Titanium alloy (Ti-6Al-4V) is widely used in a number of fields such as medical, aeronautical and nuclear fields. Since the surface finish should be enhanced as much possible in efficient way, the micro EDM process can be used for machining such materials. The process mechanism can be enhanced by utilizing the coated electrode in the process. In the present study, an effort was made to investigate the effects of process parameters on drilling titanium alloy with AlNiCr coated aluminium electrode in micro EDM process. An optimization approach was also implemented using Taguchi-DEAR approach. From the experimental interpretation, the conclusions were made as follow. The AlNiCr coated aluminium electrode can enhance the machinability of micro EDM process. The optimal combination of voltage (140V), capacitance (10nF), and Tool rotation (200RPM) could provide better quality measures among the chosen variables. The moderate voltage and capacitance can remove the material effectively with lower overcut, since the electrical conductivity can effectively enhance the conductivity of the plasma energy column. The coated electrode can create the tiny craters with uniform energy distribution.
Research Article
Multi-objects Optimization in Μ - Edm Using Alcrni Coated Tungsten Carbide Electrode for Ti-6al-4v
https://doi.org/10.21203/rs.3.rs-1634889/v1
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micro EDM
AlNiCr
Titanium
DEAR
Surface
Electrical discharge machining (EDM) plays an important role in the field of complex machining of engineering materials [1]. The limitations of machining productivity, electrode wear and machined surface quality in µ-EDM, have greatly influenced the popularity of µ-EDM in practice. Therefore, technological solutions to improve the machining efficiency of µ-EDM are still of great interest [2].Several technological methods have been proposed, including optimization of technological parameters, use of mixing powders in dielectric solutions, integrated vibration in machining, use of powder electrodes and coating electrodes [3].Currently, µ-EDM using coated electrode is a new technical solution, so studies to elucidate this solution are very necessary and its results will contribute to improving machining efficiency in µ-EDM .
Using electrodes in EDM, it has contributed to a significant improvement in TWR, and this will greatly improve the shape accuracy of the product [4]. Compared with the electrode coated with Nickel coated material, the Diamond coated electrode is able to produce better wear resistance of the electrode. The TWR of Cu and brass with Nickel and Diamond coating materials is significantly improved, and the TWR of Mo and Cu-W is slightly improved. A thin film coating of Ag was applied to the surface of the Cu electrode in the EDM for Inconel 800, to reduce electrode wear [5].However, the study has not shown an improvement in the TWR of the Ag coated electrode compared to that of the Cu electrode. The surface roughness of H13 after µ-EDM using Zn coated Brass electrode has been significantly improved compared to that of Brass electrode [6].The working ability of the coating electrode depends on the coating material, coating quality and optimal set of technological parameters in EDM [7].Compared with Cu electrode, the Gr electrode coated with Cu gives a lower quality performance [8].Compared with the coated electrode, the TWR of the Cu electrode is smaller (28.7%), the MRR is larger (8.97%) and the SR is not significantly changed. And it is necessary to have further research directions on the conductive characteristics of the electrodes to clarify these changes. Surface roughness of Ti-6Al-4V after EDM with AlCrNi coated electrode was considered, and the result of SR was good [9].Machining productivity in EDM with different types of electrodes (Al, Cu and Cu coated Al electrodes) has been compared and analyzed [10].The results show that Cu electrode has the highest machining efficiency, and the smallest is that of Al electrode.And EDM using coated electrode will bring high economic efficiency. A layer of Cu material (≤1000µm) was coated on the surface of the ABS plastic electrode by spray coating technique [11].The test results are good and it has created a new research direction in electrode manufacturing and fabrication. Compared with Cu electrode, TWR and MRR in EDM with copper-based MWCNT composite coated electrode are significantly improved, and MRR is increased by 99.52% and TWR is reduced by 57.11% [12].Results of studies in EDM with nicroloy coated Cu electrode were reviewed [13].Compared with Cu electrode, the MRR of the coated electrode is smaller, but the surface quality of the coated electrode is better. Compared with Cu electrode, TWR and overcut of Al2O3-TiO2 coated Cu electrode in EDM for Ti-6Al-4V are smaller by 92% and 62.5%, respectively [14].This shows the effectiveness of using coated electrodes in EDM. Compared with Cu electrode, the economic efficiency of EDM using coated Ak12 electrode has been improved by approximately 35% [15].The surface quality after machining after µ-EDM using Boron – CVD diamond coated electrode is significantly improved [16].The reason may be that the physico-chemical characteristics and thermal conductivity of the coating affect the spark generation process and the wear resistance of the electrode. The dimensional accuracy in µ-EDM with Cu-ZrB2 coated electrode is much higher than it in Cu electrode [17].This shows that the TWR of the coated electrode is better. Compared with WC electrode, TWR and OC of µ-EDM using TiN coated WC electrode are reduced by 16.32% and 26%, respectively [18]. And the machining productivity of the coated electrode is also improved. From the above research and survey results, it has been shown that EDM with coated electrodes can bring good economic efficiency in this machining field. The results published in the new studies stop at the initial survey level, only initially showing the feasibility of this method in practice. The number of investigated electrodes and coating materials is limited, the optimal parameter sets given in the published results are very few and especially in the field of µ-EDM.
