Machinability Performance of Powder Mixed Dielectric in Electrical Discharge Machining (EDM) of Inconel 718 With Copper Electrode

Transcription

1 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 37 Machinability Performance of Powder Mixed Dielectric in Electrical Discharge Machining (EDM) of Inconel 718 With Copper Electrode M.A. Lajis *,, S. Ahmad Sustainable Manufacturing and Recycling Technology (SMART), Advanced Manufacturing and Materials Center (AMMC), Universiti Tun Hussein Onn Malaysia (UTHM), Parit Raja, Batu Pahat, Johor, Malaysia * Corresponding author, amri@uthm.edu.my Abstract-- This study mainly explores the effects of powder additive in dielectric fluid when electrical discharge machining of Inconel 718 by employing high Peak current and Pulse duration. Copper was selected as a tool. Peak current, Pulse duration, and Concentration of the nano Alumina powder were chosen as a variable parameter to study the EDM performance in terms of Material removal rate (MRR), Electrode wear rate (EWR) and Surface roughness (R a ). The experiment results show that, the MRR has improved significantly compared to without powder concentration at a high level of Peak current and Powder concentration for both electrodes. When EDM machining at 4g/l of powder concentration, the MRR is improved about 32% in comparison to the highest MRR value obtained without powder suspension dielectric. The maximum MRR value 45.70mm3/min was obtained at 40A of Peak current, 200µs of Pulse duration and 4g/l of powder concentration. In conventional EDM, the EWR is increased at high peak current and shortest pulse duration. But when powder suspension was applied, higher peak current and longer pulse duration was decreased the EWR. The lowest EWR value mm 3 /min was achieved at the highest powder concentration 4g/l with the highest value of Peak current of 40A and the longest Pulse duration of 400µs. The negative value for EWR is indicated that the deposition effect has occurred on the electrode surface. The value of R a also increased by increasing of peak current but decreased with longer pulse duration. The R a value is worst when powder concentration was applied. The result suggested that, lower peak current with longer pulse duration and without powder additive in a dielectric is better for R a. The lowest Ra value 8.98µm is obtained at 20A of Peak current and 400µs of Pulse duration without powder suspension dielectric. Index Term-- Electrical discharge machining (EDM); Inconel 718; Aerospace material; Powder suspension dielectric; Machinability; Copper electrode 1. INTRODUCTION Up to now, EDM has been an important manufacturing process for the tool and die industry. It has proved for the machining of high toughness aerospace material alloys such Inconel 718 that are difficult to cut by conventional methods. However, compared with traditional machining processes, especially the high-speed machining (HSM), the low efficiency of EDM limits its application. The application of EDM is confined to conditions where traditional machining processes cannot be resorted to. It seems that in order to achieve a high material removal rate (MRR), the pulse offtime or pulse interval, which interrupts the material removal process, should be decreased as much as possible. Unfortunately, both of them are so indispensable that neglecting them will make EDM impossible (Fenggou and Dayong, 2004; Han et al., 2009). Since the invention of EDM in the 1940s, many efforts have been made to enhance the machining performance and stability of EDM process. It is because that the EDM process in the common dielectric oils is very unstable owing to arcing or short-circuiting. To fulfil this requirement, a relatively new method by introducing an additive of powder into the dielectric fluid of EDM and currently known as powder mixed dielectric EDM (PMEDM). This method was often reported to be effective in improving EDM performance. The results show that the PMEDM can improve the machining rate and surface quality, and decreased the tool wear (Kansal et al., 2007; Tzeng, 2008; Bhattacharya et al., 2013). Suspended powders increase the spark gap distance due to their presence between tool and workpiece. It has two outcomes: firstly, increased the spark gap is useful in effective removal of debris from the gap; secondly, it makes the powder EDM process highly stable with effective discharge dispersion throughout the gap. An increase in the distance decreases the electrostatic capacitance of the gap. Efficient discharge dispersion not only produce uniform work surface, but also prevents the occurrence of concentrated arc discharge and hence reduces finishing time (Jahan et al., 2011; Kumar et al., 2011). Suspension powder in the dielectric of EDM reduces machining time significantly and improving surface quality of work material compared to conventional EDM methods. This statement supported by a few researchers doing an experiment regarding powder suspended dielectric in EDM machining. Based on analysis done by Kumar et al., (2010) on the study of potential of graphite powder in AEDM of Nickel Based Super alloy 718 with the three type powder concentration which is 0, 6, and 12g/l, they observed that an increase in peak current and the concentration of graphite fine powder in dielectric fluid increase MRR. The observation suggests that the suspension of an appropriate amount of powder into the dielectric fluid causes greater erosion of the material. The reason for the enhancement of MRR is mainly attributed to a reduction in the breakdown strength of the

