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7xxx Series. The 7xxx series aluminum alloys contain 1 to 8% Zinc which is the main alloying element, the other major alloying elements include magnesium (Mg) , copper and chromium (Cr) ., etc. The 7xxx series aluminum alloys are used in airframe structures, mobile equipment, and other highly stressed parts.

Fig 1. Microstructure of a 7039 plate：(Left) Ingot, (Middle) as-hot rolled with 50% reduction and (Right) as-hot rolled with 83% reduction. (Metal handbook, 8th edition, vol.7.)

The high strength 7xxx series aluminum alloys exhibit reduced resistance to stress corrosion cracking (SCC) when in their peak-aged temper (T6) . To solve this problem, the 7xxx alloys usually used in a slightly overaged temper (T7x) . T7x provides a better combinations of strength, corrosion resistance, and fracture toughness. However, over-aging results in a loss in strength of 10-15%. Owing to containing more alloying elements, the second phases in 7xxx series aluminum alloys are more complexer than 2xxx series aluminum alloys. Also the precipitation process is more complex for 7xxx series aluminum alloys.

TABLE 1. MECHANICAL PROPERTIES OF 7050 AND 7075 ALUMINUM ALLOYS.

*A = Excellent. B = Good. C = Fair. D = Poor

2xxx Series. The primary alloying element is copper (Cu) and often with magnesium (Mg) as a secondary alloying element. As the increment of strength is not great and will lose elongation so much, the 2xxx series aluminum alloys usually deform and finish under the solution heat treatment condition, not under aging condition. In some application, precipitation heat treatment (aging), of which the main precipitation phase is Al2Cu or Al2CuMg, is employed to further increase mechanical properties. Iron (Fe) and silicon (Si) are impurities in 2xxx series aluminum alloys. Iron will react with copper to form insoluble compound Cu2FeAl7, resulting in reducted the amount of strengthening phase CuAl2 and CuMgAl2. Moreover, iron would also combine silicon and manganese (Mn) to form coarse and brittle (FeMnSi) Al6 and (FeMn) Al6 compounds. These would degrade the mechanical properties of the 2xxx series aluminum alloys, so the iron content should be kept lower than 0.5%. Silicon would also reduce the strengthening effect providing by other elements. The strength of 2xxx-T4 is reduced as the content of silicon is increased. So the silicon content also should be kept lower than 0.5%. The 2xxx series aluminum alloys are very suited for parts and structures requiring high structure efficiency (strength/density) and are commonly used to make truck and aircraft wheels, truck suspension parts, aircraft fuselage and wing skins, and structural parts and those parts requiring good strength at temperatures up to 150°C.

TABLE 2. MECHANICAL PROPERTIES OF 2014 AND 2024 ALUMINUM ALLOYS.

*A = Excellent. B = Good. C = Fair. D = Poor

General Information

Tempers for heat-treatable aerospace alloys:
• T3: Solution heat treated and cold-worked and naturally aged
• T4: Solution heat treated and naturally aged
• T6: Solution heat treated and artificially aged to peak strength
• T8: Solution heat treated and cold-worked and artificially aged to peak strength
• T7x: overaged (T79, T76, T74, T73 → Increasing corrosion resistance.)
   T79：Solution heat treated and then artificially overaged (very limited overaging).
   T76：Solution heat treated and then artificially overaged in order to achieve a good
          exfoliation corrosion resistance.
   T74：Solution heat treated and then artificially overaged (between T73 and T76).
   T73：Solution heat treated and then artificially overaged in order to achieve the best stress
          corrosion resistance

Stress-relief tempers：
• Tx51: Solution heat treated and stress-relieved by stretching.
• Tx52: Solution heat treated and stress-relieved by compressing.
• Tx54: Solution heat treated and stress-relieved by the combination stretching and
compressing.

Surface treatment for aluminum alloys：
• Anodizing：An electrolytic passivation process used to increase the thickness of the natural
    oxide layer on the surface of metal parts.
• Hard Anodizing： Hard coating results in a protective oxide layer that is up to 8 times
   thicker than standard anodizing.
• Chromate conversion coating：A type of conversion coating applied to passivate aluminium,
    zinc, cadmium, copper, silver, magnesium, tin and their alloys to slow corrosion.
• Electroless nickel plating：An auto-catalytic chemical technique used to deposit a layer of
    nickel- phosphorus or nickel- boron alloy on a solid work piece, such as metals or plastic.
• Powder coating and liquid paint：The main difference between liquid paint and powder
    coating is that the powder coating does not require a solvent to keep the binder and filler    parts in a liquid suspension form. The powder coating is typically applied electrostatically
   and is then cured under heat
• Tin-nickel alloy plating：An alloy of tin and nickel（atomic ratio is 1:1） can be electroplated
   directly on copper, nickel and their alloys.
• PVD：A process, carried out in a vacuum chamber, to produce a metal vapor that can be
   deposited on electrically conductive materials as a thin highly adhered pure metal or alloy
   coating. The metal vapors can react with other gases to form oxide or nitride films.