Choosing aluminium: 6082, 7075 or 5083?

Which aluminium alloys turn up on the shop floor?

If the drawing only says aluminium, that is a question for the customer before it becomes a price. An order portal that picks an alloy itself quietly takes the decision away from the shop. The shop that asks for the material specification and proposes a stock form with a reason keeps the decision in its own hands.

EN AW-6082 is Europe's structural aluminium for frames, plates and machine parts. AZoM calls it the alloy most commonly machined in plate form, and notes it has replaced 6061 in many applications. You still meet 6061 on drawings made to American standards.

EN AW-7075 is an aluminium-zinc alloy for highly stressed structural parts and moulds. EN AW-5083 resists seawater; as cast, precision-milled plate it is the base for moulds and fixtures. EN AW-2011 carries bismuth and lead alongside copper: free-machining aluminium for turning.

What does the temper after the alloy mean?

The code after the alloy is the material condition (temper). O is annealed, H strain-hardened, T6 solution heat-treated and artificially aged. T651 is T6, stretched along the way to relieve stress.

For a part milled from plate, that matters. Quenching after solution treatment leaves residual stress between skin and core, Li and colleagues write; stretching lowers it without removing it. So ask for T651 when you buy 6082 or 7075 plate. If the drawing names no temper, that is an open point in the estimate, not something for the buyer to guess.

Rolled or cast plate: what does it do to distortion?

Residual stress and distortion go together. While the block is whole, the stress is in balance. Mill a deep pocket from one side and the plate bows, often only once you unclamp it.

How you remove material counts too. Li and colleagues modelled and tested a frame part in thick plate of a 7000-series alloy. With all 10 mm taken from the top, the frame distorted by almost 2 mm. With 7 mm from the underside and 3 mm from the top, it stayed below 0.1 mm.

Cast plate tackles it at source: its datasheet describes it as homogenised and stress relieved, and it arrives milled flat to thickness. It is weaker, though, with a proof stress of 110 to 130 MPa against about 260 MPa for 6082 in T6. Plenty for a base plate or mould, not for a heavily loaded bracket.

NEN-EN 485-3 sets the dimensional and form tolerances of hot-rolled plate as delivered, not the flatness of your part after the pocket. That comes from your route: allowance, sequence and re-clamping. See sizing the blank.

Which alloy machines most easily?

The machinability of aluminium comes down to the chip and the edge. Soft, ductile materials readily form a built-up edge: material that welds onto the cutting edge and roughens the surface. A low cutting speed encourages it. That is why sharp edges and a high cutting speed belong with aluminium.

The harder alloys mill most pleasantly. 7075 gives a clean chip, 6082 in T6 or T651 machines well. On turned batches, chip control decides: one long curl round the workpiece ends an unattended night shift. Free-machining aluminium breaks its chips short, and you pay that back in anodising and welding. Whether the part belongs on a lathe at all is covered in milling or turning.

What does the alloy do to anodising?

In anodising, the alloy has a say. The Vereniging ION handbook says alloying elements set the colour, thickness and porosity of the oxide layer; silicon ends up in it as a grey constituent. Two hard anodising processes in that handbook are unsuitable for alloys with more than 3 per cent copper, and free-machining 2011 contains over 5 per cent. Choose it to turn faster, and take any hard anodising requirement to your anodiser first.

Allow for the layer itself: in typical processes half of it grows outwards. A fit or a threaded hole then needs an allowance or masking. The price comes from your finisher; how that line stays open until it arrives is in finishing without a price.

Worked example: a base plate from rolled and cast plate

A base plate of 300 by 200 by 20 mm, with a deep pocket on one side and a flatness requirement, 10 off, carrying little load. Rolled 6082 is unclamped after roughing, left to rest and re-clamped for a fresh facing pass: one setup more. Cast plate arrives milled flat to thickness, so facing both sides and the extra step drop out.

Here cast plate costs €6 more in material and still comes out €10 cheaper per part. Without the flatness requirement or the deep pocket, rolled plate skips the extra step and setup too, and wins on material price.

When do you propose a different alloy to the customer?

Proposing a different alloy is craft. The shop proposes, with the effect on price and lead time; the customer's engineer decides, because strength, corrosion and approval sit with them. Record the approval as a new revision, with the reason. An ERP that only keeps last year's price will not tell you at the repeat order why it became cast plate.

• What must the part do: strength, flatness, corrosion, appearance?
• Does the drawing name an alloy and temper, or only aluminium?
• Is there a pocket on one side that will make the plate move?
• Will it be anodised, with one colour across the batch?
• Is it a turned batch that has to run unattended?
• How old is your own purchase price, and how far apart is the price per kilo of each option?
• Who approves a different alloy, and how does it reach the revision?