Many optimization techniques have been used in EDM and µ-EDM with uncoated electrode [19]. The mathematical model of MRR, TWR and SR in EDM for hybrid 7075Al was determined by GR algorithm [20].The technological parameters and quality criteria in the multi-objective problem are established, and the machined surface quality at optimal conditions is good. Using the combined technique of RSM and Topsis to solve the multi-objective problem in EDM for Al7075/B4C/Gr, and this has contributed significantly to improve machining efficiency [21].However, the RSM technique for constructing the experimental matrix will lead to an increase in the number of experiments, and this will lead to an increase in the cost and time of the experiment.And the simplicity of the combination of Taguchi - Utility in multi-objective decision in EDM, it has contributed to overcome the above limitations [22].The single-objective and multi-objective optimization problems in EDM are solved by Taguchi method and Taguchi – GRA [23].The results show that they can be effective optimization tools in EDM. Some research results on multi-target optimization in EDM by TGRA showed that this method may be suitable for both EDM using uncoated and coated electrode [24, 25].Several quality parameters in EDM are simultaneously decided through multiple optimization methods used [26]. The research results of each optimization problem depend on the input of each type of problem and the optimization method. Therefore, it is necessary to have studies to determine the optimal results for each specific problem.
For the literatures, it was observed that process mechanism in EDM process should be further investigated. Only few literatures are available to analyze the effects of coated electrodes in EDM process. The adaptation of Envelopment Analysis based Ranking Methodology (DEAR) was reported in few studies only. Hence the present study was proposed. In the present study, an effort was made to investigate the effects of process parameters on drilling titanium alloy with AlNiCr coated aluminium electrode in micro EDM process. An optimization approach was also implemented using Taguchi-DEAR approach.
The Tungsten carbide (WC) electrode is coated with Aluminum Chromium Nitride (AlCrNi), and the coating thickness is 3.385 µm. The coating characteristics are shown in Fig. 1. And the diameter size of the coating electrode is 496.77 µm. Titanium alloy (Ti-6Al-4V) is widely used in a number of fields such as medical, aeronautical and nuclear, and it is chosen as the ingot. The dielectric fluid is EDM Oil. The micro EDM machine is the Hyper 10 Micro EDM. The technological parameters were selected according to Table 1.
Quality criterias including Z Co-ordinate – Depth of Cut (Z), tool wear rate (TWR) and overcut (OC) were selected in the study. Z is directly related to the machining productivity, TWR will characterize the electrode consumable cost and OC will characterize the machining accuracy. TWR is determined without regard to the mass of the electrode. OC will be directly related to machining accuracy. The Z Co-Ordinate was reading shown by Machine on screen related to depth of the electrode in the drilled hole cavity. The mass of the electrode before and after machining is determined through precise weighing. The amount of overcut was determined through the SEM image analyzer.
|
Process Parameters |
Levels |
||
|---|---|---|---|
|
1 |
2 |
3 |
|
|
Voltage (V) |
120 |
140 |
160 |
|
Capacitance (F) |
100pF |
1000pF |
10nF |
|
Spindle Rotation (RPM) |
200 |
400 |
600 |
2.3. Steps involved in DEAR methodology
In the present experimental investigation, Z, TWR and OC have been selected as quality measures. Since the present investigation has been performed with more than one quality measure, it is important to implement MCDM optimization in the process. In the approach, a set of original responses are mapped into a ratio to find the optimal levels. This value can be treated as multi response performance index (MRPI) value to find the optimal combination of the process parameters. The following steps are involved in Date Envelopment Analysis based Ranking Methodology (DEAR): The following steps are involved in the DEAR methodology for multi criteria decision making (MCDM) [13, 21].