2 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 38 dielectric fluid leading to early spark, and increase in frequency of sparking within the discharge. 12g/l concentration of graphite powder produced the maximum MRR in Inconel 718 with the improvement approximately 27%. Other research done by Kumar et al., (2011), regarding EDM machining of Inconel 718 with powder mixed in dielectric. This time the researcher use Aluminium as powder suspension in dielectric with three different particle size, which is fine (400 mesh = 38µm), medium (325 mesh = 44µm) and coarse (200 mesh = 74µm) and with 5 different concentrations from 0g/l to 12g/l. Medium size of aluminium additive powder enhances machining rate significantly by almost 90% of improvement and at the same time reduces wear rate (WR) by 80% and R a by 17%. Certain powder concentration in dielectric also improves machining rates. 6g/l of Aluminium with medium size additive powder in dielectric produces maximum MRR and minimum R a. However, higher contamination leads to unstable machining conditions. 4 g/l powder concentration produces minimum WR and thereafter, it increases at higher concentrations. The author also remarked that powder mixed EDM is an effective option for machining Inconel 718. According to an experiment done by Singh et al., (2010) when machining hastelloy with Cu electrode and Al powder mixed as a suspension in dielectric with three types of mesh size (fine, medium and coarse) and five levels of powder concentrations up to 12g/l, the experiment shows that MRR yielded by conventional EDM is low. Thereafter, with the addition of more powder in dielectric MRR starts increasing at a higher rate. However, too low and too high concentration of powder may reduce MRR. Highest MRR is produced at 6g/l concentration. MRR is higher by suspending medium grain size aluminium powder in EDM oil at equal concentrations. The highest MRR value is achieved at 6 g/l concentrations with medium size of powder particles. For TWR, electrode erodes at a much higher rate in a pure dielectric fluid. EWR decreases by adding 3g/l concentration aluminium powder in the dielectric fluid and increases slightly by adding more powder in dielectric fluid up to 6g/l concentration. Then, surface produced without an additive of powder in dielectric fluid has a large surface roughness (R a ) value. R a lowers down by suspending aluminium powder in a dielectric fluid. So, the researchers have made a conclusion, aluminium powder suspended in the dielectric fluid affected MRR, EWR, and R a and too low and too high concentration and grain size of aluminium powder in EDM oil reduces MRR. EWR can be lowered down by reducing the size of suspended aluminium powder particles in EDM oil and the surface finish can be enhanced by reducing the size of aluminium powder up to a certain particle size. Too small powder particles produce rough surfaces. Chromium powder is a choice made by Ojha et al.,(2011) for additive suspension in the dielectric of EDM machining EN-8 steel. He reported, current, powder concentration and electrode diameter are significant factors affecting both MRR and EWR. Both the performance measures were observed an increasing trend by increase in current for all parameter settings. MRR was increased with increasing in powder concentration. The trend shows that MRR will increase further with further increase in concentration. The highest MRR value is obtained at the highest peak current and powder concentration 8A and 6g/l respectively. EWR increases with a lower range of powder concentration, but then decrease. The authors also recommended for more workpiece, powder, electrode materials and experimental settings combinations are used to investigate further for much understanding of the process. Ming and He (1995) have reported on their research that MRR clearly increase by all additives used in the tests, especially in the middle-finish machining and the finish machining. In the condition of middle-rough machining, the MRR can be increased by about 50% due to adding additives. In the middle-finish machining the MRR can be doubled. During the finishing the MRR is even 2 to 3 times as fast as it is for pure kerosene. It has been discovered that when two kinds additive agents which is solid powder and liquid are added at the same time the MRR is higher. However, there is an optimum value of the quantity of the additive. They also found that EWR can be lower by almost all kinds of additive, especially in middle-rough machining. The essential difficulty in EDM is that the SR is not so good. One purpose of adding the additive to kerosene is to improve the surface quality. Generally the more the additive in the kerosene, the surface quality is better, but when excessive of additive used it becomes worse. The researchers also stated that the condition of fatigue stress, micro cracks, recast layer and hardness during the EDM process can be improved by adding an addition to the working fluid. The recast layer will be thinner and so the cracks will be less. For the hardness in the top layer, it was greater when adding additive. Jahan et al., (2011) has done an experiment regarding the effect of graphite nano-powder on the EDM of WC-Co and they claimed the spark gap increases significantly with the increase of powder concentration for graphite-mixed dielectric. Although the spark gap is increased, very high concentration of powder particles in the dielectric can result in series of discharging and arcing thus causing surface defects. With the increase of powder concentration also the MRR increases due to the stability of machining process at increased spark gap after adding nano-powder in dielectric. The addition of powder particles can reduce the electrical discharge power density and gap explosive pressure for a single pulse, which result in smaller craters with uniform distribution. Moreover, due to much effective flushing of debris in higher spark gap and reduced size craters, the overall MRR increases. The highest MRR is obtained at 0.8g/l of powder concentration. For EWR, an optimum range of g/l was found to provide lower EWR. Then, for the average surface roughness (R a ), it was decreases first with the increase of powder concentration. Then again tend to increase at higher concentration of powder particles. The authors also found that an addition of graphite nano-powder in dielectric oil provides smooth and defect-free surface.

