Ablation and morphological evolution of micro-holes in stainless steel with picosecond laser pulses

An experimental study is presented of micro-hole drilling on the surface of stainless steel using a 10-ps Q-switched Nd:VAN pulsed laser at two wavelengths, 532 and 1064 nm, with multiple powers and different number of pulses. Results show that two primary ablation mechanisms for ultrashort laser machining, i.e., vaporization and phase explosion, which correspond to gentle and strong ablations, are correlated mainly with the applied laser power. From measured data, two ablation thresholds are calculated using a piece-wise linear fitting. Moreover, surface recast layer associated with strong ablation can be eliminated by an appropriate selection of laser processing parameters. The Hirschegg model is applied to analyze the evolution of hole depth as a function of laser processing parameters for the two wavelengths. For the 532-nm wavelength laser ablation, the shapes and morphology of micro-holes evolve differently with low power from high power. In addition, the related reasons inducing multiple featured hole shapes are discussed.