## 2 Materials and Methods
### Closed Impeller Manufactured by Means of AM  
The same closed impeller model was fabricated by means of AM. An important pre-processing step was to add machining allowance to the closed impeller model to obtain the specific dimensional and geometrical tolerances and the clearance between the closed impeller and other components (0.1-0.15 mm), with input from the authors' previous experience [24]. Thus, a 1 mm machining allowance was added to all the exterior areas of the closed impeller. The printing process was performed using a Sisma SLM MYSINT 100 machine (Sisma, Piovene Rocchette, Italy), with aluminum, titanium, and Inconel 625, as these materials were available for the mentioned printing machine and they are also compatible with the HFE 7200 working fluid. It is acknowledged that an impeller additively manufactured from AISI316 would have been more representative; however, it was not available from the printing machine. Instead, Inconel 625 was chosen, as its roughness after printing is close to that of AISI316. The production time for one of the three closed impellers was between 14 h and 16 h. Aluminum impellers required less time, while the Inconel impeller took the highest amount of time to produce. However, as two impellers were made on each printing job, the final  
Figure 5: Manufacturing steps for the closed impeller by means of classical machining. Figure 6: Dimensional control after the welding process and final machining. production time was 18 h for the aluminum impellers, 21 h for the titanium impellers, and 26 h for the Inconel impellers. The printed closed impellers required different post-processing operations prior to being evaluated in terms of hydraulic performances and compared to the classical fabricated one. The three printed closed impellers are shown in Figure 7. The support material was removed by means of classical machining (turning). The post-processing time required for this operation was 4 to 6 h, depending on the impeller material. The visual inspection of the printed closed impellers showed no geometrical defects and no cross area that may affect the functionality of the part (and of the pump in the long term), as indicated in Figure 8. Dimensional analysis of the as-printed impellers was not required at this step, as the exterior areas of the parts still needed to be machined to their final dimensions. It should be noted that the metallic as-printed parts can have higher dimensional tolerances for the exterior areas (up to +0.5 mm), depending on the material allowance and the geometry. An average of \(\pm\)0.2 mm deviation of the as-printed closed impellers from the CAD model was found after the dimensional control. Dimensional control of the AM closed impellers after being cross-sectioned is presented in Figure 9. A \(\pm\)0.07 mm blade profile deviation was found (nominal profile deviation was \(\pm\)0.1 mm), indicating that the AM process was stable in terms of geometrical positioning. Blade thickness measurements were performed by means of a calibrated caliper. The nominal blade thickness was 1.5 mm, while for the AM closed impellers, the mean thickness was found at 1.42 mm for the aluminum closed impeller, 1.41 mm for the titanium closed impeller, and 1.37 mm for the Inconel 625 closed impeller. As the main topic of this paper is the comparison of hydraulic performance between the two fabrication methods, both the roughness and thickness influence were considered for the final evaluation on the test bench. Roughness variations of the surface of the cross-sectioned area can also be seen in the analysis in Figure 9. The aluminum and titanium closed impellers presented almost the same roughness characteristic, shown in the red areas, with the highest roughness being in the Z building direction (lowest and highest points on the building plate). Although the Inconel 625 closed impeller showed more constant roughness, in terms of surface discretization after CMM inspection, it presented the highest roughness.