Thermal-fluid characteristics and defect formation mechanisms in laser powder bed fusion of overhanging surfaces

Laser powder bed fusion (L-PBF) technology has demonstrated significant advantages in the fabrication of complex structural components due to its high design freedom, excellent mechanical properties, and short manufacturing cycles. However, when manufacturing parts with overhanging surfaces using L-PBF, the formation of incompletely molten particles on the surface significantly deteriorates surface quality and mechanical performance. This study investigates the formation mechanisms and influencing factors of incompletely molten particles through a combination of numerical simulations and experiments. A theoretical model of phase transformation, heat transfer, and mass transfer during the L-PBF process was established, and the flow and thermal characteristics of the molten pool during the formation of overhanging surfaces were simulated at the mesoscale. Additionally, the effects of laser power, number of layers, and recycled powder on the microscopic surface morphology and roughness were experimentally examined. The results reveal that overhanging surfaces exhibit a higher prevalence of incompletely molten particles and powder adhesion. The quantity and distribution of these particles are primarily influenced by molten pool subsidence, uneven heat conduction, and unstable molten pool flow. Furthermore, the heated surfaces of incompletely molten particles become wet and adhesive, leading to the adsorption of underlying powder particles and the formation of clusters, which increases the depth and volume of the incompletely molten regions. Specifically, reducing the laser power density from 49 J/mm 3 to 25 J/mm 3 decreased the surface roughness (Ra) of overhanging surfaces by approximately 25% (from 24 μ m to 18 μ m ). Moreover, the use of fresh metal powder—compared to recycled powder—significantly suppressed the formation of incompletely molten particles and mitigated oxidation-induced cracking, thereby enhancing surface integrity. This study provides theoretical insights and practical guidance for the application of L-PBF in the fabrication of overhanging structures, offering significant implications for enhancing the performance of overhanging surfaces.