Another important factor affecting the quality of the machined surface of hard cutting is the formation of white layers. The white layer is a kind of microstructure formed by the hard cutting process. It has unique wear characteristics: high hardness and good corrosion resistance on the one hand, and high brittleness on the other hand, which is easy to cause early peeling failure. Introduction Hardened steel is a kind of hard-to-machine material with hardness up to 50-65HRC, mainly including ordinary hardened steel, quenched die steel, bearing steel, roll steel and high-speed steel. Due to its typical wear-resistant structure, hardened steel is widely used in the manufacture of various basic parts requiring high hardness and high wear resistance. With the improvement of the performance of superhard tool materials, ceramics and PCBN, and the price adjustment, the contradiction between the traditional manufacturing process of hardened parts and the rapidly developing market demand has been solved, making it possible to cut hardened steel more economically.
Hard cutting refers to the machining process of precision cutting of hardened steel with hardness greater than 50HRC using superhard tools. Compared with grinding, hard cutting has good processing flexibility, economy and environmental performance. Hard cutting in the grinding process is the best choice for processing hardened steel. However, the current hard cutting technology is still not widely used by enterprises. The main reason is not only because the company does not fully understand and master the hard cutting mechanism and the tool use technology, but also because some of the hard cutting processes are not. The stability factor restricts its promotion and application. In this paper, through the synthesis of a large number of literatures at home and abroad, the characteristics of cutting force, chip formation mechanism, hard cutting force and metal softening effect, cooling lubrication technology and processed surface quality in hard cutting process are discussed to promote Popularization and application of hard cutting technology.
Hard Cutting Force Characteristics Factors affecting hard cutting forces include cutting speed, feed rate, depth of cut, flank wear and workpiece hardness. Studies by scholars at home and abroad have shown that the cutting force does not change when performing hard cutting on machine tools of different precision grades.
Dr. Abrao Mendes of Brazil selected ceramic tools, low CBN content and high CBN content of PCBN tool to cut AISI52100 bearing steel (hardness 62HRC) and found that the radial cutting force is the largest, followed by the main cutting force and axial cutting force; The cutting force is about 6 to 9 times that of the finishing; the cutting force is approximately linear with the feed amount, the cutting depth and the flank wear; when the cutting speed is increased, the cutting force is slightly decreased. Achen University of Technology, Germany Professor Konig studied the influence of cutting speed, depth of cut and feed on cutting force by comparing the cutting force of 100Cr6 hardened bearing steel with ceramic cutter and PCBN cutter. The research shows that the change of the main cutting force and the axial force has a linear growth trend and the radial force grows slowly. The different feed rates have the same influence on the change of the cutting force, and the axial force growth rate is slightly lower. The main cutting force and radial force, and when the feed rate is small, the radial force is greater than the main cutting force. Professor Masahiro Nakayama of Japan believes that the main reason for the increase in cutting speed and the decrease in cutting force is that the cutting temperature increases and the workpiece is plastically enhanced (that is, the hardness of the metal is lowered by the action of the cutting temperature). However, this change in properties is limited to a certain range of cutting speeds. When the cutting speed exceeds 20Om/min, the cutting force does not change along the descending channel. This is consistent with the findings of Professor W. Konig. Professor Masahiro Nakayama believes that although the hardened material has a higher hardness, the cutting force is smaller. The reason is that the plastic deformation is very small due to the occurrence of the fracture, and the second is because the contact area of ​​the blade and the chip is small, so that the frictional force is reduced. Professor Liu Xianli of Harbin University of Science and Technology used orthogonal test to design the influencing factors of cutting force, and obtained the three-dimensional surface of cutting force corresponding to cutting speed, cutting depth, feed rate and workpiece hardness. Under the experimental conditions, the main result was obtained. The variation of cutting force is basically consistent with the conclusion of traditional metal cutting theory.
Dr. EGNg of the University of Birmingham in the United Kingdom has solved the finite element simulation of the cutting temperature and cutting force of PCBN tool cutting AISI HI3 hardened steel. The maximum error is 25% and the precision dispersion is large. At the same time, the amount of finite element calculation is also large. Professor Zhang Hongjun used the theory of extrusion and rolling to explain the cutting mechanism of the chamfering tool according to the energy principle. The three-zone model of the chamfering tool (the first deformation zone, the metal dead zone and the second deformation zone) was proposed. The shear angle and cutting force can be predicted and simulated. According to the metallographic analysis and the rapid drop device, it is found that the existence of the metal dead zone does not depend on the cutting speed, rake angle and chamfer angle; under the same cutting conditions The shear angle of the chamfering tool is less than about 2° to 3° of the single-point cutting angle. Taiwanese scholar K. Fuh used the principle of minimum energy to correct the cutting model of Sakai Eiji. Based on the cutting area and considering the flank force, the cutting force was simulated, and the comprehensive precision was high. Due to the introduction of more empirical coefficients, these coefficients tend to vary for different tool and workpiece materials, so their practicality is limited.
