# Processing of AZ91D Magnesium Alloy by Laser Powder Bed Fusion
## 3 Results and Discussion
### Bulk Samples III  
The following series of samples were used to detect porosity changes at different heights in bulk samples. For the production, the parameter set from Table 2 sample 4-1 was used because, in the previous experiment, it led to lower porosity. After production, the individual samples were cut and metallographically prepared for the observation of porosity with a light microscope. Image analysis was used to evaluate the relative surface density of the samples in the XY planes at different sample heights (Figure 2d). An overview of the measured values can be found in Table 5. The marking of the samples corresponds to Figure 3a. Larger spherical pores at all height levels are observed in the subsurface area of the samples. The regular arrangement of pores in the subsurface area is usually the result of a local increase in energy at the end of the individual hatching vectors, which is caused by the deceleration at the end of the vector and the acceleration at the beginning of the neighboring vector (turnaround point), which can be removed with the use of so-called skywriting in newer scanning units [24]. Only very small gas pores of about 10 \(\upmu\)m size are observed in the volume of the sample, as shown in Figure 8. A specific or significantly different pore arrangement in the individual height levels was not observed.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Bulk Sample**} & \multicolumn{4}{c}{**Porosity (\%)**} & \multirow{2}{*}{**RD (\%)**} \\ \cline{2-2} \cline{4-6}  & **Plane XY** & **Plane YZ** & **Plane XZ** & **Average** \\ \hline
4-1 & 2.1 \(\pm\) 0.12 & 5.2 \(\pm\) 0.34 & 3.7 \(\pm\) 0.22 & 3.7 & 96.3 \(\pm\) 1.26 \\ \hline
4-2 & 4.1 \(\pm\) 0.25 & 3.3 \(\pm\) 0.15 & 4.7 \(\pm\) 0.39 & 4.0 & 96.0 \(\pm\) 0.57 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Porosity measured in different planes and relative density.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Height**} & \multirow{2}{*}{**5-4.4**} \\ \hline
80 mm & & & & 0.7\% \\
60 mm & & & **5-3.3** & **5-4.3** \\  & & & 0.5\% & 0.5\% \\
40 mm & & **5-2.2** & **5-3.2** & **5-4.2** \\  & & 0.5\% & 0.4\% & 0.9\% \\
20 mm & **5-1.1** & **5-2.1** & **5-3.1** & **5-4.1** \\  & 1\% & 0.7\% & 0.5\Table 6 shows the values of the relative density of the XZ planes in the individual samples (Figure 3a). The lower part of the samples (0-20 mm above the platform) shows a higher percentage of porosity than the area of the samples in the height plane \(>\) 20 mm above the platform. Figure 9 compares the lower (Figure 9a) and upper area of sample 5-4 (Figure 9b). In the lower area, a more frequent occurrence of larger spherical pores (\(100\pm 20\) \(\upmu\)m) is observed, mainly in the subsurface area of the sample, in contrast to the upper part of the sample where only the occurrence of small pores (10 \(\pm\) 6 \(\upmu\)m) is observed. Total porosity is also significantly higher in the lower areas. On the other hand, no significant influence of the sample size on the resulting porosity is observed, in contrast to the study published by [PERSON] et al. [25], where a significant decrease in porosity is detected with increasing sample size for the alloy