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

Abstract

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 diecast pure Mg was only 6.71 lm, whereas it abnormally increased to 18.93 lm 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.