Effect of two typical focus positions on microstructure shape and morphology in femtosecond laser multi-pulse ablation of metals

In femtosecond laser multi-pulse ablation, the ablated surface is changing and the fluence irradiated on the ablated surface is determined by the distance between the focal plane and the target surface. Thus, the focus position is a significant process parameter. In this paper, the effect of two typical focus positions on the microstructure shape and morphology in the ablation of steel are explored. Experimental results indicate that the microgroove ablated by the convergent beam (where the focal plane is below the target surface) has a V-shape, while that ablated by the divergent beam has a U-shape. The groove shape is analyzed by simulation in which a scanning ablation model is proposed. The simulation results are in close agreement with our experiments, and different change of beam spot size in laser propagation is confirmed as the main reason for the difference in shapes. The groove morphologies ablated by the two kinds of beams are related to the scan speed. At a higher scan speed, the grooves ablated by the convergent beam have much better morphology than that ablated by the divergent beam. However, when scan speed is slower, the morphology appears quite the contrary. The distinction of morphology can be attributed to different effects of plasma,fluence and the laser irradiation direction on groove sidewalls. As the influence of focus position on ablation shape and morphology is made clear, adopting proper focus position becomes a feasible process control method to obtain microstructures with certain shape and morphology, which can meet the demand of some particular applications. (C) 2008 Elsevier B. V. All rights reserved.