Dendrites act much like this “V”-shaped plow. In other words, growing aluminum crystals adapt a dendritic shape as a response to the alloy composition. Growing solid crystals adapt a planar or a non-planar (dendritic) shape depending on the interaction of two factors. • The growth rate of the crystal. This is usually defined as the velocity of motion of the solid/ liquid interface, in microns per second (R), and is controlled by the thermal gradient in front of the crystal (G).


Fig. 2. This shows a schematic view of silicon atoms in front of a moving aluminum crystal.


• The rate at which the “piled up” solute elements can be removed, by diffusion, from the solidifying front. Te shape of the solidifying alu- minum depends on the amount and type of solute dissolved in the alloy. Te grain size also is influenced by the presence of growth-restricting sol- utes, like Si and Cu. Tis may be seen by comparing the grains of different Al-Cu alloys in Fig. 4. Tese alloys


Fig. 3. This schematic view shows silicon atoms in front of a growing dendrite tip.


were solidified at an average cooling rate of 1.8F (1C) per second. All four figures are shown at the same magnifi- cation. Compare this to the crystals in Fig. 1, which are new and just forming. Te arms on the branches of the den- drites are fine, much like needle-shaped leaves on a Christmas tree. Also, the


40 | MODERN CASTING May 2014


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