In addition to the huge variety of alloys that are available, the temper – or hardness – of each alloy can create considerable differences in their characteristics and how they react to various fabrication processes. These include processes such as punching, forming and welding.
What are aluminum alloy temper designations?
Temper designations refer to variations of the physical properties that are achievable within an alloy.
The alloys we extrude – wrought aluminum alloys – are either heat treatable and non-heat treatable. Both types are widely used. Alloys in the 1xxx, 3xxx, and 5xxx series families are non-heat treatable. 2xxx, 6xxx, and 7xxx series alloys are heat treatable, such as the 6061 alloy. 4xxx series alloys contain both heat-treatable and non-heat treatable varieties.
Alloys in the non-heat-treatable group cannot be strengthened significantly by heat treatment, and their properties depend upon the degree of cold work. Heat-treatable alloys can.
This means the differences in the chemical and metallurgical structures of the alloy groups also have an impact on how the alloys react during the welding process, as well as the other fabrications processes I mentioned.
In other words, this wide variety of aluminum alloys and their tempers creates a complex range of materials. Understanding the basic differences can help you be more successful.
Get in touch with Hydro's engineering teamFive aluminum alloy temper designations
It is not easy, at a glance, to make sense of temper designations. But it is important to recognize and understand what the letters and numbers mean.
Aluminum products with specific properties and forms are identified by alloy and temper designations. Alloy designations are four-digit numbers. These identify the alloy chemistry.
Temper designations are alphanumeric. They are added to the alloy designations, after the four-digit alloy designation. An example is 6061-T6.
Temper designations tell both the producer and the user how the alloy has been mechanically and/or thermally treated to achieve the properties desired. The first character in the temper designation (a capital letter, F, O, H, W, or T) indicates the general class of treatment.
- F, as fabricated. Most F-temper products are “semi-finished” products. They will be used in shaping, finishing or thermal processes to achieve other finished forms or tempers.
- O, annealed. Annealing treatments are used to achieve the lowest-strength condition for the alloy. The main reason is to maximize workability or increase toughness and ductility.
- H, strain-hardened. This is for non-heat-treatable alloys that have had their strength increased by strain hardening, usually at room temperature.
- W, solution heat-treated. This designation applies only to alloys that age naturally and spontaneously after solution heat treating. It is rarely a finished temper.
- T, thermally treated. This applies to any product form of a heat-treatable alloy that has been thermally treated to produce a stable temper other than F, O, or H, sometimes including solution heat treatment and quenching.
T and H temper subdivisions explained:
T-temper subdivisions
T1: Cooled from an elevated-temperature shaping process and naturally aged
T2: Cooled from an elevated-temperature shaping process, cold worked, and naturally aged
T3: Solution heat-treated, cold worked, and naturally aged
T4: Solution heat-treated and naturally aged
T5: Cooled from an elevated-temperature shaping process and artificially aged
T6: Solution heat-treated and artificially aged
T7: Solution heat-treated and overaged/stabilized
T8: Solution heat-treated, cold worked, and artificially aged
T9: Solution heat-treated, artificially aged, and cold worked
T10: Cooled from an elevated-temperature shaping process, cold worked, and artificially aged
H-temper subdivisions (strain-hardened, non-heat-treatable alloys)
H1x: Strain-hardened only
H2x: Strain-hardened and partially annealed
H3x: Strain-hardened and stabilized
Second digit (1-8): Degree of strain hardening. 8 is the hardest full-hard temper; 9 denotes extra hard, beyond standard H8.
How aluminum heat treatment works
Most extrusion alloys are hardenable. This means they reach their final strength not through cold working alone, but through a two-step heat treatment process: solution heat treatment followed by ageing, also known as precipitation hardening. This process is what determines whether a profile ends up as a T4, T5, T6, or another T-series temper.
Solution heat treatment
Solution heat treatment involves rapid heating to a uniform temperature, generally in the range of 510 to 535°C depending on the alloy, with a tolerance of ±10°C. For most extruded profiles, this isn't a separate production step. It happens during extrusion itself, as the material is already at an elevated temperature when it leaves the press. Cooling rates after this step vary by alloy: standard alloys are cooled in water at around 1°C per second, while higher-strength alloys like 6082 require a faster quench, roughly 10°C per second, and must be transferred from furnace to water within about 10 seconds to achieve it.
Ageing
Ageing is where the alloy develops its final strength, and it can happen in one of two ways:
- Artificial ageing takes place in a furnace at approximately 180°C ±5°C for a period of several hours. This is the route to a T6 temper.
- Natural ageing happens spontaneously at room temperature, with no furnace required. For standard 6xxx series alloys, a T4 temper is typically reached within about 3 days.
Annealing
If the goal is a soft, highly formable profile rather than a strong one, the alloy is annealed instead. This means heating to roughly 380 to 420°C, holding at that temperature for about 30 minutes, then cooling slowly, ideally in the furnace down to 250°C before finishing with air cooling. This is the process behind an O temper.
Why it matters
The heat treatment route chosen changes what the profile can do. A T4 profile from naturally ageing at room temperature keeps more formability, which is useful if the part still needs to be bent or shaped after extrusion. A T6 profile from artificial ageing in a furnace trades some of that formability for meaningfully higher strength. Same alloy, same starting point, different outcome, depending entirely on which heat treatment path it takes.
How tempers impact your product
End users should understand these designations in detail so that, in any subsequent processes, they do not destroy key capabilities provided by the producer.
Here are two examples:
- You can optimize the mechanical properties of heat-treatable alloys by choosing an appropriate solution heat treatment, a suitable quench rate and age-process sequence. This can improve the corrosion resistance of certain alloys, at the expense of strength. And vice versa.
- The temper of an alloy can affect the appearance of a product after it is anodized. This is due to the combinations of elements within an alloy, which cause the alloy to react differently to the anodizing process.
It is not easy to understand the broad range of aluminum alloys and tempers – the varieties of mechanical properties that are available – particularly for structural engineers who are accustomed to working with steel. But it is important, and I hope this quick guide to temper designations is a step in the right direction.
