Cutting wear analysis and Countermeasures of the h

2022-10-18
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Analysis and Countermeasures of cutting wear of Ti (C, n) coated knives

in the process of metal cutting, the main causes of tool wear are the contact and friction between the tool and the workpiece and the extremely bad conditions of high temperature and high pressure near the blade. The change of contact conditions of any unit in the cutting area will affect the wear of the tool. These conditions mainly include tool material, tool coating and tool geometric specifications. Other conditions include workpiece material and its physical properties, tool parameters and appropriate cutting conditions, cooling and lubrication conditions, and the overload of tool fluctuation caused by large machine vibration or serious tool jump in the cutting process

due to the wear of cutting tools when cutting different materials under different cutting conditions, he also plays a leading role in the development of polimotor 2 of all plastic (and composite) internal combustion engine. The mechanism is different: in low-speed cutting, abrasive wear is often the main [1]; In high-speed cutting, with the increase of cutting speed and cutting temperature, bonding wear and chemical wear become more and more prominent. This paper mainly analyzes and studies the tool wear failure of Ti (C, n) ceramic coated cemented carbide tool when cutting Cr12 die steel

test material and conditions

1 tool material

the tool adopts Ti (C, n) coated WC based cemented carbide tool, and the cutting hardness is greater than HRC70. See Table 1 for the composition of Ti (C, n) coating materials

2 workpiece material

cr12 high carbon and high chromium steel is abbreviated as Cr12 Steel, which is characterized by high carbon content and high chromium content. See Table 2 for specific chemical composition

3 cutting test

the performance of tool cutting samples is measured as follows: the hardness of tool samples is measured by durometer, the bending strength of samples is measured by three-point bending method, and the fracture toughness of samples is measured by unilateral notched beam method. The measured and calculated material properties are shown in Table 3. In the process of tool cutting, the whole process of tool wear and even damage is monitored, and the causes of tool wear failure are analyzed

analysis of test results

1 damage analysis of coating

the main function of coating is to protect the substrate from severe friction and thermal barrier of workpiece. When cutting Cr12 die steel, the role of coating is mainly to improve wear resistance and reduce bonding wear [2]. Because the friction coefficient between the coating of coated tools and the workpiece is generally small, the wear of coated tools starts from the damage of the coating. During cutting, because the cutting edge of the tool is relatively sharp, the mechanical load on the tool per unit area is relatively large, and the strength of the coating material at the tool tip is weaker than that of the tool base, the coating here is most vulnerable to damage, and the coating gradually falls off under the action of thermal stress, mechanical stress and bonding wear

carry out energy spectrum analysis on the surface of the sample before and after tool wear, as shown in Figure 1 and Figure 2. It can be seen from the analysis that the main component Ti element decreases after tool wear, which indicates that in high-speed machining, after a period of cutting, the coating on the rake face has been damaged, and the analysis shows that the surface coating is basically peeling off, and the Ti content is very low; On the flank, the content of Ti is still very high, which proves that the coating of the flank has not been seriously damaged. The main reason for this situation is that the cutting temperature of the rake face is high, the pressure is high, the thermal and mechanical stress is high, and the chip is recycled, resulting in the rapid destruction of the tool coating; Relatively speaking, the temperature of the rear knife surface is not very high, and the stress is not as large as that of the front knife surface, so the coating damage is relatively small

after the coating on the cutting edge of the tool is damaged, the thermal barrier effect of the coating disappears, the tool matrix is impacted by temperature gradient and thermal stress, and cracks and nucleation occur in the tool under thermal shock due to defects such as pores in the manufacturing process [3]

2 diffusion wear

when cutting steel parts, the cutting temperature often reaches 800~1000 ℃, or even higher. Diffusion wear has become one of the main wear causes of cemented carbide tools [4]. Since 800 ℃, Co, C, W and other elements in cemented carbide will diffuse into the chips and be taken away; The Fe in the chip will diffuse into the cemented carbide to form a new composite carbide with low hardness and high brittleness. Due to the diffusion of CO, WC, tic and other carbides will reduce their bonding strength with the matrix due to the reduction of binder Co, which will accelerate tool wear. At the same time, as the temperature rises to a certain extent, bonding occurs, so diffusion wear and bonding wear often occur at the same time

in the process of observing the wear morphology of the tool surface, we found that there are more or less circular small particles on the back surface of the tool, as shown in Figure 3. In order to explore the composition of these pellets, EDS was used to analyze the energy spectrum of part a of the production field of vanadium in which many small and medium-sized production enterprises in China withdrew due to cost reasons in Figure 3 (a). It is found that these spheres are mainly composed of metal Fe and O, which are presumed to be oxides of iron. The shape of the ball is very round, which cannot be the chip produced in cutting. According to the theory of metal crystallography, the most possible way to form such a round sphere is through liquid-solid transformation, because liquid substances tend to shrink into spheres automatically in order to have the minimum surface energy. In high-speed cutting, the temperature is very high. Although the cutting temperature in the groove wear area is theoretically not as high as that at the tool tip and the rake face, the oxidation and diffusion phenomenon in the groove area is intense, especially the chip flows from this high speed, resulting in strong friction between the tool groove wear area and the sharp edge of the chip. The local temperature in the groove wear area and the contact micro area is very high, reaching the temperature of melting some micro areas on the chip, So many small balls are formed

