Aluminium weighs about a third as much as steel for the same size of sheet, and it doesn't rust. But it's also about a third as stiff, so a straight swap at the same thickness gives you a lighter part that flexes more.
That trade-off decides most aluminium vs steel questions. If weight matters, as on a trailer, a vehicle canopy, or a cover someone lifts every day, aluminium is often worth it. If stiffness and cost matter more, steel usually wins. Here's how the two compare on weight, stiffness, strength, corrosion, fabrication, and cost, with a worked example in millimetres.
The numbers that matter
These are typical published values. Exact figures vary with the grade and temper.
| Aluminium | Mild steel | |
|---|---|---|
| Density | About 2.7 g/cm³ | About 7.85 g/cm³ |
| Stiffness (Young's modulus) | About 69 GPa | About 200 GPa |
| Thermal expansion | Roughly 23–24 µm/m per °C | About 12 µm/m per °C |
| Rust? | No, forms a protective oxide | Yes, needs coating |
| Magnetic? | No | Yes |
| Ease of welding | Harder, needs skill and care | Easy |
| Common SA sheet sizes | 2500 × 1250 mm, 3000 × 1500 mm | 2450 × 1225 mm, 3000 × 1500 mm |
Notice the ratios. Density and stiffness both drop to roughly a third. That means, kilogram for kilogram, the two metals are similarly stiff. The difference is in how you use the material.
Weight, stiffness, and strength
Weight: a worked example
Take a flat panel 1,000 × 500 mm.
- In 2 mm mild steel, it weighs about 7.85 kg.
- In 2 mm aluminium, it weighs about 2.7 kg.
That's a saving of over 5 kg on one panel. Multiply it across the sides, floor, and lid of a trailer toolbox and the difference is real, both for fuel and for whoever has to lift it.
But that 2 mm aluminium panel is also about a third as stiff as the steel one. Push on the middle and it'll flex noticeably more.
Stiffness: why aluminium parts are often thicker
To get back the stiffness, aluminium parts are usually designed thicker, or with more shape. The bending stiffness of a flat panel rises with the cube of its thickness, so you don't need three times the material. Run the numbers on the panel above and 3 mm aluminium roughly matches the bending stiffness of 2 mm mild steel, at about 4 kg instead of 7.85 kg.
Shape helps even more. A return flange, a swage, or a bent edge on an aluminium panel stiffens it far more than extra thickness does. That's why a well-designed aluminium cover usually has folded edges rather than a flat sheet. If you're bending aluminium, the grade matters, because some alloys crack on tight bends. Our post on bending aluminium without cracking covers radii and grades.
Strength: it depends on the grade
"Aluminium is weaker than steel" is only half true. Pure aluminium like 1050 is soft and weak. Alloys like 5083 and 6082 are much stronger, and the strongest aluminium alloys beat ordinary mild steel on strength.
For sheet metal, though, what usually limits a design is stiffness and dents, not breaking strength. Soft aluminium dents more easily than steel. If your part takes knocks, pick a harder grade or a thicker sheet. Our guide to aluminium sheet grades explains the common options.
Corrosion: aluminium wins, with one catch
Aluminium forms a thin, tough oxide film on its surface, so it doesn't rust like steel. Marine grades like 5083 hold up well even in salty air. Mild steel needs a coating to survive outdoors, and once the coating is breached, it rusts.
The catch is mixing metals. Bolt aluminium to steel or stainless in wet conditions and the aluminium can corrode at the joint. Isolate the two with washers, sleeves, or sealant, or coat the parts. If you're comparing steel options too, see mild steel vs stainless steel.
Fabrication: cutting, bending, and welding
Cutting. Both cut well on a fibre laser. Aluminium is reflective and conducts heat away fast, but modern fibre lasers handle common sheet thicknesses without trouble. It's usually cut with nitrogen for a clean, bright edge.
Bending. Soft aluminium grades like 1050 and 5052 bend much like mild steel, with an inside radius around 1× thickness as a starting point. Harder grades such as 6082-T6 need larger radii, often 1.5–3× thickness or more, and are more likely to crack bending parallel to the grain.
Welding. Aluminium is harder to weld than steel. It conducts heat away quickly, has a tough oxide skin, and expands about twice as much as steel, which makes thin parts more prone to distortion. It's absolutely weldable, but it takes skill and care. Heat-treated grades like 6082 also lose some strength next to the weld.
Cost
Aluminium costs more per kilogram than mild steel. But you buy far fewer kilograms for the same size of part, so the gap per part is smaller than the per-kilogram price suggests. Add the cost of coating mild steel for outdoor use, and for some parts aluminium comes out close.
Where steel clearly wins on cost: heavy, stiff parts where weight doesn't matter, such as base plates, machine frames, and brackets bolted to a wall.
How to choose
- Weight matters (vehicles, trailers, portable equipment, lids): aluminium
- Stiffness and low cost matter (frames, brackets, base plates): mild steel
- Outdoors with no coating: aluminium or stainless, not bare mild steel
- Near the sea: marine aluminium (5083) or 316 stainless
- Heavily welded thin parts: mild steel is easiest
Getting it made
Put the material, grade if you know it, thickness, and quantity on the quote. If you're stuck between aluminium and steel, choose "Not sure yet" and tell us what the part does, and we'll help you choose. Our laser cutting service covers aluminium, mild steel, stainless steel, and 3CR12. Start a quote for laser cutting and add bending, welding, or coating to the same order.