Eroded Crater Morphology by Plasma Single Discharge

Daniel Diogo Saião Ferreira

Thesis to obtain the Master of Science Degree in

Mechanical Engineering

Supervisors: Prof. Pedro Alexandre Rodrigues Carvalho Rosa

Prof. Luís Manuel dos Santos Redondo

Examination Committee

Chairperson: Prof. Rui Manuel dos Santos Oliveira Baptista

Supervisor: Prof. Pedro Alexandre Rodrigues Carvalho Rosa

Members of the Committee: Dr. Marcos Teotónio Pereira

Eng. Afonso José de Vilhena Leitão Gregório

November 2018

i

Abstract

Electrical discharge machining (EDM) involves the generation of micro-plasmas subjected to high

temperature and pressure to promote the material removal. Hence, to understand the material removal

mechanism it is of great importance the knowledge of the interaction plasma-solid. Knowing how

physical and chemical properties of materials affect heat transfer at the electrode surface, how this

eventually affects electrical properties of the plasma channel over the discharge time are key issues to

achieve a better understanding of this machining technology.

This research attempts to provide some answers to these issues by means of single plasma discharge

tests under laboratory-controlled conditions carried out on nearly pure metals using two different type of

dielectric fluids. The main objective of this investigation is to understand the influence of current intensity

on the crater morphology of different materials. Circular geometry was the most common morphology

observed. A method to estimate the crater volume was developed to afterward calculate the specific

electrical erosion energy. Single discharges were made with two different dielectric fluids, glycerin and

deionized water for the same workpiece material, varying the current intensity to understand how

dielectric type influences the process.

Keywords: Electrical Discharge Machining, Single plasma discharge, Current intensity, Crater

morphology, Specific electrical erosion energy, Dielectric fluid

ii

Resumo

A maquinagem por electroerosão (EDM) envolve a geração de microplasmas a altas temperaturas e

pressões, promovendo a remoção do material da peça. Para se compreender melhor este mecanismo,

é importante perceber a interação plasma-sólido. É importante perceber a influência das propriedades

físicas e químicas dos materiais no que respeita à transferência de calor entre o canal de plasma e as

superfícies dos elétrodos, bem como as propriedades elétricas do canal de plasma, durante o período

de descarga.

Esta investigação procura contribuir para a compreensão da influência dos parâmetros operativos na

morfologia das crateras através de ensaios de mono-descarga em condições laboratoriais controladas,

realizadas em metais quase puros e utilizando diferentes fluidos dielétricos. O principal objetivo do

presente trabalho é compreender a influência da intensidade de corrente na morfologia das crateras. A

geometria circular foi a morfologia observada com mais frequência. Estabeleceu-se um método para

estimar o volume da cratera, posteriormente utilizado no cálculo da energia específica de erosão. Foram

efetuados ensaios com dois fluídos dielétricos, glicerina e água desionizada, utilizando o mesmo

material e variando a intensidade de corrente para tentar compreender a influência do dielétrico no

processo.

Palavras-chave: Maquinagem por electroerosão, Mono-descarga, Intensidade de corrente, Morfologia

da cratera, Energia específica de erosão, Fluído dielétrico

iii

Acknowledgments

In first place, I would like to thank the person that always believed in me, my supervisor, Professor Pedro

Rosa. A huge thank you. I would also like express my very great appreciation to my co-supervisor,

Professor Luís Redondo, for his time, interest and knowledge transmitted.

To my laboratory colleagues and friends, Luís Almeida, João Raposo, Duarte Andrade, João Pragana,

Paulo Farinha, Pedro Santos, João Sousa, Francisca Velasco, Matilde Ferreira, thank you for the

friendship and companionship among all and for the excellent laboratory atmosphere.

To my Brussels friends that always that always endorsed my decisions along this path.

Pedro Martins, simply a huge thank you for everything.

Mariana, my sister, thank you for everything. You always put up with me, motivated and endorsed in

everything.

To all my family, specially to my parents, for believing in me, the unconditional strength, support and

affection, always demonstrated. A huge thank you for allowing me to be what I am today.

iv

Table of Contents

Abstract ................................................................................................................................................... i

Resumo................................................................................................................................................... ii

Acknowledgments ................................................................................................................................ iii

Table of Contents ................................................................................................................................. iv

List of Figures ....................................................................................................................................... vi

List of Tables ....................................................................................................................................... viii

List of Annexes ..................................................................................................................................... ix

List of Acronyms ................................................................................................................................... x

1. Introduction .................................................................................................................................... 1

2. Theoretical Fundaments ............................................................................................................... 3

2.1. Material Removal Mechanism ............................................................................................... 3

2.1.1. Pre-breakdown and breakdown ................................................................................ 4

2.1.2. Phase of discharge ................................................................................................... 4

2.1.3. End of the discharge and post-discharge ................................................................. 6

2.1.4. Thermal effect on crater formation ........................................................................... 6

2.2. Technological Process ........................................................................................................... 9

2.2.1. Material Removal Rate ............................................................................................. 9

2.2.2. Superficial condition ................................................................................................ 10

2.3. Electrical Signature ............................................................................................................... 11

2.3.1. Types of discharges ................................................................................................ 12

2.3.2. Types of Power Supply ........................................................................................... 12

3. Experimental Development ......................................................................................................... 15

3.1. EDM Machine ...................................................................................................................... 15

3.2. Measuring Instruments ........................................................................................................ 23

3.3. Experimental plan ................................................................................................................ 26

4. Results .......................................................................................................................................... 27

4.1. Current influence on different materials .............................................................................. 27

4.2. Specific electrical erosion energy ........................................................................................ 32

4.3. Comparison between glycerin and deionized water ............................................................ 36

5. Conclusions ................................................................................................................................. 40

v

6. References .................................................................................................................................... 42

Annex A – Previous EDM circuit schematics ................................................................................... 44

Annex B – Tin soldering procedure ................................................................................................... 45

Annex C – Optocoupler Datasheet .................................................................................................... 46

Annex D – MOSFET Datasheet ........................................................................................................... 47

vi

List of Figures

Figure 2.1 – EDM different phases (Descoeudres, 2006) ....................................................................... 3

Figure 2.2 – Schematic representation of heat exchanges in plasma-solid interaction during an electric

discharge (Yeo, 2008).............................................................................................................................. 5

Figure 2.3 – Energy distribution in EDM process (Xia, 1996) ................................................................. 6

Figure 2.4 – Gaussian heat source model (Khan, 2011) ......................................................................... 7

Figure 2.5 – (a) Simplified schematics of the experimental apparatus used by Natsu; (b) Spatial

distribution of temperature between electrodes during the discharge (Natsu, 2004) ............................. 8

Figure 2.6 – Trend of the maximum temperature in the discharge channel with electrical conductivity . 8

Figure 2.7 – Relationship between current and MRR (Khan, 2011) ...................................................... 10

Figure 2.8 – (a) Surface layers after electrical discharge machining; (b) Relationship between the

average white layer thickness and EDM parameters; (Kruth et al., 1995) ............................................. 11

Figure 2.9 – Types of discharges: (a) Normal; (b) Open; (c) Short-circuit; (d) Arc ................................ 12

Figure 2.10 – Rotary Impulse Generator (Shah et al., 2007) ................................................................ 13

Figure 2.11 – RC Circuit (Shah et al., 2007) ......................................................................................... 14

Figure 2.12 – Pulse Generator (Shah et al., 2007) ............................................................................... 14

Figure 3.1 – (a) Woodpecker CNC used for the kinematic control of the electrode; (b) Grblcontrol

interface and explanation of main commands ....................................................................................... 16

Figure 3.2 – (a) First shaft with 10µm precision per step; (b) Manufactured shaft for EDM with 1,25µm

precision per step .................................................................................................................................. 17

Figure 3.3 – (a) Machining the base of the structure; (b) Structure of the final machine ...................... 17

Figure 3.4 – (a) TGP110 10MHz Pulse Generator (PWM); (b) PWM response observed in the

oscilloscope ........................................................................................................................................... 18

Figure 3.5 – (a) Optocoupler symbol;(b) Physical functioning of an optocoupler

(searchnetworking.techtarget.com) ....................................................................................................... 19

Figure 3.6 – (a) Front of the switch module; (b) Inner side of the switch module ................................. 20

Figure 3.7 – (a) MOSFET and Optocoupler test; (b) Testing the whole circuit...................................... 21

