In laser-based materials processing, ablation of metals using ultrafast laser pulses usually allows controllable direct removal of the metal with no heat effects because the laser pulses are too short in duration to melt the metal, entirely ablating it away instead. However, heat from ultrafast laser pulses can indeed have deleterious effects on the metal’s surface if the average power of the ultrafast laser is high enough, as researchers from Robert Bosch (Stuttgart, Germany) and Friedrich-Schiller-Universität Jena (Germany) have experimentally determined.
Their setup was built around a ytterbium:YAG (Yb:YAG) disk laser made by Trumpf (Ditzingen, Germany) that produces 6 ps pulses at an 800 kHz repetition rate and a 1030 nm wavelength. The researchers focused the circularly polarized Gaussian beam onto samples of stainless steel (EN Steel Number 1.4301), producing typical peak powers and peak intensities of 106 W and 1015 W/m2, respectively. A 1 mm square was scanned with an 80% overlap rate to produce an ablation depth of about 55 μm. Results showed two quality regimes for the ablated surfaces: 1) a smooth, flat surface with roughness <1 μm and high reflectance; and 2) a surface with reduced reflectance and irregular oxidized bumps on the order of 10 μm in size. The transition from smooth to bumpy occurred as the scanning speed was reduced. The researchers developed a heat-flow model that determined that, depending on the process parameters, heat accumulation could produce a deleterious heat-saturated state at the surface, with the transition temperature being 607°C. Knowing this, process parameters can be changed to avoid this state. Reference: Franziska Bauer et al., Opt. Express (2015); http://dx.doi.org/10.1364/OE.23.001035.