3.1 Surface topography and roughness
The surface topography as an output of EDM is a function of machining conditions and process variables like discharging voltage, current, pulse-on time, and dielectric type. Upon completion of machining operations and measuring surface roughness, the influences of different input variables on the topography of machined surfaces have been investigated using a three-dimensional optical microscope. Figures 1(a), (b) and (c) show 3D topography images of the machined surfaces when using two types of dielectric fluids, deionized water and kerosene, each at the currents of 9, 6 and 3A, respectively. In all experiments represented in Fig. 1, pulse-on time was fixed and set on 150 µs.
Based on the results it is observed that surface topography is a function of released energy in the discharge area since a rougher surface with more cavity and craters is produced when a higher current is used. Conversely, at lower peak current a finer surface is produced as in this condition a small quantity of molten material is removed from the surface due to the lower discharge energy. With current increases the discharge energy rises and this ultimately increases the intensity of the plasma channel. As plasma channel increases the discharge power also increases and results in a higher material removal rate from the surface that ultimately leads to producing a rougher surface topography.
As seen in Fig. 1, the surface cavities and holes produced on the surface of the machined samples in kerosene are deeper and possess a rougher topography in comparison with the samples machined in deionized water. This indicates that in deionized water the change of pressure over the discharge area is not as substantial as that in kerosene. When kerosene is used as the dielectric, the expansion of the generated bubbles created in discharging zone will be hampered because of the high viscosity and inertia of kerosene, while in deionized water this phenomenon is limited due to dielectric’s extremely low viscosity and density . Accordingly, the pressure over the discharge zone could experience a much more fluctuation and faster expansion speeds of the bubbles because of the constriction effect of kerosene. The higher pressure over the discharge zone in kerosene increases the impulsive force acting on the machined surface due to the contraction and expansion of the discharge generated bubbles; and this ultimately leads to higher material removal efficiency and a rougher surface with more cavities, craters and irregularly shaped appendages in comparison with the parts machined in deionized water. In addition, kerosene is a kind of hydrocarbon oil which decomposes at higher temperatures in the discharging zone and creates carbon debris during electrical discharge machining. These carbon scraps are stored in machined surface and restrain the stable and efficient discharging process, preventing generation of a smooth surface [20–21]. Whereas, in machining with deionized water, due to the lack of carbonization, the machining environment is stable and clean which results in producing parts with a smoother surface topography. Unlike distilled or pure water, in deionized water the probity of oxidation is much lower, and this enhances machining efficiency. Therefore, it can be clearly seen that the topography of the machined surfaces produced in the deionized water as dielectric is advantageous compared with those produced in the kerosene dielectric.
Figure 2 shows 3D images of the topography of the surface produced in deionized water at the current of 3A and pulse-on times of 150 (a), 100(b) and 50 µs (c), respectively. The 3D images presented in this figure clearly indicate that the pulse-on-time affects the topography of the surfaces machined; however, the change of current has a greater effect. In short-timed pulses the pressure over the discharge zone is very high and this may prevent an efficient molten material removal. As pulse time increases the pressure of discharge zone is dramatically reduced because of the faster expansion of the bubble generated in discharge point. In addition, in a higher pulse-on time, i.e. 150 µs, the heat conduction increases and the discharge plasma channel also increases as a result. In this case, the material removal rate increases, and more molten material is removed from each puddle on the workpiece surface, leading to a rougher surface topography. Moreover, longer pulse-on time results in more frequent cracking of the dielectric and higher rate of material expulsion leading to the generation of a rougher surface topography .
Figures 3(a) and 3(b) show the influence of pulse duration and current on the average surface roughness (Ra), respectively. Similar to surface topography results, the Ra value is increased when current and pulse time increase. Also, a higher value of Ra was measured in the parts machined in kerosene. Based on the results, the maximum surface roughness of 3.56 µm was reached when machining at the current of 9A, a pulse-on time of 150 µs, and when using kerosene as dielectric. Accordingly, the minimum roughness of 2.82 µm was measured with a current of 3A, a pulse-on time of 50 µs, and when using the deionized water dielectric. From the above it can be concluded that for polish machining when the smooth surface in needed, a lower current and pulse-on time should be selected with using deionized water instead of kerosene. These results are in accordance with the surface topography results obtained and analyzed as explained before. However, extracting the 3D image of machined surface reveals an excellent and comprehensive sense of machined surface morphology as parameters such as Ra and Rz are quantitative and two surfaces with totally different topographies could have the same Ra or Rz values. Therefore, use of advanced methods such as 3D optical microscope and atomic force microscopy (AFM) is beneficial for precise evaluation of the 3D morphology of machined surfaces.
