Irrespective of the toughness, strength & hardness of any conductive material, Electric Discharge Machining (EDM) is a proven machining method. Selection of most favorable process parameters for machining is desirable to avoid excessive tool wear, damaged surfaces, and unwanted roughness. For this, the main reason could be the arcing while machining. Arcing results from successive electric discharges occurring at the interface of both the electrodes. Also, presence of powder concentration makes it more susceptible to occurring. This paper emphasizes identifying the combination of parameters that lead to arcing in the machining of titanium alloy Ti-6Al-4V using copper C100 electrode on Die Sinking EDM. Graphite powder concentration of 0%, 5% and 10% in SEO-45 EDM oil is used for the investigation. To identify arcing parameters, Taguchi parameter Design approach is used. Three stage process parameters are selected for finishing, intermediate and roughing stages to study arcing. Scanning electron microscopic (SEM) images of the machined surface tells that less surface crack density (SCD) has been formed on the machined surface. Discharge current, duty cycle and graphite powder concentration are the variables and effect of arcing on Material removal rate, Electrode wear rate and Surface roughness of Ti-6Al-4V is analyzed.
Research Article
Characterization of arcing parameters in presence of graphite powder concentration in EDM of Ti-6Al-4V
https://doi.org/10.21203/rs.3.rs-3796102/v1
This work is licensed under a CC BY 4.0 License
Version 1
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EDM Dielectric
Arcing
Ti-6Al-4V
Graphite Powder
SEM
Taguchi Method
In EDM, machining occurs due to recurring high intensity sparks between tool and work piece to cavitate the material. EDM is a pervasive technique used in industry for high precision machining of all types of conductive materials [1]. Most of the products made up of hard to machine materials are EDMed. Titanium and its alloy are one of them. Many industries namely aerospace industries, bio-printing industries and like, use titanium and it’s alloys due to superior strength, resistance to fracture and corrosion [2].
Electrical Discharge Machine (EDM) is an important ‘non-traditional manufacturing method’, developed in the late 1940s and has been accepted worldwide as a standard processing manufacture of forming tools to produce plastics moldings, die castings, forging dies etc. New developments in the field of material science have led to new engineering metallic materials, composite materials, and high-tech ceramics, having good mechanical properties and thermal characteristics as well as sufficient electrical conductivity so that they can readily be machined by spark erosion. At the present time, Electrical discharge machine (EDM) is a widespread technique used in industry for high precision machining of all types of conductive materials such as, metals, Metallic alloys, graphite, or even some ceramic materials [3]. Titanium and its alloy are widely used in many industries namely aerospace industries, Bioprinting industries because of its strength, fracture resistance characteristics and their exceptional resistance towards corrosion. The high cutting temperature is due to the heat generated during machining (catastrophic thermoplastic shear process), the thin chips, a thin secondary zone, a short chip-tool contact length and the poor heat-conductivity of the metal, whilst the high stresses are due to the small contact area and the strength of titanium even at elevated temperature [2]. The EDM process materials using electrical current which generates spark erosion between the electrode and workpiece. The electrode is positioned at a fixed small distance (spark gap) above the workpiece, both submerged in a dielectric fluid. When the current starts to flow the workpiece, extreme high heat is generated in the cutting zone, sparks or current conduction are generated through the gap. Each spark results in localized heating and particles of metal become melted in a very small area and at the end of the pulse duration the molten metal particles are flashed away by the dielectric fluid and remaining liquid material resolidifies. This resolidified/recast layer has been called white layer. The process repeats itself every few milliseconds. The material removal rate and the surface finish depend on the magnitude and duration of the discharged is non-conventional machine [4]. There are various parameters in EDM which affect surface integrity while machining hard material. It has been stated that the pulse current and pulse energy directly affects the surface integrity of Ti-6Al-4V work piece. Increase in pulse energy is directly proportional to the material removal rate but in turn creates a hardened thick layer on the surface being machined [5]. Graphite powder was used in die sinking EDM machines to optimize various parameters while machining. Graphite powder is very fine and black in color. It is one of the best conductors because it contains a high level of carbon. The purification of graphite powder is done using two method first is acid and alkali treatment and second is expansion process [6]. A succession of machine pulses in the same location which leads to further damage in the workpiece and can destroy the surface finish of the workpiece by overheating is termed Arcing. The effects of Arcing can damage both the workpiece and the tool. This damage can