Chip Forms for Hard Cutting The focus and core of metal cutting process research is the formation of chips. The hard cutting process generally produces sawtooth chips. Dr. KFKoch and Dr. P. Fallbochmer believe that the chip shape of hard cutting is most affected by the thickness of the chip. When the chip thickness is less than 20μm, it is easy to produce strip-shaped chips, otherwise it will generate zigzag chips. The reason for the formation of the serrated chips is mainly that the workpiece material near the rake face of the tool is squeezed and accumulated on the rake face, and the cutter continues to cut forward, causing the chip material to suddenly break.
There are many well-known assertions about the mechanism of sawtooth chip formation. In 1964, Recht proposed a classic model of abrupt shear instability during machining. When the slope of the nominal stress-true strain curve is zero, the local rate of temperature change has a negative impact on the strength equal to or greater than the strain hardening caused by the strength. In the case of a positive influence, the plastic deformation zone inside the material undergoes a sudden change. Hou Zhen-Bin and Ranga KoII1and of Oklahoma State University in the United States proposed a thermodynamic model for the formation of sawtooth chips. Their experiments show that cutting speed and feed rate play an important role in shear instability. Samiatin and Rab found that when the ratio of the normal flow softening rate to the variable rate sensitivity value is equal to or greater than 5, the non-uniform flow of the metal cutting process occurs immediately. The instability of the thermoplastic process (strain hardening and thermal softening) leads to the shear zone, and even without the thermal softening effect, other mechanisms can significantly reduce the shear strength of the shear band. For example, when the shear band produces microcracks, the actual area under stress is reduced, which Walker and Shaw consider to be a possible mechanism for chip breaking in machining. Recent studies by Shaw and Vyas on the production of nodular swarf in the processing of AISI 4340 steel and low-speed processing titanium at lower cutting speeds have confirmed this concept more clearly. Since the cutting speed at this time is very low, the heat generated by the shearing surface can be diffused to any side, and thermal softening is quite difficult, so that it can be explained that the actual shear strength is lowered due to the presence of microcracks. Other mechanisms of shear instability include material tissue transformation, such as the reverse transformation of martensite to austenite in some steels. Zhongshan Yixiong's view on the formation mechanism of sawtooth chips during hard turning of hardened steel is that chip formation originates from the maximum shear strain value on the free surface. The deformation adjacent to the free surface is assumed to be the result of pure shearing, and the angle between the shear fracture and the free surface is 45°. Sih obtained the "strain energy density" factor S by analytical method, and simulated the formation mechanism of sawtooth chips under plane strain conditions. A new model of sawtooth chip formation during hard cutting of hardened steel was proposed, and the load angle φ was given. The relationship between the angle of fracture θ0 and the fracture angle θ0.
Research by Professor Wang Minjie and Professor Hu Rongsheng of Dalian University of Technology shows that the sawtooth chip is mainly caused by the instability of thermoplastic shear caused by high speed cutting. Thermoplastic shear instability is a material destruction phenomenon that is widely present in many dynamic plastic deformation processes. The prerequisite is that the thermal softening effect caused by the local temperature rise of the deformed material is enough to offset the deformation strengthening effect of the material. The phenomenon of thermoplastic shear instability during metal cutting refers to the intense local shear concentration occurring in the first deformation zone, which results in asymmetrical serrated chips which are different from the extruded chips formed by ordinary metal materials at low speeds. The feature is that the serrations of the chips are separated by a highly deformed thermoplastic shear band. The test results of cutting GCrl5 bearing steel with cermet blade SNMG120412N-UG (grade ZKOI) show that when the cutting depth is 0.5~4mm, the feed rate is 0.07~0.43mm/r, and the cutting speed is ≥130～160m/min, it starts to produce. Thermoplastic shear is unstable.
Processed Surface Integrity of Hard Cutting The generation of cutting heat and conduction, high-speed friction and wear during cutting process can cause some damage to the machined surface. The key to replacing the grinding process with hard cutting is how to obtain the ideal surface roughness, shape accuracy and surface condition of the machined surface. Improving the machining accuracy of the hard cutting and the performance of the hard cutting workpiece is a long-term study. Question. The integrity of the machined surface of hard cutting mainly includes the following: surface morphology and hardness, surface roughness, dimensional accuracy, distribution of residual stress and generation of white layer.
