Applied Surface Science

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1 Applied Surface Science 357 (2015) Contents lists available at ScienceDirect Applied Surface Science journal h om epa ge: Employing Ti nano-powder dielectric to enhance surface characteristics in electrical discharge machining of AISI D2 steel Houriyeh Marashi a,b,, Ahmed A.D. Sarhan a,b,c,, Mohd Hamdi a,b a Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia b Center of Advanced Manufacturing and Materials Processing (AMMP), Kuala Lumpur, Malaysia c Department of Mechanical Engineering, Faculty of Engineering, Assiut University, Assiut 71516, Egypt a r t i c l e i n f o Article history: Received 14 June 2015 Received in revised form 8 September 2015 Accepted 13 September 2015 Available online 16 September 2015 Keywords: Powder-mixed EDM Titanium nano-powder Surface characterisation Surface roughness Micro-cracks Surface defects a b s t r a c t Manufacturing components with superior surface characteristics is challenging when electrical discharge machining (EDM) is employed for mass production. The aim of this research is to enhance the characteristics of AISI D2 steel surface machined with EDM through adding Ti nano-powder to dielectric under various machining parameters, including discharge duration (T on ) and peak current (I). Surface roughness profilometer, FESEM and AFM analysis were utilized to reveal the machined surface characteristics in terms of surface roughness, surface morphology and surface micro-defects. Moreover, EDX analysis was performed in order to evaluate the atomic deposition of Ti nano-powder on the surface. The concentration of Ti nano-powder in dielectric was also examined using ESEM and EDX. According to the results, the addition of Ti nano-powder to dielectric notably enhanced the surface morphology and surface roughness at all machining parameters except T on = 340 s. Of these parameters, maximum enhancement was observed at T on = 210 s, where the material removal rate and average surface roughness improved by 69 and 35% for peak current of 6 and 12 A, respectively. Elemental analysis signified negligible Ti deposition on the machined surface while the atomic concentration of Ti was increased around the crack areas Elsevier B.V. All rights reserved. 1. Introduction One of the basic requirements for producing heavy-duty industrial parts is the ability to manufacture high-strength parts that possess fine surface quality. Of the existing non-conventional methods, electrical discharge machining (EDM) is very promising for machining difficult-to-cut conductive materials with geometrically complex shapes [1]. The operational basis of the EDM electro-thermal non-contact mechanism permits achieving near mirror-like surface finish when machining high-hardness materials [2]. An additional advantage presented by EDM is the effective ability to perform machining with no mechanical stress [3], vibration or chatter [4]. Owing to these advantages, EDM is employed in a wide range of industries, including aerospace, biomedical, automotive, as well as die and mould [5]. In EDM, material removal takes place when both electrode and workpiece are immersed in dielectric and potential difference is Corresponding authors at: Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia. addresses: houriyeh@marashi.co (H. Marashi), ah sarhan@yahoo.com (A.A.D. Sarhan). applied between them. Due to the strong local electric field in the gap, molecules and ions of dielectric get polarized and oriented. Once the dielectric strength in the gap surpasses the natural electrical resistivity, as a result of the electron avalanche a plasma discharge channel forms. Heat and pressure are thus generated due to the conversion and release of kinetic energy from the electrons and ions [6,7]. The extreme amount of heat produced causes heating, melting, vaporizing and eventually electrode and workpiece wear [8]. Despite the advanced material removal system, EDM has so far not been vastly employed on account of the limitations exhibited, particularly when higher material removal rate (MRR) is necessary. In this case, limitations such as inappropriate surface quality, surface defects, layer susceptibility to heat and residual stress result in poor EDM performance [9]. Nevertheless, it is possible to enhance EDM performance through a number of methods. In this regard, ultrasonic vibration of a silver-tungsten workpiece and using graphite powder-mixed dielectric in EDM appeared to effectively improve surface quality, eliminate micro-cracks as well as reduce machining time by 35% [10]. In another study, tool vibration in EDM yielded higher micro-hardness as well as thinner heataffected zone and recast layer on the machined surface [11]. Zhang, Liu, Ji and Cai [12] found that, by comparison to surfaces machined / 2015 Elsevier B.V. All rights reserved.

