A Study On The Quality Of Micro Hole Machining Of Tungsten Carbide.
ABSTRACT
This paper describes the study of the quality of micro-hole electrical discharge
machining (EDM) of tungsten carbide by using copper electrode. The EDM process
is a thermal machining process, which the conductive workpiece is melted through
repetitive sparks generate through ignition and cut off of voltage and current. The
micro-EDM is gaining popular in recent years due to its advantages in machining
micro-hole of hard and brittle material such as ceramic due to its non contact
machining process. Through this study, the parameters such as peak current, pulse
duration, electrode polarity and flushing were investigated. The setup of the
experiment was based on the design of experiment (DOE) method. Response surface
methodology (RSM) was used to generate the experiment matrix in this study in
order to minimize the runs of experiment. The totals of 26 runs were done by
manipulating the parameters (peak current, pulse duration and flushing). Then, the
quality of the micro-hole machined by using micro-EDM was investigated based on
the surface roughness (SR) of the micro hole and the thickness of heat affected zone
(HAZ). Based upon the experimental results, that surface roughness of machined
surface and HAZ thickness were influenced by pulsed current, pulse duration and
flushing. Lower peak current, pulse duration with flushing produces finer surface
finish. The optimum parameter in order to obtain finest surface is 4.5A peak current,
5µs pulse duration with flushing, which result to 1.264µm surface roughness.
Besides that, higher peak current, pulse duration without flushing produced higher
HAZ thickness.
i
ABSTRAK
Kertas ini membentangkan satu kajian tentang qualiti lubang bersaiz mikro dengan
menggunakan teknologi electrical discharge machining (EDM) ke atas Tungsten
Carbide dengan menggunakan elektrod
kuprum. Process EDM ialah salah satu
thermal machining process. Pengalir elektrik akan dileburkan semasa proses
pemotongan dengan bunga api yang dihasilkan daripada penjanaan voltan dan arus
sebaik sahaja voltan melebihi kekuatan dielektrik cecair di antara elektrod dengan
bahan. Kini, EDM bermikro size telah menjadi lebih popular disebabkan
kebolehanya untuk memotong bahan yang keras dan rapuh tanpa penyentuhan antara
elektrod dengan bahan. Dalam kajian ini, pembolehubah seperti peak current, pulse
duration dan flushing telah dikaji. Eksperimen ini telah dijalankan dengan
menggunakan teknik design of experiment (DOE). Response surface methodology
(RSM) diaplikasikan untuk mendapatkan matrik eksperimen. 26 kali eksperimen
telah dijalankan dengan memanipulasikan pembolehubah (peak current, pulse
duration, electrode dan flushing). Selepas itu, lubang bermikro size itu dikaji dalam
aspek kekasaran permukaan (SR) dan ketebalan heat affected zone (HAZ).
Berdasarkan kajian eksperimen yang telah dibuat, kekasaran permukaan dan
ketebalan HAZ telah dipengaruhi oleh peak current, pulse duration, electrode dan
flushing. Permukaan yang lebih licin boleh didapati dengan menggunakan peak
current dan pulse duration yang rendah dengan flushing. 4.5A peak current, 5µs
pulse duration dengan flushing telah menghasilkan permukaan yang paling licin,
iaitu 1.264µm. Selain itu, ketebalan HAZ yang lebin tebal boleh didapati dengan
menggunakan peak current dan pulse duration yang tinggi tanpa flushing.
ii
DEDICATION
For my Father & Mother
Chua Leong Bee & Cheah Yoke Chun
in gratitude
and to
A youngest brother
Chua Ka Chun
iii
ACKNOWLEDGEMENT
This Bachelor Project, with this technical report, represents a study of total two
semesters in academic year session 2009/10. The objectives could not been achieved
without the consultancy from supervisor. I would like to thank my project supervisor
Miss Liew Pay Jun for her enthusiastic support and guidance during the study.
I am happy to present the following cumulative list of all those individuals who, in
one way or another, made various contributions to this study:
Mr. Nor Mohamad bin Sulong
Mr. Zamree bin Harun
Mr. Ismail bin Ibrahim
Mr. Mohd Farihan Bin Mohammad Sabtu
Finally, many thanks to Faculty of Manufacturing Engineering, Universiti Teknikal
Malaysia Melaka and ADTEC Melaka for giving me the chance to run my Bachelor
Project in the lab.
iv
TABLE OF CONTENT
Abstract
i
Abstrak
ii
Dedication
iii
Acknowledgement
iv
Table of Content
v
List of Tables
viii
List of Figures
x
List Abbreviations
xii
1.
1
INTRODUCTION
1.1
Introduction
1
1.2
Background of Problems
3
1.3
Objectives
4
1.4
Scope
4
1.5
Importance of Study
4
1.6
Expected Result
5
2.
LITERATURE REVIEW
6
2.1
Electrical Discharge Machining (EDM)
6
2.1.1
Micro-EDM
7
2.1.2
Electrical Discharge Machining Process
7
2.1.3
Parameters of Electrical Discharge Machining
8
2.1.4
Responses of Electrical Discharge Machining
10
2.1.5
Electrode Material
11
2.1.6
Workpiece Material
12
2.1.7
Dielectric Fluid
13
2.1.8
Flushing
14
2.2
Design of Experiment (DOE)
15
2.2.1
Response Surface Methodology (RSM)
16
2.2.1.1 Central Composite Design (CCD)
17
v
2.3
Recent Study on Micro-EDM
19
2.4
Summary
23
3.
