Multi-physics modeling of plasma discharge and material removal in micro electro-discharge machining process

Mujumdar, Soham Sanjeev

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Description

Title

Multi-physics modeling of plasma discharge and material removal in micro electro-discharge machining process

Author(s)

Mujumdar, Soham Sanjeev

Issue Date

2016-04-19

Director of Research (if dissertation) or Advisor (if thesis)

Kapoor, Shiv G.

Doctoral Committee Chair(s)

Kapoor, Shiv G.

Committee Member(s)

• Ferreira, Placid
• Ruzic, David
• Curreli, Davide

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Micro electro-discharge machining (Micro-EDM)
• EDM plasma
• Material Removal

Abstract

Micro electro-discharge machining (micro-EDM) is a machining process capable of removing material in sub-grain size range and machining a range of materials irrespective of their hardness. Due to the unique capabilities offered by micro-EDM process in manufacturing high accuracy micro-scale parts with complex geometries, it has potential to meet wide spectrum of current and future needs in the electronics, automotive, optics and bio-medical industries. Despite these advantages, one of the major disadvantages of the micro-EDM process has been its low productivity in terms of material removal rate (MRR). To improve the MRR, the micro-EDM process has been subjected to numerous parametric optimization studies. However, machining characteristics of the micro-EDM process are influenced by a large number of controllable process parameters. This makes it very cumbersome to conduct full-scale parametric studies and find optimum machining parameters without understanding of material removal mechanism. Therefore, there is a strong need for development of a physics-based model to gain fundamental knowledge of the micro-EDM process. In this research work, a multi-physics model of the micro-EDM process has been developed to understand formation and expansion of micro-EDM plasma, and the process of formation of melt-pool and material removal at the workpiece. Initially, a model of micro-EDM plasma has been developed using a ’Global Model’ approach in which the plasma is assumed to be spatially uniform, and equations of mass and energy conservation are solved simultaneously along with the dynamics of the plasma bubble growth. One of the unique features of this model is the plasma chemistry module that enables understanding of chemical as well as energy interactions of various species in the plasma. Using the micro-EDM plasma model, complete temporal description of the micro-EDM plasma is obtained in terms of the composition of the plasma, temperature of electrons and other species, radius of the plasma bubble, the plasma pressure and heat flux to the electrodes. The model is also used to study the effect of electric field in the inter-electrode gap and the gap distance on the plasma characteristics. The model predicts that the application of higher field at a fixed gap increases the electron density, plasma temperature, plasma radius, plasma pressure and the heat flux to the workpiece, while increasing gap distance for a fixed electric field results in decreased overall plasma density and increased heat flux. The micro-EDM plasma model is further enhanced to predict time-transient electrical characteristics of a micro-EDM discharge such as plasma resistance, voltage, current and discharge energy. In micro-EDM, due to smaller value of the plasma resistance, it is often difficult to separate the voltage drop across stray impedances in the circuit from exact voltage drop across micro-EDM plasma alone using direct measurements. A model-based approach can be useful in this case to obtain accurate information about the time transient plasma voltage/current waveforms and the discharge energy, which plays a crucial part in material removal. Using the enhanced micro-EDM model, effects of gap voltage and gap distance on the plasma electrical characteristics are studied to reveal that the application of a higher open gap voltage decreases the plasma resistance but increases the plasma current resulting in an increase of the discharge energy for a given gap distance. Whereas, an increase in the inter-electrode gap distance increases the plasma resistance but does not affect the plasma current, thereby, increasing the discharge energy. A melt-pool model based on the plasma model predictions of plasma radius, pressure and heat flux is developed to predict material removed from the workpiece (anode) in micro-EDM. To model the melt-pool, heat transfer and fluid flow equations are solved in the domain containing dielectric and workpiece material. A level-set method is used to identify solid and liquid fractions of the workpiece material when the material is molten by micro-EDM plasma heat flux. Using the melt-pool model, a typical micro-EDM discharge is simulated to study the evolution of temperature and velocity distribution on the workpiece surface, and morphology of the resulting crater. The morphology of the crater is used to estimate volume of material removed per discharge in micro-EDM. The micro-EDM plasma model and the melt-pool model are validated with the help of a custom-designed micro-EDM machine tool equipped with single-discharge EDM circuit. Voltage and current waveforms are captured experimentally and compared with model predictions to validate the micro-EDM plasma model. Micro-EDM melt-pool model is validated by comparing the model predicted crater shapes with experimental measurements of the crater morphology. The modeling tools are employed further to study possible mechanisms of productivity improvements in micro-EDM. First, effect of electrical conductivity of the dielectric water on the dielectric breakdown, plasma characteristics and material removal in micro-EDM is investigated using both, experimentation and modeling. It is concluded that the plasma heat flux exerted on the workpiece can be increased to cause increased material removal using increased electrical conductivity of the dielectric water. Next, the melt-pool model is used to obtain evolution of temperature and velocity distributions in magnetic field-assisted micro-EDM and study the effect of Lorentz force on morphological modification of the melt-pool. It is shown that the external magnetic field exerts a Lorentz force on the micro-EDM melt-pool during the discharge, affects the morphology of the melt-pool and can be used to exert additional ejection force on the debris particles to improve productivity of the micro-EDM process.

Graduation Semester

2016-05

Type of Resource

text

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Copyright 2016 Soham Sanjeev Mujumdar. All rights reserved.

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