Effect of Electric Discharge Machining on Material Removal Rate and White Layer Composition

In this study the MRR (Material Removal Rate) of the aerospace grade (2024 T6) aluminum alloy 2024 T6 has been determined with copper electrode and kerosene oil is used as dielectric liquid. Discharge energy is controlled by electric current while keeping Pulse-ON time and Pulse-OFF time as constant. The characteristics of the EDMed (Electric Discharge Machined) surface are discussed. The sub-surface defect due to arcing has been explained. As the surface material of tool electrode and workpiece melts simultaneously and there are chances of the contamination of both surfaces by the contents of each other. Therefore, the EDS (Energy Dispersive Spectroscopy) of the white layer and base material of the workpiece was performed by SEM (Scanning Electron Microscope) at the discharge currents of 3, 6 and 12 amperes. It was conformed that the contamination of the surface of the workpiece material occurred by carbon, copper and oxygen contents. The quantitative analysis of these contents with respect to the discharge current has been presented in this paper.

T he EDM (Electrical Discharge Machining) is the non-conventional machining process that is mostly used for machining of hard to cut conductive materials [1][2]. But the complex geometries of other relatively soft materials are also machined by EDM [3]. As aluminum alloys remained highly acceptable in the aerospace and automobile industry because of high specific modulus [4]. As EDM is non-contact machining process that does not cause lateral stress and vibrations to the workpiece material and results in accurate and precise geometric dimensions [5]. The choice of proper process parameters are important for the desired surface are formed on the surface which are termed as 'Crater' [10]. The surface layer is termed as 'White layer', which have different microstructure and metallurgical conditions than the parent material [11][12][13]. Only 15% of the total quantity of the molten material is removed and the residue resolidifies on the surface [14]. Such layer was also observed in other traditional machining processes [15] but there its thickness is comparatively lower than the EDM machining [15]. Residual stresses are developed due to rapid cooling of the molten material [16]. These stresses are of tensile nature that is detrimental for cyclic loadings [17]. The residual stresses are lower at the surface which are increasing in the depth to maximum in the white layer. The generation of residual stresses depend on the cooling rate of the molten material, which is more at the surface and go on decreasing in the depth.
Then the maximum stresses should appear at the surface but these stresses are relaxed by the formation of microcracks in the surface. When the amount of residual stresses exceed the limit of rupture strength of the material then cracks generate and cause relaxation in the residual [18][19].
The MRR and tool wear rate have been the focus of different studies [5,[20][21]. Where it is was common observation that the MRR generally improved at high discharge energy, but all materials had their unique MRR with respect to the selected conditions. But it was also seen that the surface damage rate in terms of surface roughness, cracks, pits and depth of heat affected zone are also increased with the factors that result high MRR [22]. The rapid cooling rate in EDM is also associated with the refined grain structure as compared to grinding and turning process [26]. During machining the surfaces of both electrodes melt and due to the sudden change in the pressure of the plasma as well as by dielectric liquid, there are chances of the surface contamination of one material with the contents of other material [12,27].
Therefore, the resulting surface of the workpiece after EDM at different discharge currents is characterized by EDS. In this investigation, the material removal rate of aerospace grade (2024 T6) aluminum alloys has been investigated. A critical defect that occurred while machining for higher MRR has also been elaborated in this study.

EXPERIMENTAL SETUP
Aluminum alloy 2024 T6, in the form of extruded rods with 22mm diameter had been used. Its physical and mechanical properties are presented in Table 1. Small cylindrical specimen of 20x20mm were prepared on lathe machine.
Die-Sinking EDM (NEUAR) of Taiwanese origin was used and the experimental procedure is explained in Fig. 1(a-b). Where, EDM machining is being performed by the electric sparks between copper tool electrode and aluminum alloy workpiece and the machined surface is magnified to show the craters and cracks. The industrial grade kerosene oil was used as dielectric liquid. Specimens were clamped on the bed of EDM with a for k-type clamp as shown in Fig. 2. Pure commercial grade copper material was used for tool electrode with have slightly more diameter than the workpiece. The physical

Effect of Electric Discharge Machining on Material Removal Rate and White Layer Composition
properties of copper are represented in Table 2.
Side flushing of the hydro-carbon based kerosene oil was provided with the pressure of 0.9 kg/cm 2 . The machining was performed at varying current levels of 3,4.5, 6, 9, and 12 Amperes. The other constant parameters are presented in Table 3. The machined surface of the specimen have contents of kerosene oil, carbon particles and debris.
Before determination of MRR, these free particles were removed by acetone liquid in a ultrasonic vibrating chamber for ten minutes. Accelerating voltage of 20KV is used for this analysis. In this analysis all elements were analyzed with five iterations and results are shown with all elements.

Material Removal Rate
The MRR has been determined for five discharge current levels. The MRR is found less for lower discharge current and more for high discharge current, Fig. 5. As high discharge current produces intense sparks that melts more surface material in the form of large crater size. The results are shown in Table 4 and presented in Fig. 5.   Figs. 10-11 respectively. The EDS analysis shows that Al, C and Cu are the major components of the white layer and their quantity is dependent on the discharge current as summarized in Table 5 and Fig. 12. It is found that the white layer is contaminated by Cu at lowest current only, where the inter-electrode gap is smallest due to which material transfer might have occurred. Whereas, Copper is the major alloying element of 2xxx aluminum alloys that has an amount up to 4%. The contamination of the white layer by the carbon contents is observed for all discharge current levels. Where the amount of carbon contents is inversely proportional to the discharge current. These carbon content are transferred from the kerosene oil that disintegrated at the very high discharge temperature upto 40,000 K [32][33]. At low discharge current, the white layer is comparatively thinner than that of high discharge current and at lower discharge current the machining time is large to remove equal quantity of the workpiece material than higher discharge current [28]. Therefore, the concentration of carbon contents is more for lower discharge current than the higher discharge current.
Overall the total quantity of impurities is high at lower discharge current due to which the amount of aluminum element is smaller at the lower discharge current. The contamination of the EDMed surface by the transfer of copper electrode material was also observed by Guo, et. al. [34] for Fe-Mn-Al alloy. The EDS analysis of the globule that was formed at discharge current of 12A has shown that the amount of both copper and carbon is