Figure 3.8 – Complete circuit schematics ............................................................................................. 21

Figure 3.9 – Final result; (a) Top view; (b) Front view ........................................................................... 22

Figure 3.10 – (a) CLSM; (b) Optical microscope; (c) Oscilloscope ....................................................... 23

Figure 3.11 – Crater depth measurement: (a) Zinc at 4A; (b) Aluminium at 8A; (c) Tin at 16A ............ 23

vii

Figure 3.12 – (a) Sulfuric acid applied on Tin; (b) Sulfuric acid applied on Zinc ................................... 24

Figure 3.13 – (a) Microscope calibration; (b) Workpiece matrix ............................................................ 25

Figure 3.14 – (a) Voltage differential probe (200x signal attenuation); (b) Current probe..................... 25

Figure 3.15 – (a) Transistor circuit; Electrical signature: (b) Zinc (t_on=50µs, I_e=4A, U_e=200V); (c)

Aluminium (t_on=50µs, I_e=8A, U_e=200V); (d) Tin (t_on=50µs, I_e=16A, U_e=200V) ..................... 26

Figure 4.1 – Tin craters: (a) 4A; (b) 8A; (c) 16A; lower scale division is equivalent to 20µm ............... 28

Figure 4.2 – Copper craters: (d) 4A; (e) 8A; (f) 16A; lower scale division is equivalent to 20µm ......... 28

Figure 4.3 – Crater diameter (µm) vs Current intensity (A) ................................................................... 29

Figure 4.4 – Crater depth (µm) vs Current (A) ...................................................................................... 30

Figure 4.5 – Eroded craters observed through the optical microscope: (a) Tin at 16A; (b) Zinc at 16A;

lower scale division is equivalent to 100µm .......................................................................................... 31

Figure 4.6 – Ratio between diameter and depth vs current .................................................................. 32

Figure 4.7 – Crater morphology: (a) Spherical calotte; (b) Bowl-shaped crater; (Bragança, 2013) ...... 33

Figure 4.8 – Corrective factor as a function of the radius per depth ratio ............................................. 34

Figure 4.9 – Energy per volume unit (J/mm³) as a function of Current (A) ........................................... 34

Figure 4.10 – Specific electrical erosion energy as a function of the Reynaerts index ......................... 36

Figure 4.11 – Diameter (µm) vs Current using glycerin and deionized water ....................................... 37

Figure 4.12 – Eroded Tin with ionized water: (a) 4A; (b) 8A; (c) 16A; The lower scale division is equivalent

to 20µm .................................................................................................................................................. 38

Figure 4.13 – Depth vs Current for eroded Tin with deionized water and glycerin ............................... 38

viii

List of Tables

Table 3.1 – Fixed operative parameters ................................................................................................ 27

Table 3.2 – Variable operative parameters ............................................................................................ 27

Table 4.1 – Material properties and Reynaerts machinability index, data provided by MatWeb ........... 29

ix

List of Annexes

Annex A – Previous EDM circuit schematics……………………………………………….. 44

Annex B – Tin soldering procedure…………………………………………………………... 45

Annex C – Optocoupler Datasheet…………………………………………………………... 46

Annex D – MOSFET Datasheet……………………………………………………………… 47

x

List of Acronyms

AMZ Altered Metal Zone

CLSM Confocal Laser Scanning Machine

CNC Computer Numerical Control

EDM Electrical Discharge Machining

HAZ Heat Affected Zone

LED Light Emitting Diode

MRR Material Removal Rate

MT Machining Time

MOSFET Metal Oxide Semiconductor Field Effect Transistor

NOF Núcleo de Oficinas

PCB Printed Circuit Board

PWM Pulse Width Modulation

WPVA Workpiece Volume After Machining

WPVB Workpiece Volume Before Machining

xi

1

1. Introduction

First electrical discharge machining devices appeared during the Second World War. Studies regarding

material removal through electrical discharge arose in 1770 by the physicist Joseph Priestley, however,

it was only in 1943 that the scientist Boris and Natalya Lazarenko recognized the true potential of this

technology.

Electrical discharge machining is the most widely used non-conventional machining technique where

there is no direct contact between the tool and the workpiece, making it possible to machine materials

with high mechanical resistance. It has gained relevance over the years due to its ability to machine and

shape an ample range of materials, the only constraint being that they must be conductive. The electrical

energy used during the process is transformed into heat energy that will erode the workpiece. This

technology has a wide application field, such as mold and die manufacturing, small hole drilling,

electronic and optical devices, automotive industry or surgical instruments.

Electrical discharge machining holds many advantages. Not only it can machine complex shapes that

conventional machining processes cannot, but also a good surface finish can be achieved (yet, not as

good as chemical machining). Moreover, there is no mechanical stress on the machined piece. On the

other hand, the low material removal rate stands as a disadvantage, as well as surface modification of

the workpiece, heat affected zone and potential fire hazard, depending on the dielectric fluid used.

The present master thesis was marked by two distinct goals. The first objective was to characterize the

influence of several operative parameters on the crater morphology of a selected range of materials on

an existing machine (Grencho, 2015) but, the machine was not working properly and was not able to

perform discharges. The focus was to analyze all the electrical components to check if any of them had

a failure. To achieve that, a schematic of the whole electrical circuit was designed (see Annex A) because

it was not done previously, so it would be easier to track the error. Many problems were found, such as

malfunctioning voltage regulators, relays that were not supplied with enough voltage to turn both

command and power circuit on and a short-circuited PCB path that was not allowing one of the

secondary electrical circuits to function. All these problems were solved but the machine would not do a

single discharge. Therefore, the research goals were redefined to include the development of a new

testing machine and electrical power circuit. Knowledge of electrical components earnt from the previous

experience was a huge asset to project the new circuit. Many preliminary tests were performed on a

breadboard before soldering the final circuit into PCB. Finally, after the kinematic chain, structure,

command and electrical power circuit were built, trials could be performed while varying operative

parameters and materials.

This document is divided in five chapters, including the present introductory chapter. The second chapter

present the theoretical fundaments including some basic concepts of this technology. Follows the third

2

chapter, experimental development, which guides the reader through all the building phases of the new

EDM machine as well as the electrical circuit. A brief introduction to the measuring instruments used to

analyze the crater morphology is made. The last part of this chapter presents the experimental plan,

where fixed and variable operative parameters are identified as well as the materials studied in this

work. The fourth chapter aims to present and discuss the results obtained during this master thesis. A

detailed study was carried out on the material response to the operative parameter’s variation. The fifth

and last chapter brings up the main conclusions made after analyzing the results and a proposal for

future work.

3

2. Theoretical Fundaments

This chapter shows an overall vision of the Electrical Discharge Machining process. Topics such as

material removal mechanism, thermal effect on the eroded crater formation, types of electrical signature

or types of power supply are presented and explained.

2.1. Material Removal Mechanism

EDM is a process where electrical energy is transformed into thermal energy where the process

parameters, such as on-time pulse, current intensity or voltage are extremely important (Tsai, 2004). In

this technology there is no contact between the electrode tool and the workpiece leading to an absence

of mechanical stresses, vibration or chatter normally associated with conventional machining (Abbas,

2007). The electric field applied between the electrode tool and the workpiece produces a plasma

channel where the discharge takes place. The main difference between micro and macro-EDM is the

plasma channel radius (Mahendran, 2010).

The EDM process can be divided in five stages: i) pre-breakdown, ii) breakdown, iii) discharge, iv) end

of discharge and v) post-discharge, as shown in fig.2.1.

Figure 2.1 – EDM different phases (Descoeudres, 2006)

4

2.1.1. Pre-breakdown and breakdown

The material removal mechanism initiates when a potential difference is applied between the electrodes,

initiating the formation and growth of a conductive channel which is also known as the streamer. The

fast polarization of the dielectric medium is originated in the emissive electrode and forms the streamer

together with the free electrons of the dielectric fluid. Due to a transformation of kinetic energy into

thermal energy, resulting from the collision between charges and neutral particles of the fluid, the

temperature and pressure rise allowing the formation and growth of the streamer.

According to plasma physics, an electron that moves under the action of an electric field will gain kinetic

energy at some point. If this energy is superior to the gas ionization potential, it is possible that a collision

occurs between the electron and an atom or neutral molecule and, therefore, promoting ionization. The

plasma neutrality does not prevent the formation of an auto-induced field, resulting from the opposite

movement of anions and electrons. Apart from the current intensity and voltage put into the process, it

is also influenced by geometrical conditions, such as electrode and tool shape.