2 H. Marashi et al. / Applied Surface Science 357 (2015) in water or kerosene, a white layer thickness with significant hardness was produced by an emulsion of water in oil, because the machined surface displayed carbide as well as oxide. The most widely employed means of improving EDM surface characteristics is to add powder to dielectric fluid, a procedure known as powder-mixed electrical discharge machining (PMEDM). Adding semi-conductive graphite nano-powder in both milling and sinking EDM of cemented tungsten carbide provided smooth and defect-free glossy nano-surface [13]. Furthermore, Peç as and Henriques [14] observed that utilizing 10 m silicon particles suspended in dielectric reduces the craters diameter and depth as well as the white-layer thickness. Wu, Yan, Huang and Chen [15] obtained fine surface roughness by adding Al powder and surfactant to dielectric fluid, whereas surface roughness improved by 60% from m in EDM oil to m. Amongst all powder materials employed in both standard and advanced EDM systems, Ti powder has captured attention for surface modification purposes [16 18]. Following powder addition to dielectric, the dielectric s insulating strength is diminished by titanium conductivity, which leads to the occurrence of more frequent discharge [19]. Furthermore, discharge dispersion as an outcome of powder addition [20] results in non-direct striking of the workpiece surface, which improves the surface topography and hence, surface quality. Janmanee and Muttamara [18] observed that after adding Ti powder (50 g/l with grains <36 m) to hydrocarbon dielectric, the maximum Ti atomic diffusion achieved was at 20 A current and 50% duty factor. Ti deposition increased the surface hardness by 76% due to the development of a TiC layer on the machined surface, which filled and substituted the surface micro-cracks with Ti atoms from the powder and carbon atoms from dielectric decomposition. However, it is acknowledged that besides C atoms, dielectric breakdown during discharge also releases C2 molecules [21]. Furutania, Saneto, Takezawa, Mohri and Miyake [17] achieved a 150 m TiC layer with 1600 Hv hardness as an outcome of the Ti atoms reaction with C on the workpiece surface due to discharge-generated heat. Additionally, to attain a greater accretion area, the researchers proposed a rotational gear-shaped electrode to maintain high powder concentration in the machining gap. Unlike hydrocarbon oil, Ti powder added to deionized water dielectric produced a coating of TiO in micro-current EDM owing to the deposition of Ti, oxygen atoms and oxygen molecules, which resulted in the complete disappearance of surface micro-cracks [16]. Particle size, concentration and density, electrical resistivity and thermal conductivity have been asserted as the main powder properties influencing PMEDM performance [22]. Despite the significance of particle size, PMEDM with micro-level powder particles has been the focus of most studies [15,23]. Still, significant findings for the nano-level have been obtained in a handful of studies as well [20]. According to Boilard [24], smaller Ti particles have higher potential of ignition, thus nano-sized Ti additive in dielectric may result in higher spark intensity in the discharge process due to its ignition potential. Additionally, smaller particles demonstrated less discharge gap expansion, hence, it can be said that the nano size reduces spark energy loss [22]. Consequently, surfaces with shallower craters are formed on the machined surface and higher MRR can be achieved at the same time. In the present study, Ti nano-powder addition to dielectric in EDM was investigated in order to evaluate the surface characteristics of an AISI D2 steel workpiece for surface roughness, surface morphology and surface micro-defects. Using a newly designed circulation system, the Ti nano-powder dielectric was used as dielectric in the experimentation. Followed by powder addition to dielectric, in order to evaluate the efficiency, MRR and surface roughness were measured. Other surface characteristics were examined through field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). Energy dispersive X- ray spectroscopy (EDX) analysis was conducted to investigate the diffusion of Ti atoms on the machined surface. 2. Experimentation setup and procedure 2.1. Experimental setup SODICK die-sinking EDM with maximum travel range of 300 mm longitude, 250 mm latitude and 250 mm vertical travel was used to carry out the experiment. EDM has five main modules, control unit and generator, servo system, circulation system, dielectric reservoir and pump, machining tank with workpiece holder and X Y table (Fig. 1). The EDM was equipped with an external dielectric circulation system with a liquid capacity of eight litres. During the experiment, the workpiece stability was ensured by the machining chamber integrated with a holder. In the system, the dielectric liquid is extracted by the pump through a filter from the machining chamber and subsequently flushed through the nozzle into the machining gap. Direct flushing was installed to make sure that the dielectric medium was present in the machining gap. Once the circulation began, a vertical feed was implemented for 10 min. The development of an external circulation system was aimed at diminishing the quantity of powder necessary in the experiment and stopping the powder from getting in the original system of EDM filtration Preparation of nano-dielectric and materials Table 1 presents the properties of the dielectric medium of clear hydrocarbon oil. As previously mentioned, the additive particle material employed in the present experiment was Ti powder shown in Fig. 2. A stirrer was used to mix the hydrocarbon oil with the 99.9% pure Ti particles of round shape and with a mean size of nm. Before use, the liquid was subjected to ultrasonication (240 W, 40 khz, 500 W) for two days to ensure that the powder was uniformly distributed in the oil [34,35]. AISI D2 was the workpiece material of choice and its chemical composition is shown in Table 2. At first, Wire-EDM was used to cut the steel alloy into pieces measuring mm and afterwards the removal of the layer of burnt surface was undertaken with the help of the surface grinding machine. Copper was the material composition of the tool electrodes. A CNC lathe machine was used to machine the 10 mm Ø copper rods which then underwent tip grinding. To remove any potential oxidised layer that might have adversely affected electrical conductivity, grinding and polishing of the tool and workpiece surfaces was conducted prior to application of every experimental set. Fig. 1. The powder-mixed EDM designed circulation system.

