【作者】 王宇；

【导师】 刘献礼；

【作者基本信息】 哈尔滨理工大学 ， 机械制造及其自动化， 2008， 博士

【摘要】 高速切削淬硬钢作为新工艺目前被航空航天、汽车等许多工业部门所采用。高速切削淬硬钢作为单点车削方式,其优势之一就是根据机床固有的加工性能对复杂工件型面进行精加工。另外,高速切削淬硬钢比磨削加工效率高并减少对环境的污染,因此,可以代替磨削加工。但是,有些不利的因素并制约高速切削淬硬钢的应用,如刀具材料选用不恰当或加工参数选用不合理,会导致高速切削淬硬钢产生高温和很大的切削力,从而降低刀具寿命和加工表面质量。本文采用PCBN(Polycrystalline Cubic Boron Nitride)刀具对GCr15淬硬钢(HRC62~64)进行一系列高速精密硬态切削实验,并结合有限元仿真,研究高速精密硬态切削机理、刀具磨损和已加工表面质量的变化规律。首先,根据弹塑性变形理论和热力耦合有限元方法,利用大型商用有限元分析软件Deform,建立了淬硬钢高速精密硬态切削过程的二维和三维仿真模型,并设定工件材料GCr15为均质、等向强化的弹塑性材料,满足Von Mises屈服准则,在修正了库仑定律的刀/屑摩擦方程和局部网格重划分准则的基础上,采用Johnson-Cook材料模型和Cockroft-Latham断裂准则,分别研究了二维切削过程中的切屑形态和刀具磨损以及三维切削过程中切削力和切削温度的变化规律。针对高速精密硬态切削淬硬钢时,由于受金属软化效应和刃口参数的影响,尤其是切削厚度和倒棱宽度在同一数量级时,刃口的滞留层对切削过程影响很大,使切削力的变化存在特殊规律。根据能量法预报剪切角,采用滑移线场计算刃口耕犁力,对改进后Oxley模型进行了修正,建立了倒棱刀具的切削力模型;通过遗传算法建立了PCBN刀具刃口倒棱参数和切削用量对切削力影响的函数关系式,并优选了切削参数。由于PCBN刀具高速切削时刀具磨损与普通切削有很大区别,本文通过高速精密硬态切削淬硬钢GCr15试验,研究了涂层和无涂层PCBN刀具的不同刃口结构和不同切削速度对刀具磨损的影响,通过扫描电镜观察和能谱分析,系统研究了PCBN刀具的磨损机理,查明了高速精密硬态切削时PCBN刀具的磨损机理,得到了相应的PCBN磨损强度分布。最后,系统进行了高速精密硬态切削淬硬钢已加工表面完整性的研究。通过高速精密硬态切削淬硬钢GCr15实验,研究了PCBN刀具涂层、刀具几何结构和切削参数对表面粗糙度、白层和显微硬度的影响规律;研制了适合于高速精密硬态切削的新型PCBN wiper车刀,加工出了表面粗糙度为0.19μm的加工精度。更多还原

【Abstract】 The high-speed machining of hardened steel components with Polycrystalline Cubic Boron Nitride (PCBN) inserts has become a very popular machining process in the automotive industry and the aerospace industry in recent years. A significant benefit of the high-speed hard turning as a single point cutting is the capability to produce complex forms of workpiece with the inherent motion capability of modern machine tools. Moreover, because of its economic and environmental advantages in many machining tasks, fine hard turning represents an attractive alternative to grinding. However, there is still much reluctance in adopting hard turning as a finishing process because the high temperature and high levels of specific forces occurr in hard machining operations, leading to very short tool life and poor surface quality if a poor tool material or improper cutting conditions is chosen. In this thesis, the cutting mechanics of high-speed precision hardened steel cutting, tool wear and machined surface quality are studied with experiments of high speed precision hardened steel GCr15 (the hardness of HRC 62~65) with PCBN tools, and combining with finite element method (FEM).Firstly, comparing with analytical model, a two-dimensional(2D) and three-dimensional finite element model with general finite element analysis software Deform 2D&3D for high speed precision hardened steel cutting is developed by considering the chip-tool friction, the workpiece material deformed under high strain, high stain rate and high temperature using the Johnson-cook (JC) equation, local remeshing criterion combining with the Cockroft and Latham criterion used for the material damage. The 2D finite element model can predict the thermo-mechanical variables of machining process, such as saw-tooth chip and tool wear in high speed hard cutting. The 3D finite element model can predict cutting forces and cutting temperature in high speed hard cutting. The feature of cutting forces in high-speed precision hardened steel cutting is differented from conventional cutting because of the materials soften phenomena, especially in precision hard cutting the depth of cut being the same order as the width of chamfered tool, leading to the Dead Metal Zone near chamfer. By applying the minimum energy principle to predict shear angle and ploughing forces studied with a slip line field model, a theoretical model for cutting forces with chamfered cutting tools, is developed to modify the parallel shear zone of Oxley’s orthogonal cutting model. A mathematical model based on genetic algorithms is developed to study the effect of the geometries of chamfered cutting tools and cutting parameters on cutting forces and to optimize cutting parameters.PCBN tool wear mechanism in high speed precision hardened steel cutting is different from that in conventional cutting. By applying a scanning electron microscope (SEM) analysis and energy spectrum analysis, the effects of the geometries of chamfered coated cutting tools, coated cutting tools, and cutting speeds on PCBN tool wear are experimentally investigated to analyze PCBN tool wear mechanisms and to identify different tool wear mechanisms under high speed precision hard cutting.Finally, the surface integrity of GCr15 under high-speed hard precision cutting is systematically investigated. The effect of the geometries of chamfered cutting tools and cutting parameters on surface roughness, white layer and microhardness, is experimentally studied with coated PCBN tools and uncoated PCBN tools to design appropriate PCBN tools ( so called“wiper tools”) for producing very good surface quality in high-speed precision hard cutting. By applying a scanning electron microscope analysis and energy spectrum analysis, the mechanism of the formation of white layer is studied.更多还原