Investigations on geometry and productivity of micro-holes in Incoloy 800 by pulsed electrolytic jet drilling

Abstract

This paper reports on investigations on geometry and productivity of micro-holes drilled in Fe-Ni-based superalloy (i.e., Incoloy 800) by pulsed electrolytic jet drilling (PEJD) process. Depth-averaged radial overcut (DAROC) and hole taper were used to evaluate micro-hole geometry, whereas drilling time and material removal rate have been used as measures of PEJD productivity. Borosilicate glass nozzle having 0.3-mm inside diameter and 0.5-mm outside diameter was used to drill through holes of 6 mm depth in Incoloy 800 using aqueous solution of sulfuric acid (H2SO4) as an electrolyte. The experimental study involved pilot experiments to identify optimum values of standoff distance, electrolyte temperature, and electrolyte concentration to minimize radial overcut, hole taper, and drilling time using one variable at a time approach. The identified optimum values of these parameters were 1.5 mm, 35 °C, and 18 % (by wt.), respectively. These values were used during the main experiments to study effects of voltage, pulse-on time, and pulse-off time on micro-hole geometry and productivity and to identify their optimum values. The entry diameter of micro-holes ranged from 1.15 to 1.35 mm, while exit diameter was 0.46 mm. Geometry of the micro-holes was analyzed using SEM images and geometry profiles showing variation of micro-hole diameter as function of the hole depth. Based on this analysis, the micro-hole geometry was assumed to consist of three frusta of cones on the basis of which models to predict MRR and DAROC were developed. The best geometry of micro-hole was obtained using 325 V as voltage, 30 μs as pulse-on time, and 50 μs as pulse-off time having taper of 0.057 mm per unit of hole depth and DAROC as 0.302 mm. It reveals that optimization of geometry and productivity of micro-holes is conflicting in terms of voltage and pulse-on time. © 2016, Springer-Verlag London.