Dr. Bruce Banner (a.k.a. The Hulk,) had a bit of a temper, which is probably why he thought people wouldn’t like him when he was angry. Curiously though, no one thought of grading Banner’s temper. Extruded aluminum alloys on the other hand do come with a temper grading, and much like the temper of our green friend, they give an indication of behavior.
Aluminum Alloys Are Not All the Same
If you’re specifying, buying or using an extruded section understand that it comes in a range of types. Aluminum alloys are identified primarily by a series number. This is a four digit code describing it’s metallurgical composition. The most common series for extruded aluminum are the 1000’s, 6000’s and 7000’s, the last two of which can be heat-treated to improve strength and hardness.
A 1000 series alloy is nearly pure aluminum and quite soft. It extrudes readily but isn’t suitable for structural applications. Thanks to excellent electrical and thermal conductivity, aluminum alloys from this series are often used for power transmission lines.
The 6000 series material is alloyed principally with magnesium and silicon, making it stronger but still allowing it to be extruded. These alloys are weldable and formable but their most useful attribute is excellent corrosion resistance, leading to them being dubbed “marine grade.”
The 7000 series has a significant proportion of zinc and is the strongest of all yet still available in extruded form. These are generally considered as aerospace grade aluminum alloys.
The other aluminum alloy series, except the 4000’s, which is used mainly in welding wire, can be extruded but it tends to take higher forces and appearance and dimensional consistency aren’t as good. In addition, they aren’t suitable for heat treatment. The only way of increasing their hardness and strength is through cold working.
Tempering Aluminum Alloys
Processing aluminum to improve mechanical properties is referred to as “tempering.” A letter and number after the series designation indicates the type of processing an alloy has undergone. So for example, you’ll see 6061 T6 as a descriptor of a widely used alloy.
An alloy will go through one of five types of tempering. The letters indicating these are:
- F – As-fabricated, meaning no attempt was made at tempering
- O – Annealed, recrystallized, only applied to wrought products
- H – Strain or work-hardened, used on the series that can’t be heat-treated
- W – Solution heat-treated
- T – Thermally treated, a solution heat-treatment followed by aging
Solution Heat-Treatment Explained
At the temperature needed to extrude 6000 and 7000 series alloys – around 980°F – the magnesium and silicon dissolves completely in the aluminum. If the extruded section is left to air cool they’ll subsequently precipitate out and won’t contribute to strength or hardness. To avoid this, most aluminum alloys are water-quenched as they leave the extrusion die. This heating and rapid cooling is what’s called solution heat-treatment.
After quenching, in a process called “aging” the alloying elements form new crystalline structures within the aluminum extrusion that increase strength and hardness. Natural aging takes days or weeks but is accelerated by heating.
This should explain the difference between W and T tempers. T means the aging was controlled, unlike W. This is why W is rarely seen while T – temper aluminum is very common: it’s a more consistent and predictable material.
Textbooks list ten main types of thermal temper, some of which get divided into subcategories. However, when looking at aluminum extrusion the four you’re most like to encounter are:
- T3 – Solution heated-treated, followed by cold working to increase strength
- T4 – Solution heated-treated then aged naturally
- T5 – Cooled rather than quenched, followed by oven aging
- T6 – Solution heated-treated and oven-aged
Of these, for 6000 and 7000 series extruded aluminum the most common tempers are T4 and T6.
Impact of Tempering
In general terms, tempering increases strength and hardness and reduces ductility. There isn’t room here to get into the grade-by-grade details so a couple of examples will have to suffice.
- 1060 O temper – yield strength of 4 ksi (kilo-pounds per square inch) and Brinell hardness of 19
- 6061 T4 alloy – yield strength of 21 ksi and Brinell hardness of 65
- 6061 T6 alloy – yield strength of 40 ksi and Brinell hardness of 95
A good source for those wanting specifics on every series is “Understanding the Aluminum Temper Designation System” on the NIST website.
Aluminum Extrusion Tolerances
Extruded aluminum alloys always suffer a degree of bow and twist as they leave the die and cool. This is related to the exit temperature, which in turn strongly influences extrusion speed. Basically, lower temperature aluminum is extruded more slowly but has superior surface finish and dimensional stability.
There are industry standards for the maximum allowable distortion which we’ll be happy to provide. For a more general rule of thumb, you can assume the following worst-case scenarios:
Twist – 0.5 degrees per foot.
Straightness – 0.125” per foot.
You may also see some variation in the angles of extruded features, but this should not exceed +/-2 degrees.
The actual dimensions of extruded section should be very uniform. It varies slightly depending on the alloy and size of section, but a good upper limit is +/- 1% of the dimension. Wall thickness can vary by more though, as much as +/- 10%.
Tighter tolerances can be achieved if needed, although this may require a special order.
Match Alloy and Temper to the Application
If you’re thinking of using aluminum extrusion in your next fabrication design – and hopefully we’ve convinced you that it’s something to consider – remember that it’s not all the same.
If the the aluminum extrusion is for decoration or to be an electrical or thermal conductor a 1000 series alloy should work. For structural or load-bearing applications you should probably plan on using extruded aluminum alloys from the 6000 series. Even then, there are significant differences between individual alloys. Temper is something to think about as well, especially if you need higher strength.
If you need advice, we’re happy to help. Just don’t get angry, we wouldn’t like that!