【作者】 苏树朋；

【导师】 张勤河； 张建华；

【作者基本信息】 山东大学 ， 机械设计及理论， 2008， 博士

【摘要】 本文所研究的基于气体介质的电火花铣削加工技术主要指气中电火花铣削加工以及相应的超声辅助气中电火花铣削加工和浸液辅助气中电火花铣削加工。气中电火花加工技术由于具有环保安全、电极损耗率低等优点,使其适合应用于电火花铣削加工中,为电火花铣削加工工艺开辟了崭新的途径。超声辅助气中电火花铣削加工通过给工具电极施加超声振动来改善电火花加工状态以提高其加工效率的。浸液辅助气中电火花铣削加工是把工具电极与工件间的加工区域都浸没在液体中,避免了气中电火花铣削加工过程中排出的电蚀产物重新粘附在工件的加工表面等问题,使其加工效率和表面质量得以提高。从气中单脉冲放电实验研究和气中火花放电机理研究两个方面,对气中电火花铣削加工机理进行了较为深入的研究。气中单脉冲放电实验研究结果表明,正极性加工时工件上的单脉冲放电凹坑比负极性加工时的要大,且正极性加工时阳极的单脉冲蚀除量高于阴极的蚀除量;压缩空气为放电介质的单脉冲放电凹坑比常压空气下的单脉冲放电凹坑要大,且其放电凹坑的形状由于气流吹除作用而沿一定方向进行延伸;气中单脉冲放电凹坑都随着放电能量的增大而增大;通过气中单脉冲放电凹坑分析,建立了气中单脉冲放电凹坑直径的回归模型,得到了气中单脉冲放电凹坑直径与其主要因素的定量关系。分别从介质击穿、放电通道扩展、极间能量的转换分配、电蚀产物的抛出机理、介质的消电离以及连续脉冲的叠加效应等方面对气中火花放电机理进行了深入的研究。研究结果表明,气体火花放电是由Townsend击穿开始,逐步发展到流光击穿而形成的;气中放电通道的扩展要比液中放电通道扩展快;气中放电过程中电蚀产物的抛出主要是由热爆炸力、高压气流的吹除力、磁流体动力共同作用的结果,而对于硬脆材料,热应力蚀除起主导作用;气中电火花铣削加工过程中短路或拉弧现象频繁的原因主要在于放电间隙小而使得电蚀产物难以排出导致的。对气中电火花铣削加工的浸液辅助机理和超声辅助机理进行较为深入的研究。在浸液辅助气中电火花铣削加工过程中,尽管加工区域浸没在液体中,但由其极间压力状态知,充满极间放电间隙的放电介质仍为气体介质,其放电物理本质为气体介质放电;在超声振动辅助气中电火花铣削加工过程中,超声振动即使使放电间隙拉离了火花放电所允许的最大放电间隙,也不能拉断放电通道而提前中止放电脉冲,并得到了最大放电间隙公式,推得超声振动之所以改善气中放电加工状态主要是因为超声振动增大了其实际放电间隙范围。对气中电火花铣削加工的材料去除率和表面质量等基本工艺指标进行了较为系统的研究。从气中单脉冲放电蚀除特性研究出发建立了材料去除率模型,依据此模型可把各加工因素划分为影响单脉冲放电蚀除特性的加工因素和影响脉冲利用率的加工因素,并分析各加工因素对其材料去除率的影响规律,工艺实验结果验证了所建模型的有效性;还对比分析了气中电火花铣削加工和其浸液辅助加工方式不同的材料去除特性,研究结果表明,浸液辅助加工方式在较大脉冲能量参数下可获得较大的材料去除率,而在较小的脉冲能量参数下其浸液排屑效果不明显。从单脉冲放电凹坑形貌研究出发建立了气中电火花铣削加工的表面粗糙度模型,得到了重叠系数的取值规律,进行了相关参数的工艺实验分析,验证了所建模型的有效性;通过对比研究气中电火花铣削加工、其浸液辅助加工和油中电火花铣削加工的工件加工表面形貌状态,发现前两者的表面形貌要比后者平滑,前两者放电凹坑大而浅,后者的放电凹坑小而深。提出了一种新的气中电火花分层铣削加工工艺,并对其加工机床和工艺规划方法进行了较为系统的研究。开发了气中电火花分层铣削加工机床,此机床能够实现工件的X、Y伺服进给运动、工具电极的Z向伺服进给运动和工具电极的旋转超声振动。数控系统采用“IPC+PMAC”的开放式体系结构,可以实现气中电火花分层铣削加工以及相应的超声辅助加工和浸液辅助加工。根据气中电火花铣削加工特点,提出了相应的工艺规划策略。此工艺规划策略采用底面放电加工方式,初分层与次分层相结合的二次分层策略,轮廓环切与往复行切相结合的轨迹规划方式,间歇补偿的电极损耗补偿策略。依据气中电火花分层铣削加工特点及工艺规划策略,开发了相应的CAD/CAM系统。此系统采用Pro/E来完成加工工件的造型设计、工艺规划以及初始加工轨迹生成,然后依据PMAC数控系统的编码要求,对初始刀位轨迹进行后处理,生成满足加工机床要求的数控加工代码。并进行了三维结构实例的加工实验,验证了此工艺的可行性。对气中单脉冲放电的传热模型进行了数值仿真研究。分析了气中单脉冲放电的传热物理过程,推导出了热流密度载荷公式,并建立了气中单脉冲放电的传热数学模型,采用有限元数值分析软件ABAQUS对其数学模型进行了数值求解,计算结果验证了所建模型的有效性。更多还原