Step 1: Calculation of weights for all responses as per Eqs. 1, 2 and 3
A = Z * WZ (4)
B = TWR * WTWR (5)
C = OC * WOC (6)
Step 3: Compute of ratio of LTB to STB to find multi response performance index (MRPI) as per Eq. 7
Step 4: Calculation of average MRPI
In the present study, the experimental investigation has been performed based L9 based experimental matrix and tabulated in Table 2.
|
Expt. No. |
Process parameters |
Response variables |
||||
|---|---|---|---|---|---|---|
|
Voltage (V) |
Capacitance (F) |
RPM |
Z (mm) |
TWR (mg/min) |
Overcut (µm) |
|
|
1 |
120 |
100pF |
200 |
0.66 |
0.01 |
58.37 |
|
2 |
120 |
1000pF |
400 |
1.42 |
0.04 |
27.71 |
|
3 |
120 |
10nF |
600 |
2.20 |
0.07 |
53.71 |
|
4 |
140 |
100pF |
400 |
0.95 |
0.04 |
62.45 |
|
5 |
140 |
1000pF |
600 |
1.43 |
0.04 |
32.97 |
|
6 |
140 |
10nF |
200 |
2.92 |
0.05 |
45.19 |
|
7 |
160 |
100pF |
600 |
1.72 |
0.01 |
74.12 |
|
8 |
160 |
1000pF |
200 |
1.49 |
0.04 |
60.93 |
|
9 |
160 |
10nF |
400 |
2.47 |
0.08 |
73.7 |
3.1 Computation of optimal process parameters combination
The present experimental investigation was performed in micro EDM process with AlCrNi coated electrode. The values of MRPI for each trial were computed based DEAR methodology as described in section 2 and tabulated as shown in Table 3. Table 4 shows the consolidated MRPI of all the input factors with all the levels. The values have been computed by the adding the all MRPI values for corresponding level of each process parameters.
|
Expt No |
Weights |
MRPI |
||
|---|---|---|---|---|
|
Z |
TWR |
OC |
||
|
1. |
0.0433 |
263.1579 |
3.50242E-05 |
0.0108 |
|
2. |
0.0931 |
65.7895 |
7.37771E-05 |
0.0502 |
|
3. |
0.1442 |
37.5940 |
3.8063E-05 |
0.1204 |
|
4. |
0.0623 |
65.7895 |
3.2736E-05 |
0.0225 |
|
5. |
0.0937 |
65.7895 |
6.20068E-05 |
0.0509 |
|
6. |
0.1913 |
52.6316 |
4.52393E-05 |
0.2122 |
|
7. |
0.1127 |
263.1579 |
2.75818E-05 |
0.0736 |
|
8. |
0.0976 |
65.7895 |
3.35526E-05 |
0.0552 |
|
9. |
0.1619 |
32.8947 |
2.7739E-05 |
0.1518 |
|
Factors |
Levels |
Max - Min |
||
|---|---|---|---|---|
|
1 |
2 |
3 |
||
|
Voltage |
0.0605 |
0.0952 |
0.0936 |
0.0347 |
|
Capacitance |
0.0356 |
0.0521 |
0.1615 |
0.1258 |
|
Tool rotation |
0.0927 |
0.0748 |
0.0816 |
0.0179 |
The maximum level MRPI value of each process parameters indicates the optimal level of input parameters on determining the quality measures in micro EDM process. From the Table 5, it was found that voltage (Level 2), capacitance (Level 3), and Tool rotation (Level 1) could provide better quality measures, since the combination has been inferred as optimal combination of process parameters in the present study as shown Table 5.
|
Factors |
Level |
Parameter values |
|---|---|---|
|
Voltage |
2 |
140 V |
|
Capacitance |
3 |
10nF |
|
Tool rotation |
1 |
200 RPM |
3.2 Confirmation Experiment
The purpose of confirmation experiment is to validate the optimal process factors derived from the DEAR based MCDM technology. In the present study, a confirmation experiment was conducted with optimal levels of significant factors. At the optimal setting, the response values from the confirmation experiments were obtained and the deviation of corresponding MRPI has been found as 2.2% from the maximum mean value. It was the acceptable tolerance value of 5%.
