Mechanism of droplet formation during pulsed laser micro-machining

This work investigates the transport phenomena and the mechanisms of droplet formation during a laser micro-machining process. The surface of the target material was altered through a laser-induced material flow process in the molten phase induced by a tightly-focused laser energy flux. As with many other machining techniques, debris (droplets) is often generated during the laser micro-machining process. Understanding the mechanism of droplet formation is essential for developing a debris-free micro machining technique. Experimental parametric studies were carried out. It was found that a narrow range of operation parameters and target conditions existed for 'clean' structures to be fabricated on the target. Deviation of the operation parameters from that range generated debris. The transient process of the clean hole formation and the process of debris formation were studied in-situ, using the stop action photography technique. Numerical simulations of the laser-induced surface deformation were also performed to obtain the transient field variables and to track the deforming surface. The comparison between the numerical and experimental work showed that, within the energy intensity ranges investigated in this work, the surface deformation was attributed to the surface tension driven flow, and debris generation was due to the instability of the fluid flow in the laser-induced molten pool.