Bimetals: Definition, Properties, and Applications

A bimetal is an object composed of two separate metals joined together by a metallurgical process. Unlike alloys, which mix two or more metals into a single substance, bimetals consist of layers of different metals that keep their individual attributes. Bimetallic products or bicomponent materials are other names for the same idea.

Bimetals have two distinct metallic zones that work together mechanically and electrically as one unit. Their main benefit is combining the best quality of each metal in a single product. A bimetal can pair the strength of one metal with the corrosion resistance of another, or the conductivity of one metal with the low cost of another.

Bimetals appear across many industries, including electrical conductors, contacts, thermostats, thermometers, protective devices, clocks, coins, cans and blades. This page explains the working principle, common combinations and major applications.

How do bimetals work?

The working principle of bimetals relies on the different coefficients of linear thermal expansion (αL) of metals, meaning 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 is made of two strips of different metals with different thermal expansion rates, welded together lengthwise. At normal temperature, a bimetal strip stays flat.

Heating makes the two strips lengthen by different amounts. The element therefore bends into an arc with the higher-expansion metal on the outside of the curve and the lower-expansion metal on the inside, as shown below.

Cooling reverses the effect: the strip bends the other way, placing the lower-expansion metal on the outer side of the arc and the higher-expansion metal on the inner side.

This predictable bending is what turns a simple bonded strip into a temperature detector and measuring element.

What are some common combinations for making bimetals?

Various metal combinations with different thermal expansion rates can form bimetals. Common combinations for making bimetallic strips include:

• Iron and nickel, where nickel expands slightly more than iron
• Brass and steel, where brass expands noticeably more than steel
• Copper and iron, where copper expands more than iron
• Constantan and Invar, where constantan expands far more than Invar’s near-zero rate

What are some major applications of bimetals?

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

Thermostats

Bimetals make excellent thermostats for switching circuits automatically and controlling the temperature of appliances such as electric heaters, electric irons, refrigerators and electric ovens. In some designs the current flowing through the thermostat itself supplies the operating heat.

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

In service, one end of the bimetal is fixed to the supply connection while the other end carries an electrical contact and moves freely as the strip expands and bends.

At normal temperature the moving contact touches a fixed contact. Heating bends the strip away, opening or closing the controlled circuit according to the design.

Cooling lets the strip spring back to its original shape and restore the contact.

An electric-iron thermostat works exactly this way.

The sensing element need not stay straight: coiling the strip (as shown below) packs a longer active length into the same space and raises sensitivity.

Thermometers

Bimetallic strips also serve in direct-indicating thermometers, again in coiled form.

A typical thermometer of this type is shown in the figure below:

One end of the coil attaches to the instrument housing and the other end drives a pointer.

Temperature changes wind or unwind the coil, moving the pointer across the scale.

Because the scale reads directly in temperature units, the pointer position gives the temperature at a glance.

Protective devices

Bimetallic thermal relays protect electric devices from overcurrents. The monitored current passes through a heating coil wrapped around a bimetallic strip.

Sustained excess current heats the strip until it bends far enough to operate the tripping mechanism and disconnect the supply.

An example of this type of protective device is a circuit breaker.

Clocks

Mechanical clocks run fast or slow as temperature changes their parts, and a bimetallic strip can compensate for much of that error.

A typical compensated clock balance is described below:

The balance wheel, which regulates the movement’s speed, has a rim built from two metals with different coefficients of linear thermal expansion. Heat bends the rim slightly inward, reducing its diameter and speeding the wheel up; cold bends it outward, slowing the wheel down. That behavior offsets the way temperature alters the hairspring’s elasticity.

Coins

To cut costs and discourage melting coins down for their metal value, mints often cover a cheap core with a thin layer of dearer metal. The United States penny switched in 1982 from 95% copper to a 97.5% zinc core dressed in thin copper plating that preserves its familiar look. Zinc core plus copper skin makes the modern penny itself a bimetallic object. Another example is the true bi-metallic coin built from two distinct parts of different metals: the Canadian two-dollar coin pairs an outer ring of nickel-plated steel with an inner core of aluminum bronze.

Cans

Tin cans are steel covered with tin, and the tin layer keeps corrosion away from the steel beneath. Steel plus tin again forms a practical bimetallic combination. The aluminum can takes a different approach: an aluminum body carries an aluminum-alloy lid with a pull tab, which opens easily without tools but recycles less cleanly because of the mixed metals.

Blades

Bandsaw and reciprocating-saw blades often use bimetal construction. Teeth of hard high-speed steel are bonded by electron-beam or laser welding onto a softer, tougher high-carbon steel backing. Each metal ends up where it performs best: the teeth cut faster because they are harder, while the body resists cracking and breaking because it is less brittle, giving these blades a better speed-and-durability balance than single-metal alternatives.

Bimetals bond two separate metals into one body whose layers keep their own attributes yet act mechanically and electrically as a unit. Different expansion coefficients make them bend with temperature, and that simple motion powers thermostats, thermometers, protective relays, clock compensation, coins, cans and saw blades alike.

Conclusion

Bimetals combine the best qualities of two metals within one product. They sense temperature, switch circuits, compensate precision mechanisms, cut tough materials and stretch expensive metals further. Few ideas in metallurgy pay off so simply: bond two metals together and each covers the other’s weaknesses.