Tempering Designations for Aluminum

Aluminum is naturally a soft, low-density metal – too soft to be used for any structural applications in its pure form. Yet we see aluminum used regularly in high stress applications: construction, aircraft, even space shuttles. So how does such a ductile metal become so strong?
The first step is creating an aluminum alloy. Adding alloying elements such as copper, silicon and magnesium modify the mechanical properties of aluminum to produce a stronger metal. Further processing can then be done to add even more strength and durability to aluminum. One of the most popular methods of doing this is through tempering.

What is Tempering?
With controlled application of heat, metal is raised to nearly its critical temperature – a temperature just short of melting, high enough to alter a metal’s mechanical properties and relieve its internal stresses. The heat treatment is used to increase the hardness and ductility, while it decreases brittleness to prevent cracking and breakage. There are several different methods to produce the desired result, but the overall process is known as tempering.

What is an Aluminum Tempering Designation?
Once the tempering process is complete, the metal’s hardness is referred to as its temper. Its tempering designation identifies exactly what kind of heat treatment the metal has undergone. This is helpful in identifying the strength of a particular type of aluminum, and mechanical applications it is best suited for. Two pieces of aluminum alloy may have the same chemical composition, but if they have different tempering designations, their uses can be quite different.

What are the Tempering Designations?
The tempering designation is a two-character code attached after the aluminum alloy designation. The first character is a letter which identifies the type of tempering treatment used on the aluminum:

F: As fabricated. These are known as ‘semi-finished’ products, and often used in creating other finished tempers.
H: Strain hardened, used for wrought aluminum types which are non-heat treatable.
O: Annealed, which results in the lowest strength of tempered aluminum that has greater workability.
T: Thermally treated. These types of aluminum are heat-treated, quenched, and aged.

The second character of the tempering designation is a digit from 1-10, which helps to specify exactly how the tempering method was done. For instance, all T-series aluminum are thermally treated, but a naturally aged aluminum will have a different digit assignment than an artificially aged one. This level of detail helps to easily classify the type of aluminum and how it might best be used.

The T-series tempering designation is the most commonly used for aluminum, which can serve as an example showing the different tempering methods:

T1: Naturally aged after cooling from high temperatures during the forming process
T2: Cooled after a high-temperature forming process, cold worked, then naturally aged
T3: Solution heat-treated, cold worked, then naturally aged
T4: Naturally aged after a solution heat treatment
T5: Artificially aged after cooling from high temperatures during the forming process
T6: Artificially aged after a solution heat treatment
T7: Solution heat-treated, then overaged
T8: Solution heat-treated, cold worked, then artificially aged
T9: Solution heat-treated, artificially aged, then cold worked
T10: Cooled after a high-temperature forming process, cold worked, then naturally aged

Once familiar with these designations, it’s possible to quickly identify an aluminum alloy’s composition, tempering, and processing. Rather than looking up every type and grade, the tempering designation helps a buyer narrow their search for the strongest and hardest aluminum they need for the intended application.