When considering using extruded aluminum to solve a design problem, you should be familiar with aluminum alloys and tempers. Aluminum alloys are primarily identified by a serial number – a four-digit code that describes their metallurgical composition. For extruded aluminum, the most common series are 6000 and 7000.
The 6000 (Al-Mg-Si) series is the most popular alloy because it does not work harden quickly. This means that it can be extruded more easily and the aluminum profiles are more cost-effective to manufacture. The 7000 series contains a high amount of zinc, which makes it the strongest alloy and popular in marine, automotive and aerospace applications, but is more extruded.
While the alloys themselves do provide a certain level of strength, this strength alone cannot be the basis for choosing one over another based on the minor minerals or compounds added to the aluminum. Depending on the alloy composition, aluminum can be further strengthened and hardened using quenching (cooling), heat treating and/or cold working techniques. This is called tempering and appears as a hyphenated suffix to the base alloy number (e.g. 6061-T1).
How Tempering Affects Your Product
Each of these techniques allows designers to use a more extrudable, less expensive alloy to achieve the desired mechanical properties. For example, alloy 6063 often works well for decorative purposes, provides an attractive surface finish, and can be used to extrude thin-walled or fine parts. Unheat-treated 6063 has an ultimate strength of no more than 19,000 psi and no specified maximum yield strength. At first glance, its use would appear to be limited due to its low strength. However, when tempered to T6 (6063-T6), its minimum ultimate strength becomes 30,000 psi and its minimum yield strength is 25,000 psi. The increase in strength produced by a combination of solution heat treatment and artificial aging makes this alloy useful in architectural applications, particularly window and door frames.
The temper of each alloy also makes a big difference in its properties and how they react to various manufacturing processes, such as stamping,metal spinning,forming, and welding. Knowing these designations ensures that you don’t destroy critical features. For example, you can increase the corrosion resistance of some alloys at the expense of strength by selecting the appropriate solution heat treatment, the appropriate quench rate, and the aging treatment sequence. The reverse is also true.
Basic Tempering Numbers
The first designation is a letter (F, H, O, T, or W) indicating the general tempering category. The first digit after the letter indicates the basic operation. For example, 6060-T4 indicates the use of 6060 alloy and T4 temper, which means that the extrusion is heat treated (T) after extrusion and then naturally aged (4).
F As-Formed – Applies to products that have been subjected to special control of the heat or strain hardening conditions
Ø Annealed – Applies to products that have been heated to produce a minimum strength condition to improve ductility and dimensional stability
H Strain Hardened – Applies to products that have been strengthened by cold working. Supplementary heat treatment may be performed after strain hardening, which results in a reduction in strength. The “H” is always followed by two or more digits.
W Solution Heat Treated – An unstable temper, applicable only to alloys that spontaneously age at room temperature after solution heat treatment
T Heat Treated – Produces a stable temper, not F, O, or H. Applies to products that have been heat treated (sometimes with additional strain hardening) to produce a stable temper. The “T” is always followed by one or more digits.
In addition to the basic temper designations, there are two subcategories of the categories “H” Temper (strain hardening) and “T” Temper (heat treatment).
Subcategories of H Temper – Strain Hardening
The first digit after the H indicates the basic operation: The second digit after the H indicates the degree of strain hardening:
H1 – Strain hardening only HX2 – Quarter hard
H2 – Strain hardened and partially annealed HX4 – Half hard
H3 – Strain hardened and stabilized HX6 – 3 season hard
H4 – Strain hardened, lacquered or painted HX8 – Full hard
HX9 – Extra hard
Subcategories of T Temper – Heat Treatment
The first digit after the T indicates the basic operation:
T1 – Naturally aged after cooling from a high temperature forming process (e.g. extrusion).
T2 – Cold worked after cooling from a high temperature forming process, then naturally aged.
T3 – Solid solution heat treated, cold worked, and naturally aged.
T4 – Solid solution heat treated and naturally aged.
T5 – Artificially aged after cooling from a high temperature forming process.
T6 – Solution heat treated and artificially aged.
T7 – Solution heat treated and stabilized (overaged).
T8 – Solution heat treated, cold worked, and artificially aged.
T9 – Solution heat treated, artificially aged, and cold worked.
T10 – Cold worked after cooling from the high temperature forming process, then artificially aged.
The other numbers after the T indicate stress relief:
TX51 or TXX51 – Stress relief by stretching.
TX52 or TXX52 – Stress relief by compression.
For Customers Rather than trying to become an expert in aluminum alloys, it’s best to contact your aluminum extrusion manufacturer early in the design process. Discuss the end use of the aluminum component or product and your specific requirements, such as strength, environmental conditions, surface treatments, and manufacturing needs, and then let the extruder’s engineers make recommendations. These experts are waiting to help you make the best product possible, and they are a valuable resource that every designer should take advantage of.
