Most sheet-metal parts are made from one of three materials: mild or structural steel, aluminium alloy, or austenitic stainless steel. They are cut on the same lasers and folded on the same press brakes, so it is tempting to treat them as interchangeable. They are not. Swapping material without changing the design usually leads to a part that is too flexible, cracks on the bend, distorts when welded or corrodes where it touches something else.
This article compares the three from a designer’s and fabricator’s point of view, with the numbers you need to make a first choice.
The key properties side by side
| Property | Mild / structural steel | Aluminium (5xxx) | Austenitic stainless |
|---|---|---|---|
| Common sheet grades | DC01, S275 | 5754-H22, 5083-H111 | 304 (1.4301), 316 (1.4401) |
| Density | 7.85 g/cm³ | ≈ 2.66 to 2.70 g/cm³ | ≈ 7.9 to 8.0 g/cm³ |
| Young's modulus E | ≈ 210 GPa | ≈ 70 GPa | ≈ 200 GPa |
| Yield strength (typical minimum) | S275: 275 MPa. DC01: no minimum, 280 MPa maximum | 5754-H22: ≈ 130 MPa. 5083-H111: ≈ 125 MPa | 304: ≈ 230 MPa. 316: ≈ 240 MPa (cold rolled) |
| Tensile strength (typical range) | DC01: 270 to 410 MPa. S275: 410 to 560 MPa | 5754-H22: 220 to 270 MPa. 5083-H111: 275 to 350 MPa | 304: 540 to 750 MPa. 316: 530 to 680 MPa |
| Thermal expansion | ≈ 12 × 10⁻⁶ /K | ≈ 23 to 24 × 10⁻⁶ /K | ≈ 16 to 17 × 10⁻⁶ /K |
| Thermal conductivity | ≈ 45 to 55 W/m·K | ≈ 120 to 150 W/m·K | ≈ 15 W/m·K |
Two things stand out. Aluminium is about one third of the density of steel, but also one third of the stiffness. And stainless steel is close to mild steel in density and stiffness, but stronger, harder to work and a much poorer conductor of heat.
Stiffness and weight
For most sheet-metal parts, deflection governs before strength does. The bending stiffness of a plate is proportional to E × t³, so for an aluminium plate to match a steel one of the same width:
t_al = t_st × (E_st ÷ E_al)^(1/3) = t_st × (210 ÷ 70)^(1/3) ≈ 1.44 × t_st
Mass ratio = 1.44 × (2.70 ÷ 7.85) ≈ 0.50Equal plate bending stiffness. For stainless, (210 ÷ 200)^(1/3) ≈ 1.02, so practically the same thickness as steel.
So 2 mm steel becomes about 2.9 mm aluminium, which in practice means 3 mm stock at around 8.1 kg/m² against 15.7 kg/m². The saving is real, about half the weight, but only if the extra thickness is accepted. Replacing 2 mm steel with 2 mm aluminium gives a part with one third of the stiffness, which is the most common mistake in material swaps. Stiffening features such as flanges and returns help both materials equally, and are often a better route than extra thickness.
For strength, the comparison depends on the grades. In plate bending the stress scales with 1 ÷ t², so matching a 275 MPa steel with a 130 MPa aluminium needs about √(275 ÷ 130) ≈ 1.45 times the thickness, very close to the stiffness ratio in this case.
Bending, springback and cracking
After the punch is released, every bend opens out a little. The amount of springback rises with the ratio of yield strength to stiffness (σy ÷ E) and with the ratio of inside radius to thickness. Using typical actual yield values, σy ÷ E is roughly 0.001 for DC01, 0.0015 for annealed 304 and 0.002 or more for 5754-H22. That is why aluminium and stainless both need more overbend than mild steel to reach the same angle, and why stainless, which also work hardens strongly in the bend, can vary from batch to batch.
| Material | Minimum inside radius, 90° bend (rule of thumb) | Springback | Notes |
|---|---|---|---|
| DC01 cold-rolled steel | ≈ 0.5 to 1 × t | Low | Very forgiving, bends tightly |
| S275 hot-rolled steel | ≈ 1 to 2 × t | Low to moderate | Larger radii in thicker plate; check grain direction |
| 5754-H22, 5083-H111 | ≈ 1 to 2 × t | Moderate | Good formability in these tempers |
| 6082-T6 | ≈ 3 × t or more | High | Prone to cracking; bend across the grain or use a softer temper |
| 304, 316 (annealed) | ≈ 0.5 to 1 × t | Moderate to high | High tonnage; work hardens in the bend |
Harder aluminium tempers are where most bending failures happen. Fully hard tempers such as H18, and heat-treated tempers such as 6082-T6, can crack on the outside of a tight bend, particularly when the bend line runs parallel to the rolling direction. If a part needs tight bends, specify a formable temper such as H22 or H111, or bend across the grain. As a rule of thumb, stainless needs about half as much again press-brake tonnage as mild steel of the same thickness, so check the press capacity early.
