Microstructure evolution of Al-Si hypoeutectic alloys prepared by controlled diffusion solidification

The possibility of changing the dendritic microstructure associated with the conventional casting processes of hypoeutectic Al-Si alloys to non-dendritic microstructure by using the controlled diffusion solidification process (CDS) has been investigated. The successful CDS process depends on mixing two precursor alloys heated at a superheat condition near their respective liquidus temperature. Experimental work and simulation work using Ansys software were carried out in the present study by employing Al-Si and Al-Cu systems. This study investigates the effect of the content of the two precursor alloys, the mass ratio changing from 2.6 to 8.3, and the superheat of the first precursor alloy on changing the microstructure. The experimental results show that the pure aluminum used as the first precursor alloy needs more undercooling and agitation during the mixing to form the non-dendritic microstructure compared with hypoeutectic Al-Si alloys. Furthermore, mixing pure aluminum with hypereutectic alloy can change the microstructure of hypoeutectic alloys leading to extending the possibility to choose the second precursor alloy. The results also show that a higher mass ratio is preferred when mixing pure aluminum with hypoeutectic alloy. Furthermore, the microstructure of the alloy Al- 6.45Si- 4Cu- 0.5 Mg- 0.66Fe- 0.66 wt%Zn was successfully changed via the CDS process by mixing Al- 7.75Si- 0.79Fe- 0.78Zn- 0.6 wt%Mg at 2 °C superheat into Al-24 wt%Cu at around 5 °C superheat. The simulation results show that lower air bubbles and better distribution of the two precursor alloys happen during the mixing step when using the Al-Cu system.