A Coupled Thermo-Mechanical Study of Light Alloy Solidification in a Permanent Composite Mould

It is widely believed that the interfacial gap which forms between the casting and a permanent mould during the solidification process of an aluminium alloy substantially reduces heat transfer rates across the casting-mould interface. The twin aims of the present work are: (a) to investigate, through the measurement of the interfacial gap, the influence of selected casting parameters on the initiation time and growth rate of the gap, and (b) to obtain a correlation between the measured transient gap size and the calculated casting-mould interfacial heat transfer coefficient.

The strategy to achieve aim (b) above has been first to estimate the transient heat transfer coefficients by solving the inverse heat conduction problem, and then to relate the results with experimentally measured interfacial gap widths through time. In order to demonstrate the usefulness of the exercise, the correlation so obtained is fed into VULCAN, a fully coupled thermo-mechanical software model, which calculates temperature and displacement histories of the casting and the mould. VULCAN's predictions are validated with experimentally obtained values.

In the experiments, commercial aluminium alloy Al-7Si-0.3Mg is gravity cast in a composite permanent mould at two widely different mould initial temperatures. It is found that the gap initiates when the alloy temperature near the casting surface falls to approximately 540-520°C. It is also observed that the rate of growth of the gap width is greater in the lower mould preheat case. The cause for the latter is shown to be the combined effect of a higher casting shrinkage rate and a larger mould expansion rate.