Casting Skin Research Articles

One-dimensional predictive model for estimation of interfacial heat transfer coefficient during solidification of cast iron in sand mould

A one-dimensional predictive model is proposed to estimate the interfacial heat transfer coefficients during unidirectional solidification of a cast iron alloy, vertically upwards, against a sand block. The model is based on the surface roughness characteristics of the casting and sand surfaces and the concave deformation of the initial solidified casting skin towards the sand surface. The modelled interfacial heat transfer coefficients and predicted temperatures inside the casting and the sand block showed an approximate agreement with experimentally determined values. The model showed that radiation was a significant mode of casting/sand interfacial heat transfer with the predicted contribution of radiation to the overall heat transfer being nearly 50%. The evaluation of the model in comparison to the interfacial heat transfer models proposed by Zeng and Pehlke suggested that the interfacial conditions considered in this model, namely, the mean peak to valley heights of the casting/sand mould surfaces and the gap width calculated from the deformation of the initial solid skin, gave a more accurate prediction. This predictive heat transfer model has an advantage over the inverse modelling technique as the matching of experimentally measured temperatures to determine the boundary conditions is avoided and the heat transfer coefficients can be estimated as an integral part of the casting simulation.

The heat-transfer coefficient during the unidirectional solidification of an Al-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.