Aluminium–copper alloys

Aluminium–copper alloys are aluminium alloys that consist largely of aluminium and traces of copper as the main alloying elements. Important grades also contain additives of magnesium, iron, nickel and silicon, often manganese is also included to increase strength. The main area of application is aircraft construction. The alloys have medium to high strength and can be age hardened. They are both. Also available as cast alloy. Their susceptibility to corrosion and their poor weldability are disadvantageous.
Duralumin is the oldest variety in this group and goes back to Alfred Wilm, who discovered it in 1903. Aluminium could only be used as a widespread construction material thanks to the aluminium–copper alloys, as pure aluminium is much too soft for this and other hardenable alloys such as aluminium–magnesium–silicon alloys or the naturally hard alloys.
Aluminium–copper alloys were standardised in the 2000 series by the international alloy designation system which was originally created in 1970 by The Aluminum Association. The 2000 series includes 2014 and 2024 alloys used in airframe fabrication.
Copper alloys with aluminium as the main alloying metal are known as aluminium bronze, the amount of aluminium is generally less than 12%.

History

is a trade name for one of the earliest types of age-hardenable aluminium alloys. The term is a combination of Dürener and aluminium. Its use as a trade name is obsolete. Duralumin was developed by the German metallurgist Alfred Wilm at Dürener Metallwerke AG. In 1903, Wilm discovered that after quenching, an aluminium alloy containing 4% copper would harden when left at room temperature for several days. Further improvements led to the introduction of duralumin in 1909. The name is mainly used in pop-science to describe all Al–Cu alloys system.
Aluminium–copper alloys were standardised in the 2000 series by the international alloy designation system which was originally created in 1970 by the Aluminum Association. 2000s series includes 2014 and 2024 alloys used in airframe fabrication.

Pure AlCu wrought alloys

All AlCu alloys are based on the system of pure AlCu alloys.

Solubility of copper and phases

Aluminium forms a eutectic with copper at 547 °C and 33 mass percent copper, which also corresponds to the maximum solubility. At lower temperatures, the solubility drops sharply; at room temperature it is only 0.1%.
At higher copper contents, Al₂Cu is formed, an intermetallic phase. It is present in a tetragonal structure, which is so different from the cubic crystal system of aluminium that the -phase exists only as an. There are also the partially coherent ones - and -phases.

Microstructural transformations

After casting, the material is usually oversaturated – mixed crystal, which also contains more copper at room temperature than could actually be dissolved at this temperature.

After that, GP zones form at temperatures below 80 °C, in which increased concentrations of copper are present, but which do not yet have a structure or form their own phases.
At somewhat higher temperatures of up to 250 °C, this forms -phase, which increases strength.
At even higher temperatures, the partially coherent forms -Phase.
At still higher temperatures of about 300 °C the incoherent one forms -phase in which the strength decreases again.

The individual temperature ranges overlap: Even at low temperatures, there is formation of - or phases, but these form much more slowly than the GP zones. Each of the phases forms faster the higher the temperature.

GP(I) zones

The formation of GP zones is referred to as natural hardening and occurs at temperatures up to 80 °C. They are tiny disc-shaped layers just one atom thick and 2 to 5 nanometers in diameter. With time, the number of zones increases and the copper concentration in them increases, but not their diameter. They are coherent with the aluminum lattice and form on the planes.

GP(II) zones

The GP zones are largely responsible for the increase in strength of the AlCu alloys. They are coherent with the aluminium crystal and consist of alternating layers of aluminium and copper with layer thicknesses of about 10 nanometers and dimensions of up to 150 nanometers. In contrast to the GP zones, these are three-dimensional precipitations. Their layers are parallel to the aluminium plane. From the -phase forms the -phases, but there are overlaps.
The GP zones need vacancies for growth, which is why a lack of these leads to delayed growth.

Partially coherent phases

The -phase is only partially coherent with the aluminium lattice and forms at temperatures from 150 °C to 300 °C. It has the form of platelets and can arise from the GP zones. However, it can also arise directly as a precipitation from the mixed crystal. In the first case, the increasing surface tension is reduced by dislocations, in the second case, the precipitates form preferentially at dislocations.