METHODOLOGY
24
3.1
Design of Experiment (Response Surface Methodology)
24
3.1.1
Variable Machining Parameter
24
3.1.2
Design of Experiment Matrix
25
3.2
Material Preparation
26
3.2.1
Workpiece Material
26
3.2.2
Electrode Material
27
3.3
Machining Equipment and Apparatus
28
3.3.1
Die-sinker EDM Machine
28
3.3.2
Wire EDM Machine
29
3.3.3
Dielectric Fluid
30
3.4
Testing Equipment
30
3.4.1
Surface Roughness Tester
31
3.4.2
Material Microscopy
29
3.4.2.1 Preparation of Metallographic Specimen
32
3.5
Flow Chart
35
3.5.1
Project Flow Chart
35
3.5.2
EDM Flow Chart
37
4.
RESULT AND DISCUSSION
39
4.1
Surface Roughness
39
4.1.1
Experimental Result
39
4.1.2
Analysis
43
4.1.2.1 Transformation
43
4.1.2.2 Fit Summary
44
4.1.2.3 Analysis of Variance (ANOVA)
49
4.1.2.4 Model Graph
51
4.1.3
Optimization
61
4.1.3.1 Optimization Result
61
vi
4.1.4
Confirmation Run
63
4.1.4.1 Confirmation Run Result
63
4.1.4.2 Average Deviation Percentage Value
64
4.2
Heat Affected Zone (HAZ)
65
4.2.1
Effect of Peak Current to Thickness of HAZ
65
4.2.2
Effect of Pulse Duration to Thickness of HAZ
68
4.2.3
Effect of Flushing to Thickness of HAZ
71
5.
CONCLUSION AND RECOMMENDATIONS
74
5.1
Conclusion
74
5.2
Recommendations
76
5.2.1
Recommendations on Result Improvement
76
5.2.2
Recommendations on Further Study
76
REFERENCES
77
APPENDICES
A
Gantt Chart (PSM 1)
B
Gantt Chart (PSM 2)
C
Measured Surface Roughness Value
D
Verification Measurement Report
E
HAZ Micrographs
vii
LIST OF TABLES
1.1
Applications of tungsten carbide in various sectors
3
2.1
Properties of Copper
11
2.2
Methodology review of RSM for recent study
18
2.3
Recent study on Micro-EDM
19
3.1
Variable micro-hole EDM parameters
24
3.2
Experiment matrix
25
3.3
Material properties of tungsten carbide, WC
26
3.4
Material properties of copper, Cu
27
3.5
Constant micro-hole EDM parameters
28
3.6
Wire cut EDM parameters in sectioning of micro-hole
29
3.7
Etchant composition for Tungsten Carbide
34
4.1
Experimental result of surface roughness
40
4.2
Comparison between high-low peak current in response to
41
surface roughness
4.3
Comparison between high-low pulse duration in response to
41
surface roughness
4.4
Comparison of flushing in response to surface roughness
42
4.5
Fit Summary
44
4.6
Calculation of Whitcomb Score
45
4.7
Sequential Model Sum of Squares
46
4.8
Lack of Fit Tests
47
4.9
Model Summary Statistics
48
4.10
ANOVA for Response Quadratic Model
49
4.11
Summary Statistics for the Model
50
4.12
Optimization Solutions
62
4.13
Comparison of predicted result and confirmation run result
63
viii
4.14
Average Deviation Percentage of Confirmation Run
64
4.15
Readings of HAZ Thickness for different peak current
66
4.16
Readings of HAZ Thickness for different pulse duration
69
4.17
Readings of HAZ Thickness for different pulse duration
72
ix
LIST OF FIGURES
2.1
Price versus hardness graph for various materials
13
2.2
Several types of flushing system
14
2.3
Points in the factorial portion
17
2.4
Points in axial points
17
2.5
Factorial and axial portions with centre point
17
3.1
Dimension of workpiece
26
3.2
Mitsubishi Die Sinking EDM EX-30
28
3.3
Mitsubishi wire cut EDM FX-20K
29
3.4
Mitutoyo Portable Surftest, SJ-301
30
3.5
Carl Zeiss Axioskop 2 MAT equipped with Carl Zeiss
31
Axiocam MRc
3.6
Metkon MetaPress-A Automatic Mounting Press
32
3.7
Buehler Grinder and Polisher
33
3.7
Project flow chart
35
3.8
Micro-EDM flow chart
38
4.1
Box-Cox Plot for power transforms
43
4.2
One Factor Graph of Peak Current versus Surface Roughness
51
4.3
One Factor Graph of Pulse Duration versus Surface Roughness
53
4.4
One Factor Graph of Flushing versus Surface Roughness
55
4.5
Perturbation Graphs of Factors versus Surface Roughness
56
4.6
Effect of peak current and pulse duration in surface roughness
57
without flushing
4.7
Effect of peak current and pulse duration in surface roughness
58
with flushing
4.8
Box plot of Factors versus Surface Roughness
60
4.9
Optimization Selection
61
4.10
Micrograph (100x) of a sectioned hole machined at 4A peak current,
65
3µs pulse duration with flushing (Sample 18)
x
4.11
Micrograph (100x) of a sectioned hole machined at 12A peak current,
65
3µs pulse duration with flushing (Sample 22)
4.12
Micrograph (100x) of a sectioned hole machined at 20A peak current,
66
3µs pulse duration with flushing (Sample 19)
4.13
One Factor Graph of Peak Current versus HAZ Thickness
67
4.14
Micrograph (100x) of a sectioned hole machined at 20A peak current,
68
1µs pulse duration without flushing (Sample 2)
4.15
Micrograph (100x) of a sectioned hole machined at 20A peak current,
68
3µs pulse duration without flushing (Sample 6)
4.16
Micrograph (50x) of a sectioned hole machined at 20A peak current,
69
5µs pulse duration without flushing (Sample 4)
4.17
One Factor Graph of Pulse Duration versus HAZ Thickness
70
4.18
Micrograph (100x) of a sectioned hole machined at 20A peak current,
71
1µs pulse duration without flushing (Sample 2)
4.19
Micrograph (100x) of a sectioned hole machined at 20A peak current,
71
1µs pulse duration with flushing (Sample 15)
