## 2 Materials and Methods  
### Specim Preparation  
The specimens, made from 316L and 17-4 PH stainless steel, were fabricated using the BMD technique using a Studio System Desktop Metal 3D printer. The chemical composition of the specimens is presented in Table 1.  
Before the manufacturing process, a CAD model of the specimens was created using the SolidWorks program. The CAD file was uploaded on a fabricate web browser of Desktop Metal to set the print parameters, such as layer height, wall thickness, top/bottom thickness, infill density, and orientation. According to the ASTM E8/E8M standard (sheet type), the geometry and dimensions of all specimens with a constant thickness of 3 mm were designed and fabricated. Because the 3D printer used in this work can only set the infill density to a maximum of 35%, infill densities of 16%, 20%, and 24% were considered. The varying infill densities, in which a triangular infill pattern was used as default, were set via the print parameter setting. The characteristic length of the triangular pattern with infill densities of 16%, 20%, and 24% is 3.0 mm, 2.4 mm, and 2.0 mm, respectively. The cross-sections of all struts in the infill structures are rectangular sections of 0.5 \(\times\) 3.0 mm\({}^{2}\). A constant wall thickness of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Material** & \multicolumn{6}{c}{**Element 1 (\%)**} \\ \hline \multirow{2}{*}{316L} & C & Cr & Ni & Mo & Mn & Si \\  & \textless{}0.045 & 16–18 & 10–14 & 2–3 & \textless{}2 & \textless{}1 \\ \hline \multirow{2}{*}{17-4 PH} & C & Cr & Ni & Cu & Mn & Nb + Ta \\  & \textless{}0.07 & 15.5–17.5 & 3–5 & 3–5 & \textless{}1 & 0.15–0.45 \\ \hline \hline \end{tabular}  
* The data are provided on the official website of Desktop Metal.  
\end{table} 
Table 1: Chemical composition of the 316L and 17-4 PH stainless steel specimens. 0.5 mm was set to enclose all side edges of the specimens without top and bottom surfaces. The layer height was set to be a constant of 0.15 mm for all the specimens; consequently, there were 20 layers for each specimen. The print orientation was set to build in a raster horizontal direction, parallel to the build plate, as shown in Figure 2a, to ensure that all samples yield the best performance and ductile behavior [26; 27]. The parameter setting of specimens with an equivalent 100% density (the full-solid samples) was the same as those of infill specimens, except for adding the top and bottom layers with a total thickness of 3 mm. After the parameter setting, a task was created and then submitted to the printer. The specimens were printed automatically in the printing process. A specimen from the printing process is called a green part, and the dimensions were larger than the size designed. Next, the green parts were placed into a debinding tank, as shown in Figure 2b, to dissolve the binder and create an open-pore channel structure for sintering. A specimen from the debinding process is called a brown part, and these specimens were soft and breakable. The last step was the sintering process, in which the brown parts were placed into the furnace to be densified, as shown in Figure 2c. In this process, the specimens were heated to a temperature close to the melting state. Some remaining binder and wax were removed to provide micro void space for fusing the metal particles. After the sintering process, the finished parts became harder and were shrunk to the designed dimensions, as shown in Figure 3.  
The weights of the specimens with different infill densities were measured to compute the relative density of each infill specimen, which is defined by the weight of an infill specimen divided by the weight of the full-solid specimen (see Figure 3). The average relative densities of the specimens are shown in Table 2.