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 构建凝固基础

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

T

he 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. 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. • 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). Ten, 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. 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 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 com- ponents 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. 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. Te single degree of freedom means as long

56 | FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION June 2014

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