Tempers T4, T5, T6 and T66 explained

What does a temper code say?

An aluminium alloy’s strength depends on its heat-treatment history as well as on its composition. EN 515 records that history in a code placed after the alloy designation, as in EN AW-6063 T5. The letter T means the alloy has been thermally treated to a stable condition other than F, O or H, and the digits that follow describe the sequence of operations. The code therefore describes both the route taken and the properties to expect.

How do the four tempers differ?

Temper	EN 515 definition	In practice
T4	Solution heat treated and naturally aged to a substantially stable condition	Low yield strength, high elongation, good for forming
T5	Cooled from an elevated-temperature shaping process and then artificially aged	Quench at the press plus ageing, the most common temper
T6	Solution heat treated and then artificially aged	High strength, structural use
T66	A defined variant with higher properties than T6	Tighter process control, agreed at order

What are solution treatment and ageing?

These are two separate operations. In solution treatment the alloy is taken to a temperature at which magnesium and silicon dissolve in the aluminium and then cooled fast, so the elements stay trapped in solution. In ageing, that supersaturated structure is allowed to decompose into fine precipitates, either slowly at room temperature or faster in an oven. The precipitates obstruct dislocation movement and strength rises. Ageing at room temperature is natural ageing, and ageing in an oven is artificial. Artificial ageing generally runs between about 160 and 200 °C for several hours, depending on the alloy.

How can quenching at the press give a solution-treated condition?

In conventional heat treatment a product is reheated in a separate furnace and quenched. In extrusion the metal already leaves the die at high temperature. For suitable alloys and sections, controlled heating and extrusion can establish the required solid solution, and prompt, sufficiently rapid cooling directly at the press, called press quenching, retains it. The cooling rate must be high enough for the section:

• Thin-walled 6060 and 6063 sections usually cool fast enough in forced air.
• Thicker sections and quench-sensitive alloys, such as 6005A and 6082, may need a water mist or a water quench.

So the difference between T5 and T6 is not simply the cooling method. The temper designation must match both the treatment sequence defined in EN 515 and the applicable product requirements, and the same ageing oven can deliver different properties depending on the cooling rate that came before it. The temper to use follows from alloy, section and specification together.

When should you use each temper?

• T4: when bending, drawing, crimping or riveting will follow. Natural ageing stabilises within a few days, and artificial ageing can be applied after forming.
• T5: when moderate strength is enough and the section cools fast enough at the press. It is very common for architectural and general-purpose profile.
• T6: when higher strength is required and the alloy and section can deliver it. It is typical for structural parts.
• T66: when properties above T6 are needed and the alloy allows it. Check EN 755-2 for the alloys for which it is defined and for the values.

How does temper change strength?

The same alloy behaves very differently in different tempers. A profile in T4 gains markedly in yield strength when aged, and loses elongation. For that reason a design note should always carry alloy and temper together. Minimum values by wall thickness are tabulated in EN 755-2. For the differences between alloys, see 6060, 6063, 6005A and 6082 compared.

Does temper interact with welding and coating?

Welding heat spoils the ageing effect in the zone around the weld and lowers strength, so the weld zone of a T6 profile does not have the strength of the parent metal. The powder coating cure temperature is also close to ageing temperatures, so on partly aged tempers such as T4 the cure cycle can act as an additional ageing step. The guide to powder coating aluminium profiles covers the process.

What do the extra digits mean?

The first digit after T gives the main operation, and later digits describe additional treatment. Codes such as T6510 and T6511, for example, indicate that a T6 product has been stress relieved by controlled stretching. They appear mainly in rod, bar, plate and some structural profile specifications. To confirm what a code means, check EN 515 and the relevant product standard.

Which mistakes are common?

• Leaving out the temper. T4 and T6 of the same alloy give very different strength.
• Treating T6 as automatically better. Higher strength usually means lower elongation and harder forming.
• Forgetting later forming. Specifying T6 for a part that will be bent raises the risk of cracking.
• Ignoring the weld zone. Strength near a weld is below the value tabulated for the temper.

How do you state the temper in an order?

Write alloy and temper together, for example “EN AW-6063 T5”. Cite EN 755-2 as the standard, and say so clearly if you want a special condition such as T66. For the process as a whole, see the step-by-step extrusion summary.

Alloy and temper are settled against your drawing and confirmed in writing in the quotation. Send the drawing through the quote request page.