# In Situ Study of Surface Morphology Formation Mechanism During Laser Powder Bed Fusion
## 3 Results and Discussion
### Surface Morphology Evolution
#### 3.2.1. Reticular Structure and Large-Scale Protrusion  
The reticulated surface (Figure 8a) was formed by the layer-by-layer stacking of single-layer spheroidal surfaces under low-energy-density conditions. The surfaces of each layer during the stacking process are depicted in Figure 9a. These surfaces exhibit two typical local morphological characteristics: reticulated structure (Figure 9b) and large-scale protuberances (Figure 9c), with the formation of these two features being directly progressive. The evolution process of the two structures proposed based on the coaxial monitoring results is shown in Figure 10. Insufficient energy input at low-energy-density levels prevented the metal powder from fully melting into a liquid state upon heating, making it difficult to form a stable melt pool. Under the influence of surface tension, some adjacent molten metal droplets coalesced into spherical solidification particles to minimize surface energy, resulting in a surface with high roughness characterized by spheroidal morphology. Due to the consistent process parameters throughout the fabrication of the same sample, there was a persistent phenomenon of the insufficient melting of metal powder in the subsequent layers' printing process. As the manufacturing process advanced, when the metal droplets formed by the melting of the subsequent layer of powder came into contact with the previously solidified spherical solidification particles, the surface tension of the droplets drove them to merge with the particle surfaces. Concurrently, heat was transferred from the high-temperature droplets to the low-temperature solidified melt, causing the metal droplets to cool rapidly and lose their original spherical shape, solidifying onto the melt particles. At this point, the spherical solidification particles on the sample surface exhibited an increasing trend in size, and their height exceeded the thickness of the new powder layer, leading to the formation of super-elevation defects in the powder bed (Figures 9a and 10a) [31]. After stacking multiple layers, the size of the spherical solidification particles further increased, allowing the edges of particles within the same melt track or adjacent melt tracks to come into contact with the same metal melt droplet simultaneously, forming a structural connection (Figure 10b). Consequently, the initially smaller spherical particles aggregated to form a reticulated protruding structure that spanned multiple melt tracks, promoting the formation of a reticulated surface (Figure 10c). As the printing process progressed to later stages, the substantial heat accumulation within the sample not only increased the degree of metal powder melting but also slowed down the cooling rate of the molten droplets, allowing them to fill the gaps between adjacent reticulated structures. Once the reticulated structures of multiple melt tracks were fully interconnected, they formed large-scale local agglomerated protrusions.  
Figure 8: Three-dimensional surface morphology of multi-layer samples under different process parameters: (**a**) reticulated surface, (**b**) undulating surface, (**c**) uniform surface.  
Figure 9: (**a**) Morphological images of the intermediate layers of a multi-layer sample with a reticulated surface. (**b1**) Photograph of reticular structures. (**b2**) Three-dimensional morphology of reticular structures; (**c1**) Photograph of large-scale local surface agglomerate protrusions. (**c2**) Three-dimensional morphology of large-scale local surface agglomerate protrusions.