Aluminium–magnesium–silicon alloys

Aluminium–magnesium–silicon alloys (AlMgSi) are aluminium alloys—alloys that are mainly made of aluminium—that contain both magnesium and silicon as the most important alloying elements in terms of quantity. Both together account for less than 2 percent by mass. The content of magnesium is greater than that of silicon, otherwise they belong to the aluminum–silicon–magnesium alloys (AlSiMg).

AlMgSi is one of the hardenable aluminum alloys, i.e. those that can become firmer and harder through heat treatment. This curing is largely based on the excretion of magnesium silicide (Mg2Si). The AlMgSi alloys are therefore understood in the standards as a separate group (6000 series) and not as a subgroup of aluminum-magnesium alloys that cannot be hardenable.

AlMgSi is one of the aluminum alloys with medium to high strength, high fracture resistance, good welding suitability, corrosion resistance and formability. They can be processed excellently by extrusion and are therefore particularly often processed into construction profiles by this process. They are usually heated to facilitate processing; as a side effect, they can be quenched immediately afterwards, which eliminates a separate subsequent heat treatment.

Alloy constitution

Phases and balances

The AlMg2Si system forms a Eutectic at 13.9% Mg2Si and 594 °C. The maximum solubility is 583.5 °C and 1.9% Mg2Si, which is why the sum of both elements in the common alloys is below this value. The stoichiometric composition of magnesium to silicon of 2:1 corresponds to a mass ratio of 1.73:1. The solubility decreases very quickly with falling temperature and is only 0.08 percent by mass at 200 °C. Alloys without further alloying elements or impurities are then present in two phases with the-mixed crystal and thephase (Mg2Si). The latter has a melting point of 1085 °C and is therefore thermally stable. Even clusters of magnesium and silicon atoms that are only metastable dissolve only slowly, due to the high binding energy of the two elements.

Many standardised alloys have a silicon surplus. It has little influence on the solubility of magnesium silicide, increases the strength of the material more than an Mg excess or an increase in the Mg2Si content, increases the volume and the number of excretions and accelerates excretion during cold and hot curing. It also binds unwanted impurities; especially iron. A magnesium surplus, on the other hand, reduces the solubility of magnesium silicide.[1]

Alloying elements

In addition to magnesium and silicon, other elements are contained in the standardized varieties.

  • Copper is used to improve strength and hot curing in quantities of 0.2-1%. It forms the Q phase (Al4Mg8Si7Cu2). Copper leads to a denser dispersion of needle-shaped, semi-coherent excretion (cluster of magnesium and silicon). In addition, there is the phase before the for the Aluminium-copper alloys are typical. Alloys with higher copper content (alloyings 6061, 6056, 6013) are mainly used in aviation.
  • Iron occurs in all aluminium alloys as an impurity in quantities of 0.05-0.5%. It forms the phases Al8Fe2Si, Al5FeSi and Al8FeMg3Si6, which are all thermally stable, but undesirable because they brittle the material. Silicon surpluses are also used to bind iron.
  • Manganese (0.2-1%) and Chromium (0.05–0.35%) is deliberately added. If both are allocated at the same time, the sum of the two elements is less than 0.5%. After annealing, they form a dispersion of excretions at at least 400 °C and thus improve strength. Chromium is mainly effective in combination with iron.
  • As dispersion formers are coming zirconium and vanadium for use.

Dispersions
Ductile fracture of an AlMgSi alloy
Brittle fracture of an aluminum alloy

Dispersion particles have little influence on strength. If magnesium or silicon excrete on them during cooling after the solution annealing and, thus, do not form magnesium silicide as desired, they even lower the strength. They increase the sensitivity to deterrent. However, if the cooling speed is insufficient, they also bind excess silicon, which would otherwise form coarser excretions and thus reduce strength. The dispersion particles activate further even when cured. Sliding planes, so that theDuctility increases and, above all, intergranular fracture can be prevented. The alloys with higher strength therefore contain manganese and chromium and are more sensitive to deterrents.[2]

The following applies to the effect of the alloying elements with regard to dispersion formation:

