# In Situ Study of Surface Morphology Formation Mechanism During Laser Powder Bed Fusion
## 3 Results and Discussion
### Evolution of Melt Pool Characteristics
#### 3.1.3 Evolution and Remelting of Melt Pool  
Figure 6 displays images of the melt pool and melt track under three energy densities. The melting and solidification process of the powder during surface formation at a low energy density (SED = 50 Jmm\({}^{-2}\)) is shown in Figure 6a. The laser input energy was insufficient to completely melt the metal powder; upon heating, the powder partially softened and adhered to itself, forming slightly larger and separated powder agglomerates that also bonded to the base plate surface. As the processing proceeded, the region of the powder bed that was heated and melted increased, and the size of the particles formed from the agglomeration of metal powder further enlarged. Additionally, some of the powder completely melted into droplets, causing the nearby agglomerated particles to further merge into larger-sized spherical particles on the surface. Due to the absence of a melt pool, there was virtually no remelting occurring between adjacent melt tracks [27]. When the SED was increased to 75 Jmm\({}^{-2}\), the metal powder was essentially fully melted, forming a melt pool with distinct edges, significantly enhancing the melt formation effect. As depicted in Figure 6b, the evolution of the melt pool shows that as the laser spot moved forward, some trailing melt broke off during solidification, resulting in discontinuous melt tracks. This phenomenon is attributed to the Marangoni effect during the solidification of the melt pool, which generated surface tension [9] that compelled the molten metal droplets to contract, leading to the detachment of the melt pool's tail and the formation of separated balling. Given that the width of the melt pool was slightly larger than the hatch spacing, the reduced overlap area decreased heat accumulation, which was detrimental to the flow and spread of the melt pool [7]. The molten liquid could not fully fill the gaps in the overlapping areas, resulting in depressions between adjacent melt tracks, while the overall surface formed a wavy texture parallel to the scanning line direction. Figure 5: The effect of scanning speed on the temperature distribution of the melt pool: (**a**) \(V\) = 60 mm/s (Supplementary Video ESM_10); (**b**) \(V\) = 80 mm/s (Supplementary Video ESM_7); (**c**) \(V\) = 100 mm/s (Supplementary Video ESM_11); (**d**) \(V\) = 120 mm/s (Supplementary Video ESM_12); (**e**) the effect of scanning speed on the temperature distribution along the centerline of melt track (\(P\) = 300 W). When the energy density was sufficiently high (SED = 100 Jmm\({}^{-2}\)), a large amount of powder melted into molten droplets, reducing the viscosity and surface tension gradient of the melt and further enlarging the size of the melt pool [28], which was consistent with the effects of high laser power and low scanning speed on the thermal behavior of the melt pool as discussed in Section 3.1.2. The shape of the solidified melt track was uniform, and the continuity of the melt was increased. Due to the increase in the melt pool area, the overlap area between adjacent melt tracks also increased, and the remelting phenomenon was obvious. The solidified or semi-solidified melt from the previous track was expanded and covered by the melt pool of the subsequent track, and the discontinuous melt that may have existed was remelted to form a surface of good quality (Figure 6c). Because the laser dwell time at the contour edges was brief during the reciprocating scan along the path, and the wettability between the molten droplets and metal powder was poor, coupled with the low thermal conductivity of the powder particles [29], the melt at the end of the melt track did not further  
Figure 6: Continuous frames and characteristics of the melt pool corresponding to single-layer surfaces at different energy density levels: (**a**) At a low energy density (SED = 50 Jmm\({}^{-2}\)), incompletely melted powders fused with each other to form larger-sized spherical particles (Supplementary Video ESM_1).