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EFFECTS OF CHROMIUM POWDER MIXED IN ELECTRICAL

DISCHARGE MACHINING OF AISI D2 HARDENED STEELS

MUHAMMAD RIDZUAN BIN IDRIS

Project dissertation submitted in partial fulfilment of the requirement for the Master’s

Degree in Mechanical Engineering

Faculty of Mechanical and Manufacturing Engineering

Universiti Tun Hussein Onn Malaysia

APRIL, 2015

v

ABSTRACT

Nowadays, electrical discharge machining (EDM) is the non-conventional

method that has been used extensively in machining hard material that is commonly

used in mold and die industry but the limitations of EDM will cause lower productivity

and poor surface quality. EDM becomes most apparent by using powder metallurgy

electrode together with chromium (Cr) powder suspension into the dielectric fluid which

has led to the increasing of productivity and good quality performance and

characteristics. Therefore, by combining of both EDM parameters and machining

conditions by incorporating with specific magnetic system plus integrated with existing

filter system, it is highly expected that the machining speed can be enhanced. However,

there are difficulties to determine the best combination of these machining parameters in

order to increase the material removal rate (MRR) and at the same time to reduce the

electrode wear rate (EWR) with the acceptable surface integrity. This research

emphasizes the studies of Cr powder mixed in EDM machining of AISI D2 hardened

steel using copper tungsten electrode which has been done successfully. Data were

analysed using design of three factors at a time consisted of peak current (Ip), pulse-on

(Pon) and powder concentration(C). Discussions were made on the responses such as

MRR, EWR, Surface Roughness (Ra), surface morphology, recast layer (RL) and

microhardness (MH) on the selected samples from the same machining conditions.

Results have proved that Ip was the most significant parameter which has influenced the

machining responses on Cr powder mixed EDM of AISI D2. It is also found that proper

powder concentration of 2gram/litre enhanced the machining efficiency particularly in

MRR. Furthermore, introduction of proper addition of Cr powder in the dielectric also

decreased Ra and RL thickness. The EWR increased as the peak current increased, but

inversely with pulse-on. In general, the possibility of EDM process for machining AISI

D2 tool steel by incorporating Cr powder mixed in the dielectric is acceptable and the

entire objectives were successfully proven.

vi

ABSTRAK

Pada masa kini, mesin nyahcas elektrik (EDM) adalah satu kaedah tidak-

konvensional yang telah digunakan secara meluas di dalam pemesinan bahan keras

yang lazimnya digunakan dalam industri acuan tetapi kekurangan yang ada pada EDM

akan menyebabkan pengeluaran produktiviti yang rendah dan kualiti permukaan

pemesinan yang tidak baik. EDM akan menjadi lebih baik dengan menggunakan

elektrod jenis serbuk metalurgi bersama-sama dengan campuran serbuk kromium (Cr)

ke dalam cecair dielektrik yang boleh meningkatkan produktiviti dan menghasilkan

ciri-ciri pemesinan yang berkualiti. Oleh yang demikian, dengan gabungan kedua-dua

parameter EDM tersebut ditambah pula dengan sistem magnet yang menggantikan

penapis sedia ada dijangka meningkatkan kelajuan pemesinan. Walau bagaimanapun,

terdapat kesukaran untuk menentukan kombinasi parameter yang paling baik untuk

meningkatkan kadar pembuangan bahan (MRR) dan pada masa yang sama

mengurangkan kadar haus elektrod (EWR) dengan integriti permukaan yang boleh

diterima. Kajian campuran serbuk Cr dalam cecair dielektrik diuji dengan keluli keras

AISI D2 sebagai bahan kerja dan menggunakan kuprum tugsten sebagai elektrod telah

dilakukan dengan jayanya. Data akan dianalisis menggunakan reka bentuk daripada

tiga faktor iaitu arus puncak(Ip), kadar denyutan(Pon) dan kepekatan campuran(C).

Tindak balas seperti MRR, EWR, kekasaran permukaan bahan (Ra), morfologi, lapisan

kesan pemanasan haba (RL) dan mikrokeras hasil dari sampel yang dipilih dari keadaan

pemesinan sama akan dibincangkan. Keputusan menunjukkan bahawa Ip adalah

parameter yang paling berkesan mempengaruhi pemesinan. Ia juga mendapati bahawa

tambahan serbuk Cr akan membantu meningkatkan kecekapan pemesinan terutamanya

dalam keputusan MRR. Campuran serbuk Cr dengan kuantiti optimum sebanyak

2gram/liter ke dalam dielektrik akan mengurangkan Ra dan RL. Selain itu, EWR

meningkat kerana peningkatan arus tetapi menurun jika masa denyutan ditambah.

Secara umumnya, sesuatu proses EDM untuk pemesinan keluli AISI D2 dengan serbuk

Cr dicampur dalam dielektrik boleh diterima dan keseluruhan objektif telah terbukti.

vii

TABLE OF CONTENTS

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF FIGURES xii

LIST OF TABLES xvii

LIST OF ABBREVIATIONS AND SYMBOLS xix

LIST OF APPENDICES xxi

CHAPTER 1 INTRODUCTION 1

1.1 Overview 1

1.2 Background of Study 3

1.3 Problem Statement 5

1.4 Objective 6

1.5 Scope 7

1.6 Hypothesis 8

viii

CHAPTER 2 FUNDAMENTAL OF ELECTRICAL

DISCHARGE MACHINING (EDM)

