When selecting materials in engineering designs, it is not only the durability, conductivity, or price of the material that must be considered but also its reaction to temperature changes. Different metals expand or contract at different rates when exposed to heat. The primary reason for this is the arrangement of atoms (crystal lattice structure) that make up the metals and the nature of the bonds between them. In this article, we will compare the linear expansion coefficients of metals commonly used in industry and examine how these differences translate into practical applications.
What is the Coefficient of Thermal Expansion (α) in Metals?
The fundamental parameter that indicates how sensitive a material is to temperature changes is the "Coefficient of Linear Expansion (α)". This coefficient expresses how much the dimension of a 1-unit long material will change when its temperature increases by 1 degree Celsius (or 1 Kelvin).
The larger the coefficient, the faster the material reacts to heat, and the greater its dimensional change (elongation). As the coefficient decreases, the dimensional stability of the material increases. As a general rule (though there are exceptions), metals with low melting points have high expansion coefficients, while refractory metals with high melting points (like tungsten) have very low coefficients.
The standard formula used to calculate expansion is as follows:
ΔL = α · L₀ · ΔT
You can refer to our Linear Thermal Expansion Calculator to perform these calculations quickly.
Comparison of Aluminum, Copper, and Steel
Let's compare the behaviors of the three most commonly used metals in the industry:
- Aluminum: The expansion coefficient of aluminum is approximately 23-24 µm/(m·°C). This is a fairly high value. Therefore, aluminum frames or exterior claddings expand very significantly under the sun, and if care is not taken during installation, warping can occur.
- Copper: The coefficient of copper is about 16.5 - 17 µm/(m·°C). It expands less than aluminum but more than steel. Especially in electrical cables and hot water plumbing pipes, expansion allowances must be calculated based on this value.
- Steel/Iron: The coefficient of standard carbon steel is at an average level of 11-12 µm/(m·°C). It expands almost half as much as aluminum. One of the reasons it is the backbone of the construction industry is this relatively stable structure and its compatible expansion with concrete.
Which Metal Expands the Most?
When parts of the same length subjected to the same temperature difference are compared; Aluminum will be the metal that elongates the most, Copper elongates moderately, and Steel elongates the least (among this trio).
Working Principle of Bimetallic Strips
The fact that different metals expand at different rates is not always a problem in engineering; sometimes it is a source of a solution. The best example of this is "bimetallic strips."
A bimetallic strip is produced by firmly riveting or welding together two thin metal strips with different expansion coefficients (for example, brass and steel). When this strip is heated, both metals want to elongate, but according to the formula (ΔL = α·L₀·ΔT), the metal with the larger coefficient (α) tries to elongate more. Because the metals are stuck together, the side that elongates more bends over the side that elongates less.
This bending motion means that temperature is converted into mechanical motion. In older thermostats in our homes, irons, circuit breakers, or some oven thermometers, the bending property of bimetallic strips is utilized to physically break an electrical circuit when a set temperature is reached.
Elongation Examples for Different Metals Using the Calculator
To materialize the different behaviors of metals, let's do a small test using the Linear Thermal Expansion Calculator tool on our site.
Assumptions:
- Initial length (L₀) = 5000 mm (5 meters)
- Temperature change (ΔT) = 60 °C
Calculation 1: Aluminum Rod
- Coefficient (α) = 23 µm/(m·°C)
- Result (ΔL): According to our tool, the length change is 6.9 mm.
Calculation 2: Copper Rod
- Coefficient (α) = 17 µm/(m·°C)
- Result (ΔL): According to our tool, the length change is 5.1 mm.
Calculation 3: Steel Rod
- Coefficient (α) = 12 µm/(m·°C)
- Result (ΔL): According to our tool, the length change is 3.6 mm.
Despite having the same initial length and temperature change, due to the difference in material, the aluminum rod has expanded almost twice as much (6.9 mm > 3.6 mm) as the steel rod.
Summary Table (Approximate Coefficients)
The following table has been prepared to give a general idea. Actual values may vary depending on the alloy content of the material (carbon ratio, heat treatment, etc.):
- Aluminum: ~23 µm/(m·°C)
- Brass: ~19 µm/(m·°C)
- Copper: ~17 µm/(m·°C)
- Gold: ~14 µm/(m·°C)
- Steel/Iron: ~12 µm/(m·°C)
- Concrete: ~10-12 µm/(m·°C)
- Glass (Window): ~9 µm/(m·°C)
- Invar (Nickel-Steel alloy): ~1.2 µm/(m·°C) (Especially used in precision measuring devices due to its extremely stable structure against heat)
In conclusion, which material you choose for your project depends on how much thermal fluctuation that part will be exposed to and whether the mechanism can tolerate the expansion allowances. Correctly understanding the thermal identity of each material and performing quick simulations with the Linear Thermal Expansion Calculator tool is the first step to be taken for error-free and long-lasting designs.
