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Mathematical modelling of multiple pulsed laser percussion drilling

Suchatawat, Maturose

[Thesis]. Manchester, UK: The University of Manchester; 2012.

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Abstract

In laser percussion drilling, a series of laser pulses with specified energies and durations irradiate the workpiece surface to gradually heat, melt, and vaporise material until a hole with required depth and diameter is achieved. Despite being the quickest technique for producing small diameter holes, laser percussion drilling regularly suffers from difficulties in controlling the hole quality such as hole circularity, hole taper and recast layer. Therefore, in order to produce holes to a specific requirement at minimum cost and time, it is crucial to fully understand the effects of each parameter on hole quality. In this research, a new mathematical model for multiple pulsed laser drilling is developed to predict the hole depth, hole taper, and recast layer thickness, and to investigate the effects of key laser parameters on hole dimensions. The new model accounts for recoil pressure, melt ejection, O2 assist gas effects, as well as solidification of the melt. The development of the new model is divided into two stages; pulse on stage where interaction between laser beam-material takes place, and pulse off stage where solidification of the melt is modelled. Governing equations are established from heat conduction, energy, and mass equations at the solid-liquid and liquid-vapour interfaces with appropriate boundary and initial conditions. Analytical solutions are derived by using Mathematica 7 software as a tool to solve the system of non-linear equations. To validate the model, experimental work has been conducted and the measured results are compared to those calculated from the model. It is shown that the new model gives a good prediction of the hole depth and acceptable prediction of the recast layer thickness. Laser peak power and pulse width are shown to have a significant influence over the drilled hole quality whereas the changes due to pulse frequency are less pronounced.