3 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 39 Powder mixed dielectric in EDM process is still not widely used in industry. Many fundamental issues of these new processes, including machining mechanism with various additives, are still not well understood. Previous researchers also tend to use low peak current in the range 0.5A< I p < 20A and low pulse duration in range 10µs < t on < 150µs to control the tool wear and surface quality. As a consequence, the machining rate becomes slow and lead in the lower productivity. There is a researcher (Kumar et al., 2011) had used the pulse duration up to 750µs when EDM machining of Inconel 718 with powder mixed dielectric fluid but the highest peak current selected in their research is just 9A. Thus, for this research, the complexity of this process, especially from the effects of the powder mixed dielectric in relation with higher peak current and pulse duration to the EDM performance requires further investigations. 2. EXPERIMENTAL SET-UP AND PROCEDURES The EDM experiments were conducted on the CNC Sodick High Speed EDM die sink AQ55L (3 Axis Linear). The maximum travel range of the machine is 550 mm 400 mm 350 mm with the step resolution of 0.1 μm in X, Y and Z directions. All Inconel 718 specimens were standardized with a size of 40 mm 30 mm 10 mm by using an Okamoto grinding machine (ACC52DX) with a diamond-grain resinbond grinding wheel. Table 1 shows the alloy composition of Inconel 718. The tool electrode used to be cylinder-type copper with a diameter of 10 mm each. The EDM depth of cut is 3 mm was evaluated in all experiments. Kerosene was selected as a liquid dielectric with three conditions, without powder concentration, with 2g/l of powder concentration and with 4g/l of powder concentration. 99.5% purity of the nano alumina powder with an average particle size of 45nm was selected as a powder suspended in a dielectric fluid. The experimental process variables and settings are summarized in Table 2. For experiments involving powder suspension, an external tank device called high performance electrical discharge machining device (HPEDM) as shown on Figure 1 are attached on the CNC Sodick High Speed EDM. The device has its own controller and functioning as plug and play to the conventional EDM machine. Figure 2 shows the schematic diagram of HPEDM. The experiment was conducted in full factorial by using one trial for one variable approach. Table I Alloy compositions of Inconel 718 Alloy composition % Nickel (plus Cobalt) Chromium Iron Balance Niobium (plus Tantalum) Molybdenum Titanium Aluminium Cobalt Carbon Manganese Silicon Phosphorus Sulfur Boron Copper Parameters Work piece material Tool electrode Powder suspension Peak current, I p (A) Pulse duration, t on (µs) Powder concentration, C p (g/l) Pulse interval, t off (µs) Voltage, V Dielectric fluid Electrode polarity Depth of cut Table II Machining conditions and parameters 1.00 max 0.08 max 0.35 max 0.35 max max max max 0.30 max Levels Inconel 718 Copper Nano alumina powder, 45nm 20, 30, , 300, 400 0, 2, 4 Based on 80% duty factor 120 Kerosene Positive 3mm

