# Research on Microhole Processing Technology Based on the Femtosecond-Laser Spiral Trepanning Method
## 3 Results and Discussion
The microhole was relatively round, but there was a certain deviation at the microhole exit (Figure 6a,b), which was mainly due to the influence of slag discharge on the microhole at the exit. We observed the microstructure of the inner wall of the microholes to further understand the processing quality of the microholes. As shown in Figure 6c, no obvious metal residue and molten substance formation occurred at the top, center, and bottom of the inner wall surface after femtosecond-laser processing. Moreover, no obvious processing traces occurred on the inner wall after laser processing. In addition, the mass ratios of different elements at the top, center, and bottom of the microhole cross section are presented in Table 3. The mass ratios of carbon and oxygen decreased from the microhole top to bottom; this may be because the stainless steel surface temperature was relatively low at the beginning. When the laser suddenly heats up, the oxygen in the air will quickly react with the high-temperature stainless steel surface to produce black oxide. With the increase in the processing depth, the stainless steel surface temperature tends to be stable. The oxygen and carbon dioxide contents in the microhole were also relatively low and thus could not easily oxidize the high-temperature stainless steel surface. Therefore, with an increase in the microhole depth, the carbon and oxygen contents on the microhole inner wall gradually decreased. The entrance and exit of the micro-hole with a taper of 3.09\({}^{\circ}\) are displayed in Figure 7a,b. The entrance and exit surfaces were slippery, but black oxide existed close to the microhole owing to the laser processing the stainless steel. The microhole cross section (Figure 7c) shows that the microhole was not a full normal cone hole. The walls of the microhole top and center were vertical, and the taper was gradually formed at the bottom. This was because the energy gradually accumulated to the threshold value of stainless steel at the microhole top and center. However, the microhole bottom was \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Taper** & \multicolumn{3}{c}{**0.03\({}^{*}\)**} & \multicolumn{3}{c}{**3.09\({}^{*}\)**} \\ \hline
**Scanning** & **Radius Angle** & **Number of** & **Power (\%)** & **Radius Angle** & **Number of** & **Power (\%)** \\
**Parameters** & **Increment** & **Turns** & **Increment** & **Turns** & **Power (\%)** \\ \hline Lap 1 & 2 & 2 & 50 & 0.2 & 4 & 55 \\ \hline Lap 2 & 1 & 6 & 55 & 0.3 & 6 & 55 \\ \hline Lap 3 & 0.3 & 11 & 60 & 0.6 & 8 & 55 \\ \hline Lap 4 & 0.1 & 14 & 65 & 0.8 & 10 & 55 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Scanning parameters of femtosecond-laser with different tapers. Figure 5: Tapers of the inverted cone under different feeding layers: (**a**) 2, (**b**) 4, (**c**) 10, and (**d**) 50.  
tapered, because the laser energy inside the microhole was higher than that outside, and the energy penetrated the stainless steel without removing the outer material of the microhole. Figure 7 shows that the microhole top, center, and bottom had some impurities covering the inner wall; however, the inner wall was smooth where there were no impurities. Thus, we infer the impurities were sputtered material on the inner wall, not molten material of the inner wall. Furthermore, the mass ratios of different elements at the top, center, and bottom of the microhole cross section are presented in Table 4. The mass ratios of carbon and oxygen gradually decreased. One possible reason for this corresponds to the explanation of Figure 6c. The other reason is that the  
Figure 6: Morphology of microholes with a taper angle of 0.03\({}^{\circ}\): (**a**) microhole entrance; (**b**) microhole exit; (**c**) microhole cross section.