Multi-physics modeling of plasma discharge, material removal and debris ejection in electrical discharge machining with flowing dielectric film

Tanveer, Asif

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https://hdl.handle.net/2142/117641 Copy

Description

Title

Multi-physics modeling of plasma discharge, material removal and debris ejection in electrical discharge machining with flowing dielectric film

Author(s)

Tanveer, Asif

Issue Date

2022-11-07

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

Kapoor, Shiv

Doctoral Committee Chair(s)

Kapoor, Shiv

Committee Member(s)

• Curreli, Davide
• Ferreira, Placid
• Shao, Chenhui

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)

• electrical discharge machining
• plasma model

Abstract

Electrical discharge machining (EDM) is a non-conventional machining process that can remove material using a series of rapidly recurring electrical discharges between two electrodes in a dielectric liquid. Due to its unique capabilities and high-temperature discharge, EDM is used for machining complex geometries with high accuracy and can be used to machine any metal regardless of hardness. As a result, it has found a wide array of applications in electronics, automotive, biomedical and aerospace industries. However, one of the major issues with the EDM process is the low material removal rate (MRR). A reason for that is due to the accumulation of debris in the inter-electrode gap. If the debris is not flushed out properly, it can cause electric field distortion and decrease the dielectric strength. This may increase the probability of secondary discharge occurred in the gap between electrode and workpiece, and cause lower material removal rates, reduce machining accuracy and lead to poor surface finish. To improve the debris removal process, there have been many dielectric flushing techniques implemented and numerous developments to refine the existing methods. However, one of the issues with current flushing methods is the usage of excessive dielectric liquid. The liquid, which is often a hydrocarbon oil, is hazardous to health, flammable and environmentally challenging to recycle. To reduce the usage of large amount of dielectric, a novel method of utilizing spray to create a thin film of dielectric flow in the inter-electrode gap was developed. However, due to its novelty, there has not been much research on its machining characteristics or on its effectiveness in removing debris material. In addition, without any understanding of material removal mechanism makes it very difficult to conduct full-scale parametric studies and find optimum machining parameters of the process. As a result, there is a need for the development of physics-based models to provide fundamental knowledge of the material removal process in EDM with dielectric film flow. In this research work, a multi-physics model of the EDM process with dielectric film flow has been developed to understand formation of EDM plasma, formation of melt-pool and the mechanism of debris removal from the formed crater region. Initially, a 1D plasma model for a single discharge has been formulated to predict the different plasma characteristics such as electron density and plasma temperature. Equations of mass and energy conservation are solved simultaneously to obtain plasma characteristics along the length of the inter-electrode gap over time. It utilizes plasma chemical reactions, which involves energy interactions of various species in the plasma. This allows the model to create heat flux to the electrode surfaces from the ground up. In addition, the model is capable of incorporating surface reactions and surface electron emissions due to its 1D domain. The model is used to study the effect of supply voltage and inter-electrode gap on the plasma characteristics. Plasma temperature results from the model are validated using a custom-made die-sinking EDM setup. Because of the high temperature generated by the model it is assumed that the major form of heat transfer is radiation and is responsible for the rapid melting of the workpiece. A melt-pool model is developed to predict material removed and crater formed at the workpiece in EDM with flowing dielectric film using the heat flux predicted from the plasma model. To model the melt-pool, heat transfer and hydrodynamic equations are solved in a 2D domain consisting of the flowing dielectric and the workpiece material. Level-set method is used to identify the interfaces between solid workpiece and the molten workpiece. Using this model, various flow conditions of the dielectric are simulated to study the effect of the dielectric film flow on crater geometry and understand how the flow removes the molten material from the discharge location. Crater size is validated with experimental tests run on the custom-built die-sinking EDM setup with provision for dielectric film flow. The model results show that crater sizes are comparable to those from the experiments. The crater shape from the melt-pool model is utilized in the debris ejection model to observe how the flow helps in removing the smaller debris particles. Hydrodynamic equations and force-balance equations on solid spherical debris particles are used in a 2D domain to predict the trajectory of individual debris particles and their time of residency in the crater region. The model shows that dielectric film flow in the interelectrode gap enhances flushing of debris particles by carrying them away from the tool center. In addition, by changing the dielectric properties, the model indicates that liquids with higher kinematic viscosity are more effective in flushing out debris. Due to the 2D nature of the melt-pool and debris models certain characteristics including volume of material removed have not been explored even though individual models connected by their outputs to form a multi-physics model provide various insights into the film flow flushing mechanism. As a result, experimental tests are carried out to observe the effect of the flow on debris distribution around the crater and volume of material removed. Also, the plasma model uses water chemistry, so additional experiments on corn oil and water as dielectrics are conducted to compare the capability of other dielectrics in removing material using the same system of film flow. It is observed that the increased flow helps in removing larger amount molten material and disperses smaller debris particles further away from the crater center. Also, corn oil seems to perform better in regard to volume of material removed when compared to deionized water.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Asif Tanveer

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