With the rapid development of CNC machining technology and the widespread use of CNC machine tools, tool materials are constantly updated and developed. High-speed cutting has become an indispensable and important means in modern cutting. The cutting speed of high-speed cutting is 5 to 10 times that of conventional cutting, which puts forward new requirements for tool materials, tool structure, geometric parameters, etc. In particular, there are higher requirements for tool materials. The materials of high-speed cutting tools must have high strength, toughness and hardness. Good thermal stability and hot hardness, good high-temperature mechanical properties, small chemical affinity and wear resistance. Here we will discuss the tool materials and performance of high-speed cutting tools and their reasonable selection measures.
High Speed Tool Classification and Performance
Coated Milling Tools
Coated cutting tools are coated with a hard and wear-resistant metal compound film on the tool substrate to achieve the purpose of improving the hardness and wear resistance of the tool surface. Common tool substrate materials mainly include high-speed steel, cemented carbide, metal ceramics and ceramics. Coated tools can be divided into hard-coated tools and soft-coated tools. The coating materials of hard-coated tools mainly include titanium nitride (TiN), titanium carbide (TiC), aluminum oxide (Al2O3), titanium carbonitride (TiCN), titanium aluminum nitride (TiAlN), titanium aluminum carbonitride (TiAlCN), etc. The main characteristics of hard-coated tools are high hardness and good wear resistance. Soft-coated tools are high-speed steel tools with MoS2 and WS2 as coating materials. Soft-coated tools can reduce friction, cutting force and cutting temperature.
The coating can be a single coating, a double coating or a multi-coating, or even a composite coating composed of several coating materials. The composite coating can be TiC-Al2O3-TiN, TiCN and TiAlN multi-composite coatings, and the latest developments include TiN/NbN, TiN/CN and other multi-composite films. TiN coating has high hardness and good wear resistance, TiC coating has low affinity with metal and strong oxidation resistance, TiAlN has good high-speed cutting performance, and Al2O3 coating has good thermal stability. TiAlN layer has excellent performance in high-speed cutting, and the maximum cutting temperature can reach 800℃.
Ceramic Cutting Tools
The materials of ceramic cutting tools mainly include alumina-based and silicon nitride-based. Ceramic cutting tools have the advantages of high hardness (HRA91~95), high strength (bending strength 750~1000MPa), good wear resistance, good chemical stability, good anti-adhesion performance, low friction coefficient and low price. Not only that, ceramic cutting tools also have very high high-temperature hardness, reaching HRA80 at 1200°C. During normal cutting, ceramic milling tools are extremely durable, and the cutting speed can be 2 to 5 times higher than that of carbide.
In recent years, due to the control of raw material purity and particle size refinement, various carbides, nitrides, borides and oxides have been added to improve its performance. Its fracture toughness can also be improved through the synergistic effect of particles, whiskers, phase changes, microcracks and several toughening mechanisms. Not only does the flexural strength increase to 0.9~1.0GPa (the highest can reach 1.3-1.5GPa), but the fracture toughness and impact resistance are greatly improved, and the scope of application is increasingly expanded. In addition to general finishing and semi-finishing, it can also be used for roughing under impact load. It is recognized internationally as the tool with the greatest potential to improve production efficiency.
At present, ceramic tools can process materials such as steel, cast iron and their alloys at high speeds at cutting speeds of 200~1000m/min. The tool life is several times or even dozens of times longer than that of cemented carbide. Compared with super-hard tools such as diamond and CBN (cubic boron nitride), ceramics are relatively inexpensive.
Metal Ceramic Cutting Tools
Metal ceramics have high room temperature hardness, high temperature hardness, good wear resistance, strong oxidation resistance and good chemical stability. The materials of metal ceramic tools mainly include TiC-based cemented carbide (TiC+Ni or Mo) with high wear resistance, TiC-based cemented carbide (TiC+TaC+WC) with high toughness, strong TiN-based cemented carbide and high strength and toughness TiCN-based cemented carbide (TiCN+NbC). Tools made of these alloys can process steel and alloy steel at high speeds within the range of υc=300m/min~500m/min, and finely process cast iron. Metal ceramics can be made into drills, end mills and hobs. For example, the metal ceramic hob developed in Japan has a speed of υc=600m/min, which is about 10 to 20 times that of the carbide hob, and the surface roughness value Rmax of the machined surface is 2μm.
