# Powder Bed Selective Laser Processing of Alumina: Scanning Strategies Investigation
## 4 Results and Discussion
### Relative Density and Microstructure  
The Archimedes method was used to measure the relative density of alumina samples produced with different scanning strategies. Different alumina samples were printed using different laser power ranging from 95 W up to 105 W, 200 mm/s scanning speed, 100 \(\upmu\)m layer thickness, and 50 \(\upmu\)m hatching distance as shown in Figure 18a. Alumina samples have a black-colored surface resulting from the graphite which was added to increase the absorptivity of the powder. Figure 17: Top and side surface roughness of the printed sample using laser power of 95 W laser power and scanning speed of 200 mm/s. It is worth mentioning that no alumina secondary phases were formed because of graphite addition, which was reported by [PERSON] et al. [26]. For confirmation, XRD analysis was performed for the alumina sample, and that only the alpha-alumina phase existed (Figure 19). Figure 20 shows the relative density variation with laser power at a constant scanning speed of 200 mm/s and different scanning strategies. For the laser power of 105 W, the lowest relative density was obtained at linear-0\({}^{\circ}\) and concentric strategies, 69% and 67%, respectively, while for the island strategy, the highest relative density was 87.8 %. Both linear-90\({}^{\circ}\) and linear-45\({}^{\circ}\) resulted in an excellent relative density that reached 75%. The low relative density obtained using the linear-0\({}^{\circ}\) strategy can be explained by no change in the layer orientation, which led to a high porosity level inside the sample comparing to linear-90\({}^{\circ}\) and linear-45\({}^{\circ}\). The low relative density obtained at the concentric strategy is the result of the evaporation of material during the laser rotation. The island strategy produced more dense alumina samples than with the other scanning strategies, although small round channels inside the sample as shown in Figure 18b. This can be explained by the high concentration of heat resulting from the short scanning paths in tiny areas (islands), which increased the densification. However, these small channels remain an obstacle for this strategy to be of practical usage for PBSLP of alumina, and modifications are needed to avoid these holes. Furthermore, it can be noted from Figure 20 that the relative density increases with the increase in the laser power due to high energy density, which leads to more dense alumina tracks. Figure 18: (**a**) Alumina samples (10 \(\times\) 10 \(\times\) 10 mm\({}^{3}\)) printed with different scanning strategies and laser power; (**b**) a broken alumina sample printed using island-concentric out-in strategy containing the round passages. Figure 21 shows the SEM images of the studied scanning strategies, using the same process parameters, such as linear 0\({}^{\circ}\), concentric, and island-concentric out-in strategy. For linear-0\({}^{\circ}\), as shown in Figure 21a, the consolidated particles were formed intermittently and unconnected to each other, and this explains the low relative density obtained using the scanning strategy. Many cracks formation can be seen on the layer top surface. These cracks are mainly formed due to thermal shocks, and the initiation of cracks was started from places that did not contain consolidated alumina (pores). The SEM images for the side surfaces also show periodically repeated defects along the build direction resulting from the lack of adhesion between layers in some locations. The SEM images for the island-concentric out-in strategy show the small holes formed at the center of each island (Figure 21b). Cracks starting and continuing from the center of the holes are also observed, which may affect the mechanical properties of the alumina samples. Figure 21c shows  
Figure 19: XRD spectra of the alumina sample manufactured by PBSLP.