# In Situ Study of Surface Morphology Formation Mechanism During Laser Powder Bed Fusion
## 3 Results and Discussion
### Surface Morphology Evolution
#### 3.2.4 Edge Contour Protrusion  
Figure 15 displays the edge contour morphologies at the beginning and end of melt tracks for both reticulated and uniform multi-layer sample surfaces. In the reticulated surface (Figure 15a), there was a significant cavity between the initial printed layer of the built sample and the substrate surface (Figure 15(a1)), with only partial contours fused to the base plate. This was due to the irregular morphology of the reticulated surface that hindered the spread of the powder layer, preventing the depressions and gaps between adjacent reticulated structures from being adequately filled, resulting in internal cavities [30]. Both the beginning and end of the melt track exhibited peak-like protrusions lower in height than the sample surface, with internal pores and under-fusion porosities. Between the peak-like protrusions and the inner processing area, there were inclusions of unmelted powder (Figure 15(a2) and Figure 16a).  
Figure 15: Edge contour protrusion cross-sectional morphology of multi-layer samples under different process conditions: (**a**) reticular surface, (**a1,a2**) cross-sectional morphology of the contours on both sides of the sample with a reticular surface; (**b**) uniform surface, (**b1,b2**) cross-sectional morphology of the contours on both sides of the sample with a uniform surface.  
The edge contour morphology of the uniform surface is shown in Figure 15b. The height of the peak-like protrusions at both the beginning and end of the melt track was parallel to or slightly above the sample surface and was internally uniform. Compared to the reticulated surface, the edge contour protrusions of the uniform surface had a higher degree of fusion with the built sample and base plate, but there were still unfused porosities or narrow cracks present. Additionally, there was also an inclusion of unmelted powder between the contour peak-like protrusions and the inner processing area of the uniform surface. This demonstrates that within the investigated parameter range, enhanced surface uniformity did not guarantee complete defect elimination. The pores and unmelted powder observed on uniform surfaces may have originated from entrapped gas within the melt pool and insufficient wettability, while microcracks correlated with thermal stress accumulation during rapid solidification. Achieving defect-free specimens requires expanding the processing window through strategies like substrate preheating or scan pattern optimization, though this must be balanced against spatter formation risks at higher energy densities.  
The peak-like protrusions of the edge contours of both surfaces were related to the wetting behavior of the molten droplets. As the laser spot traveled along the scanning path to the end of the melt track, the melt pool was influenced by Marangoni forces causing it to overflow towards the outer edge of the contour, wetting the adjacent powder at the sample edge and causing it to melt and adhere to the sample contour melt, forming a peak-like protrusion [34]. Some powder, not fully melted, was captured between the solidifying melt and the peak-like protrusion, forming a powder inclusion (Figure 16b). Due to the lower-energy-density input during the formation of the reticulated surface compared to the uniform surface, the volume of the molten droplets formed was also correspondingly smaller, resulting in less powder being wetted and melted than by the melt pool during the formation of the uniform surface. Consequently, the height of the peak-like protrusions formed on the contour of the reticulated surface samples was lower than that of the uniform surface samples.