at 25C (73F) and 80C (176F) produced a vapor barrier which lead to a temporary slowdown of the quenching rate as seen in the quenching rate profiles in Figure 2 (b) and (c) and this vapor blanket was absent when samples were quenched in a mixture of anti-freeze solution and dry ice maintained at -40C (-40F) as shown in Figure 2 (a).


Figures 3 (a) and (b) show typical microstructures observed by optical microscopy in as-cast and after solution treatment,


respectively. In Figure 3 (a), the eutectic silicon was refined to a fine morphology due to the addition of strontium; Figure 3 (b) shows the eutectic silicon morphology transformed to a coarsened spherical shape as a result of solution heat treat- ment. The soaking time during solution heat treatment was kept at a maximum allowable time of 12 hours to ensure uni- form microstructure of the eutectic phases; the microstruc- ture shown in Figure 3 (b) is similar to those obtained for all the solution heat treated samples in the study.


(a)


(b)


(c)


Figure 2. Typical thermal data obtained during the quenching stage in the T6 heat treatment of A356.2 alloy samples. Quenching media in (a), (b) and (c) are mixture of anti-freeze and dry ice maintained at -40C (-40F), water at room temperature (~23C [~73F]), and water at 80C (176F), respectively.


(a)


(b)


Figure 3. Typical microstructures obtained in an optical microscope for (a) as-cast sample and (b) solution treated sample of A356.2 alloy.


22 International Journal of Metalcasting/Fall 2011


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