4.20
One Factor Graph of Flushing versus HAZ Thickness
xi
73
LIST OF ABBREVIATIONS
2FI
-
Two Factor Interaction Model
ANOVA
-
Analysis of Variance
ASTM
-
American Society for Testing and Materials
C. V.
-
Coefficient of Variance
Cu
-
Copper
DC
-
Direct Current
df
-
Number of Degrees of Freedom
DOE
-
Design of Experiment
DVEE
-
Diameter Variation between Entrance and Exit
EDM
-
Electrical Discharge Machining
EWR
-
Electrode Wear Rate
F
-
F Test (ANOVA)
HAZ
-
Heat Affected Zone
HCL
-
Hydrochloric Acid
HNO3
-
Nitric Acid
HOAc
-
Acetic Acid
HV
-
Hardness Vickers
L
-
Lack of Fit Score
M
-
Sequential Model Sum of Squares Score
MRR
-
Material Removal Rate
NC
-
Numerical Control
PRESS
-
Predicted Residual Sum of Squares
Prob
-
Probability
RSM
-
Response Surface Methodology
RWR
-
Relative Wear Rate
SEM
-
Scanning Electron Microscopy
SR
-
Surface Roughness
WC
-
Tungsten Carbide
xii
CHAPTER 1
INTRODUCTION
This chapter section contains the introduction of the electrical discharge machining
(EDM), problem statement, objectives, scope, importance of study and expected
result on this project.
1.1
Introduction
EDM, also known as Electrical Discharge Machining is a non-traditional machining
process, which is greatly applied in the manufacturing sector. This advance
machining process has taken the important role in the human technology
development in machining the superior hardness material, which unable to be done
by conventional machining process such as milling, turning and drilling. Fallbohmer
et al. (1996) surveyed applications of the EDM process and pointed out that almost
90% of mold and die makers employed that EDM process to finish the products in
USA, Germany and Japan.
This technology had been invented by two Russian scientists, who are Dr. B.R.
Lazarenko and Dr. N.I. Lazarenko in 1943. After 37 years of development of this
technology, a Japan scientist named Makino in Japan had successfully integrated the
EDM process with the numerical controlled (NC) system in 1980. By using the NC
system, the machining process can be done more reliability and higher productivity.
1
Kalpakjian and Schmid (2006) have stated that electrical discharge machining (EDM)
is based on the erosion of metal by spark discharges. A basic die-sinker EDM system
consists of a shaped electrode and the workpiece connected to a DC power supply
and placed in a dielectric fluid. During the machining process, the movement of the
workpiece and electrode is controlled by a servo motor, which is connected to the
numerical controller (NC) system. As the gap between the electrode and the
workpiece is sufficiently small, the current will create an ionized channel in the gap.
Then, the spark is broken down and this made the ionized channel collapse very
quickly. This has transfer a sufficient energy to melt the workpiece and the molten
metal on the surface of the material evaporates explosively, forming a small crater.
Finally, the eroded metal is flushed away by the dielectric fluid and the processes are
repeated until the shaped is formed.
In the past, the electrical discharge machining can only be done to machine large part.
In 20th century beyond, there is a new trend of the EDM development towards micro
machining in order to machine the parts with micro size texture and hole. The reason
leads to this trend is gaining popularity is due to the recent advancement in microelectro mechanical system according to Lim et al. (2003).
As development of human technology, the reduction of product size is highly
advantageous in order to suite the product with multi functional purpose, more
compact and easy to carry. Hence, the EDM process plays an important role to
machine these micro parts due to its advantages in higher precision and capability in
machining high hardness, strength and brittle part. This leads to the great advantage
in machining the new hard material such as tungsten carbide, titanium alloy, nickel
alloy and powdered metallurgy steel.
Among these hard materials, tungsten carbide is the most popular material due to its
wide range of applications from machining tools, aerospace, military, medical, sport
to domestic product for consumer as listen in Table 1.1.
2
Table 1.1: Applications of tungsten carbide in various sectors
1.2
Sector
Application
Machine Tool
Tungsten carbide tool and insert
Aerospace
Nozzle body, Fuel inlet fitting
Military
Armor-piercing ammunition and shell
Medical
Surgery Equipment
Consumer product
Ink jet nozzles in printer cartridge
Background of problems
In military sector, the expert tends to design the missile to be smaller size so that the
penetration power can be focused and higher to destroy the object such as armored
tank and concrete wall. This armor-piercing ammunition is normally made by
tungsten carbide and other alloy due to its extreme hardness and advantage in
penetration. On the structure of the missile, there are lots of parts in micro size so
that it is highly sensitive. This has lead to important role in micro machining to
machine these hard materials into precise and accurate parts.
Besides that, in aerospace sector, the nozzle body and fuel inlet fitting is also
designed with micro size hole. Hence, the machining process is very important in
fabricate these components so that it have the excellent surface quality besides
having good accuracy and precision. This is due to the rougher surface will generate
higher wall friction between the fuel and the nozzle and this will decrease the
efficiency.
By referring to the problems stated above, there is very essential to study the
parameters, which can affect the quality of the micro hole machined by using diesinker EDM. Hence, this project is very important to determine the best parameters
in machining product with good surface quality of the micro hole with diameter of
500μm on tungsten carbide by using die sinker EDM.
