# Hybrid Metal/Polymer Filaments for Fused Filament Fabrication (FFF) to Print Metal Parts
## 3 Results and Discussion
### Sintered Parts
#### 3.2.3 Comparison with Standard Metal AM Technologies  
The use of FFF for printing metal samples has been developed to have an easy, safe, and low-cost alternative to standard AM metal techniques. However, the mechanical performances are also important. Table 7 summarizes the mechanical properties for 316 L and 17-4 PH processed by standard metal AM and with no thermal post-processing applied. For comparison purposes the mechanical properties achieved using MIM were also reported. The specimens produced by FFF (Table 5) showed overall lower mechanical properties compared to analogous parts obtained by standard metal AM. For the 316L, the FFF manufactured specimens showed properties in the same range of MIM. Something similar was obtained for 17-4 PH if the real cross section areas (Table 6) were considered instead of the nominal areas. Therefore, the FFF parts manufactured by using hybrid metal/polymer filaments are better compared to MIM parts. However, to exploit the best performances, inherent defects due to processing, like voids and poor interlayer bonding (Figure 10), must be prevented. This can be achieved optimizing the printing settings [13]. Therefore, to fully exploit the potential of FFF applied to hybrid metal/polymer, more research into printing optimization is advisable. In addition to the simple tensile testing in a future work it will be important to characterize the effect of pre-crack on the printed specimens as this might have a relevant effect on applications [20]. Figure 9: EDX spectra for the raster’s surface and an inclusion observed for the 17-4 PH samples. Apart from mechanical performances, economic considerations must also be evaluated when comparing FFF of hybrid metal/polymer to standard metal AM. The manufacturing of a mold for injection molding (Figure 11) was considered as a reference part to compare the use of the Ultrafuse 316L with the production using a SLM EOS400m printing SSL316L. The cost modelling used is based on the model proposed by [PERSON] et al. [27]. The results of cost simulations are reported in Table 8. The material cost was assumed to be 129 EUR/kg and 30 EUR/kg for the Ultrafuse 316L and the SSL316L, respectively. The cost simulation yielded a higher value for SLM (EUR 330.13) compared to FFF (EUR 100.62) because the recycling fraction (95%) for SLM operating only to print this mold yields a high waste of unused and not recyclable powder. The other cost that impacted significantly on the final part price was the machine usage (18.3%) as a result of the high investment cost for the EOS M400 (about 1.7 M\(\mathrm{\SIUnitSymbolCelsius}\)). The final predicted cost for printing the mold using FFF with the Ultrafuse 316L was EUR 246.13 versus the EUR 1147.88 predicted for SLM. In the case of the FFF this was the cost for the green part. To this cost must be added the cost for sintering that is currently priced around 60 EUR/kg by Elnik as the preferred partner of the mold. Figure 10: Morphology of the fracture surface of the fractured specimen manufactured using Ultrafuse 316L: (**a**) flatwise; (**b**) upright. analysis by SEM allowed the unveiling of the presence of defects in the green state which resulted in voids and poor interlayer bonding in the sintered state. These defects caused a reduction in the tensile mechanical properties of the sintered parts compared to those declared by the materials' manufacturers. Nevertheless, the final mechanical properties obtained using FFF were in the same range of those achieved by MIM. The results of the mechanical characterization revealed that considerable gaps still exist compared to metals produced by standard metal AM techniques. However, it was demonstrated by cost modelling referring to a single mold prototype, that the use of hybrid metal/polymer filaments processed by FFF has a strong potential to allow printing of metal parts at a lower cost. This cost modelling did not account for the environmental impacts of the processes.