3 894 H. Marashi et al. / Applied Surface Science 357 (2015) Table 1 Dielectric oil properties. Type Density (15 C) Viscosity (cst) Colour Boiling point ( C) Hydrocarbon oil kg/cm Clear colourless liquid Between 200 and 250 Table 2 The chemical composition of the AISI D2 steel. AISI D2 Fe C Si Mn P S Cu Ni Cr Mo V Percentage Table 3 Experimental conditions. Parameters Value Duty factor 60% Powder concentration (g/l) 2 Voltage (V) 120 Machining time (min) 10 Electrode (mm) Cu, Ø10 Work piece Die Steel D2 Dielectric Hydrocarbon oil Powder material Titanium Powder size (nm) Fig. 2. Titanium nano-powder with average size of nm used in this research Experimental conditions and procedures A RC type pulse generator was integrated in the used EDM system. The electrical components of the pulse generator are schematically represented in Fig. 3(a). The capacitor in the RC circuit was charged by the DC power source, whilst the tool and workpiece were submerged in dielectric fluid and separated by a narrow gap. The circuit fluctuations over time in the voltage (V), charging current (I c ) and discharging current (I d ) are presented in Fig. 3(b). The rise in I c and charging go on up to the point when the voltage of the capacitor reaches dielectric breakdown voltage. Dielectric insulation is diminished as increase in potential determines an increase in the number of ionised particles in the gap as well. The discharge current is set between the tool and workpiece upon reach of breakdown voltage (V*), resulting in spark ignition. As electrical energy is converted into thermal energy, the spark occurring in the gap produces substantial heat, which causes the workpiece material to melt and evaporate. This process is repeated between the tool and the nearest workpiece point until machining is finished. Table 3 indicates the parameters and variables used in the experiment. Three signal durations with a duty cycle of 60% were chosen for the experiment. Three levels of peak current and three levels of T on, were chosen for the examination of the impact of spark size on the efficiency of the Ti nano-powder dielectric (Table 4). The experimental matrix with variables was employed to develop eighteen experiments (Table 5). For every experiment, three samples were used under time-regulated condition. Immediately after completion of every experiment, cleaning of samples was undertaken with acetone in an ultrasonic bath, after which they were left to dry in warm air. Subsequently, in order to Fig. 3. Schematic illustration of (a) a RC generator [33]; (b) voltage, charging current and discharging current fluctuations over time.

