When a metal is liquid, it is homogeneous. That is, the metal properties (especially composition) are the same everywhere. However, during freezing there is a redistribution of alloying elements and impurities called segregation. Segregation has a significant influence on casting quality. Understanding the process is useful for the casting engineer or metallurgist.
The two types of segregation are microsegregation and macrosegregation. Microsegregation is the variation in composition on a very small scale: between dendrites and dendrite arms.
The Al-Cu alloy system will be used to illustrate how segregation occurs. Researchers have examined this alloy system in great detail by researchers. It forms the basis for the high strength family of 2xx casting alloys.
Figure 1 shows the complete Al-Cu phase diagram. A number of intermetallic compounds form in this system. Aluminum casting facilities are concerned with the aluminum-rich portion of the figure and the formation of the ϴ (Al2Cu) phase. The relevant portion of the figure is shown in Fig. 2.
If an alloy containing 4.5% copper is poured and held near the eutectic temperature, the casting will be in the single phase field corresponding to solid aluminum. This is indicated by the red box in Fig. 2. According to the phase diagram, this alloy should be a single phase—aluminum with copper in solid solution.
Looking at a sample from a casting of this alloy, it is found to contain a significant amount of eutectic Al2Cu phase. This eutectic should not be present, according to the phase diagram. So, it is called a non-equilibrium eutectic.
Also, if the distribution of copper in a sample of the casting is studied by microprobe analysis in a SEM, one finds that the copper content in the aluminum phase varies. In the center of dendrite arms, which corresponds to the first solid, the copper content is low. Moving toward the outside of the arms, which corresponds to metal freezing later, the copper content increases. An example of this type of measurement is shown in Fig. 3.
Solidification scientists have studied this phenomenon for many years and have offered models to explain microsegregation. One can now calculate the amount of non-equilibrium eutectic according to a model proposed by Brody and Flemings (B-F). The B-F equations may be used to calculate the composition of liquid during solidification, and the fraction of solid at which the eutectic forms. The B-F works well for the elements commonly found in aluminum castings and for all but extremely fast freezing rates. (The model works well in other metal systems for most elements. However, a correction factor is needed for elements that diffuse rapidly—for example, carbon dissolved in iron or steel castings.)