OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 In this study, only the gap current and external magnetic fi eld was varied. It is known that the most infl uential parameter for the maximum MRR is the spark energy. During cryogenic treatment, the thermal vibration of atoms in metals decreases due to the temperature decrease. This results in a decrease in electrical resistivity and improved electrical conductivity. Owing to the cryogenic process, the homogeneity of the crystal structure increases, and the gaps and dislocations are dissolved, which improves the structural compactness and electrical conductivity. According to the Wiedmann-Franz-Lorenz law, an increase in electrical conductivity would increase thermal conductivity. Figure 2 depicts MRR varying with gap current for treated and untreated BeCu alloys with treated and untreated copper tool electrodes (four workpiece and tool combinations – U:U, T:U, U:T, and T:T). The surface temperature of the workpiece tends to increase as a result of the increase in spark energy caused by the gap current. The substance melts as a result, and the molten metal is subsequently fl ushed away by the dielectric fl uid. Due to the increased electrical conductivity of the workpiece after cryogenic treatment, the MRR increases. Debris from the undesired material removed from the workpiece is created in the machining zone during the EDM process. The machining effi ciency is decreased because arcing occurs instead of sparking if it is not removed from the cutting zone. Debris removal from the cutting zone is facilitated by the strength of the external magnetic fi eld. Additionally, this keeps the particles in the cutting zone from clogging As a result, the stability of the EDM process is improved. Figure 3 depicts MRR varying with magnetic fi eld strength for treated and untreated BeCu alloys with treated and untreated copper tool electrodes (four workpiece and tool combinations – U:U, T:U, U:T, and T:T). With an increase in the gap current, the spark energy increases, increasing the surface temperature of the workpiece, thereby melting and evaporating material from the workpiece surface and increasing MRR. Fig. 2. MRR varying with gap current for four workpiece and tool combinations Fig. 3. MRR varying with magnetic strength for four workpiece and tool combinations To understand the eff ect of input variables, namely the gap current (Ig), external magnetic fi eld strength (B), and pulse-on time (Ton), on the material removal rate (MRR) was investigated for cryogenically treated BeCu workpiece and untreated Cu electrode combination. This combination of workpiece and the tool is selected as it provided higher MRR among the other combinations of workpiece and the tool selected in the present study. Table 4 depicts the experimental matrix with MRR varying with Ig, B, and Ton. Experimentally based mathematical model (Eq. 2) was developed for the MRR for the T:U (BeCu-treated with Cu-untreated), workpiece and tool combination, for better understanding the EDM performance. The values of the coeffi cients involved in the equation were calculated using the Microsoft Advanced Excel data analysis tool. R-squared (R2) values which measure variation proportion in the data points are close to 0.912. Therefore, the developed model is reliable to predict the MRR during EDM of cryogenically treated BeCu workpiece with untreated Cu electrode. 1.339 0.00121 1.0508 0.004501( ) ( ) ( ) on MRR Ig B T = . (2)
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