## 3 Results and Discussion
### Analysis of the Hole Wall Recast Layer
The reduced power further leads to the enhancement of the heat effect near the outlet, and the liquid removal mechanism will cause the outlet to expand and reduce the orifice roundness. Therefore, it can be seen that low laser power not only leads to the deterioration in the morphology of the outlet but also affects the dimensional accuracy of the outlet. Therefore, it is advisable to avoid drilling gas film holes at low power as much as possible. As shown in Figure 11, it can also be observed that the roughness of the side wall is relatively large under high laser repetition frequency, which has a serious impact on the airflow state inside the gas film holes. Figure 12 shows the drilling results with the laser repetition frequency of 100 kHz and laser power of 10 w. As shown in Figure 12, reducing the repetition frequency can improve the roughness of the hole wall surface and change the recast layer distribution on the sidewall surface. The higher repetition frequency corresponds to the larger thermal effect, and the surface recast layer undergoes multiple melting and solidification processes, resulting in the increase of surface roughness, as shown in Figure 12b,b1. Although the recast layer still exists at the repetition frequency of 100 KHz, the continuity of the recast layer is good, and the surface of the recast layer is relatively flat, with almost no repeated processes of re-solidification after multiple melting; thus, the surface of the hole wall is relatively smooth, as shown in Figure 12a,a1. This is all attributed to the higher instantaneous power density of the laser corresponding to low repetition frequency and the consequent rapid material removal by evaporation with lower thermal effects. Figure 11: Cross-section morphology of air film holes processed with different laser powers. The outlet surfaces of the micro-holes corresponding to Figures 10(c) and 11(a),b were polished, and the thickness of the recast layer was measured by SEM; the results are shown in Figure 13. It can be observed that as the repetition frequency increases, the outlet edge begins to become irregular, with notches appearing and morphology deteriorating. This is consistent with the results observed by an optical microscope in the unpolished state of Section 3.2. The thickness of recast layers corresponding to 100 kHz and 200 KHz are about 2 \(\upmu\)m-3 \(\upmu\)m, but the thickness of recast layer corresponding to 400 KHz is locally greater than 20 \(\upmu\)m. And a large range of heat-affected zones and micro-cracks embedded in the matrix were observed; the width of the heat-affected zone is 20 \(\upmu\)m. From this, it can be seen that although the higher repetition frequency can improve the surface morphology of the inlet, they will reduce the instantaneous power density of the laser and result in significant thermal effects, then cause thicker recast layers and other substrate defects, which are extremely detrimental to the performance of the blade film cooling holes. Therefore, considering the thickness of the recast layer and the probability of defects such as cracks appearing, low repetition frequency should be given priority in actual drilling. Due to the presence of recast layer less than 5 \(\upmu\)m being allowed in practical application [25], the efficient one-step method has a large application potential in blade production. Meanwhile, the processing times in the experiments above are all between 3 and 5 s, resulting in at least twice the efficiency improvement as the multi-step method. The local three-dimensional morphology of the sidewalls at 100 kHz and 400 kHz was measured by white light interferometer, and the results are shown in Figure 14. The 3D surface roughness Sa corresponding to low repetition frequency is about 0.438 \(\upmu\)m, which is significantly smaller than that corresponding to high repetition frequency; the latter is about 0.773 \(\upmu\)m. This is consistent with the intuitive perception obtained from SEM images.