Experimental study on the development of a micro-drilling cycle using ultrashort laser pulses

Microholes for the production of high precision devices were obtained by ultrashort pulsed laser machining of martensitic stainless steels. A micro-drilling cycle based on the sequence of a drilling through phase, an enlargement and finishing phase is proposed in order to solve the trade-off between process time and quality of the ablated surfaces without making use of complex design of experiments. The three phases were studied taking into account the evolution of the microhole shape as a function of the main process parameters (number of passes per phase, incidence angle and radius of the beam trajectory respect to the hole's axis). Experiments testified that the drilling strategy was able to produce cylindrical holes with diameter of 180±2 μm on a 350 μm thick plate in total absence of burrs and debris within a drilling time of 3.75 s. Repeatability tests showed a process capability of nearly 99%. SEM inspection of the inner surface of the microholes showed the presence of elongated and periodic ripples whose size and inclination can be controlled adjusting the incidence angle of the beam over the tapered surface before the ultimate finishing phase.