Experimental study on the effect of laser overlap rate on the quality of femtosecond laser machining of micro-holes

The utilization of ultrafast lasers for ablating high-quality film cooling holes in turbine blades has long been a challenging issue. The study systematically investigates the impact of laser pulse overlap rate ( PO ) and scanning line overlap rate ( LO ) on the quality of micro-holes by varying scanning speed and line spacing, while assessing micro-hole's quality through destructive means. The PO of 92.5% is identified as a critical point in the transition of micro-hole's quality, while PO exceeding 92.5% is designated as a high pulse overlap rate. The results demonstrate that within the high pulse overlap range, dimensions of micro -hole remain relatively and have the minimum taper at PO of 92.5%, whereas lower pulse overlap rate results in improved hole wall quality with reduced occurrences of recast layers and micro -cracks. Furthermore, higher line overlap rates lead to larger inlet/outlet diameters for the micro-holes; however, optimal taper is achieved at a LO of 62.5%. Most significantly, this paper presents a novel finding that the ablation process reveals an inclined ejection angle (30 degrees ) for bottom plasma with different PO , prompting an in-depth analysis of potential influencing mechanisms. This research holds significant importance and provides valuable guidance for femtosecond laser machining of film cooling holes for turbine blades.