The Influences of Micro-Alloying Element Sn and Magnetic Field on the Microstructure Evolution of Al–Bi Immiscible Alloys

An investigation was conducted through directional solidification experiments to explore the impact of micro-alloying element Sn and a magnetic field on the solidification behavior of immiscible Al–Bi alloys, as well as the combined effect of Sn and the magnetic field. Experimental results show that the size distribution of the dispersed particles in the low-speed solidified Al–3.4 wt.%Bi alloy presents two peaks, while it only shows one peak when solidified at a relatively high speed. The addition of Sn not only can enhance the nucleation rate and the number density of the Bi-rich droplets in the sample, but also decrease the Marangoni migration velocity and the axial resultant velocity of minority phase droplets in front of the solidification interface. Thereby it promotes the formation of Al–Bi alloys with a well-dispersed microstructure. A static magnetic field with the strength of 0.2 T increases the number density of the dispersed particles and decreases the average size and the size distribution width of the dispersed particles. Under the effect of Sn addition and static magnetic field, the average radius of the dispersed particles R and the solidification velocity V0 satisfy R∝V0−1/3 when the alloy was solidified at a relatively low velocity, R and V0 satisfy R∝V0−1/2 when the alloy is solidified at a high velocity.