  • The strength at room temperature hardly changes. However, the flow limit at higher temperatures rises sharply, which makes theReformability is limited and above all unfavourable in the extrusion is because it increases the minimum wall thickness.
  • The recrystallisation is made more difficult, which prevents coarse grain formation and has a positive effect on formability.
  • Dislocation movements are blocked at low temperatures, which improved fracture toughness.
  • Dispersions of AlMn bind oversaturated silicon during cooling after solution annealing. This improves crystallization and avoids excretion-free zones that otherwise arise at the grain boundaries. This improves the fracture behaviour from brittle to ductile and intragranular.[3]
  • The sensitivity to quenching increases because precipitated silicon is required for hardening. Alloys containing Mn or Cr must therefore be cooled faster than those without these elements.

6000 series

6000 series are alloyed with magnesium and silicon. They are easy to machine, are weldable, and can be precipitation hardened, but not to the high strengths that 2000 and 7000 can reach. 6061 alloy is one of the most commonly used general-purpose aluminium alloys.[4]

6000 series aluminium alloy nominal composition (% weight) and applications
Alloy Al contents Alloying elements Uses and refs
6005 98.7 Si 0.8; Mg 0.5 Extrusions, angles
6005A 96.5 Si 0.6; Mg 0.5; Cu 0.3; Cr 0.3; Fe 0.35
6009 97.7 Si 0.8; Mg 0.6; Mn 0.5; Cu 0.35 Sheet
6010 97.3 Si 1.0; Mg 0.7; Mn 0.5; Cu 0.35 Sheet
6013 97.05 Si 0.8; Mg 1.0; Mn 0.35; Cu 0.8 Plate, aerospace, smartphone cases[5][6]
6022 97.9 Si 1.1; Mg 0.6; Mn 0.05; Cu 0.05; Fe 0.3 Sheet, automotive[7]
6060 98.9 Si 0.4; Mg 0.5; Fe 0.2 Heat-treatable
6061 97.9 Si 0.6; Mg 1.0; Cu 0.25; Cr 0.2 Universal, structural, aerospace
6063 & 646g 98.9 Si 0.4; Mg 0.7 Universal, marine, decorative
6063A 98.7 Si 0.4; Mg 0.7; Fe 0.2 Heat-treatable
6065 97.1 Si 0.6; Mg 1.0; Cu 0.25; Bi 1.0 Heat-treatable
6066 95.7 Si 1.4; Mg 1.1; Mn 0.8; Cu 1.0 Universal
6070 96.8 Si 1.4; Mg 0.8; Mn 0.7; Cu 0.28 Extrusions
6081 98.1 Si 0.9; Mg 0.8; Mn 0.2 Heat-treatable
6082 97.5 Si 1.0; Mg 0.85; Mn 0.65 Heat-treatable
6101 98.9 Si 0.5; Mg 0.6 Extrusions
6105 98.6 Si 0.8; Mg 0.65 Heat-treatable
6113 96.8 Si 0.8; Mg 1.0; Mn 0.35; Cu 0.8; O 0.2 Aerospace
6151 98.2 Si 0.9; Mg 0.6; Cr 0.25 Forgings
6162 98.6 Si 0.55; Mg 0.9 Heat-treatable
6201 98.5 Si 0.7; Mg 0.8 Rod[8]
6205 98.4 Si 0.8; Mg 0.5;Mn 0.1; Cr 0.1; Zr 0.1 Extrusions
6262 96.8 Si 0.6; Mg 1.0; Cu 0.25; Cr 0.1; Bi 0.6; Pb 0.6 Universal
6351 97.8 Si 1.0; Mg 0.6;Mn 0.6 Extrusions
6463 98.9 Si 0.4; Mg 0.7 Extrusions
6951 97.2 Si 0.5; Fe 0.8; Cu 0.3; Mg 0.7; Mn 0.1; Zn 0.2 Heat-treatable

Grain boundaries

to the grain boundaries prefer silicon to be excreted, as it has germination problems. In addition, magnesium silicide is excreted there. The processes are probably similar to those of the AlMg alloys, but still relatively unexplored for AlMgSi until 2008. The phases excreted at the grain boundaries lead to the tendency of AlMgSi to brittle grain boundary breakage.

