Grain refinement of magnesium alloys

Abstract

The literature on grain refinement of magnesium alloys is reviewed with regard to two broad groups of alloys: alloys that contain aluminum and alloys that do not contain aluminum. The alloys that are free of aluminum are generally very well refined by Zr master alloys. On the other hand, the understanding of grain refinement in aluminum bearing alloys is poor and in many cases confusing probably due to the interaction between impurity elements and aluminum in affecting the potency of nucleant particles. A grain refinement model that was developed for aluminum alloys is presented, which takes into account both alloy chemistry and nucleant particle potency. This model is applied to experimental data for a range of magnesium alloys. It is shown that by using this analytical approach, new information on the refinement of magnesium alloys is obtained as well as providing a method of characterizing the effectiveness of new refiners. The new information revealed by the model has identified new directions for further research. Future research needs to focus on gaining a better understanding of the detailed mechanisms by which refinement occurs and gathering data to improve our ability to predict grain refinement for particular combinations of alloy and impurity chemistry and nucleant particles.

Grain Refinement of Deoxidized Copper

“…The growth restriction factor Q has been used to evaluate quantitatively the role of solute in restricting crystal growth during solidification due to constitutional undercooling effects during solidification of dilute binary, [27][28][29] Eq. [1], and multicomponent alloy systems.…”

“…In the case of Mg [27,28] and Al [29] alloys, a linear semi-empirical relationship between grain size (d) and the reciprocal of Q has been determined, Eq. [2], which in turn relates the grain size to the solute content.…”

“…In binary Mg alloys with Al, Ca, Zr, Si, and Zn, it has been demonstrated [27,28] that, on increasing the Q factor, the grain size decreased, and overall the grain size was smaller than that of the unrefined alloy. The decrease in grain size occurs in three stages: an initial stage with a large reduction in grain size with small solute additions, followed by a moderate second stage that continued until stabilization was reached.…”

“…However, as can be seen in Figure 3 In comparison, an overall similar trend has previously been observed for Mg alloys. [27,28] Those results were explained by deviations due to inclusion populations that may be introduced into the Mg melt with the solute additions, variations in particle populations due to Mg reactivity and superheat modification with alloying. However, the base materials used in Reference 3 were copper (99.99 pct purity) and elemental additions (99.99 pct purity).…”

Semisolid microstructure evolution of AZ91D magnesium alloy refined by Al-Ti-B

“…The key of this method is to prepare cast ingots with fine equiaxed grains. Unfortunately, for the most commonly-used magnesium alloy of AZ91D, there is still no a commercially-used grain refining technique [4][5][6] . The result from the authors' previous investigation indicates that its grain size can be decreased from 422 µm to 79 µm after being refined by an Al-Ti-B master alloy that has been commercially used to aluminium alloys 7 .…”