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

Abstract 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 (λ1) as a function of the withdrawal rate (V) at constant GL 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 ot...