3.2 Surface micro cracks
One of the most important factors influencing the integrity of machined is surface micro-crack, and directly affecting the fatigue resistance and performance of the parts. The micro-cracks on the machined surfaces are formed as a result of residual stresses generated in the subsurface. Residual stresses during electrical discharge machining are generated because of reasons such as non-uniformity in thermal stresses, metallurgical phase transformation and local crystal plastic deformation [23–25]. However, in EDM the scale of plastic deformation and its effects on the residual stresses is very small as there is no contact between the electrode and part and local crystal plastic deformation in EDM is resulted because of very high thermal stress generated during the process.
Figure 4 presents the effect of current intensity on the surface micro-cracks created on the machined surface with deionized water at the pulse-on time of 150 µm and the current of 9 (a), 6 (b) and 3A (c), respectively. As seen in Fig. 4 there is no trace of surface micro-cracks for the current of 3 A, while relatively wide and large micro-cracks are present on the surface when a higher currents of 6 or 9 A is used. With increasing current in EDM, the size and number of micro-cracks increase rapidly. In machined parts with higher currents, the length of micro-cracks is increased, and the span width of the surface cracks is larger. In general, with increasing current in EDM machining, the density and strength of the plasma channel created between the two electrodes increases which enlarges the spark power and discharge energy. Due to higher discharge energy, the higher thermal gradient is created on the machined surface, which develops more micro cracks. As mentioned before, there is no trace of micro-crack at low current of 3 A while surface micro cracks are evident when higher currents of 6 and 9 are used. This is mainly due to a higher thermal gradient, which results in the generation of tensile stresses exceeding the maximum tensile strength of the material. According to the results, an increase in current intensity leads to the expansion of surface micro-cracks, which endangers the integrity of the surface being machined. Therefore, it is recommended that low currents are used for machining of sensitive parts that will be subjected to cyclic loads and fatigue to prevent or lower the possibility of formation of micro-cracks on the surface.
Figures 5(a), (b) and (c) show 3D images of the surface micro-cracks in two types of dielectric fluid, i.e. deionized water and kerosene, at the pulse-on times of 50, 100 and 150 µs, respectively. In all experiments represented in this image, the current was fixed at 6 A. As seen in the Fig. 5, the intensity and width of micro-cracks is relatively increased at higher pulse times and when using kerosene. The micro-cracks follow closed loops with pitting arrangements and continue to propagate when other discharges take place at the near regions.
As seen in Fig. 5, the longer pulse-on-times, i.e. 100 and 150 µs, to some extent more and larger micro-cracks are generated. However, the rate of increase in surface micro-cracks because of higher pulse-on time is much less than that of using higher currents in EDM. In higher pulse-on time, energy input in the discharge zone will be increased, which increases the supplied heat energy and thermal gradient on the machined surface; and this consequently increases the micro-cracks density.
The possibility cracking also increases when kerosene is used as the dielectric liquid especially at high pulse durations. In the machined parts with kerosene relatively more crack propagation along the grain boundary occurred in comparison with deionized water. As mentioned earlier, due to lower viscosity and density of the deionized water compared with kerosene, less pressure over the discharge zone is generated and impulsive force acting on the machined surface is also lower. Accordingly, less spark intensity and lower thermal gradient are exposed on the machined surface, which results in lower probability of micro-cracks creation in machining with deionized water. Another cause for the creation of more cracks on the machined surface in kerosene is the difference in thermal conductivities of two kinds of dielectrics that change the cooling rate in the discharge zone. The high cooling rate of deionized water efficiently dissipates the heat from the melted material and restricts the formation of micro-cracks. Moreover, in EDM machining with kerosene as the dielectric, due to carbonization, carbon derbies adhere to the electrode surface and this ultimately leads to creating carbide on the workpiece surface . This condition causes unstable machining on the discharge zone that is accompanied with higher impulsive force, resulting in the higher tendency of crack propagation in machining with kerosene.
3.3 Recast layer, HAZ and microhardness
Generally, in electrical discharge machining the dielectric fluid is inefficient in completely flushing out the entire melted material and debris. As a result, a part of melted material is deposited onto the discharged surface that is called recast layer. Recast layer and heat-affected zone are part of an altered material zone that is created on discharged surface. One of major output variables in EDM machining is the heat-affected zone (HAZ), especially in sensitive parts, which are exposed to fatigue loading. HAZ is an area of discharged subsurface where the base material was not melted; however, the subsurface mechanical properties and microstructure were dramatically changed due to exposure to high-temperature gradient in machining. These regions of machined subsurface are brittle, very hard and because of the non-homogeneities of metallurgical phases within it, are very prone to develop micro-cracks.
