Research on the matching between the hottest cutti

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Research on the matching between cutting tools and machining objects

1. Introduction

the progress of tool materials has greatly promoted the development of cutting technology. From carbon tool steel tools, high-speed steel tools, cemented carbide tools, ceramic tools to diamond and cubic boron nitride tools, almost every innovation of tool materials has brought a revolution to the cutting and processing industry. Especially in the past three decades, tool materials, which are the most basic elements of cutting, have developed rapidly. New tool materials made of various high-purity and ultra-fine oxides, nitrides, carbides, borides and alloy elements are constantly emerging. The physical and mechanical properties and cutting performance of materials have been greatly improved, and the scope of application is also expanding. The new tool materials developed, such as nanocomposite tools, nano coated tools, whisker toughened ceramic tools, functionally gradient tool materials, provide a new choice for modern cutting and processing industry. At present, the cutting tool materials widely used in the world mainly include high-speed steel cutting tools, cemented carbide cutting tools, superhard cutting tools, etc., and the brand of cutting tool materials is up to thousands

with the development of science and technology, higher and higher requirements are put forward for engineering materials. Various light and tough materials, new aerospace materials, nuclear materials, composites, biomaterials, functional materials, nano materials, rare earth materials, new metal or non-metallic materials are increasingly used. In the face of such a wide variety of workpiece materials, how to correctly select tool materials for cutting, in order to improve cutting productivity, reduce processing costs and reduce resource consumption, is a problem that needs high attention

for a long time, domestic and foreign machining enterprises choose tool materials mainly by traditional trial cutting method and referring to previous experience. When machining a new type of material, it is often necessary to use a variety of tool materials to carry out repeated cutting tests, study and analyze the wear, damage methods and causes of tools, and select the best tool materials through comparison. This method is blind, resulting in a large waste of human, financial and resources. However, many enterprises choose tools based on experience, and often cannot choose the best tool materials, resulting in low cutting productivity, increased cutting costs, and serious waste of tool material resources (especially some valuable alloy elements)

each kind of tool material has its specific processing range, which can only adapt to a certain range of workpiece materials and cutting speed. When different tools or the same tool process different workpiece materials, there are often great differences in tool life, so the so-called universal tool does not exist. The famous Chinese saying "if you want to do good work, you must sharpen your tools first" has become a consensus at home and abroad. Therefore, reasonable selection of cutting tools is the key to successful machining. Every kind of tool material has its best machining object, that is, there is a reasonable matching problem between cutting tools and machining objects. The matching of cutting tools and machining objects mainly refers to the matching of mechanical properties, physical properties and chemical properties of the two, so as to obtain the longest tool life and the largest cutting productivity. Combined with the research that the author has done, this paper will make a comprehensive comment on the reasonable matching of cutting tools and machining objects

2. Mechanical property matching between cutting tool and machining object

mechanical property matching between cutting tool and machining object mainly refers to the mutual matching of mechanical property parameters such as strength, toughness and hardness of tool and workpiece material. Tools with different mechanical properties (such as high-speed steel tools, cemented carbide tools, superhard tools, etc.) are suitable for processing different workpiece materials. Generally, the hardness of the tool material must be higher than that of the workpiece material, and the tool hardness is generally required to be above 60HRC. Workpiece materials with high hardness must be processed with tools with higher hardness, such as cubic boron nitride and ceramic tools, which can be used for finishing machining of hardened steel (45 ~ 65hrc), bearing steel (60 ~ 62Hrc), high speed steel (hrc>62), tool steel (57 ~ 60HRC) and chilled cast iron, and can realize turning instead of grinding. In addition, the higher the hardness of the tool material, the better its wear resistance

tools with excellent high-temperature mechanical properties are particularly suitable for high-speed machining. The cutting speed of high-speed cutting is several times or even more than ten times higher than that of conventional cutting, so the cutting temperature is very high. Therefore, high-speed cutting requires that tool materials not only have good mechanical properties at room temperature, but also have excellent high-temperature mechanical properties, and their high-temperature mechanical properties are more important than room temperature mechanical properties in terms of product development. Although the room temperature strength of ceramic tools is low, when the temperature increases, its bending strength decreases little. If the cutting temperature reaches about 1000 ℃, the bending strength is only slightly lower than that at room temperature. The excellent high temperature performance of ceramic tools makes them suitable for high-speed cutting, and the allowable cutting speed is 2 ~ 10 times higher than that of cemented carbide. Sialon ceramic tools with high temperature and high hardness can also be used as high temperature cutting tools. When the temperature is higher than 500 ℃, the hardness of cemented carbide decreases sharply due to the soft bonding phase transformation, so it is not suitable for high temperature cutting tools. See Fig. 1 for the change of hardness of various tool materials with temperature

Fig. 1 the change of hardness of various tool materials with temperature

the main factor determining the wear of hard and brittle tools (such as cemented carbide and ceramics) is their mechanical properties. The research of Evans et al. Shows that the inherent brittleness of hard and brittle materials is the main reason for their wear. Therefore, he established the relationship between the hardness, fracture toughness and other mechanical properties of ceramic tool materials and their wear characteristics, that is, V ∝ 1np5/4s1kic3/4h (1)

