Machining Characteristics and Parametric Optimisation of Inconel 825 during Electric Discharge Machining

This paper presents the machining characteristics and parametric optimisation of Inconel 825 during die-sinking electrical discharge machining (EDM) process. This work considers seven input parameters out of which six are of three levels and one is of one level. Metal removal rate (MRR), tool wear rate (TWR) and surface roughness (SR) have been considered as performance measures. Before carrying out physical experimental runs, the experiments have been designed using Taguchi’s L36 (21 × 36) orthogonal array (OA). In order to identify the significant input parameters, Analysis of Variance has been employed on the experimental data. Discharge current, pulse-on-time, tool material and tool electrode lift time are found as significant input parameters. The effects of these significant parameters on the performance measures have been presented using Taguchi's technique. After machining, study of surface characteristics of the electric discharged machined surface has been carried out using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDXS) and X-ray diffraction (XRD). These studies help in obtaining the information pertaining to topography of the machined surface, material transfer from tool and dielectric to the machined surface, and presence of extra element and their different phases at the machined surface. It has been found from the research that Carbon, Oxygen, Iron, Nickel, Chromium and little amount of Molybdenum are transferred to the surface of work piece. Further, for the best yield of the process, the optimal combination of input parameters has been obtained and reported using Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) as a multi-objective optimisation technique. Consideration of two different dielectric fluids and three different electrode materials is the novelty of the work.


INTRODUCTION
Inconel 825, a nickel-based super alloy, finds wide applications in the field of aerospace, marine, furnace components, orthopaedic etc. [1,2]. The intrinsic properties like work hardening, poor thermal diffusivity and high dynamic shear strength makes it difficult to be machined on conventional machines. EDM, a nonconventional machining process, is suitable to shape this alloy. EDM is a thermo-electric process in which material removal takes place due to electric discharge erosion effect of electric sparks between the tool electrode and work piece (both submerged in a dielectric medium) that are separated at a particular distance [3,4].
However, much work has been reported in literature regarding EDM of super alloys like Inconel 718 [5][6][7] SuperCo605 [8], Ni-based alloy [9], Inconel 800 [10], Nimonic 75 [11] and Monel 400 [12,13], there is scanty of data in respect of EDM of Inconel 825. Rahul et al. [14] studied the surface integrity and metallurgical characteristics of Inconel 825 using cryogenically treated tool and non-treated tool. They emphasized on the surface cracking and white layer formation at the machined work piece. Kumar et al. [15] proposed a low cost Al2O3 nano powder for the machining of Inconel 825 material. They used nano powder of size 45 to 50 nm Al 2 O 3 mixed with deionized water (dielectric) to improve surface characteristics. Mohanty et al. [16] presented the effects of various input parameters on performance measures like MRR, SR, radial overcut (ROC) and surface crack density (SCD). They employed Taguchi L9 OA for experimental design. Prabhu & Vinayagam [17] studied the effect of multiwall carbon nano (CNT) tube mixed in the dielectric fluid in EDM of Inconel 825. The experiments were planned using Taguchi's Design of experiment (DOE) to get the optimal machining parameters.
Prabhu & Vinayagam [18] presented the importance of single wall CNT mixed with dielectric fluid to analyse the surface characteristics of Inconel 825. It was found from the experimental results that surface texture is better after EDM due to discharge energy. This work presents experimental investigations during EDM of Inconel 825. During literature review, authors could not come across some work which considers three levels of electrode material and two levels of dielectric fluid for experimental investigations during EDM of Inconel 825. In this work dielectric fluid (DF), pulse-on-time (T on ), discharge current (I D ), duty cycle (ζ), gap voltage (V g ), tool electrode lift time (T L ) and tool electrode material (T M ) are taken as process parameters and MRR, SR and TWR are taken as performance measures.
Besides this, surface characterization and topography of the machined surface have also been carried out by SEM analysis. To ascertain the transfer of material from the tool and dielectric to the machines surface, EDXS has been performed. Further, XRD analysis has also been carried out for finding the presence of extra elements and their different phases. The novelty of this work is consideration of two levels of dielectric fluid and three levels of electrode materials: copper (Cu), graphite (Gr) and copper-tungsten (Cu 20%; W 80%).
The selection of optimal combination of input parameters, for the best machining performance, is an important and challenging task in the field of EDM. EDM is a very complex machining process due to involvement of many electric and non-electric parameters. More so, it becomes a multi-objective optimisation task owing to conflicting nature of performance measures. Higher MRR is desired along with lower SR and TWR. Several researchers have employed different multi-objective optimisation techniques for the parametric optimisation e.g. genetic algorithm, simulated annealing, particle swam optimisation, grey relation analysis [19][20][21][22][23]. This work employs TOPSIS method as multi-objective optimisation technique to find the optimal combination of input parameters. Some authors have employed this technique in the field of EDM for different materials [24], the authors could not come across some work that employs this technique for Inconel 825. In EDM, MRR and SR are primarily two main concerns in respect of performance improvement. Hence, in this work, optimisation has been done by considering two performance measures i.e. MRR and SR. The third performance measure (TWR) has not been considered for optimisation.

