Metallic powder is added to dielectric fluid and it fills up the gap between the electrode and workpiece as shown in Figure 1. When potential difference is applied between the electrode and workpiece, an electric field between 105 - 107 V/m will be generated. Metallic powder particles under machining zone get energized and form a bridge between the electrode and workpiece as shown in Figure 2. The energy of the conductive particle promotes the breakdown of dielectric fluid and increases the gap with between the electrode and workpiece. Hence, early discharges start under the electrode area and create fast sparks
Figure 1. Schematic diagram of PMEDM .
Figure 2. Comportment of metallic powder in the discharge .
which erode the workpiece. The presence of metallic powder ensures uniform distribution of spark and the electrical density of the spark decreases which improves the machined surface properties.
Some researchers found that accumulation of sparks between two consecutive metallic powder particles in machining area results in series of discharges     . This increases the sparking intensity within discharges leading to faster erosion from the workpiece, and therefore increases MRR and the machined surface properties.
The PMEDM process involves the use of different types of conductive or semi-conductive powders mixed with dielectric fluid in attempt to improve the EDM performance  . Metallic powder suspended in EDM dielectric fluid is another means of improving the machined surface properties. PMEDM facilitates EDM ignition phases creating higher discharge leading to low dielectric fluid breakdown strength  .
The limitations of EDM include long lead time to design and fabricate the electrode; relative electrode wear, and undesirable cracks, craters and voids; difficulty in starting the process with very clean dielectric.
Besides, excessive debris in the spark gap can result in arcing causing lack of a precise feeding mechanism and thus, leading to an unstable of EDM performance. In spite of the advantages, PMEDM has certain limitations in production application and these include effective separation of debris from the powder mixed with working fluid; powder distribution requires more investigation; high powder concentration poses a concern. The research is still ongoing in order to overcome the drawbacks of PMDEM     .
In this research work, discharge current (IP), gap voltage (GapV), ON-time (ON), aluminum powder is selected as machined variable parameters and the output responses are fatigue performance, micro-hardness and surface topography when machining molybdenum high speed steel.
2. Materials and Methods
The following sections have presented the devices and machines used.
2.1. Experiment Set-Up
Molybdenum high speed steel specifically SKH51 according to Japanese industrial Standards (JIS) designation is selected as the workpiece material. For the specimen preparation, CNC EDM Wire-cut machine FA10 brand was used to cut the raw material into block shape to the size of 9 mm × 11 mm × 5 mm required for clamping on EDM die sinker EA8. Table 1 presents the chemical compositions of molybdenum high speed steel materials and the work properties of the workpiece material are presented in Table 2.
Copper-tungsten (W70Cu30) was used as electrode material with cross-section of 9 mm × 9 mm. The selection of copper-tungsten is due to good conductivity of copper and good melting point of tungsten. High electrical conductivity of electrode promotes more electrons from electrode since electric current is the “cutting tool” and high melting point of electrode contribute low wear ratio since EDM is a thermal process. Combination of copper-tungsten gives optimal electrical and thermal conductivities to the electrode. Table 3 presents copper and copper-tungsten properties.
Nano aluminum powder was selected and each is mixed separately with dielectric fluid during PMEDM. The selection of nano aluminum metallic powder was done by considering the expected contribution of metallic powder to the PMEDM process in terms of electrical conductivity, thermal conductivity, density and melting point properties. The properties of nano aluminum powder are presented in Table 4.
2.2. Fatigue Performance Measurement
Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. During EDM or PMEDM the repeated electric sparks causes fatigue damage . Fatigue damage includes three stages such as crack initiation, crack propagation and final fracture. Fatigue of material can also be represented by hardness and some authors have established empirical correlation between hardness and fatigue and ultimate strength in steel as shown in Equation (1) and Equation (2) .
Table 1. Composition of molybdenum high speed steel.
Table 2. Properties of workpiece materials.
Table 3. Copper-tungsten (W70Cu30) properties.
Table 4. Properties of selected powders.
where σw is fatigue limit in MPa and HV is the Vickers hardness in kgf/mm2
where σu is the ultimate tensile strength of material.
In thus study, Equation (1) was used to determine the fatigue of the specimen due to specimen shape and steel base material. The shape specimen limits the use of other fatigue testing standard like standard ASTM E606 E606M using rotating cantilever bending fatigue test machine, constant deflection amplitude cantilever bending test machine.
2.3. Micro-Hardness Measurement
Hardness testing determines the mechanical property of machined component. Hardness is used to estimate the ductility and resistance to wear, fatigue and tensile strength properties. Hardness may also be shown to correlate to tensile strength and fatigue in many metals. Measurement of micro-hardness was done using micro-hardness tester. ASTM E 384 standard defines and specifies the micro-indentation hardness test method and parameters of materials . Hardness test uses forces in the 1 to 1000 gf. With the Vickers hardness test a diamond with top angle of 136˚ is used. Vickers hardness parameter is presented in this research project. Tarasov et al.  presented in their work, the details of hardness test procedure.
2.4. Surface Morphology of Machined Surface Analysis
EDM is an electro-thermal process which can result in rougher or smooth surface machined surface according to machining parameters setting . Surface roughness measurement using profile-meter is limited to quantify the surface in term of deviation from its original form . It is important to study the surface morphology of machined surface to conform the result from profile-meter in term of image. From the surface morphology image, defects on machined surface can be characterized in terms of cracks, voids, craters, phase modification which may either be acceptable or rejected depending on the application . Examination of surface morphology of machined surfaces was done on specimens machined at low and high machining parameters for each category of experiments. FESEM Zeiss SUPRA 55VP was used to examine and analyze the machined surfaces. FESEM is a microscope that uses electrons to scan in details the machined surface as compared to optical microscopy which uses light.