9

2.1 Introduction 9

2.2 Principle of EDM Process 11

2.2.1 Generator Design 12

2.2.2 EDM Control System 14

2.2.3 EDM Process Mechanism 15

2.3 EDM Parameter 17

2.3.1 Electrical Parameter 18

2.3.1.1 Peak Current 18

2.3.1.2 Pulse duration 20

2.3.1.3 Frequency 21

2.3.1.4 Polarity 22

2.3.1.5 Discharge Voltage 23

2.3.2 Non-Electrical Parameter 24

2.3.2.1 Dielectric 24

2.3.2.1 Flushing 25

2.3.3 Powder Mixed EDM 26

2.4 Electrode 30

2.4.1 Metallic Electrodes 32

2.4.1.1 Copper 32

2.4.1.2 Tungsten 33

2.4.1.3 Copper Tungsten 34

ix

2.4.1.4 Brass 36

2.4.1.5 Silver 36

2.4.2 Graphite Electrodes 37

CHAPTER 3 LITERATURE RIVIEW 39

3.1 Introduction 39

3.2 The Fabrication of AISI D2 Hardened Steel 40

3.3 EDM Machining of Hardened Steels 42

3.4 EDM Machining of AISI D2 Hardened Steels 44

3.5 Powder Mixed of EDM machining 55

3.6 Summary 63

CHAPTER 4 METHODOLOGY 66

4.1 Research Design Principles 66

4.1.1 Higher peak current, pulse duration

employed for higher MRR and productivity 68

4.1.2 Filtering system 68

4.1.3 Chromium powder as suspended material 70

4.2 Experimental design and variables 71

4.2.1 Machining parameters 71

4.2.2 Machining characteristic 73

4.3 Research procedures 79

4.3.1 Machining Setup 80

x

4.3.2 Powder Metallurgy Electrode 85

4.4 Equipment and instrumentation 86

4.4.1 Major Instrument 87

4.4.2 Measuring Instruments 88

4.4.2.1 Digital weighting balance 89

4.4.2.2 Surface Roughness Tester 90

4.4.2.3 Tool Maker Microscope 91

4.4.2.4 Scanning Electron Microscope 92

4.4.2.5 Vickers hardness tester 94

4.5 Specimen preparation 95

4.5.1 Sectioning 97

4.5.2 Mounting of specimens 99

4.5.3 Grinding 100

4.5.4 Polishing 101

4.5.5 Etching 103

CHAPTER 5 RESULTS AND DISCUSSION 106

5.1 Material Removal Rate (MRR) 108

5.2 Electrode Wear Rate (EWR) 112

5.3 Surface Roughness (Ra) 117

5.4 Surface Integrity 128

5.4.1 Recast Layer (RL) 128

5.4.2 Microhardness (MH) 139

xi

CHAPTER 6 CONCLUSION 144

REFERENCE 147

APPENDIX 158

xii

LIST OF FIGURES

1.1 Classification of EDM research 2

2.1 Schematic of conventional EDM process 12

2.2 Relaxation circuit 13

2.3 Variation of capacitor voltage with time 13

2.4 EDM spark sequence 15

2.5 Illustration of different usage of current to the different

surface area

19

2.6 Pulse-on with different duty cycle 20

2.7 Pulse-on with different frequency 21

2.8 EDM polarity 22

2.9 Actual profile of a single EDM pulse 23

2.10 Schematic diagrams of the electric discharge for single

power pulse

27

2.11 The material removal mechanism during the normal

discharge for the working fluid

28

2.12 Mechanism of PMEDM 28

2.13 Additive particle behavior 28

2.14 PMEDM spark sequence 29

2.15 Copper electrode 33

2.16 Tungsten electrode 33

2.17 Copper Tungsten electrode 35

2.18 Brass electrode 36

2.19 Silver electrode 37

2.20 Graphite electrode 38

xiii

3.1 EDM setup using CNT 49

3.2 PMEDM schematic for dispenser and pump system. 50

3.3 Diagram of a self-designed filter system for the EDM. 52

3.4 Typical waveform of voltage and current: 59

3.5 Discharging waveform of voltage 60

3.6 The apparatus used to produce a single discharge 61

3.7 Comparison of crater morphology at various machining

parameters

62

3.8 Micrographs of crater generated by a single pulse discharge 63

4.1 Overall research methodology 67

4.2 Illustration of powder and debris motion in PMEDM

process

69

4.3 Ishikawa Cause-Effect Diagram 72

4.4 Two categories of machining characteristic 73

4.5 Specimen test preparation 74

4.6 Experimental process flowchart 79

4.7 Experiment setup for HPEDM 81

4.8 Image for HPEDM 84

4.9 Sodick AQ55L 88

4.10 Mettler Toledo PL403 precision balance. 89

4.11 Mahr Perthometer PGK-120 90

4.12 Tool Maker Microscope Nikon MM-60 91

4.13 Scanning Electron Microscope (Joel-JSM 6380 L.V) 93

4.14 Vickers tester 95

4.15 Specimen preparation 97

4.16 Abrasive cut-off machine (Stuers Lobotom 3) 98

4.17 Specimens after compression mounting 99

4.18 Buehler Auto Mounting Press machine 100

xiv

4.19 Buehler Roll Grinder 101

4.20 Polish machine 102

4.21 Bright field illumination 104

4.22 Dark field illumination 105

5.1 Effect of powder concentration and pulsed duration on

material removal rate [Ip=20A]

110



110



110

5.4 Effect pulsed duration and powder concentration on

electrode wear rate [Ip=20A]

