Machining aluminium: choosing between 5083, 6061, 6082, 2017 and 7075

An aluminium designation is read in two parts, as in “EN AW-6082 T6”. EN AW-6082 identifies the alloy: “EN” for the European standard, “AW” for wrought aluminium (rolled, extruded or drawn product, as opposed to cast parts), then four digits. The first gives the series, and therefore the main alloying element:

• 2xxx: copper (2017A, 2024);
• 5xxx: magnesium (5083, 5754);
• 6xxx: magnesium and silicon (6060, 6061, 6082);
• 7xxx: zinc (7075).

T6 indicates the temper. The 2xxx, 6xxx and 7xxx series harden by heat treatment (T tempers: T4 for natural ageing, T6 for artificial ageing, T651 for a plate stress-relieved by stretching), the 5xxx series by strain hardening (H tempers). A 6082 in T6 temper does not have the same properties as an annealed 6082: the temper is part of the designation and must appear on the drawing. For a plate machined from solid, you will come across, for example, “EN AW-6082 T651” or “EN AW-7075 T651”. For an overview, see the glossary entry on aluminium alloys.

6082 and 6061: the default choice

Medium strength, good machinability, good corrosion resistance, weldable and anodisable: they cover most mechanical parts, frames and brackets. They are close and often interchangeable: 6082 is the most widely stocked in Europe, 6061 in North America. If your drawing specifies one of the two, as is common in aerospace, state that equivalence is not accepted; otherwise write “or equivalent”.

2017A: the easy-to-machine “mechanical” alloy

A copper alloy with good mechanical strength and excellent machinability, giving fine surface finishes. In return, it has lower corrosion resistance, welds very poorly and anodises less well: plan for protection. It suits mechanical parts machined from solid. The similar 2024 is a great aerospace classic.

5083: the marine, weldable alloy

A magnesium alloy, hardened by strain hardening: excellent corrosion resistance, notably in sea water, and good behaviour after welding. It is mainly found as sheet and plate. It machines less well (long chips, tendency to stick to the tool) and its strength remains moderate. It is the alloy of naval sheet-metal fabrication, tanks and welded structures (see the marine and boatbuilding sector).

7075: high strength

A zinc alloy with a much higher tensile strength (around 500 MPa and above, against about 300 MPa for a 6082 T6), with good machinability and short chips. It costs more, cannot be welded by arc processes and resists corrosion less well; in T6 temper it is susceptible to stress corrosion, hence the existence of overaged tempers such as T73. It is found in aerospace (aerospace), in tooling and in highly loaded parts.

Other alloys you will come across

5754, close to 5083 but less strong, is a classic for bent and welded sheet. 6060 is the alloy of extruded profiles, valued for decorative anodising. 2011 is a free-cutting alloy, designed for turned parts in large runs. Casting alloys, intended for cast parts, follow other rules and are not covered here.

Aluminium machines quickly, with high cutting speeds, but the alloy and temper change the result:

• Chips. Short and easy to clear on 2017A and 7075; longer and stringy on 5083 and, to a lesser degree, on 6061. This affects automatic cycles and surface finish.
• Built-up edge. Soft alloys (5083, 6061) tend to stick to the tool, which spoils the appearance. The workshop compensates with suitable tooling (sharp-edged carbide, polished surfaces) and lubrication.
• Internal stresses. A thick plate retains rolling and quenching stresses: when material is removed, the part can distort. Stress-relieved tempers (T651) limit the problem; the workshop can also rough, let the part rest, then finish.
• Expansion. Aluminium expands about twice as much as steel. A part that is still warm does not measure its dimension at 20 °C: for tight tolerances, inspection is done at the reference temperature.
• Available stock. Round bars, flats, rolled plates, extruded profiles, cast tooling plates: the stock format determines the material lost as chips, and therefore the price (see the price of CNC machining).

For details on the processes, see the page on CNC machining subcontracting.

Aluminium is very forgiving, but a few design choices reduce machine time and distortion:

• Thin walls. The thinner and taller a wall, the more it vibrates during cutting and the more it distorts. Thicken it or add ribs where the function allows, and flag very thin walls to the workshop.
• Internal radii. A milling cutter is round: a sharp corner at the bottom of a pocket requires a rework. The more generous the internal radius, the larger and faster the tool can be.
• Deep pockets. The deeper a pocket is relative to its width, the longer, more fragile and slower the tool. Discuss this ratio with the workshop before freezing the drawing.
• Stock. A thick flat from which most of the material is removed costs both in material and in time; a profile close to the final shape can change the price.

These choices interact with tolerances: only tighten the functional dimensions (see the guide on machining tolerances).

Welding is often the criterion that rules out an alloy:

• 5083: very good weldability (TIG, MIG) and good behaviour in the joint.
• 6061 and 6082: weldable, but the zone near the weld loses part of the strength of the T6 temper: the design calculation must allow for this.
• 2017A: welding not recommended (risk of cracking). Prefer mechanical fastening or bonding.
• 7075: cannot be welded by arc processes. Same solution: screws, rivets, bonding.

For the processes, see the guide on TIG, MIG and MAG welding.

As for corrosion, the mechanically strongest alloys (2017A, 7075), rich in copper or zinc, are also the most sensitive: they need protection (anodising, paint, chemical conversion). 5083 leads in marine environments; the 6xxx series behave well in a normal atmosphere.

Anodising thickens the natural oxide layer of aluminium: it protects against corrosion, hardens the surface (hard anodising) and can be dyed. The result depends on the alloy:

• 6060, 6061, 6082: good, even result, including for a decorative appearance.
• 2017A and 7075: they can be anodised, but the appearance is less uniform and the result harder to guarantee. If appearance matters, ask the anodiser for a trial.
• 5083: protective anodising works, but the appearance may differ between the base metal and the weld beads.

Three points to put on the drawing:

1. Dimensions before or after treatment. The layer grows partly into the metal and partly outwards: it changes dimensions, sometimes by several tens of microns in hard anodising.
2. Areas to mask. The layer is electrically insulating: mark contact or earthing surfaces, as well as threads and precision bores.
3. The finish required: protection, appearance, hardness, colour.

Qualitative guidelines for wrought products, to be confirmed against the standard or the supplier’s datasheet depending on the product and thickness. These alloys all have a density of around 2.7 to 2.8 g/cm³, about a third that of steel.

• No particular requirement: a 6082 (or 6061) “or equivalent”.
• Welded part or marine environment: a 5083, or a 6082 if strength takes priority.
• Heavily machined, non-welded, protected part: a 2017A.
• Maximum strength or serious weight saving: a 7075, accepting the extra cost and the surface protection.

A few habits avoid unpleasant surprises. Do not over-specify: a 7075 for a lightly loaded part pays for the material, the protection and sometimes a longer supply lead time for nothing. Put the full designation on the drawing (“EN AW-6082 T6”), not just “alu”. Only ask for a material certificate if traceability requires it (aerospace, medical, energy) and then check that it states the alloy and temper. Finally, let the workshop suggest an alternative: a grade available from stock often reduces price and lead time.

Ready to ask for prices? Post your request on Meaflow: it is free and commission-free, and you compare quotes from several workshops.