In situ study on the oscillation of mobile droplets and force analysis during the directional solidification of Al-Bi alloy

During solidifications of immiscible alloys, the motion of droplets at the solid/liquid (S/L) interface is generally driven by dragging force, gravity force, repulsion force of interface, and thermal-solutal Marangoni force. However, there is few in situ study investigating kinetics behavior to analyze the forces on droplets. The mechanism of droplet motion remains unclear due to the unavailability or uncertainty of the effect of convection and solutal Marangoni force on droplet behavior. In this study, directional solidification of immiscible Al-Bi alloy was observed via synchrotron radiography, and the horizontal oscillation of droplets at S/L interface was detected for the first time. Forces, especially solutal Marangoni force, were calculated based on the in situ measured radius of droplets and thermal-solutal gradients. The experimental results cannot be reasonably explained by the previous analysis model which neglects melt convection. The non-negligible effect of flow on droplet motion was demonstrated, and the force balance of droplet both vertically and horizontally can be obtained considering a lift force of 6.39 × 10−9 N and a modified solute-related parameter dσdc of 0.45−0.65 J m−2, respectively.