4 H. Marashi et al. / Applied Surface Science 357 (2015) Table 4 Parameters and levels. Parameters Levels Level 1 Level 2 Level 3 A Peak current (A) B T on ( s) T off ( s) C Dielectric Oil 2 g/l Ti + oil Table 5 The experimental matrix. Experiment No. A B C identify the properties of the machined surface, surface roughness profilometer, FESEM, EDX, AFM, were used. 3. Assessment of nano-dielectric concentration In order to ensure the availability of Ti powder in the process, an environmental scanning electron microscopy (ESEM) was used to analyze the Ti nano-powder dielectric. The dielectric samples were taken from the tank prior to machining and following half an hour of machining, respectively. To facilitate the investigation, after placing the Ti nano-powder dielectric sample on the ESEM testing stage, once the powder particles attached to each other, an absorbent stick was used to diminish the hydrocarbon oil content of the sample. Micrographs and the atomic concentration of elements (EDX analysis) of dielectric samples prior to and following machining are depicted in Fig. 4(a) and (b). The EDX elemental analysis was derived to make sure that the tank contained the correct powder concentration and low percentage of debris. It was found that following machining, there was a decrease of around 25% in the atomic percentage of Ti, from approximately 4% to 3%. Possible reasons for this include deposition of particles in the circulation components or the filtration process. Therefore, replacement of dielectric liquid was undertaken after every three samples (half an hour of machining) to ensure that the machining conditions remained the same all through the experiment. Since the sample was derived immediately following machining, the atomic percentage of debris elements increased for the after machining sample. Also both samples exhibited high atomic concentration of C element due to presence of hydrocarbon oil. 4. Results and discussion 4.1. Material removal rate In EDM, MRR is indirectly proportional with surface roughness, meaning that when one increases, the other decreases. Because higher material removal rate is associated with higher spark energy (given by the expression in Eq. (1)), which results in formation of bigger craters on the machined surface. Formation of large craters on the surface eventually lead to production of surface with higher roughness. Spark energy in EDM represents the quantity of electrical power present in every spark multiplied by the duration of flow of the electrical power [25]. In this equation, W t (watt time) denotes the spark energy, V (V) represents the sparking voltage, while I (A) and T on ( s) represent the peak current and discharge duration, respectively. W t VIT on (1) Thus, MRR has also been addressed along with surface quality in this experiment, to ensure the achievement of higher surface quality without deteriorating the machining efficiency. MRR is defined as the weight removed from the workpiece over time as follow: MRR = W wb W wa t m (2) Fig. 4. Micrographs and elemental analysis of Ti nano-powder dielectric samples (a) prior to machining and (b) following half an hour of machining.

5 896 H. Marashi et al. / Applied Surface Science 357 (2015) where t m is the total machining time and W wb and W wa are the workpiece weight before and after machining, respectively. Fig. 5 illustrates MRR for all the machining conditions. The figure clearly shows that at discharge durations of 120 and 210 s (Fig. 5(a) and (b)), the graphs follow similar trend and in all the conditions, Ti powder additive has led to MRR enhancement. At discharge durations of 120 and 210 s, the highest improvement of 21 and 69% was achieved at 20 and 6 A peak current as a result of powder addition, respectively. The increase in MRR value is attributed to the energy dispersion promoted by the Ti powder particles in dielectric. Unlike the other discharge durations, the MRR declined (by 32 to 43%) after using Ti nano-powder dielectric at a discharge duration of 340 s. To attain a more comprehensive understanding of the reason for this decline and the impact of the powder on the process, next sections address this effect in more detail. At any certain peak current in Fig. 5(a) (c), its observable that by increasing the discharge durations, MRR decrease. For instance at 6 A peak current, when using pure dielectric, MRR is found to drop from approximately 5.6 to 2.8 mg/min when increasing the discharge duration from 120 to 210 s. However, according to Eq. (1), the MRR is expected to increase when the discharge duration increase due to the enlargement of spark energy. This decline in MRR is explainable using Eq. (3), which indicates that an increase in the charging (T off ) or discharging duration (T on ) determines a decline in spark frequency. This means that, despite increasing the spark energy with the rise in discharge duration, it was insufficient to surmount the adverse impact of the spark frequency (F spark ). F spark (khz) = 1000 T cycle ( s) = 1000 (3) T on + T off 4.2. Surface roughness A Mitutoyo surface profilometer was employed to measure surface roughness. This feature regulates abrasion and material migration in sliding and therefore plays a significant role in machining. The profile roughness associated with 6, 12, and 20 A peak current with 120, 210 and 340 s discharge duration for pure dielectric and Ti nano-powder dielectric is illustrated in Fig. 6. The surface variation wavelength (distance separating one peak and valley from the following peak and valley) over a measured length of 2.5 mm was expanded when discharge duration (T on ) or peak current (I) was increased. Based on Eq. (1), the lower spark energy is the likely reason for the less prominent surface texture differences at lower peak current and shorter discharge duration (Eq. (2)), as the formed craters on the surface are smaller [26]. Furthermore, compared to the pure dielectric medium, the Ti nano-powder dielectric was associated with lower peaks and shallower valleys. The R a and R z of an AISI D2 surface machined by EDM with the use of pure dielectric and Ti nano-powder dielectric are indicated in Fig. 7. These two parameters denote the mean surface height variations over the measured length and the highest peakto-valley distance, respectively. Both demand examination as the extreme fluctuating surface points that might be concealed in R a are revealed in R z. Similar to the MRR graphs, the graphs are alike at discharge duration of 120 and 210 s, where addition of nano-powder improved the surface roughness in all the conditions (Fig. 7(a) and (b)). Findings show that surface roughness was considerably reduced by nano-powder addition to dielectric in every condition apart from the maximum discharge duration of 340 s. At discharge duration of 120 and 210, R a represented the highest decrease of 34 and 35% at 12 A peak current, respectively. At discharge duration of 120 s and peak current of 20 A, R z decreased by about 28%, while at 210 s discharge duration and the same peak current, R z decreased by more than 40% by addition of powder to dielectric. Fig. 5. Material removal rate (MRR) for pure dielectric and Ti nano-powder dielectric.

