSS.
② The order of thermal conductivity of various tool materials: PCD>PCBN>WC-based cemented carbide>TiC (N)-based cemented carbide>HSS>Si3N4-based ceramics>A1203-based ceramics.
③ The order of thermal expansion coefficients of various tool materials is: HSS>WC-based cemented carbide>TiC (N)>A1203-based ceramics>PCBN>Si3N4-based ceramics>PCD.
④ The order of thermal shock resistance of various tool materials is: HSS>WC-based cemented carbide>Si3N4-based ceramics>PCBN>PCD>TiC (N)-based cemented carbide>A1203-based ceramics.
3. Chemical performance matching of cutting tool materials and processing objects
The problem of chemical performance matching of cutting tool materials and processing objects mainly refers to the chemical performance parameters such as chemical affinity, chemical reaction, diffusion and dissolution of tool materials and workpiece materials. Tools with different materials are suitable for processing different workpiece materials.
① The anti-adhesion temperature of various tool materials (with steel) is: PCBN>ceramics>ceramics>HSS.
② The anti-oxidation temperature of various tool materials is: ceramics>PCBN>ceramics>diamond>HSS.
③ The diffusion strength of various tool materials (for steel) is: diamond>Si3N4-based ceramics>PCBN>A1203-based ceramics. Diffusion strength (for titanium) is: A1203-based ceramics>PCBN>SiC>Si3N4>diamond.
4. Reasonable selection of CNC tool materials
Generally speaking, PCBN, ceramic tools, coated carbide and TiCN-based carbide tools are suitable for CNC machining of ferrous metals such as steel; while PCD tools are suitable for machining non-ferrous metal materials such as Al, Mg, Cu and their alloys and non-metallic materials. The following table lists some workpiece materials suitable for machining by the above tool materials.
