Numerical investigation of grain refinement of magnesium alloys: Effects of cooling rate

Abstract Solidification experiment of Mg-4Y-3Nd-1.2Al (wt%) alloy cast in a step-like sand-mold casting shows that a slow cooling rather than a fast cooling leads to grain refinement during solidification with in-situ formed nucleant particles (Al2RE) in molten melt. A two-dimensional cellular automaton (CA) model is adopted to fundamentally understand the mechanism of grain refinement. Conservation equation of energy is solved at macro-scale. The generated thermal history serves as input for the CA simulation. The model predictions are verified against the experiment. A high cooling rate leads to fast dendrite growth and solute enrichment in the interdendritic region. The nucleant particles, in-situ formed under the fast cooling condition, have small sizes. A larger number of small-sized particles are suppressed because they do not experience enough constitutional undercooling to nucleate. A critical particle size exists, above which the grain refinement becomes less sensitive to the cooling rate measured in the experiment. The simulation of one particle transported to the interdendritic area shows it is more difficult for this particle to develop under the faster cooling condition. We expect this fundamental understanding of the physics of cooling rate for the grain refinement would have implications on future complex magnesium alloy designs as well as solidification process control.