Microstructural evolution of aluminium-copper alloys during the downward directional solidification process

The microstructural evolution of Al - Cu alloys during the downward directional solidification process was investigated. At the planar-to-cellular transformation point, the planar liquid/solid (L/S) interface broke down at the centre. This was contrasted with the behaviour in the liquid metal cooling process, where the interface broke down at the periphery. The critical withdrawal rate at this point was higher than the theoretical value. In addition to this, the variation in the primary dendrite arm spacing (λ ₁) as a function of the withdrawal rate (V) at constant G_L for the Al-2.0 wt.% Cu alloy agreed with the conventional processes. The comparison of λ₁ in our experiment to the calculated value λ₁ using the Kurz-Fisher, Ma and Trivedi models showed that λ₁, calculated by these models, overvalued our experimental results. However, the λ₁ calculated from the Hunt model agreed well with the experimental values of λ₁. When we reduced the diameter of the sample from 13 mm to 9 mm and maintained the other parameters constant, the L/S interface retained a planar shape. This indicated that the L/S interface was more stable in the smaller sample than that in the larger. This result contrasted with the result in the liquid metal cooling process.