Experimental investigation and optimization of modification during backside-water-assisted laser drilling using flowing water

Backside-water-assisted laser drilling (BWALD) is a laser drilling process in which the workpiece is partially submerged in water. The backside of the workpiece is surrounded by water, and its upside directly contacts the air. In this study, flowing water was used to solve the problem where the generated air bubbles and debris prevent water from penetrating the through-hole and achieve the fabrication of micro-holes with high aspect ratios via BWALD. Moreover, a two-step micro-hole processing strategy was proposed to divide the BWALD into punching-through and modification steps. The modification process is the key to improve the machining performance, but involves numerous processing parameters. So, the effects of modification parameters (pulse energy, modification speed, frequency and number of modifications) on the responses of the micro-hole diameters and roundness were investigated during BWALD using flowing water on the basis of the response surface methodology (RSM). To achieve the optimal combination of parameters for a given situation, the regression models for the inlet diameter, outlet diameter, inlet roundness and outlet roundness of the modified micro-holes were developed. The adequacies of the developed models were subsequently verified by the analysis of variance (ANOVA) method. Finally, the well-rounded micro-hole with an inlet diameter of 105.62 μm and an outlet diameter of 96.65 μm which taper was only 0.21° was obtained with the optimum modification parameters: pulse energy of 15.9 μJ, frequency of 52.9 kHz, modification speed of 4.7 mm/s and number of modifications of 313.