# Laser Finishing of Ti6 Al4V Additive Manufactured Parts by Electron Beam Melting
## 3 Results
### Morphological Analysis  
Figure 4 shows the surface morphology of an as-built sample. As expected, the surface appears very irregular, with high peaks and valleys very close to one other. This is due to the melting of the smaller powder particles near the surface of the component, as observable in Figure 5a,b reporting respectively the cross-section surface and its magnification. From the analysis of the cross-section, it clearly appears that the outline is very jagged and the profilometer inductive stylus, with a 2 \(\upmu\)m radius, cannot penetrate in the very narrow valleys. Thus, the roughness parameters calculated on the acquired profiles of the as-built surface may be underestimated. Figures 6 and 7 depict the cross-sections with their magnifications and the surface morphologies of laser-treated surfaces performed at Fd = 5 mm and Fd = 7.5 mm, respectively. As shown, the surfaces appear very different from the as-built one; the profile of the cross-sections appears smoother, while the peaks seem lower. Three different morphologies can be observed on the external surfaces:  
Figure 4: Surface morphology of an as-built sample. Figure 5: (**a**) Cross-section of as-built sample; (**b**) magnification of the external surface. Figure 3: Contour plot of estimated fluence vs. track width and focal distance. These morphologies are the consequence of the laser spot, rotating, and scan speed. A value that considers these parameters is the circumferential overlap percentage (COP) [25], which is defined as the following ratio:  
\[\text{COP}=1-\ \frac{60\times S\text{s}}{D\times N} \tag{1}\]  
where \(N\), \(D\), and \(S\)s represent the workpiece rotating speed in rpm, the laser spot in mm, and the scan speed in mm/s. Figure 6: (**a,d,g**) Cross-sections, (**b,e,h**) their magnifications, and (**c,f,i**) surface morphologies after laser polishing at different Ss and Fd = 5 mm. Figure 7: (**a,d,g**) Cross-sections, (**b,e,h**) their magnifications, and (**c,f,i**) surface morphologies after laser polishing at different Ss and Fd = 7.5 mm. Then, based on the preliminary results of Table 5, it is possible to calculate the COP for each test condition by replacing the laser footprint at different focal distances (the values are reported in Table 6). The results show that at the minimum focal distance, the COP is in the range 84%-97%. However, in Figure 6i the laser tracks are visible. This is due to the Gaussian shape of the beam: the central part of the laser beam results in material vaporization, while the tails just allow the melting. This effect is not visible in Figure 6c,d because of the low scan speed (i.e., very high COP). To better understand the phenomena occurring during the laser finishing, the treated surfaces were also analyzed by scanning electron microscopy (SEM). Figure 8 depicts the SEM images of the surface after laser polishing at different scan speed (Ss) and Fd. From the image, the lower focal distance (Fd = 5 mm) allows obtaining the smoothest surfaces by vaporizing the higher peaks and melting the greater ones. On the contrary, at a higher focal distance (Fd = 7.5 mm), the higher peaks are just melted and thus the surface appears more irregular. Consequently, at a lower focal distance, a higher diameter reduction of the sample is expected. In Figure 9, the sample's diameter and its ablation are reported vs. the laser parameters. The figure represents a confirmation of the aforementioned mechanism (i.e., higher ablation at Fd = 5 mm). \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Ss (mm/min)**} & \multicolumn{2}{c}{**Circumferential Overlap Percentage (\%)**} \\ \cline{2-3}  & **Fd = 5 mm** & **Fd = 7.5 mm** \\ \hline
3.6 & 97 & 98 \\
9 & 92 & 94 \\
18 & 84 & 88 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Circumferential overlap percentage (COP) at different scan speed and focal distance. Figure 8: Scanning electron microscopy (SEM) images of the surface after laser polishing at different Ss and Fd. Figure 9: Samples diameter and diameter ablation vs. laser parameters.