3.3 Identification of Significant factor
The larger Max – Min value indicates that higher influence on determining quality measures in any process. From the Table 4, it was found that capacitance was more dominant factor among all the input process parameters in micro EDM process. Since the machining mechanism produces oscillations across the machining zone in micro EDM process, the flushing process can be further enhanced. Due it is better deionization, the higher material removal rate could be obtained with better surface quality in micro EDM process.
3.4. Machining accuracy of drilled holes
The accuracy of hole has been analyzed using the SIPCON vision measuring system as shown in Fig. 2. It was found that the trial number 5 could produce better circularity with uniform discharge energy distribution. The voltage could determine the charging effect whereas the capacitance can determine the discharge phenomenon. The lower discharge energy could not remove particles from the specimen. However the higher discharge energy could produce larger craters. The larger tool rotational speed has distributed spark energy uniformly around the hole. The moderate voltage and capacitance can remove the material effectively with lower overcut. Hence trial number 5 has produced better hole accuracy as shown in Fig. 3 which has been derived from scanning electron microscope(SEM). Figure 4 shows the accuracy of the drilled specimen under trial number 7. It was found that larger voltage could affect the charging phenomenon. The larger charging energy could be discharged through the higher discharge frequency with larger tool speed. This effect could lead to distribute the discharge energy in random variation. Hence the accuracy was affected considerably to form higher deviation from the required shape with larger overcut.
While analyzing the elemental distribution over the machined specimen, it was observed that the Al, Ni and Cr were distributed over the machined specimen using EDM process as observed in Figure 5.Figure 6 represents the machined surface in EDM process under trial 5. The conductive coating can increase the tool electrode. The electrical conductivity can effectively enhance the conductivity of the plasma energy column. This coating can create the tiny craters over the specimen for creating lower surface roughness. The tiny craters were observed with uniform spark energy distribution over the machined specimen as shown in Fig. 6. Since the discharge energy could create the larger craters, the higher voltage may create the larger craters. Figure 7 represents the surface morphology of the machined specimen with larger craters under random distribution in micro EDM process.
3.5. Surface morphology of drilled holes under optimal process parameters
The accuracy of the machined hole was analyzed using vision measuring system under the optimal combination. Since the voltage could determine the charging effect and the capacitance can determine the discharge phenomenon, the moderate voltage with lower discharge energy could produce effective machining accuracy under lower tool speed as shown in Fig. 8. Hence the moderate voltage and capacitance can remove the material effectively with lower overcut. The conductive coating can increase the tool electrode. The electrical conductivity can effectively enhance the conductivity of the plasma energy column. This coating can create the tiny craters over the specimen for creating lower surface roughness. The tiny craters were observed with uniform spark energy distribution over the machined specimen as shown in Fig. 9.
In the present study, an effort was made to investigate the effects of process parameters on drilling titanium alloy with AlNiCr coated aluminium electrode in micro EDM process. An optimization approach was also implemented using Taguchi-DEAR approach. From the experimental interpretation, the conclusions were made as follow. The AlNiCr coated aluminium electrode can enhance the machinability of micro EDM process. The optimal combination of voltage (140V), capacitance (10nF), and Tool rotation (200RPM) could provide better quality measures among the chosen variables. The moderate voltage and capacitance can remove the material effectively with lower overcut, since the electrical conductivity can effectively enhance the conductivity of the plasma energy column. The coated electrode can create the tiny craters with uniform energy distribution.
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Authors Contributions:
Phan Nguyen Huu- Optimization and Surface morphology, T. Muthuramalingam - Optimization, Pham Van Dong- Optimization, Nguyen Van Thien-Experiments, Hoang Tien Dung-Experiments, Shailesh Shirguppikar -Experiments
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