## 3 Results and Discussion
### Analysis of the Influencing Factors of Inlet and Outlet Morphology
That is, the inlet goes through a long ablation process, and the orifice size increases from small to large. But the outlet edge is cut under a certain power density without obvious change, so the thermal ablation time is relatively short. Meanwhile, when the outlet edge is cut, the slag-removal channel has already been opened, which is another reason why the quality of the outlet edge is better than that of the inlet edge. However, when the deflection angle of the wedge-shaped prism is \(10^{\circ}\), it can be observed that the outlet edge begins to become irregular, with obvious notches appearing in a serrated shape and with a large amount of uncut material. The main reason for this phenomenon is that as the prism deflection angle increases, the diameter of the micro-hole increases, and thus, the overlap rate of the laser spot decreases. The decrease of spot overlap rate will lead to a decrease in the mean power density at a certain point along the cutting path, which makes it impossible to achieve complete and quick material removal, and, at last, results in serrated edge defects. It can also be found that the inlet morphology is better than the outlet morphology with the same spot overlap rate when the deflection angle of the wedge-shaped prism is \(10^{\circ}\). This is mainly because the mean power density for the cutting inlet edge is far higher than that for the outlet, considering the set of the Z-axis downward distance. Although the inlet mean power density also decreases with the decrease of the spot overlap rate, it is still above the material's ablation threshold, so the inlet morphology is not significantly affected by the spot overlap rate. According to the discussion above, the decrease of the mean power density caused by the decrease of the spot overlap rate seems to be the reason for the deterioration of the outlet morphology. To further investigate the effect of the spot overlap rate on the outlet morphology, the deflection angle of the wedge-shaped prism was fixed, and the rotation speed of the Dove prism was changed. The outlet morphology is shown in Figure 7. With the increase of Dove prism rotation speed, notches begin to appear at the outlet edge, and serrated defects gradually become apparent. From Equation (7) and Figure 4, it can be found that as the rotation speed of Dove prism increases, the laser spot overlap rate decreases; namely, the distance between adjacent pulses on the cutting trajectory increases. Because the laser parameters and the diameter of the micro-hole have not changed, the mean power density in a certain cutting area remains unchanged, but its instantaneous power density in the local area decreases. This is the fundamental reason why the material cannot be completely and quickly removed. From the discussion above, it can be seen that the decrease in spot overlap rate and mean power density, which leads to the deterioration of outlet quality, is just superficial. The fundamental reason is the decrease of instantaneous power density at a specific processing area. The instantaneous power density is a key factor in improving the quality of the outlet edge. The formula for calculating the instantaneous power is  
\[\mathrm{P_{ins}}=\mathrm{P}/\left(\mathrm{L}*\mathrm{V}\right) \tag{8}\]  
where \(V\) is the rotation speed of the Dove prism, \(L\) is the circumference of the cutting track, and \(P\) is the laser output power. To verify the effect of instantaneous power density on outlet morphology, the rotation speed of the hollow motor was fixed, and the laser power was changed. Of course, the spot overlap was also fixed under these experimental conditions. The results are shown in Figure 8. As the laser power decreases, the instantaneous power density decreases accordingly, the outlet edge gradually becomes irregular, the roundness deteriorates, and large areas of material not removed appear at the edge of the outlet. When the laser power increases, the outlet edge quality is significantly improved.