Using the calculated Froude number for quality assessment of casting filling methods

Abstract

The reliability of cast components is dependent on the quality of the casting process. This can be characterised by the robustness (repeatability) and specific fluid flow characteristics within the running system. During this transient filling phase the prevention of free surface turbulence and thus oxide entrainment is critical to ensure the mechanical integrity of the component. Past research has highlighted that return waves are major causes of free surface entrainment. To reduce the entrainment occurring during the transitional filling of the runner a steady quiescent flow must be developed. Using FLOW-3D the Froude number has been extracted from simulated casting filling to allow the quantitative prediction of air entrainment for a number of different flow conditions. Different running system geometries have been simulated and the overall quality of the running system performance assessed using the Froude number entrainment criterion. The results have been compared to real-time X-ray imaging of transient aluminum alloy flow in running systems. The results show that, for the designs used, the correctly designed geometry is advantageous. An incorrect design may reduce the Froude number but can greatly increase the persistence of the return wave and entrainment and is therefore extremely detrimental to the cast component. The addition of a filter created a deeper quiescent incoming flow and greatly reduced the persistence giving a low total entrainment value. Additionally, the in-gate design is of utmost importance in controlling the back pressure and thus the persistence of the back wave between the in-gate and the downsprue exit. This has a direct effect on the level of oxide entrainment. The quantitative Froude number data obtained from the FLOW-3D model were seen to correlate well with the qualitative real time X-ray data, where as the lowest frequency of bubble occurrence and smallest size was observed in systems containing a filter.