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7xxx Series Alloys
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7xxx series are very strong "heat treatable" alloys; they can be strengthened through heat treatment (precipitation hardening) based on the combination of zinc (mostly between 4–6 wt %) and magnesium (range 1–3 wt %).
Unfortunately these alloys seem prone to stress corrosion.
Important critical applications of these alloys are based on their superior strength, for example in aerospace, space exploration, military and nuclear applications. But also structural parts in building applications can be from the 7xxx series, as well as high strength sports' attributes such as ski poles and tennis rackets.
As with some of the 2xxx series, alloys in the 7xxx series also have additions of magnesium to maximise their age-hardening potential where the precipitating phases are typically of the type MgZn2. Such alloys give medium strength, but are relatively easily welded. Aluminium-zinc-magnesium alloys have a greater response to heat treatment than the binary aluminium-zinc alloys resulting in higher possible strengths. The additions of zinc and magnesium however decrease the corrosion resistance.
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 recrystallisation in aluminium-magnesium-silicon and aluminium-zinc alloys during hot working or heat treatment. The fibrous structures that develop reduce stress corrosion susceptibility and/or improve toughness. Chromium in solids solution or finely dispersed increases strength slightly. The disadvantage of chromium in heat-treatable alloys (6xxx and 7xxx) is the increase in quench sensitivity when the hardening phase tends to precipitate on pre-existing chromium-phase particles. Chromium also tends to colour an anodic film yellow.
The addition of copper to aluminium-zinc-magnesium alloys, together with small amounts of chromium and manganese, result in the highest strength aluminium alloys available. Alloys based on the quaternary Al–Zn–Mg–Cu system have the greatest potential of all aluminium alloys for age-hardening, and yield strengths approaching 600 MPa can be achieved in some alloys. Zinc and magnesium control the ageing process, while the effect of copper is the increase in ageing rate and the increase in quench sensitivity. Although copper decreases the general corrosion resistance, it improves the resistance to stress corrosion. In combination with other elements such as Si and Fe, zinc-containing alloys can be very strong and hard. In combination with Mg precipitation hardening is possible; however more than 10 % of Zn makes the alloy susceptible to stress corrosion. Further, the presence of Zn in aluminium increases its solution potential. It is used in protective cladding (7072) and in sacrificial anodes.
Silver at a 0.1–0.6 % level is effective for improving strength and stress-corrosion resistance of aluminium-zinc-magnesium alloys.
At levels of 0.005–0.5 % alloys cadmium reduces the time of ageing in aluminium-zinc-magnesium. It has been reported that traces of cadmium lower corrosion resistance of unalloyed aluminium.
As an unwanted trace, at small levels of 0.01 %, tin causes surface darkening on annealing and increase corrosion susceptibility due to the migration of tin to the surface. Small additions of copper (0.2 %) counteract this effect. However aluminium-zinc alloys with small levels of tin are used as sacrificial anodes in salt water. Also tin-magnesium, cadmium and silicon are additions to sacrificial anode alloys of aluminium–zinc–(indium).
Mercury has been used up to 0.05 % levels in sacrificial anodes to protect steel structures. Mercury is a toxic substance and levels should be minimized.