Settling of undissolved zirconium particles in pure magnesium melts

Zirconium is a remarkable grain refiner for magnesium alloys and is currently introduced into magnesium alloys primarily in the form of a Mg–Zr master alloy (Zirmax^® a Mg–33.3Zr alloy supplied by Magnesium Elektron). Owing to the difficulties of attaining complete dissolution, undissolved zirconium particles are often observed in Zirmax^® alloyed magnesium microstructures in the form of both isolated individual particles varying from submicrometre to greater than 10 μm, and various sizes of clusters that contain a number of zirconium particles. Aimed at improving the alloying efficiency with Zirmax^® and eliminating these undesirable large zirconium inclusions prior to pouring, this paper provides a detailed theoretical and experimental study of the settling behaviour of undissolved zirconium particles in pure magnesium melts. Various characteristics of the settling behaviour of zirconium particles are clarified based on the good agreement achieved between the experimental observations and theoretical predictions. In addition, it is also shown that wet chemical analysis of the total zirconium content in samples taken after different settling times at temperature can be an effective approach for evaluating the settling behaviour of undissolved zirconium particles in pure magnesium melts.