OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 To make a decision on the range and level of gap current and magnetic fi eld to obtain optimal values of material removal rate, pilot studies were carried out. The gap current and the gap current were varied at fi ve levels; one pass was performed at each level. Various combinations of workpiece and tool were considered: – BeCu (untreated) and Cu (untreated); – BeCu (untreated) and Cu (cryogenically treated); – BeCu (cryogenically treated) and Cu (untreated); – BeCu (cryogenically treated) and Cu (cryogenically treated). Based on the results obtained from the pilot study, the main experiments were designed using a threevariable Box–Behnken design. Results and Discussion This section illustrates the MRR experimental results and its analysis, white layer thickness and crack formation and prediction of MRR using machine learning regressions. Experimental results and analysis Experimental study was carried out in two phases. Firstly, one variable was varied over the selected levels at the fi xed average values of the other variables. These experiments were conducted to study and normalize the EDM machine settings and process responses in general. It was determined that 5 mm was the proper depth of milling to achieve stability while the process was underway. Two variables were modifi ed in this study: gap current and magnetic strength. The remaining variable was spread at equal intervals throughout its range, while the other variables were fi xed at its respective average values for the whole range of options available in the machine. In the fi rst fi ve experiments, only the gap current was changed, as is illustrated in Table 2. A similar variation was observed in the magnetic strength in fi ve experiments as shown in Table 3. The gap current and magnetic strength varied for four workpiece and tool combinations. Ta b l e 2 Experimental matrix: Varying gap current Magnetic strength (T) Gap voltage (V) Gap current (A) Pulse on time (μs) Pulse off time (μs) Workpiece and tool combinations (U:U, T:U, U:T, and T:T) 0.248 55 8 38 7 U:U (BeCu-untreated with Cu-untreated), T:U (BeCu-treated with Cu-untreated), U:T (BeCu-untreated with Cu-treated), T:T (BeCu-treated with Cu-untreated) 0.248 55 10 38 7 0.248 55 12 38 7 0.248 55 14 38 7 0.248 55 16 38 7 Ta b l e 3 Experimental matrix: Varying magnetic strength Magnetic strength (T) Gap voltage (V) Gap current (A) Pulse on time (μs) Pulse off time (μs) Workpiece and tool combinations (U:U, T:U, U:T, and T:T) 0 55 12 38 7 U:U (BeCu-untreated with Cu-untreated), T:U (BeCu-treated with Cu-untreated), U:T (BeCu-untreated with Cu-treated), T:T (BeCu-treated with Cu-untreated) 0.124 55 12 38 7 0.248 55 12 38 7 0.372 55 12 38 7 0.496 55 12 38 7
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