# Analysis of Building Platform Inhomogeneities in PBF-LB/M Process on Alloy 718
## 3 Results and Discussion
### Porosity Analysis  
Once the Track's stability analysis is completed, the specimens are prepared to perform micrographic analysis. Each specimen is cut, embedded in conductive resin, and polished, using a Strues Secotom-20, Struers CitoPress-30, and Struers Tegramin-30 machines (Struers, Ballerup, Denmark). Figure 5 shows the box plots of porosity percentage as a function of differential pressure, position and exposed area for the specimens analyzed that are listed in Table 1.  
From Figure 6 it is possible to identify for each tested parameter a distinct and clear main effect by analyzing the percentage of Porosity. As the Differential Pressure is reduced, the material density decreases while moving from the left to the right side of the building platform. On the other hand, reducing the Area Exposed, the material density increases. By evaluating the effect of each parameter individually, it is possible to state that the bottom left region of the building platform in the EOS-M290 machine is a critical zone to print a fully dense material considering Alloy 718. This criticality is primarily caused by the reduction in the laminarity of the shielding gas flow, which in turn leads to turbulent flows. This results in the improper removal of by-products and spatter particles from the powder bed during the melting process. In addition, it is evident that the production of spatter particles is strongly correlated to the size of the Exposed Area on the building platform, meaning that, as the Exposed Area is increased, the higher the formation and concentration of spatter particles on the powder bed become, which hinders the printing of a fully dense material. This is because the spatter particles that are not removed by the recoater and gas flow will accumulate above the powder bed, as shown in Figure 7a, and tend to be melted along with the powder bed in the subsequent layer. Figure 7b shows the SEM image of the powder collected in Zone 2 of Figure 7a. Numerous particles exhibit different morphologies compared to the size distribution of the virgin powder (range between 20 and 63 microns). This morphology can be classified as type 2 sputter particles [52]. The presence of spatter particles may lead to a lack of fusion defects due to a reduced absorption of energy input from the laser source, as indicated in Figure 7c, resulting in an increase in porosity. On the other hand, it is important to highlight by increasing the value of the differential pressure, and consequently the velocity of the gas flow, the removal of spatter particles from the powder bed is enhanced, thus improving the density of the printed material and reducing the presence of defects. Despite all the above-mentioned advantages, care must be taken to avoid any powder denudation effects in other areas of the building platform due to the increased gas flow velocity. Figure 8 shows the interaction between parameters for porosity formation in the material. From Figure 8, it can be inferred that the interactions between the Exposed Area, Position, and Differential Pressure are either low or zero. No interaction is evident between  
Figure 6: Overview of main effect of each tested parameter on porosity formation. the Exposed Area and Differential Pressure. In contrast, a low interaction level is registered between the Position and Exposed Area for some combinations and between the Position and Differential Pressure (i.e., Position 4 and Exposed Area of 14% and 26% and Differential Pressure Nominal and Position 4/5). This result is of utmost interest as it allows each parameter to be optimized separately without influencing the others. Figure 9 illustrates the contour plot to further analyze the effect of Porosity Formation in percentages measured as a function of Exposed Area, Position, and Differential Pressure. The contour plot shown in Figure 9 demonstrates in terms of area how the high-porosity zone (indicated in dark green) progressively decreases as the differential pressure increases.