Experimental Characterization of Electrical Discharge Machining of Aluminum 6061 T6 Alloy using Different Dielectrics

Electrical discharge machining (EDM) is one of the earliest non-traditional machining processes and is used for the machining of complex shapes and 3D profiles with great precision and accuracy. It is also called as a spark erosion process, where a series of rapidly occurring discharges between two current-carrying electrodes in the presence of a dielectric medium removes material from the workpiece. The required shape is achieved as a negative image imparting of the electrode. There is no direct contact between these two electrodes which makes this process free from mechanical stresses and chattering. EDM has distinctive advantages for machining surgical components, dies and mold-making, and in automobile and aerospace industries [1‐4].

Being a complex dynamic machining method, a small variation in parameters may vary the responses such as surface finish, material removal rate, tool wear rate, kerf width, recast layer, among other factors to a greater extent. Process parameters selection is important in the EDM process and care must be taken during selection; otherwise, the productivity and quality of parts may be damaged [5]. Furthermore, it is today’s foremost industrial requirement to produce fine-quality parts at a higher production rate, but also controlling the process cost [6]. Hence, explicated study is required to understand parameters and their responses to fully apply the EDM process effectively. Although different electrical parameters such as pulse-on-time, pulse-off-time, current and others are imperative, non-electrical parameters such as tool material, workpiece material and most importantly dielectrics play a decisive role in the EDM process. Dielectrics are very significant in EDM technology as they circulate between the electrode and the workpiece and are responsible for discharge phenomena which brings productivity and quality [7]. Firstly, dielectrics serve as an insulating medium, and during the process it acts as an ionization medium, and after machining it is responsible for washing of the chips and cooling of the working area [7‐10]. Different dielectrics have dissimilar compositions and cooling rates; hence, the selection of a suitable dielectric is essential [11, 12]. Water, hydrocarbon oils and gases are major categories of dielectrics [8‐10]. Hydrocarbon oils especially kerosene are well known in EDM, but other dielectrics need to be evaluated in order to make the EDM process more viable.

Many researchers had experimented with various parameters to investigate different performance measures during EDM. Some of them have also studied dielectric impact. Wang et al. [13] compared compound dielectric, kerosene and distilled water for machining TC4 titanium alloy. Compound dielectric resulted in highest material removal rate, lower relative electrode wear rate and smaller surface roughness values. Tang et al. [14] observed that tap water gave higher MRR due to the absence of a carbide layer in the discharge phenomena for Ti–6Al–4V alloy. Zhang et al. [15] investigated five different types of dielectrics while machining steel 8407. They reported that liquid dielectrics exhibited better material removal efficiency, while water–oil emulsion sustained pressure for a longer time. Valaki and Rathod [16] analyzed bio-dielectrics developed from vegetable oil and kerosene while machining P20 steel, the bio-dielectrics showed better results than kerosene for MRR and EWR. Furthermore, EDM process using water oil emulsion showed that both MRR and surface roughness (SR) were influenced greatly by current. Moreover, results were similar compared to that of kerosene, which confirmed the feasibility of water–oil emulsion to be used as dielectrics, as reported by Liu et al. [17] and Zhang et al. [18]. Wu et al. [19] performed a comparative study of machining in pure kerosene and kerosene with addition of powder and surfactant; 60% improved surface quality was obtained with powdered mixed dielectric. Kursad [20] performed an experimental study of hole drilling using kerosene, distilled water and water as dielectrics by changing the current level. Increasing current decreased surface quality and increased electrode wear rate, while distilled water offered better results. Baseri and Sadeghian [21] showed through experiments that EDM performance could be enhanced by adding powder in a dielectric in combination with a rotary tool, and higher current and pulse duration led to higher MRR, EWR and SR. Tebin et al. [22] evaluated parametric evaluation for steel 50CrV4. Higher energy density produced rougher surface due to more melting, and it also caused arcing phenomena which was an abnormal discharge. Guo et al. [23] presented optimization model of parameters for machining insulating zirconia 3YSZ. Increasing current and/or pulse-on-time caused an increase in both MRR and EWR. Ou and Wang [24] investigated EDM machining, using water and suspension of particles in water for medical applications. Although MRR was lower in suspension-based dielectrics, however, it resulted in good surface quality. Muthuramalingam and Mohan [25] found that SR and MRR directly depended on current and duty factor, and with uniform distribution of discharge current, surface roughness was reduced. Increasing current and pulse-on-time resulted in higher MRR, while EWR increased at higher current EDM on AISI D6 tool steel, as reported by Barenji et al. [26]. Koteswararao et al. [27] performed machining of EN31 and reported that MRR decreased with elevated values of pulse duration and continue to increase with an increase in current levels. Amorim and Weingaertner [28] compared copper and graphite electrode for machining AISI P20, and graphite resulted in better surface roughness and MRR, while copper electrode offered stable discharging. Moreover, positive polarity tool offered better MRR, while good surface was obtained using negative polarity tool.

