Effects of processing parameters on hole circularity in laser percussion drilling - A statistical model

In laser percussion drilling, holes are produced mainly by melt ejection with a series of laser pulses. The circularity of the holes not only depends on various laser-processing parameters but also their interactions that influence the characteristics of melt ejection. In this work a statistical modelling approach was used to study the effects of six processing variables (peak power, pulse width, pulse frequency, number of pulses, assist gas pressure and focal plane position) and their interactions on the hole circularity. Response surface method was used to statistically analyse the process. For each variable five levels were considered and the experiments were conducted based on a Central Composite Design (CCD). The experiments were performed using an optical fibre delivered Nd:YAG laser drilling on mild steel sheets of 2.5 mm. thickness. An analysis of variance (ANOVA) was performed to test the significance of the variables at 1% and 7% levels. It was found that pulse width has a significant effect on the hole entrance circularity. Pulse frequency and number of pulses have very little effect on the hole entrance circularity.