RHEOCASTING OF Al--Si HYPEREUTECTIC ALLOYS

Abstract

Rheocasting is a recent casting technique in which a liquid alloy is vigorously agitated during its partial solidification to yield a slurry comprising of non-- dendritic solid particles uniformly suspended in the remaining liquid. Such a slurry is subsequently cast into into desired shapes by conventional methods. Most of the investigations relating to this process deal with the structure and rheological behaviour of alloy slurries and are primarily aimed at process development rather than product characterisations. In the present investigation hypereutectic aluminium-14.05 and 20.41 weight percent of silicon are rheocast with different stirring speeds and cooling rates. The slowly cooled rheocast alloy is cast into a steel mould by bottom pouring technique and the rapidly cooled rheocast alloy is made by directly taking the slurry from the cruicible by the horn and quenching in water. The microstructural properties of these alloys are studied by optical microscope and Scanning Electron~.Microscope. The proeutectic silicon particles are present as needles and cuboids . The volume fraction of them are found by point counting technique. The mechanical properties (UTS, '/. elongation)of these alloys are tested by tensometer. The UTS of the pore free rheocast alloy, .%# is extrapolated by the linear model proposed by Ghosh etal QqJ . In case of aluminium-14.05 and 20.46 weight percent silicon alloy, 00 is expected to be 268.65 MN/m2 and 147.68 MN/m2. With increase in stirring speed 6o is expected to increase further.