高速切削材料变形及断裂行为对切屑形成的影响机理研究

发布时间：2018-06-27 21:02

本文选题：高速切削 + 锯齿状切屑　；参考：《山东大学》2016年博士论文

【摘要】：随着高速切削机床和先进刀具的迅速发展,高速切削加工技术已在汽车制造、航空航天和国防工业等领域开始获得应用。相比于普通切削速度加工,高速切削条件下的高应变率特性导致工件材料动态力学性能发生剧烈变化,进而导致切屑形成机理和切屑形态的转变。随切削速度提高时,塑性金属材料的切屑形态演化规律为带状切屑、连续型锯齿状切屑、完全分离的锯齿单元分节切屑,并最终形成类似于脆性材料切削时的碎断切屑。以往研究主要针对高速切削切屑锯齿化临界条件以及锯齿状切屑的几何表征等问题,关于材料动态力学性能对锯齿状切屑和碎断切屑形成时材料变形及断裂行为的控制机理研究较少,目前尚缺乏深入的理解和认识。金属切削可认为是切屑与工件材料之间产生目的性断裂的过程,揭示锯齿状切屑和碎断切屑形成时的材料变形和失效机理,不仅能够指导优化切削工艺参数,有助于实现高效率低能耗加工,而且可以为高速机床设计、刀具设计等提供理论基础。本文以Ti6Al4V、Inconel 718和7050-T7451等三种工件材料为研究对象,通过材料力学和切削理论分析、有限元仿真、切削实验及显微观测等手段对锯齿状切屑和碎断切屑的形成机理进行研究,重点分析在高切削速度下工件材料的动态力学性能变化——特别是塑性金属材料的塑脆性能转变——对切屑变形和失效行为的影响规律。主要研究内容包括：不同形态切屑的形成过程;碎断切屑形成的临界切削条件；材料性能对锯齿状切屑剪切局部化的敏感性分析；应力状态对锯齿状切屑断裂行为的控制机理；以及高速切削切屑形成过程的能量耗散特性分析等。通过该文研究,以期在高应变率下材料动态力学性能与高速切削工艺之间建立起研究桥梁,为高速切削机理的揭示和高速加工技术的推广奠定理论基础。首先,针对高速切削切屑形成过程进行研究,分析切削速度提高时工件材料的切屑形态演化规律,揭示不同切削速度下切屑的变形和断裂机理,建立锯齿状切屑和碎断切屑的形成模型。对获得切屑的不同部位(包括切屑横截面和自由表面、切屑断口等)进行显微观察,探索三种工件材料的切屑变形和失效机理,根据锯齿状切屑形成特点提出绝热剪切-韧性断裂复合型切屑形成模型,而碎断切屑的形成是由脆性断裂所致。针对金属材料在高应变加载下的塑脆转变机制,应用应力波传播理论,建立超高切削速度下碎断切屑形成的临界判据,获得碎断切屑形成的临界切削条件。根据切屑形态特点及其变形机理,将切削范围划分为普通速度切削、高速切削和超高速切削。然后,建立高速直角切削锯齿状切屑形成的有限元仿真模型,探索材料性能对锯齿状切屑剪切局部化影响的敏感性,揭示工件材料内廪变量(包括材料力学性能和损伤特性参数)对高速切削切屑形态的影响规律和控制机理。仿真获得不同切削速度下切屑形态的演化规律,以切屑锯齿化程度和锯齿化频率等几何参数为指标,利用高速直角切削实验对有限元仿真模型的有效性进行验证。通过调控工件材料的本构模型参数和损伤模型参数,研究不同材料性能参数变化时切屑形态的变化特性,并提出切屑锯齿化敏感性参数和切屑曲率变化敏感性参数两个评价指标,定量表征材料性能对切屑剪切局部化的影响规律。研究结果表明,本构模型参数中初始屈服强度与热软化系数对切屑剪切局部化的影响最大,损伤模型参数中初始失效应变和指数因子对切屑剪切局部化的影响最大。其次,建立高速切削锯齿状切屑形成时第一变形区的法向应力分布模型,揭示切削第一变形区的应力三轴度分布规律。将锯齿状切屑第一变形区的材料变形抽象为常剪切梯度拉伸／压缩复合加载下的材料变形与失效问题,建立综合考虑应变率和温度影响的Bao-Wierzbicki断裂应变修正模型,通过对比分析切屑变形时的等效塑性应变与材料断裂应变,获得锯齿状切屑的断裂轨迹,并讨论不同切削速度下锯齿状切屑断裂轨迹的演化规律。研究结果表明,第一变形区内法向应力呈不均匀分布,其中靠近切屑自由表面处为拉伸应力区,应力三轴度为正值且服从线性分布：而靠近刀尖区域为压缩应力区,应力三轴度为负值且服从幂函数分布。在靠近切屑自由表面处为剪切、拉伸复合加载,切屑断裂面呈现拉伸应力引起的韧性断裂模式；在靠近刀尖处为压缩、剪切复合加载,切屑发生剪切断裂并在断裂面处分布有剪切型韧窝。切削速度提高时拉伸应力区的扩大是导致锯齿状切屑内绝热剪切带裂纹扩展加剧和切屑锯齿化程度提高的本质控制因素。最后,针对高速切削形成的锯齿状切屑与超高速切削形成的碎断切屑,建立不同形态切屑形成时的能量耗散模型,探索不同切削参数下由于切屑变形行为差异引起的能量耗散变化规律,并综合利用切削力和声发射信号验证不同形态切屑的能量耗散模型。研究结果表明,锯齿状切屑形成时能量耗散主要包括第一变形区的塑性变形能、切屑与刀具前刀面之间的摩擦能和切屑动能；而碎断切屑形成时的能量耗散主要包括切屑的断裂能和局部动能。在高速切削阶段,选择大前角刀具和较大的未变形切屑厚度有利于减小切削过程的能量耗散。碎断切屑的形成实现了塑性材料的脆性域加工,使得切削能量耗散大幅降低,表明超高速加工具有高效率和低能耗的优点。切削过程中的声发射信号强度受切削能量的耗散所影响,而切削能量的耗散由工件材料的力学性能和加工参数共同决定。
[Abstract]:With the rapid development of high speed cutting machine tools and advanced cutting tools, high speed machining technology has been applied in the fields of automobile manufacturing, aerospace and national defense industry. Compared to ordinary cutting speed machining, high strain rate characteristics under high speed cutting lead to dramatic changes in the dynamic mechanical properties of the workpiece material, and then lead to cutting. When the cutting speed increases, the chip shape evolution of the plastic metal material is banded chip, continuous sawtooth chip, completely separated sawtooth chip, and eventually forming broken chips similar to brittle material cutting. The previous research was mainly aimed at high-speed cutting sawsaw. The critical conditions for the dentation and the geometric characterization of the sawtooth chips are not studied. There is little understanding and understanding of the control mechanism of the material deformation and fracture behavior of the material when the sawtooth and broken chips are formed. Metal cutting can be recognized as the purpose between the chip and the workpiece material. The process of fracture reveals the deformation and failure mechanism of the sawtooth and broken chips, which not only guides the optimization of the cutting process parameters, but also helps to achieve high efficiency and low energy consumption, and provides a theoretical basis for the design of high speed machine tools and tool design. This paper is based on three kinds of work pieces, such as Ti6Al4V, Inconel 718 and 7050-T7451. As the research object, the formation