114



114



114

5.7 Surface morphology of electrode with different pulse-on 116

5.8 Effect pulse duration and powder concentration on surface

roughness [Ip=20A]

119


roughness [Ip=32A]

119


roughness [Ip=40A]

119

5.11 Surface morphology of D2 workpiece at increasing peak

current [Pon=50µs, C=2g/l]

121

5.12 Surface morphology of D2 workpiece at increasing pulse-

on [Ip=20A, C=2g/l]

123

5.13 Surface morphology of D2 workpiece at an increasing

powder concentration [Ip=40A, Pon=50µs]

125

5.14(a) Surface texture of EDM workpiece for maximum and

minimum surface roughness

127

xv

5.15 Effect Pulse-on and Powder concentration on Recast layer

at Peak current [Ip=20A]

130



130



130

5.18(a) Optical microscope showing the cross-sectional view of the

recast layer of the EDM machined surface layer

[Pon=100µs, C= 2g/l, Ip=20A]

132

5.18(b) Optical microscope showing the cross-sectional view of the

recast layer of the EDM machined surface layer

[Pon=100µs, C=2g/l, Ip=40A]

132

5.18(c) SEM showing the cross-sectional view of the recast layer of

the EDM machined surface layer [Pon=100µs, C= 2g/l,

Ip=20A]

133

5.18(d) SEM showing the cross-sectional view of the recast layer of

the EDM machined surface layer [Pon=100µs, C=2g/l,

Ip=40A]

133


recast layer of the EDM machined surface layer [Pon=50µs,

C= 0g/l, Ip=32A]

134


recast layer of the EDM machined surface layer[Pon=100µs,

C= 0g/l, Ip=32A]

134



Ip=32A]

135


the EDM machined surface layer[Pon=100µs, C= 0g/l,

Ip=32A]

135

xvi



C= 0g/l, Ip=20A]

137



C= 2g/l, Ip=20A]

137



Ip=20A]

138



Ip=20A]

138

5.21 Microhardness of Sub-surface layer of workpiece at

different Powder concentration [Ip = 40A, Pon = 100s]

140


different Peak current[C = 2g/l, Pon = 100s]

141


different Pulse-on [Ip = 40A, C = 2g/l]

143

xvii

LIST OF TABLES

2.1 Advantages and limitations of EDM 10

2.2 Electrode material properties that effect EDM 31

2.3 Physical properties and miscellanea of copper and tungsten 35

3.1 Chemical composition of AISI D2 hardened steel 40

3.2 Physical properties of AISI D2 hardened steel 40

3.3 Procedure of heat treatment for AISI D2 hardened steel 41

4.1 Specifications of powder used 70

4.2 Thermophysical properties of powder used 70

4.3 Machining parameter 72

4.4 Experimental Design and Response 75

4.5 Data collection sample of material removal rate of each

composition of powder concentration and peak current (Ip)

setting

76

4.6 Data collection sample of tool wear rate of each


setting

77

4.7 Data collection sample of surface roughness of each


setting

78

4.8 Data collection sample of average micro-hardness of each


setting

78

4.9 Summary PMEDM part 82

xviii

4.10 The specification of Nikon MM-60 (Tools Maker

Microscope)

92

4.11 The specification of SEM (JSM 6380) 93

5.1 Overall Result of EWR, MRR, Ra and RLwith different peak

current, pulsed on duration and powder concentration

107

5.2 Results of material removal rate (MRR) with different peak

current, powder concentration and pulsed on duration

109

5.3 Result of electrode wear rate (EWR) with different peak

current pulsed on duration and powder concentration

113

5.4 Result of surface roughness (Ra) with different peak current,

powder concentration and pulsed on duration

118

5.5 Result of Recast Layer (RL) with different peak current,

powder concentration and pulsed on duration

129

6.1 Summary of optimum MRR, EWR and Ra 146

xix

LIST OF ABBREVIATIONS AND SYMBOLS

Al - Aluminium

AISI - American Iron and Steel Institute

C - Powder concentration

CNT - Carbon nano tube

Cr - Chromium

CV - Control valve

CuW - Copper tungsten

EDM - Electrical discharge machining

EWR - Electrode wear rate

FPGA - Field-programmable get array

HAZ - Heat affective zone

HRC - Hardness Rockwell unit for steel

Ip - Discharge current

MRR - Material removal rate

PMEDM - Powder mixed dielectric electrical discharge machining

Pon - Pulse duration

PLC - Programmable logic controller

PM - Powder metallurgy

SEM - Scanning electron microscopy

SA - Surface area

SI - Surface integrity

SiC - Silicon carbide

TELCO - Tellurium copper

Ra - Surface roughness

RL - Recast layer

xx

tm - Machining times

Wa - Weight of workpiece after machining

Wb - Weight of workpiece before machining

xxi

LIST OF APPENDICES

Apendix A List of Publication Related

Apendix B A Summary of Literature Review on EDM Die Sinking

(AISI D2 Hardened Steel)

1

CHAPTER I

INTRODUCTION

1.1 Overview

Following the technology of manufacturing today, machining process is dealing with

challenges from the marketing demand that needed to requires the use of advanced

materials such as composite, super alloys, and hardened steels that are so hard to be

machined. The high speed machine that can produce good surface integrity and precise

cutting with less tool wear, are among the criteria to use in the advance technology, i.e.

aerospace, automotive and medical material. Since it is impossible to use conventional

machining of hard material, non-conventional machining such as electrical discharge

machining (EDM) is one of the ideal techniques in dealing with these materials

including hardened steel.

