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Micromachining of Hard Brittle Insulating Materials Based on Electrochemical Discharge Effect

Author: WeiChenZuo
Tutor: HuDeJin; NiJun
School: Shanghai Jiaotong University
Course: Mechanical Manufacturing and Automation
Keywords: Electrochemical discharge machining (ECDM) Hard brittle insulating materials Micromachining Grinding wheel dressing Finite element simulation
CLC: TG66
Type: PhD thesis
Year: 2011
Downloads: 138
Quote: 1
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Abstract

Micro-electromechanical systems (MEMS) have been playing an important role in electronics, automotives, biomedical, renewable energy, aviation and smart weapons. Non-conductive hard brittle MEMS materials such as glass have many favorable properties such as high hardness, high brittleness, wear resistance, chemical inertness, electrical insulation, optical transparency and bio-compatibility. However, they are hard to process in micromachining. Electrochemical discharge machining (ECDM) is a micromachining process for non-conductive materials like glass, quartz and some ceramics. It is featured by high efficiency, high flexibility and low cost. Nevertheless, it has not been put to large-scale industrial applications due to its low machining accuracy and machinable depth. To solve the problems, we propose a series of micromachining techniques for insulating hard brittle materials based on electrochemical discharge effect. In this dissertation, the physics and chemistry of electrochemical discharge effect was discussed; secondly, a new mathematical model for ECDM was developed; finally, experimental investigations were carried out on thermo-assisted mechanical machining using electrochemical discharge effect, electrochemical discharge vibration and shock machining and electrochemical discharge grinding wheel dressing to understand the processes and mechanisms.In the study of physics and chemistry of electrochemical discharge effect, the static voltage–current characteristics were discovered, and it was pointed out that the onset of the effect is determined by the critical voltage. Dynamic characteristics in discharge regime was emphasized, it was found that discharge frequency and peak current follow Poisson distributions. Regarding gas film formation mechanism, an electrical model of the bubble layer and gas film were set up, and percolation theory was employed to explain the transformation mechanism from bubble layer to gas film.In the study of mathematical modeling for ECDM, the model was formulated through mathematical deduction after establishing appropriate assumptions and machining mechanism. Secondly, finite element method was used to calculate the material removal subjected to a single spark. Thirdly, experiments were conducted to obtain the critical parameters and validate the model; the model predictions were basically consistent with the experimental results. The effects of process parameters and machining mechanism were studied using the model, and it was found out that the material removal mechanism at low applied voltages is mainly chemical dissolution, and the contribution of thermal erosion increases with the increase of applied voltage. The features of this model include the rectangular distribution replaced with Gaussian distribution to model the heat source induced by one spark for high accuracy, the use of equivalent temperature as the material removal criterion to transform the complex coupled thermo-chemical problem to a simple heat conduction problem, and the exponential decay function used to model the regime effect.In the study of thermo-assisted mechanical machining using electrochemical discharge effect, the machining principle was introduced; the thermo-assisted machining process was described using mathematical language. This method was compared with conventional ECDM; the experimental results showed that thermo-assisted mechanical machining using electrochemical discharge effect increases machining efficiency in discharge regime, and reduce the dimensional error as well as roundness error. The effects of process parameters were investigated. It was found out that the machining efficiency increases with the applied voltage although the machining accuracy decreases. The machining efficiency increases with the tool rotation rate before it reaches a critical depth. The machined surface was examined using a scanning electron microscope (SEM); it was proved that the machining mechanism is a hybrid effect of thermo-assisted mechanical machining, thermo-assisted chemical machining and thermal erosion. Among them, thermo-assisted mechanical machining is predominant.In the study of electrochemical discharge vibration and shock machining, the machining principle was introduced. This method was compared with conventional ECDM; the experimental results showed that electrochemical discharge vibration and shock machining increases the machining efficiency in hydrodynamic regime. The effects of process parameters were investigated. It was demonstrated that the machining efficiency increases linearly with the vibration amplitude. The machined depth slowly increases in the range of 15–150 Hz; however, it increases from 300μm to 550μm in the range of 150–500 Hz. The machined surface was examined using a scanning electron microscope (SEM); it was proved that the machining mechanism is a hybrid effect of thermo-assisted mechanical machining, thermo-assisted chemical machining and thermal erosion. Thermo-assisted mechanical machining is predominant at high machining depths. In the study of electrochemical discharge dressing (ECDD), electrochemical discharge effect is applied to auxiliary grinding wheel conditioning in micro-grinding, and the dressing takes advantage of the tool wear in ECDM. The principle of ECDD was introduced, and the dressing process was analyzed. The analysis results showed that the dressing mechanism is the high local temperature caused by the sparks and electrochemical erosion. Through the comparison of grinding face morphology, grinding force and workpiece surface roughness before and after dressing, it was shown that the abrasives were exposed without damage after dressing; normal grinding force and surface roughness of the workpiece were reduced by 50%. The effect of process parameters was investigated, and it was discovered that the applied voltage and electrolyte concentration are two key parameters in the process. The optimal condition was founded to be the applied voltage of 32 V and electrolyte concentration of 30 wt. %.

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