Cubic Boron Nitride (CBN) Tools
Cubic boron nitride (CBN) tools have a high hardness, second only to diamond (up to 8000HV ~ 9000HV), and high thermal stability (up to 1250℃ ~ 1350℃). It is chemically inert to iron elements, has strong anti-adhesion ability, and can be ground and sharpened with a diamond grinding wheel. Therefore, it is suitable for processing various hardened steels, thermal spray materials, chilled cast iron, and cobalt-based and nickel-based materials above 35HRC. Because cubic boron nitride (CBN) tools can obtain good surface roughness when processing high-hardness parts. Therefore, the use of cubic boron nitride (CBN) tools can replace grinding with turning and milling, greatly improving processing efficiency.
Polycrystalline Diamond (PCD) Tools
Polycrystalline diamond (PCD) tools PCD tools can achieve high-speed processing of non-ferrous metals and non-metallic wear-resistant materials. It is reported that the cutting speed of PCD drill bits for processing Si-Al alloys is 300m/min~400m/min. PCD and carbide composite drill bits are used to process Al alloys, Mg alloys, composite materials FRP, graphite, and powder metallurgy blanks. Compared with carbide tools, the tool life is increased by 65~145 times. The speed υc of PCD face milling cutters using high-strength Al alloy cutter bodies for processing alloys is 3000m/min~4000m/min, and some reach 7000m/min. In recent years, diamond film tools (turning and milling inserts, twist drill bits, end mills, taps, etc.) have been developed abroad, and their life is 10~140 times that of carbide tools.
High Performance High Speed Steel and Carbide Complex Cutting Tools
Gear cutters made of high-performance cobalt high-speed steel, powder metallurgy high-speed steel and cemented carbide can be used for high-speed cutting of gears. Gear hobs made of new powder metallurgy materials made of cemented carbide powder and high-speed steel powder can reach a hobbing speed of 150m/min to 180m/min. After TiAlN coating treatment, they can be used for high-speed dry cutting of gears. When twist drills made of fine-grained cemented carbide and coated with wear-resistant, heat-resistant and lubricating coatings are added with coolant to process carbon structural steel and alloy steel, the cutting speed can reach 200m/min, and the cutting speed can also reach 150m/min when dry cutting. When taps made of fine-grained cemented carbide are used to process gray cast iron, the cutting speed can reach 100m/min.
Reasonable Selection of Cutting Tools According to Processing Materials
How to correctly select cutting tools to process the corresponding workpiece materials, thereby improving processing productivity and reducing production costs, is a very important issue. Each tool material has its specific processing range. Generally speaking, CBN, metal ceramic cutting tools, ceramic cutting tools, and coated cutting tools are suitable for processing ferrous metal materials such as steel. PCD cutting tools are suitable for high-speed processing of non-ferrous metals such as aluminum, magnesium, copper, and their alloys and non-metallic materials.
How to Reasonably Choose the Right Cutting Tool Material
Ceramic cutting tools are suitable for processing various cast irons, steel parts, and nickel-based high-temperature alloys. Different varieties and ceramic cutting tools have different processing ranges. Al2O3-based ceramic cutting tools have good wear resistance, heat resistance, and good high-temperature chemical stability, and can be used for processing cast iron, steel, and their alloys. Si3N4-based ceramic cutting tools have good fracture toughness and thermal vibration resistance, and are suitable for intermittent processing of cast iron and alloy cast iron.
Metal ceramic milling cutting tools not only have the hardness of ceramics, but also have the high strength of cemented carbide, good adhesion resistance and wear resistance. It has low affinity with steel and is suitable for medium and high-speed cutting of die steel. When the cutting speed is lower than 750 m/min, cast iron parts can be processed.
Diamond cutting tools are suitable for processing non-metallic materials, non-ferrous metals and their alloys. Because diamond has poor thermal stability and strong chemical affinity with iron. Therefore, PCD is not suitable for processing steel materials. Diamond has a small friction coefficient, no affinity with non-ferrous metals, and chips are easy to flow out. It has high thermal conductivity, is not easy to produce built-up edge during cutting, and has good surface quality. It is most suitable for ultra-precision processing of non-ferrous metal materials, non-ferrous metals and non-metallic materials. Such as copper, aluminum and other non-ferrous metals and their alloys, ceramics, powder sintered carbides, various fiber and particle reinforced composite materials, plastics, rubber, graphite, glass and various wear-resistant wood (especially composite materials such as solid wood and plywood).
Cubic boron nitride tools are suitable for processing various hardened steels, thermal spray materials, chilled cast iron and cobalt-based and nickel-based materials above 35HRC. The higher the hardness of the processed material, the more it can reflect the advantages of cubic boron nitride tools. Foreign countries have also developed CBN tools with different CBN contents to give full play to the cutting performance of CBN tools. It is reported that the speed of BN-10 processing gray cast iron can reach 2000m/min.
