Fabrication of high aspect ratio micro-holes on 304 stainless steel via backside-water-assisted laser drilling

•The two-step processing strategy based on BWALD could well reduce tapers and improve roundness of the micro-holes.•The modification process following the formation of through-holes is the key to improve machining performance.•CCM was more suitable for the modification of small size micro-hole than SCM.•Using flowing water could enable the fabrication of micro-holes with higher aspect ratio during BWALD process. Backside-water-assisted laser drilling (BWALD) is a viable and effective alternative of laser direct drilling (LDD) to fabricate micro-holes. However, it still has its own limitations, primarily due to the generated air bubbles and debris during the drilling process and the involvement of various processing parameters, especially when fabricating micro-holes with high aspect ratios on the non-transparent materials. In this study, a two-step processing strategy based on the sequence of punching through and modification was developed to fabricate the micro-holes on 304 stainless steel. Based on this strategy, two modification methods, namely single circle modification (SCM) and concentric circle modification (CCM), were proposed to amplify the advantages of BWALD during the modification process. Experiments testified that the two-step processing strategy was capable of reducing the taper and improving the roundness during BWALD process. Furthermore, a new strategy of using flowing water (backside-flowing-water-assisted laser drilling, BFWALD) was proposed to solve the problem that the generated air bubbles and debris prevent the water from penetrating the through-hole. It was found that the BFWALD could effectively improve the stability during the modification process and achieved the fabrication of micro-holes with an aspect ratio of 10:1 and a taper of only 0.38° successfully.