The dual-stage drilling scheme, i.e., first using polarization trepanning to process through holes and then using spinning beam to enlarge the hole diameter on the bottom surface, can process straight and inverted tapered holes with high efficiency. The experimental results show that this drilling scheme can decrease the processing time of straight hole by 17% compared with the conventional scheme.

The quality of the holes affects the cooling efficiency of the blade film holes and the service life of the blades. Therefore, the roundness of the holes on the top and bottom surfaces must be further improved. During the processing, adding a circular mask with a diameter of 18 mm (Fig. 1) can effectively improve the roundness of the holes (Fig.7). Without using the mask, the holes roundness is significantly greater than 1. Using the mask, the hole roundness is approximately 1. An inverted tapered hole with a roundness of ~1, a diameter of 104 μm, an aspect ratio of 22, and a taper angle of -0.29° is successfully processed.

High-aspect-ratio microholes are widely used in the film cooling of turbine-engine blades. The development of next-generation aviation turbine engines with higher thrust-to-weight ratios is an important topic related to national strategic security, and improvements to the thrust-to-weight ratio and efficiency rely on the further optimization of the engine intake temperature. Currently, gas film-cooling technology uses cooling holes with a diameter of ≥0.3 mm to form thermal protective gas films on the surface of hot components, and the cooling efficiency is typically less than 60%. The thrust-to-weight ratio of the next-generation aircraft engine can reach 15?20, which corresponds to an intake temperature greater than 2200 °C; thus, a cooling method that offers a higher cooling efficiency is necessitated. Studies show that film cooling holes with a diameter of ~100 μm and an aspect ratio of >20 can improve cooling efficiency significantly.

Laser spinning is adopted in this study to process straight and inverted tapered holes with a diameter of ~100 μm and an aspect ratio of >20 on Inconel 718 alloy. The combination of polarization trepanning and spinning beam machining improves the drilling efficiency of the straight and inverted tapered holes (Fig. 1). A femtosecond-laser amplification system is used to output femtosecond-laser pulses with a center wavelength of 800 nm, a repetition rate of 500 Hz, a pulse width of 50 fs, and a maximum single pulse energy of 4 mJ. After passing through a tilted window and a half-wave plate, the femtosecond laser is focused onto the surface of the target material using a flat convex lens (L) with a focal length of 100 mm. The angle between the normal of the tilted window and the optical axis of the focusing lens is 15°. The target material used in the experiment is Inconel 718 alloy, with dimensions of 20.0 mm×20.0 mm×2.3 mm. The circular mask (aperture) at the exit of the laser amplifier is used to correct the cross-sectional profile of the femtosecond laser.

Figure 3(a) shows the dependence of the hole diameters on the top and bottom surfaces on the processing time of the laser-spinning drilling. As shown, the diameter on the top surface does not change significantly with time and remains 99?106 μm. Meanwhile, the hole diameter on the bottom surface increases as the processing advances. At the processing time of 3120 s, a straight hole with a diameter of ~100 mm and an aspect ratio of 22 is created. As the processing time increases to 3780 s, an inverted tapered hole with a taper angle of -0.17° is created.

This study proposes using spinning beam to process straight and inverted tapered holes with a diameter of 100 μm and an aspect ratio of >20. To improve the processing efficiency, polarization trepanning is first adopted to process through holes, followed by spinning beam to enlarge the hole diameter on the bottom surface. This dual-stage drilling method can reduce the processing time of a straight hole by 17% compared with the conventional method. By employing the circular mask to modify the cross-sectional profile of the femtosecond laser, an inverted tapered hole with a roundness of ~1, a diameter of 104 μm, an aspect ratio of 22, and a taper angle of -0.29° is successfully processed. The drilling method proposed herein can promote the application of high-aspect-ratio micro-holes in the fields of microelectromechanical systems (MEMS), engine fuel nozzles, and gas film-cooling holes.