4 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 40 Fig. 1. HPEDM machining setup 3. RESPONSES The selected response variables MRR, EWR and Ra are defined as follows: The material removal rate was calculated from the difference of weight of work-piece before and after the machining process. MRR = (W b - W a / ρ 718.t) mm 3 / min (1) Where, W b is the initial weight of workpiece in g; W a is the weight of the workpiece after machining in g; t is the Fig. 2. HPEDM schematic diagram machining time in minutes and ρ 718 is the density of Inconel 718 (8.19 x 10-3 g/mm 3 ). The wear of copper electrode was calculated from the weight difference of electrode before and after the machining and is expressed as: TWR = (E b E a / ρ e t) mm 3 / min (2) Where, E b is the initial weight of electrode in g; E a is the weight of electrode after machining in g; t is the machining time in minutes and ρ e is the density of Cu electrode 8.96 x 10 -

5 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: g/mm 3. The weight of the electrodes and workpiece before and after machining needs to be measured in order to obtain MRR and EWR. The changes in weight from the tool electrode or workpiece are suspected to be small. Thus, the more decimal points are better to eliminate the possibilities of large error. For this analysis Shimadzu weight balance measurement was used. Maximum weight can be measured is 210g until five decimal point accuracy. However, for this study the decimal point of the weight balance is to set to 4 decimal. Mitutoyo SJ- 400 Surface Roughness Tester is used to measure the average surface roughness (R a ) of the machining surface. When measuring surface roughness, the only parameter to be evaluated was R a as this is the most widely used parameter in industrial applications (Baraskar et al 2011). A scanning electron microscope (SEM) JSM 6380 was used to evaluate the surface topography of machined surface after EDM machining. Table III Result of MRR, EWR, and Ra 4. RESULTS AND DISCUSSION The focus of the experiments is to determine the optimum parameters corresponding to different Peak current (I p ), Pulse duration (t on ) and Powder concentration (C p ). This various parameters have significant influence on the quality of machining of Inconel 718. It affects the Material removal rate (MRR), Electrode wear rate (EWR) and Surface roughness (R a ). These results were extracted from a series of full factorial experiment which overall trials of 27. Then the comparison of performance was made between the conventional EDM and the HPEDM with powder suspension dielectric. The experimental results for MRR, EWR and R a after EDM machining of Inconel 718 by using Copper (Cu) is shown in Table III. Trial I p, A t on, µs C p, g/l MRR EWR R a, µm

6 MRR, mm 3 /min MRR, mm 3 /min International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: Material removal rate Material removal rate (MRR) represents the average volume of material removed from the workpiece per unit time (mm³/min). There are several factors need to be considered to ensure the results gained are useful in increasing the productivity in EDM operation. The most important factor to increase the speed of the machining is due to how much the volume of the material can be removed per time taken. The MRR is a standard value that is calculated to determine the rate of production in machining. The effect of Peak current (I p ) and Pulse durations (t on ) on the MRR at a different Powder concentration (C p ) of Cu electrode is shown in Figure 3. From Figure 3(a) it is revealed that the I p affects the MRR significantly when EDM machining of Inconel 718 without powder suspension dielectric. At high I p =40A, the intensity of energy release during sparking is proportionally increased whereby higher temperature produced by the spark, melts more material and removes from the workpiece. Therefore, by increasing the I p, MRR will increase (Ghewade and Ninapikar, 2011; Rajesha et al., 2011; Sudhakara et al, 2012). Conversely, higher t on has decreased MRR for all conditions of I p when Cu was used as an electrode. The reason is with a constant setting of 80% duty factor, the pulse interval will increase with the increment of t on. This high ignition delay due to high pulse interval reduces the machining rate, thus MRR is decreased. (Kumar et, al. 2011; Sudhakara et al, 2012). The similar trend also can be observed as shown in Figure 3(b) and 3(c). By comparing the effect of powder concentration (C p ) on MRR, the MRR was increased by increasing the C p into the dielectric fluid. At C p =2g/l, I p =40A and t on =200µs the MRR enhanced significantly from 34.57mm 3 /min (Figure 3(a)) to 43.68mm 3 /min as shown in Figure 3(b). Then, as indicated in Figure 3(c), further increment in MRR value up to 45.72mm 3 /min is observed when C p =4g/l was used in the same parameter setting. This observation suggests that the addition of an appropriate amount of additives into the dielectric fluid of EDM causes greater erosion of the material. The reason for the enhancement of MRR at higher powder concentration is mainly attributed to a reduction in the breakdown strength of the dielectric fluid leading to early spark, and increase in frequency of sparking within the discharge (Kumar et al, 2010). However, effect of t on on MRR is inversely proportional. MRR is decreased by the increment value of t on. During the machining period, in addition to the expansion of plasma channel, at high pulse duration levels the localized temperature is increased and as a consequence the decomposed carbon from dielectric fluid stacked to the electrode surfaces. Thus, the discharge efficiency is reduced and become unstable, thus, the MRR is decreased (Hascalik and Caydas, 2007). Material Removal Rate (MRR) (a) C p = 0g/l (without powder suspension) Material Removal Rate (MRR)