When the auto-induced field equalizes the applied electric field, the transition between the electrode

avalanche and the streamer propagation takes place. When the streamer reaches the other electrode,

the propagation of an ionized front occurs in the opposite way (toward the emissive electrode).

There are two types of streamer, depending on the polarity of the electrodes. Once the streamer is

started, it spreads and grows due to the random nature of its propagation mechanism (Descoeudres,

2006).

2.1.2. Phase of discharge

The first plasma develops as fast as possible against the front of the dielectric fluid. Very high pressure

inside the channel results in a shock-wave distribution within the liquid. The current passing in the gap

originates high temperatures leading to material evaporation in both electrodes sites. As electrons have

smaller mass than anions and show a faster reaction, the mainly worn out material is the one belonging

to the anode. Afterwards, temperature and current density decrease fast as the plasma channel grows.

Its diameter stabilises when equilibrium between the generator and energy supply is achieved. Heat

flows to electrodes and to further evaporation of liquid and into the dielectric fluid (Descoeudres, 2006).

The quantity of material removed by the EDM process is defined by several parcels. A small quantity of

material is removed during the vaporization and the bigger part of its removal occurs when the plasma

channel collapses (Wong et al., 2003).

The discharge energy, given by Equation 1, is in function of the voltage, V(t), the intensity of the current,

I(t), and discharge time, ta. At a low current, a small quantity of heat is generated and a substantial

portion of it is absorbed by the surroundings, the left of it is utilized in melting and vaporizing the work

5

material. But, as the current increases, a stronger spark with higher energy is produced, more heat is

generated and a substantial quantity of heat strikes the workpiece, therefore, promoting more material

removal.

Material removal rate (MRR) can be approximated by the ratio between the variation in workpiece

volume and pulse time (Khan, 2011). Also, the physical properties of the materials used as electrode

and workpiece must be considered, since, the physical properties influence a larger or smaller MRR.

The physical properties of the materials influence the level of difficulty to be machined. In 1997,

Reynaerts (Reynaerts, 1997) proposed an equation that quantifies the machinability of materials by

EDM (index of machinability, Cm). The properties that Reynaerts considered essential for the calculation

of this index (Equation 2) are: the specific heat, c (J/Kg.K), the thermal conductivity, λ (W/K.m), and the

melting point, Tm (K), being the index quite affected by the last one since in the equation it is raised

squared. Materials with a high machinability index can be used as tool electrodes, while materials with

a low machinability index are used as workpiece material.

Figure 2.2 – Schematic representation of heat exchanges in plasma-solid interaction during an electric discharge (Yeo, 2008)

W= ∫ V(t) I(t) dt ta

0

[Eq. 1]

Cm= λ∙ c∙ Tm 2 [Eq. 2]

Figure 2.3 represents the energy distribution measured by Xia (1996) in the continuous pulse discharge

which shows that the bigger part of heat is lost by conduction into the electrodes. Van Dijck (1973)

indicated that more than 90% of the heat is conducted into the electrodes and the volume of metal

ejected per the volume of molten metal is between 1 and 10%. According to Xia (1996), only 34% of the

energy goes into the workpiece, while 18% is lost into the gap and the remaining 48% of energy goes

into the tool electrode.

6

Figure 2.3 – Energy distribution in EDM process (Xia, 1996)

2.1.3. End of the discharge and post-discharge

During this phase, the inductive energy accumulated by the plasma in the discharge is released leading

to material removal. This inductive energy is originated by the quick current intensity variation and

exercises electromagnetic forces in the molten material generating the crater. The plasma channel de-

ionises very fast, however, the bubble stays quite long time in the gap. With de-ionization, temperature

and pressure also decrease in the plasma channel. Molten material at the electrode sites is overheated

because of the discharge pressure and starts to boil instantaneously and ejecting liquid globules

(Schumacher, 2004).

After the bubble and plasma channel collapse and the molten material retrieved, it is necessary to re-

establish the initial conditions in order to proceed to another discharge. The dielectric fluid plays an

important role in this phase as it promotes the flushing of debris and eroded particles to clean the

material surface and avoid short-circuits.

2.1.4. Thermal effect on crater formation

With the absence of significant mechanical action in the EDM process, it is possible to assume that

this technology is essentially based on thermal action. It is hard to study the response of machined

materials in micro-EDM due to several constraints, such as the limited area of discharge, because of

its dimension, the presence of the dielectric fluid, plasma interference when comparing to conventional

EDM where thermal and spatial gradients are higher (≈103 ºC / ≈102µm),.

Several investigators adopted many predictive models for the temperature distribution linked to the

process. Initially, a model was developed based on a heat source disk shaped where the authors

assumed that the temperature increase would only occur during the on-time and only 50% would go to

7

the workpiece. In 1983, Tariq & Pandey developed a model assuming the heat transfer through the

plasma channel would only be by convection and verified that 90% of the total energy carried over the

channel was equally distributed between the workpiece and the electrode. After several studies, a lot

of authors considered that the Gaussian distribution would be the best suiting model to the action of

the plasma channel on the workpiece (Marafona and Chousal, 2006).

Figure 2.4 – Gaussian heat source model (Khan, 2011)

For a Gaussian heat distribution, if the maximum heat intensity 𝑞𝑚 (fig.2.4) is at the axis of a spark and

its radius (𝑟𝑠𝑝) are known, the heat flux 𝑞𝑓 at radius R is given by:

𝑞𝑓(𝑅) =4.45𝑊𝑀𝐼𝑉

𝜋(𝑟𝑠𝑝)2

× 𝑒[−4.5(

𝑅𝑟𝑠𝑝

)2]

[Eq. 3]

Where 𝑞𝑓 is the heat flux (W/𝑚𝑚2), 𝑊𝑀 the fraction of energy utilized by the material (%), I is the pulse

current (Amp), V the gap voltage (Volt), R the radial distance from the axis of the spark (µm) and 𝑟𝑠𝑝

the plasma channel radius(µm).

For a better understanding of a discharge behavior it is necessary to employ knowledge acquired

through plasmas physics. By using an experimental apparatus which allows to obtain variations of

radiation flux of a plasma channel (fig.2.5(a)), Natsu (2004) verified that the plasma channel

temperature reaches its maximum temperature at half way from the gap and its minimum on the

cathode surface (fig.2.5(b))

8

(a) (b)

Figure 2.5 – (a) Simplified schematics of the experimental apparatus used by Natsu; (b) Spatial distribution of temperature between electrodes during the discharge (Natsu, 2004)

The temperature achieved in a discharge is directly influenced by the amount of current intensity

passing through the channel. The discharge current is directly related to the plasma channel

dimensions and the higher the current the higher its diameter (Natsu, 2004).

The electrical conductivity between electrode and workpiece affects the maximum temperature of the

plasma channel, this being at its highest point with higher thermal conductivity, making the Joule

heating effect the main source of thermal energy raising the discharge channel temperature and, thus,

erode both electrodes.(Marafona and Chousal, 2006) (fig.2.6)

Figure 2.6 – Trend of the maximum temperature in the discharge channel with electrical conductivity

9

2.2. Technological Process

2.2.1. Material Removal Rate

The mechanism of material removal is an abrupt splatter of the molten material coinciding with the

plasma channel breakdown. When the spark collapses and the hydrostatic pressure of the arc is

released, the dielectric rushes back to fill the gaps and the pressurized molten metal splashes from the

workpiece surface leaving a crater on the surface and some particles in the neighborhood (Shuvra,

2003). One of the factors that promote material removal is flushing the dielectric after the discharge.

When the flushing is efficient, there is no adhesion of the molten material to the workpiece, decreasing

the possibility of unwanted electric arcs which could compromise the production. From what could be

observed through the literature, it is unanimous that the factors which have an impact on the material

removal rate are the voltage and the current intensity. Material removal rate (Equation 4) is expressed

as the ratio of the difference in volume of the workpiece before and after machining to the machining

time, as follows:

WPVB WPVAMRR

MT

−= [Eq. 4]

Where WPVB and WPVA are the volumes of the workpiece before and after machining and MT is the

machining time. By raising the current intensity it is possible to obtain a higher material removal rate,

but the electrode wear will also raise (Khan, 2011).

