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ENGINEERING AN EFFICIENT ZIRCONIUM-BASED GRAIN REFINER FOR MAGNESIUM ALLOYS


P. Saha and S. Viswanathan The University of Alabama, Tuscaloosa, AL, USA


Copyright © 2010 American Foundry Society Introduction


Zirconium is used as a grain refiner for magnesium alloys that do not contain aluminum.1


However, too much (about 1


wt %) zirconium is being used to ensure a peritectic reaction with magnesium. Unfortunately, the magnesium-zirconium master alloy contains many large particles that are likely not effective in grain refinement2


(see Figure 1), and most of the


larger zirconium particles settle to the bottom of the crucible as sludge.3


This work strives to develop a more efficient zir- conium-based grain refiner. Issues in Grain Refiner Development


Issues in grain refiner development include: What level of zirconium will produce grain refinement? What happens to the zirconium particles in the grain refiner master alloy? Do they dissolve, settle, or re-precipitate? Does the zirconium nucleus precipitate from the liquid or is it a pre-existing par- ticle in the grain refiner master alloy? Is it possible to engi- neer a zirconium grain refiner similar to TiBor (i.e. only use 0.02-0.05% zirconium)?


Experimental Approach


The magnesium-zirconium phase diagram is critical for analysis of grain refinement efficacy. At 0.5 wt%, zirco- nium undergoes a peritectic reaction with magnesium at


Figure 2. Magnesium-Zirconium phase diagram showing experimental additions of 0.25, 0.5, and 1 wt% zirconium corresponding to hypoperitectic and peritectic regimes.


F). Therefore, zirconium levels less than, equal to and greater than 0.5 wt% were investigated to determine whether the peritectic reaction was necessary for grain refinement.


653.4o C (1208o


Figure 1. Optical Micrograph of Mg-1wt% Zr sample poured at 8150


C immediately after addition of grain


refiner master alloy and stirring showing large zirconium particles. In the foundry, the large particles are allowed to settle and wastage as sludge.


70


Figure 3. Hockey puck experimental setup for grain refinement studies.


International Journal of Metalcasting/Winter 10


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