# Analysis of Building Platform Inhomogeneities in PBF-LB/M Process on Alloy 718
## 2 Materials and Methods  
All the specimens manufactured and analyzed in this experimental campaign were produced in Alloy 718, one of the most widely used nickel-based alloys in the PBF-LB/M process [47]. The chemical composition, mechanical, and physical properties of Alloy 718 are reported in Tables 1 and 2[20; 48]. The layer thickness used to slice and build the specimens was 40 microns. The particle size distribution (PSD) of the powder, achieved via a gas atomization process, was evaluated in the range between 20 and 63 microns. An EOS Eosint M290 (EOS GmbH, Krailling, Germany) with a single ytterbium fiber laser that had a beam wavelength of 1070 nm, a minimum laser beam spot size of 80 \(\upmu\)m, and a maximum laser power of 400 W was the apparatus utilized for all of the tests. The building platform, with dimensions of 250 mm \(\times\) 250 mm \(\times\) 30 mm, was made with C40 steel and maintains a stable temperature of 80 \({}^{\circ}\)C throughout the printing process. Continuous pumping of argon gas was utilized within the building chamber to ensure that the oxygen content remained below 100 ppm. The aim of this work, as previously mentioned and described in the Introduction section, is to investigate the inhomogeneities in terms of material printability on the building platform as a function of the area exposed and, in turn, the creation of spatter and by-products. According to previous studies carried out on EOS Eosint M-290 machines concerning the gas flow system architecture, the lower part, closed to the outlet nozzle, is identified as the most critical region during the printing phase [49]. All the tested specimens were printed with the process parameters set used in a previous paper [50] for 40 microns of layer thickness (Laser Power: 270 W; Scanning Speed: 960 mm/s; Hatch Distance: 0.11 mm) and with a constant recoater speed. The study focuses on the lower part of the building \begin{table}
\begin{tabular}{c c} \hline
**Element** & **\% Weight** \\ \hline Ni & 55.37 \\ Cr & 18.37 \\ Fe & 17.80 \\ Nb + Ta & 5.34 \\ Nb & 5.33 \\ C & 0.040 \\ Mn & 0.08 \\ Si & 0.08 \\ P & \textless{}0.015 \\ S & 0.002 \\ Co & 0.23 \\ Mo & 3.04 \\ Ti & 0.98 \\ Al & 0.5 \\ B & 0.004 \\ Ta & 0.005 \\ Cu & 0.04 \\ Ca & \textless{}0.01 \\ Mg & \textless{}0.01 \\ Pb & 0.0001 \\ Bi & 0.0001 \\ Se & \textless{}0.001 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of Alloy 718 [20]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Elastic** & **Density** & **Thermal** & **Yield** & **Tensile** & \\
**Modulus** & **(kg/m3)** & **Conductivity (W/mK)** & **Strength** & **Stress** & **Stress** \\
**(Qpa)** & **8470** & **11.2** & **1100** & **1310** & **23.3** \\ \hline \end{tabular}
\end{table} 
Table 2: Main mechanical and physical properties of Alloy 718 processed through PBF-LB/M process [20]. platform because a previous study [49] did not observe evidence of material printability degradation along the vertical direction. As illustrated in Figure 1, a customized layout of Job is defined in order to accurately explore this phenomenon. In particular, to have a clear understanding of the effect of the exposed area on printability inhomogeneities on the building platform, six levels of this parameter are investigated, as mentioned in Table 3, starting from a condition of almost total exposure of the building platform of EOS M-290. The powder bed inhomogeneities are measured and quantified in terms of the track's stability, Melt Pool, and Porosity analysis using a \"ladder\" shaped specimen. We specifically evaluate the presence of porosity for its ability to change the microstructure and subsequently deteriorate the mechanical properties of the materials [51].