# Hybrid Metal/Polymer Filaments for Fused Filament Fabrication (FFF) to Print Metal Parts
## 2 Materials and Methods  
The materials used were sourced from commercial producers. For the 17-4 PH system the technology developed by Markforged was used, while for the filaments filled with 316L the Ultrafuse 316L produced by BASF was chosen.  
The Ultrafuse 316L filament is a metal-polymer composite with a nonslip surface, allowing application in any Bowden or direct-drive extruder. In this paper, we used a desktop-size 3D printer model M200 by Zortrax (Zortrax S.A., Olsztyn, Poland) equipped with a 0.4 mm bronze nozzle. The printing conditions used for the Ultrafuse 316L were adapted from those specified by BASF in their technical data sheet testing on a calibration cube (see video S1) [15]. The printing settings are reported in Table 1. The 3D-printed samples were outsourced to a debinding and sintering service for postprocessing as recommend by BASF.  
The 17-4 PH samples were printed and sintered in outsourcing using a Metal X printer following the printing settings specified by Markforged. The part size and the whole ADAM process parameters, including the support structure, are designed automatically by proprietary software named Eiger. Eiger is a CAM (Computer Aided Manufacturing) software that manages the whole process from the design to the sintering phase. The software is closed to the user. Therefore, the process parameters are not known, and cannot be varied by the user, except for the layer thickness and the building orientation. Flatwise and upright building orientation were tested. The deposition strategy used by Markforged includes one outer contour and three inner ones as reported by [PERSON] and [PERSON] [14] (Figure S1). Like in standard FFF systems, the bottom and the top layers are printed with full density, while the strategy used for the other layers depends on the layer thickness. For the layer thickness of 0.050 mm a full-density strategy is used, while a closed triangular cell path is followed to infill the section for the higher layer thickness of 0.125 mm.  
The Ultrafuse 316L filament and all the printed green parts were analyzed by Scanning Electron Microscopy (SEM) using a SEM EVO by Zeiss. The sintered parts were analyzed using a MIRA3 by Tescan. All the samples containing polymers were gold sputtered before SEM analysis. The software Image J was used to analyze the SEM pictures to evaluate the mesostructure of the sintered parts.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Printing Parameter** & **Unit** & **Value** \\ \hline Nozzle size & mm & 0.4 \\ Retraction distance & mm & 2 \\ Retraction speed & mm/s & 40 \\ Layer height & mm & 0.14 \\ Infill & \% & 100 \\ Nozzle temperature & \({}^{\circ}\)C & 240 \\ Bed temperature & \({}^{\circ}\)C & 90 \\ Oversizing factor (xy) & \% & 19.98 \\ Oversizing factor (z) & \% & 26.03 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Printing settings for the Ultrafuse 316 LThe green parts were overdesigned with respect to ASTM D638 and E8 to account for the shrinkage upon sintering (Tables 2 and 3). All tensile tests were carried out at 1 mm/min on a Instron 5982 by Instron (Instron, Milan, Italy) with a load cell of 100 kN (Figure S2).