【作者】 邵芳；

【导师】 刘战强；

【作者基本信息】 山东大学 ， 机械制造及其自动化， 2010， 博士

【摘要】 高速切削时,尤其是高速切削钛合金及高温合金等难加工材料时,刀具和工件之间的相互作用导致工件局部产生剧烈的塑性变形,刀具工件间产生剧烈的摩擦,产生大量的切削热,导致刀具磨损过快,换刀频繁,刀具寿命短、消耗量大成为制约高速切削加工技术广泛应用的主要瓶颈之一。高速切削过程是一个局部产生高温、高压、高应变和高应变率的非线性动态随机过程。本文针对难加工材料钛合金和高温合金材料,从热力学的角度系统研究切削加工刀具的磨损机理。借助于非平衡态热力学理论,建立高速切削过程的热力学体系。分析该体系的的熵流和熵产生；对高速切削过程的热力学体系的各子过程之间的耦合进行研究,结果表明：虽然按照居里原理,不同张量阶的子过程之间不存在耦合,但通过体系结构和元素的变化仍可产生相互影响。对高速切削过程的热力学体系的刀具和工件之间的相互作用进行分析,分析结果表明扩散、粘结、氧化和塑性变形各因素之间的复杂非线性作用使切削热力学体系具有区别于一般热力学体系的显著特征：高速切削过程热力学体系各因素之间呈复杂非线性作用,而普通的热力学体系各因素之间满足居里原理,即具有不同热力学张量阶的因素间不存在耦合。采用DEFORM-3D有限元软件,建立了正交切削加工的仿真模型。采用热力学的本构方程描述材料的流动应力行为,并把刀具视为非刚体,采用单元自适应网格重划分技术与分离准则相结合实现切屑与工件的分离,选用无量纲Cockcroft Latham断裂准则实现材料的断裂,摩擦模型的建立通过划分粘结区和滑动区,分别应用不同的摩擦系数来描述。基于仿真模型预测热力耦合作用下工件、切屑中的非均匀应变场、应力场、温度场以及切削力。设计正交切削实验,验证了模型的正确性。基于扩散的热力学原理,研究硬质合金刀具材料的溶解扩散机制并进行计算,分析硬质合金刀具的扩散磨损特点。利用材料学的二元相图用反求法求解一种元素溶入另一种元素中的超额自由能,并利用此法求解碳元素在铝中的超额自由能。研究表明：WC基硬质合金刀具在高速切削铁基高温合金时抗扩散磨损的能力低于TiC、TaC和NbC含量较多的硬质合金刀具。对硬质合金刀具分别与高温合金GH907和钛合金TC4的扩散进行实验研究,实验结果与理论计算一致。以发生氧化反应的标准吉布斯自由能为判据,研究高速切削时刀具的氧化磨损。计算硬质合金刀具在不同温度下可能的氧化反应的标准吉布斯自由能,对反应发生的可能性进行排序,分析各个反应的特点,结果表明产物中WO3、CoWO4和Ti02含量较多,证明中间产物Co3O4会继续和WO及氧气发生剧烈的氧化反应,生成CoWO4,并进行实验验证,结论与实验结果一致。同时对PCBN刀具和氧化铝陶瓷刀具的氧化产物进行热力学计算,并分析这两种刀具的氧化磨损特点。建立刀具粘结磨损的热力学模型。用白光干涉仪、红外热像仪和SEM-EDS等技术手段研究刀具的粘结磨损,通过分析两种不同的工件材料形成的刀具磨损来研究刀具的磨损形态、磨损机理以及磨损过程规律,分析切削时间和切削速度对粘结磨损的影响。研究表明：高速车削铁基高温合金GH907和钛合金TC4时,刀具的粘结磨损均在切削速度v=200m／min时较为严重,在切削速度v=300m／min时,刀具和工件材料之间的粘结显著减轻,主要原因是材料热软化效应造成的。基于热力学理论核心原理—最小能耗原理,证明了一个假设三个约束条件下的切削过程热力学体系的最小熵产生,建立第一变形区剪切变形区和第二变形区刀屑摩擦区的熵产生模型,分析切削参数对熵产生的影响,并用实验验证模型的正确性。结果表明由于切削过程的复杂性和影响因素的多样性使得切削过程的流和力不满足线性关系,变形区的熵产生无最小值,呈发散状态。高速切削过程的热力学机制和理论探索有待于进一步深入研究。本课题得到国家重点基础研究发展计划(2009CB724401)、国家自然科学基金项目(50705052)、教育部博士点基金新教师项目资助(20070422032)和高档数控机床与基础制造装备科技重大专项(2009ZX04014-043、2009ZX04014-012、2009ZX04014-031)的资助。更多还原

