Laying a Foundation for Solidification 2

Geoffrey Sigworth, GKS Engineering Services, Dunedin, Florida

This article is the first in a two-part series on solidification in aluminum castings. While the series focuses on solidification principles in aluminum alloys, many can be applied to other metals, as well.

The metalcasting industry is concerned primarily with the solidification 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. This 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.
  • The composition of phases and how solute elements are distributed between the phases.
  • How difficult it will be to place a specific alloying element 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.

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

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

This 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. The liquid temperature always is above this point, and the solid temperature always is below.

One way to describe the situation 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. Therefore, the number of degrees of freedom (f) must be equal to 1. However, in practice, the pressure is fixed by the prevailing atmospheric pressure, which uses up the one degree of freedom. 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 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. The 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 dissolved in aluminum, we have an additional degree of freedom. In this case, the melting point can change. Those 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. This makes it easier to remove the ice, as long as the temperature is not far below the freezing point of water.

The same thing happens in aluminum. Adding a second element to pure aluminum usually lowers the melting point, as illustrated in the aluminum-silicon system. Silicon lowers the melting point of aluminum, but aluminum also lowers the melting point of silicon. The two curves for the melting of aluminum and silicon meet at a eutectic at a composition of 12.6 weight percent silicon and a temperature of 1,071F (577C) (Fig. 1).

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