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Bimetal
View on WikipediaBimetal 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. A bimetal bar is usually made of brass and iron.

Bimetallic strips and disks, which convert a temperature change into mechanical displacement,[1] 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.[2] 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. 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.
Blades for bandsaws and reciprocating saws are often made with bimetal construction. The teeth, made of high-speed steel, are bonded (by various methods, for example, electron beam welding or laser beam welding) to the softer high-carbon steel base. Such construction makes for blades with a better combination of cutting speed and durability than shown by non-bimetal blades, because the advantages and disadvantages of each of the metals are applied in the best locations: the teeth are harder (and thus cut better), but therefore also brittler; meanwhile, the body area of the band is softer (which would make for poorer teeth), but also less brittle, and thus more resistant to cracking and breaking (which is desirable in the body area).
See also
[edit]References
[edit]- ^ NASA. "Bimetal sensor for averaging temperature measurement of nonuniform temperature profiles" (PDF).
- ^ Gaurav Kr. Deshwal and Narender Raju Panjagari (2019). "Review on metal packaging: materials, forms, food applications, safety and recyclability". Journal of Food Science and Technology. 57 (7): 2377–2392. doi:10.1007/s13197-019-04172-z. PMC 7270472. PMID 32549588.
Further reading
[edit]Bimetal
View on GrokipediaConcept and Materials
Definition
A bimetal is a composite material formed by joining two distinct metals or alloys through metallurgical bonding, creating a laminate structure without forming a homogeneous mixture as in an alloy.[4] This bonding process ensures the layers adhere strongly, often exceeding the strength of the individual metals, and allows the material to leverage the differing physical properties of each component for engineered functionality.[4] Unlike alloys, which involve atomic-level mixing during melting, bimetals maintain distinct layers that can be tailored for specific performance advantages, such as in thermal or mechanical responses.[4] Key characteristics of bimetals include the exploitation of differential properties between the joined layers, such as varying coefficients of thermal expansion, to enable predictable behaviors like curvature changes under external stimuli.[8] These materials are typically produced in forms like strips, sheets, or layered configurations, which facilitate applications requiring reliable interface integrity and property contrast.[4] The metallurgical bond is achieved through specialized techniques that promote diffusion at the interface, ensuring durability and resistance to delamination.[4] The term "bimetal" originates from the prefix "bi-" denoting two and "metal," reflecting its dual-component nature, with the concept first applied to thermal-sensitive composites in the 18th century.[9] English clockmaker John Harrison pioneered the use of bimetallic elements in his marine chronometer designs in the mid-18th century, employing them to compensate for temperature-induced variations in timekeeping mechanisms.[10] Simple bimetal forms, such as flat strips composed of two bonded layers, provide a basic demonstration of the material's response, where heating causes differential expansion leading to visible bending.[4] This deflection arises from the mismatch in expansion rates, a principle central to the material's design without requiring complex fabrication.[4]Common Materials and Alloys
Bimetals are composite materials formed by bonding two metals or alloys with significantly different coefficients of thermal expansion (CTE), enabling differential expansion under temperature changes. Low-expansion components, such as invar (a 36% nickel-iron alloy with a CTE of approximately 1.2 × 10^{-6}/°C at room temperature), are typically paired with high-expansion materials like brass (CTE 18–19 × 10^{-6}/°C) or copper (CTE 16–17 × 10^{-6}/°C) to achieve the desired deflection.[11][12] These pairings, such as invar with brass in thermostatic applications, provide stable performance over wide temperature ranges, with selection criteria emphasizing the CTE differential to ensure reliable actuation.[4] Common pairings include steel (CTE 10.8–12.5 × 10^{-6}/°C) as a low-expansion backing with copper or aluminum (CTE 23 × 10^{-6}/°C) as the active high-expansion layer, offering cost-effective options for general use.[12][13] For precision instruments, constantan (a copper-nickel alloy with CTE 14.9–15.9 × 10^{-6}/°C) paired with invar yields minimal variability in expansion, suitable for sensitive measurements.[12] In tool configurations, high-speed steel (with enhanced heat resistance) is bonded to spring-tempered alloy steel backs to combine cutting durability with structural flexibility.[14] Operating temperature ranges vary by alloy; invar-based bimetals are limited to below 230°C due to the Curie point, where expansion increases, while certain nickel-chromium-iron pairings extend to 500°C or more for high-temperature environments.[4][15]| Material/Alloy | Approximate CTE (× 10^{-6}/°C at 25°C) | Typical Role |
|---|---|---|
| Invar (Fe-36Ni) | 1.2–1.5 | Low-expansion base |
| Steel (carbon) | 10.8–12.5 | Low-expansion backing |
| Constantan (Cu-Ni) | 14.9–15.9 | High-expansion active (precision) |
| Copper | 16–17 | High-expansion active |
| Brass (Cu-Zn) | 18–19 | High-expansion active |
| Aluminum | 23 | High-expansion active |