PERFORMANCE EVALUATION OF WIRE ELECTRODE DISCHARGE

MACHINING (WEDM) ON INCONEL 718

MOHD NIZAM BIN ALI

A project report submitted in partial fulfilment of the

requirements for the award of the degree of

Master of Engineering (Mechanical - Advance Manufacturing Technology)

Faculty of Mechanical Engineering

Universiti Teknologi Malaysia

MAY 2010

iii

To my beloved mother and father

Ali bin Mohd Jos

Badariah bt. Md. �oh

My beloved wife and son

Roslina bt. Mamat

Muhammad Ali Imran bin Mohd �izam

iv

ACK�OWLEDGEME�T

In the �ame Of Allah, the most Gracious and most Compassionate

In this opportunity, I would like to exprese my sincere appreciation to my

supervisor, Professor Dr. Safian bin Sharif for the excellence supervision, guidance and

continuous encouragement throughtout this study. Its also a pleasure to acknowledge to

Mr. Ali and other staff of Production Laboratory, Faculty of Mechanical Engineering

Universiti Teknologi Malaysia for their assistance during conducting the experiment.

I would also like to express my special thanks to my wife and son for their

patience and believing in me throughout these years. Not to forget, to all friends

especially Ameizza, Hassanudin, Suhaimi, Nazif and my other colleague who have

contributed directly or indirectly to the success of this project.

v

ABSTRACT

Superalloys are known as unique materials ever produced in manufacturing

industries. It’s capable to withstand in high temperature and the excellent resistance in

mechanical and chemical degradations. Inconel 718 is one of the superalloy material

whichs is which is widely used in aeronautical and aerospace industries. This nickel-

based superalloy is a high strength, thermal resistance with extreme toughness and work

hardening characteristics materials. It is also noted for its excellent corrosion resistance

in many conditions of engineering applications. Due to it extremely tough nature, the

machinability studies of this material had been carried out by many researchers for the

past few years. This master project presents the machining of Inconel 718 using wire

electro-discharge machining with zinc coated brass electrode wire diameter of 0.25mm.

The objective of this master project is mainly to investigate the performance of wire

electro-discharge machining on Inconel 718. This is done by observing the influence of

the various WEDM machining characteristics namely, surface roughness (Ra), sparking

gap (Gap), material removal rate (MRR) and cutting speed (CS). A full factorial design

of experiment (DOE) approach with two-level was employed to conduct this

experiment. Design expert software was used to perform the ANOVA analysis and

confirmation test was also conducted to verify and compare the results from the

theoretical prediction using software. Overall result showed that pulse duration (ON)

was the most significant factor that appeared to influence on all machining

characteristics that had been investigated. The experimental results also acceptable due

to the results obtain fall in acceptable values with less than 15% of margin error.

vi

ABSTRAK

Superaloi telah diketahui umum sebagai bahan yang unik yang pernah dihasilkan

di dalam industri pembuatan. Ianya mampu untuk bertahan pada suhu yang tinggi dan

mempunyai ketahanan yang lasak di dalam pelbagai applikasi kejuruteraan. Inconel 718

adalah salah satu bahan superaloi yang digunakan secara meluas terutamanya di dalam

industri aeronatikal dan angkasa lepas. Superaloi berasaskan bahan nickel ini

mempunyai kekuatan, rintangan haba dan ketahanan karat yang tinggi serta dicirikan

juga dengan pengerasan kerja yang baik. Berdasarkan kepada sifat Inconel 718 yang

tahan lasak, kajian kebolehmesinan bahan ini telah menjadi minat pengkaji sejak

beberapa tahun kebelakangan ini. Projek sarjana ini bertujuan untuk menyiasat prestasi

kebolehmesinan Inconel 718 menggunakan proses pemotongan nyahcas-elekrik

menggunakan wayar elektrod tembaga bersalut zink berdiameter 0.25mm. Ianya

melibatkan ujikaji serta pemerhatian terhadap ciri-ciri pemesinan Inconel 718 seperti

kekasaran permukaan (Ra), jarak percikan api (Gap), kadar pemotongan bahan (MRR)

dan kelajuan pemotongan (CS). Rekabentuk ujikaji dengan pendekatan full factorial dua

tahap telah digunakan di dalam ujikaji ini. Perisian Design Expert juga telah digunakan

untuk tujuan analisa varian (ANOVA) bagi setiap keputusan ujikaji. Bagi tujuan

penentuan ralat, ujikaji pengesahan dilaksanakan untuk menguji kesahihan dan

perbandingan diantara keputusan yang dihasilkan oleh ujikaji dan juga secara teori.

