2.7 Casting microstructure and defects
Metal castings have very specific microstructures. When a liquid metal cools and begins to solidify in a mould, grains (crystals) of the metal start to form, both on the mould walls and in the bulk of the liquid metal. The way they grow is shown schematically in Figure 25(a). As the metal solidifies, it forms curious tree-like dendrites (from dendron: the Greek for tree). This structure is maintained after the casting is fully solidified, as can be seen from Figure 25(b), which shows a typical casting microstructure. (The image is created by polishing the surface of the metal, immersing it for a short while in a dilute acid and viewing it under an optical microscope.) In addition to the dendritic structure, there are two other common defects that can be found in a cast microstructure: particles of impurities known as inclusions, and porosity which is small holes in the casting.
Some inclusions can be removed by heating the casting to a temperature somewhat below its melting point to anneal it and 'dissolve' the inclusions in the metal; but the porosity is more difficult to remove. The porosity occurs because the casting has shrunk on solidification. Most materials contract on solidification (water is one of the few liquids that expands on solidification, so that ice floats on water; bad news for the Titanic, but good news for polar bears) and this shrinkage is not always uniform, so that substantial holes and voids can be left in the casting. This reduces the load-bearing capability of the component, and in highly stressed products, where the full strength of the material is being utilised, voids can lead to failure. The shrinkage on solidification can be large, and is generally a greater effect than the thermal contraction of the solid material as it cools to room temperature.
In many casting processes, runners and risers are used as reservoirs of molten metal to prevent voids from developing in the casting as it solidifies. The runners and risers are parts of the casting which contain a 'reserve' of extra liquid to feed into the mould as the cast product contracts during cooling. However, if a volume of liquid material becomes surrounded by solid material, then a void is formed when the liquid solidifies and contracts. Figure 26 shows a section through a gravity-die casting in which the effects of this contraction can clearly be seen. The chimney-like feature is the runner, down which liquid aluminium alloy was poured into the mould. There is a hollow in the top of the runner caused by liquid flowing from the runner into the mould as the casting solidified. As well as the hollow at the top, you can see some holes in the runner and one hole within the casting itself. The runners and risers will later be cut off and discarded.
When we are using casting to form the final shape of a product, we have to live with the microstructure of our casting, including its defects. But if we are casting ingots in order to produce sheet or bar metal for further processing, then a mixture of large deformations and high temperatures is typically used to 'break down' the cast structure, remove the porosity, and create a far more uniform microstructure. Such material is the typical raw material for the forming processes we will look at in the next section.
Polymers do not produce the same cast microstructures as are seen in metals, as they are composed of long-chain molecules, rather than grains built up from an atomic lattice of metal atoms. However, polymers do shrink on solidification and in injection-moulded products, shrinkage holes can form, particularly within thick sections. Figure 27 shows such holes in an injection-moulded nylon gear. Alternatively, the contraction may take the form of depressions on the surface ('sink marks'). In an effort to 'feed' shrinkage holes with liquid, the pressure is maintained for a short time after the thermoplastic has been injected. Similar holes are found in pressure-die castings.