## 3 Results and Discussion  
Figure 1 concisely summarizes the results of the porosity quantification based on neutron microtomography and shows the 3D renderings of both samples, revealing the sample shape and the spatial distribution of the closed pores (in blue) larger than 10 micrometers in equivalent diameter. The pore size distributions of the closed pores differ significantly between the two samples, and for the large pores (larger than 10 micrometers), the fractions are 0.012% and 0.047%, respectively. Higher laser power and lower hatching distance used in the manufacturing process of Sample 2 increased the heat accumulation [41] and, as a result, the amount of sintered powder at the border of the samples. Moreover, in the crosshatch scanning strategy, which is used for Sample 2, the laser passes two times in each layer, in two directions perpendicular to each other, which also increases the total heat input and leads to the formation of large fractions of sintered powders at the sample edges. By contrast, in Sample 1, using optimal process parameters and a parallel scanning strategy, the fraction of sintered powder at the sample surface is reduced. These surface features explain the relatively large difference between the total volumes of Samples 1 and 2. The investigated samples have been printed with an in-house customized L-PBF machine that is not equipped with contour correction algorithms. Such options exist in some machines, and they may improve the sample quality (e.g., its final shape) at the borders and edges [42]. Figure 2 shows optical microscopy images acquired after neutron microtomography, following the mechanical sectioning of the samples (Figure 2b,e). Juxtaposed are the corresponding sections from the 3D neutron microtomography reconstructions (Figure 2a,d). Figure 1: 3D renderings of the L-PBF-printed red-gold samples revealing sample shapes and their closed porosities in blue. For each sample, the number of pores >10 μm (all pores which are larger than 10 μm) and their volume relative to the total sample volume has been quantified. The location of the corresponding section in the neutron tomographic datasets is difficult to precisely identify for several reasons: (a) the real metallographic sections may not necessarily be planar, (b) the preparation of the metallographic section may lead to relocation of some parts of the sample, in particular, within the sintered zones at the edges of the sample. Despite this fact, the image comparison indicates a very good match between 2D optical microscopy and 3D neutron microtomography cross-sections. Figure 2c,f show the pore size distribution based on the neutron microtomography (3D) and optical microscopy (2D). The neutron tomography data reveals the following. The number of individual pore entities in Sample 2 is almost two-times larger compared to Sample 1, with a considerable number of large pores (larger than 30 micrometers). The pore size distribution indicates that almost 88% of the total porosity in Sample 1 is composed of pores smaller than 15 \(\upmu\)m (critical pore size for watch and jewelry industry), which are generally due to entrapped gas in powder particles [43]. There are only six pores in the entire sample volume with an equivalent diameter larger than 30 \(\upmu\)m. On the other hand, Sample 2 exhibits a larger number of pores, in particular, also with increased sizes (90 pores larger than 30 \(\upmu\)m), and only 55% of the total porosity corresponds to pores smaller than 15 \(\upmu\)m. These results show a significant improvement in the choice of the optimum process parameters for printing red-gold samples and highlight the effect of the scanning strategy on the final quality of the printed samples. Although the initial porosity entrapped in powder particles cannot fully account for the total amount of porosities in the printed samples, increasing powder quality would further improve density, e.g., by using the plasma rotating electrode process (PREP) [43; 44].