10

Figure 2.7 – Relationship between current and MRR (Khan, 2011)

Relationship of MRR with current during machining of steel and aluminium using copper and brass

electrodes are shown in fig.2.7. It is to be taken in consideration that at low current MRR is very low but

increasing the current will increase MRR strongly. At a low current, a small quantity of heat is generated

and a considerable amount of it is absorbed by the surroundings and by the machine components. The

left of it is used in melting and vaporizing the workpiece material. Increasing the current leads to a

stronger spark with higher energy, therefore, more heat is produced and a substantial amount of heat is

utilized in material removal (Khan, 2011).

2.2.2. Superficial condition

The superficial integrity of a component and its dimensional accuracy are important aspects when it

comes to characterize the performance of the machining process. It was suggested by Lee (2004) that

the superficial integrity of a machined workpiece is evaluated by its surface roughness, cracks, white

layer and the residual tensions. In EDM, the thermal action of the material removal mechanism not only

does change the workpieces surface, but also does its subsurface. Kruth et al. (1995) affirm it is possible

to identify three different layers in the Altered Metal Zone (AMZ) shown in fig.2.5(a). The first layer results

from the molten material ejected from the crater and small amounts of material stemming from the

electrode. This material layer is easily removed because it is derived from a bad flushing. The next layer

is known as the white layer. It is originated by an abrupt cooling of the remaining material in the crater

which was not flushed by the dielectric fluid. Since this layer is exposed to elevated temperatures during

the discharge, its material earns high hardness and brittle characteristics and suffers crystalline and

metallurgical changes. Micro-cracks can be observed in the layer and, if not removed, the component

can fail at early stage. The last layer is in contact with the material base and denominated by the Heat

11

Affected Zone (HAZ) which is only heated up but not molten (Kumar et al., 2009). The white layers

thickness is an interesting point, because, if not controlled it could lead to a premature fail of the

component in function of its use. Lee (2004) says that the white layer is influenced by the energy

delivered to the process, backing up with his experiments that the thickness is influenced by the current

intensity and by the on-time pulse ( onT ) (fig.2.8(b)).

(a) (b)

Figure 2.8 – (a) Surface layers after electrical discharge machining; (b) Relationship between the average white layer thickness and EDM parameters; (Kruth et al., 1995)

2.3. Electrical Signature

Throughout the years, the scientific community studied several outcomes for the operative

parameters involved in this process, such as the voltage, current intensity, dielectric fluid, relating them

to the material removal rate and the electrode wear (Shankar, 1997). However, due to the elevated

complexity of the phenomena involved in EDM, the concrete characterization of the plasma channel is

not perfectly established (Descoeudres, 2006). Thus, a deeper investigation on plasma is highly

important to understand the MRR in order to enhance the characteristics of the machined surface

(Natsu, 2004) and, therefore, minimize problems related to the stochastic nature of the EDM process

(Descoeudres, 2006). Several investigators consider that the current is constant along the discharge.

However, the power supply circuit has a high influence on how the current is delivered to the process.

12

2.3.1. Types of discharges

In a normal discharge there is passage of electrical current through a spark. The ionization time should

be adequate, so a stable plasma channel can be originated allowing current to flow.

A high gap between the electrode and the workpiece will not establish the necessary conditions to

ionize the dielectric fluid. Thus, without plasma channel, there will not be a discharge and for this

reason the open discharge should be avoided.

When there is contact between the electrode and the workpiece, there is a short-circuited discharge.

The necessary conditions to ionize the dielectric fluid cannot be established and there is no plasma

channel formation, therefore, there is only current flowing between electrode tool and workpiece

material. As the current flows easily between materials, there is an increase of local temperature

through the Joule effect which does not contribute to the material removal. Theoretically, the voltage

value is slightly different than zero (due to electrode resistance), but never enough for the dielectric

fluid to ionize.

An arc discharge does not create a stable plasma channel due to a defective ionization of the

dielectric fluid. Although the voltage is higher than the short-circuit case, it does not reach the typical

value before the discharge.

(a) (b) (c) (d)

Figure 2.9 – Types of discharges: (a) Normal; (b) Open; (c) Short-circuit; (d) Arc

2.3.2. Types of Power Supply

An efficient EDM process is essentially defined by the type of power supply used to convert AC current

into a one-directional current pulse in order to have a discharge (Shah et al., 2007). The material removal

13

is directly proportional to the quantity of energy put into the system during the on-time. This energy is

controlled by the maximum current intensity and the on-time (Kumar et al., 2009). Using long time pulses

will increase the amount of molten material. Thus, craters exhibit superior diameters and depths

comparing to short time pulses. However, molten material expelled from the crater is a contributing factor

that increases the machined surface roughness. Due to an increase of the thermal energy to the

workpiece the HAZ thickness raises considerably (Kumar et al., 2009).

When machining high dimensional parts, the probability of occurring uncontrolled discharges is very

high. Nonetheless, these discharges could not cause any damage to the part because it is machining a

huge surface, in some cases it can increase the material removal rate lowering in this way the machining

time. On the other hand, in micro-EDM, these discharges are unwanted and intolerable, to avoid it there

must be a highly rigid control of the currents and voltage applied to the system (Shah et al., 2007).

Rotary Impulse Generator

The first power supply used in this kind of process was the rotary impulse generator, shown in fig.2.10.

The voltage waveform is created based on the DC generator principle. It creates a sinusoidal wave

pattern comparable to rectification. There is no way to control the waveform, so this type of power supply

is rarely used.

Figure 2.10 – Rotary Impulse Generator (Shah et al., 2007)

RC Circuit

This circuit was created by the Russian scientists Lazarenko, based on the charging and discharging of

a capacitor wired to a power supply. Accumulated energy in the capacitor is discharged when the gap

between tool and workpiece takes place, creating a spark. When there is no energy enough to sustain

the discharge, the capacitor re-charges. Current intensity can be controlled through resistors, however,

high discharge times cannot be achieved, and pulse times are hard to control.

14

Figure 2.11 – RC Circuit (Shah et al., 2007)

Pulse Generator

With the evolution of technology, the transistor appeared and turned it easy to have a more precise

control of the power and to have a better control of the on and off times (fig.2.12). Through the signal

modulation it is possible to control the ionization time and the current density of the process, restraining

the behavior of the plasma channel and, increase the production efficiency and surface finish of the part.

Joining integrated circuits to this generator has enabled to control pulses in the power supply and to

guarantee that the discharge energy is delivered to the workpiece. This makes it possible to automate

the machine and the process itself. This circuit has had many evolutions throughout the years. One of

them is called isopulse which can control the ionization time and is able to keep the off time and the

discharge time constant. This automation leads to an increase of successful discharges and, therefore,

the material removal rate. The process is thus more efficient and predictive (Gaspar, 2010).

Figure 2.12 – Pulse Generator (Shah et al., 2007)

15

3. Experimental Development

This chapter aims to present every construction stage of the experimental apparatus. It begins with an

introduction of the various modules that compose the equipment, followed by a description of the

measure instruments, which allows a deeper study on the material removal end electrical signature.

Aspects such as the circuit development, kinematic chain of the 3-axis machine, the power source

renovation will also be discussed in this chapter. Finally, an experimental plan will be proposed which

will be studied in chapter 4.

3.1. EDM Machine

In order to simplify the presentation of the equipment, this will be divided in five main modules as follows:

(i) Structure, kinematic chain and gap control; (ii) Pulse Width Modulation (PWM) apparatus; (iii) Safety

commutator; (iv) Resistor switch module (v) Power commutator.

Structure, kinematic chain and gap control

In order to control the electrode position towards the workpiece, a market study was done to find the

best suitable machine for the purpose, taking in consideration the ratio quality/cost. The option chosen

was the Woodpecker CNC (fig.3.1). This machine turned out to be an excellent choice because it is

simple to operate, easy assembly, using straightforward software, which allows the user to set several

instructions and semi-automate the process. It is equipped with three step motors which enables the

control the x,y and z axis. The milling tool was switched to a copper electrode placed at the tip of the z

axis.

16

(a) (b)

Figure 3.1 – (a) Woodpecker CNC used for the kinematic control of the electrode; (b) Grblcontrol interface and explanation of

main commands

The home button is commonly used when the discharge is completed so, the electrode goes up a few

centimeters and it is easier to prepare for the next discharge. Zero XY locks the offset at a certain

coordinate for X and/or Y. Zero Z locks the offset for the Z coordinate and it was very useful throughout

this work because it made possible to semi-automate the process, as the user only had to place the

electrode on the correct x,y position and the CNC would do the zero. After that, the user controls the

gap and the EDM machine is ready to make another discharge. The jog controller makes the machine

move to the desired point, where, 1 and 2 is the x direction, left and right respectively, 3 and 4 the y

direction, forward and backward respectively, 5 and 6 controls the z direction (gap control), up and down

respectively. There is a possibility to introduce a G-code to make the machine run by itself. This software

lets the user choose the movement step, which is crucial to control the gap, typically around 10µm.

