【作者】 王丹；

【导师】 赵万生；

【作者基本信息】 上海交通大学 ， 机械制造及其自动化， 2011， 博士

【摘要】 硬脆非金属材料的微细加工一直是机械加工中的难点。微晶云母陶瓷(Microcrystalline-mica-ceramic,简称MCMC)和聚晶金刚石( Polycrystalline Diamond,简称PCD)是比较典型并有广泛应用前景的硬脆非金属材料。以微晶云母陶瓷为代表的陶瓷材料具有优异的绝缘性、耐高温和低热导率等特性,其热导率不到硅材料的十分之一,如用作微小推力器燃烧室和喷管材料,可极大地降低推力器系统的热损失。以聚晶金刚石为代表的超硬材料具有均匀的高硬度、高耐磨性和强抗腐蚀性等优良特性,非常适用于微孔零件,在航空航天、汽车和电子行业中有广泛应用前景。本文采用微细超声加工(Micro Ultrasonic Machining,简称micro-USM)和微细电火花加工(Micro Electrical Discharge Machining,简称micro-EDM)技术,分别针对微晶云母陶瓷和聚晶金刚石的微细加工展开研究。采用块电极电火花磨削(Block Electrode Discharge Grinding,简称BEDG)和刃口电极电火花磨削(Edge Electrode Discharge Grinding,简称EEDG)方法对微细电火花加工和微细超声加工工具的在线制备技术进行研究。在BEDG研究中,提出采用带有斜面的不规则块电极进行反拷式加工的方法,并将此方法作为具有锥形轮廓的回转体粗加工工艺,以提高特殊形貌微细回转体加工速度。在EEDG研究中,采用数控加工技术规划工具进给轨迹,结合工具电极损耗补偿方法,灵活精确地制备出具有不同三维轮廓的回转体。以微晶云母陶瓷材料为例,解决微型拉瓦尔喷管加工中的关键技术问题。采用BEDG和EEDG相结合的方法,快速精确地在线制备出微细超声加工工具。采用正交实验方法研究加工工艺参数对微细超声加工中工具体积损耗率的影响,并优化加工工艺参数。通过研究超声加工工作液中磨粒的流动特点,提出在微型拉瓦尔喷管加工过程中优先加工预通孔的必要性,并在微晶云母陶瓷材料上加工出喉部直径为170μm的微型拉法瓦喷管。在分析了PCD材料的物理特性对其微细电火花加工性能影响的基础上,进行了PCD材料微细电火花加工的工艺实验研究。实验结果表明由于工具上吸附的附着物的保护作用,采用工件接负极的加工更适用于PCD材料的微细电火花加工。在工件接负极的加工中,只有当工具上吸附有适量的附着物时,附着物的保护作用比较明显,即可有效地提高材料去除率( Material Removal Rate,简称MRR)又可降低工具电极相对损耗率(Relative Tool Wear Ratio,简称TWR)。相反,如果工具电极上吸附有过量的附着物,则会使加工陷入低MRR和高TWR的恶性循环,并会加工出孔径过大的微孔。根据实验结果找出较优的加工参数范围,并采用一组优化的加工参数,成功地加工出直径为145μm且深650μm的微细通孔。在PCD材料微细电火花加工工艺研究的基础上,通过在工件上添加超声振动,可进一步提高PCD材料微细电火花加工性能。对工作液空化泡闭合过程以及工作液质点运动过程进行了力学计算。在不同加工参数下进行了PCD材料超声辅助电火花微孔加工实验。实验研究表明超声振动作用改善了加工过程中的放电状态,缩短了拉弧持续时间,提高了放电频率;超声振动会减少工具电极上吸附的附着物,对加工产生正、负两方面影响,因此在PCD超声振动辅助电火花加工中选择合适的超声振幅(2μm) ,可以使适用于PCD材料的微细电火花加工参数范围扩大,并提高加工性能和加工表面质量。更多还原

