【作者】 刘宇；

【导师】 赵福令；

【作者基本信息】 大连理工大学 ， 机械电子工程， 2011， 博士

【摘要】 微细电火花加工技术是将常规电火花加工工艺微细化,用于实现微细尺度零件特征加工的特种加工技术。由于加工具有非接触、加工过程几乎无切削力,不受材料的强度、硬度限制等特点,微细电火花加工技术特别适合于高精度、无变形的微小零件特征的加工以及硬脆难加工材料的微细加工。加之其良好的数控兼容性,被认为是加工三维复杂微细结构最具潜力的方法之一。因此,微细电火花加工技术已经成为微细制造领域的一个重要方向,并且越来越广泛的应用于航空航天、电子信息、模具以及光学和医疗器械中关键零件的加工。然而,目前国内外学者对微细电火花加工的机理和工艺研究仍然不够。在加工机理研究方面,由于电火花加工本身就是一个复杂、瞬时、随机的过程,而加工尺度的缩小,加工条件的改变,尤其是高频脉冲的作用,给微细电火花加工的物理过程增加了更多的不确定因素；在工艺研究方面,电极损耗和放电加工对表面特性的改变严重影响了微细电火花加工的加工精度和质量。这些存在的问题限制了微细电火花加工技术的进一步发展。因此,从电火花加工微细化过程中加工条件的改变入手,研究微细电火花加工独特的放电机理,探讨高频脉冲作用对加工过程的影响,分析电极损耗现象的成因和抑制损耗的方法以及总结微细电火花加工微观表面特征规律与表面质量改善方法,具有重要的意义。本论文针对上述微细电火花加工技术研究存在的问题,开展了如下四个方面的研究工作：(1)研究了微细电火花加工的放电机理。分析了电火花加工微细化过程加工条件的改变以及这些改变带来的加工过程的影响。通过研究煤油介质的工作特性,讨论了放电击穿前微观粒子受外加能量作用的一系列状态变化以及放电击穿、火花维持和消电离过程中各物质能量之间相互作用的微观情况；建立了正离子场致电子发射模型,补充了流光理论解释击穿过程时在形成电离粒子数量方面的不足；研究了电极材料的抛出过程及能量转化问题。(2)研究了高频脉冲对微细电火花加工的影响。引入高频电磁场中集肤效应的概念分析高频脉冲的特殊作用过程,理论和实验验证了集肤效应对微细电火花加工的影响；总结了高频脉冲作用下电场强度与电流密度非均匀分布的规律,并由此分析了集肤效应对加工过程中放电点位置变化、材料蚀除形式、电极形状变化的影响结果；实验研究了通过改变加工脉冲频率获得不同端面形状电极的微细电极在线修形方法,尝试了运用电极修形后不同形状电极的平滑表面加工自由曲面的加工方法。(3)电极损耗的相关技术研究。采用复合电镀工艺在工具电极表面镀覆耐电蚀材料以制备耐损耗电极。实验证明这一方法有效的抑制了深孔加工中电极棱边损耗,改善了微孔的加工精度；提出了基于图像处理技术研究工具电极损耗、端面形状变化的研究方法,通过对加工后损耗电极图像轮廓特征的提取分析及损耗数据曲线的函数拟合,开辟了数字计算、分析微细电火花加工电极损耗的新路径；分析了微细电火花铣削加工中钨工具电极材料发生意外破损现象的原因,通过微细流场的仿真研究,揭示了导致破损的交变工作液压力的影响因素,并讨论了这种电极破损造成的危害及预防措施。(4)微细电火花加工电极表面特性的研究。实验观测了微细电火花加工电极表面的微观形貌,着重讨论了表面放电凹坑、球状凸起以及球形凹坑的形成过程；分析了电极表面的裂纹现象,分析不同的表面裂纹形成过程并加以分类,讨论了各类裂纹的抑制措施；观测了电极表面剥落现象,从裂纹的交织扩展和加工表面作用力两方面分析了表面剥落现象的成因,提出了部分避免表面剥落发生的措施。以上研究工作不仅进一步揭示了微细电火花加工中放电现象的物理实质,同时也为微细电火花加工技术的广泛应用提供依据。更多还原

