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.

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 [17]. 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) [18].

σ w = ± 1.6 HV . (1)

where σ_w is fatigue limit in MPa and HV is the Vickers hardness in kgf/mm²

σ w = 0.5 σ u (2)

where σ_u 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.

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 [19]. 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. [20] presented in their work, the details of hardness test procedure.

EDM is an electro-thermal process which can result in rougher or smooth surface machined surface according to machining parameters setting [21]. Surface roughness measurement using profile-meter is limited to quantify the surface in term of deviation from its original form [22]. 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 [23]. 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.

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.

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.

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.

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 [24].

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.

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

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. [25], 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).

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. [26] concluded in their finding that the surface modification is possible by the EDM process.