## 3 Experiment and Result Analysis
### Microstructural Analysis  
The microstructure of the molten pool center of Inconel 718 formed by SLM under different laser powers perpendicular to the forming direction is shown in Figure 6. From the figure, it can be seen that the microstructure in the molten pool parallel to the scanning direction was composed of cellular dendritic structures or equiaxed crystals. As the laser power gradually increased to 310 W, 335 W, and 360 W, the size of the cellular dendritic structures or equiaxed crystals gradually increased. At a low laser power of 310 W, the edges of the cellular crystal structure were finer, the boundaries were clear, and the number of equiaxed crystals was the highest (Figure 6a). When the laser power increased to the maximum of 360 W, the edges of the cellular crystal structure were thicker, and the crystals were in a fuzzy molten state. It can be seen that the microstructure parallel to the scanning direction gradually coarsened with the increase in laser power (Figure 6c). As the laser power gradually increased, the laser energy density also increased. When a higher intensity laser beam irradiated the material surface, a large amount of heat accumulated near the growth edge of the Inconel 718 crystals. Such heat provided sufficient thermodynamic potential for the growth and boundary coarsening of the Inconel 718 crystal. Therefore, fuzzy molten dendritic cell structures with coarser edges and larger microscopic cell structures appeared. The microstructure of Inconel 718 formed by SLM in a molten pool parallel to the forming direction under different laser powers is shown in Figure 7. The SLM morphology perpendicular to the forming direction was organized in the same molten pool. Furthermore, the direction of the dendritic/cellular structure varied in different positions and was not completely perpendicular to the boundary of the molten pool. This phenomenon was mainly due to the coupling effect of solidification conditions, crystal structure, and convection inside the molten pool [28]. The heat transfer direction in a molten pool is jointly determined by the temperature gradient in the forming direction and the laser scanning direction. The heat dissipation direction of the molten pool at the bottom of the molten pool was parallel to the forming direction, and the heat was transferred to the substrate direction. Therefore, the dendritic growth direction at the bottom of the molten pool was generally parallel to the forming direction. As shown in the enlarged microstructures of Figure 7e-h, there were columnar dendrites with a single growth direction distributed in the samples, as shown in Figure 7e-g. However, there mainly was a cellular crystal structure in the sample in Figure 7h. Take the same distance S = 8.7 \(\upmu\)m as shown in Figure 7. When the laser power increased to 360 W, the microstructure in the molten pool was composed of cellular crystals. In addition, existing research shows that the precipitates located in the interdendritic region of Inconel 718 are mainly Laves phases in morphology, composed of Nb- and Mo-rich phases. As indicated by the white arrows in Figure 7e-h, the white network structures are Laves phases. Figure 6: Microstructure of Inconel 718 formed by SLM at different laser powers perpendicular to the center of the molten pool in the forming direction. (**a**) P = 310 W, (**b**) P = 335 W, (**c**) P = 360 W.  
According to existing research, the formation of a solidification structure is determined by the process parameters of the SLM forming process through changing temperature gradient (G), solidification rate (R), and undercooling conditions (\(\Delta\)T). The solidification mode of the microstructure was determined by G/R, and the size of the solidification structure was determined by G \(\cdot\) R [29]. As the laser power increased, the temperature gradient in the molten pool increased, the solidification rate decreased, and the value of G/R increased.