Where 1 is equivalent to 1cm, therefore, 0.01 is equivalent to 10µm, which is not precise enough for the

purpose of this work. The need to lathing a new shaft with a step 10 times inferior arose.

17

(a) (b)

Figure 3.2 – (a) First shaft with 10µm precision per step; (b) Manufactured shaft for EDM with 1,25µm precision per step

The previous shaft had a 2mm step, therefore, to obtain a 1µm precision, it was mandatory to

manufacture a shaft with 0,2mm step but it would not be made in time and within the budget. The best

and fastest solution found was to manufacture a 0,25mm step shaft which would meet a precision of

1,25µm.

After solving the shaft problem, another issue was found. It was not possible to guarantee the total static

stability of the structure. Therefore, a base had to be projected in order to fix the structure to it. This

solution changed considerably the stability which now is almost perfect and can secure stable trials

during the whole process. The machining stage of the base and the final result are illustrated in the

following pictures.

(a) (b)

Figure 3.3 – (a) Machining the base of the structure; (b) Structure of the final machine

18

Finally, before the machine become operational and ready to carry out an EDM process, it was

mandatory to make sure the machine was electrically insulated from the power circuit. In order to

achieve that, an acrylic base was conceived (see fig.3.4(b), number two) to separate the workpiece (i.e.

one of the electrodes) from the x,y-axis base. Another concern was the copper electrode, installed on

the z-axis. A polymer tube (see fig.3.4(b), number one) was lathed with an inner diameter equal to the

external diameter of the electrode, so it would fit by pressure. Both acrylic and polymer parts can be

observed in fig.3.4(b). Upon completion, the machine was ready to be used.

Pulse Width Modulation (PWM) apparatus

The control circuit is mainly composed by the Pulse Width Modulation (PMW) apparatus. The equipment

used for this purpose is the TGP110 10MHz Pulse generator, manufactured by Aim & Thurlby Thandar

Instruments. This equipment allows the user to select several working modes such as square wave,

double pulse, delayed pulse or single pulse. The aim of this project is to study the single pulse discharge

in EDM, thus the chosen mode was the single pulse. In addition to these options, the user can also

define the period, pulse width, pulse delay and amplitude. The period defines the frequency of the pulse.

The pulse delay is the delay from the manual trigger, and the amplitude sets the intensity of the pulse.

The most important option for this project is the pulse width because it is going to define the on-time of

the discharge. The picture below shows the apparatus used for the EDM process. (fig.3.4)

(a) (b)

Figure 3.4 – (a) TGP110 10MHz Pulse Generator (PWM); (b) PWM response observed in the oscilloscope

19

Safety commutator

Usually the power circuit is galvanically insulated from the control circuit in order to avoid interferences

from the high voltage/current side. Also, the main supply should be connected to the circuit via a circuit

breaker, or another switch which protects from any overcurrent or hazard contact. The author chose to

employ the FOD617 Phototransistor Optocoupler (see Annex C) to keep the apparatus safe. The

principle of an optocoupler is based on insulation by optical medium. The optocoupler is composed by

two sides, as shown in fig.3.5(a), which are isolated from each other and do not have electrical contact.

The first side is connected to pins 1 and 2 which contains a LED that emits infrared light when the control

system is turned on. This light will be detected by a phototransistor (pins 3 and 4) that is light sensitive

and, thus, it switches on and starts to conduct current as an ordinary transistor might.

(a) (b)

Figure 3.5 – (a) Optocoupler symbol;(b) Physical functioning of an optocoupler (searchnetworking.techtarget.com)

To better understand how the EDM circuit was built and how it works, we take in consideration fig.3.6(b).

In a first stage, the PWM sends a pulse time, previously defined by the user, to the optocoupler (“In”).

The optocoupler receives that information and the led will emit infrared light proportional to that

information to the phototransistor, which in turn will activate the power circuit (“Out”).

Resistor Switch module

This module enables the user to modulate the discharge current. Different combinations of 25Ω resistors

were used to achieve currents of 4A, 8A and 16A at 200V. The module is presented in fig.3.6.

20

(a) (b)

Figure 3.6 – (a) Front of the switch module; (b) Inner side of the switch module

To obtain a current of 4A at 200V (fig.3.6(b), number one), two 25Ω resistors were put in series, that will

give 50Ω.

𝑉 = 𝑅𝐼 ⇔ 𝐼 = 200

50= 4𝐴

To have an 8A current at 200V (fig.3.6(b), number two), a 25Ω resistor was used.

𝑉 = 𝑅𝐼 ⇔ 𝐼 = 200

25= 8𝐴

Finally, to achieve a 16A at 200V (fig.3.6(b), number three), two 25Ω resistors were put in parallel, thus,

obtaining 12,5Ω.

𝑉

𝑅𝑒𝑞=

𝑉

𝑅1+

𝑉

𝑅2⇔ 𝑅𝑒𝑞 = 12,5Ω

𝑉 = 𝑅𝐼 ⇔ 𝐼 = 200

12,5= 16𝐴

Power commutator

The power module is the most important part of the circuit. This module was the one that took more time

to be developed since it requested many tests and simulations on a breadboard before turning it into

the final circuit. The main focus of this stage was to understand how a Metal Oxide Semi-Conductor

Field Effect Transistor (MOSFET) works and how it could be inserted in a circuit to act as a switch to

control the discharge time. MOSFET is one of the most used type of transistors in electronics. It is

composed by the gate (G), drain (D) and source (S) and is widely used for switching and amplifying

electronic signals, up to several MHz. To turn a MOSFET on, as a switch, the gate-source voltage must

be greater than the threshold voltage ( VGS > VTH ).

21

Knowledge acquired by reading Electronic Principles by Albert and J., 2007 and EDM how-to-book by

Benjamin Fleming, 2006 turned out to be a huge asset.

(a) (b)

Figure 3.7 – (a) MOSFET and Optocoupler test; (b) Testing the whole circuit

Fig.3.7(a) shows a MOSFET (IRFP 450, see Annex D) being tested. The PWM would send the desired

on-time to the optocoupler and it would transmit to the MOSFET gate how long the user decided to let

the MOSFET conduct, thus, acting as a switch. The user could visualize the response through the LED

installed directly after the MOSFET, to simulate a discharge. Fig.3.7(b) is the same breadboard test, this

time with all the components of the EDM circuit being used. The discharge was simulated into a 40W

lamp. To obtain the final circuit, good knowledge of tin soldering was acquired, and an experimental

procedure is presented in Annex B.

Figure 3.8 – Complete circuit schematics

22

Colors were used to identify each module of the circuit. The yellow area represents the electrode and

the workpiece. Even though it is not represented in the schematics, this is where the kinematic structure

is located. The purple space is the PWM, used to control the discharge. The green line determines the

safety commutator, the blue line delimits the power switch module and the red line identifies the power

commutator. Fig.3.8 presents the final circuit for EDM. Firstly, a siemens transformer is connected to the

public electricity distribution to galvanically isolate the circuit from the network. This transformer is

connected to a variable transformer that allows the user to choose the value of the AC voltage to be

used. The output of the variable transformer has a fuse to protect the power system. Then this AC

voltage goes to a AC/DC bridge rectifier, with an output capacitor for filtering, allowing the setting of the

maximum experimental voltage amplitude defined by the user, through the variable transformer. In order

to do a discharge, the user needs to press the manual trigger button of the PWM which will turn the

MOSFET on during the predefined time, voltage flows through the resistor switch module, which allows

to modulate the current, and at the terminals of the electrodes a discharge occurs. Two 5408 diodes

were installed to prevent the current to flow backwards and to set a free-wiling path for any inductive

current when the MOSFET turns off. When trials are finished the capacitor should be unloaded by

switching on a 40W lamp so the energy is consumed by it.

(a) (b)

Figure 3.9 – Final result; (a) Top view; (b) Front view

23

3.2. Measuring Instruments

With the purpose of studying craters morphology obtained through the EDM process, as well as the

electrical signature of the process, several measuring instruments were used, namely, a Confocal Laser

Scanning Microscope (CLSM) used to measure the depth of the crater, an optical microscope (Union

Versamet 3) that will allow to observe and measure the crater and its diameter. An oscilloscope

(Tektronix TDS 2004B) provides the electrical signature of the discharge. The instruments will be

presented as follows.

