# 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
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **Building Orientation** & **Yield Strength** & **Tensile Strength** & **Tensile Modulus** & **Strain at Break** \\  & **Orientation** & **MPa** & **MPa** & **GPa** & **\%** \\ \hline \multirow{2}{*}{316L} & Flatwise & 148.01 \(\pm\) 4.50 & 443.90 \(\pm\) 5.87 & 157.24 \(\pm\) 4.50 & 43.33 \(\pm\) 2.53 \\  & Upright & 113.75 \(\pm\) 13.42 & 206.27 \(\pm\) 80.11 & 117.31 \(\pm\) 1.94 & 13.35 \(\pm\) 6.59 \\ \multirow{2}{*}{17-4 PH} & Flatwise & 443.00 \(\pm\) 6.90 & 497.40 \(\pm\) 9.90 & 108.00 \(\pm\) 6.90 & 0.79 \(\pm\) 0.05 \\  & Upright & 412.00 \(\pm\) 31.60 & 494.70 \(\pm\) 29.70 & 102.8 \(\pm\) 1.20 & 0.95 \(\pm\) 0.14 \\ \multirow{2}{*}{316L *} & Flatwise & 251 & 561 & – & 53 \\  & Upright & 234 & 521 & – & 36 \\ \multirow{2}{*}{17-4 PH *} & Flatwise & 1100 & 1250 & 170 & 6 \\  & 17-4 PH ** & - & 660 & 1042 & 195 & 8.5 \\ \hline \hline \end{tabular}  
* The data were extracted from the technical data sheet (TDS) released by the producer (BASF for the 316L and Markforged for the 17-4 PH).
* The data were extracted from the technical data sheet released by Desktop Metal with their Studio SystemTM.  
\end{table} 
Table 5: Tensile properties for the sintered parts produced in this paper. The properties reported in the technical data sheet of the producer are reported for comparison. Figure 6: Morphology of the fracture surface of the fractured specimens manufactured using 17-4 PH: (**a**) flatwise; (**b**) upright. The 316L samples presented tensile properties closer to those declared by BASF but lower. The cross section of these samples was characterized. The mesostructure of the flatwise sample showed the presence of micro porosity between the rasters which is clearly visible despite the full sintering cycle (Figure 10a). This porosity seems the result of the incomplete fusion between the raster observed in the green part (Figure 2). The upright sample showed even bigger pores and, as for the 17-4 PH, the presence of an interlayer fracture (Figure 10b). Using Image J, the porosity of the flatwise sample was estimated to be equal to 9.76%, leading to an estimated real load bearing area of 17.07 mm\({}^{2}\) rather than the measured 18.91 mm\({}^{2}\). The tensile properties estimated with the real load bearing area were 174.15 GPa, 163.93 MPa and 491.65 MPa for the tensile modulus, yield strength and tensile strength, respectively. Despite the area correction, the measured values were still lower than those reported by BASF. This is the consequence of the porosity that could act as a stress intensifier leading to premature failure. Similar mechanical performance drops were observed when comparing FFF to injection molded counterparts [17; 18; 19]. The upright samples, due to the higher porosity and to the building orientation that led to high loading of the interlayer region, showed lower mechanical resistance compared to the flatwise samples. As for the 17-4 PH samples, higher standard deviations were observed for the upright building orientation.