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The presence of magnesium as main alloying element in the 5xxx series (used up to 6 wt%) leads to solute hardening of the alloy, and efficient strain hardening, resulting in medium strenght. These alloys are generally stronger than the medium strength 3xxx series alloys, while having also very good formability.
Except for susceptibility to intergranular corrosion under very unfavourable conditions (when the Mg level is > 3 wt%), the 5xxx series alloys have good corrosion performance, and especially their resistance in seawater and marine atmosphere is superior to the other alloy series.
The good formability, combined with the medium strength and excellent corrosion resistance, and the high quality anodising ability and weldability, result in many applications for outdoor exposure: in building architecture sheet (anodised and electrocoloured facade panels, for example), scaffolding, and especially marine applications (ship building, platforms, etc). Also in automotive, 5xxx series alloys are used for press formed body-parts and chassis components due to their good combination of strength and formability.
Magnesium results in solute hardening of the alloy and thus increases the strength. Not only is Mg a very effective solid solution strengthening element in terms of the effect per unit weight in solution, but also its high solubility enables large additions of the element to be made. Alloys in the 5xxx series may contain from about 0.8 wt% Mg to more than 5 wt% in the most highly alloyed variants. There is the tendency in the higher Mg variants for the formation of the intermetallic phase Mg5Al8 at grain boundaries and regions of localised strain within the microstructure. This happens because the equilibrium solid solubility of Mg in Al at room temperature is only about 2 wt% (the precipitation is equivalent to that in age-hardening alloys, but in this case its nature is such that it is deleterious to the properties of the alloy). The precipitation occurs only slowly at room temperature, but is accelerated at higher temperatures or if the alloy is heavily cold worked and makes the alloy susceptible to certain types of intergranular corrosion (stress corrosion cracking and exfoliation), and/or deterioration of mechanical properties during service at elevated temperatures.
Chromium amounts generally less than 0.35 % are added to increase the electrical resistivity. At higher content levels chromium tends to form very coarse constituents with other impurities or additions such as manganese, iron and titanium. Chromium has a slow diffusion rate and form finely dispersed phases that inhibit nucleation and grain growth. Chromium is used as such to control grain structure, by preventing grain growth in aluminium-magnesium alloys. The fibrous structures that develop reduce stress corrosion susceptibility and/or improve toughness. Chromium in solid solution or finely dispersed increases strength slightly. Chromium tends to colour an anodic film yellow. Cr additions, together with Mn, are specified for high Mg containing alloys to reduce susceptibility to forms of intergranular corrosion (stress corrosion cracking and exfoliation, see Mg for explanation).
Manganese additions, together with Cr, are specified for high Mg containing alloys to reduce susceptibility to forms of intergranular corrosion (see Mg for explanation).