# Powder Bed Selective Laser Processing of Alumina: Scanning Strategies Investigation
## 4 Results and Discussion
### PBSLP of Alumina Samples  
Alumina samples were printed using 95 W laser power, 200 mm/s scanning speed, 50 \(\upmu\)m hatching distance, 100 \(\upmu\)m layer thickness for different scanning strategies as seen in Figure 13. The sample printed using island-linear strategy was unsuccessful and this was mainly due to the short and adjacent scanning paths that increased the temperature highly at the small island. All other samples have a precise cubic shape (10 \(\times\) 10 \(\times\) 10 mm\({}^{3}\)) with flat surfaces except the top surface. The top surface mirrors the used scanning strategy. The scanning strategies traces are significantly pronounced at the concentric strategy and island strategy. The concentric strategy has two crossed lines making an \"X-shape\" on the top surface, while the island strategy has small holes on the sample top surface distributed at each island center.  
Figure 12: Alumina samples (10 \(\times\) 10 \(\times\) 10 mm\({}^{3}\)) printed with different hatching distances, laser power 95 W, and scanning speed 200 mm/s.  
Figure 13: Alumina samples (10 \(\times\) 10 \(\times\) 10 mm\({}^{3}\)) printed by different scanning strategies using 95 W laser power, 200 mm/s scanning speed, and 50 \(\upmu\)m hatching distance.  
The top surface pattern for all scanning strategies except island-linear was investigated with a 3D microscope (KEYENCE VR-3000), as seen in Figure 14. An irregular structure was observed on the upper surface of all samples. This situation was attributed to the presence of regional densities resulting from different particle size distributions.  
For the concentric in-out strategy, the developed numerical model was used to analyze the two crossed lines formed on the layer top surface. It was found that they were formed mainly due to the rotation of the laser beam because when the laser changed the direction, a sharp increase in the temperature occurred, as shown in Figure 15. The sharp increase in temperature is nearly sufficient to vaporize the material at the laser's rotational position. In this strategy, the laser direction changes at four points per cycle, and the short scanning paths at the inner cycles cause the heat to be concentrated in a small area. As a result, two diagonal lines and a small hole in the center of the sample are formed. It was evaluated that this critical result can help LB-PBF printer manufacturers to make modifications that prevent the sharp return of the laser beam in such strategies.  
For island-concentric out-in strategy, since it consists of a small concentric in-out strategy, small holes are formed in the top layer surface, mainly due to the heat concentration at the minimal area as described previously. Figure 16 shows the melting contour and the temperature history for a tiny island where the concentration of the heat led to high-temperature history at the end of the scanning.  
Figure 14: 3D microscopic analysis for the alumina samples (10 \(\times\) 10 \(\times\) 10 mm\({}^{3}\)) printed using different scanning strategies.  
Figure 16: Temperature history of the small concentric in-out strategy.  
Figure 15: The sharp increase in temperature due to the change in the laser direction.