Secara keseluruhannya, keputusan ujikaji menunjukkan tempoh masa denyutan (ON)

adalah faktor yang paling signifikan mempengaruhi semua ciri pemesinan yang dikaji.

Data ujikaji juga menunjukkan perbezaan margin di bawah nilai 15% dan ianya adalah

didalam julat yang boleh diterima pakai di dalam analisa ini.

vii

CO�TE�TS

CHAPTER TITLE PAGE

DECLARATIO� ii

DEDICATIO� iii

ACK�OWLEDGEME�TS iv

ABSTRACT v

ABSTRAK vi

CO�TE�TS vii

LIST OF TABLES xi

LIST OF FIGURES viii

LIST OF APPE�DICES xv

O�E I�TRODUCTIO�

1.1 Project Background & Rationale 1

1.2 Research Statement 3

1.3 Research Objectives 4

1.4 Scope of Study 4

1.5 Expected Results 5

viii

TWO LITERATURE REVIEW

2.1 Introduction 6

2.2 Wire Electrical Discharge Machining (WEDM) 7

2.3 WEDM Machining Parameter 12

2.3.1 Pulse duration (On-time) 13

2.3.2 Pulse interval (Off-Time) 14

2.3.3 Servo voltage 14

2.3.4 Peak current 14

2.4 Machining Characteristic 15

2.4.1 Effect on surface finish, (Ra) 16

2.4.2 Effect on material removal rate, MRR 17

2.4.3 Effect on spark gap, Gap 19

2.5 Wire Electrode 20

2.5.1 Copper wire 21

2.5.2 Brass wire 21

2.5.3 Coated wire 22

2.6 Nickel Based Superalloy and Their Machinibility. 24

2.6.1 Inconel 718 physical properties and mechanical

properties 26

2.7 Design of Experiment (DOE) 27

2.7.1 Two-level full factorial design 28

THREE METHODOLOGY

3.1 Introduction 30

3.2 Research Design and Data Analysis 31

3.3 Research Design Variable 33

3.3.1 Machining parameters 33

3.3.2 Machining characteristic 36

ix

3.3.3 Surface roughness, Ra 36

3.3.4 Sparking gap, Gap 37

3.3.5 Cutting speed, Cs 37

3.3.6 Material removal rate, MRR 37

3.4 Research Procedure 38

3.5 Experimental Set-up 41

3.6 Experimental Equipment 43

FOUR EXPERIME�TAL RESULTS A�D DATA A�ALYSIS

4.1 Introduction 46

4.2 Experimental Results 47

4.3 Analysis of Results 54

4.3.1 Analysis results for surface roughness, Ra 54

4.3.2 Analysis results for sparking gap, Gap 58

4.3.3 Analysis results for material removal rate, MRR 63

4.3.4 Analysis results for cutting speed, CS 68

4.4 Confirmation Test 74

4.4.1 Confirmation test and results 75

4.5 Comparisons of the Test Results 77

4.6 Verification of Mathematical Models 79

FIVE DISCUSSIO�S

5.1 Introduction 83

5.2 Surface Roughness, Ra 84

5.3 Sparking Gap, Ra 85

5.4 Material Removal Rate, MRR 86

5.5 Cutting Speed, CS 88

x

SIX CO�CLUSIO�S A�D RECOMME�DATIO�S

6.1 Conclusions 90

6.2 Recommendations 91

REFERE�CE 92

APPE�DIX A 99

APPE�DIX B 102

APPE�DIX C 105

APPE�DIX D 108

xi

LIST OF TABLES

�O. TITLE PAGE

2.1 The designation of various types of coated wire and their applications 23

2.2 Chemical properties of Inconel 718 (%) 26

2.3 Physical properties of Inconel 718 27

2.4 Mechanical properties of Inconel 718 27

3.1 Two-level full factorial experiment with four factors 32

3.2 The design of machining parameters 34

3.3 Actual value of experiment design 35

3.4 Machining parameters set-up (constant parameters) 41

4.1 Experimental results of surface roughness (Ra) 48

4.2 Experimental results of sparking gap (Gap) 49

4.3 Experimental results for cutting speed (CS) 50

4.5 Experimental results for material removal rate (MRR) 52

4.6 Overall experimental results corresponded to each run 53

4.7 ANOVA for surface roughness (Ra) 55

4.8 ANOVA for sparking gap (Gap) 59

4.9 ANOVA for material removal rate (MMR) 63

4.10 ANOVA for cutting speed (CS) 68

4.11 Summary of the significant factors in WEDM Inconel 718 74

4.12 Quality characteristic of the machining performance 75

4.13 True value of confirmation test experiment 75

xii

�O. TITLE PAGE

4.14 Confirmation test results for surface roughness (Ra) 76

4.15 Confirmation test results for sparking gap 76