MATERIALS AND METHOD
Mechanical properties and chemical composition of Inconel 825 are given in Tab. 1 and Tab. 2. The input parameters and their levels are shown in Tab. 3.  The experiments have been designed by Taguchi's L36 OA [5] as shown in Tab. 4. The Experiments were carried out on Elektra 5535 PS die-sinking EDM as shown in Fig.  1.

Figure 1 EDM PS50
The dielectric fluid has been taken as noise factor and two levels for the same are taken. The first dielectric fluid used for the experimentation is IPOL Spark Erosion oil SEO-250 and the second one is commercial grade EDM oil. The electrodes used in this work are electrolytic Cu, CuW (Cu 20%; W 80%), and Gr as shown in Fig. 2.

Figure 2 Electrodes
Each experimental run has been repeated thrice. The machined surface is shown in Fig. 3. For analysis of machined surface, XRD patterns have been recorded by Discover X-ray powder Diffractometer. The values which were considered using XRD spectra were Cu, K αradiations (λ = 1.54060 A, the scan rate of 10 min, range -1400) having a generator setting of 40 mA and 45 kV. Besides XRD, SEM and EDX are also done for surface morphology using SEM (JSM-6610LV Joel, Japan). SR, defined as the measurement of central line average known as (Ra), was used to check the quality of the surface quantitatively. SR has been measured by using surface Roughness Tester -MITUTOYO, JAPAN SUR-FTEST SV-2100. The measurement was repeated three times and the average of the three readings was taken.

EXPERIMENTAL RESULTS AND DISCUSSION
The experimental values of the performance measures corresponding to all 36 experimental runs are shown in Tab. 4. The analysis of the experimental results has been done by Taguchi's method.

Material Removal Rate
The value of S/N ratio corresponding to MRR is shown in Tab. 4. ANOVA results for the same, shown in Tab. 5, indicate that discharge current is the most influencing parameter having percentage contribution as 41.53% followed by tool electrode lift time and tool material. Fig. 4a depicts that MRR increases with increase in discharge current. This may be attributed to the fact that the increase in discharge current increases the spark energy between the electrode and the work piece. High spark energy increases the temperature that ultimately leads to higher MRR. Fig. 4b depicts that MRR increases with increase in tool electrode lift time. Low tool electrode lift time causes carbon accumulation on the electrode face resulting in more sparks at the surface that ultimately leads to less MRR. Increment in tool lifting time increases the inter-electrode gap (IEG) between tool and work piece that leads to higher MRR. Fig. 4c shows that MRR is affected by tool material. High MRR is achieved by Gr electrode followed by CuW and Cu. This may be attributed to the fact that MRR is affected by tool material's thermo-physical properties like melting point, density, strength, electrical conductivity.

Surface Roughness
The value of S/N ratio corresponding to SR is shown in Tab. 4. ANOVA results for the same, shown in Tab. 6, indicate that pulse-on-time is the most influential parameter having percentage contribution as 54.79% followed by discharge current and tool material.   Fig. 5a and 5b indicates that SR increases with increase in discharge current and pulse-on-time. SR is nothing but the formation of small craters at the surface due to spark discharges. Spark discharges are proportional to the discharge energy and discharge duration i.e. discharge current and pulse-on-time respectively. More is the value of discharge current and pulse-on-time more will be the SR. Fig. 5c reveals that SR is also affected by tool material. SR is highest for Cu electrode and lowest for Gr electrode. The thermo-physical properties of the tool material play a crucial role in the variation of surface roughness. The parameters like thermal conductivity, density and specific heat are very much effective in deciding the SR value.

