ingot

Aluminium ingot after ejection from mold

An ingot is a material, usually metal, that is cast into a shape suitable for further processing.^[1] Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.^[2]

Uses

Ingots require a second procedure of shaping, such as cold/hot working, cutting or milling to produce a useful final product. Additionally ingots (of less common materials) can be used as currency, or as a currency reserve as with gold ingots.

Types of ingots

Metal, either pure or alloy, heated past its melting point and cast into a bar or block using a mold chill method. Polycrystalline and single crystal ingots are made from semiconductor materials by pulling from a molten melt. Uses include the formation of photovoltaic cells from silicon ingots by cutting the ingot into flats, known as wafers.

Single crystal ingots

Single crystal ingots (called boules) of materials are grown (crystal growth) using methods such as the Czochralski process or Bridgeman technique.

The boules may be either semiconductors—for the electronic industry, or non-conducting inorganic compounds for industrial and jewelry use, e.g., synthetic ruby, sapphire etc.

Single crystal ingots of metal are produced in similar fashion to that used to produce high purity semiconductor ingots^[3], i.e. by vacuum induction refining. Single crystal ingots of engineering metals are of interest due to their very high strength due to lack of grain boudaries. The method of production is via single crystal dendrite and not via simple casting. Possible uses include turbine blades.

Copper alloy ingots

In the United States, the brass and bronze ingot making industry started in the early 1800s. The US brass industry grew to be the number one producer by the 1850s.^[4] During colonial times the brass and bronze industrial was almost non-existent because the British demanded all copper ore be sent to Britain for processing.^[5] Copper based alloy ingots weighed approximately 20 pounds (9.1 kg).^[6][7]

Manufacture

Crystalline structure of mould cast ingot

Ingots are manufactured by the freezing of a molten liquid (known as the melt) in a mould. The manufacture of ingots has several aims. Firstly, the mould is designed to completely solidify and form an appropriate grain structure required for later processing, as the structure formed by the freezing melt controls the physical properties of the material. Secondly, the shape and size of the mould is designed to allow for ease of ingot handling and downstream processing. Finally the mould is designed to minimise melt wastage and aid ejection of the ingot, as losing either melt or ingot increases manufacturing costs of finished products.

A variety of designs exist for the mould, which may be selected to suit the physical properties of the liquid melt and the solidification process. Moulds may exist in top, horizontal or bottom-up pouring and may be fluted or flat walled. The fluted design increases heat transfer owing to a larger contact area. Moulds may be either solid "massive" design, sand cast (e.g. for pig iron) or water-cooled shells, depending upon heat transfer requirements. Ingot moulds are tapered to prevent the formation of cracks due to uneven cooling. Crack or void formation occurs as the liquid to solid transition has an associated volume change for a constant mass of material. Formation of these ingot defects may render the cast ingot useless, and may need to be either re-melted, recycled or discarded.

The physical structure of a crystalline material is largely determined by the method of cooling and precipitation of the molten metal. During the pouring process, metal in contact with the ingot walls rapidly cools and forms either a columnar structure, or possibly a "chill zone" of equiaxed dendrites, depending upon the liquid being cooled and the cooling rate of the mould.^[8]

For a top-poured ingot, as the liquid cools within the mould, differential volume effects cause the top of the liquid to recede leaving a curved surface at the mould top which may eventually be required to be machined from the ingot. The mould cooling effect creates an advancing solidification front, which has several associated zones, closet to the wall there is a solid zone which draws heat from the solidifying melt, for alloys there may exist a "mushy" zone, which is the result of solid-liquid equilibrium regions in the alloy's phase diagram, and a liquid region. The rate of front advancement controls the time that dendrites or nuclei have to form in the solidification region. The width of the mushy zone in an alloy may be controlled by tuning the heat transfer properties of the mould, or adjusting the liquid melt alloy compositions.

Continuous casting methods for ingot processing also exist, whereby a stationary front of solidification is formed by the continual take-off of cooled solid material, and the addition of molten liquid to the to casting process.^[9]

Historical ingots

Ancient copper ingot from Zakros, Crete. The ingot is shaped in the form of an animal skin, a typical shape of copper ingots from these times.

The mold of the Ancient Chinese gold and silver sycee, measured in tael. One of the Chinese names is 金元寶.

Lead ingots from Roman Britain on display at the Wells and Mendip Museum

Cultural references

• The Chinese New Year food Jiaozi was made to symbolize the ingot.
• The eighth letter in the Ogham alphabet is Tinne meaning "ingot".

See also

• Billet (manufacturing)—typically refers to a large ingot of less precious metal - both are forms of Bar stock
• Gold bar
• Tin ingot
• Wafer etching
• Oxhide ingot

Notes

References

• Schlenker, B.R. (1974). Introduction to Materials, Jacaranda Press.
• Chalmers, Bruce (1977). Principles of Solidification, Huntington, New York: Robert E. Krieger Publishing Company. ISBN 0-88275-446-7.