Evaluation of radially polarized femtosecond laser drilling of film cooling holes

Abstract

The low efficiency of femtosecond (fs) laser processing remains a significant obstacle to its application particularly in the field of film cooling holes of aero-engines. This paper aims to provide an understanding of the energy coupling mechanism in femtosecond vector laser drilling. In this paper, the polarization dependence of penetration efficiency in femtosecond laser drilling was investigated. Experimental results reveal that the radially polarized laser has the highest penetration efficiency in deep drilling. The penetration efficiency of the radially polarized laser can be improved by 35.2 % and 33.0 % compared with the linearly polarized laser on samples of 3 mm and 5 mm thick. At the same time, the on-line monitoring system can effectively monitor the penetration time, and the combination of ultrafast transient absorption spectroscopy (UTAS) and laser-induced breakdown spectroscopy (LIBS) further proves that the radially polarized laser has higher absorptivity and ablation efficiency. The impact of laser drilling on the morphology and microstructure evolution of the hole wall was investigated. The results indicate that the radially polarized laser exhibits superior performance compared to the other two lasers in stimulating the electronic system of the material, leading to penetration efficiency enhancement. This study provides new insights into the mechanism of femtosecond vector laser drilling and provides a basis for the application of vector laser in high aspect ratio film cooling hole machining.