Aluminium sheet cracks on the bend for two main reasons: the alloy and temper are too hard for the radius you've asked for, or the bend line runs parallel to the grain. Pick a bendable grade, give it a sensible radius, and bend across the grain, and aluminium folds as cleanly as mild steel.
It's worth getting right at the design stage, because a cracked aluminium bend can't be rescued. Welding over it looks terrible and grinding it out weakens the part. Below we go through which alloys bend well, how big the radius should be, how to deal with grain direction, and what to write on the drawing so the shop knows what it's working with.
Why aluminium cracks when other metals don't
Every bend stretches the outside surface. Mild steel will stretch a long way before it tears. Many aluminium alloys, particularly the harder heat-treated ones, will stretch much less. Push them past that point and fine cracks open along the outside of the bend, sometimes visible straight away, sometimes only after the part has been handled or vibrated for a while.
Three things decide whether you get there:
- The alloy and temper, which set how much stretch the metal can take.
- The bend radius, which sets how much stretch you're asking for. A tighter radius means more stretch on the outside surface.
- The grain direction, which changes how much stretch the metal tolerates in that particular direction.
Get all three on your side and cracking stops being a problem.
Choose an alloy that wants to bend
This is the biggest lever. The alloy you'd pick for a machined bracket is often the worst choice for a bent one.
| Alloy family | Common examples | How it bends | Starting inside radius | Typical uses |
|---|---|---|---|---|
| 1xxx (pure aluminium) | 1050 | Very easily | About 1× thickness | Covers, flashings, signage, light trays |
| 3xxx | 3003 type | Easily | About 1× thickness | General sheet work, panels |
| 5xxx in softer tempers | 5052, 5083 | Well, much like mild steel | About 1× thickness, more on harder tempers | Enclosures, trailer and marine parts, boxes |
| 6xxx in T6 temper | 6061-T6, 6082-T6 | Poorly, prone to cracking | About 1.5–3× thickness or more | Structural plate, machined parts |
| 2xxx | 2024 type | Poorly | Large radius, often not suited to bending | Aerospace, high-strength parts |
In South Africa, common aluminium sheet grades are 1050 (often in H14 temper), 5052, and 5083, with plate in 5083 and 6082. That works in your favour: the sheet grades that are easy to buy are also the ones that bend well.
My advice: if a part is going to be bent, start with 5052 or 5083. They're stronger than 1050 and still bend nicely. Only reach for 6082 when you need its strength and can give it a generous radius, or design around welding instead. Our guide to aluminium sheet grades compares them in more detail.
Give it a generous radius
The tighter the bend, the more the outside surface has to stretch. With aluminium, a slightly larger radius is the cheapest insurance there is.
As a rule of thumb:
- Soft grades (1050, 3xxx, 5xxx in soft tempers): about 1× thickness. A 2 mm 5052 cover bends happily at a 2 mm inside radius.
- Harder grades (6xxx-T6 type): about 1.5 to 3× thickness, sometimes more on thick plate. A 3 mm 6082-T6 bracket might need a 6 to 9 mm inside radius to bend reliably.
On a press brake, the radius is mostly set by the die opening, so asking for a larger radius usually just means picking a wider die. It's not a special request. Our article on sheet metal bend radius explains how the radius forms and how to call it out.
Bend across the grain, not along it
When aluminium sheet is rolled at the mill, its internal structure gets stretched out in the rolling direction, a bit like the grain in timber. Bending parallel to that grain (with the bend line running the same way as the rolling direction) makes cracking far more likely. Bending across it is much more forgiving.
Sheet is rolled lengthwise, so the grain normally runs along the long side of the sheet. On a 2500 × 1250 mm sheet, that means:
- A bend line running across the 1250 mm width (perpendicular to the grain): good.
- A bend line running along the 2500 mm length (parallel to the grain): riskier, especially on harder alloys.
On soft grades with a decent radius, grain direction rarely matters much. On 6082-T6 with a tight radius, it can be the difference between a clean bend and a cracked one.
The catch is that laser cutting nests parts to save material, which can rotate them. If grain direction is critical for your part, mark it on the drawing with an arrow, and accept that it may use a bit more sheet.
Thickness, plate, and harder tempers
Thicker aluminium is harder to bend cleanly in the same way that harder alloys are: the outside surface has further to stretch. A rough way to think about it is that the radius should grow with thickness, not stay fixed.
If you need a strong, thick bent part, it often makes sense to rethink rather than force a tight bend:
- Use 5083 plate rather than 6082-T6, if its strength is enough.
- Increase the radius, which may also look better.
- Split the part into flat pieces and weld or bolt them, instead of bending at all.
Annealing harder alloys before bending is something specialists do, but it changes the material's strength. It's not something to assume on a quote.
Look after the surface
Aluminium is soft and marks easily. Tool marks along the bend and scuffs from handling are common on visible parts. If the part will be seen, or left bare, say so on the quote. If it's going to be powder coated, light handling marks matter less, though deep scores can still show through.
Also bear in mind that aluminium springs back more than mild steel after bending, so angles need more compensation. That's the shop's job, but it's another reason to allow a realistic angle tolerance.
Getting aluminium parts bent
On your quote, give the alloy and temper (for example "5052" or "6082-T6"), the thickness, and the inside radius or "standard". Mark the grain direction if it matters, and say whether the surface is cosmetic. If you're not sure which alloy suits the job, tell us what the part does and we'll help you choose.
Read more about our CNC bending service, or start a quote for CNC bending with laser cutting and coating on the same order. If you're still deciding between aluminium and steel, see aluminium vs steel sheet.