Tool Wear Rate
The value of S/N ratio corresponding to TWR is shown in Tab. 4. ANOVA results for the same, shown in Tab. 7, indicate that tool material is the most influencing parameter having percentage contribution of 49.87% followed by pulse-on-time and discharge current.  Fig. 6a indicates that TWR increases with increase in discharge current because higher discharge current value produces more sparks resulting in high heat formation causing more material removal rate from the work piece and tool electrode. Fig. 6b indicates that increase in pulseon-time reduces tool wear reduces. This may be attributed to the reason that as pulse-on-time increases, spark (in plasma channel) spreads thus reduces the amount of heat transferred to the tool resulting in low TWR. Fig. 6c shows that TWR is minimum for CuW electrode and maximum for Gr electrode.
This may be attributed to the thermo-physical properties of the tool electrode. High melting point, higher density and good thermal conductivity of the electrode material will lead to low tool wear rate.

SURFACE CHARACTERISATION 4.1 Scanning Electron Microscopy (SEM)
EDM surface is generally matte in nature comprised of multitude layers of the crater, pockmarks, re-solidify layer, debris, spherical droplets etc. In this work, SEM analysis has been carried out to study the features like microstructure, topography, morphology and fractography with a range from millimetre to nanometre. Six samples with three different electrode materials have been taken and analysed. Fig. 7a, b and c show the SEM analysis for Inconel 825 machined using Cu electrode, (4 A, 8 A current), (20 µs, 75 µs pulse-on-time), (40 V, 60 V gap voltage), (10 tau, 11 tau duty factor), (0.1 s, 0.2 s tool lift time). The SEM images show the wavy in nature indicating the presence of crater varying in size from smaller to bigger crater due to low and high intensity of current and pulseon-time. The images also show the presence of white spot formation on a large scale indicating that are-solidification phenomenon is dominating one. The topography also presents a river bed pattern along with pockmarks which is a property of erosion phenomena [25].   . SEM images show that it corresponds to three different types of areas such as dark zone, grey zone and white zone. The area corresponding to dark zone indicates that deep craters are formed due to high pulse current and high pulse-on-time causing more evaporation and melting simultaneously resulting with more MRR from the surface, whereas the area marked with grey zone confirms the presence of shallow craters along with heat affected zone. The area with white zone indicates the formation of the white layer due to re-solidification phenomenon. The fascinating images obtained by using CuW electrode show some attractive features like minimum surface roughness amongst the other two electrodes. Fig. 8c shows the SEM image of a two cracks meeting at a point. The morphology indicates the tearing phenomena causing the residual stress to exceed the ultimate tensile stress.  The micrograph shows the presence of overlapping craters with variation in sizes along with micro crack present throughout work piece. It is evident from the machining process that by sudden heating there is loosening of the substance all along the grain boundary. Since the grain boundaries are weak, the material gets easily chipped off [26]. This fact is confirmed from the SEM image Fig. 9b taken at higher magnification which indicates that there are patches of craters of bigger size lying on the surface. The SEM image also confirms the presence of cracks formed due to thermal stresses.

Energy-Dispersive X-Ray Spectroscopy (EDXS)
EDXS is used for observing the transfer/migration of material from the dielectric and tool electrode. In the present study, the work piece is machined by three types of electrode material i.e. Cu, CuW and Gr. An EDXS analysis shows that material is transferred to the work piece from electrodes surface which is continuously flushed out by the flow of dielectric fluid. It also confirms that percentage of carbon in EDM surface is high due to low pulse current and high pulse-on-time i.e. 4 A and 75 µs as shown in Fig.  10a-d. At low current and high pulse on time, pyrolysis effect of the dielectric fluid increases resulting in more carbon percentage on the EDMed surface. The formation of oxide takes place when oxygen attacks and degrades the EDM fluids. From Fig. 10e-f of EDXS analysis, the higher peaks represent the concentrated elements found in the work sample. EDXS results are indicated by the percentage of elements transfer from electrode and dielectric to the work piece. The EDX analysis also reveals that major contributions are Carbon (C), Oxygen (O2), Iron (Fe), Nickel (Ni), Chromium (Cr) and little amount of Molybdenum (Mo) which is transferred to the surface of the work piece. The higher percentage contribution is shown by the oxygen and carbon which is generally due to the disintegration of dielectric, oxidation and the amount of debris removed at a high temperature in EDM process.