Compositions of standardised varieties

All information in mass percent. EN stands for European standard, AW for aluminium wrought alloy; the number has no other meaning.

Numerically Chemical Silicon Iron Copper Manganese Magnesium Chrome Zinc titanium other Other (individual) Other (total) Aluminum
EN AW-6005 AlSiMg 0.6–0.9 0.35 0.10 0.10 0.40–0.6 0.10 - - - 0.05 0.15 Rest
EN AW-6005A AlSiMg(A) 0.50–0.9 0.35 0.3 0.50 0.40–0.7 0.30 0.20 0.10 0.12–0.5 Mn+Cr 0.05 0.15 Rest
EN AW-6008 AlSiMgV 0.50–0.9 0.35 0.30 0.30 0.40–0.7 0.30 0.20 0.10 0.05–0.20 V 0.05 0.15 Rest
EN AW-6013 AlMg1Si0.8CuMn 0.6-1.0 0.5 0.6-1.1 0.20 - 0.8 0.8-1.2 0.10 0.25 0.10 - 0.05 0.15 Rest
EN AW-6056 AlSi1MgCuMn 0.7-1.3 0.50 0.50-1.1 0.40 - 1.0 0.6-1.2 0.25 0.10–0.7 - 0.20 Ti+Zr 0.05 0.15 Rest
EN AW-6060 AlMgSi 0.30–0.6 0.10 - 0.30 0.10 0.10 0.35–0.6 0.05 0.15 0.10 - 0.05 0.15 Rest
EN AW-6061 AlMg1SiCu 0.40–0.8 0.7 0.15–0.40 0.15 0.8-1.2 0.04 - 0.35 0.25 0.15 - 0.05 0.15 Rest
EN AW-6106 AlMgSiMn 0.30–0.6 0.35 0.25 0.05–0.20 0.40 - 0.8 0.20 0.10 - - 0.05 0.15 Rest

Mechanical properties

Conditions:

  • O soft (soft annealed, whether or not warmly formed with the same strength limits).
  • T1: quenched by the hot forming temperature and cold outsourced
  • T4: solution annealed and cold outsourced
  • T5: quenched from the hot forming temperature and warm outsourced
  • T6: solution annealed, quenched and warmly outsourced
  • T7: solution annealed, quenched, hot outsourced and overhardened
  • T8: solution annealed, cold solidified and hot outsourced
Numerical[9] Chemical (CEN) Condition E-module/MPa G-module/MPa Elongation limit/MPa Tensile strength/MPa Elongation at break/% Brinell hardness Bending change resistance/MPa
EN AW-6005 AlSiMg T5 69500 26500 255 280 11 85 n.b.
EN AW-6005A AlSiMg(A) T1 69500 26200 100 200 25 52 n.b.
T4 69500 26200 110 210 16 60 n.b.
T5 69500 26200 240 270 13 80 n.b.
T6 69500 26200 260 285 12 90 n.b.
EN AW-6008 AlSiMgV T6 69500 26200 255 285 14 90 n.b.
EN AW-6056 AlSi1MgCuMn T78 69000 25900 330 355 n.b. 105 n.b.
EN AW-6060 AlMgSi 0 69000 25900 50 100 27 25 n.b.
T1 69000 25900 90 150 25 45 n.b.
T4 69000 25900 90 160 20 50 40
T5 69000 25900 185 220 13 75 n.b.
T6 69000 25900 215 245 13 85 65
EN AW-6061 AlMg1SiCu T4 70000 26300 140 235 21 65 60
EN AW-6106 AlMgSiMn T4 69500 26500 80 150 24 45 n.b.
T6 69500 26200 240 275 14 75 <75

Heat treatment and curing

AlMgSi can be used in two different ways through aHeat treatment can be hardened, whereby hardness and Strength rise, while ductility and Elongation at break. Both begin with the Solution annealing and can also be used with mechanical processes (Forging), with different effects:

