Cylindrical holes were once the industry standard for air film cooling on turbine engine components. The cooling holes were traditionally formed using an electro-discharge machine (EDM) with a round electrode that slowly eroded the component alloy away until a complete hole was formed. The task was time-consuming and costly, because a typical combustion turbine blade required hundreds of holes.
As coatings technology progressed, turbine manufacturers added a ceramic thermal barrier coating (TBC) to protect blades as the pursuit of higher operating efficiencies pushed up gas temperatures some years ago. Unfortunately, that made EDM shaping through a TBC coating applied to blades and vanes impossible. One major limitation of the EDM process is that it requires a surface material to be conductive, because the process requires an electrical circuit path to function.
The process of applying shaped holes on turbine airfoils then evolved into a two-step machining process sandwiched around the application of a TBC. Before a ceramic nonconductive coating could be applied, cylindrical airfoil holes were drilled with a laser. Laser drilling focuses a high-energy beam of coherent monochromatic light at the surface of the airfoil to melt the alloy.
Next, the external TBC was applied to the surface of the turbine components to protect the alloy. However, the laser-drilled holes could be blocked if overcoated, thus reducing the efficiency of the intended blade cooling design. Normally, a mathematical formula was used to determine hole coatdown based on hole size and amount of coating thickness. The final application of the TBC also created yet another airflow disturbance step.
In the second machining step, a specially shaped EDM electrode was superimposed in each laser hole to produce a round metering hole with a diffuser shape at the surface. This diffuser shape (unlike the traditional round exit hole) spreads the exiting airflow and promotes more uniform surface cooling effect. Because it was a two-step process, there was a possibility for many hole-to-shape mismatches that impede the smooth transition of the airflow. The entire process was time-consuming, and rework was often required to produce a blade that met specification. At the time, this process technology helped to usher in the next generation of advanced turbine engine components.
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