Figure 6 Effect of current and dielectric type on the recast layer thickness at the pulse-on times of 50 (a), 100 (b) and 150 µs (c)
In all experiments presented in Fig. 7, the pulse-on-time was fixed at 150 µs. These images clearly demonstrate the effect of different EDM conditions on the thickness of recast layer. These images are taken at 1200x magnification. Figure 7 shows that the thickness of the recast layer on the machined subsurface varies from 7 to 18 µm and increases with increasing current and using kerosene as the dielectric. However, the influence of current on recast layer thickness is greater than that of the dielectric type.
In addition, in order to characterize the mechanical properties of machined subsurface, microhardness test has been performed using a Vickers microhardness tester. The hardness profiles of machined samples under different machining conditions are presented in Fig. 8. These microhardness profiles correspond to cross-sections of the machined samples shown in Fig. 7. The maximum hardness values of machined subsurface were measured 415 HV, whereas microhardness of the un-affected base material was about 170 HV. It can be observed in Fig. 8 that these hardness profiles can be divided into three sections. The first section correspond to the recast layer where the formation of carbon is resulted when the hardness of recast layer is between 416 and 397 HV and that is significantly higher than the hardness of base material. Beneath the recast layer, the HAZ area is created in the machining subsurface, which its hardness is lower than that of the recast layer but is higher than base material. Clearly the subsurface mechanical properties of HAZ and its hardness were dramatically changed due to exposure to high-temperature gradient in EDM machining. As can be seen in Fig. 8, HAZ layers have different slopes of hardness variation in different machining conditions and the hardness value in HAZ zone decreased in the higher depth form the recast layer. This indicates that changes in mechanical properties in higher depths were lower due to a lower temperature gradient. These results also support the fact that microhardness of the recast layer and HAZ can be approximately 2–3 times larger than that of the unaffected base metal [27–28]. As shown in Fig. 7, there is a direct relationship between the heat-affected zone and recast layer thickness as the thickness of recast layer and HAZ was increased with increasing the current density.
From the above discussion, it is perceived that when for electrical discharge machining of AISI 1045 steel at the current of 3 A, the lowest thickness of recast layer and HAZ thicknesses are achieved; and it becomes even lower when deionized water is used as the dielectric. On the contrary, the highest thickness of recast layer and HAZ thicknesses are resulted at the current of 9 A and when kerosene is used as the dielectric. The relation between discharge energy and as pulse-on time as well as current is directional. An increase in peak current leads to increased discharge energy that in turn creates a thicker recast layer and consequently produces a larger HAZ. In fact, in the higher values of current and pulse duration, discharge energy is enlarged, and this results in a higher material removal rate (MRR). The higher MRR results in an increase in material debris in the discharge zone that smooths the way for secondary sparking. Therefore, higher discharge energy and secondary sparking lead to an increase in the recast layer and HAZ thicknesses. On the other hand, an increase in pulse-on time causes incomplete flushing of material derbies. Partial flushing of debris due to higher pulse-on time results in higher heat accumulation and thermal gradient on electrically discharge machined subsurface, leading to an increase in the recast layer thickness.
In all machining conditions with deionized water, the depth of recast layer and HAZ were reduced. This reduction is more evident in the higher currents (i.e. 6 and 9 A). Based on Figs. 7 (c) and 8 (c), there was little difference between the recast layer and HAZ thickness in the parts machined with a current of 3 A as the measured recast layer in EDM with kerosene and deionized water was 8 and 7 µm, respectively. However, as the current was increased the difference became more evident. For instance, at the current of 9 A the recast layer thickness was 15 and 18 µm with kerosene and deionized water dielectrics, respectively (Figs. 7 (a) and 8(a)). Because of the higher thermal conductivity of water than kerosene and faster cooling in the presence of deionized water, the length of plasma channel in deionized water is more than kerosene. In addition, the rate of igniting breakdown and plasma channel formation is faster in water dielectric fluid. In this condition, discharge energy is dispersed to a larger zone and there is less accumulated energy prior the breakdown point, leading to the reduction of recast layer and HAZ thicknesses in EDM machining with deionized water as the dielectric fluid . In addition, effective cooling of material during EDM with deionized water prevents the formation of thick recast layer leading to a lower HAZ thickness.