(a) (b) (c)

Figure 3.10 – (a) CLSM; (b) Optical microscope; (c) Oscilloscope

The CLSM (fig.3.10(a)) machine is equipped with a beam laser, MG 35 STILSA, which sends photons

to the surface and computes the relief of the material, based on the time it takes for the photons to reach

the surface. It was used to read the depth and compile the acquired data in a graphic, shown in fig.3.11.

Several measurements were taken for each of the variable operative parameter to obtain an average

depth. The x axis represents the material surface and the z axis the crater depth.

(a) (b) (c)

Figure 3.11 – Crater depth measurement: (a) Zinc at 4A; (b) Aluminium at 8A; (c) Tin at 16A

24

Acquiring data from the CLSM was not always easy. When a surface is very well polished, it becomes

very reflective and, therefore, very hard for the beam laser to focus in a single point since it gets reflected

into different directions, making the scanning unsuccessful. This problem appeared while scanning both

Tin and Zinc, which correspond to the craters with greater depth. To solve this problem, sulfuric acid

was applied on the surface, in order to corrode it and turning it not reflective. The result can be observed

in fig.3.12.

(a) (b)

Figure 3.12 – (a) Sulfuric acid applied on Tin; (b) Sulfuric acid applied on Zinc

In fig.3.12(a) the acid was let settle for ten minutes, while in fig.3.12(b) it settled for five minutes. Both

experiments turned out to let settle for too long, as this technique is not mastered. It is recommended to

let the sulfuric acid act not longer than a minute. This recommendation is based on other experiments

done during this project.

The optical microscope needed to be calibrated in order to have a better reading of the crater diameter.

This calibration is shown in fig.3.13(a). Before doing a set of trials, it was designed a matrix on the

workpiece to know exactly which operative parameter belonged to each parcel, displayed in fig.3.13(b).

25

(a) (b)

Figure 3.13 – (a) Microscope calibration; (b) Workpiece matrix

To have an accurate measurement of the voltage and current flow during the process, a voltage probe

(Hameg Differential Probe HZ100, fig3.14(a)) and a current probe (Bergoz CT-B1.0-B, fig.3.14(b)) were

used. Both probes are directly connected to the oscilloscope to provide the reading of the electrical

signature.

(a) (b)

Figure 3.14 – (a) Voltage differential probe (200x signal attenuation); (b) Current probe

26

The type of circuit used to study the mono-discharge is the transistor circuit shown in fig.3.15(a). Some

examples of electrical signature obtained by the oscilloscope are presented in fig.3.15(b); (c); (d). The

electrical signatures expose the voltage and current put into the process and it is a way of monitoring

the discharge.

(a) (b) (c) (d)

Figure 3.15 – (a) Transistor circuit; Electrical signature: (b) Zinc (t_on=50µs, I_e=4A, U_e=200V); (c) Aluminium (t_on=50µs, I_e=8A, U_e=200V); (d) Tin (t_on=50µs, I_e=16A, U_e=200V)

3.3. Experimental plan

The purpose of this work is to study the erosion process and the craters morphology. To achieve that,

an experimental plan was suggested to have a clear and organized idea of the work to be done.

Materials in study are nearly pure materials, as listed: Zinc (99,99%), Tin (99,99%), Copper (99,99%),

Aluminium (99,99%) and AISI 304. The material choice was mostly influenced by the fact that these

elements have known thermodynamic properties making it easier to compute a theoretical estimation of

the crater morphology. To make sure the discharges were successfully observed and read by both

optical microscope and confocal laser scanning microscope, all material surfaces were carefully

polished using normal sandpaper in a first stage and after diamond paste with varied grain size, from

the highest to the lowest, for a better polishment.

Relatively to the operative parameters, the open voltage, on-time, gap and polarity were fixed and

constant along the trials. It was decided to vary the current intensity during the diverse trials. Both fixed

and variable parameters are displayed in tables 3.1 and 3.2, respectively.

The dielectric fluid used in this work was the glycerin. A good dielectric fluid should have stable dielectric

strength, its flashing point should be high enough to avoid any fire hazard, not emit any toxic vapours,

chemically neutral to the electrode and workpiece and should maintain its properties under all working

conditions (Khan, 2011). Glycerin is not the typical dielectric fluid used in this kind of process, but it

seemed to be a good idea to bring a different case study. To study glycerin’s changing factor, some tests

were made with deionized water, to have comparison between both dielectric fluids.

27

Table 3.1 – Fixed operative parameters

Fixed operative parameters

Open voltage (𝑼𝟎) On-time (𝒕𝒐𝒏) Gap Polarity

200V 50µs 10µm Reverse

Table 3.2 – Variable operative parameters

Variable operative parameters

Current intensity (𝑰𝒆) Dielectric fluid

4A, 8A, 16A Glycerin and Deionized water

4. Results

This chapter presents and discusses the main results of the experimental research. Response from

different metallic materials was carefully studied when machined by the EDM process. The current

intensity was varied for the same gap, on-time and open voltage for each of the materials. The first

analysis is mainly visual as the crater’s morphology is observed through the microscope. The second

analysis is more detailed, the diameter and depth of the crater was measured and studied in order to

make some comments and conclusions about the influence of the operative parameters, as well as the

dielectric fluid. Thirdly, the specific electrical erosion energy was calculated and a relation between this

energy and the Reynaerts index was made. Lastly, a comparison between two dielectric fluids used

during this work, deionized water and glycerin, has been carried out. It is important to highlight that all

results are based in average measurements to obtain more accurate results.

4.1. Current influence on different materials

Several tests were made on different materials to quantify the influence of the electrical current on the

material removal mechanism of the EDM process. According to the available literature, several authors

affirm that discharges with high current produce large craters and remove far more material than low

current discharges. In this investigation, eroded craters were observed through an optical microscope

and some of the samples acquired are illustrated in figures 4.1 and 4.2, regarding Tin and Copper, for

different electrical current intensities (4,8 and 16A).

28

Picture of the craters were taken by the optical oscilloscope’s software AMCap and, thereafter,

measured with the help of an incorporated microscope scale. Copper and Tin crater examples were

chosen to be displayed in fig.4.1 and fig.4.2 to show their morphology and how they were measured.

(a) (b) (c)

Figure 4.1 – Tin craters: (a) 4A; (b) 8A; (c) 16A; lower scale division is equivalent to 20µm

(d) (e) (f)

Figure 4.2 – Copper craters: (d) 4A; (e) 8A; (f) 16A; lower scale division is equivalent to 20µm

These two materials were chosen as reference in this research because, according to Reynaerts and

his machinability index, Copper is the hardest material to be machined and Tin the easiest. Copper

having the highest machinability index is more suitable for electrode tool and Tin, with the lower index,

is more suitable for workpiece, despite its mechanical properties. Looking at the pictures, the crater

diameter of Tin is clearly wider than Copper.

29

Table 4.1 – Material properties and Reynaerts machinability index, data provided by MatWeb

AISI 304 Aluminium Copper Tin Zinc

Melting point (K) 𝑻𝒎 1430 933,5 1360 505,1 692,7

Heat conductivity

(𝑾 𝑲−𝟏𝒎−𝟏) λ 401 210 385 63,2 112

Specific heat ( 𝑱 𝑲𝒈−𝟏𝑲−𝟏 ) 𝑪𝒑 700 900 398 232 390

𝑪𝒎 (𝟏𝟎𝟏𝟏𝑱𝟐(𝒎. 𝒔. 𝒌𝒈)−𝟏) 2,81 1,65 2,83 0,0374 0,21

Average diameters were used to build fig.4.3. Overall, circular morphology has been the typical

observation in the optical microscope, yet, EDM is a stochastic process and, craters could be random-

shaped instead of having a circular morphology.

Figure 4.3 – Crater diameter (µm) vs Current intensity (A)

Figure 4.3 shows that most of the eroded crater diameters tend to increase linearly with electrical current

intensity with some convergence to the graphic origin, since an eroded crater cannot be achieved with

no current. Schumacher (2004) has found in his experiments that a minimum of 3A is needed to start

discharges, which is the requested power for particle evaporation.

