Effect of mixing rate and temperature on primary Si phase of hypereutectic Al-20Si alloy during controlled diffusion solidification (CDS) process

Controlled Diffusion Solidification (CDS) is a promising process relied on mixing two liquid alloys of precisely controlled chemistry and temperature in order to produce a predetermined alloy composition. In this study, the CDS was employed to prepare hypereutectic Al-20%Si (mass fraction) alloy using Al-30%Si and pure Al of different temperatures. The mixing rate was controlled using three small crucibles with a hole of different diameters in their bottom. The effect of mixing rate and temperature on the microstructure of the primary Si-phase during the mixing of molten Al and Al-30%Si was studied. The results showed that when the diameter of the small crucible bottom hole is 16 mm, a higher mass mixing rate 0.217 kg.s(-1) would results in a lower stream velocity 0.414 m.s(-1). Conversely a lower mass mixing rate 0.114 kg.s(-1) (the diameter of the small crucible bottom hole is 8 mm) would result in a higher fluid stream velocity 0.879 m.s(-1). A lower mass mixing rate would be better to refine the primary Si than a higher mass mixing rate. Meanwhile, the morphology and distribution of primary Si could also be improved. Especially, when Al-30%Si alloy at 820 degrees C was mixed with pure Al at 670 degrees C in the case of a mass mixing rate of 0.114 kg.s(-1) and a pouring temperature of 680 degrees C, the average size of the primary Si phase would be only 18.2 mu m. Its morphology would mostly be octahedral and the primary Si would distribute uniformly in the matrix microstructure. The lower mass mixing rate (0.114 kg.s(-1)) will enhance the broken tendency of Al-30%Si steam and the mixing agitation of resultant melt, so the primary Si phase can be better refined.