Aluminium–magnesium alloys
Most standardised alloys also contain small additives of manganese (AlMg(Mn)).
Discovery of aluminium–magnesium alloys dates back to late 19th century.
They get cold dwell transform, i.e., by rolling and forging and are easily weldable at Mg levels of at least 3%.
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.
[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.
-phase occurs with pure AlMg alloys after a four-stage process.
[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.
The speed of the process depends on the Mg content and the temperature and increases with both.
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.
-phase transform into numerous small particles, referred to in the specialist literature as "bead line-like".
Aluminium-magnesium alloys are considered to be very corrosion-resistant, making them suitable for marine applications, but this is only true if the
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.
AlMg therefore tends to intergranular corrosion if[16] Alloys in states susceptible to intergranular corrosion are annealed at temperatures of 200 °C to 250 °C with slow cooling (heterogeneisation annealing).
Even with medium Mg content, the increase in strength by alloying manganese is higher than by additional magnesium, which is also one reason why most AlMg alloys still contain manganese.
[23] In the case of pure aluminium, the grain size has a minor influence on the strength for metals.
[24] In the case of very high degrees of deformation with heavily work-hardened alloys, softening can also occur at room temperature.
In a long-term study over 50 years, a decrease in strength could be measured by the end.