Tin provides the larger eroded craters for high electrical current values. On the contrary, Copper usually

presents the smaller overall dimensions for the eroded crater. Copper is followed by Aluminium, that

displays the second lowest crater diameter, but, for the intensity of 8A it is lower than Copper. Zinc and

AISI 304 have a very similar behavior when currents of 8A and 16A are used, however, this behavior

diverges when the current of the process is set to 4A. If we take into consideration table 4.1, where

material properties are displayed, and, afterward, the Reynaerts resistance index is calculated, materials

0

50

100

150

200

250

300

350

0 5 10 15 20

Cra

ter

Dia

mete

r (µ

m)

Current intensity (A)

Zinc

Copper

Tin

Aluminium

AISI 304

30

that appear to be more suitable to be workpieces are Tin and Zinc. The best material to be used as a

tool electrode is Copper. This assumption is backed by the graphic in fig.4.3, which shows a concordant

achievement, as the bigger craters are both Tin and Zinc, while the shortest craters belong to Copper.

Another important aspect to be studied about the eroded crater morphology is the crater depth. This

aspect also seems to verify Reynaerts machinability index because, for a material with lower

machinability index, a deeper crater is obtained. For the material with lower index (Tin) it is possible to

see in figures 4.1(a), (b) and (c) a light reflex, which indicates that the crater is deep. These assumptions

can be verified observing fig.4.4.

Figure 4.4 – Crater depth (µm) vs Current (A)

As it was said and shown earlier, AISI 304, Copper and Aluminium have the highest machinability index

(table 4.1) and, therefore, the Reynaerts theory is corroborated by analyzing fig.4.4, as these materials

have the lower depth among the others. Although AISI 304 has higher machinability index than

Aluminium, its crater depth is almost as deep. A possible reason for this fact is that AISI 304 possesses

a lower specific heat value and, consequently, less energy must be delivered to the process to remove

material from it. All three materials show an almost regular behavior while the electrical current is varied.

Zinc has the second lowest machinability index and, by looking at fig.4.4 it is easy to affirm that it stands

out from the materials mentioned above. This is a good indicator that its material removal rate is

generally higher than the others.

Special attention should be given to Tin. Its crater depth is almost twice as big as Zincs, nearly five times

deeper than Aluminium and AISI 304 and ten times Copper’s depth. This probably happens because of

Tin’s thermal and electrical properties: (i) very low melting point, making it easier to remove material

through heat; (ii) low heat conductivity, so, instead of spreading heat along the material surface, it is

more likely to concentrate heat where electrons strike, originating a deeper crater; (iii) low specific heat,

0

10

20

30

40

50

60

0 5 10 15 20

Cra

ter

depth

(µ

m)

Current (A)

Zinc

Copper

Tin

Aluminium

AISI

31

the amount of heat required for a single unit of mass to be raised by one degree of temperature.

Moreover, a very neat and clean crater was formed, which can be seen in pictures below.

(a) (b)

Figure 4.5 – Eroded craters observed through the optical microscope: (a) Tin at 16A; (b) Zinc at 16A; lower scale division is equivalent to 100µm

The craters showed in fig.4.5 did not have a cleaning procedure (i.e. no ultrasonic cleaning), they were

eroded and directly put in the microscope to be observed. This figure sums up the results obtained from

the present study, which are very divergent from the previous works regarding this subject (Grencho,

2015 and Sousa, 2017). The percentage of circular craters for each set of assays was considerably

elevated and craters with higher dimensions, deeper and well defined were attained. The present

kinematic chain and machine structure were projected with the aim of providing total stability and

perpendicularity between axis while doing mono-discharge experiments. When comparing with

preceding works, the difference lays on this exact matter, perpendicularity. The z-axis of the previous

machine, which controls the gap, was unstable which led to a process instability and would not allow

the plasma channel to form naturally, it would develop along its slope and, thus, different craters would

result from it.

32

Figure 4.6 – Ratio between diameter and depth vs current

As it can be seen in fig.4.6, Copper, Aluminium and AISI 304 have the highest diameter/depth ratio, even

though Copper is highlighted from the other two materials. All three material curves follow a similar

pattern. The reason behind this might be the fact that Copper, Aluminium and AISI 304 have a much

higher melting point, specific heat and thermal conductivity than Tin and Zinc. As a result, the plasma

channel tends to expand radially instead of axially. Once again, the Reynaerts number is a good

qualitative index for 8A and 16A.

Tin and Zinc’s ratio between the diameter and depth of the eroded crater tend to decrease as the current

is increased. In contrast with the other three materials, these two have a low melting point and heat

conductivity. Additionally, the combination of both material and thermomechanical properties causes this

ratio to decrease, because, in Tin’s case, the evaporation temperature of glycerin is near its melting

point. This fact makes heat to not only flow radially, but also axially into the workpiece. It is possible to

speculate that both curves tend asymptotically to 1 as the current increases. To conclude, for Tin and

Zinc’s case, the more the current increases the more diameter and depth approach from each other.

4.2. Specific electrical erosion energy

Specific electrical erosion energy is the energy required to remove a volume unit of material. Considering

the previous section, where both crater diameter and depth were studied in detail, it was possible to

estimate the removed volume from the eroded crater. To do so, it was necessary to find a model that

intend to express the geometrical morphology of the eroded crater. The model considered was the

spherical calotte and it is represented in the next figure alongside with the bowl-shaped crater.

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20

Dia

mete

r per

depth

Current (A)

Zinc

Copper

Tin

Aluminium

AISI 304

33

(a) (b)

Figure 4.7 – Crater morphology: (a) Spherical calotte; (b) Bowl-shaped crater; (Bragança, 2013)

Previous works decided to compute the eroded crater volume based on average values while simplifying

its calculation by approximating the volume to a cylinder volume. It was decided not to do so, because

it would not be an accurate result. The way used to compute the eroded crater volume was by making

a relation between the spherical calotte and the volume of a cylinder, obtaining a corrective factor whose

formula is:

𝑓𝑐(𝑟, ℎ) =𝑉𝑐𝑎𝑙𝑜𝑡𝑡𝑒

𝑉𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟= 0,5 +

ℎ2

6𝑟2 [Eq. 5]

Where h is the eroded crater depth and r the eroded crater radius. Using the cylinders volume as a

reference, the eroded crater volume is going to be computed by calculating the cylinder volume and

applying the corrective factor in the same equation, as follows:

𝑉𝑐𝑟𝑎𝑡𝑒𝑟 = 𝜋𝑟2ℎ𝑓𝑐 [Eq. 6]

The corrective factor is always in function of each crater radius and depth. It can be withdrawn from

fig.4.8.

34

Figure 4.8 – Corrective factor as a function of the radius per depth ratio

All the elements are present to compute the eroded crater volume so, the next step is quantifying the

energy put into the process. Using equation [1] and consulting the electrical signature of each discharge,

it is possible to calculate the energy delivered to the process. The specific electrical erosion energy is

the ratio between the energy delivered to the discharge and the volume removed during the process,

expressed in (J/mm³). Combining all these values it is now possible to build a plot to express the energy

per volume as a function of the current, shown in fig.4.9.

Figure 4.9 – Energy per volume unit (J/mm³) as a function of Current (A)

The energy input is in function of the pulse duration, current intensity and polarity of the electrodes and

it depends upon the fraction of energy transferred to the workpiece (Singh, 2012). As it can be observed

in fig.4.9, Copper is the material that needs more energy to remove one unit of volume which is in

0,4

0,5

0,6

0,7

0 5 10 15 20 25

Fc

r/h

Factor correctivo

1

10

100

1 000

0 5 10 15 20

Energ

y per

volu

me u

nit (

J/m

m³)

Current (A)

Zinc

Copper

Tin

Aluminium

AISI 304

35

agreement with the Reynaerts theory. Both Aluminium and AISI 304 behaviors are considered to be

typical engineering materials, because they possess an elevated machinability index. Copper and Tin

assume an asymptotic performance when the current intensity rises. Values for Tin and Zinc, at 4A do

not seem to be in concordance with the other testing materials. A clear example is, at 4A, where Tin

needs more energy to remove material than Aluminium, AISI 304 and Zinc. In one way, an error

associated with measurements could be the source of the problem because, this error is greater when

measuring smaller crater dimensions, in comparison to bigger ones. To back this theory up, if we have

a look at 16A, values are in agreement with Reynaerts machinability index and physically possible, as

it is easier to quantify the removed material. On the other hand, discharge times for 4A were not as

constant as 8A and even less constant as 16A. This affirmation is based on electrical signatures

collected and analyzed from the oscilloscope. This fact has an impact on the energy delivered to the

process for each discharge and, therefore, on the ratio mentioned above. The discharge voltage is not

always regular, but it is not a controllable parameter of EDM. It can also be speculated that the MOSFET

might not be switching on and off as fast as desired, leading to different on-times and, thus, to different

energy delivery to the process, for each single discharge, because higher on-time pulses generate more

powerful discharges.

