# 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  
Samples with uniform surfaces had good surface quality and no significant large-scale morphological features. Therefore, based on the micro-surface images and surface roughness measurement results of the samples, a quantitative analysis of the microstructural changes present in the uniform surface was conducted, and the formation and evolution of different microstructures were studied in conjunction with the melt pool image data collected through real-time monitoring. Figure 12a shows the measurement results of single-layer surface roughness under different process parameters. When the scanning speed was constant, the arithmetic mean height (Sa) of the sample surface generally showed a trend of decreasing and then increasing with the increase in laser power. This is because defects such as balling, unmelt powder, porosities, and cracks caused by low energy input gradually decreased with the increase in energy density, the surface quality continuously improved, and Sa showed a downward trend [33; 34]. When the laser power was 300 W, an optimal process window was achieved. As the laser power continued to rise, metal vaporization formed unstable keynholes, causing Sa to increase. The scanning speed was inversely related to surface roughness Sa. As the scanning speed increased, the dwell time of the laser spot on the powder bed surface at the same location was shortened, the powder's heat absorption rate decreased accordingly, and surface roughness increased due to the incomplete melting of the powder, forming defects such as balling. In addition, high-speed scanning intensified the oscillation of the melt pool, triggering spatters that sputtered onto the sample surface [23; 32], also increasing the surface roughness Sa. Figure 11: Variation in melt pool temperature and high-temperature area in transitional surface samples during printing process: (**a**) SED = 62.5 Jmm\({}^{-2}\); (**b**) SED = 70 Jmm\({}^{-2}\). The color images represent the temperature distribution of the melt pool at different moments. We measured seven surface roughness parameters for the single-layer samples [34]: the (1) arithmetic mean height (Sa); (2) maximum height (Sz); (3) root mean square height (Sq); (4) skewness (Ssk); (5) kurtosis (Sku); (6) maximum peak height (Sp); and (7) maximum valley depth (Sv) [34]. The roughness values at different energy densities were normalized to characterize the quality level of the corresponding surfaces. As shown in Figure 12b, the weight ranged from 0 to 1, indicating roughness values increasing from low to high. Among them, the arithmetic mean height (Sa), already discussed in Figure 12a, is sensitive to the values of the maximum height (Sz), maximum peak height (Sp), and maximum valley depth (Sv). Skewness (Ssk) and kurtosis (Sku) reflect the degree and quantity of surface peak and depression undulations; the higher the values are, the greater the number of surface peaks and depressions is, the more pronounced the steep changes at the edges are, and the rougher the overall surface is. Figure 12b indicates that, considering all roughness parameters, the optimal single-layer sample printing surface results correspond to an energy density between 80 Jmm\({}^{-2}\) and 100 Jmm\({}^{-2}\), which is considered a uniform surface. Compared to the single-layer spheroidal surface and transitional surface at low energy densities, the uniform single-layer surface, due to ample energy input and thorough powder melting, formed a melt pool with a clearly defined contour and a significantly expanded melting area. The rapid temperature gradients within the melt pool caused thermal convection inside the melt pool, leading to melt pool oscillations. Under the combined effect of the steam plumes formed by metal evaporation in high-temperature areas, the liquid melt was excited to form droplet spatters [35] (Figure 13c).