7 MRR, mm 3 /min International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 43 (b) C p = 2g/l Material Removal Rate (MRR) (c) C p = 4g/l Fig. 3. Effect of Peak current (I p) and Pulse duration (t on) on MRR at different Powder concentration (C p) [(a) C p=0g/l, (b) C p=2g/l and (c) C p=4g/l] Within a selected parameter, the highest value for MRR is 45.70mm 3 /min obtained when C p =4g/l was suspended in the dielectric fluid at 40A and 200µs of I p and t on, respectively. The improvement is about 32% as shown in Figure 4 in compared to without powder concentration at the same parameter setting I p =40A and t on =200µs. 32% C p =0g/l C p =4g/l Fig. 4. The improvement of MRR when EDM machining of Inconel 718 employing powder suspension dielectric On EDM applications, the usage amount of current is depending by the surface area of the cut and the process requirement. Higher peak current generally used in roughing operations with large surface areas and the lowest Peak current used for the finishing process. High Peak current (I p ) improved the MRR but it will affect the severe conditions of the machined surface topography of the workpiece. The surface topography of the material that has been machined closely related to the I p supplied. During the EDM process, the high temperature in every spark causes material melt and evaporation, and then leaves a crater on the machined surfaces (Li et al., 2013). From the Figure 5(a-1) shows that, at lower I p =20A, longest t on =400µs, and without powder concentration (C p ) of the dielectric, the conditions of craters are shallow and flatten, whereas at high I p =40A as indicated in Figure 5(a-2), the severe surface conditions such as the large crater formation and more nodules were appeared. This is due when increase of current intensity, the working energy increased, so that discharge craters become deeper and wider, thus contributing to a more noticeable surface topography (Theisen and Schuerman, 2004). The existence of micro-voids is due the stress released from the underneath of machined surface at high temperatures. The temperature at machined surface is raised when the high I p is applied and the hot bubble air trapped on the machine surface is exploding, thus created the micro-void (Li et al., 2013). There are also nodules on the machined surface produced from reattachment of molten metal during an improper flushing condition. The effect of powder suspension dielectric on the surface topography of Inconel 718 also more profound.