EDM is generally known as a ‘vertical’, ‘ram’ or ‘sinker’ electrical discharge

machining that has been used in the industry over fifty years ago. According to the

current world developments, EDM has also increased technology that enables the EDM

to produce superficial, thin and compact mechanical elements that are used by the

advanced manufacturer. The power supply on a previously effective EDM has improved

according to the changes in electric and electronic fields. It is capable to produce more

power efficiently, generate various types of waves and high frequency. Due to the

2

existence of rapidly growing control systems, EDM is suitable for producing work that

requires a high level of accuracy. Most of EDM use computer numerically method

(CNC) as their programming to communicate with human to improve the machining

process.

EDM is a machine that can provide many benefits for high accuracy and

precision of machining process. It requires further research to produce a more realistic

EDM. Numerous researchers have optimized the process variable to improve the

machining workpiece characteristics such as by increasing the material removal rate due

to increase in machining speed, increasing the tool life and obtain the best surface

integrity. Some of the researchers modified the EDM system to fulfill the requirement

global trend of machining process. Figure 1.1 shows some of the research area in EDM.

Figure 1.1: Classification of EDM research (Pandey and Singh, 2010)

High potential of EDM can help to improve productivity in the industry. Productivity

is one of the important elements in the industry today. Higher productivity can lead to

improved industry performance such as lower average costs, higher profits, higher

EDM ResearchArea

Improve PerformanceMeasure

EDMApplication

Area

HybridMachining

Process

SurfaceRoughness

EWRMRR

PMEDM

USEDMECDM

Pulse-on Peak Current Discharge

Voltage Polarity

Rotational Speed Flushing Pressure Volume Friction of

Powder Mixed

Electrode DesignModification

Optimization ofProcess Variable

EDMDevelopment

Incr

ease

Red

uce

Red

uce

3

wages and improved competitiveness and trade performance. Hence, to fulfill this

demand, the machines in the industry must have a high speed productivity including

EDM.

1.2 Background of Study

Electrical discharge machining (EDM) is the non-conventional method that has been

used extensively in the machining process of hard material that is commonly used in

mold and die industry. The basic process of EDM is to remove metal through the action

of an electrical discharge energy and high current intensity between the tool (electrode)

and the workpiece without physical cutting force. This is one of the advance methods to

replace traditional machine method that able to machine difficult-to-cut or hard materials

such as titanium, inconel and hardened steel. It can be successfully employed to

electrically conductive machine parts regardless of their hardness and toughness. In spite

of remarkable process capabilities, limitations such as low volumetric material removal

and poor surface quality are associated with EDM (Kansal et al, 2007).

Limitation of EDM will cause lower productivity, which is due to low cutting

speed. In that case, researchers of EDM explored a new method to improve the sparking

efficiency phenomenon including some modification of experimental concepts and their

existing system. The researchers needed to consider the machining process condition

and parameters such as discharge voltage, peak current, pulse-on, flushing and dielectric

medium before executing the new method of the machining process. To date, most of

the studies in EDM have undergone the normal effect of machining process condition

which applies low level of machining parameters. Yet, few studies have been

implemented in greater current and long pulse-on and at the same time to maintain the

EDM machining stability and efficiency to obtain the best material removal rate (MRR),

lower electrode wear rate (EWR) and the good performance of the workpiece surface

4

integrity. Therefore, there is a keen interest to look forward into this new phenomena in

EDM machining and hence for greater productivity.

Nowadays, research becomes most apparent by using powder metallurgy

electrode together with powder suspension into the dielectric fluid which has increased

the productivity, and good quality performance and characteristics. Therefore, by using a

combination of both EDM parameters and machining process conditions, and

incorporating with the specific magnetic system plus integrated with the existing filter

system, it is highly expected that this combination can enhance the machining speed.

The filter system integrated with magnetic force is used to separate debris or

contaminants from dielectric fluid and hence the efficiency of spark erosion becomes

more stable and easy to cut the materials. In order to realize the full potential of new

setting of EDM machining by incorporating magnetic and pump system, the new

dielectric tank was developed which was integrated with a continuous circulation

system.

Many studies have been applied through this process in several types of

materials, but lack of work has been carried out to incorporate powder mixed in the

EDM machining of AISI D2 hardened steels. It is also found that a comprehensive study

in EDM machining of AISI D2 is still scarce. New machining data on the EDM of AISI

D2 serves a great significance and could be further exploited especially for hole making

operation. Further research on powder mixed EDM machining mechanism and its

characteristics of this kind of material is expected to give better rate and efficiency of the

machining process. This project was undertaken to study the effect of machining process

conditions and incorporated with powder mixed in EDM machining of AISI D2

hardened steels and by employing advanced powder metallurgy electrode of copper

tungsten (CuW).

5

1.3 Problem Statement

The low cutting speed for machining process of difficult-to-cut material which is due to

low productivity of electrical discharge machining (EDM) is one of the EDM

limitations. According to this problem, researchers associated to EDM have explored a

new method to improve the efficiency and cutting speed of the machining process

including some modification of experimental concepts to improve their existing system.

The machining process condition and parameters such as discharge voltage, peak

current, pulse-on, flushing and dielectric medium, need to be considered before

executing the new method or some modification.

