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How and why alloying elements are added to aluminum

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Q - I have been informed that pure aluminum is not usually used for structural applications and that in order to produce aluminum that is of adequate strength for the manufacture of structural components, it is necessary to add other elements to the aluminum.  What elements are added to these aluminum alloys? What affect do they have on the material’s performance? And in what applications are these alloys used?

A - Your acquired information is essentially correct.  It would be very unusual to find pure aluminum (1xxx series of alloys) chosen for structural fabrication because of their strength characteristics.  Although the 1xxx series are almost pure aluminum, they will respond to strain hardening and especially so if they contain appreciable amounts of impurities such as iron and silicon.  However, even in the strain-hardened condition, the 1xxx series alloys have very low strength when compared to the other series of aluminum alloys. When the 1xxx series alloys are chosen for a structural application, they are most often chosen for their superior corrosion resistance and/or their high electrical conductivity.  The most common applications for the 1xxx series alloys are aluminum foil, electrical buss bars, metallizing wire and chemical tanks and piping systems.

The addition of alloying elements to aluminum is the principal method used to produce a selection of different materials that can be used in a wide assortment of structural applications.

If we consider the seven designated aluminum alloy series used for wrought alloys, we can immediately identify the main alloying elements used for producing each of the alloy series.  We can then go further and examine each of these elements’ effects on aluminum.  I have also added some other commonly used elements and their effects on aluminum.

Series              Primary Alloying Element

1xxx                 Aluminum - 99.00% or Greater

2xxx                 Copper           

3xxx                 Manganese

4xxx                 Silicon

5xxx                 Magnesium

6xxx                 Magnesium and Silicon

7xxx                 Zinc

The principal effects of alloying elements in 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.

Chromium (Cr) – Chromium is added to aluminum to control grain structure, to prevent grain growth in aluminum-magnesium alloys, and to prevent recrystallization in aluminum-magnesium-silicon or aluminum-magnesium-zinc alloys during heat treatment.  Chromium will also reduce stress corrosion susceptibility and improves toughness.

Nickel (Ni) – Nickel is added to aluminum-copper and to aluminum-silicon alloys to improve hardness and strength at elevated temperatures and to reduce the coefficient of expansion.

Titanium (Ti) – Titanium is added to aluminum primarily as a grain refiner.  The grain refining effect of titanium is enhanced if boron is present in the melt or if it is added as a master alloy containing boron largely combined as TiB2.  Titanium is a common addition to aluminum weld filler wire as it refines the weld structure and helps to prevent weld cracking.

Zirconium (Zr) – Zirconium is added to aluminum to form a fine precipitate of intermatallic particles that inhibit recrystallization.

Lithium (Li) - The addition of lithium to aluminum can substantially increase strength and, Young’s modulus, provide precipitation hardening and decreases density.

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.

Summary:

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.  Certainly one of the most important considerations encountered during the welding of aluminum is the identification of the aluminum base alloy type to be welded.  If the base material type of the component to be welded is not available through a reliable source, it can be difficult to select a suitable welding procedure.  There are some general guidelines as to the most probable type of aluminum used in different applications, such as those mentioned above.  However, it is very important to be aware that incorrect assumptions as to the chemistry of an aluminum alloy can result in very serious effects on the weld performance.  It is strongly recommended that positive identification of the type of aluminum be made and that welding procedures be developed and tested in order to verify weld performance.

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