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10: Casting
10.4: Microstructure and Segregation in Castings

10.4: Microstructure and Segregation in Castings

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Microstructure of castings

The animation below shows how the microstructure of an ingot develops during solidification.

For a closer look at dendrite formation look at this animation in the Solidification of alloys TLP.

Below is a picture of a real cast Al ingot. Move your mouse over the various parts to see how they are formed.

Segregation in castings

When casting an alloy, segregation occurs, whereby the concentration of solute is not constant throughout the casting. This can be caused by a variety of processes, which can be classified into two types:

Microsegregation; which occurs over distances comparable to the size of the dendrite arm spacing. This occurs as a result of the first solid formed being of a lower concentration than the final equilibrium concentration, resulting in partitioning of the excess solute into the liquid, so that solid formed later has a higher concentration. More about microsegregation can be found in the TLP on Solidification of alloys.

Macrosegregation occurs over similar distances to the size of the casting. This can be caused by a number of complex processes involving shrinkage effects as the casting solidifies, and a variation in the density of the liquid as solute is partitioned. We will not discuss these processes further.

It is desirable to prevent segregation during casting, to give a solid billet that has uniform properties throughout. Microsegregation effects can be removed after casting, by homogenisation, carried out at by annealing at high temperatures where the diffusivity is higher. Macrosegregation effects occur over larger distances so cannot be removed in this way, but can be reduced by control of the casting process and mixing during solidification, often by electromagnetic stirrers. Ultrasound is sometimes used to break up dendrites as they grow, reducing the scale of the dendritic structure and the extent of microsegregation.