## 3 Results and Discussion
### Characterization of the BJ Impeller  
After casting and finishing, the BJ impeller was subjected to a series of dimensional and mechanical tests (Table 3). The finished part had a weight of 4.03 kg and a Brinell hardness of 183 as compared with the 212 of the original KSB impeller. The results of two-plane impeller dynamic balancing were within the permissible unbalance calculated according to ISO1940/1 for grade G 2.5 at an operating speed of 3000 rpm.  
Figure 4: Workflow employed for the fabrication of the BJ impeller.  
Figure 5: Impeller digitizing and design: (**a**) 3D scan with curves, (**b**) solid surface view, (**c,d**) PA12 communication model.  
Figure 6: (**a**) Transparent and (**b**) cross-section 3D views of the sand moulds. (**c**) Model views of the impeller fills at the initial (**left**) and final (**right**) moments. (**d**) Photographs of the manufactured 3D sand moulds indicating the different parts.  
Figure 8: View of the final BJ impellers manufactured before (**a**) and after (**b**) machining finish. (**c**) Original KSB impeller.  
Figure 7: View of the impellers after casting, cooling, and mould separation: (**a**) top view, (**b**) downside view.  
Table 4 shows the results of the dimensional characterization of the BJ impeller and a comparison with the dimensions specified by the manufacturer of the KSB impeller. A geometrical precision of \(\pm\)0.1 mm was obtained with standard deviations for each dimension lower than 1%. The highest absolute dimensional discrepancy was obtained for dimension B, corresponding to the outside diameter of the impeller inlet, which may have been due to the finishing operations carried out after casting and removal of the excess materials (although in relative values, the deviation was only 0.28%). These results suggest that the 3D printing of the sand mould and subsequent metal casting are appropriate for generating relatively complex parts with sufficient precision to pass dimensional testing.  
Examination via scanning electron microscope of the surface of a residual portion of casted material revealed the presence of cavities due to metal contraction during the cooling process that followed casting (Figure 9). These shrinkage voids [42] had variable shapes and sizes, from a few \(\upmu\)m to \(\sim\)400 \(\upmu\)m, but accounted only for a very small portion of the material. The \(\upmu\)m diameter of the sample was \(\sim\)400 \(\upmu\)m, and the \(\upmu\)m diameter of the sample was \(\sim\)400 \(\upmu\)m. The \(\upmu\)m diameter of the sample was \(\sim\)400 \(\upmu\)m.  
of the surface (less than 0.5%). Assuming that these discontinuities were also present in the finished BJ impeller, they did not affect its performance (see Section 3.3).  
A metallographic study carried out after thermal treatment gave some hints on its microstructure (Figure 10). Analysis of the resulting images showed a microstructure typical of a casting stainless steel 316 [43], with an austenitic matrix and elongated ferrite grains of 40 \(\pm\) 10 \(\upmu\)m length and 6-8 \(\upmu\)m width. The proportion of the ferritic phase was 6.3%, as suggested by image analysis, and no chromium carbide precipitation was observed [44]. Comparison of the microstructure with a sample of a discarded KSB impeller revealed a similar pattern of a Cr/C-free austenitic matrix with ferrite islands but differing in size (120 \(\pm\) 30 \(\upmu\)m long, 10-15 \(\upmu\)m width) and proportion (8.1%). These variations, although not considerable, indicate a faster cooling rate in the BJ impeller, resulting in the formation of a smaller ferrite precipitate.