Abstract
Grain refinement of magnesium (Mg) alloys has been a major research topic over the past decades as one of the effective approaches to increase their strength and ductility simultaneously. In this study, a brief review of the grain refinement of aluminum-bearing Mg alloys is included to provide an in-depth understanding of the detailed mechanisms of grain refinement of Mg alloys. Additionally, the effect of melt superheating on the grain refining of Mg–Al-based alloys has been investigated. It was confirmed that melt superheating caused a significant grain refining effect in the commercial purity (CP) of AZ91 alloy (0.25% Mn). Undercooling of 1.3 °C was observed before superheating and was noticeably reduced after the superheating process. A vacuum filtering experiment was conducted, which involves filtering the melts using fine metal porous filters to separate the particles in the melts. It was observed that a large amount of Al8Mn5 particles were generated in the commercial purity AZ91 alloy by the superheating process. However, because of the poor crystallographic matching between Al8Mn5 and Mg, Al8Mn5 was not considered the nucleation site for Mg grains. A master alloy containing ε-AlMn particles, which are in good crystallographic matching with Mg, was added, and it was found that the grain size of the commercial-grade AZ91 alloy was reduced. Therefore, it is suggested that Al8Mn5 particles, existing as a solid phase in the molten metal of the commercial AZ91 alloy could be transformed into ε-AlMn particles by the superheating process, and these particles can be effective nucleation sites for Mg grains. The transformation of Al8Mn5 into ε-AlMn is considered the main mechanism of grain refinement of the commercial purity of AZ91 alloy by superheating. Notably, the effect of grain refinement by superheating was not observed in the high-purity (HP) AZ91 alloy (0.006% Mn) because Al–Mn particles were likely not formed due to a very small quantity of manganese.
Highlights
Introduction iationsMagnesium (Mg) alloys are the lightest structural metals, with several remarkable properties such as outstanding strength/weight ratio, 1.7 g/cm3 density at room temperature, and good castability and machinability [1]
We have focused on the grain refinement of commercial AZ91 alloy by superheating because the effect of grain refinement by superheating was not observed clearly in the HP AZ91 alloy
The average grain size of commercial-purity AZ91 alloy with 0.25 wt.% Mn was 290 μm before superheating, and it was reduced to 114 μm by the superheating process
Summary
Sung Su Jung 1,2 , Yong Guk Son 1,2 , Yong Ho Park 2, * and Young Cheol Lee 1, *. +82-05-309-7410 (Y.C.L.); Fax: +82-51-514-4457 (Y.H.P.); +82-05-309-7422 (Y.C.L.)
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