3
1.3
Objectives
The objectives of this project are to:
1.
Study the effect of using different machining parameters (peak current, pulse
duration and flushing) to the responses of surface roughness and thickness of
heat affected zone by EDM process.
2.
Determine the optimum parameters to obtain finest surface roughness in
micro-hole EDM by using Response Surface Methodology (RSM).
1.4
Scope
This project includes manipulating of the machining parameter of EDM such as peak
current, pulse duration and with/without jet flushing. The type of electrode is copper
and the material is tungsten carbide, WC. This project studied the effect of the
responses of surface roughness (SR) and the thickness of the heat affected zone
(HAZ), machined by different parameters. The responses will then be measured and
inspected by using surface roughness tester and optical microscopy. The optimization
in this study was only being done to surface roughness. Along this project, the design
of experiment (DOE) was applied together with response surface methodology
(RSM).
1.5
Importance of Study
This project is very important to the operating of the die sinker EDM process in order
to run the machining process in most effective way to get the best surface finish by
manipulating the machining parameter. Besides that, this is very important especially
to the aerospace and automotive industry since many of their parts require excellent
surface finishing and these industries using lots of extremely hard material, which is
hard to machine with conventional machining process.
4
1.6
Expected Result
At the end of this study, it was expected that the effect of the machining parameter
(peak current, pulse duration and with/without flushing) to the response (surface
roughness and thickness of HAZ). Besides that, an optimum machining parameter for
surface roughness will be determined through the response surface methodology.
5
CHAPTER 2
LITERATURE REVIEW
This chapter summarizes the theory and review of this project. Besides that, it also
includes the studies made by the researchers recently.
2.1
Electrical Discharge Machining (EDM)
Electrical Discharge Machining (EDM) is an advanced manufacturing process,
which is used commonly in machining the high hardness, very brittle and electrical
conductive material. Suleiman and Ahsan (2007) have stated the example of hard
material such as silicon, titanium, stainless steel, hardened steel, nickel, tungsten
alloys etc. that are difficult to machine with traditional methods. This machining
process has a unique machining process, which different from others. Norliana et al.
(2006) have stated that EDM does not make direct contact between the electrode and
the workpiece. Therefore, the machining is carried out by inducing thermoelectric
energy created between workpiece and electrode submerged in a dielectric fluid
according to Pham et al. (2004). Tlusty (2000) has noted that a typical Electrical
Discharge machine consist of a servo controlled motor system, electrode, dielectric
fluid, direct current pulse generator and the fixture to hold workpiece.
6
2.1.1
Micro EDM
Micro EDM is the machining process to machine micro parts and drill small hole
without contact among workpiece and electrode. Micro EDM is basically scaled
down version of the EDM process according to Allen et al. (1999). Lim et al. (2003)
verified that the micro EDM is the most efficient technology for fabricating micro
components. It means that the electrode used is in micro size from as low as 50μm.
Pham et al. (2004) also stated that the micro EDM is the best means for achieving
high aspect ratio micro features. It means that the length of the tool is usually 3-5
times of its diameter according to Lim et al. (2003). Therefore, it is hard to run the
machining by traditional machining process due to high probability of tool vibration
of high aspect ratio micro tool. The high tool vibration of traditional machining will
result of out of tolerance, rougher surface and sometimes damage the workpiece.
Hence, EDM is the best choice for this type of process due to no contact between the
tool and workpiece along the machining process.
2.1.2 Electrical Discharge Machining Process
Luis et al. (2005) mentioned that the EDM process is based on removing material
from a part by means of series of repeated electrical discharge between tool called
the electrode and the workpiece in the presence of dielectric fluid. Bojorquez et al.
(2002) stated the electrode is moved toward the workpiece until the gap is small
enough so that the impressed voltage is great enough to ionize the dielectric. Luis et
al. (2005) had shown that the machining is accomplished through spark, which can
generate a temperature between 10,000oC – 12,000oC. This temperature is large
enough to melt most types of material. According to Marafona and Chousal (2005),
the material is removed with the erosive effect of the electrical discharges tool to
workpiece. Finally, the eroded debris is flushed away by dielectric fluid and the
machining processes are repeated until the shape is formed.
7
2.1.3
Parameters of Electrical Discharge Machining
The parameters of the EDM must be controlled in very effective in order to get the
optimum tolerance and surface finish of the machined workpiece. According to Todd
et al. (1994), the parameters that affect the EDM process include accuracy and
alignment of electrode, pulse frequency, types of dielectric fluid, spark gap between
electrode and workpiece. Besides that, Tlusty (2000) also stated that magnitude of
peak current and power source characteristics will also affect the machined part
response. Liu et al. (2008) has proved that the machining polarity of electrode, pulse
duration also affect the machining result. Yao and Masuzawa (2004) had also
showed that the types of material used for electrode is also important parameter
during machining.
The parameters such as peak current, dielectric used and frequency have great effect
on the machined surface roughness according to Schey (2000). He had noted that
surface finish rougher by higher current density (which gives high discharge
energies), more viscous dielectric and lower pulse frequency. The rougher finish will
result to more overcut and deeper Heat Affected Zone (HAZ). Besides that, the
debris should also be flushed out to prevent short circuit.
The EDM parameters according to Kumar et al. (2009):
(a)
Discharge voltage
Before current can flow, the open gap voltage increases until it has created an
ionization path through the dielectric. Once the current flow, voltage drops
and stabilizes at the working gap level. The preset voltage determines the
width of the spark gap between electrode and workpiece. Higher voltage
settings increase the gap, which improve the flushing conditions and helps to
stabilize the cut.
