Grain Refinement of High-Purity Mg-Al Alloy Ingots and Influences of Minor Amounts of Iron and Manganese on Cast Grain Size

Abstract

Grain refining mechanisms of Fe- and Mn-free high-purity Mg–Al alloy ingots and the influence of a small addition of Fe or Mn on their cast structures are investigated. In order to prepare the alloys, distilled pure magnesium (<99.99%) and commercial high-purity aluminum (99.99%) were melted, and given amounts of Fe and Mn were added to the molten alloys. The grain size of the high-purity alloy is refined naturally without the use of a grain refiner or superheat treatment, and the addition of small amounts of Fe and Mn to the alloy coarsens the grains. In the composition range of Fe or Mn where magnesium does not crystallize from the melt as the primary crystal, both grain-coarsening and the effect of superheat treatment are remarkable. EPMA and AES analyses reveal a dispersion of fine particles composed of Mg, O, Al, and C elements in the high-purity alloy. Some combination of these elements seems to be an effective nucleation substance for magnesium crystal, and the nucleation substance is similar to those whose presence has been confirmed in AZ91E alloy to which a proper amount of carbide was added. In coarse-grained alloys, Fe and Mn elements coexist with the above-mentioned nucleation substances, and this disturbs the grain refinement.