be irreparable and can lead to cost damaging for the workpiece. There are two types of Arcing “coking” and “burring.” In this case, the discharges are concentrated at the same location on the workpiece surface and carbon growths begin to form on the surface of the electrode and the workpiece. This phenomenon is called as “coking.” The residue of the carbon cannot be easily discarded by standard EDM machining. This growth of carbon is not advised to be formed and can also lead to damage to the filtration system. “Burring” takes form in machined cavity. The severity of “burring” leads to leads to damaging the piece and possibility of destroying it also. Damage done by “burring” can be discarded with the help of expensive hand finishing[7]. Arcing is undesirable as it damages the power supply of the machine. Arcing phenomenon occurs by unusual current values and rate of current changes in the spark gap of the EDM machining. It led to a short-circuit in the gap which leads to dissipative discharges and uncontrolled machining. Arcing is the counterproductive phenomenon which generally results in damaging and destroying the workpiece and the tool. Improper selection of the EDM parameters can lead to Arcing causing conditions. It can give undesirable pulse type, severely affect the process performance, and appropriate finish, accuracy, and geometry. EDM sparks can be converted into Arcing when proper parameters are chosen. Also, Arcing can be formed when there is improper flushing. Machining of larger depth with less cross-sectional area causes complexity for debris removal movement and that leads to Arcing[8]. In EDM, Copper electrodes are generally suitable for rough machining of the workpiece. Graphite electrode are generally used for better surface [9]. Research on EDM process in increased in due time due to efficiency of machine and reliability of manufacturing process. [10]. Arcing in EDM is undesirable for both machine and workpiece and electrode surface [11]. It is essential to find the parameters which cause arcing while machining Ti-6Al-4V using copper C-100 electrode and it is shown in following experiment results.
All the experiments were conducted in die sinking EDM machine of (M/S Electronica Machine Tools LTD) model controlled by (SINO SDS2-3E) controller. The experiment was conducted on an originally configured Die sinking EDM machine. For conducting an experiment, the work material used was Ti-6Al-4V alloy. The chemical composition (wt%) of Ti–6Al–4V is Al: 5.11%, V: 3.33%, Fe: 0.14%, O: 0.0027%, C: 0.04%, N: 0.0012%, H: 0.0053%, and balance is Ti. Salient properties of Ti-6Al-4V alloy were shown in Table 1. Titanium alloy was cut into shape of circular disc of φ 2.8 cm. Copper electrode with dimension φ 0.5 cm and 30.48 cm length is selected as a tool material to machine Ti-6Al-4V alloy on Die sinking EDM machine. Chemical composition (wt%) of copper electrode was Cu: 99.8%, Mn: 0.18%, S: 0.01%, P: 0.001%. The physical properties of copper electrode are mentioned in Table 1. EDM oil was used as a dielectric fluid. Specification of SEO − 45 EDM oil is mentioned in Table 2. Weighing Scale (ME Analytical Balance – manufactured by METTLER TOLEDO) was used to calculate before and after weight of workpiece as well as weight of Electrode for MRR and EWR. A stopwatch was placed to calculate machine running time while experiments are done. Graphite Powder of 325 mesh i.e., 44-micron size was used in experimentation. Specification of Graphite Powder is mentioned in Table 3. During experiments, straight polarity was maintained.
Machining time has been chosen for conducting each experiment is 15 minutes. Before machining, the work pieces and electrodes were cleaned and polished. The work piece was firmly clamped in the vice and immersed in the dielectric. The copper electrode has been mounted on EDM tool post. Before and After weight of Tool as well as workpiece has been noted down before each experiment. 10L tank has been fabricated to do the experiment using graphite concentration (Fig. 1). For 0 g/l concentration of Graphite powder experimentation everything has been placed as per original configuration. 8L EDM oil has been placed in that tank. For 4 g/l concentration of graphite powder experimentation, 9 equal weighted pouches have been made for each trail which is eventually mixed with EDM oil. A pump has been placed in that tank which maintains the circulation and spray mechanism during EDM operation on workpiece. For 8 g/l concentration of graphite powder experimentation additional 9 equal weighted pouches have been made for each trail which is eventually mixed with EDM oil. The pump has been placed in that tank which maintains the circulation during EDM operation on workpiece. Stopwatch has been placed to measure the exact 15-minute machine time duration. The ranges of each factor were taken based on the capability of the machine, and preliminary experiments were conducted on their performance’s effects. In this study the number of process parameters which were varying are concentration of graphite powder, Duty Cycle, and Input Current. The level of each parameter was three. The remaining parameters were kept constant. Table 4 specified which parameters are kept constant. Table 5 specified varying parameters. In this study, the number of process parameters considered were three, and the level of each parameter was three. L27 (313) is considered for the present study. It is presented in Table 5.
