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How do you turn aluminium, one of the weakest metals, into a metal alloy that can be used structurally?

In the previous frequently asked question article the malleability of aluminum alloys was identified as a key attribute to enable aluminium to be extruded. Whilst the malleability of aluminum is a great attribute to facilitate the extrusion process the downside is that malleability is also associated with softness and low yield strengths, in fact aluminum in its unalloyed form is amongst the weakest of all metals.  
How is it possible to turn one of the weakest metals into a metal that is capable of being used for structural applications? The answer to this question arrived by accident at the turn of the 1900’s. A German engineer, Alfred Wilm, started to experiment with aluminium alloys but found no significant strength advantage straight after the alloys were formed. It was discovered by accident however that a significant increase in strength occurred simply by leaving the metal alloy over a weekend at room temperature. Age hardening had been discovered, although it would be some time before the processes that enabled it to occur was understood. 
The age hardening process, also known as precipitation hardening or particle hardening, is enabled by the addition of metals, such as manganese, copper and silicon that have similar (but critically different) sized atoms to aluminium that can disperse and dissolve throughout the aluminum atoms when the alloy is heated. This process is known as Solution Heat Treatment. When the alloy is subsequently rapidly cooled (Quenched) the atoms are frozen into position. Over time at room temperature the atoms of the added metals start to group together forming precipitates that provide the increased strength. The process can be accelerated and better implemented by artificially heating the aluminum alloy, known as Precipitation Heat Treatment. 


Solution heat treated alloy atoms (red) dispersed and dissolved within aluminium atoms that are aligned, creating low force slip planes.


Natural or artificial age hardening allows the alloy atoms to group into participates (red) that obstruct slip planes, requiring a higher force to move.



A small change in chemical composition to create an alloy, in combination with age hardening unlocks the potential of aluminum and enables it to be soft enough to extrude, yet strong enough to be used structurally. As an example of this transformation pure aluminum has a yield strength of 7 to 11 Mpa. The aluminium alloy 6063 has a minimum aluminium content of 97.35 % and when naturally age hardened (T4) it can achieve a yield strength of 69 Mpa. When artificially age hardened (T6) the yield strength goes up to 172 Mpa. 

Subsequent heat treatment, such as welding or prolonged exposure to high temperatures can impact the strength of aluminium alloys as it can disrupt the participates, however this can be mitigated by the extent and inclusion of different metals into the alloy. The extent and use of different metals have enabled many different grades of highly engineered aluminum alloys to be developed that serve different purposes dependent on the intended application. As an example, the inclusion of lead and bismuth provides an aluminium alloy that is more suitable for machining. Due to the extent and wide variety of engineered aluminum alloys that are available they are grouped into a set of series: 

1xxx series – 99% aluminium content (pure) that offers high thermal and electrical conductivity and corrosion resistance.

2xxx series - The main alloying element is copper (4-5%) offering excellent strength over a broad range of temperatures.

3xxx series - The main alloying element is manganese (0.9-1.5%) offering offer good corrosion resistance and moderate strength.

5xxx series - The main alloying element is magnesium (1.7-4.9%) offering good fatigue strength and very good resistance to corrosion, especially in marine environments.

6xxx series – The main alloying elements are manganese and silicon.  Ideal for extruding with medium strength.

7xxx series – The main alloying element is zinc (5.7 to 8.3%) offing the highest strength and stress corrosion resistance.

Typical applications for three of the commonly used grades in the 6xxx series are provided below.


Key advantage

Trade off

Typical Applications

Maximum weight % of metal content other than aluminium


Ease of extrudability

Lower strength

Window Frames & Doors, Curtain Walling Louvres, Brise Soleil, Fins, Balcony Decking, Balustrading, Bespoke Light Fittings, Heatsinks and Housings, Display Equipment, Partitions, Solar Panels, Shop Fittings and Signage

Manganese (0.1), Iron (0.35), Magnesium (0.9), Silicon (0.6), Zinc (0.1), Titanium (0.1), Copper (0.1), Chromium (0.1)


Structural strength

Lower quality surface finish

Roof trusses, Scaffolding, Bridges, Cranes, Aluminium Beams

Manganese (1.0), Iron (0.5), Magnesium (1.2), Silicon (1.3), Zinc (0.2), Titanium (0.1), Copper (0.1), Chromium (0.25)

6026 (Replacement for 6262)


Harder to weld and anodise

Valve Screws, Marine Fittings, Nuts, Hinges and Couplings, Oil Line Fittings, Decorative Hardware, Appliance Fittings

Manganese (1.0), Iron (0.7), Magnesium (1.2), Silicon (1.4), Zinc (0.3), Titanium (0.2), Chromium (0.3), Copper (0.5), Bismuth (1.5), Lead (0.4), Tin (0.05)

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