Results


Grain Size and fading AZ91E


An optical micrograph of the base AZ91E alloy is shown in Figure 2. The grain size was measured over 100 grains using the linear intercept method. The average grain size for AZ91E without any grain refiner addition was 1000 µm. There were no columnar grains at the edges of the casting because the molds were heated above the pouring tempera- ture of the melt. Consequently, solidification proceeded evenly in all directions.


AZ91E + Al-5Ti-1B


After a holding time of five minutes, the addition of 0.1 wt.% Al-5Ti-1B grain refiner provided a significant reduc- tion in grain size from 1000 to 323 µm. Figure 3A depicts the change in microstructure with increasing holding time. The small grain size was maintained with increasing holding time. The optical micrographs for the other addition levels are in Figures 3B-3D. Addition levels of 0.1 and 0.2 wt.% provided the largest degree of grain refinement with very little signs of fading, while addition levels of 0.5 and 1.0 wt.% had grain sizes comparable to the unrefined alloy after 20 minutes of holding.


A quantitative summary of the grain size measurements is shown in Figure 4. Previous crystallographic work8,9


Figure 4. Influence of Al-5Ti-1B addition levels and holding time on the grain size of AZ91E.


nucleating sites in Mg alloys. In addition, Wang et al.10 and Liu et al.12


determined that TiB2


for nucleation at longer holding times. AZ91E + Al-1Ti-3B


the bottom of the less dense (1.74 g/cm3 Mg melt. Therefore, fewer TiB2


of grains. If a similar scenario is occurring for the cur- rent study then the fading of the grain refiner could be explained by density differences between the Mg and the grain refiner. For each pour, the liquid metal was ladled from the top of the crucible. As time progressed, the dens- er TiB2


observed TiB2 particles (4.38 g/cm3 , 0.16 lb/in3


) likely sunk to , 0.0629 lb/in3


)1 particles were available


particles could act as heterogeneous particles within the center


As shown in Figure 5, significant grain refinement was observed while using the Al-1Ti-3B grain refiner. The


(a)


(b)


(c)


(d)


Figure 5. Optical micrographs of AZ91E + Al-1TI-3B castings with increasing holding time (a) 0.1 wt.% Al-1Ti-3B (b) 0.2 wt.% Al-1Ti-3B (c) 0.5 wt.% Al-1Ti-3B (d) 1.0 wt.% Al-1Ti-3B.


International Journal of Metalcasting/Spring 11 33


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