## 3 Materials and Experimentation  
The commercially available nickel-based alloy Monel 400 was used as a test specimen in this investigation. The chemical composition of the Monel 400 alloys is shown in Table 1[36]. The machinability of Monel 400 alloy is very difficult, as it work hardens during machining. Therefore, it is a tough task to machine these alloys using conventional machining techniques. The experiments were carried out on the eco-friendly electrochemical machining (EECM) tool. The schematic diagram of the EECM tool is shown in Figure 1. EECM comprises a power supply system, electrolyte supply system, filtering system, tool feed mechanism, work holding and position system, control panel, frame, and housing. The electrolyte (e.g., NaCl aqueous solution) is allowed to flow at the rate between 1 L/min and 10 L/min through the inter-electrode gap (IEG) of 0.1-0.6 mm. The direct current (DC) potential (11-15 V) with a current density of 20-110 A/cm\({}^{2}\) is applied across IEG between a cathodic copper tool and an anodic workpiece. At low current densities, the MRR is low [37]. The higher current density of 110 A/cm\({}^{2}\) was used to attain high current efficiency. According to [PERSON]'s laws of electrolysis, the anodic dissolution rate of workpiece depends on the electrochemical properties of the workpiece metal, the electrolyte properties, and the power supply conditions. The tool is fed against the anodic workpiece, which is firmly fixed on the vice. The machining conditions of IEG, voltage (V), and current (C) are set in the control panel. The filtration system consists of layers of bio-absorbents compartments and membrane filter. A photograph of the indigenously developed EECM is shown in Figure 2. The tool's diameter and electrolyte exit hole were 8 mm and 2 mm, respectively. The operating conditions of EECM are summarized in Table 2. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & & & \multicolumn{6}{c}{**Monel 400 Alloys**} & \\ \hline  & C & Si & Mn & P & S & Cr & Mo & Fe & V \\ \cline{2-10} Composition and weight (\%) & 0.047 & 0.172 & 1.03 & 0.012 & 0.01 & 0.1 & 0.1 & 1.66 & 0.029 \\  & W & Cu & Al & Co & Nb & Ti & Mg & Ni & \\  & 0.1 & 29.24 & 0.01 & 0.103 & 0.1 & 0.047 & 0.031 & 67.4 & \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of Monel 400 alloys. Figure 1: Schematic diagram of EECM. Experimental Design and Measurements  
The MRR is directly proportionate to the feed rate of the tool [36]. A high feed rate leads to electrolyte boiling or choking in the tool and workpiece gap [37]. Therefore, the tool feed rate was set to the possible minimum of 0.1 mm/min to have stable movement of the tool through the workpiece. When the applied voltage is high, the current machining increases to a high MRR. A low voltage results in poor machining performance [10,11]. The voltage range of 11-16 V was considered in many such investigations in ECM to have better performance in terms of the MRR and surface finish [38,39]. Therefore, the voltage range (10-15 V) available in the set-up was used for this investigation. When the voltage is raised above a particular level, it increases the hydrogen gas bubble generation at the tool electrode, resulting in high resistivity of the electrolyte and a decrease in the current density at the workpiece [40, 41, 42, 43]. A high concentration decreases the mobility of ions, which results in poor anodic dissolution. The concentration levels in the range of around 100-200 g/L influence the MRR significantly [10]. According to the literature, the main process parameters governing the ECM are the voltage (V), flow rate, and EC. Therefore, these parameters were considered in this investigation, and their levels are shown in Table 3.