Numerical simulation and X-ray direct observation of mould filling during vacuum suction casting

The mould filling of the aluminium casting alloy AC4C was directly observed by X-ray radiography in order to evaluate the accuracy of simulated results for the mould filling behaviour of a vacuum suction casting process. A numerical method has been applied to simulate mould filling behaviour, in which evacuation of gas through the suction vent is taken into account and the backpressure of gas is coupled with the pressure field of the melt. Backpressure of gas in the empty mould cavity was also measured using a semiconductor pressure sensor. Experiments have been carried out at two different suction pressures (10 and 20 kPa) and three different decompression rates (1-2, 4-2 and 80 kPa s(-1)), controlled by the aperture of the valve between the gas pipe of the vacuum suction system. Experimental results showed that the effects of suction pressure and decompression rate on the mould filling behaviour were significant. The higher the suction pressure and decompression rate, the more turbulent the free surface of molten metal, resulting in earlier occurrence of gas entrapment, larger volumes of entrapped gas and higher risk of misrun. By comparing simulated results with experimental ones, it was found that good agreement could be obtained in terms of mould filling patterns, positions of gas entrapments and mould filling time. It was also found that results simulated by the conventional method, in which gas evacuation and the effects of backpressure on mould filling are not taken into account, showed poor agreement with experimental values.