## 5 Third Phase: Case Studies
### Case Study 1: Impeller  
The first case study involved the Ti6 Al4V impeller shown in Figure 12a. In order to understand how the processing time affects the surface conditioning, the six blades were shielded by steel elements, locally avoiding the BF action (Figure 12b). One by one, the shields were removed, and the surfaces were measured at different BF working times. The zones were selected according to the different geometrical features shown in Figure 12c: the external side of the turbine (A), the internal surface of the inner cylinder (B), the top surface of the blade (C), the top blade surface (D), and the bottom blade surface (E). The measurements were taken after 6, 12, 24, 48, and 96 h of BF operation in order to understand the trend with processing time. At the beginning, the component had a roughness dependent upon the local stratification angle. A and C were vertical walls with a starting roughness of about 14 \(\upmu\)m; D was inclined and showed an Ra of about 13 \(\upmu\)m; B was horizontal and had the lowest value of about 5.5 \(\upmu\)m. Figure 13 shows the feature trends as a function of the working time. Feature A was external and marked Ra was observed, as expected. At 12 h, the Ra was reduced by 67%, and, at 24 h, the Ra was \(2.05\pm 0.9\)\(\upmu\)m. At 48 h, the Ra decreased below 1 \(\upmu\)m. Feature B was an internal hole 30 mm in diameter; a lower BF action was expected since the big media were 20 mm in diameter. At 12 h, the Ra showed a reduction of only 49%, and, after 96 h, the surface reached an Ra of about 5 \(\upmu\)m. This descending speed was similar to that observed for small media and/or small \(Ba\) number; the hole slowed down the media and reduced the big media hits inside. Feature C showed a roughness reduction of more than 80% at 12 h of working time, and, after 24 h, it was reduced below 1 \(\upmu\)m. Feature D was an inclined surface and a partially confined zone; in this case, an 80% reduction was obtained after 48 h, with an Ra of about 3 \(\upmu\)m. At 96 h, the Ra was lowered to 2 \(\upmu\)m. Feature E was an inclined overhanging surface, which, as expected, showed the higher Ra value. The trend was similar to feature D; after 48 h, the Ra was about 3.5 \(\upmu\)m, and, after 96 h, the Ra was 2.5 \(\upmu\)m, corresponding to an 85% reduction. All curves show that the difference in Ra decreased with time. Notwithstanding this finding, the behavior did not follow a first-order negative exponential function. It can be assessed that, within 96 h, the Ra difference was less than 10%, highlighting the presence of an Ra threshold nearby. Figure 12: Selective laser melting (SLM) Ti6 Al4V impeller (**a**), shielded blades (**b**), and measured zones (**c**). The profile analysis confirmed the previous observations. As shown in Figure 14a, feature A before BF operation was characterized by a pseudo-Gaussian profile (Rsk and Rku paired to 0.57 and 2.77, respectively), an Rt of 100 \(\upmu\)m, and an Ra of 15.5 \(\upmu\)m. After 48 h, the valleys were almost removed with a dramatic reduction of Ra and Rt to 0.19 \(\upmu\)m and 1.2 \(\upmu\)m, respectively. Moreover, the RDq moved from 0.73 to 0.03, which was a good indicator of the surface reflectance. By calculating and aligning the AFCs, Figure 14b was obtained. It is evident that the BF mechanism was peak cutting, suggesting that the process was correctly undertaken. In the case of feature B (Figure 14c), the starting profile was similar to the previous one (both surfaces were vertical walls); however, after 48 h, the valleys were still pronounced. The Rsk was negative and Rt was more than 50 \(\upmu\)m. The AFCs (Figure 14d) showed a slow action on the peaks, which were rounded at the beginning and then partially cut away. The horizontal feature C started with an Ra of 5.5 \(\upmu\)m, and, after 48 h, the Ra decrease was impressive; however, the negative Rsk and the high Rku indicate that some scratches were still present (Figure 14e). The aligned cumulative distributions in Figure 14f show good peak chipping.