Inverse segregation and near-surface microstructure development in aluminum-copper alloy castings

The strip casting of aluminum is a common, low-cost method of achieving a thin gauge casting that is suitable for further processing into sheet. However, if Al-4.5wt%Cu or similar high strength wrought alloys are produced by this method, they all tend to show inverse segregation. This produces significant degradation of the surface quality of the casting, rendering it unsuitable for further processing.; This project has been directed towards developing an improved understanding of the influence of the mold surface conditions and the thermal behaviour at the mold/casting interface on inverse segregation. The experimental work was focused on the analysis of near-surface microstructures and inverse segregation obtained under different process parameters. A slider and different thermal barriers were applied at the mold/casting interface. The temperature profiles in the molds were measured and this data was used to calculate the heat flux into the molds for correlation with the near-surface microstructure and the local inverse segregation.; The Monte Carlo method was applied to simulate the nucleation and grain growth process taking place in the liquid near the surface of the casting. The micro-model developed in this way was coupled with a macro-model which described the heat transfer, species transfer and fluid flow in the casting during the solidification process.; The results show that the application of the slider improved the heat transfer conditions between the mold and the casting, producing equiaxed grains at the surface of the casting; this microstructure was associated with a reduction in the observed inverse segregation. An as-cast surface with less inverse segregation also could be achieved if the heat flux through the mold-wall was restricted to be within a certain range. Therefore, if a mold is designed to provide the optimum heat transfer conditions, sounder castings should result.; The thermosolutal buoyancy flow and shrinkage-driven flow during solidification have been calculated. The flow patterns indicated that thermosolutal buoyancy flow enhances normal segregation, and the segregation in the vertical direction at the centre of the castings; shrinkage-driven flow causes inverse segregation by driving the residual liquid with high solute content back to the semi-solidified surface.