Grain Refinement of Aluminium Alloys: Recent Developments in Predicting the As-Cast Grain Size of Alloys Refined by Al-Ti-B Master Alloys

This paper presents the results of recent research on the grain refinement of aluminium alloys. There has been considerable development in our understanding of the mechanisms controlling grain refinement over the last decade and these will be briefly described. In particular, the Interdependence Theory has clearly explained the interdependence between the growth of grains and the nucleation potency of refining particles in causing a wave of nucleation events throughout a casting. This interaction determines the final as-cast grain size. A key factor identified by this research is the formation of a nucleation-free zone in front of a growing grain that prevents nucleation up to the point where a critical amount of constitutional supercooling is established allowing the next nucleation event to occur. Thus, the grain size is determined by the size of the nucleation-free zone and the distance to the next most potent particle. The Interdependence Model can, therefore, be used to predict grain size changes with composition and nucleant particle density. In this paper the Interdependence Model is used to develop equations for predicting the grain size of aluminium alloys when refined by Al-Ti-B master alloys. The performance of Al-5Ti-1B and Al-3Ti-1B master alloys are compared in the light of the Model.