## 3 Microstructure and Mechanical Properties  
Optical microscopy (OM) conducted on the cube samples cut along the building direction show nominally fully dense samples (see Figure 3). A density of around 99.8% was achieved following the analysis using ImageJ v1.54 software. Each OM-stitched map  
Figure 3: Bending fatigue sample after machining: (**a**) dog bone sample geometry; (**b**) test setup for the four-point bending test.  
represented \(\sim\)100 [mm\({}^{2}\)] area, with each image taken with a 10\(\times\) objective lens; thus, each pixel represented a feature of \(\sim\)1 [mm]. Automatic thresholding was conducted in ImageJ to establish the number of pores. After the OM, the sample was then introduced in an SEM and was shown to possess a fine microstructure overall, with nanometric precipitates rich in Mn, as shown in Figure 4. The primary strengthening contribution comes from even finer L1\({}_{2}\)-type Al\({}_{3}\)Zr nanoprecipitates (not reported). The sizes of these precipitates were on average \(<\)5 [nm] [64, 65]. These fine precipitates increased the yield strength from 250 [MPa] in the as-printed condition to \(\sim\)350 [MPa] in the heat-treated condition (+40% increase).  
Relatively coarser precipitates were observed at the grain boundaries and the melt pool boundaries of the material, which could affect the ductility of these precipitation-hardened materials, which has been widely discussed in the literature [66, 67, 68]. The uniaxial tensile tests, as presented by the authors in [68], are summarized in Table 2. The bending fatigue testing conducted as part of this study resulted in an S-N curve, as shown in Figure 5. Thus, the fatigue strength of the material with a 50% probability was 140.3 [MPa] and with a 90% probability it was 130.7 [MPa]. This seems to be on the higher side of reported values for similar tests conducted on AlSi10 Mg material in fatigue tests conducted at R = \(-\)1.0 [69, 70, 71].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Alloy**} & \multirow{2}{*}{**Direction**} & **Yield Strength,** & **Tensile Strength,** & **Elongation to** \\  & & **f\({}_{\text{tr}}\)** & **f\({}_{\text{tr}}\)** & **Failure** & **Density** \\ \cline{2-5}  & **(Z), (XY)** & **[MPa]** & **[MPa]** & **[\%]** & **[g/cm\({}^{2}\)]** \\ \hline \multirow{2}{*}{Al-HS1} & Vertical (\(Z\)) & 343 & 430 & 4.8 & \multirow{2}{*}{2.99} \\ \cline{2-5}  & Horizontal (\(\mathcal{OX}\)) & 356 & 441 & 10.4 & \\ \hline \multirow{2}{*}{AlSi10 Mg} & Vertical (\(Z\)) & 220 & 468 & 10.7 & \multirow{2}{*}{2.645} \\ \cline{2-5}  & Horizontal (\(\mathcal{OX}\)) & 261 & 449 & 15.7 & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of the tensile properties of Al-HS1 reported previously [68] and compared to AlSi10 Mg [69]. All samples of Al-HS1 were tested in a heat-treated state (375 \({}^{\circ}\)C 14 h) and the AlSi10 Mg in the as-printed state.  
Figure 4: (**a**) Optical microscopic image of a sample along the building direction (BD); (**b**) SEM images showing the microstructure with insets focusing on precipitates at melt pool boundaries (red) and grain boundaries (yellow) respectively.  
The results of the fractography tests conducted on the samples, as shown in Figure 6, reveal that for low cycle fatigue, larger pores are visible (up to 50 [\(\upmu\)m]), with a relatively smaller area of fatigue failure. In the case of high cycle fatigue, the damage initiation was seen to be originating from the edge of the sample (point of highest stress in bending loads) and relatively larger area of fatigue failures were observed.  
Figure 5: Bending fatigue data at R = \(-\)1.0 for the Al-HS1 printed along the building direction. Run-out samples (at 2 \(\times\) 10\({}^{6}\) cycles) are spread along the X-axis for the visualization. The fatigue strength range is as reported for AlSi10 Mg in the literature [69].  
Figure 6: Fractography tests on Al-HS1 after the bending fatigue tests: (**a**) low cycle fatigue sample; (**b**) high cycle fatigue sample.