Numerical study of melt pool dynamics and bubble behavior in the laser powder bed fusion remelting process

• A high-fidelity model simulates melt pool and bubble behavior in laser melting • Distinct flow and thermal characteristics are observed in fusion and remelting • Two bubble escape mechanisms in remelting contribute to pore suppression • Bubble and pore evolution is tracked and analyzed over time and position Laser remelting (LR) is an effective method for reducing porosity defects formed during laser powder bed fusion (LPBF). However, the underlying mechanisms of pore formation and suppression, as well as the relationship between melt pool dynamics and bubble behavior during the LPBF and subsequent LR processes, remain inadequately understood. In this study, melt pool flow and bubble dynamics under depression-mode conditions during LPBF and LR were systematically investigated using a computational fluid dynamics (CFD) model coupled with a laser ray tracing algorithm. The results indicate that during both the LPBF and LR processes, the molten liquid near the melt pool surface exhibits centrifugal flow patterns. The molten flow within the melt pool is influenced by both Marangoni convection and the spatial distribution of vapor recoil pressure. During the LPBF process, bubbles formed within the melt pool move synchronously with the molten liquid, and some bubbles coalesce during their migration, eventually becoming trapped as pores within the solidified layer. Subsequently, in the LR process, these pores are reactivated into bubbles due to reheating and again move synchronously with the molten liquid. The escape mechanisms of these bubbles depend on their initial positions within the melt pool: bubbles located deeper tend to merge into the depression region and escape, while bubbles situated closer to the melt pool surface escape directly upward. This study enhances the understanding of the complex multi-physical interactions occurring during LPBF and LR processes.