Grain Refinement of Magnesium Alloys: A Review of Recent Research, Theoretical Developments, and Their Application

Abstract

This paper builds on the ''Grain Refinement of Mg Alloys'' published in 2005 and reviews the grain refinement research on Mg alloys that has been undertaken since then with an emphasis on the theoretical and analytical methods that have been developed. Consideration of recent research results and current theoretical knowledge has highlighted two important factors that affect an alloy's as-cast grain size. The first factor applies to commercial Mg-Al alloys where it is concluded that impurity and minor elements such as Fe and Mn have a substantially negative impact on grain size because, in combination with Al, intermetallic phases can be formed that tend to poison the more potent native or deliberately added nucleant particles present in the melt. This factor appears to explain the contradictory experimental outcomes reported in the literature and suggests that the search for a more potent and reliable grain refining technology may need to take a different approach. The second factor applies to all alloys and is related to the role of constitutional supercooling which, on the one hand, promotes grain nucleation and, on the other hand, forms a nucleation-free zone preventing further nucleation within this zone, consequently limiting the grain refinement achievable, particularly in low solute-containing alloys. Strategies to reduce the negative impact of these two factors are discussed. Further, the Interdependence model has been shown to apply to a broad range of casting methods from slow cooling gravity die casting to fast cooling high pressure die casting and dynamic methods such as ultrasonic treatment.

Effects of Cooling Rate and Solute Content on the Grain Refinement of Mg-Gd-Y Alloys by Aluminum

“…[1,2,11] [2,12,15,36] A similar approach will be used in the present work to understand the grain refining behavior of Mg-Gd-Y(-Al) alloys. The variation of Al 2 (Gd x Y 1Àx ) nucleant particles in Mg-Gd-Y-Al alloys is discussed, and the grain refining behavior by Zr and Al additions is also compared.…”

“…By replacing DT n in Eq. [1] by DT fg and substituting Eqs. [2] into [1], the grain size can be expressed as …”

“…In terms of the effect of cooling rate ( _ T) on grain size of cast samples, Johnsson found that the grain size is proportional to the reciprocal of square root of cooling rate, 1 ffiffi ffi _ T p . [25] Chai et al [12] found that with both solute content and cooling rate considered, the grain size of Al alloys is proportional to StJohn [15] found that with the presence of active nucleants, grain sizes are also linearly related to…”

“…[1] The formation of grains consists of two steps: nucleation and subsequent growth. Enhancing the nucleation rate and/or reducing the growth rate of grains refine the grains.…”

Active Mg Estimation Using Thermal Analysis: A Rapid Method to Control Nodularity in Ductile Cast Iron Production

“…[9] This is possibly because some of them do not fulfill the requirements to be efficient nuclei [10] and certainly because very little of the nucleation sites are effectively activated as shown for aluminum alloys. [11] This blocking of the grain refiner action has been rationalized in the case of aluminum and magnesium alloys, [12][13][14] and this certainly applies to cast irons as well.…”