Phase separation and formation of core-type microstructure of Al-65.5 mass% Bi immiscible alloys

Core-type microstructures offer a unique combination of properties of both layers. Phase separation and formation of core-type microstructure in Al-65.5 mass% Bi hyper-monotectic alloys were studied by melting in quartz crucibles with orifices at the bottom and then ejecting the melt into silicon oil. Effects of processing parameters were investigated by changing the free fall distance of droplets and the temperature of silicon oil where the droplets solidified. Based on ANSYS simulation, the cooling rate and temperature gradient of droplets were estimated. As the free fall distance and silicon oil temperature decrease, both the temperature gradient and cooling rate tend to increase, leading to great changes in Marangoni and Stokes motions, and hence causing the transitions of morphology between a crescent-shape shell and a regular shell. The solidification paths under different processing parameters were analyzed, and it was concluded and exemplified that only if the effects of Marangoni and Stokes are discreetly balanced can the regular core-type particles be obtained. (C) 2010 Elsevier B.V. All rights reserved.