8
This paper describes the study of the quality of micro-hole electrical discharge
machining (EDM) of tungsten carbide by using copper electrode. The EDM process
is a thermal machining process, which the conductive workpiece is melted through
repetitive sparks generate through ignition and cut off of voltage and current. The
micro-EDM is gaining popular in recent years due to its advantages in machining
micro-hole of hard and brittle material such as ceramic due to its non contact
machining process. Through this study, the parameters such as peak current, pulse
duration, electrode polarity and flushing were investigated. The setup of the
experiment was based on the design of experiment (DOE) method. Response surface
methodology (RSM) was used to generate the experiment matrix in this study in
order to minimize the runs of experiment. The totals of 26 runs were done by
manipulating the parameters (peak current, pulse duration and flushing). Then, the
quality of the micro-hole machined by using micro-EDM was investigated based on
the surface roughness (SR) of the micro hole and the thickness of heat affected zone
(HAZ). Based upon the experimental results, that surface roughness of machined
surface and HAZ thickness were influenced by pulsed current, pulse duration and
flushing. Lower peak current, pulse duration with flushing produces finer surface
finish. The optimum parameter in order to obtain finest surface is 4.5A peak current,
5µs pulse duration with flushing, which result to 1.264µm surface roughness.
Besides that, higher peak current, pulse duration without flushing produced higher
HAZ thickness.
i
ABSTRAK
Kertas ini membentangkan satu kajian tentang qualiti lubang bersaiz mikro dengan
menggunakan teknologi electrical discharge machining (EDM) ke atas Tungsten
Carbide dengan menggunakan elektrod
kuprum. Process EDM ialah salah satu
thermal machining process. Pengalir elektrik akan dileburkan semasa proses
pemotongan dengan bunga api yang dihasilkan daripada penjanaan voltan dan arus
sebaik sahaja voltan melebihi kekuatan dielektrik cecair di antara elektrod dengan
bahan. Kini, EDM bermikro size telah menjadi lebih popular disebabkan
kebolehanya untuk memotong bahan yang keras dan rapuh tanpa penyentuhan antara
elektrod dengan bahan. Dalam kajian ini, pembolehubah seperti peak current, pulse
duration dan flushing telah dikaji. Eksperimen ini telah dijalankan dengan
menggunakan teknik design of experiment (DOE). Response surface methodology
(RSM) diaplikasikan untuk mendapatkan matrik eksperimen. 26 kali eksperimen
telah dijalankan dengan memanipulasikan pembolehubah (peak current, pulse
duration, electrode dan flushing). Selepas itu, lubang bermikro size itu dikaji dalam
aspek kekasaran permukaan (SR) dan ketebalan heat affected zone (HAZ).
Berdasarkan kajian eksperimen yang telah dibuat, kekasaran permukaan dan
ketebalan HAZ telah dipengaruhi oleh peak current, pulse duration, electrode dan
flushing. Permukaan yang lebih licin boleh didapati dengan menggunakan peak
current dan pulse duration yang rendah dengan flushing. 4.5A peak current, 5µs
pulse duration dengan flushing telah menghasilkan permukaan yang paling licin,
iaitu 1.264µm. Selain itu, ketebalan HAZ yang lebin tebal boleh didapati dengan
menggunakan peak current dan pulse duration yang tinggi tanpa flushing.
ii
DEDICATION
For my Father & Mother
Chua Leong Bee & Cheah Yoke Chun
in gratitude
and to
A youngest brother
Chua Ka Chun
iii
ACKNOWLEDGEMENT
This Bachelor Project, with this technical report, represents a study of total two
semesters in academic year session 2009/10. The objectives could not been achieved
without the consultancy from supervisor. I would like to thank my project supervisor
Miss Liew Pay Jun for her enthusiastic support and guidance during the study.
I am happy to present the following cumulative list of all those individuals who, in
one way or another, made various contributions to this study:
Mr. Nor Mohamad bin Sulong
Mr. Zamree bin Harun
Mr. Ismail bin Ibrahim
Mr. Mohd Farihan Bin Mohammad Sabtu
Finally, many thanks to Faculty of Manufacturing Engineering, Universiti Teknikal
Malaysia Melaka and ADTEC Melaka for giving me the chance to run my Bachelor
Project in the lab.
iv
TABLE OF CONTENT
Abstract
i
Abstrak
ii
Dedication
iii
Acknowledgement
iv
Table of Content
v
List of Tables
viii
List of Figures
x
List Abbreviations
xii
1.
1
INTRODUCTION
1.1
Introduction
1
1.2
Background of Problems
3
1.3
Objectives
4
1.4
Scope
4
1.5
Importance of Study
4
1.6
Expected Result
5
2.
LITERATURE REVIEW
6
2.1
Electrical Discharge Machining (EDM)
6
2.1.1
Micro-EDM
7
2.1.2
Electrical Discharge Machining Process
7
2.1.3
Parameters of Electrical Discharge Machining
8
2.1.4
Responses of Electrical Discharge Machining
10
2.1.5
Electrode Material
11
2.1.6
Workpiece Material
12
2.1.7
Dielectric Fluid
13
2.1.8
Flushing
14
2.2
Design of Experiment (DOE)
15
2.2.1
Response Surface Methodology (RSM)
16
2.2.1.1 Central Composite Design (CCD)
17
v
2.3
Recent Study on Micro-EDM
19
2.4
Summary
23
3.
