Aluminum-copper alloy

Aluminum-copper alloys ( AlCu ) are aluminum alloys - alloys that largely consist of aluminum (Al) - which contain copper (Cu) as the main alloying element . Important types still contain additives of magnesium and silicon (AlCu (Mg, Si)), and manganese is often also included to increase strength (see AlMn ). The main area of ​​application is aircraft construction . The alloys have medium to high strengths and are age- hardenable . They are available both as wrought and cast alloys . Disadvantages are their susceptibility to corrosion and their poor weldability . They are standardized in the 2000 series. Duralumin is the oldest type of this group and goes back to Alfred Wilm who discovered it in 1903. It was only through the use of aluminum-copper alloys that aluminum could be used as a widespread construction material, since pure aluminum is far too soft for this and other hardenable alloys such as aluminum-magnesium-silicon alloys (AlMgSi) or the naturally hard (non-hardenable) alloys are still were not known.

Types, alloy elements and contents

As with almost all aluminum alloys, a distinction is made between wrought alloys , for rolling and forging, and cast alloys for casting .

The copper content is usually between 3 and 6%. Between 0.3% and 6% they are considered to be impossible or very difficult to weld (by fusion welding ), with higher Cu contents they are weldable. Most types still contain magnesium , manganese and silicon additives to increase strength. Lead and bismuth form small inclusions that melt at low temperatures and thus lead to better chip formation , similar to free-cutting steel . The heat resistance is increased by adding nickel and iron.

Iron, which is contained in technical alloys as an impurity, prevents cold hardening . It becomes possible again by adding magnesium. Larger amounts of magnesium up to 1.5% increase the strength and elongation at break (see AlMg ). Manganese is also used to increase strength (see AlMn ). However, larger amounts have negative side effects, so that the content is limited to about 1% Mn. Smaller additions of silicon are added to bind iron because it preferentially forms the AlFeSi phase, while the formation of Al ₇ Cu ₂ Fe would remove larger amounts of copper from the material, which would then no longer lead to the formation of actually desired phases (especially Al ₂ Cu, copper aluminide ) are present. Larger amounts of silicon are added in order to form Mg ₂ Si ( magnesium silicide ) with magnesium , which, like AlMgSi, improves strength and hardenability.

Some alloys still contain lithium with contents between 1.5% and 2.5%. Because of the very low density of Li (0.53 g / cm³ compared to 2.7 g / cm³ of aluminum), this leads to lighter components, which is particularly advantageous in aviation. For details see aluminum-lithium alloy .

Casting alloys

Casting alloys contain around 4% copper and other additives in small amounts that improve castability , including titanium and magnesium . The starting material is primary aluminum ; Secondary aluminum (made from scrap), unlike other aluminum casting alloys, is not used because it reduces elongation at break and toughness. The AlCu casting alloys tend to hot cracks and are used in the hardening states T4 and T6.

The following table shows the composition of some grades according to DIN EN 1706. All data in percent by mass , the rest is aluminum.

number	Chemical ( CEN designation)	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	0.2	4.2-5.2	0.55	-	0.07	0.15-0.30

Wrought alloys

number	Chemical ( CEN designation)	Silicon	iron	copper	manganese	magnesium	chrome	zinc	titanium	Others
EN AW-2007	AlCu4PbMgMn	0.8	0.8	3.3-4.6	0.50-1.0	0.4-1.8	0.10	0.8	0.20	0.20 Bi 0.8-1.5 Pb 0.2 Sn 0.2 Ni
EN AW-2011	AlCu6BiPb	0.40	0.7	5.0-6.0	-	-	-	0.30	-	0.20-0.6 Bi 0.2-0.6 Pb
EN AW-2014 (EN AW-2014A)	AlCu4SiMg AlCu4MgSi (A)	0.5-1.2 (0.5-0.9)	0.7 (0.5)	3.9-5.0	0.40-1.2	0.20-0.8	0.10	0.25	0.15	0.2 Zr + Ti (0.2 (Zr + Ti), 0.10 Ni)
EN AW-2017	AlCu4MgSi (A)	0.2-0.8	0.7	3-5-4.5	0.4-1.0	0.4-1.0	0.10	0.25	-	0.25 Zr + Ti
EN AW-2024	AlCu4Mg1	0.50	0.5	3.8-4.9	0.30-0.9	1.2-1.8	0.10	0.25	0.15	0.2 Zr + Ti
AA 2026	AlCu4Mg1Zr	0.05	0.07	3.6-4.3	0.30-0.8	1.0-1.6	-	0.10	0.06	0.05-0.25 Zr

