Electrical Discharge Machining on Biodegradable AZ31 Magnesium Alloy Using Taguchi Method

Abstract

Magnesium alloy is highly potential material for biodegradable implant application. Due to limitations in conventional machining methods, non-traditional machining method such as electrical discharge machining (EDM) die sinking process is proposed to produce intricate shape with tight tolerance on magnesium alloy. Nine EDM experiments with three levels and four parameters were conducted using Taguchi method on AZ31 magnesium alloy to explore the optimum machining parameters. It was found that pulse on-time was the most significant parameter affecting the surface roughness (Ra) of the machined surface. The optimum EDM condition obtained was 47 A peak current, 80V voltage, 16μs pulse on-time and 512μs pulse off-time. A confirmation test was conducted and the result shows 95.5% similarity with the predicted Ra. However, the formation of cracks and craters were found on the machined surface area. It is proposed to solve this problem by applying powder mixed EDM method in future research work.

Full text

Procedia Engineering 148 ( 2016 ) 916 – 922
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of ICPEAM 2016
doi: 10.1016/j.proeng.2016.06.501
ScienceDirect
Available online at www.sciencedirect.com
4th International Conference on Process Engineering and Advanced Materials
Electrical Discharge Machining on Biodegradable AZ31
Magnesium Alloy using Taguchi method
M.A. Razaka,c,
*
, A.M. Abdul-Rania, T.V.V.L.N. Raoa, S.R. Pedapatia, S. Kamalb
aMechanical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Perak, Malaysia
bPetroleum Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Perak, Malaysia
cManufacturing Section, Universiti Kuala Lumpur Malaysian Spanish Institute, Kulim Hi-Tech Park, 09000 Kedah, Malaysia
Abstract
Magnesium alloy is highly potential material for biodegradable implant application. Due to limitations in conventional machining
methods, non-traditional machining method such as electrical discharge machining (EDM) die sinking process is proposed to
produce intricate shape with tight tolerance on magnesium alloy. Nine EDM experiments with three levels and four parameters
were conducted using Taguchi method on AZ31 magnesium alloy to explore the optimum machining parameters. It was found
that pulse on-time was the most significant parameter affecting the surface roughness (Ra) of the machined surface. The optimum
EDM condition obtained was 47 A peak current, 80 V voltage, 16 μs pulse on-time and 512 μs pulse off-time. A confirmation
test was conducted and the result shows 95.5% similarity with the predicted Ra. However, the formation of cracks and craters
were found on the machined surface area. It is proposed to solve this problem by applying powder mixed EDM method in future
research work.
© 2016 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the organizing committee of ICPEAM 2016.
Keywords: Biodegradable material; EDM; Magnesium alloy; Taguchi method
1. Introduction
Magnesium alloy is a biocompatible material that can be used as temporary biomedical implant with high
potential for in-vivo degradation [1]. Magnesium has the ability to dissolve in biological environment such as a
* Corresponding author. Tel.: +604-403-5199; fax: +604-403-5201.
E-mail address: alhapis@unikl.edu.my
© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of ICPEAM 2016