8 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 44 Low MRR High MRR Craters a-1) MRR=14.96mm 3 /min; I p =20A; t on =400µs; C p =0g/l - [Trial 3] Low MRR a-2) MRR=34.57mm 3 /min; I p =40A; t on =200µs; C p =0g/l - [Trial 7] High MRR Nodules Globules b-1) MRR=18.54mm 3 /min; I p =20A; t on =400µs; C p =2 g/l - [Trial 12] Low MRR b-2) MRR=43.68mm 3 /min; I p =40A; t on =200µs; C p =2 g/l - [Trial 16] High MRR Micro-voids c-1) MRR=17.57mm 3 /min; I p =20A; t on =400µs; C p =4 g/l - [Trial 21] c-2) MRR=45.70mm 3 /min; I p =40A; t on =200µs; C p =4 g/l - [Trial 25] Fig. 5. The EDM machined surface topography of Inconel 718 at a different powder concentration (C p) [a) C p=0g/l, b) C p=2g/l, and c) C p=4g/l] Based on Figure 5(b-1), at low I p =20A and C p =2g/l the surface looks rough and the size of nodules is bigger compared to C p =0g/l at low I p (Figure 5(a-1)), and the condition is better when C p =4g/l was used as shown in Figure 5(c-1). This is due to the suspension of powder particles in the dielectric fluid enlarged the plasma channel, caused an electric density decrease and hence uniform distribution of the sparking takes place, thus shallow craters was produced (Sharma et al., 2010). When the highest I p =40A was used, the topography looks worst. This is because powder settling is a common problem at higher powder concentration due to the dielectric loses its ability to distribute uniformly all the powder particles and because of that, the bridging of powder particles may occur, which results in short-circuiting and arcing more frequently. This bridging effect can result in more concentrated discharge energy (Sharma et al, 2010; Kumar et al, 2010; Jahan et al., 2011). Due to increase in frequency of discharging and high I p, faster erosion takes place from the workpiece thus damaged the machined surface. The formations of nodules are bigger and existence of micro-voids are clearly visible at higher MRR as indicated in Figure 5(b-2) and 5(c-2).

9 EWR, mm3/min EWR, mm3/min International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: Electrode wear rate In EDM applications, higher amperage is used in roughing operations with large surface areas and the lower current used for the finishing process. The higher current improve the MRR but the electrode wear and the quality of surface finish will be decreased. The high rate of electrode wear is one of the main problems in EDM. Electrode wear must be effectively compensated in order to achieve the required accuracy of the machined features. The effect of Peak current (I p ), Pulse durations (t on ) and Powder concentration (C p ) on the Electrode wear rate (EWR) of Cu is shown in Figure 6. Higher Peak 0.15 Electrode Wear Rate (EWR) current, I p is resulting an increasing in EWR for Cu electrodes at the constant Pulse duration (t on ) when EDM machined without powder concentration, C p =0g/l as indicated in Figure 6(a). The reason is due to high discharge current promoted to high spark energy eroded more material from the workpiece and the electrode which in effect increases the EWR. However, EWR was decreased when increasing of t on for each of the I p used respectively. The reason is that, because of the deposition effect of decomposed carbon from dielectric oil on the tool electrode at a high temperature for the longer pulse duration (Kang and Kim, 2003; Hascalik and Caydas, 2007) (a) C p = 0g/l Electrode Wear Rate (EWR) (b) C p = 2g/l

10 EWR, mm3/min International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: Electrode Wear Rate (EWR) (c) C p = 4g/l Fig. 6. Effect of Peak current (I p) and Pulse duration (t on) on EWR of Cu electrode at a different Powder concentration (C p) [(a) C p=0g/l, (b) C p=2g/l, and (c) C p=4g/l] As observed in Figure 6 (b), the effect of powder concentration is significant on the EWR of Cu electrode. The EWR is slightly increased when C p =2g/l was used at the highest I p and the lowest t on compared to C p =0g/l, but then decreased with the increment of t on. The similar trend also can be observed when 4g/l of C p was supplied in the dielectric fluid as shown in Figure 6(c), the EWR is increased at the highest I p and the lowest t on, but further decreased when high I p with high t on were promoted. The reason behind is that by suspended nano aluminium powder into dielectric fluid the machining process become more reactive and generated more heat, as a consequence the decomposed of Carbon from dielectric embedded on the electrode surfaces which functions as a wear resistant layer for electrode and helps to decrease the Carbon deposited electrode wear (Murray et al., 2012). In overall, t on and C p were identified as the two factors that improved EWR. EWR decreases with an increase in t on and further decrease with suspension of powder in the dielectric fluid. Previous works have found that the wear of tool electrodes is a dynamic process that is influenced by two opposite factors; electrical discharges erode materials from both the tool electrode and the workpiece and cracked carbon from the dielectric oil may be deposited on the surface of the electrode (Han et al 2009). The lowest EWR produced from this experiment is approximately mm 3 /min obtained at I p =40A, t on =400µs and C p =4g/l. A dissolved metal from the workpiece also revealed deposited on the electrode surface as indicated in Figure 7, 8 and 9. a-1) Low EWR [I p =20A; t on =400µs; C p =0g/l]- Trial 3 a-2) EDX test [I p =20A; t on =400µs; C p =0g/l]- Trial 3

