# In Situ Study of Surface Morphology Formation Mechanism During Laser Powder Bed Fusion
## 3 Results and Discussion
### Surface Morphology Evolution
#### 3.2.3 Surface Roughness and Morphology Analysis of Uniform Surface
Therefore, the spherical protruding particles formed by spatters landing on the already-formed surface, as well as the large-scale powder agglomerate particles formed by spatters landing in the area to be processed (Figure 13a,b), became the main factors affecting surface quality in this stage. The micrograph of the multi-layer sample surface stacked from a single uniform-layer surface (SED = 100 Jmm\({}^{-2}\)) is shown in Figure 14(a1). The relatively smooth base surface and uniform melt track facilitated the further spread of the melt pool, resulting in a well-formed intermediate layer surface. However, since the melt pool width was slightly greater than the hatch spacing, the lower overlap area reduced the accumulation of heat within the sample, which was not conducive to the flow and spread of the melt pool. The molten liquid failed to adequately fill the gaps in the overlapping areas, leading to depressions between adjacent melt tracks, and the overall surface formed a wavy morphology parallel to the scanning line direction [7] (Figure 14(a2)). Figure 12: The quantification of surface roughness under different process parameters. (**a**) The effect of laser power and scanning speed on the arithmetic mean height (Sa). (**b**) The impact of varying energy densities on multiple surface roughness parameters: the arithmetic mean height (Sa), maximum height (Sz), root mean square height (Sq), skewness (Ssk), kurtosis (Sku), maximum peak height (Sp), and maximum valley depth (Sv). When the SED was further increased to 116.67 Jmm\({}^{-2}\), the temperature of the melt pool rose, reducing the viscosity and surface tension gradient of the melt, which was conducive to the flow and spread of molten metal liquid. Additionally, a significant amount of metal powder melted into liquid droplets, causing the melt pool size to expand further. This was consistent with the previous analysis regarding the impact of high laser power and low scanning speed on the thermal gradient of the melt pool. After solidification, the shape of the melt track was uniform, and the continuity of the melt increased. As the melt pool area increased without changing the hatch spacing, the overlapping area between adjacent melt tracks also increased, leading to a noticeable remelting phenomenon. That is, the solidified or semi-solidified melt in the previous melt track was expanded and covered by the melt pool of the subsequent track, and any discontinuous melt was remelted to form a surface of good quality [7] (Figure 14(b1,b2)). Additionally, spatter particles that existed  
Figure 14: Local surface morphology of uniform surface on multi-layer samples: (**a1,a2**) wavy morphology; (**b1,b2**) uniform surface. Figure 13: Spattes and unmelted powder particles on a uniform single-layer surface: (**a**) SED = 100 Jmm\({}^{-2}\); (**b**) SED = 116.67 Jmm\({}^{-2}\); (**c**) the process of spatter formation. on the processed surface of the previous layer were also fully remelted into metal droplets due to the high-energy-density input.