The Heat-Transfer Coefficient during the Unidirectional Solidification of an AI-Si Alloy Casting

The heat-transfer coefficient was measured during the unidirectional solidification of Al-7 wt pct Si alloy castings against a water-cooled Cu chill. Heat-transfer coefficients in the range of 2.5 to 9 kW m^-2 K^-1 were obtained with solidification vertically upward associated with higher values than solidification vertically downward. Horizontal solidification was associated with intermediate values. Profiles taken across the diameters of the casting surfaces at the interface with the chill showed them to be convex toward the chill by an amount which would produce a mean gap between the casting and the chill that would account for a significant proportion of the measured heat-transfer coefficient. The convex casting surfaces were attributed to the deformation of the initial casting skin by the thermal stress produced during solidification. Heat transfer during casting solidification is shown to be a complex mechanism controlled by the microscale surface roughness of the respective surfaces, mesoscale deformation of the casting skin by thermal stress, and macroscale movements of the casting and the chill due to their relative thermal expansion and contraction.