4.16 Confirmation test results for cutting speed (CS) 76

4.17 Confirmation test results for material removal rates (MRR) 76

4.18 Comparison test results for surface roughness (Ra) 78

4.19 Comparison test results for sparking gap (Gap) 78

4.20 Comparison test results for material removal rate (MRR) 78

4.21 Comparison test results for cutting speed (CS) 78

4.22 Margin of error for actual results and predicted values (%) 82

xiii

LIST OF FIGURES

�O. TITLE PAGE

2.1 Typical product by WEDM (AGIE Charmilles groups,

Charmilles the solution when to EDM, Geneva 2004) 7

2.2 Classification of EDM processes 8

2.3 Wire electrical discharge machining (WEDM) processes 9

2.4 Schematic of the thermal removal processes of WEDM

(Spark between electrode and workpiece perform the material removal) 10

2.5 Classification of major EDM research areas 11

2.6 Definition of kerf and overcut in WEDM 19

3.1 Research methodology 31

3.2 Flowchart of experiment steping 40

3.3 WEDM linear motor 5 axes – Sodick series AQ537L 44

3.4 Mitutoyo surface roughness measuring machine 44

3.5 Zeiss Axiotech high power optical microscope 45

4.1 Normal probability plot of residuals for surface roughness (Ra) 56

4.2 Residual vs. predict response for surface roughness (Ra) 57

4.3 Main effect plot for surface roughness (Ra) 57

4.4 Normal probability plot of residuals for sparking gap (Gap) 60

4.5 Residual vs. predict response for sparking gap (Gap) 61

4.6 Main effect plot for sparking gap (Gap) 61

4.7 Normal probability plot of residuals for material removal rate (MRR) 65

xiv

�O. TITLE PAGE

4.8 Residual vs. predict response for material removal rate (MRR) 65

4.9 Main effect plot for material removal rate (MRR) 66

4.10 Interaction between SV*OFF for material removal rate (MRR) 67

4.11 Normal probability plot of residuals for cutting speed (CS) 70

4.12 Normal probability plot of residuals for cutting speed (CS) 70

4.13 Main effect plot for cutting speed (CS) 71

4.14 Interaction plot of cutting speed (CS) 72

5.1 3D interaction graph for surface roughness (Ra) 84

5.2 3D interaction graph for sparking gap (Gap) 85

5.3 3D interaction graph for material removal rate 87

5.4 3D interaction graph of IP*SV for cutting speed (CS) 88

5.5 3D interaction graph of OFF*SV for cutting speed (CS) 89

xv

LIST OF APPE�DICES

�O. TITLE PAGE

A1 Schedule for Master project part I (Semester 1 – 2009/2010) 100

A2 Schedule for Master project part I (Semester 1 – 2009/2010) 101

B1 Summary of finding related to EDM performance 103

C1 Experimental results of sparking gap (top surface) 106

C2 Experimental results of sparking gap (bottom surface) 107

D1 Confirmation experimental results of sparking gap (top surface) 109

D2 Confirmation experimental results of sparking gap (bottom surface) 109

1

CHAPTER OE

ITRODUCTIO

1.1 Project Background and Rationale

Electrical discharge machining (EDM) is a non-traditional concept of

machining which has been widely used to produce dies and molds. This technique

has been developed in the late 1940s and has been one of the fast growing methods

in manufacturing during 1980s and 1990s [1].

This non-traditional machining method is commonly used for very hard metals

that would be impossible to machine with traditional techniques. It has been

extensively used, especially for cutting intricate contours or delicate cavities that also

would be difficult to produce with a conventional machining methods or tools.

However, one critical limitation is that EDM only works with electrically conductive

materials. Metal that can be machined by using EDM include nickel-based alloy

(such as inconel), hardened tool steels and carbides.

Wire electrical discharge machining (WEDM) is introduced in the late 60’s.

