B required 2.2 wt.% of Ti to form 3.2 wt.% of TiB2 The remaining Ti formed TiAl3 of TiB2
particles. . Therefore, a total of 3.2 wt.%
wt.% of B reacted with 2.19 wt.% of Al to form 3.94 wt.% of AlB2
ing particles, Al-1Ti-3B is expected to be more efficient than Al-5Ti-1B. The 1.0 wt.% addition of Al-1Ti-3B grain refiner required to match the grain sizes obtained using 0.1 wt.% Al- 5Ti-1B could be a result of the residual salt (Figure 11) that was unable to form additional nucleating particles. Another reason for the poorer grain refinement efficiency of Al-1Ti-3B as compared to the Al-5Ti-1B grain refiner can be TiB2 as a more potent grain refining particle than AlB2 al salt and the better refining effect of TiB2
AlB2 acting . The residu- particles could be
two factors that caused the poorer grain refinement efficiency of Al-1Ti-3B as compared to Al-5Ti-1B.
A comparison of the present results with a study by Kumar and Mahanty24
using hexachloroethane addition revealed that
the Al-Ti-B based refiners were more efficient than hexachlo- roethane. Using a Mg-0.5Al alloy, the addition of 0.01 wt.% hexachloroethane reduced the casting grain size from 900 to 400 µm (55 % reduction) in the base and refined castings re- spectively. Further addition of hexachloroethane increased the grain size. In comparison with the Al-5Ti-1B grain refiner, the largest decrease in grain size was from 1000 to 323 µm (67 % reduction) using 0.1 wt.% after five minutes of hold- ing. The largest decrease in grain size using the Al-1Ti-3B grain refiner was 1000 to 361 µm (64 % reduction) after five minutes of holding and an addition level of 1.0 wt.%. The results showed that both Al-5Ti-1B and Al-1Ti-3B addition can produce grain sizes smaller than that of hexachloroethane addition without the release of harmful fumes.
Conclusions
The addition of Al-5Ti-1B and Al-1Ti-3B grain refiners led to significant grain refinement compared to the base AZ91E Mg alloy. Some of the key results were:
1. After five minutes of holding, the smallest average grain size obtained using the Al-5Ti-1B grain re- finer was 323 µm at an addition level of 0.1 wt.%.
2. After five minutes of holding, the smallest average grain size obtained using the Al-1Ti-3B grain re- finer was 361 µm at an addition level of 1.0 wt.%.
3. Minimum fading was observed with 0.1 wt.% of the Al-5Ti-1B grain refiner and 1.0 wt.% of the Al- 1Ti-3B grain refiner.
4. The Al-5Ti-1B grain refiner consisted of mostly fine (< 3 μm) TiB2
particles and large (> 30 µm) TiAl3 particles. International Journal of Metalcasting/Spring 11
formed, the Ti content was depleted. The remaining B (2.21- 1/2.2=1.75) reacted with Al to form AlB2
grain refiner, the 1 wt.% of Ti reacted with (1/2.2) wt.% of B to form 1.45 wt.% of TiB2
nucleating particles were available. For the Al-1Ti-3B particles. After the TiB2
particles
resulting in 5.39 wt.% (3.94 + 1.45 = 5.39) of TiB2 nucleating particles. With a greater number of nucleat-
. Therefore, 1.75 and
6. Grain refinement with Al-5Ti-1B addition was at- tributed to TiB2
5. The Al-1Ti-3B grain refiner consisted of both AlB2 (3-10 μm) and TiB2
(< 3 µm) particles. particles providing nucleating sites and grain growth restriction.
7. Grain refinement with Al-1Ti-3B addition was at- tributed to TiB2
and grain growth restriction. In addition, AlB2 ticles provided nucleating sites.
particles on mechanical properties.
particles providing nucleating sites par-
8. The Al-5Ti-1B was found to be a more effective grain refiner than Al-1Ti-3B but mechanical test- ing is required to determine the effects of the large TiAl3
Acknowledgments
The authors thank Ryerson University Research and Inter- national Affairs for financial support and Meridian Tech- nologies (Mr. I. Kosi) and Haley Industries (Mr. G. Mar- zano) for material support. A. Elsayed thanks NSERC for a postgraduate scholarship and the Michael Smith Foreign Study Supplement. Finally, the authors thank the members of the Centre for Near-net-shape Processing of Materials at Ryerson University (in particular, Mr. A. Machin and Mr. M. Vlasceanu) and the Micro-Nano Group at the IIT-Madras (India) for technical support.
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