【Abstract】 The research of this dissertation mainly focus on the technology of electrical discharge milling(ED-Milling) based on gas dielectric which is composed of three ED-Milling methods including conventional ED-Milling in gas,ultrasonic vibration aided ED-Milling in gas as well as gas medium ED-Milling submerged in liquid. Electrical discharge machining(EDM) in gas is suitable for ED-Milling as it owns numerous advantages such as safe and green dielectric,no fire trouble,low tool electrode wear,etc.So it provides a new path for ED-Milling.The principle of ultrasonic vibration aided ED-Milling in gas is that the machining performances and the machining status can be enhanced by means of ultrasonic vibration of the tool electrode. Gas medium ED-Milling submerged in liquid,a new-type ED-Milling process,immerse the machining area between the tool electrode and the workpiece during ED-Milling in gas to solve the machining debris adherence problem on the machining surface of the workpiece.The mechanism of ED-Milling in gas was studied through conducting single pulse discharge experimental research and analyzing the mechanism of EDM in gas.The following conclusions can be drawn from numerous single pulse discharge experiments. The single pulse discharge crater of the workpiece on the condition of the positive polarity is bigger than that on the condition of the negative polarity.The single pulse discharge crater of the workpiece is bigger than that of the tool electrode when the tool electrode is cathode and the workpiece is anode.The crater produced by single pulse discharge in the compressed air is bigger than that produced in the air and its form changes according to the fluid of the compressed air.The volume of the single pulse discharge crater increases as the pulse energy strengthens.The mechanism of ED-Milling in gas is further studied from the aspects of the breakdown of the dielectric, the spread of the discharging channel,the conversion and distribution of the discharging energy,the removal of the machining debris,the deionization as well as the integrated effect of the impulsive discharges.The results indicate that the discharging process during EDM in gas initially comes from Townsend breakdown process and then changes to streaming breakdown process.The discharging channel during EDM in gas expands more quickly than it does during conventional EDM.The removal of the machining debris during EDM in gas is concurrent results of the thermal expansion,the fluid of compressed gas,magnetic fluid dynamic force,while the thermal stress removal plays a dominant role in machining the hard-brittle material.The principles for ultrasonic vibration aided ED-Milling in gas and gas medium ED-Milling submerged in liquid were analyzed.According to the characteristics of the pressure status between electrodes,the medium between the tool electrode and the work piece for gas medium ED-Milling submerged in liquid is still gas dielectric.The mechanism of the effect of the tool electrode’s ultrasonic vibration is that the machining efficiency and the machining status can be improved because ultrasonic vibration of the tool electrode can enlarge the practical discharge gap range.The fundamental machining performances such as material removal rate(MRR) and surface quality were studied systematically.The model of MRR for ED-Milling in gas was developed based on the characteristics of material removal of single pulse discharge from which the machining parameters can be divided into two groups including the parameters affecting the characteristics of material removal of single pulse discharge and the parameters affecting pulse usage factor.The model has been verified by process experimental research.By comparing the rule of MRR for gas medium ED-Milling submerged in liquid with the MRR for ED-Milling in gas,the MRR of the former is much greater than that of the latter when the large pulse energy is chosen, while the MRR of the former is close to the MRR of the latter when the small pulse energy is chosen.The model of surface roughness for ED-Milling in gas was developed based on the form of single pulse discharge crater and its overlap coefficient has been deduced.The model has been verified by the process experimental research.By comparing the surface of the workpiece of ED-Milling in gas,gas medium ED-Milling submerged in liquid,EDM in oil dielectric,the shape of the firth two is smoother than that of the last one,and the craters of the former two items are big and shallow while the craters of the last one are small and deep.A novel ED-Milling method,electrical discharge layered milling in gas (EDL-Milling in gas),was developed.The machine tool and process plan strategy of the technology were studied systematically.The machine tool can accomplish the following functions,X-axis and Y-axis feed movement of the workpiece,and Z-axis feed movement of the tool electrode,the generation movement of the rotary movement and ultrasonic vibration of the tool electrode.The machining process is controlled by an open system architecture CNC system based on "IPC+PMAC".The machine tool is applicable in these processes including EDL-Milling in gas,ultrasonic vibration aided EDL-Milling in gas as well as gas medium EDL-Milling submerged in liquid.Process plan strategies for EDL-Milling in gas were developed according to the machining features of the process.Bottom milling method,twice-layered slicing strategy,tool path planning algorithm with the couple of direction-parallel machining and contour machining path as well as the semi-online periodical compensation method were developed.CAD/CAM system can be accomplished through the following procedures. Drawing of the workpiece,process plan strategy,and initial tool path generation can be accomplished by the commercial software Pro/E.NC code accepted by CNC system can be acquired through post-processing procedure.3-D machining samples have verified the feasibility of the approach.Numerical solutions to the thermal model of EDM in gas were studied.The mathematical model of the thermo-physical procedure was built and the formula of distributed surface flux was deduced.Mathematical model can be calculated through FEM(finite element method) software ABAQUS.The results of the FEM analysis have verified the feasibility of the mathematical model of EDM in gas.更多还原