Extruded Aluminium Tube: Seamless and Porthole Tube Explained

What is extruded aluminium tube?

An extruded tube is a hollow section made in a single extrusion pass. The die forms the outside of the tube and a mandrel forms the bore, so the wall thickness is the gap between them. The tube leaves the press as a continuous length, is quenched, stretched straight, cut and aged, exactly like a solid profile.

Extrusion is the most economical way to make aluminium tube in medium and large sizes and in non-round shapes. It produces round tube, square and rectangular hollow sections, oval and streamline tube, and hollows with internal webs, fins or slots, all from relatively inexpensive dies. Smaller, thin-walled and precision tubes are usually extruded first and then cold drawn to size; see the comparison later in this guide.

Tube and pipe are often used interchangeably, but in the aluminium trade "pipe" refers to the standard nominal pipe sizes (NPS) and schedules used in piping systems, while "tube" is specified by actual outside diameter and wall thickness. Both can be extruded, and both can be seamless or porthole, which is the distinction that matters most for performance.

Typical uses include structural frames and handrails, truck and trailer bodies, machine guarding, bicycle and sports equipment, hydraulic and pneumatic cylinders, heat-exchanger and condenser tubes, aerospace and defence components, and pipework for chemical, cryogenic and gas service.

Seamless vs porthole extruded tube

There are two ways to put the hole in an extruded tube, and they produce materially different products.

ASTM B429 makes the point explicitly: structural pipe and tube made through a bridge or porthole die is not for fluid-carrying applications involving pressure; for those, the specification directs users to seamless products under ASTM B241. If your application involves internal pressure, bending to tight radii, flaring, swaging or expansion, specify seamless tube. Our seamless aluminium tube guide goes deeper into seamless production and testing.

Extruded tube shapes and special tubes

Non-round tubes are almost always made in porthole dies, since a mandrel shaped like the bore is easier to hold with bridges. Custom hollow sections with internal webs, screw ports or attachment features are covered in our custom extrusion guide.

Large-diameter tube deserves a separate note. Round tube above roughly 300 mm OD needs a large press, and the very largest seamless tubes are made on specialised vertical presses or by alternative routes such as forging and boring, or rolling and welding plate. Lead time, minimum quantities and test requirements all grow with diameter, so it pays to confirm early whether a standard size from stock, a heavier-wall tube machined to size, or a fabricated alternative would serve. For heavy-wall cylinders and sleeves, bored bar or a seamless rolled ring can also be an economical substitute, depending on length.

Alloys, tempers and properties for extruded tube

Typical properties (Aluminum Association / ASM handbook values) and selected ASTM minimums:

Design to the specified minimums for the actual wall thickness, not to typical values. Mill test certificates report the measured results for each lot.

Welding reduces the strength of heat-treated tube near the weld; 6061-T6, for example, falls to roughly T4-level strength in the heat-affected zone. Structural design codes such as the Aluminum Design Manual and EN 1999 give reduced allowable stresses for welded zones.

Standards for extruded aluminium tube and pipe

EN 755-7 and EN 755-8 cover the same size ranges (round tube about 8 – 450 mm OD, and shaped tube within a circumscribing circle of about 10 – 350 mm), but each applies only to its own production method. If a European drawing calls up EN 755-7, porthole tube is not acceptable even if it meets the dimensions.

Extruded tube vs drawn tube

Much of the small-diameter tube on the market is extruded and then cold drawn over a mandrel or plug through a sizing die. The drawing step changes the product significantly.

A useful rule: if you need tight tolerances or thin walls on a tube under about 50 – 75 mm OD, ask for drawn tube; for larger diameters, heavy walls, non-round shapes or structural use, extruded tube is usually the right and cheaper answer. See our guide to drawn tube and wire.

Typical sizes and theoretical weights

Weight per metre of round tube in kg = π × (OD − wall) × wall × 0.0027, with OD and wall in mm (density 2.70 g/cm³ for 6061 and 6063). The tables below use that formula; 7075 and 2024 are about 3 – 4 % heavier. Use the tube weight calculator for other sizes, including square and rectangular tube.

Aluminium pipe uses the same nominal sizes and schedules as steel pipe (ANSI H35.2 references ASME B36.10 dimensions). Weights below are calculated for 6061 from the nominal OD and wall:

Tolerances, testing and inspection

Extruded tube is supplied to the tolerance tables of its order standard: ANSI H35.2 for ASTM orders, EN 755-7 or EN 755-8 for EN orders. Tolerances are set on OD (or width across flats), wall thickness, ovality, straightness, twist (for shaped tube), corner radii and cut length.

• Wall thickness and eccentricity: the most important tolerance for pressure tube; seamless tube is checked for wall variation around the circumference
• Straightness: controlled by stretching; specify a tighter limit if the tube is used for shafts, rollers or cylinders
• Hydrostatic or pneumatic testing: required by some pressure specifications; state the test pressure on the order
• Eddy-current and ultrasonic testing: available to detect discontinuities in seamless tube for critical service
• Flattening and flare tests: used to show ductility and the absence of weak seams
• Certification: EN 10204 3.1 mill test certificate with chemistry and mechanical properties; third-party inspection on request

The most frequent ordering error with tube is mixing up dimensions: specifying OD and ID and wall together over-defines the tube, and specifying ID tolerance on porthole tube when the standard controls OD and wall. Specify two of the three (normally OD and wall) and name the standard.

Bending, welding, joining and finishing

Where extruded tube is used, by industry

For industry-specific details see the automotive, aerospace and marine pages.

How to choose and order extruded tube

1. Pressure or not?If the tube carries fluid or gas under pressure, or will be flared, expanded or tightly bent, specify seamless tube to ASTM B241 or EN 755-7. Otherwise porthole tube to B221, B429 or EN 755-8 is usually fine.
2. Pick the alloy and temper6063-T5/T6 for appearance and light loads, 6061-T6 or 6082-T6 for structural and pressure tube, 3003 for heat exchangers, 5083 for marine and cryogenic, 7075/2024 for aerospace strength.
3. Define the sizeOD and wall (or NPS and schedule for pipe), shape and corner radius for square and rectangular tube, and length.
4. Choose route and toleranceExtruded for general and large tube; drawn for small, thin-walled and precision tube. Name the tolerance standard.
5. Add tests and documentsHydrostatic, eddy-current or ultrasonic tests if required, EN 10204 3.1 certificate, export packing and delivery terms (EXW, FOB, CIF or DAP).

Related tube products and guides

Extruded aluminium (aluminum) tube: frequently asked questions