Investigation of the laser drilling process by emission spectroscopy

Pulsed laser drilling has recently become of increasing importance to the aerospace industry as a method of creating cooling holes in a variety of high temperature gas turbine engine components. The potential for improved processing speed, accuracy, and reproducibility compared to conventional machining methods make laser drilling attractive from both an economic and product quality standpoint. Previous experimentation has shown that a relationship exists between laser irradiance values (W/cm²) and metallurgical hole quality. Higher irradiance values were observed to lead to smaller amounts of both residual resolidified material and hole plugging while requiring fewer pulses to complete a hole. It is suspected that the improved metallurgical hole quality is associated with the vaporization and liquid ejection behavior of the irradiated material. To further investigate this, emission spectroscopy is used to probe and monitor vaporization and liquid ejection that result from interaction between the laser and a metallic workpiece. In this paper, an overview of laser-induced plume characteristics is presented, and the dependence of these characteristics on processing conditions are reported.