Instability of the liquid metal-pattern interface in the lost foam casting of aluminium alloys

The nature of the liquid-metal pattern interface during mould filling in the Lost Foam casting of aluminium alloys was investigated using real-time X-ray radiography for both normal expanded polystyrene (EPS), and brominated polystyrene (Br-EPS) foam patterns. Filling the pattern under the action of gravity from above or below had little effect on properties, both cases resulting in a large scatter of tensile strength values, (quantified by their Weibull Modulus). Countergravity filling at different velocities demonstrated that the least scatter of tensile strength values (highest Weibull Modulus) was associated with the slowest filling, when a planar liquid metal – pattern interface occurred. Real-time X-ray radiography showed that the advancing liquid metal front became unstable above a certain critical velocity, leading to the entrainment of the degrading pattern material and associated defects. It has been suggested that the transition of the advancing liquid metal – pattern interface into an unstable regime may be a result of Saffman-Taylor Instability.