  1. Solution annealing: At temperatures of about 510-540 °C, annealing is made, with the alloying elements in solution.
  2. Quenching almost always follows immediately . As a result, the alloying elements initially remain in solution even at room temperature, whereas they would form precipitates if they cooled down slowly.
    • Cold curing: At room temperature, excretions gradually form that increase strength and hardness. In the first hours after quenching, the increase is very high, lower in the next few days, then only creeping, but not yet completed even after several years.
    • Hot curing: At temperatures of 80-250 °C (usual are 160-150 °C), the materials are reheated in the oven. The hardening times are usually 5–8 hours. The alloying elements thus excrete faster and increase hardness and strength. The higher the temperature, the faster the maximum strength possible for this temperature is reached, but the lower the higher the temperature, the lower.

Interim storage and stabilisation

If time passes after quenching and hot curing (so-called interim storage), then the achievable strength decreases during hot curing and only occurs later. The reasons are the change in the material cold curing during temporary storage. However, the effect only affects alloys with more than 0.8% Mg2Si (excluding Mg or Si surpluses) and alloys with more than 0.6% Mg2Si if Mg or Si surpluses are present.

To prevent these negative effects, AlMgSi can be annealed after quenching at 80 °C for 5–30 minutes, which stabilizes the material condition and temporarily does not change. The heat curing is then maintained. Alternatively, a step quenching is possible in which temperatures are initially quenched to be applied during hot curing. The temperatures are maintained for a few minutes to several hours (depending on temperature and alloy) and then completely cooled to room temperature. Both variants allow the workpieces to be processed in the deterred state for some time. Cold curing begins in the event of a longer waiting time. Longer treatment times increase the possible storage period, but reduce the formability. Some of these procedures are protected by patents.

Stabilization has other advantages: The material is then in a definable state, which allows repeatable results in the subsequent processing. Otherwise, for example, the time of interim outsourcing would have an impact on theRebound at theBending so that a constant bending angle would not be possible over several workpieces.

Influence of cold forming

A transformation (forging, rolling, bending) leads to metals and alloys strain hardening, an important form of increasing strength. With AlMgSi, however, it also has an influence on the subsequent warming. Cold forming in the hot-cured state, on the other hand, is not possible due to the low ductility in this state.

Although cold forming directly after quenching increases the strength through strain hardening, it reduces the increase in strength through strain hardening and largely prevents it for degrees of deformation from 10%.

On the other hand, cold forming in a partially or fully cold-hardened state also increases the strength, so that both effects add up.

If cold forming (in the quenched or cold-hardened state) is followed by hot forming, this takes place more quickly, but the strength that can be achieved is reduced. The higher the strain hardening, the higher the yield point, but the tensile strength does not increase. If, on the other hand, the cold forming takes place in the stabilized state, the achievable strength values improve.[10]

Applications

AlMgSi is one of the aluminum alloys with medium to high strength, high fracture resistance, good welding suitability, corrosion resistance and formability.[11]

They are used, among other things, for bumper, bodies and for large profiles in the Rail vehicle construction. In the latter case, they were largely responsible for the changed design of rail vehicles in the 1970s: previously, riveted pipe structures were used. Thanks to the good extrusion compatibility of AlMgSi, large profiles can now be produced, which then can be welded.[12] They are also used in aircraft construction, but there they are AlCu and AlZnMg preferred, but not or only difficult are weldable. The weldable higher-strong AlMgSiCu alloys (AA6013 and AA6056) are used in the Airbus models A318 and A380 for ribbed sheets in the aircraft hull used, where through the Laser welding, weight and cost savings are possible.[13] Swelding is cheaper than the usual in aircraft construction Rivets; The overlaps required during riveting can be eliminated during welding, which saves component mass.[14][15][16]