In EDM process, the pulse-on, peak current and dielectric condition are among

the very important machining parameters because they can directly control the result of

material removal rate (MRR), electrode wear rate (EWR), surface roughness (Ra) and

surface integrity. However, there are difficulties to determine the best combination of

these machining process parameters to increase the MRR and at the same time to reduce

the EWR with the acceptable surface integrity. In addition, the unsuitable dielectric

condition used with longer pulse-on and higher peak current will also increase the

production cost. The high hardness properties of AISI D2 hardened steel and high wear

of tool electrode EDM material will also affect the performance of EDM machining

characteristic. Therefore, it is estimated that by incorporating Chromium (Cr) powder in

the dielectric fluid can deliver better result in terms of material removal rate, electrode

wear rate and acceptable surface integrity in machining process of AISI D2 hardened

steel. Likewise, the use of copper tungsten (CuW) electrode is also expected to deliver

minimum EWR and maximum MRR in the Cr suspended dielectric electrode.

6

1.4 Objective

The aim of the research is to investigate the effect of machining process parameter at

different concentration of Chromium (Cr) powder mixture of dielectric in electrical

discharge machining (EDM) machining of AISI D2 hardened steel with specific aims:

i). To determine the effect of machining process parameter on the following

machinability such as electrode wear rate (EWR), material removal rate (MRR)

and surface roughness (Ra).

ii). To identify the effect of machining process parameter on the following

machining characteristics such as;

a) Topmost workpiece surface morphology

b) Subsurface layer changes which include recast layer (RL) and heat

affected zone (HAZ)

c) Microhardness at deeper layer of the machined surface

iii). To explore the performance of powder mixed dielectric in EDM machining of

AISI D2 hardened steel.

1.5 Scope

i. Electrical discharge machining (EDM) die sinking AQ55L Sodick machine is

used to carry out the experiments designed.

ii. The main parameters investigated are: powder concentration and pulse-on.

a) Peak current: 20A, 32A, 40A.

b) Powder concentration: 0g/l, 2g/l, 4g/l.

c) Pulse-on: 50µs, 75µs, 100µs.

iii. Powder metallurgy tool electrode (W-Cu: Copper-Tungsten 35%C65%W) is

selected as the cutting tool for machining the work piece.

7

iv. The machining process operation is conducted on AISI D2 hardened steel

(having a typical hardness range of 56-62 HRC) as work material.

v. The experiment is carried out in hydrocarbon oils (i.e. Kerosene) as a dielectric

medium with Chromium (Cr) powder suspended.

vi. Magnetic system attached with filter system is applied in the EDM machine.

vii. Experimental trials are conducted to investigate and evaluate the following

responses:

a) Subsurface layer changes which include recast layer (white layer) and

heat affected zone (HAZ) using scanning electron microscope (SEM)

b) Microhardness at deeper layers of the machined surface using the Vickers

Hardness Tester

c) Machining removal rate and tool electrode wear rate by Digital Weighing

Machine

d) Workpiece morphology by SEM

viii. Collection of data through experiments are analyzed with:

a) Observation of machining process characteristics of workpiece materials

due to surface modification. The machining process characteristics are

composed of EWR, MRR, Ra and RL.

b) Evaluation and comparison of the effect of cutting conditions on the

machinability of the cutting tool material with the effects of workpiece

surface modification or alteration.

1.6 Hypothesis

Powder mixed of electrical discharge machining (PMEDM) is one of the methods to

improve the quality of the machined component. It is expected that the investigation on

machining process performance for different concentration of Chromium (Cr) powder

of PMEDM will determine the best amount of Cr powder concentration in the machining

process of AISI D2 hardened steel particularly for high productivity of EDM machining

8

operation. The introduction of Cr powder as an additive in dielectric can help to

improve the machining process capability to remove material from workpiece material

efficiently. This research concentrates on high productivity of machining process by

achieving faster and more efficient metal removal rate (MRR) by using higher peak

current and pulse-on without compromising the electrode wear rate (EWR) and surface

machining characteristics. In addition, the evaluation of surface morphology images

using scanning electron microscope (SEM) will give good information about the

characteristics of the workpiece after machining process. Hence, the information could

be used to analyze the electrode and surface integrity of the machined workpiece

surface.

9

CHAPTER II

FUNDAMENTAL OF ELECTRICAL DISCHARGE

MACHINING

2.1 Introduction

The history of electrical discharge machining (EDM) goes far back to the 1770’s when

an English scientist named Joseph Priestly, discovered the eroding effect of electricity

on various metals. Based on Priestley’s research, the soviet researchers named

Lazarenko B. R and Lazarenko N.I had the idea to use the destructive effect of the

electrical charge for difficult-to-cut machining. In 1943, they developed the discharge

generator of EDM known as Larazenko Circuit. It is based on the relaxation or RC

circuit that was used today. They also developed a working procedure with spark

erosion, where electrical discharges in a dielectric liquid take place between two

conductors (Fleming, 2005). Since the introduction of EDM over fifty years ago, the

technology for speed and precision has grown tremendously and, likewise, its capability

and manufacturing applications have followed suit (Guitrau, 1997).

EDM is one of the non-traditional machining process that uses electrochemical

concept, where electrical energy is used to generate an electrical spark and vaporizing a

part of the material. The material removal mainly occurs due to the thermal energy of

the spark. In fact, EDM can create precise shapes, burn-free and intricate in machining

material. Because of that, EDM has been widely used not only in molds and dies, but

10

also in aerospace application, making extrusion dies and production in small holes.

EDM is selected in many cutting applications because of the advantages offered over the

conventional method, but EDM is available only for conductive material. The

advantages and limitations of EDM are listed in Table 2.1.

