## 2 Materials and Methods
### Closed Impeller Manufactured by Means of AM
The roughness of the halved sections of the closed impellers was measured using the same  
Figure 8: Cross-sectioned closed impellers for visual inspection and dimensional control. Figure 7: Closed impellers manufactured with different materials, before and after exterior machining. Mars Surf apparatus. A mean roughness was found at 7.45 Ra for the aluminum impeller, 8.12 Ra for the titanium impeller, and 14.42 Ra for the Inconel 625 impeller. The X-ray CT scan was performed for all AM closed impellers to identify any potential internal defects that could not be found using visual inspection. The analysis was carried out using Micro-CT System Diondo d2 (Diondo, Hattingen, Germany) equipment. The CT scan was performed with a low number of voxels (0.041 mm voxel size in all directions) so that any inclusions or defects could be identified. The Inconel 625 closed impeller was the only AM part that showed small voids, varying from 0.08 mm to 0.09 mm, measured using MyVGL 2.2 software, as shown in Figure 10.  
Figure 10: X-ray CT scan for the Inconel 625 closed impeller. Figure 6:0 Dimensional control for the three AM closed impellers after cross-sectioning—disc with hub. Due to their low size and number, these pores were considered superficial (i.e., scratches of 0.07 mm depth, 2.2 mm length, and 0.16 mm wide, pores < 0.1 mm), and it can be stated that they did not impose any risk on the mechanical or physical properties of the part. The machining (turning operation) was performed on the same CNC horizontal lathe machine as the classical machined closed impeller. The AM closed impellers ready for testing are shown in Figure 11, together with their weight (compared to the classical one). Roughness measurements were performed on the external surfaces, and the values were between 0.8 Ra and 1.6 Ra, within the nominal values imposed by the pump assembly configuration (maximum 1.6 Ra). Dimensional control was also performed on the machined impellers to verify the dimensional and geometrical accuracies (Figure 12). The aluminum closed impeller exhibited the highest dimensional fidelity (\(-\)0.054 mm to +0.052) compared to the other AM closed impellers, followed by the titanium closed impeller (\(-\)0.063 mm to +0.059) and Inconel 625 closed impeller (\(-\)0.068 mm to +0.067). Although the dimensional accuracies were within the imposed tolerance of \(\pm\)0.1 mm, the differences may have been caused by the work regime for each of the impellers (lathe speed and feed). A non-destructive test (NDT) method using dye penetrant inspection (DPI) was performed to assure there were no defects after machining. This procedure was applied on all closed impellers (fabricated by means of AM and classical machining). The examination considered three solutions: MR71 cleaner (a mixture of solvents based on ketone to clean the surfaces and to remove any traces of debris), MR 98 NF Dye Penetrant (left for a 10 min soak time to fill into cracks and voids), and MR 70 Developer (allowed 15 min to exhibit any defects/flaws). After a close-up evaluation, it was found that the none of the closed impellers presented any flaws. A balancing class of G2.5 was identified according to [25]. Balancing operations followed one-plane correction analysis at speeds of 2200 rot/min. The identified unbalances were below the permissible residual unbalance value of 0.1 g\(\cdot\)mm [25], and the highest unbalance value was obtained for the Inconel 625 closed impeller at 0.075 g\(\cdot\)mm. Figure 6:1 AM impellers for evaluation and comparison of hydraulic performance. Figure 6:2 AM closed impellers for evaluation and comparison of hydraulic performance.