Femtosecond Laser Micro Drilling of Cooling Holes on Thermal Barrier Coated Titanium Alloy for Aeroengine Application

Drilling cooling holes on the turbine blade is essential for enhancing aero engine performance and to protect it from elevated temperature. So, the objective of this work is to produce micro holes on the thermal barrier coated titanium alloy using a femtosecond laser. An in-depth experimental investigation has been carried out to understand the influence of femtosecond laser parameters, circular and zig-zag scanning modes on geometrical characteristics and surface integrity of the micro holes. It is inferred that better hole circularity and less taper angle are observed for circular scanning method at laser energy density (LED) of 2.04 J/cm(2), pulse repetition rate (PRR) of 20 kHz, and scanning speed of 300 mm/s. The surface finish and hole quality are also improved by this method, although at higher LED and PRR the inner hole wall surface degrades because of flaws including microcracks, re-solidified material, and micropores. When compared to circular scanning, the zig-zag scanning strategy reduces the drilling time by 51.87%. Conical spike with micro protrusion and micro trench type microstructure, is noticed for the zig-zag path. In contrast, micropores with micro protrusion type microstructures are obtained for concentric path with minimal surface flaws.