# Laser Finishing of Ti6 Al4V Additive Manufactured Parts by Electron Beam Melting
## 2 Materials, Equipment, and Methods
### Experimental Procedures  
The work was divided into two phases. In the first phase, preliminary tests were performed aimed at identifying the laser parameters (i.e., the control factors) to allow the treatment (i.e., the process windows) and the levels adopted in the second phase in which a 2 \(\times\) 3 factorial plan was defined and tested. It is known that the laser energy density (i.e., the so called fluence) affects the interaction between laser and material. When the density increases, it is possible to pass from heating up to melting or even vaporizing the material. In other words, the fluence affects the amount of treated surface, the final roughness, and the heat affected zone (HAZ). The adopted solution to decrease the energy density is to vary the focal distance. At the increase of the focal distance, an increase of the laser footprint/track on the surface of the specimens occurs, and accordingly, a decrease of the fluence. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Characteristics** & **Units** & **Value** \\ \hline Source type & – & Yb-YAG \\ Wavelength & nm & 1070 \\ M \({}^{2}\) factor & – & 5.87 \\ Regime & – & CW/QCW \\ Maximal average power & W & 450 (CW/QCW) \\ Maximal peak power & W & 4500 (in QCW) \\ Pulse duration & ms & 0.05–50 ms (in QCW) \\ Focal length & mm & 100 \\ Dedared focal spot & \(\upmu\)m & \(\approx\)60 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main laser characteristics. Abbreviations: YAG: yttrium aluminum garnet; CW: continuous wave; QCW: quasi continuous wave. Figure 1: Experimental setup for laser finishing: (**a**) real image; (**b**) schematic view. Preliminary tests were carried out keeping the laser radiation on the samples at maximum power (i.e., 450 W) with three different focal distances (+5, +7.5, and +10 mm) on the surface of the samples during rotation. In the second phase, a 2 \(\times\) 3 factorial plan was defined and tested based on the results of the first phase. Table 3 shows the control factors and their levels. The plan was repeated three times; thus, a total of 18 tests were performed. The influence of the process parameters on the roughness of the samples was assessed via analysis of variance (ANOVA). The main effect and the interaction plots were plotted to see the effect of the control factors on the laser finishing. In order to assess the quality of the treatment, the surface morphology of as-built and treated samples were studied by means of a digital microscope (Hirox KH 8700). In addition, roughness measurements were performed on all samples using a 3D surface profiling system (Taylor Hobson Tayssurf CLI 2000). More specifically, in order to characterize the surface topography [24], the amplitude roughness parameters were calculated on the acquired profile: the arithmetic mean surface roughness (Ra) and the root mean squared surface roughness (Rz), skewness (Rsk), and kurtosis (Rku) were calculated in height; the mean width of profile elements (RSm) was calculated horizontally. In addition, the root mean square slope (RDq), defined as a hybrid parameter (i.e., a combination of amplitude and spacing), was calculated. The latter parameter was studied as it is known to influence tribological properties. For what concerns roughness analysis, the cut-off and evaluation length were chosen as foreseen by the UNI EN ISO 4288:2000 standard reported in Table 4. Accordingly, for the as-built sample, the cut-off was set at 8, while for the finished samples, when Ra was in the range 2-10 \(\upmu\)m, it was set at 2.5. In addition, 3D maps of 4 \(\times\) 4 mm\({}^{2}\) areas of the samples were also acquired using a resolution of 2 \(\upmu\)m along both directions. After the acquisitions, the samples were cut with a diamond saw, embedded in an epoxy resin and polished. Then, micro-indentation tests (CSM Instruments Micro-Combi) were performed on the cross-sections, using a load of 20 N, starting at 0.75 mm from the edge up to the center of the samples with a step of 0.2 mm.