Incoherent phases

The -phase is incoherent with the lattice of the mixed crystal. It forms at temperatures of 300 °C and more. It usually forms larger particles with a larger spacing than the other phases and thus does not lead to any increase in strength or even to a drop if its formation takes place at the expense of the other phases. The -phase also occurs at temperatures between 150 °C and 250 °C as precipitation at grain boundaries, as this reduces the surface tension.
The -phase leads to a partial intergranular fracture; however, the fracture behavior remains ductile overall. The change in fracture behavior is caused by precipitation-free zones at the grain boundaries.
The -phase has a greater potential difference compared to the mixed crystal, so that layer corrosion and intergranular corrosion can occur. With longer annealing times, the inside of the grains also separate the -phases, the potential difference is additionally lower.

Grades, alloying elements and contents

As with almost all aluminium alloys, a distinction is made between for rolling and forging and for casting.
The copper content is usually between 3 and 6%. Between 0.3% and 6% the alloys are regarded as not weldable or very difficult to weld, with higher copper contents they become weldable again. Most types also contain additives of magnesium, manganese and silicon to increase strength. Lead and bismuth form small inclusions that melt at low temperatures, resulting in better chip formation, similar to free machining steel. The heat resistance is increased by adding nickel and iron.
Iron, is found as an impurity in engineering alloys, preventing strain hardening, but adding magnesium makes the aforementioned process possible again. Larger amounts of magnesium up to 1.5% increase strength and elongation at break. Manganese is also used to increase strength. Larger amounts, however, have negative side effects, so the content is limited to around 1% manganese. Smaller additions of silicon are added to bind iron, since it prefers to form the AlFeSi phase, while the formation of Al₇Cu₂Fe would remove larger amounts of copper from the material, which then no longer leads to the formation of phases that are actually desired. Larger amounts of silicon are alloyed to form with magnesium Mg₂Si which, like aluminium-magnesium-silicon alloy, improves strength and hardenability.
Lithium is added to some alloys with contents between 1.5% and 2.5%. Due to the very low density of Li, this leads to lighter components, which is particularly advantageous in aviation. See aluminium-lithium alloy for details.

Cast alloys

Cast alloys contain about 4% copper and small amounts of other additives that improve castability, including titanium and magnesium. The starting material is primary aluminium; in contrast to other cast aluminium alloys, secondary aluminium is not used because it reduces elongation and toughness at break. The AlCu cast alloys are prone to hot cracking and are used in the T4 and T6 hardening states.
The following table shows the composition of some grades according to DIN EN 1706. All data is shown in percent by mass, the rest of the materials is aluminium.

number	Chemical	silicon	iron	copper	manganese	magnesium	zinc	titanium
21000	AlCu4TiMg	0.2	0.4	4.2–5.0	0.10	0.15–0.35	0.1	0.15–0.30
21100	AlCu4Ti	0.18	0.19	4.2–5.2	0.55	–	0.07	0.15–0.30

Wrought alloys

AlCuMg(Si,Mn) wrought alloys

The AlCuMg alloys represent the most important group of AlCu alloys. Many other phases can form in them:

Al₈ Mg₅
Al₂ CuMg, the S phase
Al₆ Mg₄ Cu, the T phase

The addition of magnesium accelerates the process of cold hardening. Which phases are formed depends primarily on the ratio of copper to magnesium. If the ratio is less than 1/1, clusters containing Cu and Mg are eliminated. At a ratio above 1.5/1, which is the case with most engineering alloys, the forms preferentially phase. These kinds of alloys have significantly higher hardness and strength.

Mechanical properties

Conditions:

O soft
T3: solution annealed, quenched, work hardened and naturally aged
T4: solution annealed, quenched and artificially aged
T6: solution heat treated, quenched and artificially aged
T8: solution annealed, cold worked and artificially aged

Numeric	Chemical	Condition	Elastic modulus /MPa	Shear modulus /MPa	Yield strength /MPa	Tensile strength /MPa	Elongation at break /%
EN AW-2007	AlCu4PbMgMn	T3 T8	72,500	27,300	300 310	380 405	16 14
EN AW-2011	AlCu6BiPb	T3 T4 T6 T8	72,500	27,300	290 270 300 315	365 350 395 420	15 18 12 13
EN AW-2014	AlCu4Mg	0 T4 T6	73,000	27,400	85 275 425	190 430 485	20 18 12
EN AW-2017A	AlCu4MgSi	0 T4	72,500	27,200	70 275	180 425	20 21
EN AW-2024	AlCu4Mg1	0 T8	73,000	27,400	75 450	185 485	20 nb