Professor CRLiu of Purdue University in the United States published a paper on the mechanical state of the subsurface of the processed surface during the chip formation process in 1976. The main influences of the sharp edge cutter and the worn tool on the residual stress were analyzed. Recently, CRLiu also demonstrated the feasibility and cutting conditions of ultra-precision hard-cut hardened bearing steel. A lot of work has been done on the residual stress model, simulation and optimization of ultra-precision hard-cut surfaces. The research work of P Leskovar in Germany shows that the micro-hardness of the machined surface is greatly affected by the feed rate and the amount of flank wear. The smaller the feed, the larger the wear and the higher the surface hardness. Professor Liu Xianli's orthogonal hard cutting test results show that the effects of cutting speed, feed rate and depth of cut on surface hardness have a single change law. That is, the hardness of the machined surface increases as the cutting speed increases. Decreased as the feed rate and depth of cut increase. Moreover, the higher the hardness of the machined surface, the greater the depth of the hardened layer. Through the comparative analysis of the scanning electron micrographs of the matrix and surface structure of the test piece, it is considered that although the hardness of the machined surface during the hard cutting process is improved, a certain hardening depth is produced, but the metallographic structure of the surface layer is not damaged. .
Professor DKAspinwsll of the University of Birmingham used ceramic and PCBN tools to cut hardened AISI E521O0 bearing steel on a high-rigidity CNC lathe. The microstructure of the surface and subsurface of the workpiece changed. The microstructure was changed from white untempered layer and black. The tempering layer is composed. The experimental results show that the surface of the workpiece after hard cutting is residual compressive stress, and the maximum compressive stress of the workpiece after grinding is mainly concentrated on the surface of the workpiece.
The residual stress is the same as the composition, structure and defects of the material. It has a great influence on the mechanical properties of the workpiece. In most cases, the magnitude of the residual stress and its distribution law must be controlled. The generation of residual stress during hard cutting is considered to be closely related to the formation of cutting heat and the moving speed of the heat source, the geometry of the cutting edge, the workpiece material and the tool wear. Many foreign scholars have tried to calculate the residual stress by simulating the generation and movement of cutting heat, but the complexity of cutting heat formation and the measurement error of residual stress lead to large simulation errors. Recently, Kurt Jacobus of Canada used the theory of plane strain viscoelasticity and S.Mittal of Purdue University to predict the influence of cutting parameters on residual stress distribution by using polynomial fitting principle. The disadvantage is that a large number of calibration experiments are needed to estimate the coefficient. . JDThiele et al. studied the influence of cutting edge geometry and workpiece hardness on the residual stress on the workpiece surface during precision hard cutting. In the experiment, PCBN tools with sharp edge, chamfer and obtuse were selected. The test results showed: tool The larger the radius of the blunt circle, the larger the residual compressive stress value; the higher the hardness of the workpiece, the larger the residual compressive stress value. Y.Matsumoto and DWWu also believe that the hardness of the workpiece has a great influence on the surface integrity of the workpiece. The larger the hardness value of the workpiece, the more favorable the formation of residual compressive stress. Y. Matsumoto also believes that tool geometry also affects the formation of residual stresses. The residual compressive stresses created by double chamfering and large blunt tools are far superior to single chamfer and sharp edge cutters, but cutting parameters (depth and penetration) The amount) has no significant effect on the residual stress.
Another important factor affecting the quality of the machined surface of hard cutting is the formation of white layers. The white layer is a kind of microstructure formed by the hard cutting process. It has unique wear characteristics: high hardness and good corrosion resistance on the one hand, and high brittleness on the other hand, which is easy to cause early peeling failure. The white layer is thinner in size, and it is difficult to accurately analyze its tissue characteristics. Its formation mechanism is still controversial. One view is that the white layer is the result of phase transformation and consists of fine grained martensite formed by rapid heating and sudden cooling of the material during the cutting process. Another point of view is that the formation of the white layer is only a deformation mechanism, but an unconventional martensite obtained by plastic deformation. At present, the view that the white layer is regarded as a martensite structure is unanimously recognized, and the main controversy lies in the fine structure of the white layer. YKChou and cJEvans believe that the formation of white layer in the hard cutting process is related to the heat of cutting. The increase of the amount of flank wear will lead to an increase in the depth of the white layer. When the VB reaches 0.31 mm, the depth of the white layer is as high as lOμm. B. J. Griffiths believes that the self-layer phenomenon in the cutting process is caused by high-speed sliding wear. The microstructure of the white layer is a mixture of austenite and martensite of ultra-fine grain structure, and is closely related to tool wear. Therefore, it is necessary to further study the formation mechanism of the white layer and its influence on the life of the parts.
The development trend of hard cutting technology At present, the hard cutting technology has attracted great attention and great interest from the manufacturing industry and scientific research institutions all over the world. However, there are still some obstacles in promoting the application of hard cutting technology. The main problems are: how to make The processed surface maintains a stable surface roughness and dimensional accuracy; whether the processed surface quality can meet the working conditions of the part and has a certain life; how to select, use, and control the hardness of the cutting tool. Therefore, the research focus of future solid cutting mechanism and its technology is: control the cutting force during the cutting process and maintain its stability; eliminate and reduce the effect of cutting heat on the dimensional accuracy of the workpiece; cooling lubrication during the hard cutting process The rationalization of technology; the gradient of processed surface hardness, the distribution of residual stress, the morphology of the surface layer and the formation mechanism of white layer.
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