11 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 47 Material deposited b-1) High EWR [I p =40A; t on =200µs; C p =0g/l]- Trial 7 b-2) EDX test [I p =40A; t on =200µs; C p =0g/l]- Trial 7 Fig. 7. Surface morphology and EDX testing of the Cu electrode at a powder concentration, C p=0g/l [a) Low EWR and b) High EWR] a-1) Low EWR [I p =40A; t on =400µs; C p =2g/l]- Trial 18 a-2) EDX test [I p =40A; t on =400µs; C p =2g/l]- Trial 18 b-1) High EWR [I p =40A; t on =200µs; C p =2g/l]-Trial 16 b-2) EDX test [I p =40A; t on =200µs; C p =2g/l]- Trial 16 Fig. 8. Surface morphology and EDX testing of the Cu electrode at a powder concentration, C p=2g/l [a) Low EWR and b) High EWR] a-1) Low EWR [I p =40A; t on =400µs; C p =4g/l]- Trial 27 a-2) EDX test [I p =20A; t on =400µs; C p =4g/l]- Trial 27

12 R a, µm International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 48 b-1) High EWR [I p =40A; t on =200µs; C p =4g/l]-Trial 25 b-2) EDX test [I p =40A; t on =200µs; C p =4g/l]- Trial 25 Fig. 9. Surface morphology and EDX testing of the Cu electrode at a powder concentration, C p=4g/l [a) Low EWR and b) High EWR] Based on the Figure 7-9, the deposited material was analyzed according to the lowest and the highest EWR value condition at a different C p conditions. It is observed that, the distribution of the deposited material is wider and more at the low EWR condition compared to at the high EWR at C p =0g/l as shown in Figure 7. The similar condition also can be observed at C p =2g/l and C p =4g/l as indicated in Figure 8 and 9, respectively. The EDX testing has been performed accordingly in order to clarify the elements of the deposited material on electrode surfaces. Based on the graph on the Figure 7(a-2) and 7(b-2), Carbon and the alloy elements of the material of Inconel 718 were deposited on the electrode surface. The counts of Carbon on the electrode surface were increased when powder concentration was used for EDM of Inconel 718 as shown in Figure 8(a-2) and 9(a-2). The negative value for the lowest EWR is indicating that the electrode was deposited by the carbon and material from the workpiece is more than the wear of electrode. Thus, the increment in the electrode mass after machining can be explained by this deposition effect. 4.3 Surface roughness Figure 10 shows the effect of peak current, pulse durations and powder concentration on Surface roughness (R a ) of Inconel 718. Without powder concentration which is C p =0g/l the R a value is increased when peak current is increased. By increasing the I p, the amount of energy in the EDM process will increase as shown in Figure 10(a). This can be attributed to the fact that the high I p may cause massive expulsion due to high discharge density leading to the formation of deeper and larger craters on the surface of the workpiece, thus the R a value is increased (Theisen and Schuermann, 2004; Patel, et al., 2009; Li et al., 2013). The result also showed that with increasing of the t on the R a was decreased at all of I p conditions. This is due to the fact that an increase in t on was equivalent to a decrease in the frequency of the pulse, which is lead in reducing of sparking intensity on the machined surface and produced a shallower crater as shown in Figure 11. This result was contrasted with the previous researcher (Bharti et al., 2010). According to Bharti et al., the R a will increase at the higher level of t on. Then, at C p =2g/l, it is observed that, the R a value was increased with high level peak current and but decreased when longest t on was applied. Surface Roughness (R a ) (a) C p = 0g/l