It is evident from the literature review that the selection of a proper dielectric is indispensable for the efficiency and stability of the EDM process. Dielectrics are not only providing medium for discharging, but also control the mechanism of the process. Different surface characteristics can be obtained using different dielectrics. Many researchers have compared different dielectrics for steels, titanium alloys and other engineering materials. However, little or no studies have been reported on aluminum alloy with different dielectric media. Moreover, all the above-mentioned researchers utilized copper electrodes. Hence, in this present work, the performances of distilled water and kerosene are compared. Graphite electrode was used during machining of aluminum 6061 T6 alloy. The aluminum was machined as the workpiece for the experimental study. In addition, the effects of process parameters: pulse-on-time and current, are also evaluated, using surface plots and line graphs to compare both dielectrics in order to select better dielectric. Furthermore, a comparison was performed on scanning electron microscope (SEM) micrographs to evaluate the effects of both change of parameters and dielectrics.

Experiments were performed on NEUAR Diesinker EDM with the jet flushing method as depicted in Fig. 1. Aluminum 6061 T6 alloy was used as a workpiece. The chemical composition of the material is shown in Table 1. The materials have equal dimensions and sizes. The diameter and length were 22 and 20 mm, respectively, as shown in Fig. 2.

A hollow cylindrical graphite electrode with external and internal diameters of 25 and 15 mm, respectively, were used (Fig. 3). The workpiece and electrode were attached to positive and negative polarities, respectively. Taguchi technique was used for experimental design (planning and implementation). It has a distinct advantage of offering a fewer number of experimental runs [29].

An L9 orthogonal array was employed for the design of the experimental sequence with three levels of parameters: Pulse-on-time (Pon) and current (Table 2). Process parameters (Current and Pon) have been selected based on their significant importance in EDM [23‐27]. The correct selection of the ranges of these parameters was based on literature review and preliminary trials in such a way that the machined parts were defect free. Some parameters were set at a constant level as shown in Table 3. The experimental or test runs are provided in Table 4. Firstly, a separate analysis of electrical parameters (pulse-on-time and current) was studied for EWR and MRR using distilled water and kerosene; then, MRR and EWR were compared for both dielectrics.

A separate electrode and workpiece were used for each experimental run. The weights of the workpiece and electrode were measured using a weight balance before and after each experiment. Machining time was recorded using a stopwatch. After each run, all the workpieces and electrodes were cleaned with acetone to remove dielectric remains. For the calculations of the MRR and EWR, the following equations were employed [30]:where W_a and W_b represent the weight of the workpieces before and after machining for each experiment; E_b and E_a denote weight values of electrode prior and subsequent to the experiment, ρ denotes density; and T_m represents time utilized for machining. After computing MRR and EWR values, signal-to-noise ratios (S/N) were estimated to study precisely the trend of parameters. Similarly, Taguchi approach was used to measure S/N ratios, using larger the better trend for MRR and smaller the better trend for EWR. These two expressions are shown as Eqs. (3) and (4), as adopted from [31]:where y represents values obtained after experiments for both MRR and EWR in each experiment and n indicates the number of experiments.

For micrographs, samples were prepared by first dipping them into hardener and resin as depicted in Fig. 4; then, dried to stabilize them. TESCAN (MIRA 3 XMU type) scanning electron microscope (Fig. 5) was used to obtain clear micrographs.

In this experimental work, the influences of distilled water and kerosene dielectrics have been investigated for EDM machining of aluminum 6061 T6 alloy using a graphite electrode. Moreover, the impact of pulse-on-time and current on MRR, SR and microstructures has been evaluated separately for both dielectrics. The following important conclusions are drawn from this innovative experimental study:

Evidently, EDM is an adequate process for machining aluminum 6061 T6 alloy. The results obtained from this study clearly show that distilled water has better dielectric properties in comparison with kerosene. Also, distilled water offers better MRR values in addition to being an environmentally friendly dielectric. Therefore, it should be a preferred dielectric for EDM. Moreover, graphite, as an electrode offers a better MRR with low processing cost and good thermal stability. Hence, it should be employed widely for this non-conventional machining (EDM). For future work, different aluminum alloys can be explored along with different dielectrics, such as dry EDM setup and dielectrics mixed with nanoparticles. Lastly, a study on the effects of different electrode shapes is also recommended.