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M.A. Razak et al. / Procedia Engineering 148 ( 2016 ) 916 – 922
human body and yet remain non-toxic [2-4]. In average, human body contain 35 g magnesium per 70 kg body
weight. Human has been established to require up to 375 mg of magnesium daily [5]. The density of magnesium is
1.738 g/cm3 while the density of magnesium alloys is between 1.75 g/cm3 and 1.85 g/cm3. They are very similar to
human cortical bone density which is 1.75 g/cm3 which made it ideal [6]. Magnesium alloy possess mechanical
tensile strength between 185 to 230 MPa and fracture toughness between 17.6 to 20.7 MPa·m1/2 suitable for bone
fixation device [7, 8]. Several magnesium alloys suitable for biomedical applications including magnesium-calcium
(Mg-Ca), magnesium-zinc (Mg-Zn) and magnesium-aluminum-zinc (AZ31 and AZ91) [9-12].
However, machining magnesium alloys using conventional methods such as milling, turning and drilling cause
formation of cracks, built-up edge and chatter. The most important precaution need to bear in mind while machining
magnesium alloy is that the formation of fine chips and the dust is highly flammable [13]. Melting point of
magnesium is 650°C and this metal is only stable below its melting point. Therefore, non-traditional machining such
as EDM die sinker and EDM wire cut are preferable especially to produce intricate shapes with tight tolerance. This
paper aims at establishing the optimum parameters for EDM process on magnesium alloy. In this research, design of
experiment and result is analyzed using Taguchi method.
2. Literature review
EDM process uses spark erosion principle for material removal from the workpiece as shown in Fig. 1. The
sparks occur across a small gap between electrode and workpiece surface which takes place in a dielectric fluid
which pumped through the gap at a pressure of 2 kg/cm2 or less [14]. A suitable gap between 0.01 to 0.5 mm known
as spark gap is maintained between the electrode and the workpiece by a servomotor. The dielectric fluid becomes
ionized in the gap to create a path for each discharge. Kerosene oil is the most widely used dielectric fluid.
Alternatively, paraffin and mineral oil can also be used. The dielectric fluid in EDM process functioned as spark
conductor, flushing medium and also to remove particles of eroded metal [15].
Basic rule in the EDM process is that the electrode and the workpiece materials must be electrically conductive.
The current may vary from 0.5 to 400 A and the pulse duration can be varied from 2 to 2000 μ sec. A spark
generator performs the important functions of supplying sufficient voltage to maintain the discharge. The direct
current used for getting rapidly recurring discharges. The voltage of pulse generator is between 40 to 300 V and the
sparks frequency can be achieved up to 10,000 sparks per second [16]. The energy in the form of local heat released
during repetitive sparks. High temperature up to 20,000°C reached at the spot hit causes some metal melted and
eroded. A true replica of the electrode shape is produced on the workpiece surface in the process. A sudden
temperature reduction occurs when approximately 20,000 to 30,000 Hz of pulsating direct current is turned off and
the plasma channel breaks down. This allows the circulation of the dielectric fluid at the pole surfaces and flushes
away the molten material [17].
Fig. 1. Spark occurs between electrode and workpiece [18].

918 M.A. Razak et al. / Procedia Engineering 148 ( 2016 ) 916 – 922
3. Methodology
Taguchi method involves reducing the variation in a process through robust design of experiments. This method
uses orthogonal arrays to organize the parameters affecting the process and the levels at which they should be
varied. Only necessary data collected to determine which factors are most affecting the result with minimum number
of experiment, thus saving time and resources. New parameter values to optimize the performance characteristic can
be obtained by analyzing data using Taguchi approach [19, 20]. In general, Taguchi method involves steps as
follows:
x Determine the process objective and parameters affecting the process.
x Create orthogonal arrays for the parameter design.
x Conduct the experiments.
x Complete data analysis to determine the effect of the different parameters on the performance measure.
x Predict the optimum parameters and conduct a confirmation test.
There were nine EDM experiments conducted with three levels and four parameters as indicated in Table 1. The
orthogonal array for the experiment is shown in Table 2. Parameter values as suggested in machine user manual
were used in the experiments. Each experiment was repeated diligently three times to ensure data accuracy.
Workpiece material used in this research was biocompatible AZ31 magnesium alloy which was suitable for
temporary implant application while copper was chosen as EDM electrode. A constant cutting depth of 2 mm
maintained through-out the experiment. Surface roughness, Ra, was measured and analyzed using Mitutoyo SV3000
Surface Roughness Tester at three different locations on each specimen. Optimum EDM parameters for smoothest
surface were obtained using Taguchi approach and validated via confirmation test.
Table 1. Variable process parameters.
Parameter
Level 1
Level 2
Level 3
Peak current
38 A
47 A
55 A
Voltage
80 V
220 V
320 V
Pulse on-time
16 µs
32 µs
64 µs
Pulse off-time
128 µs
256 µs
512 µs
Table 2. Orthogonal array for EDM experiment.
Experiment
Peak Current (A)
Voltage (V)
Pulse on-time (µs)
Pulse off-time (µs)
1
38
80
16
128
2
38
220
32
256
3
38
320
64
512
4
47
80
32
512
5
47
220
64
128
6
47
320
16
256
7
55
80
64
256
8
55
220
16
512
9
55
320
32
128