The details of experimental setup are shown in the Fig. 2
Weights of workpiece as well as tool (before and after machining) were measured by the digital weighing balance (ME Analytical Balance – mfg. by METTLER TOLEDO). Material removal rate (MRR) and tool wear rate (TWR) were computed by weight loss criteria. Density of Ti–6Al–4V (ρ ∼ 4.415 g/cc) as well copper (ρ ∼ 8.9858 g/cc) were obtained through Archimedes principle.
Material removal rate (MRR) was defined as the volume of workpiece material removed per unit machining time, generally measured in [mm3/min]. It was basically the volumetric material removal rate used to calculate the time required to remove the specified quantity of material from the workpiece during the machining process. It can be expressed as:
here M1, mass of the workpiece before EDM operation, in (g); M2, mass of the workpiece after EDM operation, in (g); ρ, density of work material, in (g/mm3); tm machining time, in (sec). Tool wear rate (TWR) was defined as the loss of volume of electrode material per unit machining time. Generally, the value of TWR depends on the tool material properties and the discharge energy supplied during EDM. It can be expressed as:
here Mt1, mass of the tool before EDM operation, in (g); Mt2, mass of the tool after EDM operation, in (g); ρt, density of tool material, in (g/mm3); tm, machining time, in (sec).
Measurement of Surface Roughness was done using Mitutoyo SJ410 (Fig. 3).
The values of MRR, EWR, SR and Power Consumption for each trial (run) were presented in Table 7.
Effect of Arcing on MRR, EWR and S/R –
Arcing and non-arcing parameters of different graphite concentration (0%, 5%, 10%) are identified.
1. Material Removal Rate (MRR) – In Fig. 5, with respect to process parameters MRR was calculated, and arcing parameters were identified based on results. On X axis Process parameters like Input current, Duty cycle and Graphite powder concentration were mentioned, and Y axis represents MRR value for each trail. Nine Trials were identified with an arcing phenomenon based on Material Removal Rate. Low material removal rate was observed during arcing.
2. Electrode Wear Rate (EWR) – In Fig. 6, with respect to process parameters EWR was calculated, and arcing parameters were identified based on results. On X axis Process parameters like Input current, Duty cycle and Graphite powder concentration were mentioned, and Y axis represents EWR value for each trail. Nine Trials were identified with arcing phenomenon on basis on Electrode wear rate. High electrode wear was observed during arcing.
3. Surface Roughness(S/R) – In Fig. 7, with respect to process parameters SR was calculated and arcing parameters were identified based on results. On X axis process parameters like Input current, Duty cycle and Graphite powder concentration were mentioned, and Y axis represents SR value for each trail. Nine Trials were identified with an arcing phenomenon on the basis on Surface Roughness. Surface Roughness was higher on trials where arcing occurs during the experimentation.
4. SEM Images of Cu and Ti-6Al-4V samples – Scanning electron microscope helped to capture images of surface of Ti-6Al-4V and Copper electrode. Carbon deposition, Fractured surface was visible on surface.
Figure 8 (a), (b), (c) highlighted SEM images of copper electrodes taken under the Scanning electron microscope, when used for different graphite concentrations. In the 8 a) image copper electrode was used for 0% graphite concentration and after the trials have been taken surface fracture is observable. In 8 b) image carbon deposition was slightly seen for an eroded copper electrode in 5% graphite concentration. In the 8 c) image a high amount of carbon was deposited due to arcing phenomenon when used under 10% graphite concentration. The tool wear was less when used in 0% and 5% as compared to 10% graphite concentration and carbon deposition was high due to arcing.
Figure 9 (a) represents the 8th trial of experiment in which duty cycle was 9, input current 6 A and concentration of graphite 0%. Figure 9 (b) represents the 9th trial of experiment in which duty cycle was 9, input current 9 A and concentration of graphite 0%. Carbon deposition around the machined surface was seen when images taken under Scanning electron microscope, Tool Wear is measured, and Arcing is observed during these trials.