6 H. Marashi et al. / Applied Surface Science 357 (2015) Fig. 6. Surface profile for samples machined in pure dielectric and Ti nano-powder dielectric. It can be concluded that at 210 s discharge duration, Ti nanopowder had the highest improvement effect, as shown by Fig. 7. Nonetheless, for both discharge durations, R a and R z improved to a similar extent. Second highest improvement is evident at 120 s discharge duration, whereas at 12 A, R a decreased by around 34% (Fig. 7(a)). As shown in both Fig. 7(a) and (b), R a and R z exhibited the greatest improvement at peak current of 12 A and 20 A, respectively. However, compared to lower discharge durations, a completely different trend was observed at 340 s (Fig. 7(c)). It seems that the additive determines deterioration in surface roughness at high discharge durations. At 12 A for R a and at 12 and 20 A for R z, the results are unsatisfactory. The need to monitor both parameters of R a and R z is emphasised when the decrease in R a and increase in R z by almost 20% at 340 s discharge duration and 20 A peak current is observable, which validates that irregular surface points inadequately represented by R a do indeed exist. In order to gain understanding of this complications, further investigations on the machined surface is conducted in the next section Surface morphology In order to evaluate the surface texture, FESEM and AFM analysis were carried out. To determine what caused extreme surface discrepancies, samples with complications and highest improvement in R z value were chosen for further analysis. These samples were subjected to machining at a peak current of 20 A in pure and Ti nano-powder dielectric, which are presented in Fig. 8. The clear dissimilarities in the surface texture of the samples highlight the importance of the dielectric in spark behaviour and surface texture. By comparison to the surface machined with pure dielectric, the one machined with Ti nano-powder dielectric seemed flatter at both 120 and 210 s discharge duration. In pure dielectric, the surface was hit by the spark directly, resulting in the formation of high ridges at the edge of the crater, whilst in Ti nano-powder dielectric, the surface texture was smoother due to discharge dispersion [27]. Increased electrical conductivity of dielectric after addition of powder leads to reduced breakdown voltage which results in more even distribution of spark energy, thus reducing the magnitude of impact force [28]. Moreover, the adherence of re-solidified particles to the surface determined the formation of black spots on the surface in the case of Ti nano-powder dielectric at discharge duration of 340 s. Based on the previous explanations, the reduced height of ridges has a favourable effect on machined surface morphology. However, the use of Ti powder demonstrates debris embedding in the machined surface at discharge duration of 340 s and current of 20 A (Fig. 9). After EDX analysis of the black spots, as expected, the presence of workpiece material in the black spots was revealed by elemental analysis which confirms the adherence of the debris particles on the surface. Debris embeddedness in the machined surface might have been caused by the greater density of Ti nano-powder dielectric, which has led to inadequate flushing in the gap. Furthermore, according to Eq. (1), the explosion size is expanded by peak current and discharge duration increase, resulting in debris particles of larger size. Thus, at 20 A and 340 s discharge duration, formation of the heavy debris particles with low dielectric flow rate of Ti nano-powder dielectric in the discharge gap results in improper ejection of the debris from the gap. Therefore, MRR and surface quality deterioration is caused by the debris particles re-solidification on the machined surface The structure of craters on surface A surface machined in pure dielectric and Ti nano-powder dielectric is represented in three-dimension in Fig. 10. More specifically, in the case of the surface machined in pure dielectric for discharge duration of 120 s, the dark blue colour in Fig. 10(a) denotes that the centre of the crater can reach 30 m; in the case