Table 2.1: Advantages and limitations of EDM

Advantages Limitation

EDM can be used to all conductive

materials

The workpiece must be conductive

Able to create complex shapes Slower material removal rate for hard

material

No force between tool electrode and

workpiece

Electrode wear may require the use of

several tools

Automated Undesirable recast layer needs to be

removed

High precision machine Equipment is expensive

Repeatability Leave a very shallow, highly stresses,

surface layer

High accuracy machine Cavities may slightly taper

EDM has been proven to be applicable for electrically conductive machine

materials such as carbide, stainless steel, hastalloy, naturally, waspalloynomonic and

others regardless of their physical and metallurgical properties (Kumar et al., 2011).

EDM technology is expanding as being used in tool, dies and mold making industries for

machining process of heat treated tool steel and advanced material (super alloys,

ceramics and metal composites) which require high precision, complex shapes and high

surface finish (Prabhu and Vinayagam, 2008). This has made the EDM technology to

grow with more accurate and dependable process.

11

2.2 Principle of EDM Process

Electrical discharge machining (EDM) is a process that uses electrical discharge to

remove metal from the workpiece. The electrode and workpiece are separated by a small

gap and connected to the power supply probe. These two parts are immersed in dielectric

fluid such as kerosene which is functioning as integral to the process. By applying high

frequency of AC or DC current to workpiece through an electrode, the workpiece melts

and vaporizes. Positioned very precisely near the workpiece, the electrode never touch

the workpiece but discharges its potential current through an insulating fluid (dielectric)

across a very small spark gap (Guitrau, 1997). The electric spark is generated and

controlled by a machine power supply.

An EDM system comprised of a generator, servo system, dielectric fluid system

and dielectric work tank. Figure 2.1 shows the schematic diagram of EDM system. The

workpiece is placed in the dielectric tank and affixed to the metal plate in the tank. The

tank is filled with hydrocarbon dielectric which is ionized in the presence of an electrical

field. The dielectric fluid breakdown electrically, after short ionization period, assuming

that the electricity is high enough. The electric field is created by applying voltage

between electrode and workpiece. The breakdown of dielectric fluid is much like the

breakdown of air when the large voltage is supplied from coil in an automotive ignition

system to the spark plug (Fleming, 2005). The spark of EDM is reported to be in the

range of 8000C to 12000C (14432F to 21632F), and it vaporizes and melts the

workpiece material (Guitrau, 1997).

The effect of using EDM machining is that the ‘chip’ from the workpiece is

produced and not allowed to accumulate at the workpiece. Flushing is needed to flush

away the ‘chip’ and debris from the spark gap because the accumulated debris will

disturb the machining process. The debris is filtered by the EDM filter to maintain the

cleanliness of dielectric in the work tank.

12

Figure 2.1: Schematic of conventional EDM process

2.2.1 Generator Design

The electrical discharge machining (EDM) generator is the device to control electrical

discharge. In 1943, B.R and N.I Lazarenko exploited the destructive effect of an

electrical discharge machining and developed the relaxation circuits known as lazarenko

relaxation (RC) circuit. They realized that the spark energy would have to be harnessed

and controlled if the discharges are to be efficiently utilized for machining (McGeough,

1988). Figure 2.2 below shows relaxation or RC circuit. The electrical energy of the

capacitor is charged from the DC source voltage until it breaks down and discharges into

the electrode and workpiece that are immersed in the dielectric fluid. Figure 2.3 below

shows the output voltage generated by relaxation circuit.

FILTER

PUMP

DIELECTRICFLUID SYSTEM

DIELECTRIC

MACHININGSERVO HEAD

POWER SUPPLY

HYDRAULICPOWER UNIT

RESERVOIR

MACHINE TOOL

WORKPIECE

WORKTANK

ELECTRODE

13

Figure 2.2: Relaxation circuit (Lazarenko,1943)

Figure 2.3: Variation of capacitor voltage with time.

The basic of EDM generator is the controlled pulse generator. The pulse

generator system can control their ON/OFF time, frequency and current level. The

advantages of pulse generator are the electrode wear can be greatly reduced and higher

metal removal rate under certain condition (Fleming, 2005). The development of control

pulse generator has been improved every decade and century. Their efficiency has

improved from 33% (1970) to >80% (today). With the development of electronics power

system, the power supply of EDM is now using field programmable get array (FPGA), a

technology that can control the distance of spark gap tightly, high frequency and high

current and constant result. The EDM generator has improved drastically which is

Chargingvoltage,V

BreakdownVoltage Vc

Gap filled with

dielectricWorkpiece

Toolelectrode

DC voltagesource

Resistor

Capasitor

Voltage Vc

Capasitorvoltage

Time

14

progressing from RC circuit power supply and vacuum tubes to solid-state-transistor

with nanosecond pulsing (Guitrau, 1997).

2.2.2 EDM Control System.

Almost all of EDM machining use drive system in their manufacturing. Some builders

use stepper motors and others use servo motor. With the presence of sophisticated

technology today, high resolution of an encoder can move the electrode in micron scale.

To support the increase in technology movement that can produce high potential

machine, this driver must be provided. Over the past few years, the trends in control

system have grown from 16 bit to 32 bit processor for increasing the computing speed

(Guitrau 1997).

The investigation of the control systems of EDM machines has been an

important effort in the field of electric discharge machining. Each EDM can be seen as a

stochastic time-dependent nonlinear system involving many parameters. Applications

mainly include those situations when the conventional controls of linear, constant

coefficient systems could not produce adequate outcomes. The current commonly

employed EDM control systems are established on the modern control theory. The

control systems often applied as self-adaptive control systems generally employ the

mathematical models estimates accompanied by high costs without actually realize the

truly significant optimal results (Liu, 2010).

