## 3 Results and Discussion
### Analysis of the Influencing Factors of Inlet and Outlet Morphology  
When the laser power and repetition frequency are constant, namely, the laser parameters are fixed, the influence of the helical drilling system parameters on the inlet morphology of the micro-hole is as shown in Figure 6. As shown in Figure 6, the inlet diameter gradually increases as the deflection angle of the wedge prism increases, but it is not a linear relationship. This is consistent with the helical drilling principle introduced in Section 2. In fact, the diameter of the hole inlet is also related to another factor, which is the distance at which the focus spot moves down during the drilling process. In order to completely cut off the material at the outlet of the micro-hole on the thick plate, the  
Figure 5: The inherent errors of helical drilling system. (**a**) Light spots at different positions along the rotation trajectory. (**b**) Cutting path when the light spot rotates at a high speed. focus spot must be lowered to the position approximately one Rayleigh length from the outlet. When the focus spot shifts downwards, the size of the hole inlet will increase, and the increase extent is related to the axial light intensity distribution of the Gaussian spot. In order to study the effects of helical drilling system parameters on the size of the micro-hole, the downward distance of the focus spot was set as 0.43 mm, according to the thickness of the processed plate and the Rayleigh length of the laser. In Figure 6, although the inlet sizes are different, the edge morphology of all micro-holes is similar. There is always a certain degree of roundness error, and the error direction is also the same. This can prove that roundness error is mainly caused by mechanical rotation system errors and spot non-uniformity errors, and the reasons have been discussed in Section 3.1. In addition, it can be observed that there are obvious discoloration regions at the edges of the hole inlets with different wedge prism deflection angles, but the width difference of the discoloration regions is not dramatic. This is mainly because the laser mean power density at the inlet of each micro-hole is theoretically the same, if the move-down distance of the focus spot is fixed, and the morphology differences of focal spots with different wedge prism deflection angles are ignored. The discoloration zone is actually composed of two parts. One part is the heat affected zone, and the other part is the surface oxidation zone, as shown in the blue area in Figure 6. Surface oxidation is an inevitable defect in the air environment for ablation processing methods, but it can be eliminated by using inert gas protection. The outlet of the processed hole is a more important feature than the inlet, because, from the perspective of practical application of film cooling holes, the outlet of laser drilling is exactly the inlet of cooling airflow, and the flowing state of airflow inside the hole is directly affected by the shape of the inlet. In addition, outlets are also most susceptible to process factors such as blowing pressure, slag discharge conditions, depth to diameter ratio,  
Figure 6: Inlet and outlet morphology of high-temperature alloy hole processed with different deflection angles of wedge prism. etc. The influencing factors of outlet morphology should be analyzed in detail. In Figure 6, the diameter of the outlet increases with the increasing of the wedge prism deflection angle, and there is also an oxidation discoloration area around the outlet, which is consistent with the inlet. When the deflection angle is less than \(10^{\circ}\), it can be clearly observed that the cutting quality of the outlet edge is significantly better than that of the inlet edge. The outlets have clear edges, good roundness, and few notches. This is mainly because when the Z-axis descends to complete the drilling, the laser power density at the inlet gradually decreases, although it is all above the material's ablation threshold.