# Hybrid Metal/Polymer Filaments for Fused Filament Fabrication (FFF) to Print Metal Parts
## 3 Results and Discussion
### Sintered Parts
#### 3.2.2 Tensile Test of the Sintered Parts  
The sintered parts were tested in tensile mode. Exemplary curves for the samples printed with the Ultrafuse 316L and the Markforged 17-4 PH are shown in Figure 5. The data refer to the stress calculated as load divided by the nominal area Width*Thickness (Tables 2 and 3). The 316 L samples showed higher strain at break values (i.e., \(\approx\)43%) compared to the 17-4 PH samples (below 1%). The tensile properties measured for sintered 316L and 17-4 PH are summarized in Table 5. The properties declared by the filaments' manufacturers are also reported for comparison purposes. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Build Orientation** & **Printing Axis** & **Measure** & **Green Part** & **Sintering Axis** & **Sintered Part** & **Shrinkage** \\  & & & **(mm)** & & **(mm)** & **(\%)** \\ \hline \multirow{3}{*}{Flatwise} & Y & L & 22.80 & Y & 19.04 & 19.73 \\  & Z & W & 3.78 & Z & 3.02 & 25.20 \\ \cline{1-1}  & X & T & 137.98 & X & 115.25 & 19.72 \\ \cline{1-1}  & X & L & 22.80 & Y & 19.35 & 17.81 \\ \cline{1-1}  & Y & W & 3.78 & Z & 3.48 & 8.57 \\ \cline{1-1}  & Z & T & 137.98 & X & 114.31 & 20.70 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Dimensional analysis of green part versus sintered part for 316L. Figure 5: Tensile stress–strain curves for the sintered parts: (**a**) Ultrafuse 316L and (**b**) Markforged 17-4 PH. (Flat: red; upright: blue). It is clear from Table 5 that the measured properties for the 17-4 PH samples are lower compared to those reported by Markforged. However, these specimens presented a closed triangular cell infill [14] which resulted in a lower load bearing area (Figure 6). The cross-section area was estimated using Image J to be equal to 20.89 mm\({}^{2}\) rather than 36 mm\({}^{2}\) which was calculated considering the external sides of the cross section. If the real load bearing area (20.89 mm\({}^{2}\)) is considered, the tensile properties were higher and close to those reported by Markforged (Table 6). Desktop Metal has also released some values for their 17-4 PH system (Table 5) that lie in between the values found in this paper and those declared by Markforged. In the open literature, [PERSON] et al. [12] for their lab-made filament filled with 55 v% of 17-4 PH metal particles and printed with a 100% infill density reported a tensile modulus between 174 GPa and 208 GPa, while the maximum stress ranged from 660 MPa to 729 MPa. The wide range of the mechanical properties observed was explained as the result of the presence of defects in the structure of the specimens. The fractured surface of the 17-4 PH samples characterized in this paper showed the presence of two types of defects: pores between the raster (Figure 7a) and the presence of inclusions (Figure 7b). The inclusions were observed on the raster's surface too (Figure 8). The EDX analysis (Figure 9) showed the presence of a higher carbon concentration on the raster's surface leading to the conclusion that the surface might be rich in carbon-based residues of the polymeric binder. The presence of these defects and the higher carbon