Aluminium–magnesium alloys

Discovery of aluminium–magnesium alloys dates back to late 19th century.[1] AlMg alloys are among the most important aluminium alloys for construction materials. They get cold dwell transform, i.e., by rolling and forging and are easily weldable at Mg levels of at least 3%. AlMg is rarely processed through extrusion presses, as subsequent strength changes in extrusion profiles must be avoided. The majority of AlMg alloys are processed into rolled products as well as pipes, rods, wires and free-form or drop-forged parts. Parts are also processed into extrusion profiles with simple cross-sections.[2]

Due to the good corrosion resistance and high strength at low temperatures, AlMg is used in shipbuilding, in the construction of chemical apparatus and pipelines, and for refrigeration technology and automobiles. The good weldability is crucial for use in the aircraft construction, there also with additions of scandium and zirconium for better weldability.[3]

The solubility of magnesium is very high in aluminium and reaches a maximum at 450 °C with 14% to 17% depending on the literature reference. At 34.5%, there is a Eutectic with Al₈Mg₅ (sometimes referred to as Al₃Mg₂), an intermetallic phase (${\displaystyle \beta }$-phase). The solubility of Mg decreases sharply with falling temperature, i.e., at 100 °C it is still 2%, at room temperature 0.2%.

The elimination of the ${\displaystyle \beta }$-phase occurs with pure AlMg alloys after a four-stage process. With technically used alloys with other alloying elements and impurities, the process is much more complicated:[4]

• First of all, clusters form, in the case of aluminium as GP zones. These are local accumulations of magnesium atoms in the aluminium grid, which do not yet form their own phase and do not have a regular arrangement.
• Formation of the coherent ${\displaystyle \beta ''}$-phase. Their crystals have the same spatial orientation as those of the aluminium mixing crystal.
• Formation of the semi-coherent ${\displaystyle \beta '}$-phase. It is only partially oriented towards the lattice of the Al mixed crystal.
• Formation of incoherent ${\displaystyle \beta }$-phase. It has no spatial orientation with the Al mixed crystal.

In the case of technical alloys, the excretion differs from this for the following reasons:[5]

• Low diffusion of magnesium in aluminium
• For the formation of GP zones and ${\displaystyle \beta ''}$-phase, a high oversaturation of 7% Mg and more is required, which is not achieved in most all>oys. In AlMg_4.5Mn_0.7, no GP zones were used even after prolonged glow at temperatures up to 250 °C or ${\displaystyle \beta ''}$-phase found, although after just a few days ${\displaystyle \beta }$-phase is present.
• Dislocations are not sufficient terms for the formation of ${\displaystyle \beta ''}$-phase, ${\displaystyle \beta '}$-phase or ${\displaystyle \beta }$-phase. The reason is the small volume difference between these phases and the matrix.

round test specimen made of AlMg₃, without grain fineness (2×)

round test specimen made of AlMg₃, grain-fight

The diffusion of magnesium in aluminium is very low. The reason is the high size difference between the radius of the aluminium atoms and that of the magnesium atoms (${\displaystyle r_{Al}:r_{Mg}=1.43:1.6}$).[6] Therefore, after watering, only part of the magnesium is removed from the mixed crystal, while most of it is present as an oversaturated solution in aluminium. Even with prolonged annealing treatment, this condition cannot be eliminated.

Excess magnesium is excreted mainly at the grain boundaries as well as on dispersion particles in the grain. The speed of the process depends on the Mg content and the temperature and increases with both. At the grain boundaries, so-called plaques are initially excreted, thin plates that are not connected, i.e. do not yet form a continuous layer around the grain. At 70 °C, they form after 3 months, at 100 °C after 3 days and at 150 °C after one to nine hours. If further time passes at elevated temperature, the plaques grow together to form a contiguous film. This has a negative effect on corrosion resistance, but can be dissolved by heat treatment. Annealing at 420 °C for one hour followed by slow cooling of 20 °C/h or starting annealing at 200 °C to 240 °C is suitable. The plaques of the ${\displaystyle \beta }$-phase transform into numerous small particles, referred to in the specialist literature as "bead line-like". They no longer form a coherent film.[7]

The compositions of some standardised varieties are contained in the following table. Proportions of alloying elements in mass percent. Of the available varieties, there are fine gradations of Mg and Mn levels. Mn-free are very rare. Standard alloys are AlMg₃Mn, AlMg_4.5Mn_0.7, as well as for bodywork AlMg_4.5Mn_0.4. Magnesium levels of up to 5% and manganese content up to 1% are used for wrought alloys.[8][9]

Mg contents up to 10% are also possible for cast alloys; however, contents of 7% and more are considered heavypourable.[10]

5000 series are alloyed with magnesium. 5083 alloy has the highest strength of non-heat-treated alloys. Most 5000 series alloys include manganese as well.

Aluminium-magnesium alloys are considered to be very corrosion-resistant, making them suitable for marine applications, but this is only true if the ${\displaystyle \beta }$-phase exists as a non-contiguous phase. Alloys with Mg contents below 3% are therefore always corrosion-resistant, with higher contents, appropriate heat treatment must ensure that this phase is not present as a continuous film at the grain boundaries.[15]

The ${\displaystyle \beta '}$-phase and the ${\displaystyle \beta }$-phase are very base compared to aluminium and have an anodic characteristic. AlMg therefore tends to intergranular corrosion if[16]

1. The ${\displaystyle \beta }$-phase is excreted as a continuous film at the grain boundaries and at the same time
2. the material is in an aggressive environment.

Alloys in states susceptible to intergranular corrosion are annealed at temperatures of 200 °C to 250 °C with slow cooling (heterogeneisation annealing). This changes the ${\displaystyle \beta }$-phase film in globulite ${\displaystyle \beta }$-phase and the material is resistant to intergranular corrosion.[17]

• Ghali, Edward (2020-05-05). Revie, R. Winston (ed.). Corrosion Resistance of Aluminum and Magnesium Alloys: Understanding, Performance, and Testing. John Wiley & Sons. doi:10.1002/9780470531778. ISBN 9780470531761.
• Wieser, Dietrich; Schütze, Michael; Bender, Roman, eds. (2010-12-28). Corrosion Resistance of Aluminium and Aluminium Alloys. Wiley. ISBN 978-3-527-33001-0.