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Wrought Aluminium Alloys

Leonardo da Vinci, Helsinki Award 2006, Design and execution: Gerold Fink, Austria. Click to open PDF document about this award

Leonardo Da Vinci Helsinki Award 2006

Leonardo da Vinci, Helsinki Award 2006, Design and execution: Gerold Fink, Austria. Click to open PDF document about this award

Bronze medal for an outstanding project promoting and supporting the LifeLong Learning EU policy. Award Berlin 2007

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2xxx Series Alloys

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  Major elements
  Minor elements

2xxx series are high strength alloys.

The addition of copper as main alloying element (mostly range 3–6 wt%, but can be much higher), with or without magesium as alloying consituent (range 0–2 %), allows material strengthening by precipitation hardening, resulting in very strong alloys. Also the fatigue properties are very good for this series. The presence of copper is however very bad for the corrosion resistance. Copper tends to precipitate at grain boundaries, making the metal very susceptible to pitting, intergranular corrosion and stress corrosion. These copper rich zones are more noble/cathodic than the surrounding aluminium matrix and act as preferred sites for corrosion through galvanic coupling. (learn more in the section on corrosion) Copper is also very bad for anodising. Copper precipitates dissolve in the anodising electrolytes (acid electrolytes for porous film formation) leaving holes in the oxide, and solute copper migrates under the high electric field towards the aluminium/oxide interface compromising the anodic film properties.

Up to 12 wt% copper the strength of the alloy can increase through precipitation hardening, with or without the presence of Mg; Hardening is achieved through the precipitation of Al2Cu or Al2CuMg intermetallic phases during ageing which leads to strengths second only to the highest strength 7xxx series alloys. Above 12 wt% Cu the alloy becomes brittle. Copper also improves the fatigue properties, the high-temperature properties and the machinability of the alloy. Lower copper content levels then in the conventional 2024 and 2014 type alloys are required for the automotive industry. These alloys have sufficient formability, spot weldability and good corrosion resistance (as opposed to the higher copper containing alloys). The paint baking cycle in the automotive sheet application provides the precipitation treatment and imparts the final mechanical values.

The 2xxx series alloys are used for high strength structural applications such as aircraft fittings and wheels, military vehicles and bridges, forgings for trucks, etc. The low melting phase elements, lead and/or bismuth, facilitate machining of the 2xxx series alloys, making them also suitable for applications where hard extruded and machined parts are required (screws, bolts, fittings, machinery components, etc).

Vanadium, Zirconium and Titanium raise the recrystallisation temperature of copper containing alloys to retain their properties at elevated temperatures, fabricate readily and have good casting and welding behaviour.

Manganese has a substantial effect on the tensile properties of aluminium-copper-magnesium alloys. Tensile strength and yield strength increase with increases of manganese and magnesium levels. Manganese causes a loss in ductility, hence its content should be maximum 1 %. Mn also raises the recrystallisation temperature of copper containing alloys to retain their properties at elevated temperatures, fabricate readily and have good casting and welding behaviour.

Iron in aluminium-copper-magnesium alloy Al–4%Cu–0.5%Mg, even at very low levels of 0.5 % reduces the tensile properties in the heat-treated condition if there is an excess of iron that is not tied up by silicon in alpha-Fe-Si precipitates. This iron excess then forms Cu2FeAl7 constituents thereby reducing the amount of available copper for heat-treating effects.

Iron is added to aluminium-copper-nickel alloys to increase strength at elevated temperatures. The properties are due to the fine grain size.

Nickel improves strength and hardness in aluminium-copper-magnesium alloys at elevated temperatures although additions of about 0.5 % nickel lowers the tensile properties of heat-treated Al–4%Cu–0.5%Mg alloy at room temperature.
Nickel also reduces the coefficient of expansion.

Silver at trace level substantially increases the strength of heat-treated and aged aluminium-copper-magnesium alloys.

Cadmium is a relatively low-melting element. Up to 0.3 % Cd may be added to aluminium-copper alloys to accelerate the rate of age hardening, increase strength and corrosion resistance.

Indium is added in small amounts (0.05–0.2 %) to aluminium-copper alloys, particularly those of low copper content (2–3 % Cu). Indium reduces room temperature ageing and increases the effect of artificial ageing. When magnesium is present in the alloy, these effects are counteracted.

Small amounts of tin (0.05 %) greatly increase the artificial ageing response of aluminium-copper alloys following solution heat treatment. Higher strength results and also an improvement in corrosion resistance. This effect is counteracted by the presence of magnesium that probably forms a non-coherent second phase with tin. Higher concentrations of tin however cause hot cracking of aluminium-copper alloys. Tin is also used in alloys for bearing applications requiring a high resistance to high speeds, loads and temperatures. Copper, nickel and silicon together with tin improve the load-carrying capacity and wear resistance, and the soft tin phase provides antiscoring properties.

Lead is added to improve machinability of alloys such as in EN–AW 2011. Lead is like bismuth, tin and cadmium a low melting-point metal that with its restricted solubility in aluminium forms a soft, low-melting phase that promotes chip breaking and helps tool lubrication.

At present lead is however being restricted by law in automotive and electronic applications to a maximum of 0.4 % due to its toxicity. For example the EN–AW 6262 free-machining alloy with a 1–to–1 ratio of lead and bismuth needs to be replaced for these applications by a lower lead containing alloy. More bismuth or tin are being added to this alternative alloy to compensate for the lower lead content.

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