# In Situ Study of Surface Morphology Formation Mechanism During Laser Powder Bed Fusion
## 3 Results and Discussion
### Surface Morphology Evolution  
An ultra-depth-of-field scanner was used to image the surface morphology of all samples, and the results indicate that samples with different SEDs or VEDs exhibited a variety of morphological features. Based on the overall three-dimensional surface morphology, the single-layer sample surfaces can be categorized into three typical types: spheroidal surface, transitional surface, and uniform surface, as shown in the high-resolution optical images in Figure 7.  
Figure 7: Three-dimensional surface morphology of single-layer samples under different process parameters: (**a1,a2**) spheroidal surface, SED = 50 Jmm\({}^{-2}\); (**b1,b2**) transitional surface, SED = 62.5 Jmm\({}^{-2}\); (**c1,c2**) uniform surface, SED = 100 Jmm\({}^{-2}\). Figure 7 indicates that the surface quality of the samples improved with the increase in SED. When SED = 50 Jmm\({}^{-2}\), combining the analysis of melt pool morphology from Section 3.1.3, it can be deduced that the metal powder did not fully melt into a liquid state upon heating; instead, it only partially softened and adhered to the base plate surface. Partially melted powder adjacent other aggregates formed dense balling micro-particle protrusions (Figure 7(a1,a2)). As the scanning speed decreased and the laser power increased, the energy input rose, and the sample with SED = 62.5 Jmm\({}^{-2}\) formed a transitional surface (Figure 7(b1,b2)). The starting part of the sample still exhibited a spheroidal surface, but the degree of powder melting increased, and the size of the particles formed by powder aggregation enlarged, with the size of the surface spheroidal being larger than that of the spheroidal surfaces under low-SED conditions. The surface quality of the latter half of the transitional surface samples improved, with a sharp decrease in the number of spheroidal particles. As the SED was further increased to 100 Jmm\({}^{-2}\), the samples exhibited a flat and uniform surface (Figure 7(c1,c2)), with almost no observable unmelted powder or spheroidization, resulting in a significant improvement in surface quality. The surface morphology characteristics of multi-layer printed samples were not only influenced by process parameters but also related to the surface quality of the previous layer and interlayer bonding. When process parameter conditions were kept constant, the surfaces of each layer of the same sample usually exhibited similar characteristics. Under multi-layer printing accumulation, the morphology that ultimately affected the top layer surface also made the intermediate layer surface morphology during the printing process partially an indicator of the part construction quality [30]. As shown in Figure 8, multi-layer samples accumulated from stacking three types of single-layer surfaces also exhibited different surface morphologies. The continuous stacking of single-layer spheroidal surfaces formed an irregular reticulated surface (Figure 8a), characterized by the alternating distribution of grooves and protrusions. These concave-convex features often connected and were adjacent or spanned multiple melt tracks, forming locally continuous irregular morphologies [5]. The stacking of single-layer transitional surfaces formed an undulating surface (Figure 8b), with significantly better surface quality than the reticulated surface. The central area of the sample was relatively smooth but still had noticeable undulations and troughs. The contour edges exhibited a dense distribution of spheroidization and protrusions. When single-layer uniform surfaces were stacked, the multi-layer sample formed a similarly smooth and uniform surface (Figure 8c). Compared to the other two surface types, the uniform surface had significantly reduced peak height values, and the edge contours were clear and continuous.