Effect of precipitation and sedimentation of primary α-Fe phase on liquid metal quality of cast Al–11.1Si–0.4Mg alloy

Abstract

A novel approach, the heat treatment of liquid metal, was initiated to investigate the possibility of removing oxide films and primary intermetallics from aluminium melts and its effectiveness. The concept was to remove oxide cracks from liquid metal by precipitation of primary intermetallics onto oxide films, causing them to sink under the weight of the particles. The sedimentation of primary α-Fe particles was investigated at 600°C using an Al–11·1Si–0·4Mg alloy. The liquid metal quality was assessed by reduced pressure and pressure filtration tests. After sedimentation processing there is a clear improvement in liquid metal quality due to the sedimentation and separation of primary α-Fe particles, causing the removal of some oxide films from aluminium melts. Thus the heat treatment of liquid metal has the potential to improve liquid metal quality. Primary α-Fe phase seems to precipitate onto the wetted sides (i.e. the outer interfaces) of double oxide films. However, the gap between the two dry sides of a double film constitutes a crack, thus explaining the common observation of cracks in and around such intermetallics. Hydrogen precipitation into the central oxide films in intermetallics may open the oxide films, or even expand and convert the cracks into open porosity. The oxide films may further open during a later stage of freezing because of shrinkage and cooling stress in the solid state.