Understanding Aluminium Alloy Tempers: The Complete Guide

Pure aluminium is soft, but mixing it with other metals creates strong aluminium alloys. To make these alloys ready for factories, they go through thermal or mechanical treatments. This final state of the metal is called its temper.

The temper designation system uses letters and numbers added after the alloy name (for example, 6061-T6).

1. Main Temper Letter Codes

There are five basic temper treatments. Each has a specific letter code.

[Alloy Number]-[Basic Letter Code][Specific Sub-Numbers]

Example: 6063-T66

• F (As Fabricated): Metal used exactly as it comes out of shaping machines. No extra heating or hardening is done.
• O (Annealed): The metal is heated to a high temperature and cooled down very slowly. This makes it soft and easy to bend.
• H (Strain Hardened): The metal is strengthened at room temperature by bending, rolling, or pressing. This is only used for alloys that cannot be hardened by heat (like the 3xxx and 5xxx series).
• W (Solution Heat Treated): An unstable temper. The metal is heated and cooled fast, but its properties change over time at room temperature.
• T (Thermally Treated): The metal is heated in special ovens to become stable and much stronger. This is used for structural alloys (like the 6xxx and 7xxx series).

2. Deep Dive: Strain Hardened Sub-Codes (H)

The letter H is always followed by two or more numbers to explain the exact process:

First Digit (The Method)

• H1: Strain hardened only.
• H2: Strain hardened and then partially annealed (softened slightly to prevent cracking).
• H3: Strain hardened and stabilized by low heat.

Second Digit (The Hardness Level)

• H_2: 1/4 hard (very easy to form).
• H_4: 1/2 hard (medium strength).
• H_8: Full hard (maximum strength from rolling).

3. Deep Dive: Thermally Treated Sub-Codes (T)

The letter T is followed by numbers from 1 to 10. These show the specific heat-treating recipe:

4. Deep Dive: Tempers for Extruded Aluminium Profiles

Extruded aluminium profiles (like tubes, window frames, and structural beams) mostly use heat-treatable alloys from the 6xxx series (such as 6060, 6063, or 6082). The choice of temper changes how strong the profile is and how easily it can be bent or machined.

The Main Extrusion Tempers Explained

• T4 (Solution heat-treated and naturally aged): The profile is cooled quickly after extrusion, then left at room temperature. It stays relatively soft and highly ductile, making it the best choice if you need to bend the profile into arches or complex shapes without cracking.
• T5 (Cooled from an elevated temperature shaping process and artificially aged): The profile is cooled quickly using air fans right at the extrusion press, then placed in a hot oven. It gives good strength with minimal distortion, commonly used for standard window frames.
• T6 (Solution heat-treated and artificially aged): The profile is fully quenched (cooled fast with water or heavy air blast) to freeze the internal structure, then aged in a hot oven for several hours. This achieves the maximum standard strength for the alloy.
• T66 (Higher mechanical properties variant): This is a special European standard temper. By strictly controlling the chemical mixture and the speed of cooling at the factory, manufacturers achieve higher strength values than standard T6, without adding extra production steps.

Key Differences: How to Choose the Right Profile Temper

5. Real-World Practical Examples

• Aluminium Foil (1200-O): It must be soft and flexible to wrap around food. Annealing (O) makes it soft.
• Bicycle Frames (6061-T6): The frame must survive heavy jumps. Artificial aging (T6) gives the 6061 alloy its maximum tensile strength.
• Beverage Cans (3104-H19): Can walls are super thin but must hold gas pressure. Extreme strain hardening (H19) makes the thin metal stiff.