【Abstract】 Micro electrical discharge machining (micro EDM) is a nontraditional machining method to realize the manufacturing of micro scale parts and features by miniaturizing conventional electrical discharge machining (EDM) technology. Because of the advantages of noncontact machining and almost no cutting force within the processing, micro EDM can be applied in fabricating smaller parts and micro features with high precision and no distortion. For micro EDM processing is not limited by the strength and hardness of materials, it is very fit for the micro fabrication of hard and brittle materials. Additionally, micro EDM has been regards as one of the most potential methods to produce three-dimensional (3D) microstructures with complex structure owing to its outstanding compatibility with numerical control techniques. Therefore, micro EDM has become an important direction in micro manufacturing field, and has been more and more widely used to machining key parts in the fields of aerospace, electrommunication, mould, optical and medical instruments.However, the research on machining mechanism and technology of micro EDM is still not enough. In the term of mechanism research, as the EDM itself is a complex, transient and random process, and the decrease of machining scale, changes of machining conditions, especially the effect of high frequency pulses, add more uncertainty to the physical process of micro EDM; at the respect of technology research, the tool wears and the influences of discharge machining on surface properties seriously affect the machining accuracy and quality of micro EDM. These existing problems limit the further development of micro EDM technology. Thereby, by starting from the machining condition changes in miniaturization of EDM, it has an important significance to study unique discharge mechanism of micro EDM, discuss the influence of high frequency pulses on machining process, analyze the causes of tool wear and methods to reduce wear, and summarize the law of surface characteristics and methods to improve surface quality in micro EDM.The above problems in micro EDM are focused on in this thesis, and four main research aspects have been carried out as follows:(1) Study on the discharge mechanism of micro EDM. The machining condition changes in the process of EDM miniaturization and the influences of these changes on micro EDM processing are analyzed. Though the characteristic analysis of kerosene dielectric, a series of state changes of microscopic particles caused by the impact of external energy are discussed before the discharge breakdown, and the microscopic situations that various micro materials and energy interacts in discharge breakdown, spark maintenance and deionization processes are proposed; the model of positive ion field electron emission is present forward, this complement the streamer theory in the lack of ionized particle number; the electrode material removal process and energy conversion problem are conducted.(2) Study on the influences of high frequency pulses on micro EDM. The skin effect in high frequency electromagnetic field is introduced to analyze the special impact of high frequency pulses, and the influences of skin effect on micro EDM is demonstrated with electromagnetic field theory and experiments; nonuniform distribution laws of electric field intensity and current density under high frequency pulses are summarized, and based on this, the spark location change, form of material removal, tool shape change are analyzed; the online electrode modification method to obtain different tool end shape from changing machining pulse frequency is experimental studied, and the method through adopting the smooth surface of different tool shapes after electrode modification is tried to manufacture freeform surface.(3) Study on relative technology of tool wear. Composite plating is used to plating electrical erosion resistant material on tool surface for the fabrication of wear-resist electrodes. The experimental study proves that this method effectively reduces the edge wear in deep hole machining and improves the processing precision of micro holes; a way to study tool wear, tool end shape change based on image processing is put forward, through the contour extraction of worn tool from electrode image after machining and curve fitting of contour data, a new path to digital computation and analysis tool wear in micro EDM is opened; the reasons causing unpredictable failure of tungsten tool in micro electrical discharge milling are conducted, by simulation study of micro flow field, the influencing factors of alternating pressure of dielectric fluid causing the failure are revealed, and the harm of this unpredictable failure and its preventive measures are discussed.(4) Study on surface properties of electrode in micro EDM. The electrode surface topography after micro EDM is experimentally observed, the forming processes of surface discharge craters, globules of debris and micro spherical holes are focused on; the crack phenomena on electrode surface are investigated, different crack formations of various cracks are analyzed, and based on this, cracks are divided into groups, the restraining measures of every type of crack are discussed; surface exfoliation phenomenon is observed, the causing reasons of surface exfoliation are analyzed through the interlacing and expanding of cracks and surface forces, some methods to avoid surface exfoliation are proposed.These studies not only further reveal the physical essence of discharge process in micro EDM, but also provide a basis for widespread applications of micro EDM technology.更多还原