# In Situ Study of Surface Morphology Formation Mechanism During Laser Powder Bed Fusion
## 4 Conclusions  
Our research, based on the self-developed dual coaxial imaging monitoring system for LPBF, captured the evolution of melt pool characteristics and temperature distribution during the LPBF process. Combined with micro-images of the sample surface structure, we investigated the impact of different process parameters on the surface morphology of the samples and the evolution mechanism of the melt pool. Samples across different layers were categorized based on their surface morphology into the following: single-layer  
Figure 16: (**a**) Inclusions of unmelted powder within the part due to insufficient melting. (**b**) The formation mechanism of edge contour protrusions and powder inclusions. samples with a spheroidized surface, transitional surface, and uniform surface; and multilayer samples with a reticulated surface, undulating surface, and uniform surface. Our study elaborated on the formation principles of different types of surfaces and focused on discussing the evolution mechanisms of local features in multi-layer samples, such as edge contour protrusions, reticulated structures, and local surface large-scale protrusions. 1. The melt pool underwent phased evolution with increasing energy density; unmelted powders formed spherical droplets at low energy, progressively developing into hemispherical shapes and ultimately teardrop shape accompanied by keyhole formation. Melt pool area was predominantly controlled by laser power. Temperature field analysis revealed higher front-end temperature gradients due to the powder's low thermal conductivity, while laser power exerted stronger control over gradient distribution than scanning speed. Elevated temperature gradients induced by increased energy input intensified the Marangoni effect, promoting melt pool expansion and enhancing inter-track remelting to achieve surface homogenization. Concurrently, the combined effects of the powder's low thermal conductivity and insufficient melt wettability resulted in melt accumulation at edges perpendicular to the scanning direction. 2. Low-energy input induces partial powder melting, forming spheroidal surfaces, with spheroidized particles self-organizing into inter-track reticulated structures as layer count increases. Thermal accumulation reduces melt cooling rates, facilitating adjacent track expansion and large-scale protrusion formation. 3. In multi-layer uniform surfaces, surface tension drives melt pool adhesion to solidified melt, creating contour-rough yet internally smooth morphologies, where spatters from melt pool oscillations predominantly affect surface uniformity. 4. Wettability differentials govern contour protrusion formation modulated by melt pool dimensions, while unmelted powders generate inclusions between protrusions and substrate. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app15052550/s1](https://www.mdpi.com/article/10.3390/app15052550/s1)]([https://www.mdpi.com/article/10.3390/app15052550/s1](https://www.mdpi.com/article/10.3390/app15052550/s1)). Video S1: High-speed camera images acquired during multi-layer LPBF of 316 L under a power P = 250 W, a scan speed \(v\) = 80 mms\({}^{-1}\), and a surface energy density SED = 62.5 Jmm\({}^{-1}\) showing the small-sized melt pool and incomplete powder melting phenomenon at low energy density. Video S2: High-speed camera images acquired during multi-layer LPBF of 316 L under a power P = 300 W, a scan speed \(v\) = 80 mms\({}^{-1}\), and a surface energy density SED = 75 Jmm\({}^{-1}\) showing the increase in melt pool size and the phenomenon of balling as energy density increases. Video S3: High-speed camera images acquired during multi-layer LPBF of 316 L under a power P = 350 W, a scan speed \(v\) = 60 mms\({}^{-1}\), and a surface energy density SED = 116.67 Jmm\({}^{-1}\) showing the large-sized teardrop-shaped melt pool at high energy density and spatter flying towards the powder bed.