The primary consideration in designing cutting-edge high thrust-to-weight and bypass ratios aero engine is turbine entry temperature, leading to the application of Ni-based superalloy as the major materials for manufacturing turbine blades. In order to withstand the extremely severe environment during service, film-cooling technology is one of the major improvements on cooling configurations of turbine blade. However, processing quality of film-cooling holes is crucial to service hours of the turbine blades. Therefore, it is vital to study the corresponding processing techniques for manufacturing film-cooling holes. Compared with the conventional drilling methods, the novel ultrafast laser machining technology possesses promising advantages, including “cold processing” property, material versatility, and higher drilling precision and quality. Thus, it has received national and international attentions. This study investigates the effects of four processing parameters of picosecond laser, i.e., single-pulse energy, repetition frequency, scanning speed, and scanning time, on dimensions, quality, and morphology of produced holes using controlled variables, the optimal parameters can be determined accordingly. Furthermore, hole profiles have been characterized by a scanning electron microscopy equipped with energy dispersive spectroscopy.

The second generation Ni-based superalloy, IC10, was used as the experimental material. The air-film holes were processed by a picosecond fiber laser with a wavelength of 1064 nm and a pulse width of 10?15 ps. The laser repetition frequency ranges from 250 to 1000 kHz, with an average output power of 35 W and a maximum single pulse energy of 135 μJ. The laser spot size and focus depth are 40 μm and 1.82 mm, respectively. The laser system is integrated with a five-axis platform to conduct experiments. Holes with diameter of 0.6 mm were prepared using a layer-by-layer circumferential drilling method at ambient temperature and pressure. Macroscopic characterization was performed using a metallographic microscope to characterize the size and morphology of the air-film holes, while microscopic characterization was performed using a scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) to microstructurally characterize the morphology and chemical composition of the hole wall.

The results show that the optimal parameters using a picosecond laser for hole processing are single-pulse energy, 120 μJ; repetition frequency, 63 kHz; scanning speed, 40 mm/s; and scanning times, 400. The hole profile can be achieved at an inlet diameter of 0.725 mm, an outlet diameter of 0.515 mm, an inlet roundness of 0.992, an outlet roundness of 0.965, and a taper of 4.753°. It is deduced that scanning speed affects the overlap rate of laser spot and duration of the unit area of the material affected by picosecond laser. The number of processing times determines the ablation efficiency of the laser on the material. The pulse repetition frequency determines heat accumulation during processing. Further microstructural characterisation shows a crack-free and minor remelt or oxidised surface along the processed hole using optimal laser parameters. When the repetition frequency is 250 kHz, or above, an oxidation layer with a thickness of 5?6 μm formed on the hole walls, which contain numerous transverse and longitudinal cracks. EDS line scanning results verify homogeneous distribution of elements in the sidewall of the air-film holes processed in 50 kHz while elemental fluctuation of O confirms formation of oxides in the sidewall processed in 250 kHz.

In this study, we demonstrated the effects of picosecond laser scanning speed, processing time, repetition frequency, and single-pulse energy on the quality of air-film holes and identified the optimal combination of processing parameters. The results show that the single pulse energy and repetition frequency determine the diameter of hole, roundness and taper, while scanning times and scanning speed mainly affect the laser ablation rate. For 1 mm thickness IC10 samples, it shows that the optimal parameters are 120 μJ pulse energy, 63 kHz repetition rate, 400 processing times and 40 mm/s scanning speed. In this range, the entrance diameter of the air-film hole is 0.725 mm, the exit diameter is 0.515 mm, the hole roundness is improved by 0.02, and the hole taper is reduced by 3°?4°. Meanwhile, there is negligible amount of recast or oxide layer on the inner wall of the hole.