Mechanical properties

Conditions:

• O soft ( soft annealed , also hot-formed with the same strength limit values).
• T3: solution annealed, quenched, strain hardened and artificially aged
• T4: solution annealed, quenched and artificially aged
• T6: solution annealed, quenched and artificially aged
• T8: solution annealed, strain hardened and artificially aged

Numerically	Chemical (CEN)	Status	Modulus of elasticity / MPa	G module / MPa	Yield strength / MPa	Tensile strength / MPa	Elongation at break /%
EN AW-2007	AlCu4PbMgMn	T3 T8	72500	27300	300 310	380 405	16 14th
EN AW-2011	AlCu6BiPb	T3 T4 T6 T8	72500	27300	290 270 300 315	365 350 395 420	15th 18th 12 13
EN AW-2014	AlCu4Mg	0 T4 T6	73000	27400	85 275 425	190 430 485	20th 18th 12
EN AW-2017A	AlCu4MgSi (A)	0 T4	72500	27200	70 275	180 425	20th 21st
EN AW-2024	AlCu4Mg1	0 T8	73000	27400	75 450	185 485	20th nb

Applications

Aluminum-copper alloys are mainly used in aircraft construction, where their low corrosion resistance plays a subordinate role. The alloys are processed by rolling , forging , extrusion and sometimes by casting .

Pure AlCu wrought alloys

Excerpt from the phase diagram relevant for technically used alloys

Complete phase diagram

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

Solubility of copper and phases

Aluminum forms a eutectic with copper at 547 ° C and 33 percent by mass 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, Al ₂ Cu the phase, an intermetallic phase, is formed . It is in a tetragonal structure that is so different from the face-centered cubic of aluminum that the phase only exists as an incoherent phase. There are also the partially coherent and phases. ${\displaystyle \theta }$ ${\displaystyle \theta }$ ${\displaystyle \theta ''}$ ${\displaystyle \theta '}$

Structural transformations

After casting, the material is usually a supersaturated - solid solution before which even at room temperature longer contains copper could be considered actually solved at this temperature. ${\displaystyle \alpha }$

• Thereafter, at temperatures below 80 ° C, GP zones (GP (I) zones) are formed in which increased concentrations of copper are present, but which do not yet have a structure or form their own phases.
• At slightly higher temperatures of up to 250 ° C, the phase (also called GP (II) zones) forms, which increases strength. ${\displaystyle \theta ''}$
• At even higher temperatures, the partially coherent phase is formed ${\displaystyle \theta '}$
• and at higher temperatures of around 300 ° C, the incoherent phase forms in which the strength drops again. ${\displaystyle \theta }$

The individual temperature ranges overlap: Even at low temperatures, - or - phases form, but these form much more slowly than the GP (I / II) zones. Each of the phases forms faster the higher the temperature. ${\displaystyle \theta '}$ ${\displaystyle \theta }$

GP (I) zones

The formation of GP (I) zones is called cold hardening and occurs at temperatures up to 80 ° C. They are tiny, disk-shaped layers just one atom thick and 2 to 5 nanometers in diameter. Over time, the number of zones and the copper concentration in them increase, but not their diameter. They are coherent with the lattice of aluminum and form on the {100} planes.

GP (II) zones

The GP (II) zones ( phases) are largely responsible for increasing the strength of the AlCu alloys. They are coherent with the aluminum crystal and consist of alternating layers of aluminum and copper with layer thicknesses of around 10 nanometers and dimensions of up to 150 nanometers. In contrast to the GP (I) zones, these are three-dimensional precipitates. Their layers are parallel to the {100} plane of the aluminum. The phase forms the phases, but there are overlaps. ${\displaystyle \theta ''}$ ${\displaystyle \theta ''}$ ${\displaystyle \theta '}$

The GP (II) zones require vacancies for growth , which is why a lack of these ( e.g. due to magnesium) leads to delayed growth.

