Crystallographic studies8,9 have determined that TiB2
grain refiners for Mg alloys. According to the edge to edge model, a good grain refiner is one where a row of atoms in a closed packed direction in a closed packed plane for both the refining particle and nucleating substrate will result in a minimal strain energy and misfit between the planes and interatomic spacings, respectively.9
, Al4
using the edge to edge model C3
and Al2
tween the directions should be less than 10 % while the mis- fit between the planes should be less than 6 %.9
Generally, the misfit be- These crys-
tallography studies give good starting points for determining effective grain refiners, but detailed information about the crystallography of the particles in question is required. Fur- thermore, experimental validation was also not usually car- ried out alongside these studies.
Aluminum-titanium-boron (Al-Ti-B) based refiners have been
gaining attention because their microstructure contains TiB2 particles which meet the characteristics outlined by Campbell4 and the edge to edge model.8,9
Wang et al.10 utilized an Al-4Ti-
ticles by energy dispersive x-ray (EDX) and crystallographic calculations. The ratio of titanium (Ti) to boron (B) also plays a role in the behavior of the Al-Ti-B refiner. If the ratio of Ti:B is equal to 1:2, then coarse and brittle TiAl3
particles are ex-
particles. Aluminum-titanium-boron refiners have been seen to be effective nucleating agents, but careful control on the composition of grain refiner is required to minimize the formation of detrimental intermetallics such as TiAl3
pected11 TiAl3
. Liu et al.12 ticles acting as sites for heterogeneous nucleation.
powder sintering process. With an addition of 1.8 wt.% grain refiner to AZ91D Mg alloy, the grain size reduced from 240 µm to 50 µm (79 % reduction) in the permanent mold cast samples. The grain refinement was attributed to TiB2
developed a Mg-50TiB2 par-
An Al-B based refiner has been shown to be effective in re- ducing the grain size of Mg-Al alloys. Suresh et al.13
studied parameters of a = 0.300 nm and c = 0.325 nm14
the grain refinement of AZ91E using Al-4B. The permanent mold cast AZ91 alloy had a grain size of 100 µm while 0.032 wt.% addition of B refined the grains to 30 μm (70 % re- duction). The nucleating particle was AlB2
which has lattice similar to
that of Mg with lattice parameters of a = 0.323 nm and c = 0.523 nm.6
Fading of Grain Refiners
In addition to reducing the casting grain size, the grain refiner must be able to maintain its grain refining effect even for long holding times. A fade resistant grain re-
30 grain refiner using a while a Ti:B ratio of 1:2.5 results in the elimination of
5B grain refiner to decrease the grain size of AZ31 Mg alloy. The addition of 0.3 wt.% Al-4Ti-5B grain refiner resulted in a grain size reduction from 1100 µm in the base AZ31 alloy to 80 µm (93 % reduction) in the grain refined alloy, respective- ly. The source of the refinement was confirmed to be TiB2
par- Present Study
This study investigated the effects of Al-5Ti-1B and Al- 1Ti-3B based grain refiners at various addition levels on the grain size of AZ91E Mg alloy. One objective of this study was to determine the optimal addition level to mini- mize grain size. Another objective was to characterize the nucleating particles within the castings. Fading phe- nomenon of the refiners was also investigated to observe the composition and addition level to minimize fading. The grain refinement was characterized using grain size measurements, optical microscopy, scanning electron mi- croscopy (SEM) and transmission electron microscopy (TEM).
Experimental Procedure Materials
The composition of the AZ91E alloy is in Table 1 and the compositions the Al-5Ti-1B and Al-1Ti-3B grain re- finers are in Tables 2 and 3, respectively. The composi- tion of the Mg alloy and Al-5Ti-1B grain refiner were obtained from their respective suppliers. The Al-1Ti-3B grain refiner was synthesized at the Indian Institute of Technology-Madras. Commercially pure Al was melted in an induction furnace and degassed at 800C (1472F) us- ing hexachloroethane. The Ti and B were added as K2 and KBF4
were wrapped in Al foil and plunged into the Al melt us- ing a zirconia coated steel plunger. The melt was manu- ally stirred using a zirconia coated steel stirrer until the mixture was homogeneous. The melt was allowed to react with the salts for 60 minutes with no additional stirring. The melt was then skimmed and poured into a cylindrical graphite mold 25 mm (0.98 in) in diameter and 250 mm (9.8 in) in height.
salts respectively. Both K2 TiF6 and KBF4 International Journal of Metalcasting/Spring 11
CO are potential
finer is one that requires little to no stirring after initial addition to maintain its grain refining effect. Within the foundry, a pause in the production line to stir the melt and redistribute the grain refiner is an expensive procedure. Fading investigations involving Al and an Al-7Si alloy were conducted by Vinod Kumar et al.15,16
using Al-Ti-
C and Al-Ti-B based grain refiners. The study concluded that Al-Ti-C based refiners were more beneficial than Al- Ti-B based refiners because of the lower agglomeration tendency of TiC as compared to TiB2 glomeration of TiB2
particles. The ag- particles in Al melts was also noted by McCartney.17 In another study by Vinod Kumar et al,.18 Al-7Si was found
Al-1Ti-3B grain refiner were found to be the source of the grain refinement.
to respond poorly to grain refinement by Al-5Ti-1B, but very sharply to Al-1Ti-3B even at holding times of two hours. Aluminum diboride (AlB2
) particles that were present in the
TiF6 salts
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