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Te phase diagram tells us that, at


equilibrium, the silicon content in solid aluminum is 13% of that found in the surrounding liquid. Te other 87% remains in the liquid, where it accumu- lates. And as the silicon content increases in the liquid, its melting point decreases. Hence, the composition and temperature of both solid and liquid phases follow the arrows in Fig. 2. Tis segregation continues until the liquid contains 12.6% Si and cools to the eutectic temperature. At this point, a eutectic mixture of solid Al and Si forms. Another important factor that can


be determined from the phase diagram is the depression of the melting point of aluminum. Tis is defined by the slope of the liquidus curve and by this equation for the Al-Si system:


Fig. 3. This diagram shows the liquidus surface for aluminum-rich alloys in the ternary Al-Zn-Mg system. (Temperatures are given in C.)


Te last important factor is the solu-


bility of the element in liquid aluminum at typical furnace temperatures. For silicon, this maximum concentration is equal to the eutectic composition, or12.6 weight percent Si. Tese three factors have been


For silicon in aluminum, m is equal to 11.9F (6.6C) per weight percent Si.


tabulated for a number of important or interesting alloying elements and are shown in Table 1. Several important and interesting


things may be gleaned from Table 1. • Nickel, iron, silicon and copper segre- gate strongly during solidification.


• Zinc and manganese segregate only moderately.


• Manganese hardly segregates at all. The concentration of manganese in solid aluminum is 94% of the liquid. This is an important factor in the improved performance of diecasting alloys, where manganese replaces iron


36 | MODERN CASTING April 2014


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