Laser metal additive manufacturing has the potential to revolutionize the production of complex geometries with high precision across various industrial applications. To optimize the reliability of the process, a detailed understanding of the melt pool dynamics during the process is essential, particularly in the nearly pore-free regimes. In this study, the melt pool dynamics across conduction mode to keyhole mode in laser powder bed fusion processing of stainless steel 316 L have been investigated by means of in-situ synchrotron X-ray imaging utilizing tungsten particles as tracers. The spatial distribution of the fluid flow in the melt pool has been quantified with a resolution of ∼10 µm through automatic tracing all moving particles in the melt pool. The results identified the influence of the interplay between laser power and scanning speed on melt flow velocities. The measurements also revealed a pronounced impact of the inward Marangoni convection on the conduction-keyhole threshold, offering a new degree of freedom to broaden the pore-free process window of laser-based additive manufacturing. These findings contribute to a more comprehensive understanding of the melt pool dynamics during laser metal additive manufacturing and provide a valuable reference for calibrating high-fidelity computational fluid dynamics models.