Femtosecond laser high-quality drilling of film cooling holes in nickel-based single superalloy for turbine blades with a two-step helical drilling method

A femtosecond laser two-step helical drilling method was proposed, which realized the extremely high-quality machining of film cooling holes (FCHs) on nickel-based single crystal superalloy for turbine blades. The two-step helical drilling increased the exhaust capacity of machining residues, steam and laser-induced plasma, so it avoided the damage and defects, such as recast layers, microcracks and microvoids caused by laser energy shielding and heat accumulation. Furthermore, since more aggressive parameters can be adopted in the first step, and the amount of material removal in the second step was reduced, the two-step helical drilling improved the machining efficiency compared with one-time helical drilling. After two-step helical drilling, sub-wavelength nano-stripes were formed on the hole wall at a certain regular inclination angle along the hole depth direction. This characteristic feature was caused by the combined action of the periodic distribution of energy due to the interference of the laser and the plasmon, and the high-speed rotation and downward movement of laser beam. With this fundamental understanding of the mechanism, three processing parameters, i.e., laser output power (P), dwell time of single-layer scanning (T) and amount of focal plane drop for single-layer scanning (H), were coherently determined, such that the hole wall surface with nano-roughness without ablation can be realized.