METHODOLOGY
24
3.1
Design of Experiment (Response Surface Methodology)
24
3.1.1
Variable Machining Parameter
24
3.1.2
Design of Experiment Matrix
25
3.2
Material Preparation
26
3.2.1
Workpiece Material
26
3.2.2
Electrode Material
27
3.3
Machining Equipment and Apparatus
28
3.3.1
Die-sinker EDM Machine
28
3.3.2
Wire EDM Machine
29
3.3.3
Dielectric Fluid
30
3.4
Testing Equipment
30
3.4.1
Surface Roughness Tester
31
3.4.2
Material Microscopy
29
3.4.2.1 Preparation of Metallographic Specimen
32
3.5
Flow Chart
35
3.5.1
Project Flow Chart
35
3.5.2
EDM Flow Chart
37
4.
RESULT AND DISCUSSION
39
4.1
Surface Roughness
39
4.1.1
Experimental Result
39
4.1.2
Analysis
43
4.1.2.1 Transformation
43
4.1.2.2 Fit Summary
44
4.1.2.3 Analysis of Variance (ANOVA)
49
4.1.2.4 Model Graph
51
4.1.3
Optimization
61
4.1.3.1 Optimization Result
61
vi
4.1.4
Confirmation Run
63
4.1.4.1 Confirmation Run Result
63
4.1.4.2 Average Deviation Percentage Value
64
4.2
Heat Affected Zone (HAZ)
65
4.2.1
Effect of Peak Current to Thickness of HAZ
65
4.2.2
Effect of Pulse Duration to Thickness of HAZ
68
4.2.3
Effect of Flushing to Thickness of HAZ
71
5.
CONCLUSION AND RECOMMENDATIONS
74
5.1
Conclusion
74
5.2
Recommendations
76
5.2.1
Recommendations on Result Improvement
76
5.2.2
Recommendations on Further Study
76
REFERENCES
77
APPENDICES
A
Gantt Chart (PSM 1)
B
Gantt Chart (PSM 2)
C
Measured Surface Roughness Value
D
Verification Measurement Report
E
HAZ Micrographs
vii
LIST OF TABLES
1.1
Applications of tungsten carbide in various sectors
3
2.1
Properties of Copper
11
2.2
Methodology review of RSM for recent study
18
2.3
Recent study on Micro-EDM
19
3.1
Variable micro-hole EDM parameters
24
3.2
Experiment matrix
25
3.3
Material properties of tungsten carbide, WC
26
3.4
Material properties of copper, Cu
27
3.5
Constant micro-hole EDM parameters
28
3.6
Wire cut EDM parameters in sectioning of micro-hole
29
3.7
Etchant composition for Tungsten Carbide
34
4.1
Experimental result of surface roughness
40
4.2
Comparison between high-low peak current in response to
41
surface roughness
4.3
Comparison between high-low pulse duration in response to
41
surface roughness
4.4
Comparison of flushing in response to surface roughness
42
4.5
Fit Summary
44
4.6
Calculation of Whitcomb Score
45
4.7
Sequential Model Sum of Squares
46
4.8
Lack of Fit Tests
47
4.9
Model Summary Statistics
48
4.10
ANOVA for Response Quadratic Model
49
4.11
Summary Statistics for the Model
50
4.12
Optimization Solutions
62
4.13
Comparison of predicted result and confirmation run result
63
viii
4.14
Average Deviation Percentage of Confirmation Run
64
4.15
Readings of HAZ Thickness for different peak current
66
4.16
Readings of HAZ Thickness for different pulse duration
69
4.17
Readings of HAZ Thickness for different pulse duration
72
ix
LIST OF FIGURES
2.1
Price versus hardness graph for various materials
13
2.2
Several types of flushing system
14
2.3
Points in the factorial portion
17
2.4
Points in axial points
17
2.5
Factorial and axial portions with centre point
17
3.1
Dimension of workpiece
26
3.2
Mitsubishi Die Sinking EDM EX-30
28
3.3
Mitsubishi wire cut EDM FX-20K
29
3.4
Mitutoyo Portable Surftest, SJ-301
30
3.5
Carl Zeiss Axioskop 2 MAT equipped with Carl Zeiss
31
Axiocam MRc
3.6
Metkon MetaPress-A Automatic Mounting Press
32
3.7
Buehler Grinder and Polisher
33
3.7
Project flow chart
35
3.8
Micro-EDM flow chart
38
4.1
Box-Cox Plot for power transforms
43
4.2
One Factor Graph of Peak Current versus Surface Roughness
51
4.3
One Factor Graph of Pulse Duration versus Surface Roughness
53
4.4
One Factor Graph of Flushing versus Surface Roughness
55
4.5
Perturbation Graphs of Factors versus Surface Roughness
56
4.6
Effect of peak current and pulse duration in surface roughness
57
without flushing
4.7
Effect of peak current and pulse duration in surface roughness
58
with flushing
4.8
Box plot of Factors versus Surface Roughness
60
4.9
Optimization Selection
61
4.10
Micrograph (100x) of a sectioned hole machined at 4A peak current,
65
3µs pulse duration with flushing (Sample 18)
x
4.11
Micrograph (100x) of a sectioned hole machined at 12A peak current,
65
3µs pulse duration with flushing (Sample 22)
4.12
Micrograph (100x) of a sectioned hole machined at 20A peak current,
66
3µs pulse duration with flushing (Sample 19)
4.13
One Factor Graph of Peak Current versus HAZ Thickness
67
4.14
Micrograph (100x) of a sectioned hole machined at 20A peak current,
68
1µs pulse duration without flushing (Sample 2)
4.15
Micrograph (100x) of a sectioned hole machined at 20A peak current,
68
3µs pulse duration without flushing (Sample 6)
4.16
Micrograph (50x) of a sectioned hole machined at 20A peak current,
69
5µs pulse duration without flushing (Sample 4)
4.17
One Factor Graph of Pulse Duration versus HAZ Thickness
70
4.18
Micrograph (100x) of a sectioned hole machined at 20A peak current,
71
1µs pulse duration without flushing (Sample 2)
4.19
Micrograph (100x) of a sectioned hole machined at 20A peak current,
71
1µs pulse duration with flushing (Sample 15)
