Certain alloying elements, such as copper, magnesium, zinc, and silicon, 

 show increasing solid solubility in aluminum with increasing temperatures. 

 Many aluminum alloys containing these elements can be heat treated to enhance 

 the initial strength. These alloys are heat treated by first raising the 

 alloy to an elevated temperature below the melting point, called the solu- 

 tioning temperature, which puts the soluble element or elements into solid 

 solution. This is followed by quickly cooling the material, usually by 

 quenching in water, to retain the elements in solid solution at room tempera- 

 ture. At this stage the freshly quenched alloy structure is very workable. 

 By storing such material at below-freezing temperatures, this workable alloy 

 structure can be retained until the fabrication is ready to form the alloy 

 into the desired final shape. Such alloys after quenching are not stable at 

 room or elevated temperatures because precipitation of the constituents from 

 the supersaturated solution takes place. After a period of several days at 

 room temperature or hours at an elevated temperature, the alloy is consider- 

 ably stronger. This process is called age hardening or precipitation harden- 

 ing. The degree of hardening or temper produced by heat treatment is denoted 

 by a "T" followed by a number. 



C2) Identification of Aluminum Alloys . Aluminum alloys are identi- 

 fied by specific numbers. Alloys belong to certain series depending on the 

 particular alloying elements. The 1000 series consists of the high purity 

 aluminums containing at least 99 percent aluminum. These alloys are char- 

 acterized by having high thermal and electrical conductivity, excellent 

 corrosion resistance, excellent workability, but low strength. These alloys 

 can only be hardened by cold working. Major impurities are iron and silicon. 



Copper is the major alloying element of the 2000 series. These alloys 

 are solution heat treated to obtain optimum properties. Some alloys of this 

 series are aged at slightly elevated temperatures, a process called artifi- 

 cial aging, to obtain increased yield strength. The corrosion resistance of 

 the alloys in the 2000 series is less than most of the other aluminum alloys. 

 Sheet forms of these alloys are often clad with high purity alloy or a 

 magnesium-silicon alloy of the 6000 series which provides galvanic protection 

 to the core material and therefore increases resistance to corrosion. 

 Manganese is the principal alloying element of the 3000 series alloys. 

 Alloys of this group generally cannot be heat treated, but can be hardened 

 by cold working. 



Silicon is the major alloying element of the 4000 series, which in 

 sufficient quantities, lowers the melting point without producing brittle- 

 ness. Aluminum-silicon alloys are used in welding and brazing wire where 

 the lower melting point is beneficial in joining other aluminum alloys. 

 Although most alloys of this group are nonheat- treatable, during welding of 

 heat-treatable alloys, some elements from the parent material may be picked 

 up by the weld metal providing joints that may be strengthened by heat 

 treatment . 



The 5000 series of alloys contains magnesium. Although these alloys are 

 nonheat-treatable, the addition of magnesium produces alloys having moderate 

 to high strength, good welding characteristics, and good corrosion resistance 

 to marine atmospheres. These alloys are subject to stress corrosion cracking 

 if employed in the cold-worked condition in services where the temperature 

 exceeds about 65 Celsius (150 Fahrenheit). 



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