Melt flow in a mushy zone - barrier effect of intermetallic phases

Abstract

Iron and manganese are common impurity elements in cast aluminium alloys, especially in secondary aluminium. During casting Fe/Mn-containing intermetallics are formed between the aluminium dendrites, which cause porosity and shrinkage defects. In this paper an experimental study on the influence of controlled convection during solidification on the spatial arrangement of intermetallic phases and their interaction with the dendritic microstructure in Al-7Si-1Fe (AlSiFe) and Al-7Si-1Mn (AlSiMn) alloys (wt-%) is presented. Forced convection is induced by a rotating magnetic field. The alloys are solidified directionally over a range of constant solidification velocities (0·015-0·18 mm/s) at a constant temperature gradient G of 3 K/mm. The results indicate that the primary spacing and the secondary dendrite arm spacing are affected by the presence of Fe and Mn intermetallic phases. In samples solidified under forced convections the primary dendrite arm spacing did not depend on the solidification velocity and no obvious fluid flow effect on the secondary spacing could be detected. These observations are in contrast to Fe and Mn free alloys. It seems that the intermetallics act as a barrier for the flow into the mushy zone.