Some ram machines are equipped with fuzzy logic. Unlike bilevel logic, which

recognizes a statement as either true or false, fuzzy logic allows a statement to be

partially true or false. Fuzzy logic allows the machine to think and react quickly to

various machining conditions. This machine can lower the increased power setting to

obtain the optimum combination of speed, precision and finish. Fuzzy logic system will

constantly control the power setting to maximize the efficiency (Sommer, 2005).

15

2.2.3 EDM Process Mechanism

In electrical discharge machining (EDM), the removal of material is based upon the

effect of electro discharge erosion of electric spark occurring between the electrode and

workpiece separated by dielectric as their type of gap. The spark is unstable, irreversible

and transient phenomenon sometimes marked the transition from a state of more or less

stable for the current between the electrodes in the gas to other more stable under

conditions imposed (Leonard and Meek, 1941). For the EDM process, the transition

spark from the electrode will occur because the condition of the workpiece is more

stable. An electric spark is a type of electrostatic discharge that occurs when an electric

field creates an electrically conductive ionized channel in the air producing a brief

emission of light and sound. A spark is formed when the electric field strength exceeds

the dielectric field strength of air (Leonard and Meek, 1941). Figure 2.4 shows the

illustrations of relative values of voltage and current at the represented point and the

description of the process.

Process Description

(a)

“Open-Gap” voltage. The electrode is seeking

the workpiece while “cutting air”. The graph

shows high potential voltage only and no

current. Time line runs horizontal. The

capacitor starts to charge until voltage

breakdown.

(b)

Displays the electromagnetic field created

between electrode and workpiece. Dielectric

within this field becomes polarized as

resistance decreases. Voltage level off because

capacitor stop to charge.

16

(c)

“On-time” begins. Dielectric resistance is

overcome and the spark occurs, generating

current which vaporizes the workpiece. As

amperage increases, voltage decreases.

(d)

The spark is plasma hot and enclosed within a

sheath of gases. Vaporization of workpiece

continues.

(e)

Gas bubble continues to expand rapidly (vapor

pressure). At a certain point, vaporization will

cease and melting begins. Dielectric

contamination increases.

(f)

Amperage and voltage have leveled off as

contamination and thermal damage of dielectric

increase. Dielectric is now severely

compromised and its electrical resistivity

continues to rise. If allowed to continue,

conditions will cause “DC arcing”.

17

(g)

Power is interrupted during “off time” part of

EDM cycle. Current drops to zero. The gas

bubble collapses upon removal of heat source.

(h)

Gases and contaminated dielectric will

naturally disperse, but using forced or sealed

flushing is the best method and will

significantly reduce dielectric recovery time

and increase cutting speed.

(i)

Contamination and damaged dielectric are

expelled, revealing EDM crater on workpiece

and wear on the electrode. Dielectric begins

reionization, allowing a repeat of the cycle.

Figure 2.4: EDM spark sequence

2.3 EDM Parameter

In EDM process, various input parameters affected the measured experimental

parameter. Electrical parameter on EDM can controlthe power supply system or

generator of EDM machine, while non-electrical parameter is not related to discharge

energy such as flushing and dielectric type. Each distinctly different, but must be used

18

together in different combination to obtain the desired result (Guitrau, 1997). Generally,

EDM parameters consist of two functional groups which are:

i) Electrical parameter

(a) Peak current

(b) Pulse-on

(c) Frequency

(d) Polarity

(d) Discharge voltage

ii) Non electrical parameters

(a) Flushing

(b) Dielectric

2.3.1 Electrical Parameter

Major electrical parameters are discharged voltage, peak current, pulse-on and pulse

interval, pulse waveform and polarity. The EDM process is a stochastic thermal nature

which has complicated discharge mechanism. Therefore, it is difficult to explain all the

effect of these parameters on performance measures. However, researchers are now

relying on the process analysis for optimization of parameters to identify the effect of

operating variables in achieving the desired machining characteristics.

2.3.1.1 Peak Current

Current is the amount of power used in the electrical discharge machining. The

utilization of current in EDM is related to the area of machining “cut”. It is like cutting

wood, if we cut a thick wood, we need a lot of energy to swing the ax. The greater

amount of surface area needs more power to cut. According to Guitrau (1997), the

19

maximum power selection is approximately 65 amps per square inch of electrode

engagement. This is a simple formula to be based upon surface area (SA) of electrode

engagement multiplies by a constant of 65 amps per square inch or

Surface Area(SA) 65 = maximum amperage (2.1)

Based on the formula, the electrode has an area of 0.1 inch and 0.2 inch, 6.5A

and 13A current, respectively. Figure 2.5 below shows the illustration of different usage

of current to different surface area (Guitrau, 1997).

