Droplet evolution and rotational flow mechanisms during in-situ fusion in powder bed with millimeter-scale large spot

Conventional Laser Powder Bed Fusion (LPBF) technology uses a ∼100 μm single laser spot to melt metal powder line by line, facing efficiency/cost bottlenecks for large components. Millimeter-scale large-area exposure boosts efficiency but lacks clarity on melt pool dynamics/regulation. This work studies droplet evolution/flow in 3.2 mm laser in-situ fusion. High-speed photography was employed to comprehensively investigate the influence of laser power (1400–2000 W), exposure duration (30–200 ms), and powder preheating (20/300 °C) on the dynamic features of the melt pool. The findings demonstrate that droplet evolution is characterised primarily by vortical rotational flow, influenced collaboratively by laser recoil pressure, viscous shear force, and surface tension. Both laser power and exposure duration regulate the ultimate droplet size via total energy input; nevertheless, elevated power amplifies melt pool oscillations and spatter, with spatter diameters reaching around 700 μm at 2000 W. Preheating the powder to 300 °C markedly enhances droplet size and development rate (achieving a final diameter of around 3380 μm at 1700 W) and efficiently mitigates spatter (maximum spatter diameter decreased to around 560 μm). Additionally, numerical simulations and theoretical research demonstrated that the initial vorticity, influenced by the baroclinic term, and radial pressure gradients are the fundamental mechanisms for vortex generation. They clarified the mechanisms by which powder preheating facilitates droplet development and improves precision control. This research offers theoretical backing for the advancement of large-area melting powder bed technology and holds significant engineering importance for enhancing the efficiency and stability of metal additive manufacturing.