4.20
One Factor Graph of Flushing versus HAZ Thickness
xi
73
LIST OF ABBREVIATIONS
2FI
-
Two Factor Interaction Model
ANOVA
-
Analysis of Variance
ASTM
-
American Society for Testing and Materials
C. V.
-
Coefficient of Variance
Cu
-
Copper
DC
-
Direct Current
df
-
Number of Degrees of Freedom
DOE
-
Design of Experiment
DVEE
-
Diameter Variation between Entrance and Exit
EDM
-
Electrical Discharge Machining
EWR
-
Electrode Wear Rate
F
-
F Test (ANOVA)
HAZ
-
Heat Affected Zone
HCL
-
Hydrochloric Acid
HNO3
-
Nitric Acid
HOAc
-
Acetic Acid
HV
-
Hardness Vickers
L
-
Lack of Fit Score
M
-
Sequential Model Sum of Squares Score
MRR
-
Material Removal Rate
NC
-
Numerical Control
PRESS
-
Predicted Residual Sum of Squares
Prob
-
Probability
RSM
-
Response Surface Methodology
RWR
-
Relative Wear Rate
SEM
-
Scanning Electron Microscopy
SR
-
Surface Roughness
WC
-
Tungsten Carbide
xii
CHAPTER 1
INTRODUCTION
This chapter section contains the introduction of the electrical discharge machining
(EDM), problem statement, objectives, scope, importance of study and expected
result on this project.
1.1
Introduction
EDM, also known as Electrical Discharge Machining is a non-traditional machining
process, which is greatly applied in the manufacturing sector. This advance
machining process has taken the important role in the human technology
development in machining the superior hardness material, which unable to be done
by conventional machining process such as milling, turning and drilling. Fallbohmer
et al. (1996) surveyed applications of the EDM process and pointed out that almost
90% of mold and die makers employed that EDM process to finish the products in
USA, Germany and Japan.
This technology had been invented by two Russian scientists, who are Dr. B.R.
Lazarenko and Dr. N.I. Lazarenko in 1943. After 37 years of development of this
technology, a Japan scientist named Makino in Japan had successfully integrated the
EDM process with the numerical controlled (NC) system in 1980. By using the NC
system, the machining process can be done more reliability and higher productivity.
1
Kalpakjian and Schmid (2006) have stated that electrical discharge machining (EDM)
is based on the erosion of metal by spark discharges. A basic die-sinker EDM system
consists of a shaped electrode and the workpiece connected to a DC power supply
and placed in a dielectric fluid. During the machining process, the movement of the
workpiece and electrode is controlled by a servo motor, which is connected to the
numerical controller (NC) system. As the gap between the electrode and the
workpiece is sufficiently small, the current will create an ionized channel in the gap.
Then, the spark is broken down and this made the ionized channel collapse very
quickly. This has transfer a sufficient energy to melt the workpiece and the molten
metal on the surface of the material evaporates explosively, forming a small crater.
Finally, the eroded metal is flushed away by the dielectric fluid and the processes are
repeated until the shaped is formed.
In the past, the electrical discharge machining can only be done to machine large part.
In 20th century beyond, there is a new trend of the EDM development towards micro
machining in order to machine the parts with micro size texture and hole. The reason
leads to this trend is gaining popularity is due to the recent advancement in microelectro mechanical system according to Lim et al. (2003).
As development of human technology, the reduction of product size is highly
advantageous in order to suite the product with multi functional purpose, more
compact and easy to carry. Hence, the EDM process plays an important role to
machine these micro parts due to its advantages in higher precision and capability in
machining high hardness, strength and brittle part. This leads to the great advantage
in machining the new hard material such as tungsten carbide, titanium alloy, nickel
alloy and powdered metallurgy steel.
Among these hard materials, tungsten carbide is the most popular material due to its
wide range of applications from machining tools, aerospace, military, medical, sport
to domestic product for consumer as listen in Table 1.1.
2
Table 1.1: Applications of tungsten carbide in various sectors
1.2
Sector
Application
Machine Tool
Tungsten carbide tool and insert
Aerospace
Nozzle body, Fuel inlet fitting
Military
Armor-piercing ammunition and shell
Medical
Surgery Equipment
Consumer product
Ink jet nozzles in printer cartridge
Background of problems
In military sector, the expert tends to design the missile to be smaller size so that the
penetration power can be focused and higher to destroy the object such as armored
tank and concrete wall. This armor-piercing ammunition is normally made by
tungsten carbide and other alloy due to its extreme hardness and advantage in
penetration. On the structure of the missile, there are lots of parts in micro size so
that it is highly sensitive. This has lead to important role in micro machining to
machine these hard materials into precise and accurate parts.
Besides that, in aerospace sector, the nozzle body and fuel inlet fitting is also
designed with micro size hole. Hence, the machining process is very important in
fabricate these components so that it have the excellent surface quality besides
having good accuracy and precision. This is due to the rougher surface will generate
higher wall friction between the fuel and the nozzle and this will decrease the
efficiency.
By referring to the problems stated above, there is very essential to study the
parameters, which can affect the quality of the micro hole machined by using diesinker EDM. Hence, this project is very important to determine the best parameters
in machining product with good surface quality of the micro hole with diameter of
500μm on tungsten carbide by using die sinker EDM.
