Modelling of the potential for oxide film entrainment in light metal alloy castings

It has been established that the entrapment of surface oxide film originating from a poor running system design has a critical role in determining final casting quality. One of the clearest applications of this work is the concept of a critical velocity of 0.5 ms^-1 at the ingate. The consequence of exceeding this velocity is that the liquid metal jets into the mould cavity leading to the entrainment of the surface oxide film. However, the relative quality of the liquid metal as a result of previous metal transfer stages is unclear. A modelling approach was used to assess the decrease in quality in the liquid aluminium to be inferred from ingate velocities of from 0.1 ms^-1 to 1.0 ms^-1 by examining the changes in the modelled free surface area. Other metal transfer stages in the casting process were then modelled for comparison, such as the transfer from the furnace to the crucible, from the crucible to the pouring basin and etc. The results show that the free surface area produced during filling of the crucible and the pouring basin was much greater than the free surface area associated with filling a casting with an excessive ingate velocity. Hence these results indicate that a reduction in liquid metal quality can easily occur in prior metal transfer stages before entering the mould cavity. Monitoring the free surface area during modelling is a useful indicator for optimising a casting process.