## 3 Results and Discussion
### Surface Roughness  
Figure 6 shows the surface roughness (Sa) of different L-PBF samples. The results demonstrated that among all scanning strategies, the surface roughness of the sample with the CHESS combination scanning strategy was the lowest, and the surface roughness of the CHESS&45\({}^{\circ}\) sample under this strategy was 14 \(\upmu\)m, which was the lowest value under this scanning strategy. The surface roughness value of the sample was the highest with the combined LINE scanning strategy, and the surface roughness value of the LINE&90\({}^{\circ}\) sample was the highest, reaching 42 \(\upmu\)m. Overall, under a specific general scanning strategy, the increase in the angle offset improves the surface roughness value of the sample to a certain extent. This phenomenon may be because the increase in the offset angle reduces the distance of a single laser scan; the laser acts on the sample's surface more frequently, accumulates more heat, and thus causes a higher temperature gradient. As a result, defects such as sputtering increase significantly on the surface of the sample, and the surface roughness value rises, as shown in Figure 6. Compared with the LINE scan strategy, the surface roughness of the formed sample was lower with the CHESS scan strategy. The reason was that the CHESS scanning strategy could reduce heat accumulation and make the heat distribution on the sample surface more uniform [34; 35]. The value of surface roughness and defects had been decreased.  
To further explore the surface micro-morphology and surface roughness of the sample under the condition of a single scanning strategy, the micro-morphology of the sample was obtained using a laser confocal device, as shown in Figure 7. The results show that the surface roughness value of the samples formed with a single scanning strategy was relatively higher than that of the samples formed with different scanning strategies. With the increase in the angle offset, the surface roughness of the samples had an upward trend. Among them, the surface roughness value of the 90\({}^{\circ}\) sample was the largest, reaching 83 \(\upmu\)m. In addition, the study also found that under the scanning strategy of a single angle offset, corresponding protrusions appeared on the surface of the sample, the direction was parallel to the offset direction, and the width of the melt channel was about 100-200 \(\upmu\)m, as shown in Figure 7b. This phenomenon occurred because some gaps would appear in the overlap between each melt track during L-PBF forming. At the same time, the laser scanning path of each forming layer was the same. The gaps continuously accumulated during the stacking process and finally expanded into macroscopic ravines. The direction was the same as the laser scanning direction, as shown in Figure 7c. The surface roughness of the sample was relatively low, around 25 \(\upmu\)m under the general scanning strategy. Compared with the scanning strategy of angle offset, the general scanning strategy rotates 67\({}^{\circ}\) per layer, compensating for the gap between the forming layers so that less surface roughness can be obtained, as shown in Figure 7f. However, under the general scanning strategy, the melt channels on the sample's surface were visible and evenly distributed without obvious ravines. The size of the melt channels was maintained at about 100 \(\upmu\)m, as shown in Figure 7e. Compared with the single-angle offset scanning strategy, the melting channel bulge width of the general scanning strategy is smaller, and the distribution is uniform without obvious grooves.  
Figure 6: Surface roughness of L-PBF samples under different combinations of scanning strategies. (**a**) Surface roughness of L-PBF samples under different combinations of scanning strategies; (**b**) surface micro-morphologies of samples under different combinations of scanning strategies.