, where V is the wear volume, KIC is the fracture toughness, h is the hardness, n is the number of abrasive particles, and P is the force acting vertically on the abrasive particles

wayne et al. Tested the abrasive wear characteristics of Al2O3, al2o3/tic and al2o3/tib2 ceramic tools. The results showed that the relationship between the mechanical properties such as hardness and fracture toughness of ceramic materials and their wear characteristics could qualitatively reflect the relationship between the wear of tool materials and their mechanical properties, but the theoretical calculation results were different from the actual measured values, mainly because the influence of the microstructure of ceramic materials was not considered

3. physical property matching between cutting tool and machining object

physical property matching between cutting tool and machining object mainly refers to that the melting point, elastic modulus, thermal conductivity, thermal expansion coefficient, thermal shock resistance and other physical property parameters of tool and workpiece material should match each other. 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 machining workpieces with poor thermal conductivity, tools with good thermal conductivity should be used, so that the cutting heat can be quickly transmitted and the cutting temperature can be reduced

the thermal conductivity of diamond is 1.5 ~ 9 times that of cemented carbide and 2 ~ 6 times that of copper. Due to the high thermal conductivity and thermal diffusivity, the cutting heat is easy to dissipate, so the temperature of the cutting part of the tool is low. The coefficient of thermal expansion of diamond is several times smaller than that of cemented carbide, which is about 1/10 of that of high-speed steel. Therefore, diamond tools will not produce great thermal deformation, which is particularly important for precision machining tools with high dimensional accuracy requirements. Although the thermal conductivity of cubic boron nitride is not as good as that of diamond, it is much higher than that of high-speed steel and cemented carbide. With the increase of cutting temperature, the thermal conductivity of CBN tools increases gradually, which can reduce the cutting temperature at the tool tip, reduce the diffusion wear of tools, and improve the machining accuracy during high-speed finishing. The heat resistance of CBN can reach 1400 ~ 1500 ℃, which is almost twice that of diamond (700 ~ 800 ℃)

because the cutting speed used in high-speed cutting is several times or even ten times higher than that in conventional cutting, and the cutting temperature is very high, the failure of high-speed cutting tools mainly depends on the thermal properties of tool materials (including melting point, heat resistance, oxidation resistance, high-temperature mechanical properties, thermal shock resistance, etc.). The maximum cutting speed of high-speed dry cutting, high-speed hard cutting and high-speed machining of ferrous metals is mainly limited by the heat resistance of tool materials. Therefore, tool materials are required to have high melting point, good thermal conductivity, high oxidation temperature, good heat resistance and strong thermal shock resistance. For example, when machining ferrous metals such as steel and cast iron at high speed, the maximum cutting speed can only reach 1/3~1/5 of that when machining aluminum alloy, because the cutting heat is easy to cause thermal damage to the tool tip. When cutting low thermal conductivity and high hardness materials (such as titanium alloy, heat-resistant nickel base alloy, high hardness alloy steel, etc.) at high speed, it is easy to form serrated chips, while intermittent chips with varying thickness will be generated in the process of high-speed milling, which will lead to high-frequency periodic changes in the thermal stress in the tool, thus accelerating the wear of the tool

4. Matching of chemical properties between cutting tools and machining objects

tool wear is the result of the combined action of mechanical wear and chemical wear. Mechanical wear mainly includes abrasive wear, adhesive wear, plastic wear and micro fracture. Chemical wear mainly refers to the chemical reaction and chemical dissolution between the components of tool material and workpiece material at high temperature, as well as the diffusion of elements between tool and workpiece. Previous studies have shown that the wear of cutting tools is closely related to the workpiece materials and cutting conditions. When machining different workpiece materials under different cutting conditions, the dominant wear mechanism is different. For example, in low-speed cutting, due to low temperature, the wear mechanism is often abrasive wear; In high-speed cutting, chemical reaction, oxidation wear and diffusion wear caused by high temperature are dominant. As the hardness of the workpiece material decreases at high temperature, the abrasive wear gradually decreases, as shown in Figure 2. Chemical wear is closely related to cutting temperature, and its expression is:

Figure 2. Wear of tools during cutting

k = aexp-[e/(RT)] (2)

the chemical property matching of cutting tools and machining objects mainly refers to the chemical compatibility, chemical reaction, diffusion, bonding, dissolution and other chemical property parameters of tools and workpiece materials should match each other. Tools with different components (such as high-speed steel tools, cemented carbide tools, superhard tools, etc.) are suitable for processing different workpiece materials. When the chemical affinity between the cutting tool and the elements in the workpiece is strong from the current research and development results of graphene (easy to produce chemical reaction, mutual adhesion or diffusion), we should try to avoid it. For example, the tool material containing SiC particles or SiC whiskers shows excellent cutting performance when machining nickel based alloys, but the tool material is rapidly worn when machining steel parts. This is because SiC is easy to react with Fe in the workpiece material under the action of high cutting temperature. The reaction formula is 4Fe + SiC → FeSi + Fe3C

the chemical inertia of Al2O3 ceramics is greater than that of tic and WC. Even at the melting temperature, Al2O3 does not react with steel; Secondly, the dissolution rate of Al2O3 in iron is 4 ~ 5 times lower than that of WC. Therefore, when cutting steel parts, the diffusion wear of Al2O3 ceramic tools is very small. In addition, Al2O3 ceramics contain aluminum element, so Al2O3 ceramic tools have large chemical affinity when processing aluminum and aluminum alloys, and are prone to large bonding wear and diffusion wear. Ceramic tools such as al2o3/tic and Al2O3 (/w, Ti) C contain aluminum and titanium elements, and there is also a large reduction in the processing of titanium and titanium alloys, aluminum and aluminum alloys with these ceramic tools

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