smallest average grain size after a holding time of five minutes was 361 µm at an addition level of 1.0 wt.%. As shown in Figure 5D, increasing holding time resulted in only a small increase in grain size. Figures 5A-5C show micrographs representing addition levels of 0.1, 0.2 and 0.5 wt.% Al-1Ti-3B grain refiner. The Al-1Ti-3B grain refiner showed little fading and provided small grain sizes at higher addition levels.
nucleants for Mg as described by Suresh et al,.13 et al.10 and Liu et al.12
Aluminum diboride (AlB2 TiB2 and AlB2 ) and TiB2
are documented Wang
. The different concentrations of
particles are possible causes for the dis- similar addition levels required for comparable grain refinement using each refiner. A summary of the grain size measurements of the Al-1Ti-3B treated castings is in Figure 6.
SEM Analysis Al-5Ti-1B Al-5Ti-1B Grain Refiner
Samples for SEM analysis of the Al-5Ti-1B grain refiner were prepared to observe the particles responsible for grain refinement. Figure 7 is an SEM image of the Al- 5Ti-1B grain refiner.
particles in Figure 7 are also similar in morphology to the TiAl3
particles observed by Wang et al.11 concluded that the Ti-Al particles were in fact TiAl3
. It was therefore . Fine
(< 3 µm) sized Ti bearing particles were also observed. The fine Ti bearing particles were compared to the par-
Figure 6. Influence of Al-1Ti-3B addition levels and holding time on the grain size of AZ91E.
The SEM image shows large (> 30 µm) Ti-Al particles. The Al-5Ti-1B grain refiner had a Ti:B ratio greater than 1:2 which results in the formation of TiAl3
. The Ti-Al 3K2 TiF6
2KBF4 AlB2
+ TiAl3
+ 13Al → 3TiAl3 + 3Al → AlB2 → TiB2
+ 3KAlF4
+ 2KAlF4 + 4Al
Equation 3 would continue until all the AlB2 were exhausted.
During grain refiner preparation, the AlB2 react with TiAl3
to form TiB2 + K3 AlF6
Equation 1 Equation 2 Equation 3
particles would
. The chemical reaction in particles
ticle morphology observed in the study by Fjellstedt et al.20
detection of B in small concentrations is difficult because of low intensity of B and the limitations of EDX analysis. The Al-5Ti-1B grain refiner in Figure 7A contained many clusters of fine TiB2
and deduced to be TiB2 . Using SEM, the accurate particles. The requirement for only a
particles within the grain refiner can be ex- plained using the following chemical reactions21
small concentration of Al-5Ti-1B grain refiner to provide a large reduction in grain size can then be explained by the high concentration of TiB2 lack of AlB2
nucleating particles. The :
(b) (a)
Figure 7. SEM Image of Al-5Ti-1B grain refiner showing TiAl3 analysis of TiAl3
particle and (c) EDX analysis of TiB2 34 particle. and TiB2 particles (a) SEM image of Al-5Ti-1B (b) EDX International Journal of Metalcasting/Spring 11
(c)
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