Bimetal

Bi-metal refers to an object that is composed of two separate metals joined together. Instead of being a mixture of two or more metals, like alloys, bimetallic objects consist of layers of different metals. Trimetal and tetrametal refer to objects composed of three and four separate metals respectively.

Bi-metallic strips and disks, which convert a temperature change into mechanical displacement, are the most recognized bimetallic objects due to their name. However, there are other common bimetallic objects. For example, tin cans consist of steel covered with tin. The tin prevents the can from rusting. To cut costs and prevent people from melting them down for their metal, coins are often composed of a cheap metal covered with a more expensive metal. For example, the United States penny was changed from 95% copper to 95% zinc, with a thin copper plating to retain its appearance [http://en.wikipedia.org/wiki/Cent_%28United_States_coin%29#References] . A common type of trimetallic object (before the all-aluminium can) was a tin-plated steel can with an aluminum lid with a pull tab. Making the lid out of aluminum allowed it to be pulled off by hand instead of using a can opener, but these cans proved difficult to recycle owing to their mix of metals.