Process modelling of grain selection during the solidification of single crystal superalloy castings

A process model is described for the grain selection occurring during the solidification of single crystal investment castings, which are now used widely for a number of critical applications in gas turbine engines. The basis of the model is a thermal analysis of the heat transfer in the vicinity of the chill region onto which the molten metal is poured. Subsequently the competitive growth of grains during directional solidification is simulated via a cellular-automaton technique. For the purpose of model validation, processing trials have been carried out on a commercial single crystal casting furnace. The thermal cycles set up in and around the vicinity of the grain selector have been measured, and these are used to choose a number of critical parameters in the thermal model. The evolution of grain structure during competitive growth has been characterised using a number of analytical techniques, including orientation imaging microscopy. The results are compared critically with the predictions from the model. It is shown that the model is able to reproduce the statistical distribution describing the final casting orientation, measured with respect to the 〈001〉 crystallographic pole. The model is used to study the geometrical factors influencing competitive growth and the efficacy of two designs of grain selector, and in particular the conferral of any control of the secondary 〈001〉 orientation.