Investigation of the effect of fluid flow on microstructure evolution in Al-Si-Fe alloys: The MICAST project

Abstract

In secondary Al-Si-alloys, the presence of small amounts of Fe causes the formation of Fe-intermetallic phases which have a negative effect on mechanical and physical properties of castings. To understand the interaction of fluid flow and precipitating intermetallic phases and their effect on microstructure formation, unidirectional solidification experiments have been carried out on AlSi7 and AlSi7Fe1 alloys as a function of temperature gradient, G, and growth rate, v, under diffusive and controlled fluid flow conditions in the ESA payload Materials Science Lab (MSL) onboard the ISS. A strong segregation of Fe and Si towards the sample centre under convective conditions is observed. The influence of fluid flow and intermetallic phases on the solidifying microstructure was characterized by changes of primary and secondary dendrite arm spacing. In the pure binary alloy the secondary dendrite arm spacing increases under convective conditions, clearly revealing the dramatic effects of convection on solidification kinetics. Instead of a cube root relation it is observed that the secondary arms coarsen as the square root of solidification time. In contrast to this, even small amounts of iron eliminate possible effects of flow on the secondary dendrite arm spacing. These results are explained from the viewpoint of the blocking effect of the Fe-intermetallic phases on fluid flow leading to a loss in permeability of the mushy zone and a decreased solute transfer.