Figure 2.5: Illustration of different usage of current of the different surface area

Dc source

Dc source

Dc PulseCurrent

6.5A

13A

Dc PulseCurrent

Dc PulseCurrent

0.1”Electrode

Workpiece

0.2”Electrode

Workpiece(a)

(b)

20

2.3.1.2 Pulse Duration

Knowing the EDM process and how it works is not enough. The pulse duration of EDM

needs to be explored because it relates to know how to get good machining speed and

good finishes with minimum wear, high machining rate and the lowest possible chance

of d.c arcing. The pulse duration is controlled by pulse generator that can adjust the

value of ON time and OFF time. In the ON-time cycle, current is generated and spark

gap is bridged, and the work accomplishes. The longer duration of ON-time will create

the longest spark and this will cause more workpiece material to melt. With the

occurrence of longer discharge energy, the resulted creators will be broader and deeper,

therefore the surface finish will be rougher. In positive polarity, the spark leaves

electrode and strikes the workpiece. More sparks produced within a unit time will

produce proportionately wear. OFF-time is the duration of rest required for reionization

of dielectric. The longer period of OFF-time is not good because the process will take

longer time, but the machine stability increases (Guitrau, 1997). The ON-time as a

percentage of total cycle time (inverse of the frequency) is call duty cyle. Figure 2.6

shows three examples of pulse-on with different duty cycle.

(a)

(b)

(c)

Figure 2.6: Pulse-on with different duty cycle (a)25% (b)50% (c)75 % (Guitrau, 1997)

50µs

0

0

I

OFF-time

I

I

025µs

25µs

50µs

0

25µs

ON-time25µs

21

2.3.1.3 Frequency

Frequency is the number of total cycles in one second. Low frequency is usually used in

the roughing operation because it consists of long pulse-on of the spark that yield a

larger, deeper and broader crater. In addition, the recast layer will be thicker and Heat

Affected Zone (HAZ) is deeper because of the length of heat transfer. While, high

frequency is usually for finishing state that can reduce the size of the crater and less

recast (Guitrau, 1997). Figure 2.7 shows three different frequencies of pulse-on.

(a)

(b)

(c)

Figure 2.7: Pulse-on with different frequency (a)low (b)moderate (c)high

(Guitrau, 1997)

100µs

ON

CURRENT

OFF

f=6.67 KHz

50µs

50µs

200µs

ON

CURRENT

OFF

f=4 KHz

50µs

f=10 KHz

50µs

ON

CURRENT

OFF

22

2.3.1.4 Polarity

This parameter determines electric polarity of the electrode and the workpiece. Polarity

refers to the direction of current flow in relation to the electrode (Sommer, 2005).

Polarity can be either reverse (positive) or normal (negative). Polarity can affect speed,

finish, wear and stability. In most cases, positive polarity (reverse polarity) will machine

slower than negative polarity. Despite this disadvantage, positive polarity is often used

to protect the electrode from excessive wear (Fleming, 2005). According to the research

from Khan (2011), higher metal removal rate and lower relative electrode wear are

achieved with normal polarity (positive polarity) but better surface finish is achieved

with reverse polarity (negative polarity). Figure 2.8 shows normal polarity and reverse

polarity in EDM.

(a) (b)

Figure 2.8: EDM polarity (a) Reverse polarity (b) Normal polarity

+ve

Electrode

Workpiece

-ve

PL=-

-ve

Electrode

Workpiece

PL=+

+ve

23

2.3.1.5 Discharge Voltage

Discharge voltage in Electrical Discharge Machining (EDM) is related to the spark gap

and break-down strength of the dielectric (Kansal et al., 2005). Prior to flow of current,

the open gap voltage increases until it has created an ionized path through the dielectric.

Once the current starts to flow, the voltage drops and stabilizes at the working gap level.

The preset voltage determines the width of the spark gap between the leading edge of the

electrode and workpiece. Higher voltage settings increase the gap, which improves the

flushing conditions and helps to stabilize the cut. Electrode wear rate (EWR) and surface

roughness increase by increasing the open circuit voltage because the electric field

strength increases. However, the impact of changing open circuit voltage on surface

hardness after machining has been found to be only marginal (Kansal et al., 2005).

Figure 2.8 shows the actual profile of the single EDM pulse; (a) ionization time, (b)

discharge time, (c) deionization time, and (d) idle time.

Figure 2.9: Actual profile of a single EDM pulse (Fuller, 1996).

On-Time Off-Time

Voltage

Current

Time

a b c d

24

2.3.2 Non-Electrical Parameter

Main non-electrical parameters are flushing the dielectric and type of dielectric. These

non-electrical parameters play a critical role in optimizing the performance measures.

Researches on flushing pressure revealed that it affects the surface roughness, tool wear

rate, acts as a coolant and also plays a vital role in flushing away the debris from the

machining gap (Leonardo and Bruzzone, 1999). Workpiece rotary motion improves the

circulation of the dielectric fluid in the spark gap and temperature distribution of the

workpiece yielding better material removal rate (MRR) and surface roughness (Guu and

Hocheng, 2001).

2.3.2.1 Dielectric

Basic characteristics required for dielectric used in EDM are high dielectric strength and

quick recovery after a breakdown (Wong et al., 1995). The dielectric oils are the best

coolant for most of the EDM machining and impact for machining speed. In EDM,

material removal mainly occurs due to thermal evaporation and melting as thermal

processing is required to be carried out in the absence of oxygen so that the process can

be controlled and avoid oxidation. Oxidation often leads to poor surface conductivity

(electrical) of the workpiece hindering further machining. Hence, dielectric fluid should

provide an oxygen free machining environment. Further, it should have enough strong

dielectric resistance so that it will not electrically breakdown too easily, but at the same

time ionizes when electrons collide with its molecule. Moreover during sparking, it

should be thermally resistant as well (Dewangan, 2010). The sinker EDM process has

primarily used oil in the dielectric fluid. According to Roger Ken from EDM Magazine

Today, the dielectric oil in a Sinker EDM serves a number of functions:

• The dielectric oil acts as a medium through which control the occurrence of

electrical discharges.

147

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