Study on the variation of recast layer in laser trepan drilling by millisecond Nd:YAG laser

Abstract

Recast layer (RL) is a major defect in laser drilling of micro holes. In this study, some methods to reduce RL in micro-hole drilling were carried out by using a pulsed Nd:YAG laser (λ = 1.06 µm) on a type 304 stainless steel. Based on the previous optimization of laser parameters, trepanning velocity and rotation number in laser trepan drilling were specially studied of their effect on RL. Single-factor experiment and orthogonal experiment were both employed in the experimental research. It was found that the thickness of RL dropped rapidly with lower trepanning velocity and fewer beam rotations. However, when the two factors deviated from some range, cracks might emerge on entrance edges and inner walls, and the further they deviated, the more serious the cracks became. An equation was constructed to evaluate the optimizing effect simultaneously considering four factors: (a) the total volume of RL in a hole; (b) the sum of entrance crack length (CL); (c) the sum of inner CL; and (d) drilling efficiency. Eventually, in this article, an optimum set of parameters was obtained, which might generate micro-holes almost free of RL.