Aluminium Billet: Types, Alloys, Sizes and Quality

What is an aluminium billet?

In the aluminium industry, billet means a cast cylinder intended for further hot working. It sits between primary metal (ingot, sow or molten metal) and finished wrought products. Rolling mills use rectangular slabs or rolling ingots; extruders and forgers use round billet.

The word is used in two other ways that cause confusion. Machinists talk about "billet aluminium" parts, meaning parts CNC-machined from solid wrought bar, plate or block rather than cast to shape. And forgers sometimes call a sawn length of extruded or rolled bar a "forging billet" or "forging stock". This guide covers all three, but focuses on cast billet for extrusion and forging.

Billet quality matters more than its humble appearance suggests. Inclusions, porosity, coarse grains or poor homogenisation in the billet show up downstream as tearing and pick-up on extrusions, surface streaks after anodising, cracks during forging or ultrasonic rejects in finished parts. Buyers of aerospace forgings and high-finish architectural extrusions therefore specify billet as carefully as the final product.

How aluminium billet is cast and processed

1. Melting and alloyingPrimary aluminium and controlled scrap are melted, and alloying elements (Mg, Si, Cu, Zn, Mn) are added to hit the target chemistry, checked by spectrometer.
2. Melt treatmentThe melt is degassed (usually with argon in an in-line unit) to remove hydrogen that would cause porosity, and filtered through ceramic foam filters to remove oxide inclusions. A grain refiner such as Al-Ti-B rod is added for a fine, even cast structure.
3. Direct-chill (DC) castingMetal is poured into short water-cooled moulds on a casting table. As the shell freezes, the starting block is lowered and water sprays directly on the surface, solidifying the logs vertically. Hot-top and air-assisted moulds give a smooth surface with a thin segregated shell.
4. HomogenisationLogs are soaked at high temperature (for 6063 roughly 560 – 580 °C for several hours; lower for 2xxx and 7xxx alloys), then cooled at a controlled rate. This dissolves coarse phases, evens out segregation and gives the right precipitate size for extrusion speed and surface finish.
5. Sawing, scalping and inspectionLogs are sawn to length or to press-ready billets. Billet for forging and aerospace use is often scalped (peeled) to remove the cast surface. Inspection covers chemistry, dimensions, surface, macro-etch and ultrasonic tests as specified.

Older and smaller plants may use horizontal continuous casting for small diameters, and some forging stock is produced by extruding a cast billet into bar first, which refines the structure further. For critical aerospace forgings, the specification often defines which route is allowed.

Billet alloys and what they are used for

The large majority of extrusion billet is 6xxx, because 6060 and 6063 extrude fast with an excellent surface and 6061, 6082 and 6005A give structural strength. 2xxx and 7xxx billets extrude several times more slowly and need tighter casting control, which is why they cost more and are often bought from specialist casthouses. For a single alloy in depth, see our 7075 billet guide.

Chemistry within the registered limits also matters. Extruders often specify a narrower window, for example controlled Mg and Si in 6063 for consistent anodising, or a defined Mg₂Si balance for strength. Aerospace billet is typically cast to AMS or customer specifications with tighter impurity limits (especially Fe and Si) for better toughness.

Standard billet diameters, lengths and weights

Billet diameter is chosen to fit the extrusion press container or the forging operation. Common sizes are quoted in inches. Logs are typically cast around 4.8 – 5.8 m (about 190 – 230 in) long and sawn to press length, often 0.5 – 1.5 m. Weights below use 2.70 g/cm³; 7xxx and 2xxx alloys are 2 – 4 % heavier.

Smaller diameters (3 – 5 in) and larger ones (up to about 24 in or more) are made for special presses and forging work. Scalped billet is supplied with a diameter tolerance agreed to suit the container clearance; as-cast logs carry a wider tolerance. For extra-large round stock, see large diameter bar and billet.

How billet quality is checked

A good billet extrudes at high speed without tearing, gives a uniform anodised finish, and leaves little back-end scrap. A poor billet shows its problems as die lines, blisters, streaks, low press speeds or cracked forgings. If you see these, ask for the cast numbers and homogenisation records: most root causes trace to melt cleanliness, hydrogen or homogenising practice.

Where aluminium billet ends up

How to specify aluminium billet, and common mistakes

• State alloy and any narrowed chemistry (for example 6063 with Mg 0.45 – 0.55 %), not only the registered grade.
• Specify diameter, length (log or cut), tolerance and whether the billet is as-cast or scalped.
• Require homogenisation and say whether a controlled cooling rate is needed for your press.
• Define inspection: chemistry per cast, macro-etch, ultrasonic class for forging billet, and the certificate type (EN 10204 3.1).
• Agree packing: banded bundles or lengths on dunnage, with cast numbers marked on each billet for traceability.

Common mistakes include buying unhomogenised billet to save cost (and then losing far more in press speed and scrap), choosing a diameter that does not match the container, and accepting billet without cast-number traceability, which makes it impossible to investigate downstream defects. Metline supplies billet in all common alloys with mill test certificates and export packing, on EXW, FOB, CIF or DAP terms worldwide. Full product data is on our extrusion billet product page.

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