Partially coherent phases

The phase is only partially coherent with the aluminum grid and forms at temperatures from 150 ° C to 300 ° C. It has the shape of platelets and can arise from the GP (II) zones. However, it can also arise directly as precipitation from the mixed crystal. In the first case, the increasing interfacial energy is reduced by dislocations , in the second case the precipitates are formed preferentially on dislocations. ${\displaystyle \theta '}$

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 at a greater distance than the other phases and therefore does not lead to an increase in strength or even to a decrease if its formation occurs at the expense of the other phases. The phase also arises at temperatures between 150 ° C and 250 ° C as precipitation at grain boundaries, as this reduces the interfacial energy. ${\displaystyle \theta }$ ${\displaystyle \theta }$

The phase leads to a partially intergranular break ; the fracture behavior remains ductile overall. The change in fracture behavior is caused by precipitation-free zones at the grain boundaries. ${\displaystyle \theta }$

The phase has a greater potential difference compared to the mixed crystal, so that layer corrosion and intergranular corrosion can occur. In the case of longer annealing times, the grain phases also separate out and the potential difference is lower. ${\displaystyle \theta }$ ${\displaystyle \theta }$

AlCuMg (Si, Mn) wrought alloys

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

• Al ₈ Mg ₅ ( phase, see AlMg ) ${\displaystyle \beta }$
• Al ₂ CuMg, the S phase
• Al ₆ Mg ₄ Cu, the T phase

Magnesium additions accelerate the cold hardening process. Which phases are formed mainly depends on the ratio of copper to magnesium. If the ratio is below 1/1, clusters containing Cu and Mg are eliminated. At a ratio of more than 1.5 / 1, which is the case with most technical alloys, the phase is preferentially formed . These alloys have significantly higher hardnesses and strengths. ${\displaystyle \theta }$

literature

• Aluminum-Taschenbuch - Volume 1. 16th edition, 2002, Aluminum-Verlag, Düsseldorf, pp. 101 f., 114-116, 121, 139-141.
• George E. Totten, D. Scott MacKenzie: Handbook of Aluminum - Volume 1: Physical Metallurgy and Processes . Marcel Dekker, New York / Basel, 2003, pp. 140–152.
• Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, ISBN 978-3-662-43806-0 , pp. 117–124.

Individual evidence

1. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 117 f.
2. ↑ Aluminum-Taschenbuch - Volume 1. 16th edition, 2002, Aluminum-Verlag, Düsseldorf, p. 439.
3. ^ Aluminum-Taschenbuch - Volume 1. 16th edition, 2002, Aluminum-Verlag, Düsseldorf, pp. 140 f.
4. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 185.
5. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, Appendix.
6. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, Appendix.
7. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, Appendix.
8. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 118.
9. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 119.
10. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 119.
11. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 119 f.
12. ↑ George E. Totten, D. Scott MacKenzie: Handbook of Aluminum - Volume 1: Physical Metallurgy and Processes. Marcel Dekker, New York / Basel, 2003, p. 140 f.
13. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 120.
14. ↑ George E. Totten, D. Scott MacKenzie: Handbook of Aluminum - Volume 1: Physical Metallurgy and Processes. Marcel Dekker, New York / Basel, 2003, p. 141.
15. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 120.
16. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 120.
17. ↑ George E. Totten, D. Scott MacKenzie: Handbook of Aluminum - Volume 1: Physical Metallurgy and Processes. Marcel Dekker, New York / Basel, 2003, pp. 141–143.
18. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 120 f.
19. ↑ George E. Totten, D. Scott MacKenzie: Handbook of Aluminum - Volume 1: Physical Metallurgy and Processes. Marcel Dekker, New York / Basel, 2003, p. 143.
20. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 121.
21. ↑ George E. Totten, D. Scott MacKenzie: Handbook of Aluminum - Volume 1: Physical Metallurgy and Processes. Marcel Dekker, New York / Basel, 2003, pp. 146–149.
22. ^ Aluminum-Taschenbuch - Volume 1. 16th edition, 2002, Aluminum-Verlag, Düsseldorf, p. 114 f.