Shape characteristics of microstructures in femtosecond laser multi-pulse ablation of metals

In femtosecond laser multi-pulse ablation, the ablated surface and the fluence irradiated on this surface are ever changing. Thus, the simple summarization of single pulse ablation is not an accurate shape forecast method and the influence of beam spot change must be considered. In this paper, a new simulation method is proposed to forecast the microstructure shape, in which fluence distribution, ablation directions, and ablation rates of each pulse on the ablated surface are determined according to the spatial propagation characteristics of laser and two logarithmic ablation models. Further, the microstructure shape is obtained by accumulating the effects of all pulses. Three kinds of ablation - on-focus ablation, off-focus convergent beam ablation, and divergent beam ablation - are investigated using the proposed method. Simulation results show that the influence of beam spot change on microstructure shape is obvious. In convergent beam ablation, the structure has a sharp bottom, and the sidewall changes from concave to convex with the increase of pulses. In contrast, the structure obtained by divergent beam ablation has a much higher draft-angle and flatter bottom. On-focus ablation can be regarded as parallel beam ablation when ablations proceed within the Rayleigh range and the profile keeps to the parabola function. In experiments, microstructures are machined with three kinds of ablation in typical metals, and it is proved that the proposed structure-developing model is a valid shape forecast method when vaporization is the main ablation mechanism.