The special operating parameters used in this study and the process quality data obtained in line with these parameters are given in Table 3. In this table, current, voltage, Ton and Toff input parameters; machining time, MRR, EWR, SR and hardness show the output parameters. When machining a blind hole, or applying bigger diameter of electrode tube, the selected processing parameters and electrode are very important. Both GAP (gap voltage setting key) and CAP (parallel capactor selection key) values were used as 3 in accordance with the parameters given in the table and machine safe operating parameters. The lower of gap voltage, the higher of machining current, the faster of machining speed, but the more difficulty of carbon removing and accordingly this may affect the machining speed. The application of parallel capacitor may increase the machining speed, but cause electrode wear. On the other hand, if a parallel capacitor is not applied, especially when machining a extra hard material, low machining speed will occur.
It was observed that the surface roughness was high at the highest current values among the experiments shown in Table 3. With the increase of the machining current, the sparking between the workpiece and the electrode increased and the point melting and destruction increased, as a result of which the surface roughness increased. As a result of the increased metal removal rate, the machining time was significantly reduced, but the EWR increased due to the electrode doing more work as shown figure 2a. Singh et al, in their study on experimental examination and modeling of surface treatments with ANSYS in EDM method, confirmed that the parameters that affect the surface roughness the most are discharge current, tool speed, pulse-on time and duty cycle, respectively . However, according to both analytical and simulation results in Ahmed et.al 's study, they reported that the effect of tool electrode rotation on debris removal is negligible . The most important point to be considered here is that the viscosity of the fluid used, the pressure and the processing parameters affect the debris evacuation; as a result, the debris in the environment affects the EWR and surface roughness. In this study, increasing voltage value showed a graph similar to the results caused by current increase as shown figure 2b. The machining voltage has a significant influence on the tool electrode wear followed by the rotating speed of the electrode and the diameter of tool electrode was also reported by Huang et al . In low arc duration, the electrode was operated more in unit time; excessive electrical density and sparking-induced abrasions occurred at the ends of the electrode, as a result of these abrasions, the abraded particles adhered to the machined surface, causing an increase in the surface roughness as shown in figure 2c.
In Figures 3 and 4, respectively, the SEM morphology of the machined surfaces and the effects of wearing electrodes are visualized using a Thermo Fisher Scientific Apreo S brand/model scanning electron microscope.
Before each experiment, holes were drilled using a new electrode measuring Ø 2 mm x 400 mm. At the beginning of the process, the diameter of the area where the electrode approaches the material and the hole diameter at the end of the process are not the same, this is due to the wear on the tip areas of the electrode. In Figure 4b and 4d show the entrance sections of the electrode as well as 4a and 4c show the hole bottom sections, respectively. As can be seen from Scanning Electron Microscopy (SEM) images and measurements, there was a change in size due to electrode wear. The macro images of the worn electrodes used in the experiments are shown in Fig. 5.
When the surfaces of the holes drilled into the workpiece were examined, eroded electrode residues were found on the surface. The EDAX analysis of the hole surface drilled in experiment number four is given in Fig. 6. Energy Dispersive X-Ray Analysis (EDX), referred to as EDS or EDAX, is an x-ray technique used to identify the elemental composition of materials. According to the measurements made at three different points of the machined surface, brass elements that it does not contain in its own structure were detected on the surface of the workpiece. The chemical components of X153CrMoV12 workpiece are given in Table 1, Carbon (C), Chrome (Cr), Molybdenum (Mo), Vanadium (V) are the chemical components of this steel. Brass is the general name of yellow alloys obtained by adding zinc (Zn) to copper (Cu). Some other elements that can be found in brass are tin (Sn), lead (Pb), nickel (Ni), manganese (Mn), iron (Fe), aluminum (Al), silicon (Si), arsenic (As), antimony (Sb) and phosphorus (P). However, as seen in Fig. 6c, 6d and 6e graphs, Cu, Fe, Mn, Zn elements were found on the machined surface, which is proof that the materials separated from the worn electrode were carried to the workpiece surface.
Although the hardness of the 1.2379 (X153CrMoV12) workpiece used in the experiment was approximately 720 HV0.10, the hardness value decreased to values between 580-626 HV0.10 in the parts close to the drilled areas with different machining parameters due to thermal effects. As seen in Fig. 7, three different measurements were made near each hole drilled and average values were reported. The hardness values obtained from experiments 10 and 7, where the surface roughness is the highest and the lowest, respectively, and measured at 75µm intervals from the machined surface, are shown in Fig. 8.
As can be seen from the measured values, the hardness values gradually decrease as they approach the machined surfaces as shown fig. 9. This decrease is affected by the increase in the processing time. As the processing time increases, the hardness change increases as the material is more exposed to thermal effects. In the literature, it has been reported that hardness change is observed in the workpiece because thermal effects are revealed in the machining processes using the EDM method. The melting and resolidification of the material causes formation of white layer (WL) onto the top of the machined surface . It was emphasized that the increase in hardness in these WL regions was more due to the iron carbon carried by the dielectric fluid [41-42]. Since the material used as the workpiece in this study is heat-treated steel, hardness changes were detected in the areas close to the processed areas with the effect of energy and heat applied during drilling with EDM. As a result of the hardness measurements, it was seen that the main hardness of the material was reached as it moved away from the machined area as shown fig. 9.