## 4 Second Phase of Experimentation: Results and Discussion
Figure 8: Ti6 Al4V roughness profiles before BF conditioning (**a**,**e**) and at different working times by using parameters set 1 with charge 1 (**b**–**d**) and charge 3 (**f**–**h**). The AFC allows graphing the cumulative distribution of the profile height. In Figure 10, this outcome is shown for the original profile (Figure 10a) and after processing for 24 h (Figure 10b). Under the assumption that the valley shape is not affected by the BF conditioning, the curves were aligned according to the procedure described in Reference [35]. Figure 10c shows the outcome; approximately 20% of the curves overlapped and more than 50% were almost horizontal. This demonstrates that a mechanism of peak cutting without valley modification took place. Moreover, the area between curves represents the area removed by BF operation. The value was 15,990 \(\upmu\)m\({}^{2}\)/mm, equivalent to 72 \(\upmu\)g/mm\({}^{3}\), corresponding to the amount of Ti6 Al4V removed in 24 h.  
This calculation was repeated five times for each processing condition and for each charge. The overall outcome is reported in the box-and-whiskers charts in Figure 11.  
It is evident that charge 3 was characterized by a low material removal rate for all BF parameter sets. Charge 1 and 2 had similar behavior; by engaging parameter sets 3 and 4, which correspond to regimes characterized by low \(Ba\), the material removal rates were 40 \(\upmu\)g/mm\({}^{3}\) and 80 \(\upmu\)g/mm\({}^{3}\) for surfaces inclined at 0\({}^{\circ}\) and 45\({}^{\circ}\)/90\({}^{\circ}\), respectively. Sets 1 and 2 allowed achieving the best results for these charges, and two observations can be provided. Firstly, charge 2, which was composed of mixed media, had the possibility to access small holes and, thus, it was preferred to charge 1. Secondly, set 2 was characterized by a higher filling percentage than set 1. A higher filling percentage results in greater active layer thickness; hence, big components are processed faster in relatively small barrel machines. Thus, charge 2 and BF parameter set 2 were chosen in this work. Figure 9: Roughness profiles before and after BF operation with charge 1 and parameter set 3 for Ti6 Al4V vertical surfaces (**a**,**b**), for Ti6 Al4V horizontal surface with parameter set 1 (**c**,**d**), for Inconel718 vertical surfaces with parameter set 1 (**e**,**f**). Figure 11: Box-and-whiskers charts of the material removal rate (MRR) for different surface inclinations (0\({}^{\circ}\), 45\({}^{\circ}\), 90\({}^{\circ}\)), using different BF process parameter sets (1, 2, 3, 4) and charge types (1, 2, 3), for Ti6 AL4V (**a**) and Inconel718 (**b**). Figure 10: Roughness profile and AFC for a Ti6 AL4V specimen: original profile (**a**), same surface after 24h BF (**b**), aligned profiles (**c**). In the case of Inconel718, the trends were very similar; charge 3 and/or small \(Ba\) were not able to provide adequate speed; sets 1 and 2 with charges 1 and 2 allowed good performance, albeit slightly lower than the Ti6 Al4V case. This was probably related to the lower initial roughness of the parts made by this material. Hence, both materials were processed by employing charge 2 and BF processing set 2.