7 898 H. Marashi et al. / Applied Surface Science 357 (2015) Fig. 7. Average (R a) and peak-to-valley (R z) surface roughness for samples machined in pure dielectric and Ti nano-powder dielectric. of the surface machined in Ti nano-powder dielectric, the light blue colour in Fig. 10(b) indicates that the centre of the crater can be as low as 10 to 15 m. This means that, by comparison to the surface machined in Ti nano-powder dielectric, craters of greater depth form on the surface machined in pure dielectric. Additionally, there are close similarities in terms of ridge height on the surface machined in pure dielectric and the surface machined in Ti nanopowder dielectric (up to 30 m), whereas the valley on the surface machined with Ti nano-powder dielectric is almost 10 m higher than the valley on the surface machined in pure dielectric. This is why the use of Ti nano-powder dielectric resulted in lower R a and R z values. Surfaces machined in pure dielectric and Ti nano-powder dielectric at peak current of 20 A and discharge duration of 210 s are represented in 3D in Fig. 10(c) and (d), respectively. Likewise, the dark blue colour of the crater centre and the high ridges of up to 40 m indicate that crater depth was greater in the case of surface machined in pure dielectric than in surface machined in Ti

8 H. Marashi et al. / Applied Surface Science 357 (2015) Fig. 8. Morphology of surface machined in pure dielectric and Ti nano-powder dielectric at peak current of 20 A. Fig. 9. Adherence of debris particles to surface machined in Ti nano-powder dielectric at I = 20 A and T on = 340 s.

9 900 H. Marashi et al. / Applied Surface Science 357 (2015) Fig. 10. Three-dimensional representation of machined surface in pure dielectric and Ti nano-powder mixed oil at peak current of 20 A. nano-powder dielectric. Fig. 11 schematically illustrates the surface formation in EDM before and after adding powder to the dielectric. Since impurities contaminate the plasma channel in the EDM gap [21], the discharge process and plasma channel formation are influenced by the addition of conductive powder to dielectric. Is well-established that adding powder to dielectric increases the dielectric constant (K), resulting in discharge gap expansion. On one hand, larger gap width means decreased heat flux, which reduces the MRR and surface roughness [30]. On the other hand, higher dielectric electrical conductivity accelerates dielectric ionization and the spark frequency between tool and workpiece [28,31]. It seems that the higher discharge frequency overcomes the lower MRR effect resulting from the gap expansion, because the sparks do not hit the workpiece surface directly; instead, the primary discharge takes place between the tool and additive particles. Moreover, discharge dispersion markedly reduces ridges emerging on the surface due to formation of shallower craters with lower borders. As shown in Fig. 10(e) and (f), at 340 s discharge duration, the use of Ti nano-powder dielectric is associated with a distinct surface texture. The surface defects of black spots visible in the micrograph of the surface machined in Ti nano-powder dielectric in Fig. 9 are likely the reason for such texture variability. The results derived from R a and R z are corroborated by the surface texture micrographs Surface micro-defects Formation of micro-droplets, micro-voids, and micro-cracks Images obtained with FESEM constitute the basis of investigation of the EDM machine surface defects. The mechanisms for EDM

