Aluminum Alloy Families: Strength Metallurgy
There are many different types of wrought aluminum alloys used in a wide variety of applications such as wire for power distribution, automobile radiators, airplane fuselage, fasteners, and soda cans. The main criteria for selecting a particular alloy are strength, electrical conductivity, corrosion resistance, ease of manufacturing and assembly, and cost. This article discusses the effects of alloy composition on strength.
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 and magnesium and 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.
For the 1xxx group, 10xx is used to designate unalloyed compositions. The last two digits in the designation indicate the impurity content. Designations having a second digit other than zero indicate special control of one or more impurity elements.
For the 2xxx through 7xxx alloy groups, the second digit indicates alloy modification. A second digit of zero indicates the original alloy. Integers 1 through 9 indicate modifications of the original alloy. The last two digits have no special significance other than to identify the different aluminum alloys in the group.
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. Furthermore, except for 1xxx alloys, additional strength can be obtained by solid solution strengthening, dispersion strengthening, and precipitation strengthening. The particular strengthening mechanism possible depends on the alloy.
Solid solution strengthening occurs in 3xxx and 5xxx alloys through the addition of manganese (3xxx) and magnesium (5xxx). Aluminum can hold more magnesium than manganese in solid solution. Consequently, greater solid solution strengthening is possible in 5xxx alloys than in 3xxx alloys. However, in the 3xxx alloys, the formation of Al-Mn-Si particles results in increased strength. These particles are obstacles to dislocation motion through the metal.
The alloys can be divided into two groups based on whether the alloys can be precipitation strengthened. The 2xxx, 6xxx, and 7xxx alloys can be precipitation strengthened and the 3xxx, 4xxx, and 5xxx alloys cannot. Precipitation strengthening in 2xxx, 6xxx, and 7xxx alloys occurs through the formation of Al-Cu (2xxx), Al-Mg-Si (6xxx), and Al-Zn-Mg-(Cu) (7xxx) sub-micron sized particles in the alloys. The particles form as a result of a series of heat treating processes and are obstacles to dislocation motion through the metal.
The yield and tensile strengths possible in the different alloy families depends on the strengthening mechanisms available. The 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.
Selecting a particular alloy requires consideration of other requirements such as electrical conductivity, corrosion resistance, ease of manufacturing and assembly, and cost. A future article will discuss the general properties of the alloys from the different alloy families.
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