Metal-foam interface stability during the filling of lost foam moulds with aluminium alloys

Abstract

Aluminium Lost Foam castings were made using gravity and counter-gravity filling techniques. Tensile strength was found to be most uniform in those castings which had been filled slowly from the bottom and where the metal front had remained stable throughout filling sequence. Pores containing carbon deposits were found on the fracture surfaces of all the castings made and this suggested that the defect was caused by polymer entrapment. A Saffman-Taylor instability was observed at the interface between Hg and a viscous glucose syrup which were contained in an analogue, that was used simulate the casting of Lost Foam moulds at room temperature. The liquid degradation products of the polystyrene patterns were also found to be viscous, although this was reduced by treatment with Br. Under the same conditions of temperature and velocity, the interface observed during the filling of a Br-treated pattern was planar whereas that of an untreated pattern was unstable. This demonstrated not only that the interfacial instability entrapped polymer degradation products, which adversely affected casting quality, but was probably of the Saffman-Taylor type