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M.A. Razak et al. / Procedia Engineering 148 ( 2016 ) 916 – 922
4. Result and Discussion
Data taken from three repeated experiments were re-arranged into lower average, medium average and higher
average as shown in Table 3. Average Ra of all three repetitions is indicated in most right column. Experiment eight
with parameters combination of 55 A peak current, 220 V voltage, 16 μs pulse on-time and 512 μs pulse off-time
obtained the lowest Ra value with 5.926 µm. On the other hand, experiment five with parameters combination of 47
A peak current, 220 V voltage, 64 μs pulse on-time and 128 μs pulse off-time obtained the highest Ra value with
13.149 µm.
Table 3. Average Ra of EDM experiment.
Experiment
Lower Ra (µ m)
Medium Ra (µm)
Higher Ra (µm)
Average Ra (µm)
1
6.237
6.393
6.889
6.506
2
8.199
8.313
8.390
8.301
3
11.654
11.872
12.887
12.138
4
6.661
6.739
7.999
7.133
5
12.412
12.766
14.404
13.194
6
5.997
6.033
6.942
6.324
7
12.327
13.356
13.764
13.149
8
5.366
6.186
6.227
5.926
9
8.310
8.725
9.228
8.754
Fig. 2. Main effects plot (data means) for means.
Graph for main effects of data means is shown in Fig. 2 and the response of mean is shown in Table 4. The most
significant parameter is pulse on-time and followed by pulse off-time. Among four parameters, voltage is less
significant compared to others. Optimum condition for smaller-is-better was selected from lowest mean value from
each parameter which is A2, B1, C1 and D3. Predicted optimum Ra by mean was computed with equation (1) and
the result obtained was 5.322 µm. The signal to noise ratios for smaller-is-better were derived from Taguchi loss
function as shown in equation (2). Main effect plot for signal to noise ratios is shown in Fig. 3 and the response for
signal to noise is shown in Table 5. The maximum value from each parameter in the plot was selected to calculate
prediction value by signal to noise ratios. The predicted optimum value by signal to noise ratios computed with
equation (3) is -14.8818.

920 M.A. Razak et al. / Procedia Engineering 148 ( 2016 ) 916 – 922
Table 4. Response of mean.
Level
A
Peak current (A)
B
Voltage (V)
C
On-time (µs)
D
Off-time (µs)
1
8.982
8.929
6.252
9.485
2
8.884
9.140
8.063
9.258
3
9.276
9.072
12.827
8.399
Delta
0.393
0.211
6.575
1.086
Rank
3
4
1
2
(1)
(2)
(3)
Fig. 3. Main effects plot (data means) for signal to noise ratios.
Table 5. Response of signal to noise ratios for smaller-is-better.
Level
A
Peak current (A)
B
Voltage (V)
C
On-time (µs)
D
Off-time (µs)
1
-18.78
-18.57
-15.91
-19.17
2
-18.50
-18.75
-18.10
-18.93
3
-18.89
-18.85
-22.16
-18.07
Delta
0.39
0.28
6.24
1.10
Rank
3
4
1
2
A confirmation test has been conducted and Ra of 5.561 µm was obtained with 95.5% similarity to the predicted
value. However, one of the drawbacks found from the machined surface area was the formation of cracks and craters
in consequences of electrical sparks as shown in Fig. 4. The size of cracks and craters increase when the energy
content per pulse increased. They can cause disruption of surface oxide layer and expedites the corrosion process

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[21, 22]. This phenomenon is not good for biomedical implant application due to direct contact and reaction with
body fluid such as water, proteins and amino acids. In-vitro study reported by Wong et al. shows that uncoated
magnesium alloy corrodes 12 mg per two months [23]. Even though magnesium alloy is suggested for
biodegradable implant, high corrosion rate of the implant causes degrade whilst the defect has yet to recuperate.
Further investigation is needed to avoid or reduce the formation of cracks and craters during EDM process.
Fig. 4. Scanning electron microscope image of machined surface.
5. Conclusion
As a conclusion, there are three important points can be drawn. Firstly, among four EDM parameters, the most
significant effect to the Ra was pulse on-time and followed by pulse off-time. Secondly, the optimum EDM
parameters to machine AZ31 magnesium alloy are 47 A peak current, 80 V voltage, 16 μs pulse on-time and 512 μs
pulse off-time. Finally, even though EDM is excellent in machining intricate shapes with tight tolerance and burr-
free, the undesirable cracks and craters were found on the machined surface area. It is proposed to have further
investigation to solve this problem. In recent years, new exploratory research works have been initiated to improve
the efficiency of EDM process using powder mixed EDM method which also known as PMEDM. This method has
potential to reduce or omit the formation of cracks and craters during EDM process.
Acknowledgements
We would like to express our gratitude to all academic and support staff from Mechanical Engineering
Department and Research and Innovation Office, Universiti Teknologi PETRONAS for the assistance provided
during conducting this research.
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