Automated Laser Re-Opening of Film Cooling Holes

Abstract

The re-opening of film cooling holes is currently one of the major challenges for the commercial repair of latest generation gas turbine noble parts. Protective coatings consumed in the course of a C-Interval must be replaced. An undesired side effect is that the new coating material plugs existing cooling channels. The use of reconditioned parts critically depends upon the capability to restore the original efficiency of the advanced cooling system. We present a novel process for that purpose, which has been developed at the ALSTOM Technology Center combining robotic vision and laser material processing. As a first step accurate information about cooling hole positions and orientation is obtained from a robotic vision system. This data is used to guide a short-pulse drilling laser in a subsequent machining operation. Typical overspray conditions for a great variety of film cooling geometries have been simulated and the results have been used to create a database of 3D overspray models. From these models a CAD/CAM postprocessor automatically generates individualized laser machining programs. Overspray coating material is then removed through an ablation process using a tailored 5+2 axes laser machining center equipped with an industrial q-switch Nd-YAG laser and a galvano-scanner. Airflow measurement results show that the new process is capable of meeting the cooling airflow requirements for heavily loaded hot-gas parts of the first gas turbine stage. This allows high quality reconditioning of film-cooled gas turbine components such as those in ALSTOM’s GT24/GT26 fleet.