【作者】 张高峰；

【导师】 张璧； 邓朝晖；

【作者基本信息】 湖南大学 ， 机械制造及其自动化， 2008， 博士

【摘要】 本文提出了一种新型结构的有序化微刃刀具,采用方条形金刚石纤维取代传统砂轮的磨粒与传统切削刀具的刀片,通过金刚石纤维在基体材料中的定向排布和对金刚石纤维的刃磨,保证有序化微刃刀具的每个金刚石纤维均能以一定的切削角度参与切削,并且每个金刚石纤维均具有锋利的刀刃。众所周知,传统砂轮是由磨粒、结合剂与气孔三部分组成。由于磨粒形状及其分布的随机性,造成砂轮磨削时存在法向力与切向力之比大、磨削比能高、磨削温度高、对机床刚度要求高等不足,由于砂轮同时参与切削的磨粒数量多,单个磨粒的切深很小,一般在亚微米级,对于超精密磨削,砂轮的单个磨粒切深甚至可达到纳米级,因而利用砂轮磨削能获得较高的表面质量与加工精度。传统的切削加工,刀具的切削部分因经过人为刃磨而具有锋利的刀刃,刀具的形状与切削角度均可人为控制,因而法向力与切向力之比小、切削比能低、对机床刚度要求也相对较低,然而,切削刀具同时参与切削的刀刃数量少,为了保证一定的加工效率,往往单个刀刃的切削深度较大,一般在几百微米至几毫米,从而导致切削力比较大(一般可达几百牛),因而加工质量相对要差一些。有序化微刃刀具的金刚石纤维其宽度在0.2-0.5mm之间,比传统的切削刀具小得多,同时参与切削的金刚石纤维数量可远大于普通切削刀具的刀刃数,并且各纤维同样具有锋利的刀刃与确定的切削角度,因而可获得较高的加工精度与保证较好的加工效率。有序化微刃刀具是一种集砂轮与切削刀具优点于一身的新型刀具,开展有序化微刃刀具的设计及其基础研究具有很好的应用价值与理论价值。本课题首先对传统的磨削加工与切削加工进行了比较,对目前国内外砂轮结构创新现状以及切削刀具的研究现状进行了全面综述,在此基础上提出了有序化微刃具的概念与构想。金刚石纤维的制备是开发有序化微刃刀具最关键的一步,由于金刚石具有极高的硬度与耐磨性,并且在大气压下当温度升高至1000K以上时金刚石会出现石墨化,因此,普通的纤维制备方法很难适于金刚石纤维的制备。本课题提出了减薄聚晶金刚石复合片(Polycrystalline Diamond Compacts,简称PDC)与高能切割技术相结合以制备金刚石纤维的方案。开展了PDC的电火线花切割加工与Nd:YAG激光切割加工的实验研究,采用扫描电镜(Scanning Electric Microscopy,简称SEM)与拉曼光谱仪对切割试件进行了显微观察与分析,发现纳秒级脉宽的KTP/Nd:YAG激光所切割的试件热损伤小,并且切割速度比较快,因而采用纳秒级脉宽的KTP/Nd:YAG激光切割PDC的方法制备金刚石纤维,金刚石纤维的截面尺寸为0.3mm宽×0.6mm厚×10mm长。为了保证有序化微刃刀具在切削加工过程中其金刚石纤维具有一定的切削角度,采用了自制模具对金刚石纤维进行定向、均匀、有序排布。为了保证每个金刚石纤维具有锋利的刀刃,开展了金刚石纤维的固结磨粒刃磨与游离磨粒刃磨实验研究,通过SEM的显微观察发现,固结磨粒刃磨很容易造成刃口的崩刃,采用游离磨粒刃磨则可以获得较好的刃口质量,最终确定以游离磨粒刃磨的方法实现金刚石纤维的刃磨加工。氧化铝、碳化硅等陶瓷颗粒增强铝合金因具有材质轻、耐磨性好、比强度高等优点而在汽车与航空领域得到广泛地应用,然而,硬质颗粒的加入也给其加工带来了极大的困难。金刚石刀具是目前公认的加工陶瓷颗粒增强铝合金的最有效工具,由于金刚石刀具无法制备成复杂的形状,严重影响了陶瓷颗粒增强铝合金更为广泛地应用。本课题以碳化硅增强高硅铝合金为试件材料,开展了有序化微刃刀具对塑性金属材料的精密切削实验研究,单个纤维切削深度为1-10μm,介于传统的切削加工与磨削加工之间。经SEM与表面轮廓仪对试件表面的检测与分析发现,有序化微刃刀具在加工碳化硅增强高硅铝合金时获得了较高的加工质量,当单纤维切深为4μm、进给速度为10mm/sec时,表面粗糙度达到了Ra0.08。并开展了与数控铣削加工的对比性实验研究,在相同的材料去除率下,有序化微刃刀具的加工表面质量明显好于数控铣削加工,其加工表面没有微裂纹,而数控铣削加工在较大切削效率时表面存在明显的微裂纹。对于普通的工程陶瓷材料,如碳化钨/钴、氧化铝,材料硬度越高则其断裂韧性越低,然而,当材料晶粒减小至纳米级时,如纳米碳化钨/钴,材料硬度提高时断裂韧性降低较少。纳米碳钨/钴涂层作为一种新型的耐磨涂层材料其优越的物理力学性能已获得广泛的认同,在机械制造、工具、国防、航空航天、地质勘探领域获得了广泛地应用。目前,纳米碳化钨/钴涂层主要是采用金刚石砂轮进行磨削加工。本文采用有序化微刃刀具开展了纳米碳化钨钴涂层材料的精密切削加工试验研究,并开展了与金刚石砂轮磨削加工的对比分析,经SEM显微观察分析发现,采用0°前角的有序化微刃刀具进行切削加工,最大未变形切削层厚度存在临界值,当最大未变形切削层厚度小于临界值时,其加工表面质量明显好于SD600N100V金刚石砂轮磨削表面,只要控制单个微刃的最大未变形切削层厚度,既可获得脆性材料的无损伤加工,因此,通过增加参与切削的微刃数量,采用0°前角的有序化微刃刀具可以实现脆性材料的高效精密加工;当切削前角为-30°时,法向力与切向力之比较大,并且当切削深度与进给速度较大时,加工表面存在碎粒状形貌;更多还原

【Abstract】 This study is aimed at developing a new engineered cutter which combines the advantages of grinding wheels and cutting tools through replacing the abrasives in a grinding wheel or the inserts in a cutting tool with superabrasive fibers. Similar to a cutting tool with inserts, the engineered cutter has superabrasive fibers that are artificially ordered so as to have the desired geometric angles and edge space among themselves. Since superabrasive fibers are normally preferred for the engineered cutter, diamond fibers have been chosen in this study. For the purpose of achieving the desired geometric angles and sharp cutting edges, diamond fibers are first artificially aligned along the desired direction and distributed in a mold and fixed with epoxy resin. The spacially ordered diamond fibers are then lapped with diamond abrasives to get sharp cutting edges and the necessary clearance