What Are Ingots?
Ingots are blocks of cast metal from which other things are made. You have probably heard of cast iron. Casting is the process by which molten metal is passed into a specially made mold. It then hardens into the shape of the mold, like jelly or blancmange does. Molds can be for any shape, from enormous sections of railway trains or bridges to small pieces of sculpture. But if it is a plain block shape, it is called an ingot mold.
Many different metals can be cast, iron being perhaps the most important. So can alloys, or metal mixtures, such as bronze which is a combination of copper and tin.
Most casting is done in molds made of special hard-packed sand, which resists very high temperatures. But metal molds may be used if their melting point is higher than that of the substance being cast. When a metal mold is used the process is called die-casting. Usually when we use the word ingot we think of gold ingots. This is because gold has for centuries been stored in the form of ingots.
The manufacture of ingots has several aims. Firstly, the mold 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 mold is designed to allow for ease of ingot handling and downstream processing. Finally the mold is designed to minimize 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 mold, which may be selected to suit the physical properties of the liquid melt and the solidification process. Molds 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.
Molds may be either solid “massive” design, sand cast (e.g. for pig iron) or water-cooled shells, depending upon heat transfer requirements. Ingot molds 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 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 mold.
For a top-poured ingot, as the liquid cools within the mold, differential volume effects cause the top of the liquid to recede leaving a curved surface at the mold top which may eventually be required to be machined from the ingot. The mold cooling effect creates an advancing solidification front, which has several associated zones, closer 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 mold, 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 casting process. Approximately 70 percent of aluminium ingots in the U.S. are cast using the direct chill casting process, which reduces cracking. A total of 5 percent of ingots must be scrapped because of stress induced cracks and butt deformation.