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Steel and aluminium are the two metals mechanical designers reach for most. Steel is strong, stiff, cheap and easy to weld. Aluminium is light, doesn’t rust and machines quickly. Neither is better in general; the right one depends on what limits the design. This guide compares them property by property and shows how to decide.
The numbers that matter
| Property | Structural steel (S355) | Aluminium (6061-T6) | What it means |
|---|---|---|---|
| Density | 7850 kg/m³ | 2700 kg/m³ | Aluminium weighs about a third as much for the same volume |
| Young’s modulus | 210 GPa | 69 GPa | Aluminium bends three times as much under the same load and shape |
| Yield strength | 355 MPa | 276 MPa | Similar; high-strength grades of both go much higher |
| Strength per kg | 45 kJ/kg | 102 kJ/kg | Aluminium carries about twice the load per kilogram |
| Stiffness per kg | 26.8 MJ/kg | 25.6 MJ/kg | Almost identical |
| Thermal expansion | 12 × 10⁻⁶ /K | 23 × 10⁻⁶ /K | Aluminium grows twice as much when heated |
| Thermal conductivity | about 50 W/m·K | about 167 W/m·K | Aluminium is used for heat sinks |
| Usable temperature | up to about 400 °C | strength falls above about 150 °C | Steel for hot parts |
| Fatigue | Has an endurance limit (many steels) | No true endurance limit | Aluminium needs more care under cyclic loads |
| Magnetic | Yes (carbon steels) | No |
Function and load: strength or stiffness?
The first question is what limits the part.
- If strength limits it (it must not break or yield), aluminium often wins on weight: per kilogram it carries about twice the load of structural steel.
- If stiffness limits it (it must not bend or vibrate too much), there is little difference per kilogram, but aluminium needs a bigger section. A bigger section takes more space, and in a tube or a profile that isn’t always available.
- If space limits it (the outside dimensions are fixed), steel wins: it is three times stiffer in the same envelope.
Worked example: equal stiffness
A flat steel bar 10 mm thick is used as a cantilever bracket. How thick must an aluminium bar of the same width be to bend the same amount, and how much does it weigh in comparison?
Bending stiffness of a flat bar is proportional to , so for equal stiffness:
Environment
- Corrosion. Carbon steel rusts and needs paint, powder coating or galvanising. Aluminium forms a thin, tough oxide layer that protects it in most atmospheres; anodising thickens it. Stainless steel is the choice when you need steel’s strength without coating.
- Contact with other metals. Aluminium bolted to stainless steel or copper in wet conditions corrodes galvanically. Isolate them with washers, sleeves or sealant (corrosion guide).
- Temperature. Aluminium alloys lose strength quickly above about 150 °C and melt at about 600 °C. Steel keeps its strength much hotter.
- Thermal expansion. Aluminium parts bolted to steel frames grow twice as much with temperature; long assemblies need slotted holes or allowance.
Cost
Per kilogram, aluminium costs several times as much as structural steel. Per part, the gap is smaller: an aluminium part weighs about a third as much for the same volume, machines several times faster, and often needs no coating. For welded frames and large structures, steel is almost always cheaper. For machined parts, enclosures and anything that has to be carried or moved, aluminium is often cheaper overall.
Manufacturability
| Process | Steel | Aluminium |
|---|---|---|
| Machining | Slower; harder grades wear tools | Fast; high speeds and feeds, good finish |
| Welding | Easy with MIG, TIG or stick | Needs TIG or MIG with pure argon and clean metal; heat-treated alloys lose strength at the weld |
| Sheet metal | Bends well, small bend radii | Bends well in 5xxx alloys; 6061-T6 cracks on tight bends |
| Casting | Cast iron and cast steel | Excellent for sand and die casting |
| Extrusion | Limited to simple shapes | Complex profiles at low tooling cost |
| Surface finish | Paint, powder coat, galvanise, plate | Anodise, powder coat, or leave bare |
Grades to specify
Steels
- S235 / S275 / S355 (EN 10025): structural plate, sections and welded frames.
- C45 / 1045: shafts, pins and machined parts; can be hardened.
- 42CrMo4 / 4140: highly loaded shafts and gears, quenched and tempered.
- 304 and 316 stainless: corrosion-resistant parts; 316 for salt and chemicals.
Aluminium alloys
- 6082-T6 / 6061-T6: the general-purpose choice for machined parts and profiles.
- 5052-H32 / 5083: sheet metal and marine parts; bend and weld well.
- 7075-T6: highest strength, for light, highly loaded parts; not for welding.
- 6063-T6: extrusions with good surface finish, such as frames and covers.
Quick decision guide
| Choose steel when | Choose aluminium when |
|---|---|
| Space is tight and stiffness matters | Weight matters most |
| The part is welded into a large frame | The part is machined in quantity |
| It runs hot | It must resist corrosion without coating |
| Cost per kilogram decides | It must conduct heat or be non-magnetic |
| Fatigue loads are high | Complex extruded profiles are useful |
Key points
- Aluminium is a third of steel’s density and a third of its stiffness, so stiffness per kilogram is about equal.
- For strength-limited parts aluminium saves weight; in a fixed space, steel is stiffer.
- Steel is cheaper to buy and weld; aluminium is quicker to machine and needs no coating.
- Always specify the grade and condition, not just “steel” or “aluminium”.