Abnormal Grain Refinement Behavior in High-Pressure Die Casting of Pure Mg with Addition of Zr as Grain Refiner

Abnormal grain refinement behavior was found during high-pressure die casting of pure Mg with addition of Zr as grain refiner. The grain size of die-cast pure Mg was only 6.71 µm, whereas it abnormally increased to 18.93 µm after addition of 1 wt.% Zr particles, indicating loss of the grain refinement effect. Further experimental results showed that a melt quenching effect in combination with an increased number of effective nuclei could result in extremely refined grains in die-cast pure Mg, because many MgO particles could be activated simultaneously as nuclei. Therefore, it can be suggested that, before nucleation in high-pressure die-casting conditions, in the absence of effective (Zr) particles with small misfit, particles (MgO) with larger misfit and high number density might represent the most effective grain refiner.