3. Results and Discussions
3.1. Micro-Hardness of EDM on MHSS
Figure 3 shows the trend of micro-hardness of molybdenum high speed steel after EDM process and measured using micro-hardness tester on three different locations on the machined surface. The micro-hardness of as received molybdenum high speed steel is 316.7 HV. The micro-hardness of molybdenum high speed steel after EDM is higher than the micro-hardness of bulk material as. Micro-hardness of molybdenum high speed steel after EDM can reach 458 HV which is increased about 44.9% from as received molybdenum high speed steel micro-hardness.
This is due to the presence of alloying elements deposited, embedded and recast layer on the machined surface.
3.2. Fatigue Performance of EDM on MHSS
Figure 4 shows the effect of EDM machining parameters on fatigue performance after EDM process. Fatigue performance of as received molybdenum high speed steel is about 505.60 MPa.
Figure 3. Micro-hardness of EDM on MHSS.
Figure 4. Fatigue of EDM on MHSS.
Using the Equation (1), a correlation between hardness in HV and fatigue in MPa stated earlier, it can be found that there is a significant improvement on fatigue performance as compare to the fatigue of bulk molybdenum high speed material. Improvement of fatigue is due to the handing of the machined surface during EDM process.
3.3. Micro-Hardness of Nano Aluminum PMEDM on MHSS
Figure 5 shows the trend of micro-hardness of molybdenum high speed steel after PMEDM. The hardness of molybdenum high speed steel after PMEDM is significantly improvement as compared to conventional EDM shown in Figure 5. As explained above, this improvement is due to the presence of alloying elements deposited on machined surface. The hard layer containing TiC can be formed on the machined surface by EDM using titanium electrodes .
3.4. Fatigue Performance of Nano Aluminum PMEDM on MHSS
Figure 6 presents the effect of nano aluminum PMEDM machining parameters on fatigue performance. It can be found that there is a significant improvement on fatigue performance as compare to the fatigue of EDM on molybdenum high speed material. Improvement of fatigue is due to the nano aluminum embedded on the machined surface and handing of the machined surface during PMEDM process.
3.5. Surface Morphology of Nano Aluminum PMEDM on MHSS
Figure 7 presents the surface morphology of machined surfaces as the result of changes in IP, ON and gap voltage at low (Figure 7(a)) and high parameter setting (Figure 7(b)) when machining molybdenum high speed steel with PMEDM
Figure 5. Micro-hardness of molybdenum high speed steel after PMEDM process.
Figure 6. Fatigue performance on MHSS after PMEDM process.
Figure 7. Surface morphology of nano aluminum PMEDM on MHSS at low parameters: IP = 27 A, ON = 64 µs, GapV = 80 V, Pcon = 1 g/l (Figure 7(a)) and at high machining parameters IP = 51 A, ON = 128 µs, GapV = 150 V, Pcon = 3 g/l (Figure 7(b)).
mixed. Machined surface micrograph at low peak current, ON-time and gap voltage (Figure 7(a)) shows a less rough surface. Craters, voids, and micro-cracks can be seen especially for samples machined at high peak current, ON-time and gap voltage (Figure 7(b)). The addition of nano aluminum particles can reduce the electrical discharge power density and gap explosive pressure, which result in smaller craters with uniform distribution.
Chrishna et al. , showed the influence of machining parameters on EDM of maraging steels where cracks were formed due to high thermal energy. The energy dispersive spectroscopy (EDS) spectrum characterization of molybdenum high speed steel (MHSS) is presented in Figure 8.
Figure 9 presents EDS spectrum of respected machined surfaces EDM at low (Figure 9(a)) and high (Figure 9(b)) machining parameters setting when machining molybdenum high speed steel. From Figure 9, it can be analyzed the presence of carbon, oxygen, copper and increase in alloying elements compared to as received molybdenum high speed steel (Figure 8).
Figure 8. EDS of molybdenum high speed steel as received.
Figure 9. EDS of nano aluminum PMEDM on MHSS at low parameters: IP = 27 A, ON = 64 µs, GapV = 80 V, Pcon = 1 g/l (Figure 9(a)) and at high parameters IP = 51 A, ON = 128 µs, GapV = 150 V, Pcon = 3 g/l (Figure 9(b)).
Percentage of aluminum, tungsten, and copper increase about 3.4% compared to the based molybdenum high speed steel material. The alloying elements are transferred, deposited and embedded onto the machined surface improving the micro-hardness and the fatigue of machined surface. Kumar et al.  concluded in their finding that the surface modification is possible by the EDM process.
In this research work, analysis of fatigue and micro-hardness was done when machining molybdenum high speed steel in nano aluminum PMEDM for biomedical and industrial applications. The use of nano aluminum in PMEDM on molybdenum high speed steel results in improving fatigue and micro-hardness as compared to conventional EDM. This is attributed due to transfer of alloying deposited elements and uniform distribution of particles from nano aluminum onto the workpiece machined surface. The created carbon enriched surface layer also improves the properties of workpiece machined surface.
Further analysis on crack initiation, crack growth and osteointegration will be useful in order to establish the potential used of PMEDM for biomaterials.
Authors thank the Universiti Teknologi PETRONAS for funding this research trough ERGS grant.
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