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How And Why Alloying Elements Are Added To Aluminum
There are many aluminum alloys used in industry today - over 400 wrought alloys and over 200 casting allloys are currently registered with the Aluminum Association.The principal effects of alloying elements in hypereutectic aluminum are as follows:
Copper (Cu) 2xxx – The aluminum-copper alloys typically contain between 2 to 10% copper, with smaller additions of other elements. The copper provides substantial increases in strength and facilitates precipitation hardening. The introduction of copper to aluminum can also reduce ductility and corrosion resistance. The susceptibility to solidification cracking of aluminum-copper alloys is increased; consequently, some of these alloys can be the most challenging aluminum alloys to weld. These alloys include some of the highest strength heat treatable aluminum alloys. The most common applications for the 2xxx series alloys are aerospace, military vehicles and rocket fins.
Manganese (Mn) 3xxx – The addition of manganese to aluminum increases strength somewhat through solution strengthening and improves strain hardening while not appreciably reducing ductility or corrosion resistance. These are moderate strength nonheat-treatable materials that retain strength at elevated temperatures and are seldom used for major structural applications. The most common applications for the 3xxx series alloys are cooking utensils, radiators, air conditioning condensers, evaporators, heat exchangers and associated piping systems.
Silicon (Si) 4xxx – The addition of silicon to aluminum reduces melting temperature and improves fluidity. Silicon alone in aluminum produces a nonheat-treatable alloy; however, in combination with magnesium it produces a precipitation hardening heat-treatable alloy. Consequently, there are both heat-treatable and nonheat-treatable alloys within the 4xxx series. Silicon additions to aluminum are commonly used for the manufacturing of castings. The most common applications for the 4xxx series alloys are filler wires for fusion welding and brazing of aluminum.
Magnesium (Mg) 5xxx - The addition of magnesium to aluminum increases strength through solid solution strengthening and improves their strain hardening ability. These alloys are the highest strength nonheat-treatable aluminum alloys and are, therefore, used extensively for structural applications. The 5xxx series alloys are produced mainly as sheet and plate and only occasionally as extrusions. The reason for this is that these alloys strain harden quickly and, are, therefore difficult and expensive to extrude. Some common applications for the 5xxx series alloys are truck and train bodies, buildings, armored vehicles, ship and boat building, chemical tankers, pressure vessels and cryogenic tanks.
Magnesium and Silicon (Mg2Si) 6xxx – The addition of magnesium and silicon to aluminum produces the compound magnesium-silicide (Mg2Si). The formation of this compound provides the 6xxx series their heat-treatability. The 6xxx series alloys are easily and economically extruded and for this reason are most often found in an extensive selection of extruded shapes. These alloys form an important complementary system with the 5xxx series alloy. The 5xxx series alloy used in the form of plate and the 6xxx are often joined to the plate in some extruded form. Some of the common applications for the 6xxx series alloys are handrails, drive shafts, automotive frame sections, bicycle frames, tubular lawn furniture, scaffolding, stiffeners and braces used on trucks, boats and many other structural fabrications.
Zinc (Zn) 7xxx – The addition of zinc to aluminum (in conjunction with some other elements, primarily magnesium and/or copper) produces heat-treatable aluminum alloys of the highest strength. The zinc substantially increases strength and permits precipitation hardening. Some of these alloys can be susceptible to stress corrosion cracking and for this reason are not usually fusion welded. Other alloys within this series are often fusion welded with excellent results. Some of the common applications of the 7xxx series alloys are aerospace, armored vehicles, baseball bats and bicycle frames.
Iron (Fe) – Iron is the most common impurity found in aluminum and is intentionally added to some pure (1xxx series) alloys to provide a slight increase in strength.
Lead (Pb) and Bismuth (Bi) – Lead and bismuth are added to aluminum to assist in chip formation and improve machinability. These free machining alloys are often not weldable because the lead and bismuth produce low melting constituents and can produce poor mechanical properties and/or high crack sensitivity on solidification.