## 3 Results and Discussion
### Influences of the Assisting Gas Type on Cutting Quality
#### 3.1.2 Element Compositions of the Cutting Surface  
Tables 4 and 5, respectively, show the chemical composition of the oxygen cuts surface and the argon cuts surface. \"Original\" in the table indicates the laser cuts without any post process, and \"Pickling\" represents the laser cuts processed using acid pickling.  
About 33% oxygen was observed on the original oxygen cuts surface, and the contents of Cr, Co, as well as other elements, had changed. The chemical composition of the argon cuts surfaces showed little change, except for small amounts of oxygen (about 3.43%) and carbon (about 4.03%). The results demonstrated that the oxygen conditions can change the chemical composition of the cuts on the surface, while the argon conditions did not.  
Acid is usually used for eliminating cross formation in the manufacturing process of stents, which was induced by laser cutting. The samples were etched in a solution (HF:HNO\({}_{3}\):H\({}_{2}\)O, 2:15:83) for three hours at room temperature.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Original**} & **Cr** & **Co** & **O** & **Mn** & **Ni** & **Fe** & **W** \\ \cline{2-9}  & 30.11 \% & 21.80 \% & 32.93 \% & 1.61 \% & 2.83 \% & 1.09 \% & 9.63 \% \\ \hline \multirow{2}{*}{**Pickling**} & **Cr** & **Co** & **O** & **Mn** & **Ni** & **Fe** & **W** \\ \cline{2-9}  & 12.64 \% & 53.72 \% & 3.94 \% & 1.29 \% & 11.44 \% & 2.17 \% & 14.80\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Chemical composition on the oxygen cuts surface.  
Figure 3: Surface topography of the sample that was cut with argon: (**a**) macro view; and (**b**) micro view.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Original**} & **Cr** & **Co** & **O** & **C** & **S** & **Mn** & **Ni** & **Fe** & **W** \\ \cline{2-9}  & 19.29 \% & 45.51\% & 3.43\% & 4.03 \% & 0.02\% & 0.91 \% & 10.43\% & 1.31 \% & 14.89 \% \\ \hline \multirow{2}{*}{**Pickling**} & **Cr** & **Co** & **O** & **C** & **S** & **Mn** & **Ni** & **Fe** & **W** \\ \cline{2-9}  & 19.75 \% & 46.73 \% & 4.38 \% & 0 & 0 & 1.55 \% & 9.39 \% & 2.18 \% & 16.01 \% \\ \hline \hline \end{tabular}
\end{table}
Table 5: Chemical composition on the argon cuts surface.  
Figure 2: Surface topography of the sample that was cut with oxygen: (**a**) macro view; and (**b**) micro view.  
Figure 4 shows an image of the oxygen cuts with acid pickling. The particles and voids disappeared, and the surface topography changed compared with the original topography. Figure 5 shows the argon cuts with acid pickling, and the results showed that the acid pickling process had no influences on the surface topography.  
The energy dispersion spectrum (EDS) results of the cuts processed with acid pickling are also shown in Tables 3 and 4. For the oxygen cuts, the content of Cr and oxygen decreased remarkably. For the argon cuts, the elements composition was similar to the original surface. Chromic oxide is easy to dissolve in acid, and it could then be concluded that the particles on the oxygen cuts surface are oxides. Thereby, the dorsal formation on the oxygen cuts was easily removed by the acid pickling process. However, the acid pickling process had a limited influence on the argon cuts surface, and the dorsal formation was not eliminated.