mechanism of sawtooth chip and broken chip is studied by means of material mechanics and cutting theory, finite element simulation, cutting experiment and microscopic observation. The change of dynamic mechanical properties of the workpiece material at high cutting speed, especially the plastic brittle properties of the plastic metal material, is emphatically analyzed. Change - the law of influence on chip deformation and failure behavior. The main contents include the formation of different forms of chip, the critical cutting conditions of broken chip formation, sensitivity analysis of the shear localization of the sawtooth chip, the control mechanism of the serrated chip fracture behavior of the stress state, and the high speed cutting. The energy dissipation characteristics of chipper forming process are analyzed. Through this study, a study bridge is established between the dynamic mechanical properties of the material and the high speed cutting process at high strain rate, which lays a theoretical foundation for the discovery of the mechanism of high speed cutting and the popularization of high speed machining technology. First, the research on the formation process of high speed cutting chips is carried out. After analyzing the evolution law of chip shape of the workpiece material when cutting speed is raised, the deformation and fracture mechanism of chip in different cutting speed are revealed, and the formation model of sawtooth and broken chip is established. The microscopic observation on the different parts of the chip (including the chip cross section, free surface surface, chip fracture, etc.) is observed, and three kinds of work are explored. According to the formation characteristics of sawtooth chip, the adiabatic shear ductile fracture complex type of chip formation model is put forward, and the formation of broken chip is caused by brittle fracture. In view of the mechanism of brittle transition of metal material under high strain loading, the stress wave propagation theory is applied to establish the ultra high cutting speed. Critical cutting conditions for the formation of broken chips are obtained, and the critical cutting conditions for the formation of broken chips are obtained. According to the characteristics and deformation mechanism of the chip, the cutting range is divided into ordinary speed cutting, high speed cutting and ultra high speed cutting. Then, a finite element simulation model of high speed right angle cutting sawtooth shape is established to explore the material properties to the saw. The sensitivity of the shear localization of the toothed chip, which reveals the influence law and the control mechanism of the grain storage variables (including material mechanical properties and damage parameters) on the cutting chip shape in high speed cutting. The evolution of chip morphology under different cutting speeds is obtained, and the geometric parameters, such as the degree of sawtooth serration and the serrated frequency, are obtained. For the index, the validity of the finite element simulation model is verified by the high speed right angle cutting experiment. By adjusting the parameters of the constitutive model and the damage model of the workpiece material, the change characteristics of the chip shape when the performance parameters of different materials are changed, and the sensitivity parameters of the chip sawtooth sensitivity and the change of the chip curvature are proposed. The results show that the initial yield strength and the thermal softening coefficient have the greatest influence on the chip shear localization, and the initial failure strain and the exponential factor have the greatest influence on the chip shear localization in the parameters of the damage model. Secondly, the results show that the influence of initial