The continuous updating of tool materials has made cutting processing develop towards high speed and ultra-high speed. Various tool materials are matched with the corresponding workpiece materials. And with different cutting speed ranges, coated tools will dominate the field of high-speed cutting. The inherent advantages of ceramic tools will be more widely developed. CBN and PCD will occupy an important position in high-speed cutting, and their applications will become more and more extensive. Strengthening the research and development of high-speed cutting tool materials will be of great significance to the application and development of high-speed cutting technology.
From the study of high-speed cutting mechanism, we can see that with the increase of cutting speed, the metal removal rate is greatly improved. The high strain rate of the material (about 1.67×105/s at a cutting speed of 500m/min and a cutting temperature of 1400℃) makes the chip forming process and the various phenomena occurring on the contact surface between the tool and the workpiece different from those under traditional cutting conditions. The hot hardness and tool wear of the tool become the key. In order to achieve high-speed cutting, tool materials and tool manufacturing technologies suitable for high-speed cutting must be supported.
Currently used metal cutting tool materials can be divided into nine categories. They range from high-speed steel with the lowest cutting speed and the worst wear resistance to polycrystalline diamond tools with the best high-speed performance. High-speed cutting also requires tool materials with good toughness, strong impact resistance and good wear resistance.
Generally speaking, low-performance tools, such as high-speed steel and uncoated cemented carbide, are widely used in ordinary cutting processes, but their high-speed performance is not good. High-performance tools, such as polycrystalline diamond tools, are the hardest of all tool materials. They have very high wear resistance and speed capabilities, and cutting heat has almost no effect on them. Its superiority is fully reflected in the high-speed processing of non-ferrous metals and non-metallic materials. However, its impact resistance is poor, and its thermal performance is very poor when cutting steel and cast iron materials. Because when cutting ferrous metals at high speed, the carbon atoms in the tool material will diffuse into the workpiece, causing the tool wear to increase.
Carbide tools are the most widely used, accounting for 80% of the total sales of various tools. 90% of metal cutting at normal speeds uses carbide tools. Carbide has relatively good impact toughness, but poor high-speed performance. In addition, high-speed cutting tools must also have extremely high thermal hardness and chemical stability, which carbide cannot meet.
CBN tools have excellent heat resistance and can cut hard materials at high temperatures. However, they are expensive and cannot process soft metal materials. Aluminum-based ceramic materials with good chemical stability are just suitable for this type of processing, but lack thermal hardness and cannot be used for high-speed processing.
Concentrate on the advantages of tool materials with different characteristics and develop new tool materials. Making them have both impact toughness and high wear resistance is one of the main problems to be solved in the research of high-speed cutting tools. On the tool base material with relatively good impact toughness, the surface is coated with materials with good thermal hardness and wear resistance. The new tool often has the advantages of two materials, and this type of tool has a wide range of applications in high-speed cutting.
Under the guidance of this idea, new composite tools such as carbide coated tools, CBN coated tools and PCD coated tools have appeared one after another. It is suitable for high-speed cutting of different metal materials.
Coated carbide tools suitable for high-speed cutting use coating materials with good heat resistance and high hardness and multi-layer coating technology. The cutting range of coated carbide tools is large and the service life is long. Its cutting performance is much better than that of uncoated carbide. Therefore, coated carbide has a tendency to replace ordinary uncoated carbide.
Tool coating technology can not only be used on hardened carbide tools, but also has good effects on other tool materials, such as metal ceramics and ceramics.
Ceramic tools are also tools that can be used for high-speed cutting, and are an important part of tool material research and development in recent years. Whisker-reinforced ceramic tools are tools made of special materials. Because they have good impact toughness and strong thermal shock resistance, they are very suitable for high-speed processing.
Another problem that high-speed cutting tools need to solve is to grind or press out chip breaker grooves of a certain geometry on the blade. In order to achieve chip breaking and control the chip direction, this is also an important technology to improve processing efficiency and tool durability.
High-speed cutting tools should have excellent impact toughness and thermal shock resistance, better heat resistance, wear resistance and chemical stability. And better chip breaking effect, and suitable for a wider metal cutting range.
Development Trend of High Speed Cutting
Develop and manufacture a new generation of high-speed and high-power machine tools to further improve and enhance spindle performance.
New high-wear-resistant and heat-resistant tool materials need to be developed, and the structure of high-speed tools needs to be further improved. Because high-speed cutting tools directly determine the benefits, quality and safety of high-speed cutting, they are the basis for promoting and implementing high-speed cutting technology.
Develop and promote new processes of dry cutting and hard cutting. In recent years, foreign countries have developed and applied new processes of dry cutting and hard cutting (i.e. cutting hardened workpieces) in the process of developing high-speed cutting. Since dry cutting has the effect of protecting the environment, hard cutting can replace grinding to improve work efficiency. And it has gained new development, but both types of cutting require high-performance tool materials.
