Comparative electrical characterization of spark-erosion of silicon and steel as a base and its implications on equivalent circuit

Abstract

This study uses electrical characterization of sparks to advance the understanding of complex mechanism involved in wire electrical discharge machining (WEDM) of photovoltaic grade silicon. Specifically, WED machining of silicon is compared with that of steel as a base, with two approaches of experimentation — (a) one factor at a time (OFAT) and (b) design of experiments (DoE). The responses are measured in terms of four pulse parameters — maximum current Imax, maximum voltage Vmax, average voltage Vavg, pulse energy Ep; and three summary statistics. It was evident that the current required for steel workpiece is more by ≈ 10–15 A than that for silicon, whereas the spark voltage is more by ≈ 10 V for silicon than that for steel. Analysis of variance (ANOVA) showed that, current setting primarily affects the mean of Imax, Vavg, Ep and the effect is >60% for both silicon and steel. Moreover, quantitative effect of input factors on the pulse-to-pulse variation was also obtained for the first time. The pulse-to-pulse variation in Vavg for silicon is influenced by several parameters: current, pulse period, ON-time, and wire speed (cumulative effect ≈55%), whereas that for steel only current setting influences and the effect is >51%. The role of workpiece thickness (height of cut) has not been studied in detail in the available literature and is important for slicing cylindrical ingots as the length of cut changes continuously while slicing the cylindrical ingots. Therefore, the effect of workpiece thickness was studied, especially from perspective of the current and voltage pulse parameters during slicing. For silicon, an increase in workpiece thickness results in an increase in Vavg, and a decrease in Imax, whereas for steel, such effect was not seen. The equivalent circuit of spark as observed from the pulse generator showed that besides electrical conductivity of the materials, the junction between spark plasma and silicon plays a dominant role in deciding the current drawn by the spark-erosion process.