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Laying a Foundation for Solidification


Phase diagrams of aluminum alloys provide a picture of castability and expected properties before you pour the casting. GEOFFREY SIGWORTH, GKS ENGINEERING SERVICES, DUNEDIN, FLORIDA


Tis article is the first in a three-part


series on solidification in aluminum cast- ings. While the series focuses on solidifica- tion principles in aluminum alloys, many can be applied to other metals, as well.


process, which essentially is a phase transformation from the hot, liquid state to a colder, solid state. Phase diagrams tell us a great deal about how this transformation occurs. Tis gives us clues about castability as well as the properties in the finished product. For example, they tell us about: • What phases form. • At what temperatures the phases form.


T


• The composition of phases and how solute elements are distrib- uted between the phases.


• How difficult it will be to place a specific alloying ele- ment into aluminum. If pure aluminum is slowly


heated, it remains solid until it reaches 1,220F (660C). Then, it starts to melt but remains at 1,220F until all the metal is molten. Once it is fully liquid, it can be heated to higher temperatures.


34 | MODERN CASTING April 2014


he metalcasting industry is concerned primarily with the solidification


Tis situation is akin to melting ice


or placing ice cubes in a glass of water. Ice and liquid water coexist only at a single temperature: the melting point. Te liquid temperature always is above this point, and the solid temperature always is below. One way to describe the situa-


tion is the phase rule (p + f = n + 2); where p is the number of phases present, f is the number of degrees


of freedom, and n is the number of components present). For a pure metal, the number of


components (n) is equal to one. When both solid and liquid are present, the number of phases (p) is equal to 2. Terefore, the number of degrees of freedom (f) must be equal to 1. How- ever, in practice, the pressure is fixed by the prevailing atmospheric pressure, which uses up the one degree of free- dom. In other words, the melting temperature is not free to vary or change, as long as two phases are present in a pure material. If a pure metal was melted in


Fig. 1. Shown is the phase diagram for the aluminum-silicon system.


a high pressure furnace in a lab, the melting point would increase. Aluminum exhibits about a 7% volume increase when it melts. Higher pressures would make it more difficult to melt metal by opposing this volume increase. Te single degree of freedom means as long as the pressure is fixed, the melting point is fixed. According to the phase rule, when a second element is dis- solved in aluminum, we have an additional degree of freedom. In this case, the melting point can change. Tose who live, or have lived, in cold climates are familiar with the practice of adding salt to icy sidewalks and driveways to melt ice in the winter. Salt dissolves in water, lowering its melting point. Tis makes it easier


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