failure strain and index factor on the chip shear localization is the largest in the parameters of the constitutive model. Secondly, the effect of the initial failure strain and index factor on the chip shear localization is the most. Secondly, the influence of the initial failure strain and the index factor on the chip shear localization is the most. Secondly, the effect of the initial failure model parameters on the chip shear localization is the most. To establish the normal stress distribution model of the first deformation zone in the formation of the high speed cutting sawtooth chip, the distribution of the stress three axis in the first deformation zone is revealed. The material deformation of the first deformation zone of the sawtooth chip is abstracted into the deformation and failure of the material under the constant shear gradient tensile / compression composite loading, and the comprehensive consideration should be established. The Bao-Wierzbicki fracture strain correction model affected by the variable rate and the temperature is obtained by comparing and analyzing the equivalent plastic strain and the fracture strain of the material in the chip deformation. The fracture trajectory of the sawtooth chip is obtained, and the evolution law of the serrated chip fracture trajectory under different cutting speeds is discussed. The results show that the normal stress in the first deformation zone is in the first deformation zone. There is an uneven distribution, in which the free surface near the chip is a tensile stress zone, and the stress three axis is positive and obeys the linear distribution: while the area near the tip area is the compressive stress area, the stress three axis is negative and obeys the power function distribution. The shear, tensile composite loading near the free surface of the chip and the tensile stress of the chip fracture surface are presented. The ductile fracture mode caused by force is compressed at the tip of the knife, shear composite loading, shear fracture of the chip and the shear type dimple at the fracture surface. The expansion of the tensile stress zone is the essential control of the expansion of the crack expansion in the sawtooth chip and the increase of the serration degree of the chip when the cutting speed is raised. Finally, in view of the broken chips formed by the sawtooth chip and the ultra high speed cutting formed by high speed cutting, the energy dissipation model of different forms of chip formation is established, and the energy dissipation changes caused by the difference of chip deformation behavior under different cutting parameters are explored, and the different shapes of the cutting force and acoustic emission signal are used to verify the different shapes. The energy dissipation model of the state chip shows that the energy dissipation mainly includes the plastic deformation energy in the first deformation zone, the friction energy and the chip kinetic energy between the chip and the tool front surface, while the energy dissipation of the broken chip formation mainly includes the chip fracture energy and the local kinetic energy. In the high speed cutting stage, The selection of the large front angle tool and the larger undeformed chip thickness is beneficial to reduce the energy dissipation in the cutting process. The formation of broken chip has realized the brittle domain processing of the plastic material, making the energy dissipation of the cutting greatly reduced, indicating the high efficiency and low energy consumption. The dissipation of cutting energy is affected, and the dissipation of cutting energy is determined by the mechanical properties and machining parameters of workpiece materials.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG506.1

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