as the cutting process proceeds, these melted balls are brought into the wear marks on the flank of the tool. According to the iron carbon phase diagram, when the carbon content is 0.45%, the melting point of steel is 1450 ℃, which shows that the maximum temperature during cutting is not lower than 1450 ℃

after analyzing the energy spectrum of the tool wear part, it is found that the material of the workpiece is transferred to the wear area, as shown in Figure 3 (b). The content of the original elements of the tool (such as Ti, Mo, W, Ni, etc.) is low, while the content of Fe and O increases. This shows that under the action of high temperature and high pressure, mutual diffusion occurs between tool material and workpiece material [5]. In the wear area of the tool surface, almost all the original components of the material diffuse into the chips of the workpiece material, in which w and Mo elements may be oxidized into volatile oxides and lost at high temperature, while carbonitride particles decompose due to the diffusion of C into the workpiece, and oxidize into low hardness TiO2, which is taken away by the workpiece and chips. The energy spectrum of C and n elements was not detected in the wear zone of the tool, mainly because C and N had been transformed into gaseous N2 and co/co2 at high temperature, and the Fe diffused into the tool material was oxidized into iron oxide

wear reduction measures

1 tool material

the tool adopts Ti (C, n) coating, which aims to improve the friction and wear resistance of the tool by using the high hardness and wear resistance of the coating material, especially the high temperature characteristics; The good chemical stability, low friction coefficient and certain solid lubrication of the coating material are used to reduce the cutting force, reduce the generation of cutting heat, and indirectly improve the friction and wear resistance of the tool

from the above analysis, it can be seen that the change of material composition in the coating material has a great impact on the performance of the material itself, so the coating material with good cutting performance should be selected to maximize its cutting performance

the matrix of the tool is the basis for maintaining the overall strength of the tool, supporting the coating and forming the blade shape of the tool. Therefore, although it does not directly contact with the chip and workpiece, it plays a very important basic role in the integrity of the tool

and the adhesion between the coating and the substrate is the most fundamental problem of coated tools, which largely depends on the nature and surface state of the tool substrate. Before using the coating, the substrate surface should be mechanically ground, polished, ultrasonic cleaned, chemical cleaned, ion sputtering cleaned, etc; And adopt advanced multi-element, multi-layer and gradient coating system to improve the adhesion between the coating and the substrate by adding intermediate transition layer [6]; Proper pretreatment of high-speed steel substrate, such as ion nitriding and ion implantation, can prevent the coating cracking and spalling caused by matrix softening and deformation during cutting

2 cutting parameters

the service life of coated tools is higher than that of uncoated tools, mainly because the wear resistance and lubrication performance of the coating are obvious under a small cutting amount, while under a large cutting amount, the coating falls off quickly, and its cutting performance decreases, which is equivalent to uncoated tools, or even worse than uncoated tools. Therefore, cutting parameters should be optimized to reduce tool wear

conclusion

in order to reduce tool wear, the cutting parameters can be optimized, and the cutting parameters with small cutting force can be selected for cutting under the condition of ensuring product quality and processing efficiency; It can also optimize the composition of coating materials, improve the machinability of coating materials, adopt advanced multi-element, multi-layer and gradient coating system, improve the adhesion between the coating and the substrate by adding intermediate transition layer, and properly pretreat the tool substrate during coating

references

[1] Zhang Xiaobo, Liu Ning, Li Yong Carbon nanotubes toughen ultrafine Ti (C, n) - based cermets Journal of composite materials, 2009 (1):

[2] Li Tao Mechanical modeling and computer simulation of orthogonal cutting of hardened die steel [d] Nanjing: Nanjing University of Aeronautics and Astronautics, 2007

[3]Zhang F,Shen J,Sun ocessingand properties of carbon nanotubes-nano-WC-Co composites. Materials Science andEngineering(A),2004(381):.

[4] Xiong Weihao, Hu Zhenhua, Cui Kun Phase interface transition layer of Ti (C, n) cermet Acta Metallica Sinica, 1996,32 (10):

[5]Zheng Y,Liu W,Wang S,fectof carbon content on the microstructure andmechanical properties of Ti(C,N)-based ramics international,2004(30):.

[6] Yan Yonglin Research on preparation technology of nanocomposite Ti (C, n) - based ceramics and coatings [d] Nanjing: Nanjing University of Aeronautics and Astronautics, 2008 (end)

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