【Abstract】 Micro machining of hard and brittle non-metallic material has been a challenging issue in manufacturing areas. Microcrystalline-mica-ceramics (MCMC) and polycrystalline diamond (PCD) are two typical hard and brittle non-metallic materials and have broad application prospects. As a kind of ceramics, MCMC provides excellent insulation, high temperature resistance, low thermal conductivity and other good properties. The thermal conductivity of MCMC is less than one-tenth of that of silicon. The heat loss in a thrust system will be greatly reduced, if MCMC is used as combustion chamber and nozzle materials in a micro-thruster. As a superhard material, polycrystalline diamond (PCD), is of uniform high hardness, superior wear and corrosion resistance and many other excellent properties, and it is an ideal material for micro-hole parts and has wide potential applications in aerospace, automotive and electronics industries. The micromachining performance of MCMC and PCD were studied in this dissertation by using micro ultrasonic machining (micro-USM) and micro electrical discharge machining (micro-EDM), respectively.For generating the tools for micro-EDM and micro-USM, block electrode discharge grinding (BEDG) and edge electrode discharge grinding (EEDG) methods were investigated. In the study of BEDG, an irregular shaped block with a slope was used as a sacrificial electrode that copied its shape onto the workpiece. This method was used in rough machining of conical electrode to improve the machining efficiency. The flexibility of EEDG process was investigated using different tool paths designed for CNC. Different methods of tool electrode wear compensation for fabricating micro electrodes were carried out for forming various electrode shapes.MCMC was chosen as a case study to resolve the major problems of micro Laval-nozzle machining techniques. Micro-USM tools were prepared online quickly and accurately by a method combined with BEDG and EEDG. The experiments designed by orthogonal-experiment method were adopted to investigate the influences of machining conditions on tool wear ratio in the micro-USM of MCMC. Base on the experiments, a group of optimized micro-USM parameters were chosen. The behavior of abrasives in micro-USM slurry flow was analyzed. The necessity of pre-drilled hole was emphasized in the machining of micro Laval-nozzle. Furthermore, a micro Laval-nozzle was made on a MCMC piece with a throat diameter of 170μm.Base on the analysis of the relationship between physical properties of a PCD and micro-EDM performance, a series of experiments were carried out to investigate the micro-hole micro-EDM machining performance of PCD. Experimental results indicate that positive polarity machining, in which the workpiece is connected to the cathode, is suitable for micro-EDM of PCD because of the protection brought over by the adhesion on the electrode. In positive polarity machining, an appropriate volume of material adhesion onto the tool electrode can help to increase the material removal rate (MRR) and reduce the relative tool wear ratio (TWR). In contrast, an excessive volume of material adhesion can lead the machining into a vicious circle in which MRR is low, TWR is high and micro-holes are drilled with oversized diameters. An optimal machining conditions were chosen, and a through-hole with a diameter of 145μm and a depth of 650μm on PCD machined under the chosen optimal machining conditions shows satisfactory machining results.On the basis of the investigation into the micro-hole machining performance of PCD by micro-EDM, an approach of micro-EDM combined with ultrasonic vibration act upon workpiece was proposed to improve micro-hole machining performances of PCD. Mechanical calculation of the cavitation bubble collapsing process and the particle motion in the dielectric fluid was carried out. The drop-off effect of diamond particles caused by ultrasonic vibration was analysed with the consideration of the special allographic structure of PCD. Under different machining conditions, a series of experiments were carried out to investigate the micro-hole machining performance of PCD by using ultrasonic enhanced micro-EDM. The experimental result indicates that the discharge status is improved,the arcing duration is reduced and the discharge frequency is increased by the effect of ultrasonic vibration. The adhesion onto the electrode is reduced due to the ultrasonic vibration act upon workpiece, which brings both positive and negative impacts in the machining performance. Therefore, in ultrasonic enhanced micro-EDM, the optimal amplitude chosen as 2μm can expand the optimal ranges of EDM machining conditions, hence improving both machining performance and surface quality.更多还原