The process was fairly simple, not complicated and wire choices were limited to

copper and brass only. WEDM is a thermo-electrical process in which material is

2

eroded from the workpiece by a series of discrete sparks between the workpiece and

the wire electrode (tool) separated by a thin film of dielectric fluid (deionized water)

that is continuously fed to the machining zone to flush away the eroded particles. The

movement of wire is controlled numerically to achieve the desired three-dimensional

shape and accuracy of the workpiece [2]. The degree of accuracy of workpiece

dimensions obtainable and the fine surface finishes make WEDM particularly

valuable for applications involving manufacture of stamping dies, extrusion dies and

prototype parts. Without WEDM, the fabrication of precision workpieces requires

many hours of manual grinding and polishing [3].

In recent years, the technology of wire electrical discharge machining

(WEDM) has been improved significantly to meet the requirements in various

manufacturing needs, especially in the precision mold and die industry. WEDM is

being used to machine a wide variety of miniature and micro-parts from metals,

alloys, sintered materials, cemented carbides, ceramics and silicon. This tremendous

achievement in WEDM technology has been achieved by many researchers from

some of the world leading institution and research centre, but still cannot coped with

the new materials introduced to the market.

The selection of cutting parameters for obtaining higher cutting efficiency or

accuracy in WEDM is still not fully solved, even with the most up-to-date CNC

WEDM machine. This is mainly due to the nature of the complicated stochastic

process mechanisms in wire-EDM. As a result, the relationships between the cutting

parameters and the process performance are hard to achieve accurately [4]. There is

still lack of research on WEDming of material such as nickel based super alloy

which include Inconel 718. It is widely used; mostly in aerospace and marine

applications which are classified as difficult to machine material by conventional

method due to high cutting temperature and rapid tool wear [5].

3

1.2 Research Statement

Studies on WEDM using coated wire somehow is limited and manufactures

claimed that the outstanding performance is achieved through their lab test. But the

results are not disclosed to the public and researcher for further study and

understanding. As such the machine materials information and the WEDM

parameters setting for the subjected wire are somehow limited. The only information

given/set by manufactures is commonly applicable to the common steel grades [6].

Inconel 718 is a high strength and thermal resistance (HSTR) [7] known to

play increasingly important in the aviation, space navigation and shipping industries

because of its outstanding multi-properties [8]. Broad bases of Inconel 718

knowledge are now exist due to its great acceptance in industries. However, the

parameter setting on WEDM of Inconel 718 is still lacking. The available

technological data which is based on manufacturers for in house experimentation is

helpful but insufficient.

Inconel 718 is assigned to be machined with WEDM in this project with the

attention to study the parameters setting for an optimum machining. A comparative

study will be carried out between previous study using brass wire and the proposed

study using coated wire electrode.

4

1.3 Research Objectives

The objectives of the research are:

a. To determine the significant parameters that influences the machining

responses during Wire Electro-Discharge Machining (WEDM) of Inconel 718.

b. To evaluate the performance of Electro-Discharge Machining (WEDM) on

Inconel 718 with respect to various responses such as spark gap, material

removal rate, cutting speed and surface finish.

c. To establish mathematical model for spark gap, surface finish, cutting speed

and material removal rate during WEDM of Inconel 718 alloy.

1.4 Scope of Study

The scope of the research consists of:

a. Wire Electro-Discharge Machining, (WEDM) linear motor 5-axis – Sodick

series AQ537L will be employed.

b. Nickel based superalloy, Inconel 718 will be used as the workpiece material.

c. Zinc coated brass wire of diameter 0.25mm will be used as electrode.

d. Parameters to be studied include voltage, peak current, pulse duration and

interval time.

e. Response variable to be study are surface finish, spark gap, material removal

rate and cutting speed.

f. The DOE and analysis of variance (ANOVA) will be processed using Design

Expert software version 7.0.0.

5

1.5 Expected Results

The expected outcomes of this study are as follows:

a. To obtain the optimum condition for WEDM on Inconel 718 in various

parameters setting using zinc coated brass wire.

b. Establishment of mathematical models for various responses during WEDM on

Inconel 718

c. The outcome of the study can be used to assist the industrial practitioners that

involved in machining of superalloy materials such as nickel alloys and to

select the most suitable cutting parameters for machining nickel alloys

application.

d. This will help in improving the quality of Inconel products as well as

minimizing the machining cost to realize the economical potential to the

fullest.