Laser drilling of effusion holes: Process optimization and quality control using industrial micro-computed tomography

This work targets the development and validation of a micro-computed tomography (CT) inspection technique for assessing the quality of effusion holes in thermal barrier-coated metallic liners of combustion chambers. This automated inspection workflow is implemented using the Dragonfly 3D World software to extract geometric metrics from CT scans, including the hole angle, the volume of material removed, and the inlet/outlet hole diameters and surface areas, all in a bid to establish correlations between key laser drilling parameters such as the focal position, the drilling angle, the number of holes per sample, the drilling side (metallic or thermal barrier-coated), and the cooling air mass flow, which serves as a standard criterion for effusion holes qualification. Comparing the CT with standard metallographic measurements demonstrated an adequate precision of the proposed approach in capturing critical geometric details of the laser-drilled effusion holes: ∼0.55° ± 0.25° mean deviation in the assessment of drilling angles, and ∼50/75 μm (mean absolute deviations) in the assessment of inlet/outlet hole diameters. This approach significantly reduces the analysis time as compared to conventional metallographic and air flow testing methods, eliminating the need for destructive testing and specialized test rigs. Moreover, correlations established in this work between laser drilling parameters, the hole geometry, and the air mass flow allow to optimize the laser drilling process and establish quality control guidelines for the effusion holes used in combustion chambers.