Fabrication and quality improvement of film cooling holes via picosecond laser drilling assisted by waterjet and post-electrochemical machining

Film cooling holes (FCHs) are of superior importance for blade protection and performance improvement in aircraft engine, while their fabrication requires high efficiency, high precision and extremely good surface quality, making it a challenging task. In this study, the ultrashort picosecond (ps) laser is employed to drill the FCHs on the DD6 single crystal superalloy, based on which the assisted waterjet is further introduced to reduce thermal damage, and the electrochemical machining (ECM) is also added as a post-processing manner for further quality improvement. It has been found through orthogonal tests that the pulse repetition frequency possessing most significant influence on hole taper during direct ps-laser drilling. The introduced low-pressure waterjet effectively eliminates the thermal damage such as slag attachment and recast layer, and the waterjet angle affects the hole profile obviously. After conducting post-ECM, the hole taper decreases continuously with an increase in processing time and applied voltage. In addition, the laser-induced periodic surface structure (LIPSS) has been observed on the sidewall, while an obvious decrease in LIPSS wavelength has been confirmed after post processing. Moreover, simulation study of temperature evolution is carried out by developing a two-temperature model, demonstrating that the lattice temperature surpasses the boiling point within 0.182  ps, and the ablation can be precisely localized within laser irradiation zone. The simulation of flow field on DD6 surface has also been conducted, and the distributions of flow velocity, pressure and thickness are obtained, which helps understand the effect of waterjet on the drilling process.