Figure 10 (a) represents the 17th trial of experiment in which duty cycle was 9, input current 6 A and concentration of graphite 5%. And Fig. 10 (b) represents the 9th trial of experiment in which duty cycle was 9, input current 9 A and concentration of graphite 5%. Heavy carbon deposition was observed in images when taken with the help of scanning electron microscope. This was mainly because of the arcing phenomenon.
Figure 11 (a) represents the 26th trial of experiment in which duty cycle was 9, input current 6 A and concentration of graphite 10%. And Fig. 11 (b) represents the 9th trial of experiment in which duty cycle was 9, input current 9 A and concentration of graphite 10%. Heavy Arcing was observed during these trials which results in ruptured tool and electrode surface, ultimately results to high Surface roughness.
Based on the experimental results in the present work, the following conclusions were drawn, Arcing parameters contribute significantly to decreasing machining characteristics namely material removal rate, electrode wear rate. Surface of the electrode is fractured due to arcing which ultimately results in high electrode wear. Arcing results in a poor surface finish and damages the electrode and workpiece during EDM operation. Arcing has been observed when the duty cycle is kept at max value. To prevent damage due to arcing, we can avoid high duty cycle and high input current and finally we can conclude that the arcing phenomenon is not suitable from a sustainability manufacturing view. Furthermore, material wastage and excess energy consumption that happened due to arcing phenomenon is directly affecting the sustainability goals considering the future manufacturing. To access more efficiency and more stability in controlled EDM process while cutting the Ti-6Al-4V, it is recommended to avoid high duty cycle and high input current. The controlled strategy of adjusting duty cycle, input current might minimize the arcing related issue and ensure the efficiency in EDM manufacturing process. The study from this research concludes that this research in future needs more exploration in EDM process considering the sustainability and efficiency in manufacturing sector.
Acknowledgment
Authors are grateful to Mrunal Mohatkar, Kashish Wanjari, Atharva Ketkar , Prashansa Khandelwal for their assistance in the experimental work. Authors like to thank Shri Ramdeobaba College of Engineering and Management Nagpur along with Visvesvaraya National Institute of Technology, Nagpur for their constant support.
Funding
The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.
Competing Interests
The authors have no relevant financial or non-financial interests to disclose.
Author Contributions
“All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Yadnesh Abhaykumar Kuthe, Yashraj Rathod and Anshuman Prajapati . The first draft of the manuscript was written by Yadnesh Abhaykumar Kuthe, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.”
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|
Properties
|
Ti-6Al-4V alloy
|
Copper
|
|---|---|---|
|
Density (g/cc)
|
4.43
|
8.91
|
|
Thermal Conductivity (W/mK)
|
6.7–6.9
|
391.1
|
|
Melting Point (0C)
|
1649–1660
|
1082
|
|
Specific Heat (J/kg0K)
|
560
|
0.39
|
|
Hardness (HRC)
|
36
|
3
|
|
Characteristics
|
SEO 45
|
|---|---|
|
Appearance
|
Bright and Clear
|
|
Density @15°C
|
0.85
|
|
Flash Point (COC)°C
|
130
|
|
Viscosity @40°C cSt
|
4.8
|
|
Copper Corrosion
|
< 1
|
|
Di-electric Strength
|
40
|
|
Product Name
|
Graphite Synthetic (Type 3) pure, 96% C, 325mesh
|
|---|---|
|
Physical State
|
Solid
|
|
Molecular Weight
|
12.01
|
|
Melting Point
|
3652.2-3697.2 OC / 6606–6687
|
|
Size
|
325 Mesh / 44 micron
|
|
Parameters
|
Unit
|
Value
|
|---|---|---|
|
Bipulse Current
|
A
|
1
|
|
Spark Time
|
Seconds
|
5
|
|
Lift Time
|
Seconds
|
2
|
|
Pulse On
|
microseconds
|
25
|
|
Pulse Off
|
microseconds
|
9
|
|
Polarity
|
-
|
Straight
|
|
Parameters
|
Units
|
Levels
|
|||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
|
Graphite Powder Concentration (C)
|
g/L
|
|
|||||||||
|
Duty Cycle
|
%
|
||||||||||
|
Input Current
|
A
|
|
Experiment No.