X-Ray Diffraction (XRD)
XRD analysis has been done for the machined surface to examine the nature of compounds formed. For this,  The experimental runs are designed in such a way that, for every trial, there is a variation of input factors such as pulse-on time (10 µs -75 µs), discharge current from (4 A -12 A) and gap voltage (40 V -80 V) which results into the formation of bcc structure of the γ-phase of Inconel 825 along with some δ (Ni 3 Al) phase at a diffraction angle of 43.5°. The formation of intermetallic phase at 63.6° is because of the transfer of elements from tool as well as from dielectric used in the experiment which is a similar observation reported by Talla G. et al. [27]. The intermetallic phase (Ni 3 Al) will offer superior material properties such as high hardness, tensile strength and high creep strength [28]. The lattice planes are indexed as (300), (422), (622), (642) and the compound as Cr 12 Fe 22 Mo 4 NbNi 10 Si 9 . The pattern shows traces of chromium, iron, molybdenum, niobium, silicon and nickel on the machined surface. The chromium acts as an oxidizing agent whereas nickel, molybdenum and niobium act as resistance to reducing environment and for pitting and crevice corrosion. In the second observation with Gr electrode and oil 1 (Exp. No.5) is shown in Fig. 11b. The variation in the peak with varying input factors showed the presence of a BCC structure with martensitic phase. The martensitic phase occurs during the EDM process as the machining takes place at high heat energy (high current and high pulse-on-time) causing the material to get melt down and simultaneously get cooled down with the help of dielectric. The lattice planes are indexed as (110), (200), (211) and the compound formed as C0.60Cr1.13Fe8.5Mn0.8Mo0.04Nb0.01Ni0.08Si0.7V0.02.
The pattern shows the presence of carbon on the machined surface which is acting as hardening agent. It, therefore, increases wear resistance to the work sample as well as reduces toughness and ductility. The carbon has the tendency for maximum solubility in austenite region. Similarly, when machined with CuW electrode and oil 1 (Exp. No.11) is shown in Fig. 11c. It is forming an FCC structure with change in phase transformation from BCC to FCC. This may be due to generation of high temperature (high peak current) at the machining zone which causes the material to become soft and ductile and causes more carbon to absorb. The lattice planes are indexed as (111)

OPTIMISATION BY TOPSIS METHOD 5.1 TOPSIS Method
TOPSIS Method is one of the multi criteria decision making methods [29]. It awards rank to different solutions. Highest ranked solutions are taken as the optimal solution. The steps to solve a problem using TOPSIS are briefly discussed in the following section along with a mathematical illustration. In this work, authors have evaluated factors (which are input parameters) with an objective to achieve higher MRR and lower SR for 36 experimental runs that are considered as alternatives. The best alternative is the one closest to the ideal solution and worst is the one farthest from the ideal solution.
Step 1. The results obtained after conducting experiments were used to make a decision matrix. The decision matrix consists of values of MRR and SR corresponding to different combinations of input parameters. Hence, the decision matrix, in this work, has been taken from Tab. 4.
Step 2. The above decision matrix is normalized using Eq. (1) and is shown in Tab. 8.
(for i = 1 to m; j = 1 to n) Step 3. Weight is considered 0.5 for both MRR and SR. Weighted normalized decision matrix is calculated using Eq. (2).
(i = 1 to m; j = 1 to n; w j is the weight of j criterion) Step Step 5. Separation distance for each alternative from Positive and Negative Ideal Solution is calculated using Eq. (4) and is shown in Tab. 9.
( ) where, i = 1 to m Step 6. Relative closeness to ideal solution i C * is calculated using Eq. (5) and is shown in Tab. 9.    By employing TOPSIS method, as explained above, the best solution obtained is experiment No.20 that corresponds to D F -second oil, T on -40 micro seconds, I d -12 amps, ζ -11%, V g -40 V, T M -Cu, T L -0.1 s. This combination gives the values of MRR and SR as 4.8870 and 7.426 respectively.

CONCLUSIONS
This work presents the experimental investigations during EDM of Inconel 825. A total of seven parameters have been considered in this work. After analysis of experimental results, the following conclusions may be drawn: 1. Discharge current, tool electrode lifts time and tool materials have significant influence on the MRR. The most influencing factor is found to be discharge