Suitability of laser drilling models containing melt eject mechanisms

A reliable model of the laser drilling process is potentially an important tool for operators of industrial laser drilling systems, allowing the effects of altering material and laser parameters to be studied without the need for extensive and costly, drilling trials. A number of models have been developed for laser drilling, but the reliability of their predictive capability depends on the specific drilling regime of the process (Le. whether material removal is predominately by vaporisation or by vaporisation and melt expulsion). Melt expulsion can account for up to 70% of the material removed, and occurs because the vapour ablation pressure inside the hole is large enough to expel the surrounding molten material from the hole. In this paper the predictions of analytical models with and without the melt ejection mechanism are compared with experimental measurements made using a free running Nd:YAG laser over a wide range of drilling conditions in aluminium and titanium. The importance of modelling the melt expulsion process is discussed. The models also consider the time dependence of the drilling pulse. The importance of modelling the time dependence of the drilling pulse for obtaining reliable model predictions is considered. The effect of the reactive nature of titanium is discussed as a possible source of discrepancy between the experimental measurements and the predictions of the models, which do not consider this effect.