Aluminum Archives - Industrial Metallurgists

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.

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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.

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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.

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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.

© DoITPoMS Micrograph Library, University of Cambridge.

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.

Data from Aluminum Standards and Data 2009, The Aluminum Association

Data from Aluminum Standards and Data 2009, The Aluminum Association

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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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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The strength of metals is improved by impeding the motion of dislocations through the metals. One approach to achieving this improvement is to form a uniform distribution of closely spaced sub-micron sized particles throughout an alloy. The particles, which are called precipitates, impede dislocation motion through the alloy. Not every alloy can be precipitation strengthened. Alloys that can be precipitation strengthened include Al-Cu, Al-Mg-Si, Cu-Be, and 17-8 PH steel. The figure shows precipitates in a Al-Cu alloy.

Al2Cu precipitates in a Al-4% Cu alloy. © DoITPoMS Micrograph Library, Univ. of Cambridge

Al2Cu precipitates in a Al-4% Cu alloy. © DoITPoMS Micrograph Library, Univ. of Cambridge

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The particles are formed by precipitation, which involves a series of heat treatment steps. The first step is solution heat treatment. This involves heating the alloy up to a temperature that results in the atoms of the alloying element being dissolved within the solid structure formed by the array of atoms of the main element. For Al-Cu alloys, the copper atoms dissolve into the array of aluminum atoms. The dissolved structure is then retained at ambient temperatures by cooling the alloy rapidly, such as by water quenching.

After cooling, precipitates are formed either by natural aging or artificial aging.  With natural aging, the precipitates form at room temperature. With artificial aging, the precipitates form when an alloy is heated to a temperature lower than the solution heat treatment temperature. Only certain alloys will undergo natural aging. The other alloys must be artificially aged.

Regardless of the aging process, as the precipitation process proceeds the precipitates go through a series of stages, with changes in the size, form, and composition of the precipitates. The particular stage of the precipitates has a direct influence on the strength of the alloy. For artificially aged alloys, this is controlled by the aging temperature and time. At any particular aging temperature, there is an aging time at which the alloy will reach its maximum strength. This maximum strength corresponds to a specific stage of the form and composition of the precipitates. Aging for a time that is too short or too long will result in less than maximum alloy strength.

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For artificially aged alloys, the aging temperature affects the maximum strength that can be obtained, and the time required to reach maximum strength. For naturally aged alloys, the strength increases over time. The time required to reach maximum strength depends on the alloy.

Finally, precipitation strengthening can be combined with cold-working to give even greater alloy strength.

More information about the metallurgy of precipitation strengthening and precipitation strengthening heat treatment is in our Precipitation Strengthening course. Also, Heat Treatment: Structure and Properties of Nonferrous Alloys by C. R. Brooks, Precipitation Hardening by J.W. Martin, and ASM Handbook, Volume 4: Heat Treating discusses precipitation strengthening.

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The metallurgical phases present in an alloy have a huge impact on the properties of a metal component.  Phases are distinct materials that are comprised of the elements in the alloy.  These distinct materials have distinct properties that have an impact on the overall properties of the entire alloy.  Additionally, the size, shape, and location of the phases within the alloy also effect on the overall properties of an alloy.

Within many common alloys it is possible to alter the phases present with heat treatment.  Forming one or more phases from a different phase is called a phase transformation.   Phase transformations occur when in an alloy is heated or during cooling from an elevated temperature.

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There are several types of phase transformations that can occur when an alloy is cooled from an elevated temperature.  The type of transformation that can occur depends on the specific alloy.  Not all types of transformations occur in all alloys, and in some alloys no transformations are possible, other than the solid-liquid transformation.

Two of the most common phase transformations encountered with common alloys are eutectoid and precipitation.  For both types of transformation, the transformation involves the movement of atoms through the metal to rearrange themselves to form the new phase or phases.

Ferrite (white) and cementite (dark) in steel.

Ferrite (white) and cementite (dark) in steel.

A eutectoid transformation involves a change from a single phase to two other phases when the initial phase is cooled form an elevated temperature.  The most common alloy in which this transformation is encountered is steel.  The transformation occurs when steel is cooled from the austentizing temperature.  During slow to moderate cooling, the austenite transforms to ferrite and cementite.  The microstructure consists of cementite plates with ferrite between the plates.  This is commonly referred to as pearlite.  A micrograph of a steel alloy with 0.6% carbon is shown here.

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During faster cooling of some alloys, the ferrite bainiteforms in the shape of needles or plates and the cementite forms as particles.  This structure is referred to as bainite.  A micrograph of a steel with bainite is shown in Figure 2.

The reverse transformation occurs when steel with ferrite and cementite is heated.  When the temperature is high enough, the ferrite and cementite transform to austenite.  So, the austenite to ferrite + cementite phase transformation is reversible, and repeatable.

Precipitation transformations involve the formation of particles of one phase within an already existing phase.  These particles are called precipitates.  This transformation occurs when an alloy is cooled from an elevated temperature.  At the elevated temperature the phase present consists of the main element in the alloy with the alloying elements in solid solution.  When the alloy is cooled the solid solution is not able to hold all the atoms of the alloying elements in solution, so precipitates form that consist of the solute atoms and possibly the atoms of the main element in the alloy.

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

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

For engineered metal components, precipitation  during cooling is undesirable because of the resulting size and location of the precipitates.  So, the process is modified by first quenching the alloy to room temperature to suppress the atom motion.  Then the alloy is either allowed to transformation at room temperature, if room temperature transformation is possible, or the alloy is reheated to an intermediate temperature to speed up the transformation.  An example of a common alloy system in which precipitation is used is the aluminum-copper system.  This figure shows a micrograph of Al2Cu precipitates in an aluminum matrix.

The precipitation transformation occurs in a number of alloys including aluminum alloys (Al-Cu, Al-Mg-Si, Al-Zn-Mg, and Al-Zn-Mg-Cu), precipitation hardened steels (e.g. 17-4 PH, 15-5 PH, and 13-8), some copper alloys (Cu-Be and Cu-Cr), and Zn-Al alloys.

Regardless of the particular transformation, control of the heating temperature, heating time, cooling rate, and, if necessary, reheating temperature and time are all important factors for controlling whether the desired transformation is complete and the shape, size, and location of the phases that form.  These in turn have a big impact on the properties of a metal component.  The relationship between heat treating process conditions, final microstructure, and properties is discussed in our Metallurgy of Steel, Metallurgy of Steel Heat Treating, and Precipitation Strengthening courses.

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