Abstract: The Marangoni effect critically governs two key movements within the molten pool of immiscible alloys undergoing liquid phase separation: the migration of minor droplets toward high-temperature regions (driven by interfacial energy minimization) and concurrent bulk melt flow toward low-temperature regions (due to a negative surface tension temperature coefficient). However, the competition between these opposing Marangoni-driven flows, significantly influencing mass transport dynamics, has received limited attention. Here, utilizing synchrotron high-speed X-ray imaging, we investigated mass transport within the molten pool of an Fe-Cu immiscible alloy. Observations reveal that Marangoni flow can drive the molten pool to stretch until fracture during stationary laser melting, and can change the morphology of the solute-enriched zone at the trailing edge of the molten pool during scanning. Further analysis revealed that the melt Marangoni flow velocity significantly exceeds the Marangoni migration rate of droplets, except for sufficiently large droplets. Consequently, the Marangoni flow predominantly governs mass transport within the molten pool and determines the final distribution of droplets. These findings resolve the long-standing ambiguity regarding the competing roles of melt Marangoni flow versus droplet Marangoni migration, providing crucial insights for tailoring microstructures in processes such as laser welding and additive manufacturing.

Key words: Synchrotron X-ray imaging, Immiscible alloy, Marangoni effect, Matter migration