3
1.3
Objectives
The objectives of this project are to:
1.
Study the effect of using different machining parameters (peak current, pulse
duration and flushing) to the responses of surface roughness and thickness of
heat affected zone by EDM process.
2.
Determine the optimum parameters to obtain finest surface roughness in
micro-hole EDM by using Response Surface Methodology (RSM).
1.4
Scope
This project includes manipulating of the machining parameter of EDM such as peak
current, pulse duration and with/without jet flushing. The type of electrode is copper
and the material is tungsten carbide, WC. This project studied the effect of the
responses of surface roughness (SR) and the thickness of the heat affected zone
(HAZ), machined by different parameters. The responses will then be measured and
inspected by using surface roughness tester and optical microscopy. The optimization
in this study was only being done to surface roughness. Along this project, the design
of experiment (DOE) was applied together with response surface methodology
(RSM).
1.5
Importance of Study
This project is very important to the operating of the die sinker EDM process in order
to run the machining process in most effective way to get the best surface finish by
manipulating the machining parameter. Besides that, this is very important especially
to the aerospace and automotive industry since many of their parts require excellent
surface finishing and these industries using lots of extremely hard material, which is
hard to machine with conventional machining process.
4
1.6
Expected Result
At the end of this study, it was expected that the effect of the machining parameter
(peak current, pulse duration and with/without flushing) to the response (surface
roughness and thickness of HAZ). Besides that, an optimum machining parameter for
surface roughness will be determined through the response surface methodology.
5
CHAPTER 2
LITERATURE REVIEW
This chapter summarizes the theory and review of this project. Besides that, it also
includes the studies made by the researchers recently.
2.1
Electrical Discharge Machining (EDM)
Electrical Discharge Machining (EDM) is an advanced manufacturing process,
which is used commonly in machining the high hardness, very brittle and electrical
conductive material. Suleiman and Ahsan (2007) have stated the example of hard
material such as silicon, titanium, stainless steel, hardened steel, nickel, tungsten
alloys etc. that are difficult to machine with traditional methods. This machining
process has a unique machining process, which different from others. Norliana et al.
(2006) have stated that EDM does not make direct contact between the electrode and
the workpiece. Therefore, the machining is carried out by inducing thermoelectric
energy created between workpiece and electrode submerged in a dielectric fluid
according to Pham et al. (2004). Tlusty (2000) has noted that a typical Electrical
Discharge machine consist of a servo controlled motor system, electrode, dielectric
fluid, direct current pulse generator and the fixture to hold workpiece.
6
2.1.1
Micro EDM
Micro EDM is the machining process to machine micro parts and drill small hole
without contact among workpiece and electrode. Micro EDM is basically scaled
down version of the EDM process according to Allen et al. (1999). Lim et al. (2003)
verified that the micro EDM is the most efficient technology for fabricating micro
components. It means that the electrode used is in micro size from as low as 50μm.
Pham et al. (2004) also stated that the micro EDM is the best means for achieving
high aspect ratio micro features. It means that the length of the tool is usually 3-5
times of its diameter according to Lim et al. (2003). Therefore, it is hard to run the
machining by traditional machining process due to high probability of tool vibration
of high aspect ratio micro tool. The high tool vibration of traditional machining will
result of out of tolerance, rougher surface and sometimes damage the workpiece.
Hence, EDM is the best choice for this type of process due to no contact between the
tool and workpiece along the machining process.
2.1.2 Electrical Discharge Machining Process
Luis et al. (2005) mentioned that the EDM process is based on removing material
from a part by means of series of repeated electrical discharge between tool called
the electrode and the workpiece in the presence of dielectric fluid. Bojorquez et al.
(2002) stated the electrode is moved toward the workpiece until the gap is small
enough so that the impressed voltage is great enough to ionize the dielectric. Luis et
al. (2005) had shown that the machining is accomplished through spark, which can
generate a temperature between 10,000oC – 12,000oC. This temperature is large
enough to melt most types of material. According to Marafona and Chousal (2005),
the material is removed with the erosive effect of the electrical discharges tool to
workpiece. Finally, the eroded debris is flushed away by dielectric fluid and the
machining processes are repeated until the shape is formed.
7
2.1.3
Parameters of Electrical Discharge Machining
The parameters of the EDM must be controlled in very effective in order to get the
optimum tolerance and surface finish of the machined workpiece. According to Todd
et al. (1994), the parameters that affect the EDM process include accuracy and
alignment of electrode, pulse frequency, types of dielectric fluid, spark gap between
electrode and workpiece. Besides that, Tlusty (2000) also stated that magnitude of
peak current and power source characteristics will also affect the machined part
response. Liu et al. (2008) has proved that the machining polarity of electrode, pulse
duration also affect the machining result. Yao and Masuzawa (2004) had also
showed that the types of material used for electrode is also important parameter
during machining.
The parameters such as peak current, dielectric used and frequency have great effect
on the machined surface roughness according to Schey (2000). He had noted that
surface finish rougher by higher current density (which gives high discharge
energies), more viscous dielectric and lower pulse frequency. The rougher finish will
result to more overcut and deeper Heat Affected Zone (HAZ). Besides that, the
debris should also be flushed out to prevent short circuit.
The EDM parameters according to Kumar et al. (2009):
(a)
Discharge voltage
Before current can flow, the open gap voltage increases until it has created an
ionization path through the dielectric. Once the current flow, voltage drops
and stabilizes at the working gap level. The preset voltage determines the
width of the spark gap between electrode and workpiece. Higher voltage
settings increase the gap, which improve the flushing conditions and helps to
stabilize the cut.
8