## 4 Discussion
### Influence of Scanning Strategy on Porosity  
The degree of porosity in the three specimens was calculated from the total volume of pores relative to the investigated volume (Table 2). This was done for both the full specimen CT scan and \begin{table}
\begin{tabular}{c c c c} \hline
**Specimen Designation** & \(\mathrm{\SIUnitSymbolMicro S_{a}}\) (\(\mathrm{\SIUnitSymbolMicro m}\)) & \(\mathrm{\SIUnitSymbolMicro S_{q}}\) (\(\mathrm{\SIUnitSymbolMicro m}\)) & \(\mathrm{\SIUnitSymbolMicro S_{z}}\) (\(\mathrm{\SIUnitSymbolMicro m}\)) \\ \hline Line Scan & 48.4 & 66.9 & 678.0 \\ Island Scan Stripes & 66.4 & 82.4 & 722.0 \\ Island Scan Modeled & 82.8 & 101.6 & 743.5 \\ \hline \end{tabular}
\end{table} 
Table 1: Surface roughness parameters (arithmetic mean height, \(\mathrm{S_{a}}\); root mean square height, \(\mathrm{S_{q}}\); and maximum height, \(\mathrm{S_{z}}\)) calculated in SPIP using IFM data for specimens fabricated by three different scan strategies. Figure 9: Micro hardness profiles of the three different SLM specimens. Average of five profiles shown with trendlines constructed by Gaussian fitting. A representative error bar is added, calculated as 2\(\sigma\) of all measurements for each sample, averaged. the 4\(\times\) zoomed CT scan on the cropped regions (note the different cropped regions for the different specimens in Figures 2-4). The highest apparent density was achieved in the line scan specimen (-99.6%), while the island scan stripes specimen was the most porous of the three (-98.6%). The island scan modeled specimen exhibits an apparent density in-between the island scan stripes and the line scan specimen (-99.2%). The relatively large difference between the porosity percentages determined with the LFOV and 4\(\times\) zoom for the line stripes specimen is ascribed to the inhomogeneous distribution of porosities (Figure 2g); the cropped volume investigated with the 4\(\times\) zoom objective contains a relatively small fraction of the part of the specimen that contains the most porosity (Figure 2a). The enhanced fraction of spherical porosities in the beginning of the line scan specimen (in the green valley in Figure 2a) is explained as follows. On depositing a layer of new powder, the solidified part will be cooled before scanning of the next layer begins. On scanning the next layer, reheating occurs. The longer the heat input continues, the higher the average temperature in the sample will be and the smaller the local temperature gradients. Accordingly, the temperature gradient during deposition decreases with increasing y. Consequently, faster cooling occurs in the first scanned lines than in the last scanned lines. Faster solidification implies a greater risk for pore development associated with gas precipitation. It is suggested that changing the print direction between successive layers could mitigate the accumulation of porosity on one side of the specimen. A gentle thermal gradient also leads to lower thermal stresses and hence reduces the risk for deformation proceeding in the y-direction. Accordingly, the largest deflections are expected at the beginning of the build (Figure 2f). The \"deflection\" at the edge is limited to the first 1 mm and is thought to be caused by melt digging into the powder bed [24], possibly assisted by deflection as a result of thermal stresses. The pores were divided into spherical and irregular pores, using a sphericity value of 0.7 as the cut-off. The irregular pores can be assumed to be mainly lack-of-fusion pores. The percentages of spherical and irregular pores, shown in Table 2, shows that for the island scan stripes specimen, irregular pores take up a higher volume than the spherical pores, while the opposite was the case for the line scan and island scan modeled specimens. Even though the irregular pores represent a large fraction of the total amount of pores in the islands scan stripes specimen, the equation diameter is very close to the equation diameter for the spherical pores.