The effect of applied pressure from gravity up to 1.7 GPa during solidification of Al-5.4wt%, 35wt%Cu and 5wt% Fe alloy on the microstructure and heat transfer phenomena have been investigated.
The cooling rate of Al-Cu alloy increased with increasing the pressure up to GPa level, but it was not linearly increased. The dendrite arm spacing of Al and eutectic spacing of $Al-CuAl_2$ refined with the increase of pressure up to 1.7 GPa during solidification, and also it's microstructures were more refined than thoses of gravity cast under the condition of the same cooling rate. From the results of experimental work, applied pressure during solidification enhanced both cooling rate and undercooling. The thermodynamic undercooling by Clausius-Claypeyron equation was dominant factor of microstructural refinement under GPa pressure. In Al-5.4wt%Cu alloy solidified under GPa pressure, volume fration of interdendritic eutectics were decreased and large of metastable $\theta'$ at interdendritic Al boundary was formed. It can be thought by increasing of solid solubility by high pressure phase equilibria.
The primary $CuAl_2$ in hypereutectic Al-35wt%Cu and $Al_{13}Fe_4$ in Al-5wt%Fe alloy were transition to regular eutectic under the pressure of GPa, and it was due to the enhanced thermodynamic undercooling by changing of the growth mechanism to coupled zone of eutectic. The eutectics of $Al-CuAl_2$ were grown near by the condition of maximum stability and the irregularity of eutectic growth, $\psi(\overline{\lambda}/lambda_m$, was 3.2. The growth of primary $CuAl_2$ was controlled by facets at the tip and dendritic growth of $CuAl_2$ under GPa pressure was promoted.
A theoretical model was developed and analysed to predict the interfacial heat transfer coefficient during solidification as function of interface contact ratio, contact gap thickness, thermal properties and interfacial conditions.
The changes of heat transfer coefficient according to applied pressures were approximated by the cooling rates measured, comparing with the theoretical cooling curves calculated by 2-dimensional solidification analysis. The heat transfer coefficient of Al-Si alloys was about $10^3W/m^2K$ in gravity and increased to $10^4W/m^2K$ under the pressure of 110 MPa, to $10^5W/m$^2K$ under the GPa.
