# In Situ Study of Surface Morphology Formation Mechanism During Laser Powder Bed Fusion
## 1 Introduction
However, due to the differences in optical bands between the laser and monitoring equipment, as well as spatial constraints of LPBF machinery, these multi-camera coaxial monitoring methods often suffer from issues such as excessive imaging focal length, low magnification, and insufficient input light intensity. These factors result in poor image quality, small object size in the captured images, and low pixel resolution, which are not conducive to subsequent analysis. Furthermore, the coaxial setup of multiple cameras may lead to redundancy in the overall optical components and a complex installation process. Some studies are based on dual-wavelength thermometry, using a single camera for dual-wavelength imaging to calculate the temperature field distribution in the monitoring images [15; 20; 21]. While this method can simultaneously acquire visible light and infrared information, it requires high calibration accuracy in the monitoring equipment and necessitates preliminary blackbody radiation experiments to match light intensity and temperature, thus making it prone to errors in temperature field calculations due to poor calibration [20]. Therefore, current common monitoring methods struggle to effectively achieve real-time in situ data acquisition in the complete dynamic process of melt pool melting and solidification, as well as the transient changes in surface morphology for each intermediate layer surface in LPBF. Furthermore, while widely adopted simulation methods can effectively simulate the process under specified parameters, they demand substantial computational resources, and the accuracy of the models is influenced by various factors, including material properties, process parameters, and environmental conditions. The uncertainty of these factors may lead to discrepancies between simulation results and actual conditions [21; 22]. This study develops an innovative coaxial monitoring system incorporating a visible-light high-speed camera and infrared thermometer, effectively mitigating inherent limitations of off-axis configurations in viewing angle and image fidelity. Through modular compact design and optical magnification optimization, the system achieves the synchronized high-resolution monitoring of melt pool morphology dynamics, temperature gradient distribution, and defect formation processes (e.g., balling and spattering). By systematically correlating multi-modal monitoring data with quantitative surface characterization, the intrinsic mechanisms linking energy density-regulated melt pool dynamics to multi-scale surface morphology evolution are revealed. The spatiotemporal coordination between morphological transitions and thermal behavior during solid-liquid phase changes is decoded, elucidating the layer-wise accumulation mechanisms of characteristic surface features (edge protrusions, reticulated structures, and large-scale local surface agglomerations). Furthermore, a cross-scale quantitative mapping framework connecting process parameters, melt pool behavior, and surface topography is established, providing theoretical foundations for predictive quality control in LPBF.