The heat-transfer coefficient during the unidirectional solidification of an Al-Si alloy casting

The heat-transfer coefficient was measured during the unidirectional solidi fication of Al-7 wt pet 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 obtai ned with solidification vertically upward associated with higher values tha n solidification vertically downward. Horizontal solidification was associa ted with intermediate values. Profiles taken across the diameters of the ca sting surfaces at the interface with the chill showed them to be convex tow ard the chill by an amount which would produce a mean gap between the casti ng and the chill that would account for a significant proportion of the mea sured heat-transfer coefficient. The convex casting surfaces were attribute d to the deformation of the initial casting skin by the thermal stress prod uced during solidification. Heat transfer during casting solidification is shown to be a complex mechanism controlled by the microscale surface roughn ess of the respective surfaces, mesoscale deformation of the casting skin b y thermal stress, and macroscale movements of the casting and the chill due to their relative thermal expansion and contraction.