Flattening of surface roughness in ultrashort pulsed laser micro-milling

Micro-milling of AISI 316L was attempted by pulsed (800 fs) laser ablation using a scan strategy based on spiral trajectories. The aim is twofold: reducing the areal roughness of the ablated surface with respect to those obtainable by simply flanking parallel grooves and ensuring an uniform depth of cut per layer. The influence of the laser parameters (pulse energy, repetition rate) and the trajectory parameters (beam speed, number of turns) on the areal roughness Sa were investigated by means of experiments removing circular layers with a diameter of 300 μm. Ablated morphologies were investigated by coherence correlation interferometry and surface metrology was carried out according to ISO 25178-2.Guidelines were derived to remove larger layers with a diameter of 600 μm and the dependence of areal roughness and depth of cut was studied as a function of the energy density parameter, to include laser parameters in a simplified term. The removal process of layers having a depth of cut ranging between 1 and 2 μm and areal roughness of about 100 nm was ascertained and used for a straightforward micro-machining case study.