Welding and finishing
- Mild steel: the easiest to weld, usually MIG/MAG or TIG. It needs a protective finish: powder coating, wet paint, pre-galvanised sheet, or hot-dip galvanising after fabrication. Hot-dip galvanised fabrications need vent and drain holes, and should be welded before galvanising.
- Aluminium: welded by MIG or TIG, with the oxide layer removed first. High thermal conductivity means more heat input, and high expansion means more distortion. Welding locally softens work-hardened tempers such as H22 towards the annealed condition, so design welded joints for the softened strength. 5xxx alloys are commonly welded with a 5xxx filler wire. Finishes include anodising, powder coating over a suitable pretreatment, or plain mill finish.
- Stainless steel: welds well by TIG or MIG, but low thermal conductivity and higher expansion make thin sheet prone to distortion, so use stitch welds and good jigging. Heat tint should be removed by pickling and passivation where corrosion resistance matters. Keep separate tools and abrasives to avoid contaminating it with carbon steel particles, which rust on the surface. Many stainless parts are left in the mill finish (for example 2B) or brushed.
Corrosion and galvanic contact
Mild steel will rust without a coating. Aluminium forms a thin protective oxide and performs well in most atmospheres; 5xxx alloys are widely used in marine environments. Stainless resists corrosion through its chromium-rich passive film: 304 suits most indoor and many outdoor uses, while 316, with added molybdenum, is the usual choice near the coast or where chlorides are present.
When two different metals touch in the presence of moisture, the less noble one corrodes faster. Aluminium is anodic to both steel and stainless, so it is the one that suffers. The effect is worst when a small area of aluminium is in contact with a large area of stainless, and when the joint stays wet. Stainless fasteners in a large aluminium sheet are often acceptable in dry or sheltered conditions, but in wet or marine conditions the joint should be isolated.
Cutting behaviour
All three cut well on a fibre laser. Mild steel is often cut with oxygen, which is fast but leaves an oxide layer on the edge that may need removing before painting or welding; nitrogen gives a clean edge. Stainless and aluminium are normally cut with nitrogen for a bright, oxide-free edge. Aluminium reflects and conducts heat strongly, so cutting speeds fall off with thickness faster than for steel, and dross on the underside is more common.
For punching, the force needed is roughly the cut perimeter times the thickness times the shear strength, which as a rule of thumb is about 0.8 times the tensile strength:
F ≈ L × t × 0.8 × RmExample: a 20 mm round hole (L = 62.8 mm) in 2 mm sheet needs about 25 kN in 5754-H22 (Rm ≈ 245 MPa), 35 kN in DC01 (Rm ≈ 350 MPa) and 65 kN in 304 (Rm ≈ 650 MPa).
Stainless also work hardens around the punch and wears tooling faster. Aluminium is soft and can gall or pick up on tooling, so punches need to be sharp and suitably lubricated.
When to choose each
- Mild steel: the default for brackets, frames, enclosures and guards where weight is not critical. Lowest material cost, easiest to fold and weld, but it needs a finish.
- Aluminium: where weight matters, such as vehicles, handheld or portable equipment, and large panels that people lift. Also where good thermal conductivity or natural corrosion resistance is useful. Allow for the extra thickness, the softer bend tempers and the welding distortion.
- Stainless steel: hygienic surfaces, food and pharmaceutical equipment, wet or coastal environments, and parts that must stay clean without coating. Expect higher material cost, higher bending tonnage and more care in welding.
Checklist for a material change
- Has the thickness been revised for equal stiffness, not just copied?
- Are the bend radii suitable for the new material and temper?
- Has the flat pattern been redeveloped with the right K-factor and springback?
- Are welded joints designed for heat-affected strength and distortion?
- Are dissimilar metals isolated where the joint can get wet?
- Is the finish specified, and is it compatible with the fabrication sequence?
If you are weighing up a change of material on an existing design and would like a second pair of eyes on the thickness, bends and joints, send us your drawings.