A Comparison in laser precision drilling of stainless steel 304 with nanosecond and picosecond laser pulses

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Abstract
Precision drilling with picosecond laser has been advocated to significantly improve the quality of micro-holes with reduced recast layer thickness and almost no heat affected zone. However, a detailed comparison between nanosecond and picosecond laser drilling techniques has rarely been reported in previous research. In the present study, a series of micro-holes are manufactured on stainless steel 304 using a nanosecond and a picosecond laser drilling system, respectively. The quality of the micro-holes, e.g., recast layer, micro-crack, circularity, and conicity, etc, is evaluated by employing an optical microscope, an optical interferometer, and a scanning electron microscope. Additionally, the micro-structure of the samples between the edges of the micro-holes and the parent material is compared following etching treatment. The researching results show that a great amount of spattering material accumulated at the entrance ends of the nanosecond laser drilled micro-holes. The formation of a recast layer with a thickness of ∼25 μm is detected on the side walls, associated with initiation of micro-cracks. Tapering phenomenon is also observed and the circularity of the micro-holes is rather poor. With regard to the micro-holes drilled by picosecond laser, the entrance ends, the exit ends, and the side walls are quite smooth without accumulation of spattering material, formation of recast layer and micro-cracks. The circularity of the micro-holes is fairly good without observation of tapering phenomenon. Furthermore, there is no obvious difference as for the micro-structure between the edges of the micro-holes and the parent material. This study proposes a picosecond laser helical drilling technique which can be used for effective manufacturing of high quality micro-holes.

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