## 3 Results and Discussion
### Analysis of the Influencing Factors of Inlet and Outlet Morphology
Thus, the important influence of instantaneous power density on the outlet morphology was verified. When the laser power and Dove prism rotation speed remain constant, the instantaneous power density will be affected by the laser repetition frequency. As the repetition frequency increases, the number of pulses increases, and the overlap rate of the light spot increases, but the energy of a single pulse decreases; that is, the instantaneous power density at a certain area along the cutting path will decrease, which prefigures that the processing quality at the outlet edge of the hole will also decrease. Figure 9 shows the processing results of changing the laser repetition frequency, which well confirms the above conclusions. If the rotation speed of the Dove prism is 2000 rpm when the repetition frequency is 100 kHz, the overlap rate of the laser spot is 0, and there is no overlapping area between the two adjacent laser pulses, but the morphology of the hole exit is better. When the repetition frequency is 400 kHz, the overlap rate of the spot is 45.49%, but the morphology of the hole exit is poor. This is mainly because when the repetition frequency changes from 100 kHz to 400 kHz, the peak power density of the laser will decrease to a quarter of the original. Although the spot overlap rate increases about 45%, the instantaneous power density still decreases in a certain processed area. The instantaneous power density directly determines whether the material can be removed quickly. Figure 8: Surface morphology of hole outlets processed at different laser powers. Figure 7: Surface morphology of hole outlets processed at different rotation speeds of Dove prism. Figure 9: Surface morphology of hole outlets processed at different laser repetition frequencies. Based on the discussion above, if the speed of the hollow motor is fixed at the state with a good dynamic balance, and in order to obtain the better outlet morphology, a lower repetition frequency should be selected in combination with higher laser power. But the entrance morphology is also something we need to consider comprehensively. Figure 10 shows the original inlet morphology of the micro-hole obtained by different repetition frequencies at a power of 10 w. It can be observed that the lower the repetition frequency, the more severe the splashing phenomenon on the surface. A large amount of melted and re-solidified slag accumulates on the inlet surface, while these defects are much milder when the repetition frequencies are higher. This is mainly because the lower the laser repetition frequency, the higher the peak power density, and the more intense the interaction between the laser and the material. The higher the temperature and pressure inside the molten pool, the greater the amount of spraying towards the surroundings. In the case where the slag discharge channel has not yet been opened, the sprayed slag will accumulate on the inlet surface, leading to a deterioration of the entrance morphology. After the outlet is opened, the slag can be discharged in a timely manner with the protective gas, so it does not have significant impact on the morphology of the outlet. During the actual drilling process of blade air film holes, a layer of protective material is pre-coated on the surface of the blade to prevent oxidation and discoloration of the substrate. After the processing is finished, the coating material is removed, and part of the slag at the hole inlet is also removed, and the morphology of the inlet edge is improved. Therefore, the influence of repetition frequency on the inlet morphology can sometimes be disregarded in the actual drilling process. Figure 5:0 The original morphology of the entrance surface of micro-holes processed with different laser repetition rates. In summary, the rotation speed of the Dove prism, laser power, and laser repetition frequency all affect the quality of the helical drilling hole by changing the spot overlap rate or mean power density.