Aluminium–copper alloys (AlCu) are aluminium alloys that consist largely of aluminium (Al) and a small percentage of copper (Cu) as the main alloying elements. Important variants also contain additives of magnesium, iron, nickel and/or silicon (AlCu(Mg, Fe, Ni, Si)). Often manganese is also included to increase strength (see aluminium–manganese alloys). The main area of application is aircraft construction. The alloys have medium to high strength and can be age hardened. There are both and cast alloys. The susceptibility of these alloys to corrosion and their poor weldability are disadvantages.
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 (AlMgSi) or the naturally hard (non-hardenable) alloys.
Aluminium–copper alloys were standardised in the 2000 series by the international alloy designation system (IADS) 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
Duralumin 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 Its use as a trade name is obsolete.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.J. Dwight. Aluminium Design and Construction. Routledge, 1999. 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 (IADS) 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.Friedrich Ostermann: Anwendungstechnologie Aluminium. 3. Auflage, Springer, 2014, S. 119.
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 (GP(I) 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 (also called GP(II) zones), 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(I/II) zones. Each of the phases forms faster the higher the temperature.Friedrich Ostermann: Anwendungstechnologie Aluminium. 3. Auflage, Springer, 2014, S. 119 f.George E. Totten, D. Scott MacKenzie: Handbook of Aluminum – Band 1: Physical Metallurgy and Processes. Marcel Dekker, New York/Basel 2003, S. 140 f.
GP(I) zones The formation of GP(I) 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 {100} planes.Friedrich Ostermann: Anwendungstechnologie Aluminium. 3. Auflage, Springer, 2014, S. 120.George E. Totten, D. Scott MacKenzie: Handbook of Aluminum – Band 1: Physical Metallurgy and Processes. Marcel Dekker, New York/Basel 2003, S. 141.
GP(II) zones The GP(II) zones (-phases) 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(I) zones, these are three-dimensional precipitations. Their layers are parallel to the aluminium {100} plane. From the -phase forms the -phases, but there are overlaps.
The GP(II) zones need vacancies for growth, which is why a lack of these (e.g. due to magnesium) leads to delayed growth.George E. Totten, D. Scott MacKenzie: Handbook of Aluminum – Band 1: Physical Metallurgy and Processes. Marcel Dekker, New York/Basel 2003, S. 141–143.
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(II) 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.Friedrich Ostermann: Anwendungstechnologie Aluminium. 3. Auflage, Springer, 2014, S. 120 f.George E. Totten, D. Scott MacKenzie: Handbook of Aluminum – Band 1: Physical Metallurgy and Processes. Marcel Dekker, New York/Basel 2003, S. 143.
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.Friedrich Ostermann: Anwendungstechnologie Aluminium. 3. Auflage, Springer, 2014, S. 121.
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 (by fusion welding), 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.Friedrich Ostermann: Anwendungstechnologie Aluminium. 3. Auflage, Springer, 2014, S. 117 f.
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 (see Aluminium-magnesium alloy). Manganese is also used to increase strength (see AlMn). 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 (especially Al2Cu, copper aluminideAluminium-Taschenbuch – Band 1. 16. Auflage, Aluminium-Verlag, Düsseldorf 2002, S. 439.). Larger amounts of silicon are alloyed to form with magnesium Mg2Si (magnesium silicide) which, like aluminium-magnesium-silicon alloy, improves strength and hardenability.Aluminium-Taschenbuch – Band 1. 16. Auflage, Aluminium-Verlag, Düsseldorf 2002, S. 140 f.
Lithium is added to some alloys with contents between 1.5% and 2.5%. Due to the very low density of Li (0.53 g/cm3 compared to 2.7 g/cm3 of aluminium), 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 (made from scrap) 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.Friedrich Ostermann: Anwendungstechnologie Aluminium. 3. Auflage, Springer, 2014, S. 185.
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.Friedrich Ostermann: Anwendungstechnologie Aluminium. 3. Auflage, Springer, 2014, Anhang. numberChemical (CEN designation)siliconironcoppermanganesemagnesiumzinctitanium21000AlCu4TiMg0.20.44.2–5.00.100.15–0.350.10.15–0.3021100AlCu4Ti0.180.194.2–5.20.55–0.070.15–0.30
Wrought alloys numberChemical (CENdesignation)siliconironcoppermanganesemagnesiumchromezinctitaniumOtherEN AW-2007AlCu4PbMgMn0.80.83.3–4.60.50–1.00.4–1.80.100.80.200.20 Bi 0.8–1.5 Pb 0.2 Sn 0.2 NiEN AW-2011AlCu6BiPb0.400.75.0–6.0–––0.30–0.20–0.6 Bi 0.2–0.6 PbEN AW-2014 (EN AW-2014A)AlCu4SiMg AlCu4MgSi(A)0.5–1.2 (0.5–0.9)0.7 (0.5)3.9–5.00.40–1.20.20–0.80.100.250.150.2 Zr+Ti (0.2 (Zr+Ti), 0.10 Ni)EN AW-2017AlCu4MgSi(A)0.2–0.80.73.5–4.50.4–1.00.4–1.00.100.25–0.25Zr+TiEN AW-2024AlCu4Mg10.500.53.8–4.90.30–0.91.2–1.80.100.250.150.2Zr+TiAA 2026AlCu4Mg1Zr0.050.073.6–4.30.30–0.81.0–1.6–0.100.060.05–0.25 zr
AlCuMg(Si,Mn) wrought alloys The AlCuMg alloys represent the most important group of AlCu alloys. Many other phases can form in them:George E. Totten, D. Scott MacKenzie: Handbook of Aluminum – Band 1: Physical Metallurgy and Processes. Marcel Dekker, New York/Basel 2003, S. 146–149.Aluminium-Taschenbuch – Band 1. 16. Auflage, Aluminium-Verlag, Düsseldorf 2002, S. 114 f.
Al8 Mg5 (-phase, see ) 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 (, also hot-formed with the same strength limit values) T3: solution annealed, quenched, work hardened and naturally aged T4: solution annealed, quenched and artificially aged