13 R a, µm R a, µm International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 49 Surface Roughness (R a ) (b) C p = 2g/l Surface Roughness (R a ) (c) C p = 4g/l Fig. 10. Effect of Peak current (I p) and Pulse duration (t on) on Surface roughness at a different Powder concentration (C p) [(a) C p=0g/l, (b) C p=2g/l and (c) C p=4g/l] The similar condition also can be observed when the highest C p =4g/l was applied. At C p =4g/l the R a was increased as the I p and t on increases as shown in Figure 10(c). At very high concentrations, the dielectric loses its ability to distribute uniformly all the powder materials. Therefore, powder settling is a common problem at higher concentration, although spark gap increases. In addition, at higher concentration of alumina nano-powder, the bridging of powder particles may occur, which results in arcing and short-circuiting more frequently (Jahan et al, 2011). The bridging effect can result in more concentrated discharge energy and, finally, deteriorating the R a as indicated in Figure 12. The lowest R a value of 8.98µm is achieved at I p =20A and t on =400μs and C p =0g/l. Then, the highest R a value is 21µm was obtained when the highest level of I p =40A, shortest t on =200µs with C p =4g/l. Thus, for this study the highest variable parameter setting is not suggested when good surface finish is desirable and the existence of powder suspension in the dielectric did not improve the R a.

14 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 50 Low magnification High magnification Craters a-1) Low R a [I p =20A; t on =400µs; C p =0g/l]- Trial 3 a-2) Low R a [I p =20A; t on =400µs; C p =0g/l]- Trial 3 Low magnification High magnification Nodules Craters b-1) High R a [I p =40A; t on =200µs; C p =0g/l]- Trial 7 b-2) High R a [I p =40A; t on =200µs; C p =0g/l]- Trial 7 Fig. 11. Surface topography of machined surface at Powder concentration, C p=0g/l at [a) low R a, and b) High R a] Low magnification High magnification Craters a-1) Low R a [I p =20A; t on =400µs; C p =4g/l]- Trial 21 a-2) Low R a [I p =20A; t on =400µs; C p =4g/l]- Trial 21 Low magnification High magnification Craters b-1) High R a [I p =40A; t on =200µs; C p =4g/l]- Trial 25 b-2) High R a [I p =40A; t on =200µs; C p =4g/l]- Trial 25 Fig. 12. Surface topography of machined surface at Powder concentration, C p=4g/l at [a) low R a, and b) High R a]

15 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: CONCLUSIONS By employing high peak current and pulse duration, the EDM machinability of Inconel 718 at a different powder concentration was studied. Based on the experimental result, the following conclusion can be made: i) The peak current and powder concentration is the most contributing factor that improves the MRR. The metal removal rate will increase as the peak current and powder concentration increase. The result shows the highest of MRR as Cu as electrode is 45.7mm 3 /min at peak current 40A. With 4g/l of powder concentration The improvement is almost 32% in comparison to without powder concentration at the same parameter setting. The result shows that the introduction of powder concentration in dielectric fluid will helps to enhance the machining efficiency. It also found that within selected parameters, 4g/l is the best powder concentration to achieve high MRR. ii) The electrode wear rate (EWR) is increased when peak current is increased, but inversely proportional with pulse duration. However, the EWR is decreased when high concentrations of powder additive with the combination of high peak current, and longer pulse duration. The lowest EWR of Cu is mm 3 /min obtained at I p =40A, t on =400µs and C p =4g/l. iii) Through the EDX analysis, it was found that the carbon from dielectric and the workpiece material has been deposited on the electrode surface. The negative value for the EWR is due to the this deposited effect on the electrode after machining. iv) High I p is not recommended for surface roughness. The surface roughness will increase when the increase of the I p. The result also shows that an increasing in the t on value the surface roughness will decrease and there is no improvement in surface roughness when powder additive was suspended in a dielectric fluid. The lowest R a value 8.98µm was obtained at, t on =400µs, Cp=0g/l and then the highest R a value 21µm achieved at, t on =200µs, and C p =2g/l. ACKNOWLEDGEMENTS This research was supported by Exploitary Research Grant (ERGS) and Fundamental Research Grants (FRGS) under Ministry of Education, Malaysia. The authors would like also to thank to the Sustainable Manufacturing and Recycling Technology (SMART) research cluster, Advanced Manufacturing and Materials Center (AMMC), and Advanced Machining Laboratory (AML), Universiti Tun Hussein Onn (UTHM) for providing the facilities. REFERENCES [1] Ezugwu, E. O. (2005). Key improvements in the machining of difficult-to-cut aerospace superalloys. International Journal of Machine Tools & Manufacture [2] Rajesha, S., Sharma, A., and Kumar, P. (2011). 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