## 3 Results and Discussion
### Analysis of the Hole Wall Recast Layer  
The recast layer inside the film cooling holes of turbine blades in aircraft engines has a significant impact on the performance of the blades. On the one hand, the recast layer will damage the integrity of the hole wall surface. The protrusions and pits in the recast layer will increase the roughness of the processed surface, affecting the state of the airflow inside the gas film hole and thus affecting the cooling effect. On the other hand, micro-cracks inside the recast layer will gradually expand and fracture during the high-speed rotation of the blade, which seriously reduces the expected life of the blade. Therefore, the recast layer is a necessary element to be considered in the drilling process of gas film holes. For long pulse lasers, the energy absorbed by the material has sufficient time to be converted from light energy to thermal energy and expands around the processed area to form a liquid melt pool. Part of the liquid metal in the molten pool is removed in the form of steam, while another part is directly blown away by the protective gas. The remaining part solidifies and adheres to the hole wall after the laser spot is moved away and then forms a recast layer. For ultra-fast lasers such as femtosecond lasers, the pulse irradiation time usually is shorter than the material's thermal relaxation time. The material removal mechanism changes from thermal melting removal to direct removal by plasma vapor based on the multi-photon absorption. Therefore, theoretically, the ultra-fast laser processing will not produce thermal damage, such as recast layers and heat-affected zones. However, for micro-holes with large depth-to-diameter ratios, such as gas film holes, as the processing depth increases, the protective air pressure decreases, and the metal vapor cannot be discharged from the hole interior in a timely manner, which will condense and adhere to the hole wall. At the same time, the shielding effect of the hole wall on the incident laser will also reduce the instantaneous power density, leading to a strong thermal accumulation effect, promoting the thermal melting of the material, and forming a recast layer after cooling. Although the multi-step method is able to reduce the recast layer to some extent, it obviously increases the processing time about three times that of the one-step method and seriously decreases the efficiency, which is an important element in practical application. Figure 11 shows the cross-sections of gas film holes processed with different laser powers at a laser repetition frequency of 400 kHz. From the figure, it can be observed that there are recast layers on the sidewall surfaces. The presence of the recast layer increases the roughness of the sidewall surface. The severity of the recast layer near the inlet and outlet is different, and the re-condensation phenomenon near the outlet is significantly more severe than that near the inlet. This is mainly because the processing depth at the outlet is larger. The laser instantaneous power density is reduced due to the obstruction of the hole wall. Then, the thermal effect is enhanced, and the proportion of material removed as liquid increases, thereby aggravating the surface re-condensation phenomenon after cooling. In addition, it can also be found that the lower the power, the more severe the recast defects near the outlet. This is because the decrease in power leads to a decrease  
in instantaneous power density, thereby enhancing the thermal effect and removing the material in a molten state. This corresponds to the observed deterioration in low-power outlet morphology in the previous section. Another phenomenon that needs to be noted is that the inlet diameters of the micro-holes are the same under three different laser powers, but the outlet diameter of the micro-hole increases under low power, such as 7 W, which is related to the melting removal state of the material near the outlet under low power.