How aluminium compares with steel in high volume manufacturing13 January 2021
Two of the most commonly used metals in industry, aluminium and steel, can be found in factories all over the world. Most process engineers and materials scientists designing manufacturing tooling or industrial components rely upon them heavily.
While these two materials are very well known, recent advances in technology have rewritten the rules for how these are best used. In some cases, the optimal metal for manufacturing plants and machinery is often overlooked. The properties of the two metals can be changed and enhanced by various techniques, such as alloying and surface coatings, making them an optimal choice for a wide variety of uses.
So, the decision to specify steel or aluminium, or one of their alloys, can be complex. Here are some of the factors to consider, and myths to bust, when evaluating aluminium versus steel for high volume manufacturing.
Properties of steel versus aluminium compared
The mechanical properties useful to process engineers designing high volume manufacturing plant equipment include yield stress, tensile strength, Young’s modulus and Poisson’s ratio, among others. Example data is shown in the table below.
|Yield stress (MPa)||Tensile stress (MPa)||Young’s modulus (GPa)||Poisson’s ratio||Density (kg/m3)||Thermal conductivity
|Hardness (Moh scale)|
|Medium carbon steel||305-900||410-1200||190-215||0.27-0.3||7850||11.2-65.2||5-8.5|
From the yield and tensile stress data shown above, it is clear that steel is typically stronger than aluminium. Density data shows that steel is also much heavier than aluminium. However, the Young’s modulus shows aluminium to be less stiff, more ductile and therefore more workable than steel. Although not as strong, its low density means aluminium has a high strength to weight ratio when compared to steel.
Aluminium has very high thermal conductivity when compared to steel. If a lower thermal conductivity suits the application, a thermal barrier coating (TBC) can be added to aluminium, using a technique such as Plasma Electrolytic Oxidation (PEO), to significantly decrease its thermal conductivity.
Steel is a poor conductor when compared to aluminium, which is an excellent conductor. Again, if insulating properties are required, a PEO coating can be applied to aluminium to enhance its dielectric properties. Steel, being ferrous, can be magnetised.
Aluminium does not rust, but can be seriously corroded under some operating conditions, whereas stainless steels that include 12% chromium are highly rust and corrosion resistant but are more expensive. The chromium forms a corrosion resistant oxide layer on the surface of the steel.
Corrosion is a major cost to industry. Although a similar aluminium oxide layer forms on pure aluminium in air, providing a level of corrosion resistance, in harsh operating environments this is not sufficient. To generate higher corrosion resistance, a PEO layer can be added that significantly enhances aluminium’s natural corrosion resistance. This is ideal for harsh production environments in industries such as food and beverage, textiles and packaging and plastics production.
Finally, steel is much harder than aluminium, which gives superior wear resistance characteristics. However, applying a surface coating, such as PEO, to aluminium, increases its wear resistance significantly, as the surface characteristics of the coating reduce both adhesive and abrasive wear.
Which is more expensive, steel or aluminium?
By weight, mild and carbon steel is lower cost than aluminium, whereas stainless steel is much more expensive than aluminium. Some aluminium alloys are more expensive than steels. Being globally traded commodities, the costs of both also fluctuate and are driven by global market forces.
The costs must also be considered over the lifecycle of a plant. Carbon steel is heavy and stiff, making it harder and costlier to work into complex machinery components, when compared to the more ductile and lighter aluminium. On the other hand, steel is easier to weld. It is a trade-off between these different factors and the application should be the main driver for the material choice.
Being heavier, steel components require more energy to move, particularly rotational components in high-speed plants. So, a process engineer might start with a lower cost raw material but building and operating the plant using steel may be more expensive than the higher cost by unit weight aluminium.
Which metal should I choose for my high-volume manufacturing application?
As shown above, the decision is complex and clearly the end-application drives the selection. Also, mild steel and pure aluminium are just the starting point. Carbon and stainless steels are very different from mild steel, and coated aluminium alloys have very different properties to basic aluminium.
For long-term use in heavy industries, the innate strength of steel means it is often the best choice. Adding a surface coating can radically change chemical and mechanical properties, making aluminium in particular a better choice in many high-speed high-tech manufacturing applications.
To learn more about PEO, download our white paper ‘What is Plasma Electrolytic Oxidation?’ or get in touch with one of our materials scientists today.