Aluminium grades explained: what 6061 means on a spec sheet

A four-digit number and a temper code tell you how a metal part was made and how it will finish, and a listing without them tells you very little.

The four digits

Wrought aluminium alloys, the kind that are rolled, extruded or machined from solid, carry a four-digit number under the system maintained by the Aluminum Association in the US and used internationally. The first digit names the main alloying element:

• 1xxx: at least 99% aluminium. Soft, conductive, used for foil and electrical parts.
• 2xxx: copper. High strength, used in aircraft structures.
• 3xxx: manganese. Drink cans and cookware.
• 4xxx: silicon. Welding wire and brazing.
• 5xxx: magnesium. Sheet metal, marine parts.
• 6xxx: magnesium and silicon. Extrusions and machined parts.
• 7xxx: zinc. The highest-strength alloys, used in aerospace.
• 8xxx: other elements, including lithium.

The remaining digits identify the specific alloy within the series. Cast alloys use a different format, written with a decimal point, such as 380.0.

What 6061 is

6061 is a 6xxx alloy: mostly aluminium, with roughly 1% magnesium, under 1% silicon, and small amounts of copper and chromium. It is one of the most widely used aluminium alloys in the world. Bicycle frames, machined brackets and structural parts are commonly made from it.

It earns that position because it does several things well at once. It machines cleanly, so edges and holes come out crisp. It resists corrosion. It anodises evenly. And it can be heat treated to a useful strength.

Like all aluminium, it is light. Its density is about 2.7 grams per cubic centimetre, roughly a third of steel.

The letters after the number

The temper code follows a dash and tells you how the metal was treated.

• O: annealed. The softest state.
• H: strain hardened by working. Used for alloys that cannot be heat treated, like 5xxx sheet.
• T: heat treated. The digit after the T names the process.

6061-T6 is the common form: solution heat treated, then artificially aged. Standard materials references list its typical tensile strength at around 310 MPa (about 45 ksi). Annealed 6061-O is far softer. So "6061" alone is an incomplete spec. "6061-T6" is a complete one.

Other grades you will see

• 6063. Another 6xxx alloy, built for extrusion. It gives a smooth surface finish and is lower in strength than 6061. Common in window frames and extruded stands.
• 5052. A magnesium alloy used as sheet. It bends without cracking, so it suits stands folded from a single sheet. It cannot be heat treated.
• 7075. A zinc alloy, much stronger than 6061 and more expensive. Standard references list 7075-T6 at around 570 MPa tensile strength. It is rarely needed in a desk accessory.
• Die-cast alloys. A380 in US notation and ADC12 in the Japanese standard are common. Casting makes complex shapes cheaply. These alloys contain a lot of silicon, which anodises poorly, so die-cast parts are usually painted or powder coated instead.

Does the grade matter on a desk?

Honestly, for strength, not much. A laptop weighs a few pounds. Almost any aluminium alloy is strong enough to hold one.

Stiffness is a separate question, and the grade barely changes it. Aluminium alloys share roughly the same elastic modulus, about 69 GPa. That number governs how much a part flexes under load. A wobbly stand made of 7075 would wobble almost as much in 6061. Wobble is a design problem: wall thickness, cross-section, joints.

What the grade does tell you is how the part was made and how it will look. 6061 points to a machined or extruded part with crisp edges and an even anodised finish. A die-cast alloy points to a moulded shape with a painted finish. Both can be fine. They are not the same thing at the same price.

Anodising

Anodising is an electrochemical process that thickens the oxide layer that forms on aluminium naturally. The layer grows out of the metal itself rather than sitting on top like paint, so it does not peel. It can be dyed before it is sealed, which is how coloured aluminium is made.

The US military specification MIL-A-8625 is the common reference. Type II is the standard decorative process. Type III, called hardcoat, is thicker and more wear resistant. A deep scratch through either shows bright bare metal underneath.

How to check a claim

• Ask for grade and temper. A seller who knows the part will answer with both.
• Ignore "aerospace grade" and "aviation aluminium". They are marketing phrases, not standards. Several alloys are used in aircraft. The phrase needs a number.
• Treat "aluminium alloy" with no number as unknown. It could be any of the above.
• Use a magnet. Aluminium is not magnetic. If a magnet sticks to the body of a part, and there is no magnet built into it, the body is steel.
• Feel the weight. Zinc die-cast alloys are roughly two and a half times as dense as aluminium and are often plated to look like it. A small part that feels heavy for its size is worth asking about.
• Look for casting marks. A thin seam line or small round marks on hidden faces are signs the part was cast, not machined.

From Undivided

The Undivided Laptop Stand is made from 6061 aluminium, anodised, and raises a 10 to 16 inch laptop through six height positions.