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A bimetal is defined as an object that is composed of two separate metals joined together by a metallurgical process. Unlike alloys, which are mixtures of two or more metals, bimetals consist of layers of different metals that retain their individual attributes. Bimetals can also be called bimetallic products or bicomponent materials.

Bimetals are characterized by two distinct metallic zones of the parent metals, which perform mechanically and electrically as a single unit. The benefit of bimetals is the ability to fully utilize the best qualities of each metal within one single product. For example, bimetals can combine the strength of one metal with the corrosion resistance of another, or the conductivity of one metal with the cost-effectiveness of another.

Bimetals are widely used in various industries and applications, such as electrical conductors, electrical contacts, thermostats, thermometers, protective devices, clocks, coins, cans, blades, and more. In this article, we will explore the working principle, common combinations, and major applications of bimetals.

How do bimetals work?

The working principle of bimetals is based on the fact that different metals have different coefficients of linear thermal expansion (αL), which means they expand or contract at different rates when heated or cooled. The coefficient of linear thermal expansion is defined as the fractional change in length per degree change in temperature.

Where,

l is the initial length of the object,

Δl is the change in length,

Δt is the change in temperature,

The unit of αL is per °C.

A bimetal consists of two strips of two different metals having different coefficients of linear thermal expansion, welded together lengthwise. A bimetal at normal temperature is shown in the figure below.

When heated, the expansions in the length of both metal strips are different. This causes the bimetallic element to bend and form an arc in such a way that the metal with a higher coefficient of linear thermal expansion is on the outer side of the arc, and the metal with a lower coefficient of linear thermal expansion is on the inner side of the arc as shown in the figure below.

When cooled, the bimetallic element bends and forms an arc in such a way that the metal with lower coefficient of linear thermal expansion is on the outer side of the arc, and the metal with higher coefficient of linear thermal expansion is on the inner side of the arc as shown in the figure below.

The above phenomenon can be used to produce a useful device for detecting and measuring changes in temperature.

What are some common combinations for making bimetals?

Many combinations of metals with different coefficients of linear thermal expansion can be used to form bimetals. Some of the commonly used combinations for making bimetallic strips are listed below:

• Iron (high αL) and nickel (low αL)

• Brass (high αL) and steel (low αL)

• Copper (high αL) and iron (low αL)

• Constantan (high αL) and Invar (low αL)

What are some major applications of bimetals?

Bimetals have many applications in various fields. Some of these are listed below:

Thermostats

Bimetals are very useful for making thermostats for automatic switching of circuits to control the temperature of certain appliances such as electric heaters, electric irons, refrigerators, electric ovens, etc. In some circuits, the current passed through the thermostat itself produces heat for its operation.

A typical bimetallic thermostat for this operation is shown in the figure below:

In operation as a thermostat, one end of the bimetal is fixed and connected to the source of supply. The other end is free to move. An electrical contact is attached to the free end of the bimetal, which moves with its expansion and bending.

At normal temperature, this moving contact makes contact with a fixed contact, as shown in the figure above. When heated, this bimetallic strip bends and disconnects from the fixed contact, as shown in the figure below. This opens or closes a circuit depending on its design.

When cooled down to normal temperature again, the bimetal returns to its original shape and closes or opens a circuit depending on its design.

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