References
  1. Smith, Andrew W. F. (2002). The Recrystallization and Texture of Aluminium-Magnesium-Silicon Alloys (Thesis). OCLC 643209928. Archived from the original on 11 March 2023. Retrieved 10 March 2023.
  2. Jacobs, M. H. (August 1969). The nucleation and growth of precipitates in aluminium alloys (Thesis). OCLC 921020401. Archived from the original on 9 December 2022. Retrieved 11 March 2023.
  3. Harris, I. R.; Varley, P. C. (April 1954). "Factors influencing brittleness in aluminium-magnesium-silicon alloys". Journal of the Institute of Metals. 82: 379–393. OCLC 4434286733. OSTI 4402272.
  4. "Aluminium in Marine Applications – Aluminium Alloys Used in Boat Building". AZoM.com. 1 May 2008. Archived from the original on 2 October 2022. Retrieved 10 March 2023.
  5. "Alloy 6013 Sheet Higher Strength With Improved Formability" (PDF). Archived from the original (PDF) on 22 December 2017. Retrieved 8 March 2023.
  6. "New, Sleeker Samsung Smartphone Built Stronger with Alcoa's Aerospace-Grade Aluminum". Business Wire (Press release). Alcoa. 4 June 2015.
  7. "Alloy 6022 Sheet Higher Strength with Improved Formability" (PDF). Archived from the original (PDF) on 27 August 2017. Retrieved 8 March 2023.
  8. Davies, G. (November 1988). Aluminium alloy (6201, 6101A) conductors. 1989 International Conference on Overhead Line Design and Construction: Theory and Practice. London. pp. 93–98. ISBN 978-0-85296-371-5. Archived from the original on 11 March 2023. Retrieved 11 March 2023.
  9. Ostermann, Friedrich (2014). Anwendungstechnologie Aluminium [Application technology aluminum] (in German). doi:10.1007/978-3-662-43807-7. ISBN 978-3-662-43806-0.
  10. Swindells, N.; Sykes, C. (1938). "Specific Heat-Temperature Curves of Some Age-Hardening Alloys". Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences. 168 (933): 237–264. Bibcode:1938RSPSA.168..237S. doi:10.1098/rspa.1938.0172. JSTOR 97238. S2CID 94528199.
  11. Weser, A (2010). "Alkaline Earth Hydroxides". In Schütze, Michael; Wieser, Dietrich; Bender, Roman (eds.). Corrosion Resistance of Aluminium and Aluminium Alloys. John Wiley & Sons. pp. 37–45 [39]. ISBN 978-3-527-33001-0. Archived from the original on 11 March 2023. Retrieved 11 March 2023.
  12. Ekşi, Murat (2012). Optimization of mechanical and microstructural properties of weld joints between aluminium - magnesium and aluminium - magnesium - silicon alloys with different thicknesses (Thesis). hdl:11511/22296.
  13. Mathers, Gene (2002). "Material standards, designations and alloys". The Welding of Aluminium and its Alloys. pp. 35–50 [44]. doi:10.1533/9781855737631.35. ISBN 978-1-85573-567-5. Archived from the original on 11 March 2023. Retrieved 11 March 2023.
  14. Ostermann, Friedrich (2014). "Märkte und Anwendungen" [Markets and Applications]. Anwendungstechnologie Aluminium [Application technology aluminum] (in German). pp. 9–67. doi:10.1007/978-3-662-43807-7_2. ISBN 978-3-662-43806-0.
  15. Guilhaudis, A. (1 March 1975). "Some Aspects of the Corrosion Resistance of Aluminium Alloys in a Marine Atmosphere". Anti-Corrosion Methods and Materials. 22 (3): 12–16. doi:10.1108/eb006978.
  16. Rambabu, P.; Eswara Prasad, N.; Kutumbarao, V. V.; Wanhill, R. J. H. (2017). "Aluminium Alloys for Aerospace Applications". Aerospace Materials and Material Technologies. Indian Institute of Metals Series. pp. 29–52. doi:10.1007/978-981-10-2134-3_2. ISBN 978-981-10-2133-6.

Further reading
  • Hirsch, Jürgen; Skrotzki, Birgit; Gottstein, Günter, eds. (2008). Aluminium Alloys: The Physical and Mechanical Properties. John Wiley & Sons. ISBN 978-3-527-32367-8.
  • Ghali, Edward (2010). Corrosion Resistance of Aluminum and Magnesium Alloys: Understanding, Performance, and Testing. John Wiley & Sons. ISBN 978-0-470-53176-1.
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