# Research on Microhole Processing Technology Based on the Femtosecond-Laser Spiral Trepanning Method
## 3 Results and Discussion
Therefore, the microholes roundness can be calculated as follows:  
\[\Phi_{1}=\frac{D_{MAX}-D_{MIN}}{2}=\frac{0.3082-0.3012}{2}=0.0035\;\text{mm},\]  
\[\Phi_{2}=\frac{d_{MAX}-d_{MIN}}{2}=\frac{0.3600-0.3364}{2}=0.0118\;\text{mm}.\]  
The microholes taper can be calculated as follows:  
\[\text{C}=\tan\alpha=\frac{\text{D}-\text{d}}{\text{L}}=\frac{0.3524-0.3056}{2 }=0.0234,\]  
\[\alpha=\arctan(0.0234)=1.34^{\circ}. \tag{1}\]  
where \(\Phi_{1}\) and \(\Phi_{2}\) are the roundness at the microhole entrance and exit, respectively; \(\alpha\) is the taper of the inverted cone hole; D is the microhole entrance diameter; d is the microhole exit diameter; and L is the material thickness. The roundness and taper of micro-holes can be obtained by the above formula. It can be found that the entrance had a smaller circularity than the exit; this was mainly because the air pump could blow debris at the microhole entrance and reduce the surface temperature; therefore, the inner hole wall and the exit had a higher amount of debris and temperature than the entrance, which caused the difference in circularity between the entrance and exit. Finally, the microhole taper angle was obtained as 1.34\({}^{\circ}\). To analyze the microhole formation process, the micromorphology of the whole inverted cone hole was observed by adjusting the number of feeding layers during femtosecond-laser processing. The scanning electron microscopy (SEM) images of feeding layers 2, 4, 10 and 50 are shown in Figure 5. With the increase in the feeding layers, the taper of the inverted cone hole was gradually formed. From the path distribution and energy distribution of the spiral scanning, it can be seen that the number density of the inner ring was small, and the laser energy was relatively low; however, the number density of the outer ring was high, and the laser energy was high. It can be observed that when the laser processing started, the inner ring etching trace was relatively shallow, and the outer ring etching depth was relatively deeper. As the depth increased, the depth difference between the inner and outer circles also increased. Moreover, with the increase in the number of feed layers, the hole depth gradually increased, and the energy increased gradually in the outer ring will diffuse to the outer ring. Therefore, the energy of the outer ring presented a gradient change between the initial front processing and the complete processing of the microhole. This change in the energy gradient caused the formation of the microhole taper. In addition, the low laser-scanning speed can result in higher-quality and more symmetrical holes than a high scanning speed [25]. However, low scanning speed results in a low machining efficiency; therefore, it is necessary to improve the scanning speed to meet the requirements of microholes machining. Furthermore, we also explored microholes with different tapers formed under different scanning trajectory parameters. We obtained the scanning parameters of microholes with taper angles of 0.03\({}^{\circ}\) and 3.09\({}^{\circ}\) formed under different scanning paths and power distributions (Table 2). The air pressure is set to 0.3 Pa. The microhole taper size was mainly controlled by changing the laser scanning path and power. In the scanning paths of the normal cone hole, the inner ring was dense and the energy was high, while the outer ring was sparse and the energy was low. In contrast, in the scanning paths of the straight hole and the inverted cone hole, the inner ring was sparse and the energy was small, while the outer ring was dense and the energy was high. The most important thing is that the taper of the microhole is controllable. Besides, compared with the microhole that [PERSON] et al. [22] used femtosecond-laser spiral trepenning processing on, K24 superalloy, the microholes have a high level of precision. The microhole images obtained by the experimental parameters in Table 2 are shown in Figures 6 and 7.