|
Graphite Powder Concentration (C) g/L
|
Duty Cycle
|
Input Current
|
|---|---|---|---|
|
1
|
0
|
3
|
3
|
|
2
|
0
|
3
|
6
|
|
3
|
0
|
3
|
9
|
|
4
|
0
|
6
|
3
|
|
5
|
0
|
6
|
6
|
|
6
|
0
|
6
|
9
|
|
7
|
0
|
9
|
3
|
|
8
|
0
|
9
|
6
|
|
9
|
0
|
9
|
9
|
|
10
|
4
|
3
|
3
|
|
11
|
4
|
3
|
6
|
|
12
|
4
|
3
|
9
|
|
13
|
4
|
6
|
3
|
|
14
|
4
|
6
|
6
|
|
15
|
4
|
6
|
9
|
|
16
|
4
|
9
|
3
|
|
17
|
4
|
9
|
6
|
|
18
|
4
|
9
|
9
|
|
19
|
8
|
3
|
3
|
|
20
|
8
|
3
|
6
|
|
21
|
8
|
3
|
9
|
|
22
|
8
|
6
|
3
|
|
23
|
8
|
6
|
6
|
|
24
|
8
|
6
|
9
|
|
25
|
8
|
9
|
3
|
|
26
|
8
|
9
|
6
|
|
27
|
8
|
9
|
9
|
|
Sr. No
|
Concentration
|
Duty Cycle
|
Input Current
|
Time (Sec)
|
MRR (Ti-6Al-4V)
|
EWR (Cu)
|
Surface Roughness (µm)
|
|---|---|---|---|---|---|---|---|
|
1
|
0
|
3
|
3
|
900
|
0.0046
|
0.0002
|
4.8732
|
|
2
|
0
|
3
|
6
|
900
|
0.0068
|
0.0004
|
7.1085
|
|
3
|
0
|
3
|
9
|
900
|
0.0108
|
0.0010
|
7.8825
|
|
4
|
0
|
6
|
3
|
900
|
0.0049
|
0.0002
|
9.0735
|
|
5
|
0
|
6
|
6
|
900
|
0.0057
|
0.0001
|
11.5955
|
|
6
|
0
|
6
|
9
|
900
|
0.0108
|
0.0004
|
14.9200
|
|
7*
|
0
|
9
|
3
|
900
|
0.0057
|
0.0004
|
6.3695
|
|
8*
|
0
|
9
|
6
|
900
|
0.0084
|
0.0001
|
7.8120
|
|
9*
|
0
|
9
|
9
|
900
|
0.0056
|
0.0003
|
23.4065
|
|
10
|
5
|
3
|
3
|
900
|
0.0047
|
0.0004
|
4.9050
|
|
11
|
5
|
3
|
6
|
900
|
0.0063
|
0.0005
|
5.6575
|
|
12
|
5
|
3
|
9
|
900
|
0.0105
|
0.0011
|
8.5860
|
|
13
|
5
|
6
|
3
|
900
|
0.0036
|
0.0001
|
8.8035
|
|
14
|
5
|
6
|
6
|
900
|
0.0071
|
0.0002
|
11.3070
|
|
15
|
5
|
6
|
9
|
900
|
0.0090
|
0.0004
|
18.6670
|
|
16*
|
5
|
9
|
3
|
900
|
0.0051
|
0.0003
|
7.0175
|
|
17*
|
5
|
9
|
6
|
900
|
0.0059
|
0.0002
|
18.4480
|
|
18*
|
5
|
9
|
9
|
900
|
0.0041
|
0.0001
|
23.9955
|
|
19
|
10
|
3
|
3
|
900
|
0.0320
|
0.0004
|
6.1230
|
|
20
|
10
|
3
|
6
|
900
|
0.0073
|
0.0008
|
6.2430
|
|
21
|
10
|
3
|
9
|
900
|
0.0095
|
0.0009
|
8.5780
|
|
22
|
10
|
6
|
3
|
900
|
0.0150
|
0.0007
|
6.6680
|
|
23
|
10
|
6
|
6
|
900
|
0.0069
|
0.0001
|
7.6340
|
|
24
|
10
|
6
|
9
|
900
|
0.0126
|
0.0003
|
13.6980
|
|
25*
|
10
|
9
|
3
|
900
|
0.0087
|
0.0013
|
6.5390
|
|
26*
|
10
|
9
|
6
|
900
|
0.0079
|
0.0004
|
16.7300
|
|
27*
|
10
|
9
|
9
|
900
|
0.0088
|
0.000657976
|
20.7600
|
| *Represent Heavy Arcing Parameters | |||||||