angles.It is well known that a grinding wheel is composed of abrasives, bond and pores, and due to the random nature of the abrasive geometries and their distribution in a grinding wheel, grinding is characterized by good surface finish and quality, good dimensional accuracies, but a high ratio of normal to tangential grinding forces, high specific energy, high temperature rise, and a high demand in grinding machine stiffness. The good surface finish and quality in grinding is largely due to the small depth of cut in individual abrasive grits which is usually in the submicron or even nanometer range. On the other hand, due to its favorable geometric parameters, such as positive rake angle, a milling cutter usually results in a low normal to tangential force ratio, low specific energy, and high material removal efficiency as compared to grinding. However, machined workpiece surface finish and quality in milling is inferior to that in grinding.Machining with an engineered cutter is expected to provide both good surface finish and high machining efficiency at the same time. The development of the engineered cutter, which combines the advantages of both grinding and cutting, is considered a technological breakthrough in cutting tools. The engineered cutter is therefore of technological importance. In this study, the current state-of-the-art in the grinding wheel structures is discussed. Presented in this study are also the recent advances in cutting tools through analyzing and comparing grinding against cutting processes. The concept of an enineered cutter is proposed and verified.The fabrication of diamond fibers is a key step towards the successful preparation of an engineered cutter. This is due to the fact that it is difficult to obtain diamond fibers through the conventional processes of fiber preparation because of the graphization tendency of diamond in a temperature range of 700-800℃or above. In order to prepare diamond fibers, this study deals with the cutting mechanisms of Polycrystalline Diamond Compacts (PDC) with an Nd:YAG laser and Wire Electric Discharge Machining (WEDM). Based on the observations with Scanning Electron Microscopy (SEM) and the analysis with Raman spectra, it is discovered that the samples cut by the Nd:YAG laser with pulse width in nanoseconds have better surface quality, less thermal damage, moreover, and a higher efficiency. Therefore, the diamond fibers with cross-sectional dimensions of 0.3×0.6×10 mm are prepared by the Nd:YAG laser cutting of PDC.The artificially oriented diamond fibers are fabricated into blocks of 3-5mm thickness. Each block contains spacially distributed fibers in epoxy resin. It is very important for a diamond fiber cutter to have sharp cutting edges so as to improve its cutting efficiency. In this regard, sharpening of diamond fibers is carried out in the preparation of diamond fiber cutter. In this study, the diamond fibers have been ground by fixed diamond abrasives and polished by free diamond abrasives, respectively, and then investigated through SEM observations. It is found that the quality of polished PDC cutter edges is better with the free