Matter transport and phase separation behavior in laser-processed Al-Bi immiscible alloy revealed by high-speed X-ray imaging

The unique phase separation characteristics of immiscible alloys enable creating new materials with distinctive microstructures and properties. However, traditional casting methods tend to introduce macro segregation and lack the capability for fine-tuning microstructure. While laser additive manufacturing provides a feasible way to dynamically regulate phase separation, precisely tailor second-phase distribution, and directly fabricate complex components. The mechanisms governing melt flow, mass transport, and their influence on phase separation, however, are not yet fully understood. Using synchrotron high-speed X-ray imaging technique, we comparatively investigated the mass transport behavior and solidification microstructure evolution in Al-Bi immiscible alloy during static laser melting and laser scanning. During static laser melting, two distinct transport modes of Bi-rich phase exist within molten pool, bubble-induced solute transport and Marangoni motion. After laser deactivation, liquid phase separation initiates at molten pool bottom and is followed by occurrence at the top, with both fronts propagating inward to eventually form a uniformly distributed microstructure. During laser scanning, liquid phase separation is triggered at the molten pool rear due to rapid cooling, generating numerous fine Bi-rich droplets. These droplets are driven to rotate continuously by the clockwise vortex flow, while exhibiting an overall net displacement tendency along the scanning direction. This study provides a theoretical basis for regulating second-phase distribution and optimizing the performance of immiscible materials through laser processing.