The different methods for strengthening aluminum make it possible to achieve a wide variety of strengths, making aluminum a versatile material that can be used in a wide variety of applications. Because of this, 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 good corrosion resistance, it has very good electrical and thermal conductivity, and it can be engineered to have a wide range of strengths.
This article explains about the different families of aluminum alloys and the metallurgical mechanisms for strengthening 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
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.
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.
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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.
Dislocations are line defects within the crystal lattice of a metal. They are essentially imperfections that disrupt the regular arrangement of atoms. While they may seem detrimental, they play a crucial role in the mechanical behavior of metals, particularly in terms of their strength and ductility.
When a metal is subjected to stress, dislocations can move through the crystal lattice. This movement, known as slip, allows the metal to deform plastically. The stress required to initiate plastic deformation is known as yield strength.
To improve the strength of a metal, it is necessary to hinder the movement of dislocations. Several techniques can be employed such as grain size reduction, alloying, cold working, and heat treating. How alloying, cold working, and heat treating are used to strengthen aluminum alloys is discussed next. More information about dislocations and metal strengthening is in our online course Principles of Metallurgy.
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. Actually, cold working is not only used for strengthening aluminum. It is also used to strengthen many other alloys.
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.
The increase in the number of dislocations due to cold working is responsible for the increase in strength. Dislocations can interfere with each others movement through a metal. Increasing the number of dislocations results in more interference, resulting in a strength increase. Pure aluminum at room temperature has yield strength of 4 ksi (30 MPa). In the fully cold-worked state its yield strength can be as high as 24 ksi (165 MPa).
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. The presence of the alloying elements distorts the atomic bonds between the aluminum atoms. The distortion impedes dislocation motion, resulting in a strength increase as the concentration of alloying element increases in the alloy. 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 aluminum. Solid solution strengthening occurs in 3xxx and 5xxx alloys through the addition of manganese (3xxx) and magnesium (5xxx) to aluminum.
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. Precipitates interfere with the motion of dislocations through a metal, resulting in an increase in strength compared to the same alloy without precipitates. 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.
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.
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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Michael Pfeifer, Ph.D., P.E. is President of Industrial Metallurgists and a metallurgical engineering consultant with more than 30 years of experience in product design, manufacturing, failure analysis, and metallurgy training. He helps clients worldwide solve design and manufacturing problems and is the author of Materials Enabled Designs.
