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Dr. Dmitri Kopeliovich
Wrought aluminum alloys are used in the shaping processes: rolling, forging, extrusion, pressing, stamping.
There are two principal groups of the wrought aluminum alloys:
Non-heat-treatable alloys cannot be strengthened by heat treatment.
Alloys of 1xxx, 3xxx, 4xxx and 5xxx series are non-heat-treatable.
The initial strength of these alloys is achieved due to the hardening effect of the alloying elements: manganese (Mn), silicon (Si), magnesium (Mg).
Additional hardening of these alloys is done by cold work (strain hardening).
Non-heat-treatable alloys are ductile and moderately strong (depending on the alloying elements concentration).
The alloys from this group are supplied either in O condition (annealed) or in H condition (cold worked at various degrees).
Non-heat-treatable alloys are used for manufacturing deep drawn parts, sheets, foil, tubes, wire, extruded parts, pressure vessels.
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Heat-treatable alloys can be strengthened by heat treatment.
Alloys of 2xxx, 6xxx and 7xxx series are heat-treatable.
The initial strength of these alloys is achieved due to the hardening effect of the alloying elements: copper (Cu), silicon (Si), magnesium (Mg) and zinc(Zn).
Since solubility of these elements (or intermetallic compounds formed from these elements) in solid aluminum depends on the temperature, it is possible to harden the alloys from this group by a heat treatment, called precipitation hardening (age hardening).
Precipitation hardening (age hardening) – strengthening by precipitation of fine particles of a second phase from a supersaturated solid solution.
The precipitation hardening heat treatment involves the following stages:
At this stage a part is heated to a temperature above the solvus temperature in order to dissolve the second phase in the solid solution.
The part is held at this temperature for a time varying from 1hour to 20 hrs. until the dissolving has been accomplished.
The temperature and the soaking time of solution treatment should not be too high to prevent excessive growth of the grains.
Quenching is carried out in water, water-air mixture or sometimes in air.
Object of the quenching operation is obtaining supersaturation solid solution at room temperature.
Since the second phase retains dissolved at this stage, hardness of the quenched alloy is lower than after age precipitation, however higher than hardness of the alloy in annealedstate.
Depending on the temperature at which this operation is carried out aging may be artificial or natural.
1. Artificial aging.
At this stage the part is heated up to a temperature below the solvus temperature, followed by soaking for a time varying between 2 to 20 hours.
The soaking time depends on the aging temperature (the higher the temperature, the lower the soaking time).
The aging temperature and the soaking time are also determined by the desired resulted combination of the strength and ductility of the alloy.
Too high aging temperature and time result in overaging – decrease of the strength and increase of ductility.
Natural aging is conducted at room temperature and it takes a relatively long period of time (from several days to several weeks).
The alloys from this group are supplied either in O condition (annealed) or in T condition (heat treated by age hardening).
Heat-treatable alloys are used for manufacturing aircraft parts and structures, screw machine parts, automotive body panels, welded structures.
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Microstructure (phase composition) of an aluminum alloy consists of the aluminum base solid solution grains and intermetallic (and/or metallic) inclusions, composition of which is determined by the composition of the alloy.
Despite the fact that the compositions of alloys from a group (eg. 2xxx) are similar (the main constituents are the same), the composition and amount of intermetallic/metallic inclusions differ for different alloys.
Fo example the alloy 2011 is generally binary (not taking into account the impurities like iron and silicon) - it consists of aluminum and 5.5% copper. Its intermetallic inclusions have the composition Al2Cu.
The alloy 2014 contains also Mg, Si and Mn, presence of which changes the composition of intermetallic inclusions to more complex particles (Al4CuMg5Si4, Al12(Fe,Mn)3Si, Al20Cu2Mn3).
The phase composition also depends on the temper of the alloy. Most of intermetallic particles are dissolved during solution treatment and stay in supersaturated solution after quenching. Aging results in precipitation of the intermetallics.
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