Uptake of iron and its effect on grain refinement of pure magnesium by zirconium

The uptake of iron by molten magnesium from uncoated new mild steel crucibles at temperatures 680 degreesC, 730 degreesC, and 780 degreesC has been investigated. It was shown that the uptake of iron was sluggish at 680 degreesC and the use of 0(.)05% zirconium addition could effectively suppress the increase in iron content within the first 2 h of holding at temperature. Rapid and severe uptake of iron was observed at 780 degreesC. As a consequence, it was found that the grain refinement of pure magnesium achieved by 1% zirconium addition nearly vanished after 60 min hold at 780 degreesC due to the depletion of soluble zirconium. The uptake of iron at 730 degreesC was conspicuous but it was still controllable by use of 0(.)05'% zirconium addition within the first 60 min of holding at temperature. The work conducted using an aluminium titanite crucible and a boron nitride coated mild steel crucible at 730 degreesC further confirmed the highly detrimental influence of the uptake of iron on the grain refinement of pure magnesium by zirconium. The characteristic zirconium rich coring structures developed from circular to rosette like when the melt was held at 730 degreesC in an uncoated mild steel crucible, while no such evolution was observed when held in an aluminium titanite crucible at the same temperature. Recommendations to minimise the consumption of zirconium by the uptake of iron were made based on the results obtained from this investigation. The mechanism of grain refinement of magnesium by a low concentration of zirconium is discussed.