(1) Characteristics of 5xxx aluminum alloys
5xxx aluminum alloys are aluminum alloys with Mg as the main alloying element and are non-heat-treatable aluminum alloys. This series of alloys has a low density and higher strength than 1xxx and 3xxx aluminum alloys. They are medium-to-high strength aluminum alloys with good fatigue and welding properties and good resistance to marine and atmospheric corrosion. In order to avoid stress corrosion of high-magnesium alloys, the final cold-worked products must be stabilized, or the final cold-work amount must be controlled, and the use temperature must be controlled (not exceeding 65°C). This series of alloys is mainly used to make welded structural parts and is used in the field of ships.
(2) Main alloy and impurity elements and their roles
The main component of 5XXx series aluminum alloy is Mg, with a small amount of Mn, Cr, Ti and other elements added, while the impurity elements are mainly Fe, Si, Cu, Zn and so on.
Mg: Mg mainly exists in the solid solution state and β (Mg2Al3 or Mg5Al8) phase. Although the solubility of Mg in the alloy decreases rapidly with decreasing temperature, the alloy has low aging strengthening effect due to the difficulty of precipitation nucleation, small number of cores and coarse precipitation phase. It is generally used in annealing or cold working state. Therefore, this series of alloys is also called non-strengthening aluminum alloys. The strength of this series of alloys increases with the increase of Mg content, while the plasticity decreases accordingly, and its processing performance also deteriorates accordingly. The Mg content has a great influence on the recrystallization temperature of the alloy. When the Mg content is less than 5%, the recrystallization temperature decreases with the increase of Mg content; when the Mg content exceeds 5%, the recrystallization temperature increases with the increase of Mg content. Mg content also has a significant effect on the welding performance of the alloy. When the Mg content is less than 6%, the welding crack tendency of the alloy decreases with the increase of Mg content; when the Mg content exceeds 6%, the opposite is true; when the Mg content is less than 9%, the strength of the weld increases significantly with the increase of Mg content. At this time, although the plasticity and welding coefficient gradually decrease slightly, the change is not large. When the Mg content is greater than 9%, its strength, plasticity and welding coefficient are all significantly reduced.
Mn: 5XXX series aluminum alloys usually contain less than 1.0% Mn. Part of the Mn in the alloy is dissolved in the matrix, and the rest exists in the structure in the form of MnAls phase. Mn can increase the recrystallization temperature of the alloy to prevent grain coarsening, and slightly increase the strength of the alloy, especially the yield strength. In high Mg alloys, adding Mn can reduce the solubility of Mg in the matrix, reduce the tendency of weld cracks, and increase the strength of the weld and base metal.
Cr: Cr and Mn have similar effects, which can increase the strength of the base metal and weld, reduce the tendency of weld cracking, and improve stress corrosion resistance, but slightly reduce plasticity. Cr can be used instead of Mn in some alloys. In terms of strengthening effect, Cr is not as good as Mn. If both elements are added at the same time, the effect is greater than that of a single addition.
Be: Adding a small amount of Be (0.0001%~0.005%) to high Mg alloy can reduce the crack tendency of ingots and improve the surface quality of rolled plates, while reducing the burning loss of Mg during smelting, and can also reduce the oxides formed on the surface of the material during heating.
Ti: Adding a small amount of Ti to high Mg alloys is mainly to refine the grains.
Fe: Fe can form insoluble compounds with Mn and Cr, thereby reducing the role of Mn and Cr in the alloy. When more hard and brittle compounds are formed in the ingot structure, processing cracks are prone to occur. In addition, Fe also reduces the corrosion resistance of the alloy, so the Fe content should generally be controlled below 0.4%, and for welding wire material F, it is best to limit it to below 0.2%.
Si: Si is a harmful impurity (except for 5A03 alloy). Si and Mg form Mg2Si phase. Due to the excess Mg content, the solubility of Mg2Si phase in the matrix is reduced, so not only the strengthening effect is not great, but also the plasticity of the alloy is reduced. During rolling, Si has a greater negative effect than Fe, so the Si content should generally be limited to less than 0.5%. 5A03 alloy contains 0.5%~0.8% Si, which can reduce the tendency of welding cracks and improve the welding performance of the alloy.
Cu: A trace amount of Cu can make the corrosion resistance of the alloy worse, so the Cu content should be limited to less than 0.2%, and some alloys are more strictly restricted.
Zn: When the Zn content is less than 0.2%, it has no significant effect on the mechanical properties and corrosion resistance of the alloy. Adding a small amount of Zn to high Mg alloys can increase the tensile strength by 10~20MPa. The impurity Zn in the alloy should be limited to less than 0.2%.
Na: Trace impurity Na can strongly damage the hot deformation properties of the alloy, resulting in "sodium brittleness", which is more prominent in high Mg alloys. The way to eliminate sodium brittleness is to convert the free Na enriched in the grain boundary into a compound. The chlorination method can be used to produce sodium chloride and remove it with the slag, or the method of adding trace Sb can be used.