Mechanisms for Strengthening Aluminum

 

Abstract:  This article discusses the different aluminum alloy families and  the different methods for strengthening aluminum.  This includes a discussion of cold working, solid solution strengthening, precipitation strengthening, and dispersion strengthening.  This article is an abbreviated version of our on-demand course Aluminum Metallurgy.

Aluminum is the second most commonly used metal after steel.  Common engineering applications of aluminum include aerospace, automotive, buildings, and soda and beer cans.  Aluminum has some unique properties: it is very light compared to steel, it has very good electrical and thermal conductivity, and it does not rust like steel if left in air.  However, pure aluminum is soft.  So, strengthening aluminum is required in order to use it for engineering structures.

This article explains about the different families of aluminum alloys and the metallurgical mechanisms for strengthening aluminum alloys.

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Families of Aluminum Alloys

There are several families of wrought aluminum alloys.  Each family is based on specific major alloying elements added to the aluminum.  These alloying elements have a large influence on the properties.  The different families of alloys and the major alloying elements are

  • 1xxx: no alloying elements
  • 2xxx: Copper
  • 3xxx: Manganese
  • 4xxx: Silicon
  • 5xxx: Magnesium
  • 6xxx: Magnesium and silicon
  • 7xxx: Zinc, magnesium, and copper

The first number in the alloy designation indicates the particular alloy family. Within each family there are different alloys based on the amounts of the major alloying elements present and the types and amounts of minor alloying elements that have been added. The XXX’s are used to indicate the different alloys in each family.

The strength of aluminum alloys can be modified through various combinations of cold working, alloying, and heat treating. All the alloys can be strengthened by cold working processes such as cold rolling or wire drawing. Except for the 1xxx alloys, additional strength can be obtained by solid solution strengthening, dispersion strengthening, and precipitation strengthening. The particular strengthening mechanisms possible depend on the alloy.

This table shows the maximum nominal yield and tensile strengths for the different alloy families and the methods by which the strength is increased. There is a wide range of strengths possible with aluminum alloys. The yield and tensile strengths possible in the different alloy families depends on the strengthening mechanisms available.

Alloy series Methods for increasing strength Yield Strength ksi (MPa) Tensile Strength, ksi (MPa)
1xxx Cold-working 4-24 (30-165) 10-27 (70-185)
2xxx Cold-working, Precipitation 11-64 (75-440) 27-70 (185-485)
3xxx Cold working, solid solution, dispersion 6-36 (40-250) 16-41 (110-285)
4xxx Cold working, dispersion 46 (315) 55 (380)
5xxx Cold working, solid solution 6-59 (40-405) 18-63 (125-435)
6xxx Cold working, precipitation 7-55 (50-380) 13-58 (90-400)
7xxx Cold working, precipitation 15-78 (105-540) 33-88 (230-605)
Cold working

Cold working involves the reduction in thickness of a material. Plate and sheet of different thickness are produced by cold rolling. Wire and tubes of different diameter and wall thickness are produced by drawing. All aluminum alloys can be strengthened by cold working.

During the cold working, the strength of a metal increases due to the increase in the number of dislocations in the metal compared to its pre-cold-worked condition.  Dislocations are defects in the arrangement of atoms within a metal (discussed in Principles of Metallurgy).

The increase in the number of dislocations due to cold working is responsible for the increase in strength. Pure aluminum at room temperature has yield strength of 4 ksi (30 MPa).  In the fully cold-worked state the yield strength can be as high as 24 ksi (165 MPa).

Solid solution strengthening

Certain alloying elements added to aluminum mix with the aluminum atoms in a way that results in increased metal strength.  This mixture is called a solid solution because the alloying atoms are mixed in with the aluminum atoms. This is discussed in detail in Principles of Metallurgy and Aluminum Metallurgy. The extent of strengthening depends on the type and amount of the alloying elements.  Manganese and magnesium are examples of elements added to aluminum for the purpose of strengthening. Solid solution strengthening occurs in 3xxx and 5xxx alloys through the addition of manganese (3xxx) and magnesium (5xxx) to aluminum.

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Precipitation strengthening

Al2Cu precipitates in an aluminum matrix.
© DoITPoMS Micrograph Library, Univ. of Cambridge

With precipitation strengthening, particles less than 0.001 mm in diameter form inside the metal.  These particles are called precipitates and consist of compounds of aluminum and alloying elements or compounds of the alloying elements.  This figure shows Al-Cu precipitates in an Al-Cu alloy.

Precipitates form as a result of a series of heat treating processes.  The step of the process during which precipitates form is called aging.

Precipitation strengthening can increase the yield strength of aluminum from about five times up to about fifteen times that of unalloyed aluminum.   The strength depends on the specific alloy and the aging heat treatment temperature.

Only certain alloys can be precipitation strengthened.  The 2xxx, 6xxx, and 7xxx alloys can be precipitation strengthened through the formation of Al-Cu (2xxx), Mg-Si (6xxx), and Al-Zn-Mg-(Cu) (7xxx) precipitates.  The 1xxx, 3xxx, 4xxx, and 5xxx alloys cannot be precipitation strengthened.

Dispersion strengthening

Dispersoid particles form during the aluminum casting process when manganese in 3xxx series alloys reacts with aluminum and iron and silicon. These particles are less than 0.001 mm in diameter.  Dispersoid particles influence the grain structure that forms during heat treating so that there is increased strength compared to an alloy without dispersoids.  Fully-annealed 1100 aluminum has tensile strength of 13 ksi and yield strength of 5 ksi.  Fully-annealed 3003 has minimum tensile strength of 16 ksi and minimum yield strength of 6 ksi.  This increase in strength is due to the grain structure formed as a result of the presence of dispersoids.

 Additive strengthening

Finally, the methods of strengthening aluminum discussed here are often combined to provide even higher strength alloys.  Solid solution strengthened alloys are often cold-worked and precipitation strengthening is sometimes combined with cold working prior to the aging step.

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