Fig. 4. These micrographs show grain morphology in (a)Al-5% Cu; (b) Al-9.6% Cu; (c) Al-16.2% Cu; and (d) Al-25% Cu.


dendrites are growing freely into liquid metal. Tey are still largely unimpeded by neighboring grains. At some point, however, the “trunks” of the dendrites come in contact with neighboring grains. (Tis type of contact is called “dendrite coherency”.)


Fig. 5. Measuring SDAS by linear intercepts is shown.


After this time, any further solidifica- tion (and growth of dendrites) can occur only by thickening of the leaves and branches on the dendrite. As a result, the dendrites in the final casting are thicker. Te spacing between arms also becomes larger.


It has long been known that the


spacing of arms of the dendrite in the casting depends on the solidification time. One of the first detailed stud- ies was published in 1963 by Alcoa researchers who related dendrite cell size to the solidification time.


May 2014 MODERN CASTING | 41


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