Extrusion as the Base Process
Almost all aluminum profiles are produced by extrusion. A cast billet is preheated to between 450 °C and 500 °C, pushed through a shaped die, quenched in air or water and then aged to the ordered temper. Rolling and forging produce aluminium products too, but extrusion dominates because a single die can deliver a hollow, multi-void, constant cross-section at low tooling cost, which is exactly what window systems, heat sinks and machine frames need. Because a profile is a constant-section product, it can be cut to any length and joined mechanically, so design flexibility comes from the die rather than from the mill rolling schedule.
The alloy and temper are chosen together with the section. EN 573-3 defines the chemical composition of each alloy and EN 755-2 defines the mechanical properties of extruded bar, tube and profile. ASTM B221 covers the same product family for North American projects.
Classification by Function
| Family | Typical sections | Common alloy and temper | Main use |
|---|---|---|---|
| Structural | I-beams, angles, channels, gusset profiles | 6061-T6, 6082-T6 | Bridges, frames, support structures |
| Architectural | Window and door frames, curtain wall mullions, louvres | 6063-T5, 6063-T6 | Building envelopes and interiors |
| Transport | Body frame rails, floor beams, roof bows | 6005A-T6, 6061-T6 | Bus, rail carriage, trailer bodies |
| Industrial | Machine frames, conveyor rails, racking | 6063-T6, 6061-T6 | Production equipment |
| Electronic and thermal | Heat sinks, LED housings, enclosure frames | 6063-T5, 1050A | Dissipation and enclosures |
Architectural work accounts for the largest tonnage, and 6063 is the standard alloy there because its magnesium and silicon content, typically 0.45 to 0.9% Mg and 0.20 to 0.6% Si, gives a good balance of extrudability, strength and anodising response. Where higher load capacity is required, 6061 with 0.8 to 1.2% Mg and 0.15 to 0.40% Cu is selected and supplied in the T6 temper.
Mechanical Values and Tolerance Classes
Minimum mechanical properties are quoted from EN 755-2 for wall thickness below 25 mm. 6063-T5 delivers a tensile strength of at least 150 MPa, a 0.2% proof stress of at least 110 MPa and 8% elongation. 6063-T6 raises those figures to 205 MPa tensile and 170 MPa proof stress. 6061-T6 reaches 260 MPa tensile and 240 MPa proof stress, and 6082-T6 reaches 295 MPa tensile with a 250 MPa proof stress. Designers working to a structural code will normally apply the proof stress rather than the tensile value, with the buckling check governed by the second moment of area of the extruded shape.
Tolerances follow the same standards and are expressed as classes. EN 755-9 and EN 12020-2 define dimensional tolerances for general and precision profiles respectively, covering wall thickness, cross-section, straightness and twist. Precision classes are specified for profiles that will be cut, punched and assembled without further machining, while general classes are adequate for architectural sections that are mitre-cut and gasketed on site. Wall thickness asymmetry is the most common source of assembly problems, so it is checked on the first article rather than assumed.
Surface Finishes
Three finishes dominate. Anodising produces a controlled aluminium oxide layer, specified under EN ISO 7599 or GB/T 5237.2 with classes such as AA10 and AA15 denoting 10 µm and 15 µm average coating thickness on the visible surface and a local minimum of 8 µm and 12 µm respectively. Powder coating is specified under EN 12206-1 or GB/T 5237.4 and normally builds 60 to 80 µm of film, with 40 µm the practical local minimum, giving wide colour choice and good impact resistance. Mill finish is used where appearance is secondary or where the profile will be machined and painted downstream. Mechanical finishing such as brushing, polishing and etching is applied before anodising when a specific texture is required.
Ordering a Profile to Fit the Application
An export order should state the alloy and temper, the cross-section drawing with tolerance class, the length and cut accuracy, the surface treatment and colour or anodising class, and the packing method. Profiles are normally supplied in lengths of 3 m to 6 m, protected with interleaved paper or shrink film, and boxed or bundled on wooden supports. Inspection covers chemical composition against EN 573-3, mechanical properties from a tensile test on each production lot, dimensional checking against the drawing, and surface examination for die lines, streaks and handling marks. A mill test certificate to EN 10204 3.1 accompanies the shipment and the heat number is traceable through the bundle labels.
Frequently Asked Questions
Q: Which alloy should be used for an anodised window frame?
A: 6063 in the T5 or T6 temper is the standard choice. It extrudes cleanly to fine wall thickness and takes a uniform anodic film, which 6061 does not achieve as consistently because of its copper content.
Q: What does T5 mean compared with T6?
A: Both are artificial ageing tempers, but T5 is aged after cooling from the press without a separate solution treatment, while T6 involves a controlled solution treatment and ageing. T6 gives higher strength and tighter property control.
Q: How thick is a normal anodised layer?
A: Architectural anodising is usually specified as AA15, meaning 15 µm average on the visible face with a 12 µm local minimum. Interior work is often AA10 at 10 µm average with an 8 µm minimum.
Q: What elongation should be expected from an extruded profile?
A: Alloy tempers in the 6061-T6 and 6082-T6 class are typically certified at 8% minimum elongation in EN 755-2, while the softer 6063-T5 is also certified at 8% minimum across the usual wall thickness range.
Q: Can profiles be supplied cut to final length?
A: Yes. Cut-to-length supply with punched holes and machined ends is a standard service, and it is preferred for transport projects because it reduces on-site saw work and scrap. Cut tolerance and end squareness are stated on the drawing.