10 H. Marashi et al. / Applied Surface Science 357 (2015) Fig. 11. Discharge gap in (a) pure dielectric and (b) Ti nano-powder dielectric. material removal are indicated by the machined surface micrographs, in which the material that has melted or evaporated is washed away from the surface by dielectric. Melted/evaporated material is pushed away by the shock wave triggered by the heatproducing blast that accompanies discharge. The molten material moves on the surface and gives rise to a ligament affixed to the droplet until the explosion force or dielectric washing effect causes complete detachment (Fig. 12). However, because of the low dielectric temperature, complete detachment of the molten material before re-solidification is sometimes unsuccessful. Consequently, micro-droplets form and cause surface quality to deteriorate. An additional factor of EDM surface quality degeneration is micro-void formation, which is the outcome of the entrapment of gas bubbles in the discharge gap. This, in turn, is due to deionisation of hydrocarbon pure dielectric induced by high plasma temperature. In addition, due to the gas bubbles expelled from the molten material during the solidification, micro-voids form on the machined surface [29]. However, the Ti powder addition leads to gap expansion which facilitates easier expulsion of gas as well as debris. Beside micro-droplets and micro-voids, an EDM surface can also suffer micro-cracks (Fig. 12). Micro-cracks can form in this very hard, brittle layer owing to the increased non-homogeneities of the metallurgical phases within the recast layer [12]. Cracks occur primarily due to thermal stress on the workpiece [29] engendered by the significant fluctuations in temperature over a minimal time interval as well as the non-uniform temperature distribution over the surface area [30]. Tensile stress occurs because not all surface molten material is washed away. Due to the presence of carbon atoms (from dielectric breakdown), the molten material contracts more than the unaffected base substrate during cooling, and when the stress in the surface exceeds the material s ultimate tensile strength, cracks form [12]. Micro-cracks deteriorate the surface quality due to the high possibility of crack propagation through the substrate under stress, which could lead to product failure Investigation of surface micro-defects The micrograph in Figs show the machined surface at 120, 210 and 340 s discharge durations, respectively. Obviously, increasing the peak current rises the micro-defects Fig. 12. Surface micro-defects including crack, void and droplet (machined surface in pure dielectric at 120 s charging duration and 20 A peak current).

11 902 H. Marashi et al. / Applied Surface Science 357 (2015) Fig. 13. Surface micro-defects on the surface machined in pure dielectric and Ti nano-powder dielectric at T on = 120 s and T off = 80 s. availability on the surface. As demonstrated in Fig. 13, by comparison to pure dielectric, the use of Ti nano-powder dielectric at 120 s discharge duration significantly diminished the number of micro-voids. Micro-voids increase at higher peak currents, this effect does not seem to persist when using mixed dielectric though, owing to easier ejection of gas bubbles from the gap. Moreover, the addition of Ti powder also moderately reduced the number and size of micro-droplets at peak current of 20 A. In discharge gap, the plasma increase when increasing the discharge current [31], which made droplets more easily obtainable on the surface (Fig. 14). At 210 s discharge duration, Ti powder addition decreased number and size of droplets on the machined surface which resulted in a remarkable enhancement in surface quality, specially at 12 and 20 A peak current. Nonetheless, the use of Ti nano-powder dielectric at this discharge duration did not have a notable effect on the occurrence of micro-voids on the machined surface. When Ti nano-powder dielectric is used in EDM, the Ti particles do not trigger just one spark, but multiple dissipated ones. The number and size of micro-droplets on the surface are smaller as less molten material is blasted by the small explosions and undergoes solidification faster. At Fig. 15, the surface machined at 340 s discharge duration is demonstrated. Unlike the surface machined

12 H. Marashi et al. / Applied Surface Science 357 (2015) Fig. 14. Surface micro-defects on the surface machined in pure dielectric and Ti nano-powder dielectric at T on = 210 s and T off = 140 s. in 20 A peak current, slight improvement of surface is indicated at 6 and 12 A Micro-cracks Surface cracks visual inspection was applied using FESEM. Results indicated that Ti nano-powder dielectric has a noticeable impact on surface cracks. The number and width of the microcracks was found to be the smallest at 120 s discharge duration and 6 A peak current (Fig. 16). Discharge between the tool and workpiece was disrupted by the powder particles acting as barriers. This reduced the effect of thermal shock on the machined surface. A role in incidence of cracks is also played by discharge gap width, which is wider in the EDM process with added powder particles. The broader gap, the farther away from the machined surface the spark is likely to occur, reducing the thermal shock on the surface Material migration and elemental analysis Elemental analysis of the machined surface was conducted using EDX to evaluate the titanium transfer in atomic scale from the mixed dielectric to the machined surface (Fig. 17). As schematically demonstrated in Fig. 18, as a result of spark, plasma channel forms,

