Laser Powder Bed Fusion (LPBF) is a promising metal additive manufacturing (AM) technology that enables high-precision fabrication of high-performance parts with unprecedented design flexibility. However, its widespread adoption remains hindered by the occurrence of various defects, which significantly degrade the mechanical properties and reproducibility of fabricated parts. These defects are inherently linked to the complex and dynamic nature of melt pool behavior, governed by a combination of laser-material interaction, heat transfer, fluid flow, and phase transformations.
This doctoral research integrates in-situ X-ray experiments and high-fidelity computational fluid dynamics (CFD) modeling to investigate the underlying mechanisms of melt pool dynamics and laser absorption during LPBF. The work focuses on understanding fluid flow behaviors, laser energy distributions, and pore-related phenomena under various processing conditions, with particular emphasis on the role of the complex Marangoni effect.
The melt flow in LPBF of SS316L under nearly pore-free regimes was statistically quantified with a high resolution of ~10 µm by employing tungsten tracer particles during in-situ synchrotron X-ray imaging, revealing inward Marangoni convection due to surface active elements. This convection was observed to increase the conduction-keyhole threshold, thereby expanding the pore-free process window. Complementary in-situ X-ray diffraction (XRD) of laser melting provided additional insight into temperature evolution, and a new approach was proposed to estimate the melt pool width from the amorphous intensity of the liquid phase.
A high-fidelity CFD model at the powder scale was developed with the OpenFOAM framework and validated against experimental data for both Ti-6Al-4V and SS316L. By incorporating advanced physical modules such as multi-element vaporization, multiple laser reflections, temperature-dependent absorptivity, and an enhanced powder bed model, the simulation can accurately predict keyhole and melt pool morphologies, replicate measured laser energy absorption, and reproduce melt flow and pore formation observed experimentally. Moreover, offset effects of inward Marangoni flow on keyhole formation observed experimentally were captured by simulations and are shown to be valid at practical laser scanning speeds of several hundred mm/s.
A systematic parameter study across different materials and melt pool regimes using the high-fidelity CFD model provided quantitative insights into melt pool dynamics and laser energy absorption under various processing conditions, leading to the derivation of universal scaling laws for laser absorption. In addition, the model was extended to LPBF with copper, which is difficult to process using near-infrared lasers due to its high reflectivity, offering insights into strategies to improve its printability by increasing the laser absorptivity of powder and substrate.
Overall, this work advances the funda