【Abstract】 High speed cutting technology has been widely used in industry for its outstanding advantages. However, severe tool-work friction and plastic deformation result in huge heat generation and early tool failure. Short tool life in high speed cutting difficult-to-cut materials, such as titanium alloys and super alloys, is one of the most difficult problems for its further impliation. Frequent tool exchange decreases machining efficiency and cutting tool consumption increases cost. Most phenomena in cutting system relates with thermal or thermodynamic theory. In this research, tool wear in high speed cutting titanium alloy and super alloys have been studied based on thermodynamical theory.Thermodynamic system in high speed cutting process was developed based on non-equilibrium thermodynamic theory. Entropy generation and entropy flow were analyzed in this system. The coupling between the microscopic sub-processes of the thermodynamic system were studied. The results showed that mutual effects between different sub-progresses did exit through structure and element changes, while there is no coupling between those progresses with different tensor orders according to curie principle. The interaction between the chip tool and the workpiece were analyzed, the results showed that complicated nonlinear interaction is the remarkebale characteristic of high speed machining thermodynamic system which is different with the general thermodynamic systems whose various factors meet the Curie principle, that is different thermodynamic order tensor coupling does not exist between the factors.Orthogonal cutting simulation model was built developed with finite element analysis software DEFORM-3D. The material’s flow stress behavior was described with thermodynamical constitutive equation. Separation of chips from workpiece was realized by the combination of adaptive remeshing technique and separation criterion with tool as a non-rigid solid. The material’s fracture was defined by adopting Cockcroft and Coulomb friction model and shear friction model. To validate the finite element model, the orthogonal cutting tests were conducted. With this simulation model stresses, strains distribution in the chips and the workpiece can be predicted as well as cutting temperature and cutting force.Diffusion and solubility mechanisms of cemented carbide tools were studied and calculations were conducted based on thermodynamics principles. Reverse method was firstly used to calculate the excess free energy of one element dissolving into another element with binary phase diagram. For instance, with this method, the excess free energy of carbon dissolving into aluminum was calculated. Results showed that diffusion wear resistance ability of WC-based cemented carbide tool was poorer than those cemented carbide tool with more TiC、TaC and NbC when high speed cutting iron-based alloy. This analysis was proved with experiments during high speed cuting titanium alloy (Ti6A14V) and super alloy (GH907) with cemented carbide tools.The oxidation wear of cutting tool during high speed machining were studied with Gibbs free energy criterion. Oxidation products of carbide tool were calculated based on thermodynamics during high speed machining. The possible oxidation reactions for cemented carbide tool elements at different temperature were calculated with substances Gibbs free energy function method. And the orders of reaction was sorted with reaction degree as well as characteristics of every reaction were analyzed. Theoretical calculation showed that the quantities of reaction products of WO3、CO3O4 and COWO4 were large, especially,the middle reaction product Co3O4 will continue to react with WO and O2 to generate the product CoWO4. Experimental results proved the theoretical calculation. Furthermore, the oxidation products of PCBN and LT55 ceramic tools were also calculated and their wear characteristics were analyzed.The thermodynamical model of tool adhesion wear was developed. The adhesion wear was studied by turning experiments and investigated with white light interferometer, infrared imaging, and SEM-EDS and so on. Adhesion wear patterns,, wear mechanism were studied in cutting GH907 and TC4. Adhesive wear was analyzed in different velocity and cutting time. Results showed that the tool adhesion wear was more serious when cutting speed was 200m/min, while the adhesion wear was significantly reduced when cutting speed was 300m/mindue to thermal softening effect.The principle of minimum entropy production in cutting thermodynamics system was proved based on minimum energy consumption under the condition of one assumption and three constraints. The entropy generation model of shear zone and tool chip contact zone were deduced and the effects of cutting parameters on entropy generation were analyzed and proved by experiments. The results indicated that there was no minimum value of entropy generation because generalized flow and generalized force of cutting process did not satisfy the linear relationship as the complexity of cutting process and the diversity of the factors. So, the mechanisms and theoretical exploration of the machining friction thermodynamics system need to be further explored.This disseratation is supported by National Basic Research Program of China (2009CB724401)、Natural Science Foundation of China (50705052)、Doctorate Fund of the Ministry of Education project funding of new teachers(20070422032) and Major Science and Technology Program for High-End CNC Machine Tools and Basic Manufacturing Equipment(2009ZX04014-043、2009ZX04014-012 2009ZX04014-031).更多还原