# Hybrid Metal/Polymer Filaments for Fused Filament Fabrication (FFF) to Print Metal Parts  
[PERSON]  
1 Department of Civil Engineering and Architecture, University of Catania, Viale [PERSON] 6, 95125 Catania, Italy; [EMAIL_ADDRESS]  
[PERSON]  
2 Department of Chemical Engineering Materials Environment, Sapienza--Universita di Roma and UdR INSTM, Via Eudossiana 18, 00184 Roma, Italy; [EMAIL_ADDRESS] (J.T. ); [EMAIL_ADDRESS] (F.S.) [PERSON]  
2 Department of Chemical Engineering Materials Environment, Sapienza--Universita di Roma and UdR INSTM, Via Eudossiana 18, 00184 Roma, Italy; [EMAIL_ADDRESS] (J.T. ); [EMAIL_ADDRESS] (F.S.) [PERSON]  
1 Department of Civil Engineering and Architecture, University of Catania, [PERSON] 6, 95125 Catania, Italy; [EMAIL_ADDRESS]  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/pdf/1](https://www.mdpi.com/journals/pdf/1)]([https://www.mdpi.com/journals/pdf/1](https://www.mdpi.com/journals/pdf/1))  
were applied on green printed parts to remove the polymeric binder and to obtain fully dense metal objects. The AM manufacturing techniques used so far are fused filament fabrication or vat photopolymerization. In the first case, metal/polymer filaments or rods are printed with FFF-like printers to obtain the green part. Desktop Metal Inc, Markforged Inc and Nanoe systems rely on this approach. Other companies, like Pollen AM and AIM3D GmbH, patented machines processing pellets directly like those used for Metal Injection Molding (MIM). Admatec, Incus and Photocentric Ltd. are developing metal AM using photopolymerizable resins highly filled with metal particles. The seminal works that introduced the use of FFF with metal filled filaments date to the 1990s when Fused Deposition of Metals (FDMet) was first presented [5; 6; 7]. However, only recently this process has received increasing interest with some papers addressing the use of different metals. Among all of them, two types of stainless steels were investigated: 316L and 17-4 PH. [PERSON] et al. [8] studied the effects of different building orientations for 316L printed parts using their own filament. All the parts were printed with a rectilinear infill pattern and at 100% infill density varying the layer thickness. The highest density for the green printed part was 92.9% with a layer thickness of 0.1 mm. The sintered parts showed anisotropic behavior with shrinkage degree varying between 15 and 17% and with the highest shrinkage along the layer direction. Higher shrinkage values were reported in other studies with values between 15 and 23% [9]. The specimens printed up-right displayed the lowest values for the Ultimate Tensile Strength (i.e.,100 MPa), while the highest values (i.e., 453 MPa) were shown by samples printed flatwise and on the edge. The large difference was explained as the result of the voids laying perpendicular to the tensile direction for the upright specimens. No data on the tensile modulus were reported. [PERSON] et al. [9] printed their specimens using the filament Ultrafuse 316L, measuring a residual porosity around 1.5%, which contrasted to the fully dense structure obtained by SLM. The sintered specimens showed equiaxed grains and austenitic microstructure such as those shown during annealing. The yield strength measured was 167 MPa while UTS (Ultimate Tensile Strength) was 465 MPa and the tensile modulus 152 GPa. All these values were lower than those measured for samples manufactured by SLM and for those reported with AISI 316 L. The low mechanical properties were explained as the results of the porosity which also caused a lower ductility (i.e., 31%) compared to standard annealed AISI 316 L (i.e., 60%). [PERSON] et al. [10] focused on the characterization of the flexural behavior for 316L filaments produced on their own. They used over-extrusion to produce high green body density samples.