Meso-scale modelling of directional solidification and comparison with in situ X-ray radiographic observations made during the MASER-12 XRMON microgravity experiment

Computational modelling of advanced solidification processes has made considerable advances over the last half century, with ever increasing levels of modelling complexity. There is, therefore, an increasing need for state of the art experimental investigation to provide suitable validation for these model predictions. In situ X-ray radiography has become a powerful tool for solidification experimentation. Using either synchrotron or microfocus X-ray sources, thin samples, encased in X-ray transparent crucibles, can be directionally or isothermally solidified, allowing for direct real time observation of dynamic solidification phenomena. This paper presents the results of a meso-scale Front Tracking simulation of a directional solidification experiment, performed using an Al-20 wt.% Cu alloy, carried out under microgravity conditions on board the MASER 12 sounding rocket. The sample was mounted in a Bridgman type gradient furnace and solidified using a prescribed cooling regime with a constant gradient, thus promoting directional solidification in the field of view. The actual thermal gradient in the sample was found to be lower than the nominal thermal gradient, as set/recorded by thermocouples embedded in the heater elements. The adjusted thermal data were supplied as inputs to the Front Tracking model and good agreement was then observed between the model predictions and the in situ observations. The extent and amplitude of the undercooled zone ahead of the columnar front was predicted based on analytical growth kinetics laws and the results were also compared to analytical models of columnar-to-equiaxed transition (CET) prediction.