diamond abrasives than that with the fixed diamond abrasives. Therefore, polishing with the free diamond abrasives is performed throughout the study.Aluminum metals reinforced by alumina or silicon carbide particles are now widely used in automotive and aerospace industries because of their enhanced mechanical properties, low density and high thermal conductivity. Nevertheless, the addition of hard particles poses great difficulty for the machining, because the hard particles, which intermittently come into contact with the cutting edges, act as abrasives against the cutting edges, which causes rapid tool wear and poor surface finish. So far, as to the machining of aluminum alloy reinforced by hard particles, a diamond cutter is recognized as the most appropriate cutting tools due to its hardness and wear resistance. However, it is difficult to fabricate a complicated diamond tool due to the extreme properties of diamond, which prevents the application of aluminum alloy in industry. To investigate the material removal mechanisms with the diamond fiber cutter, aluminum-silicon alloy is chosen as the sample material. Depth of cut is set in the range of 1-10μm for the diamond fiber cutter, which is smaller than that used in the conventional machining and larger than in grinding. Based on the SEM obervations and inspections with a surface profilometer, it is discovered that the diamond fiber cutter results in a surface roughness of Ra0.08μm on the aluminum-silicon alloy workpiece. Moreover, in a comparison study, the diamond fiber cutter is used to compare with conventioal milling cutters. The results are promising. At the similar material removal rates, the diamond fiber cutter provided obviously better surface quality than the conventioal milling cutters. The former generated smooth workpiece surface without observable machine damage such as microcracks, while the latter showed many microcracks which are perpendicular to the direction of cutting at increased depths of cut.For common engineering ceramics, such as tungsten carbide/coabalt and alumina, an increase in hardness is often accompanied by a decrease in toughness. However, there is no decrease in toughness for a nanostructured material with high hardness. Nanostructured WC/Co coatings are finding a wide application in many industries, such as manufacturing, machine tools, cutting tools, military and areospace. Because of their enhanced hardness and toughness, nanostructured materials are difficult to machine. Grinding with a diamond wheel is widely used in machining these materials. A comparative investigation is carried out in this study on the machining mechanisms of the nanostructured WC/Co coatings by the diamond fiber cutter and by diamond wheels, respectively. The study verifies that the diamond fiber cutter with zero rake angle results in better surface finish than the 600# diamond wheel. On the other hand, the diamond fiber cutter with -30°rake angle results in large ratio of normal to tangential forces. The resulting machined workpiece showed a surface with granular surface roughness and also obvious microcracks at an increased depth of cut.更多还原