A minimum energy for the plasma channel formation must be considered. After the correct increase of

the plasma volume, the energy will be shared between the plasma stabilization, losses in the dielectric

and the electrodes. When using low current, in EDM, the plasma channel struggles to form and if it does

so, few energy will be left for an effective material removal. As the current increases, more energy is put

into the process and, thus, more material will be removed. This is the reason why all material curves

tend to descend as the current raises and for a certain current value they start to be constant.

36

Figure 4.10 – Specific electrical erosion energy as a function of the Reynaerts index

Fig.4.10 shows the relation between the specific electrical erosion energy and the Reynaerts

machinability index, and it demonstrates that a correlation exists between the specific electrical erosion

energy and the Reynaerts machinability index. There is a minimum energy that needs to be provided to

the process in order to be able to machine materials. It also tells that there is no need to input more

energy than the red line, which should be the maximum value for the manufacturing process to be

profitable. The values in between the stripes show a linear correlation. This could be helpful when trying

to machine different materials by EDM, once the person knows the Reynaerts machinability index, the

specific electrical erosion energy that should be put into the process is located between the stripe’s

boundaries.

4.3. Comparison between glycerin and deionized

water

The dielectric fluid chosen to be used in this project was glycerin. It is not the typical dielectric found in

the literature, so it was interesting to compare results obtained with a more often used dielectric fluid,

deionized water. Water based dielectric in EDM process brings high thermal stability with tolerance of

1,00E+00

1,00E+01

1,00E+02

1,00E+03

1,00E+04

0,00E+00 1,00E+11 2,00E+11 3,00E+11

E/V

(J/m

m³)

Reynaerts machinability index

Zinc 4A

Zinc 8A

Zinc 16A

Tin 4A

Tin 8A

Tin 16A

Copper 4A

Copper 8A

Copper 16A

AISI 304 4A

AISI 304 8A

AISI 304 16A

Aluminium 4A

Aluminium 8A

Aluminium 16A

37

high input parameters allowing a higher material removal rate (Singh, 2012). These two fluids have

different chemical properties and, therefore, will have different actions along the process. As it is known,

deionized water has a boiling point of 100ºC while glycerin 290ºC. Also, deionized water has a density

of 1 g/cmᶟ against 1,26 g/cmᶟ for glycerin. The higher density of dielectric fluids makes them more difficult

to breakdown and radial plasma expansion, therefore, it requires a higher electric field (Descoeudres,

2006). As glycerin density is higher, it restricts the plasma channel and the electrons struggle more to

pass through it. According to König (1987), working with glycerin is an advantage, compared with

hydrocarbon dielectrics, when using long pulse durations and high pulse duty factors and discharge

currents for roughing range and positively polarized electrode. These electrons will strike the workpiece

at a higher speed and, thus, produce a higher amount of heat. Next will follow a comparison of both

dielectric fluids using Tin as a workpiece material (fig.4.11).

Figure 4.11 – Diameter (µm) vs Current using glycerin and deionized water

Experiments show that for this current range, deionized water as a dielectric generates a larger crater

than glycerin on Tin because deionized water has a lower boiling point than glycerin so, the plasma

channel tends to have a bigger expansion resulting in wider craters. Both curves follow a similar pattern.

At 4A they have a diameter difference of about 50 µm, when eroding Tin with glycerin at 8A the diameter

gets closer to deionized water performance and at 16A there is again about 50µm diameter variation.

As the density of glycerin is higher than deionized water, the plasma channel restriction phenomenon

occurs, leading the electrons to strike the workpiece at lower velocity, resulting in less heat, and thus

creating smaller crater sizes. Some samples of eroded Tin with dielectric ionized water taken by the

optical microscope are presented in fig.4.12.

0

50

100

150

200

250

300

350

0 5 10 15 20

Dia

mete

r (µ

m)

Current (A)

Tin deionizedwater

Tin glycerin

38

(a) (b) (c)

Figure 4.12 – Eroded Tin with ionized water: (a) 4A; (b) 8A; (c) 16A; The lower scale division is equivalent to 20µm

Next step is comparing depths of Tin when using deionized water and glycerin. To do so, the CLSM was

used to get an accurate measurement of both material depths and it is going to be analyzed in fig.4.13.

Figure 4.13 – Depth vs Current for eroded Tin with deionized water and glycerin

Due to differences in the boiling point and density of the two fluids, it was expected that the Tin crater

depth would be deeper with the use of glycerin in comparison with the deionized water, although, the

results do not meet the expectations. The evolution of the curves does not take a similar behavior. While

eroded Tin with deionized water almost triplicates its depth from 4 to 8A, eroded Tin with glycerin only

duplicates. The reason behind this, in accordance with König, is that using glycerin is advantageous

when working with long pulse durations, which in this experiment are not, for roughing range, i.e. direct

polarity. The present tests were done with reverse polarity, low pulse duration and a low range of current

intensity and it is believed that for these operative parameters glycerin is less advantageous. König used

0

10

20

30

40

50

60

0 5 10 15 20

Depth

(µ

m)

Current (A)

Tin deionizedwater

Tin glycerin

39

current in the range of 50~90A and in that interval glycerin is an optimum choice but, when working with

low currents deionized water proves to be more efficient. However, analyzing Tin eroded with glycerin

depth from 8 to 16A, a huge leap can be observed, almost matching the depth for Tin eroded with

deionized water. This can be a good indicator that glycerin is a good dielectric when working with higher

currents. Glycerin seems to consume more energy but is better for finishing because it produces smaller

craters than deionized water. Another positive point is that Tin eroded craters with glycerin seem to be

less susceptible to current variations and other disturbs.

40

5. Conclusions

The focus of the present work was to study the influence of the plasma single-discharge EDM operative

parameters on the crater morphology. A range of electrical current intensity was used (4, 8 and 16A) as

well as two dielectric fluids (glycerin and deionized water), while gap, on-time and open voltage were

kept constant. Through an optical microscope and a confocal laser scanning machine crater diameter

and depth were registered and further analyzed. Results using glycerin as a dielectric fluid show that as

the current increases, wider and deeper craters are obtained, leading to more material removal. The

range of crater diameters obtained were between 67,45µm and 296,5µm whilst crater depth was

between 2µm and 52,5µm. For both cases the lowest measure was obtained when testing Copper while

the highest value was for Tin.

The first conclusion is that when the current intensity raises, more material is removed from the

workpiece, and is in accordance with the available literature on the subject. Another conclusion is that

results analyzed in this work corroborate the Reynaerts theory, as low machining index indicates a

material is more suitable for workpiece and high index is more suitable for electrode tool.

Although it is a stochastic process and thus, unpredictable to know if a trial is going to be successful, a

great amount of discharges was accomplished with success, since around 75% of the craters presented

a circular geometry. Moreover, a way to estimate the eroded crater volume is presented.

Energy per volume unit was also analyzed and a relation between this energy and the Reynaerts number

was made with the objective of quantifying the minimum energy that needs to be put into the process

for a material, depending on its Reynaerts number. The result was that Tin is the material that needs

less energy input to remove a volume unit because of its material properties.

There are few EDM works using glycerin as a dielectric fluid so, some trials with a different dielectric

fluid (deionized water) were made to have a term of comparison. The result is that, for the tested current

intensity range, larger crater diameter was obtained when using deionized water, although crater width

formed while using glycerin are very near the mentioned above. Concerning the crater depth, deeper

craters were attained using deionized water. Furthermore, at the current intensity of 8A a huge

discrepancy in the results was observed, 29,5µm more precisely. Studies performed by other authors

defend that glycerin is advantageous when machining with high currents, high pulse times and for

roughing. The conclusion drawn from this experiment is that when working with lower currents deionized

water is a good fluid to work with, in terms of material removal. On the other hand, in agreement with

the works mentioned above, glycerin seems to be a valid option since the depth results obtained for 16A

are very close to deionized water and follow an increasing tendency as the current raises. For a surface

finish purpose, glycerin i