Magnesium in Aluminum
Introducing aluminum magnesium in an amount up to 6 % as the main alloying element provides strengthening of the solid solution alloy and a high strain hardening efficiency. it 5xxx series alloys provides relatively high strength properties - higher, than the 3xxx series alloys – while maintaining good formability.
With a certain susceptibility to intergranular corrosion (with a magnesium content of more 3 %With a certain susceptibility to intergranular corrosion (with a magnesium content of more, particularly corrosion resistance in sea water and marine atmosphere, which is considerably higher, than the other alloy series.
The use of aluminum-magnesium alloys
These alloys combine good formability, relatively high strength, excellent corrosion resistance, anodized good and best of all alloys weldability. Therefore, these Aluminum alloys are used in many designs, subject to severe weathering, for example, in facing panels of buildings, scaffolding and, special, - in the shipbuilding and constructions in the coastal areas and on the high seas, including oil platforms. welded aluminum boats and boats made entirely of alloys of this series. In the automotive industry are made from these alloys pressed housing parts and the chassis due to the good combinations of strength and formability.
Achieving a high strength by strengthening the solid solution magnesium possible because, that magnesium in this role is very effective. Moreover, its high solubility allows to increase its content to 5 % in most doped alloys.
However, alloys with high magnesium content there is a tendency to form intermetallic phases Mg5Al8 along the grain boundaries and in the areas of localized strain inside the microstructure. This is because, that the equilibrium solubility of magnesium in aluminum only about 2 %. Isolation of excess phase in this case is equivalent to, what happens in alloys, strengthened by aging, but with a negative effect to properties of the alloy. Isolation of the particles occurs slowly at room temperature, but it accelerates with increasing temperature, or, if the alloy is subjected to intense cold plastic deformation. This phenomenon makes the alloy susceptible to certain types of intergranular corrosion, for example, stress corrosion, and / or deterioration of mechanical properties during operation at elevated temperatures.
The role of chromium
Chromium in quantities not exceeding 0,35 % is added to improve electrical resistance. At high chromium content tends to form very tough compound with other impurities or additives, following, as manganese, iron and titanium. Chromium has a low diffusion rate and forms very fine phase, which inhibit the nucleation and growth of grains. Therefore, it is used to prevent the grain growth. Thus formed fibrous structures reduce susceptibility to stress corrosion and improve viscosity. Chromium in solid solution or as fine particles contributes to a certain increase in strength. Chromium tends to stain anodic coating these alloys yellow.
Effect of manganese
manganese additives, as well as chromium, designed to reduce the susceptibility of alloys with a high content of magnesium to various forms of intergranular corrosion.
5xxx series in European Standard EN 573-3 It includes 44 alloy and their modifications, in GOST 4784-97 – 13.