Heat Treatment Methods For Aluminium Alloys

Every aluminium alloy possesses distinct physical properties that make it ideal for various applications, and heat treatment can be used to further refine these properties.

Heat treatment involves controlled heating and cooling of an entire product to alter its microstructure in a precise manner, thereby changing its properties.

While you might be familiar with heat treating steel, where the metal is heated to very high temperatures, aluminium requires different handling due to its lower melting point and higher thermal conductivity.

This article offers a general overview of heat treating aluminium and its alloys to help you determine if heat treatment is beneficial for your project.

Properties Of Heat Treated Aluminium Alloys:

Certain aluminium alloys can undergo heat treatment to enhance their properties beyond the as-manufactured state. Heat treatments can:

– Strengthen the alloy
– Increase ductility
– Improve hardness
– Relieve internal stresses

Which Alloy Series Numbers Can Undergo Heat Treatment?

Many alloys in the following series’ can be heat treated to enhance their strength:

– 2XXX
– 6XXX
– 7XXX
– 8XXX
– 4XXX

The chemical composition of an alloy determines its heat treatability. Elements that are more soluble in aluminium at elevated temperatures can redistribute more effectively, allowing for an even distribution.

You can read more about what alloy serial numbers mean in our previous blog (https://alloyheat.co.uk/what-do-aluminium-alloy-series-numbers-mean/)

4 Types of Heat Treatments for Aluminium Alloys:

1) Annealing

Annealing is used to counteract the effects of cold working (deforming aluminium without heat). Cold working builds up internal stresses, increasing strength and hardness but reducing ductility and formability, a phenomenon known as work hardening.

While work hardening can make a product stronger, further forming processes can lead to cracking if formability is too low. Annealing resets the crystalline structure, relieving internal stresses and improving formability, allowing for easier shaping and greater deformation resistance.

In cast aluminium parts, annealing also helps prevent future cracking by relieving internal stresses. The process involves heating the alloy to a specific temperature, holding it there, and then slowly cooling it.

Annealing is also used to counteract the effects of previous heat treatments.

2) Solution Heat Treatment

Solution heat treatment is similar to annealing but includes quenching the alloy (typically in water) to “freeze” the microstructure before atoms can redistribute. This prepares the aluminium for hardening through ageing and allows further forming processes.

Solution heat treatment maximises the concentration of hardening elements in the solid solution, preventing precipitation until ageing occurs. The ideal temperature for this is just below the alloy’s melting point, requiring precise furnace control.

3) Natural and Artificial Aging

Ageing increases the strength and hardness of a solution heat-treated alloy. During ageing, the alloy’s microstructure locks into place as solid solution elements precipitate.

Natural ageing occurs mostly within 24 hours and reaches full effectiveness in four to five days. Some alloys, however, need additional heating to complete precipitation, known as artificial ageing or precipitation hardening. This involves heating the alloy to a specific temperature for six to 24 hours, resulting in increased strength and hardness but reduced ductility.

4) Homogenising

Homogenizing (aka solution annealing) aims to evenly distribute alloying elements for a uniform product chemistry, especially in castings that solidify at different rates. Without homogenising, the product may have inconsistent properties, being too soft in some areas and too hard and brittle in others.

The process involves heating the part just below its melting point and allowing it to cool slowly. Unlike annealing, which promotes recrystallization through grain boundary movement, homogenising also enables the migration of alloying elements, ensuring uniform chemistry.