Material removal rate and Surface roughness
From the experimental analysis, the amount of material removed from the SMA alloy is calculated and illustrated in the form of graph as shown in Figure 2. Graph shows the performance of electro spark process for different pulse on process timings. Results found with wide variations for 10 µs and the removal of material varied from 2.532 mm3/min to 7.136 mm3/min. For 3 µs, material removal found steadily increasing in trend with respect to the applied voltage. On other two conditions (6 and 9 µs) material loss is gradual variation with minimum variations. Subsequently, for 30 µs the material removal is in the range of 3.01 mm3/min to 4.132 mm3/min. Influence of applied voltage is less with maximum pulse on time. The variation in material removal for 30 µs has a very small variation (1 mm3/min) compared to 10 µs (4.604 mm3/min). While machining the electro spark induced at minimum pulse on has high intensity to fuse and vaporize the material under die sinking. Due to increase in pulse of time the intensity of the spark has prolonged to diffuse material in bulk and MRR found increased. It is inclined towards the current density, which is generated due to the discharge current [15, 16]. The accumulated discharge energy will have severe melting and evaporation of the material. However, for maximum pulse time material behavior varied and it starts to conduct the energy between the electrode and work. The presence of carbon (alloying) elements in SMA alloy has categorized the material to have high electrical conductivity. Specifically, for prolonged electro spark the presence of carbon in the SMA intended to conduct the amount of energy used. During machining the high conductivity of the material leads to transform energy (electrical conductivity) between the electrode (pure copper material) and the SMA work material . It also defeats the production of electro spark and the intensity of the spark turns weak and form inefficient process. During electro spark production, the electrode or the tool used becomes vulnerable with respect the machining conditions. The tool in contact with the work will also possess slight melting and vaporization. Figure 3. indicates the measure of tool wear rate with respect to applied voltage at different pulse on and pulse of time. The behavior of the electrode tool material is varying strongly with respect to minimum pulse on time. Reflection of SMA alloy material removal is directly reflecting with the performance of copper electrode material. Uniform and linear result for maximum pulse on time noticed with tool wear. The metallurgical ion transmission and conductivity due to current density are discussed with surface topographical analysis.
The material removal and surface roughness of SMA material under electro spark machining has performed with the similar pattern. Figure 4. illustrates the measured average surface roughness value for different pulse duration and applied voltage. As discussed in material removal, the surface roughness of the SMA material has produced results with wide range for low pulse on time (10 µs). Minimum surface roughness recorded is 1.04 µm for an applied voltage of 40V and maximum of 3.47 µm for 60 V at 3 µs (pulse off time). It has to be understood that the surface roughness found increased with respect to increase in applied voltage. This is due to the interruptions in discharge current and strikes from electro spark. On extension of pulse during electro spark, the melting of material is with highly intense and deep craters initiated. The variations in material removal is due to the low pulse on time and floatation in pulse off time. For maximum pulse on time, the material removal found uniform and gradual Similarly, the variation in surface roughness is due to the intensity of spark voltage. When applied voltage increases the material removal reveals catastrophic with maximum roughness. Since the maximum surface roughness recorded for maximum applied voltage and pulse off time. As a result, the pulse on time has played vital role in material removal and applied voltage for surface roughness. To confirm the influence of process parameters, the experimental data is evaluated through mathematical analysis.
Influence of input process parameters over MRR and Ra
The data extracted from the experimental analysis is mathematically evaluated to find the influence of input process parameters. analysis is performed through the MINITAB software with the eighteen combinations of experimental design and their corresponding results. From the analysis of experimental design (ANOVA), the influence of is process parameters is predicted and plotted in the form graph (Figure 5). It is clear to infer that the material removal is highly influenced by the pulse on time with a maximum contribution of 60.7%. Pulse off and applied voltage are less and equal in contribution (15%) for the response on material removal. While comparing the SMA alloy and copper electrode, the influence of pulse on time is severe with copper electrode. For better understanding, the electrode ion transmission at low pulse on time has high resistance and the material (tool wear) loss occurred. At the same with increase in pulse on time, the current density was diminished and act as a conducting material to transfer electrical energy from the electrode to work piece. Since the maximum contribution of 79.18% recorded for tool wear rate. In both the case (MRR and TWR), the influence of applied voltage is 6 to 8%. Similarly, for surface roughness, the intensity of electro spark in terms of applied voltage has maximum contribution of 92.4%. The significance in the research finding is that the intensity of the spark depends on the applied voltage. Based on the spark intensity, the SMA alloy melts and vaporized with the pressurized dielectric fluid. Surface profile reveals crater with strong electro spark and partially melted cast layer due to in – efficient spark. The fitness of the equation (R2 Adj value) for the proposed experimental design is 91.9% for MRR, 97.83% for surface roughness and 93.83% for TWR.
Surface Topographical analysis
The surface morphology of the EDM process SMA alloy entirely depends on the machining parameters. Figure 6 (a) – (f). Shows SMA alloy's surface morphology image under different machining parameters such as pulse on time of 10 – 30, pulse off time of 3, 6, and 9, a voltage of 40 – 80, and Cu electrode. The performed EDM process was characterized by electro-discharge carters, recast material, and melting point. Fewer micro-cracks occurred along the EDM machined surface, and the recast materials will attribute a quick shrinkage. These micro-cracks occur due to thermal shock stress. The electro-discharge carter and micro-crack and recast material formation will change the hardness of the machined surface [17, 18]. There is a change in machining energy with the increasing voltage, so the electro-discharge craters have become broader and more profound, contributing to evident surface roughness (Ra) in µm [18 – 20]. When the voltage increases, the electro-discharge carter's extension is high and decreases crack density. At high voltage, few micro-cracks were parallel along the machined surface. The recast material layer thickness was increased initially and then decreased with increased pulse duration, as shown in Figure 6 (c) - (d). The high pulse duration relatively will have increased electro-discharge energy. This increased electro-discharge energy will resolve and melt the material, as well as dissolve in the dielectric medium, which settles along the EDM machined surface . Thus, the recast material layer thickness was widened. An extra high electro-discharge energy will have sufficient impact energy to effectively remove the molten materials and accumulated materials from the EDM machined surface by increasing the pulse duration. Thus, it will reduce the thickness of the recast material layer.
Figure 7 (a – f) represents the secondary electron (SE) image and energy-dispersive X-ray spectroscopy (EDX) analysis of the EDM machined surface of the SMA alloy. The EDX analysis shows significant elements such as Ti, Ni, C, Cu, and O in the EDM machined surface layer. Thus, it shows the EDM machined surface layer has TiO2, TiC, Cu81 Ni19, Cu2O, C, and rich phase of Ni. The recast material layer consists of oxides TiO2 and the consumed Cu electrode deposition particles, and the dissolved dielectric medium. Alidoosti et al.  describe the TiO2, TiC, and Cu81 Ni19 compounds attributed to Ni and Ti atoms' great action. Due to the deposition of the consumed Cu electrode, Cu2O and Cu81 Ni19 occurs. Also, TiC and C are the effects of the dielectric medium.