13 904 H. Marashi et al. / Applied Surface Science 357 (2015) Fig. 15. Surface micro-defects on the surface machined in pure dielectric and Ti nano-powder dielectric at T on = 340 s and T off = 225 s. which leads to the ionization and decomposition of atoms, particles and other species present in the gap. Eventually, the released particles are either washed away from the gap by dielectric flow or may be transferred to the machined surface. The thermal energy released from the discharge incident leads to the melting and deposition of Ti atoms onto the surface [18]. This energy also facilitates the diffusion of Ti atoms according to the Arrhenius principle [32]. As shown in Fig. 17, discharge duration and peak current are influential in deposition of Ti atoms due to thermal energy possessed by the atoms and ions. 12 A peak current was associated with the highest amount of Ti deposition on the machined surface, but 6 A peak current presented the best condition for preventing Ti deposition. The maximum deposition of 0.27% Ti atomic percentage was recorded at 12 A peak current when the discharge duration was set at 340 s, followed by 0.24 and 0.2% atomic percentage at 210 and 120 s, respectively. Moreover, Ti atomic concentration was investigated in more detail using EDX analysis around the crack areas, as these are

14 H. Marashi et al. / Applied Surface Science 357 (2015) Fig. 16. Micro-cracks formation on the surface machined at 120 s charging time and 6 A, with (a) 500 magnification in pure dielectric (b) 20,000 magnification in pure dielectric (c) 500 magnification in Ti nano-powder dielectric (d) 20,000 magnification in Ti nano-powder dielectric. Fig. 17. Deposition of Ti atoms on the machined surface. susceptible to higher deposition. It was found that the concentration of Ti atoms transferred around the crack area significantly varies from the Ti atomic concentration on the surface demonstrated in Fig. 17. The example of a surface crack selected for elemental analysis and the EDX spectrum are presented for discharge duration of 120 s and peak current of 20 A in Fig. 19. It is clear that the peaks related to Ti element are considerably high. The results show approximately 3% deposition of Ti atoms, which is significantly higher than 0.18% on the surface (Fig. 17). Also, by closely observing the crack edges in Fig. 17 and comparing them with the crack edges seen in Fig. 16, crack shape differs potentially on account of Ti atom deposition. This observation could be attributed to the higher local accumulation of Ti particles around the crack zones and ingress of Ti particles into the crack area where C acts as a bonder that increases the Ti deposition concentration [18]. This reveals the ability of Ti powder to reduce machined surface cracks and ultimately improve the surface quality. Fig. 18. Schematic illustration of particles decomposition and deposition on the machined surface.

15 906 H. Marashi et al. / Applied Surface Science 357 (2015) Fig. 19. Typical micrograph of crack on the machined surface and its related EDX spectrum (T on = 120 s and I = 20 A). 5. Conclusions In this research, the influence of adding Ti nano-powder to dielectric was investigated considering discharge durations (T on ) of 120, 210 and 340 s and peak current (I) of 6, 12 and 20 A. In order to utilize the Ti nano-powder dielectric in the machining area, a new circulation system made in-house was developed. To check the efficiency of the developed system, environmental scanning electron microscopy (ESEM) and energy dispersive X-ray spectroscopy (EDX) analyses were carried out to examine the concentration of Ti nano-particles. Furthermore, the surface characteristics were evaluated using a surface roughness profilometer, field emission scanning electron microscopy (FESEM), EDX and atomic force microscopy (AFM). The following findings were derived from the analysis: It was observed that due to addition of Ti nano-powder to dielectric, both material removal rate (MRR) and surface roughness significantly improved in all machining conditions except 340 s discharge duration. Ti nano-powder dielectric presented the highest enhancement at 210 s discharge duration, where the MRR and average surface roughness improved the most by 69 and 35% at 6 and 12 A peak current, respectively. Using Ti nano-powder dielectric enhanced the morphology of D2 steel surface as a result of shallower craters and the formation of low-height ridges. Furthermore, surface micro-defects, including voids, droplets and cracks diminished. However, higher density of Ti nano-powder dielectric at discharge duration of 340 s led to the adhesion of debris particles to the surface. 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