Selection of aluminum alloys: metallurgical factors

Industrial aluminum processing technologies

Most aluminum products and semi-finished products, which are used in mechanical engineering, construction and other engineering fields are shaped, which they receive by methods of metal forming by pressure - rolling, pressing (extrusion), drawing, forging, punching. Examples are rolled sheet (thick, thin and foil), extruded profile, trumpet, bar and wire. aluminum alloys, which are processed by these methods, called deformable.

Almost all other aluminum products – these are mainly cast products, which are obtained by various casting methods, such as sand casting, chill casting, high pressure die casting, low-pressure casting, investment casting and others. aluminum alloys, which are used for casting, called foundries.

A relatively small amount of aluminum alloys and products from them are obtained by special methods, such as hot isostatic treatment, powder technology, ultrafast solidification method and others.

Wrought alloys

The number of wrought aluminum alloys, which the design engineer has at his disposal, is very large. All wrought aluminum alloys are classified into:

  • thermally unhardenable alloys and
  • thermally hardenable alloys.

Before considering the characteristics of these two classes of wrought aluminum alloys, it is useful to recall the modern international system of designation of the deformable alloys themselves and their states, that is, that technological processing, which they receive during the manufacture of products and semi-finished products.

Designation of wrought alloys

The international designation system for wrought aluminum alloys subdivides all wrought alloys into eight classes (series, groups) for the main alloying elements (table 1).

Table 1 - International classification and
notation system for wrought aluminum alloys [1]

In this international designation system for wrought aluminum alloys, which consists of four numbers:

  • the first digit identifies the main alloying elements
  • the other three digits are used as serial numbers to identify the various individual alloys.

The 1xxx series formally includes non-alloys (no alloying elements), and aluminum grades. Certain information is included in the designation of aluminum grades. for instance, in the designation of industrial grade aluminum grades, the last three digits xxx indicate the grade of purity: 1080 indicates the purity of aluminum 99,80 %. In other series of aluminum alloys (2xxx, 3xxx, 4xxx, 5xxx, 6xxx, 7xxx and 8xxx) numbers xxx do not carry direct information about the chemical composition of a particular alloy. To do this, you need to refer to standards and other regulatory documents..

Tempers of aluminum alloys

The same alloy can be processed by different technological operations: heat treatment, cold deformation processing (autofrettage) and their combinations. As a result of such processing, the alloy can obtain different mechanical properties., microstructure and other performance characteristics. These features of the technological processing of aluminum alloy and the properties obtained by it as a result of this processing reflect the characteristics of the alloy, which is called "alloy state".

International classification of states of aluminum alloys is presented in the table 2.

Table 2 - International classification
states of aluminum alloys [1, 2]

Groups of states (basic states) of deformed aluminum alloys

State F:

  • Applies to wrought alloy products and semi-finished products and cast products, which are made without special control of thermal conditions or the degree of work-hardening. For products made of wrought alloys, mechanical properties are not controlled.

States About:

  • Applies to wrought alloy products, which anneal, to obtain a condition with minimum strength and to cast products, which anneal, to improve plastic properties and stabilize dimensions. The letter O may be followed by a number, nonzero.

State H:

  • Applies only to wrought alloy products. It indicates products, which have received work hardening by cold deformation with additional heat treatment for some reduction in strength or without it. The letter H is always followed by two or more digits, which reflect the degree of autofrettage and additional operations. for instance, Н24 indicates that the aluminum product (semi-finished product) was subjected to cold working to the degree of ½, then partial reduction annealing. See. more here.

States T:

  • It is used for products made of thermally hardened wrought and cast alloys. The letter T can be followed by one to four digits. for instance, T64 - treatment (heating) for solid solution and incomplete artificial aging (to increase plasticity compared to the T6 state). See. more here.

Reference

Heat treatment for solid solution [2]:

  • To maximize the efficiency of thermal hardening by precipitating hardening phases, you must first get a solid solution. Process, as a result of which this is achieved, called solution heat treatment. The purpose of this treatment is to solidify the maximum amount of soluble strengthening elements of the alloy.. It consists in keeping the alloy at a sufficiently high temperature and for a time, which is sufficient to obtain an almost homogeneous solid solution.

Stabilization heat treatment (stabilization) [2]:

  • This processing is applied, to stabilize the properties of an aluminum product at a temperature slightly higher, than the real temperature, in which this product will be used. This prevents a change in the properties of the product when it is heated in operation.. This treatment is also used for cast products to reduce residual stresses..

Full designation aluminum alloy

The complete designation of the alloy consists of the designation of the alloy and the designation of its condition. This entry is spelled differently in different regions and countries:

  • Hyphenated: 6060-T6 - North America
  • Space separated: 6060 T6 - Europe
  • Through a dot: AD31.T5 - for example, GOST 8617-2018
  • Continuously: AD31T5 - for example, SNIP 2.03.06-85 Aluminum structures (SP 128.13330.2011).

In our opinion, “American” the most convenient and unambiguous option.

Selection of wrought aluminum alloys

Wrought aluminum alloys for products are selected on the basis of:

  • properties, required for the product during its service life
  • degree of complexity of product manufacturing
  • total alloy cost, its availability on the market, heat treatment technology and surface finishing

Wrought aluminum alloys are divided into:

  • Non-thermally hardened alloys
  • Thermally hardenable alloys

Non-thermally hardened alloys

Non-thermally hardened alloys can be hardened by solution treatment and / or by cold deformation working (autofrettage), but not able to harden due to the aging mechanism, that is, precipitation of strengthening phases.

  • Aluminum grades 1xxx: technically pure aluminum (electrical conductors, chemical equipment, decorative architectural elements). The tensile strength in the annealed state is low (about 50 MPa) and can be doubled by cold working.
  • Alloys 3xxx: aluminum-manganese ("beer" cans, car radiators). They are used for products with moderate strength (about 180 MPa) with good plasticity (for ease of forming products) and good corrosion resistance. Alloy 3003 in the form of sheets is used for the manufacture of automotive radiators. Alloy 3004, with manganese and magnesium, used for the manufacture of cans for soft drinks and beer.
  • 5xxx alloys: aluminum-magnesium: used in transport engineering, shipbuilding, automotive industry, in decorative building elements. High strength and high corrosion resistance. Weld well.

Thermally hardenable alloys

These are alloys of the 2xxx series, 6xxx, 7xxx and 8xxx, which gives ample opportunities for choosing a suitable alloy very wide.

  • Alloys series 2xxx, especially those, which are based on the aluminum-copper-magnesium system, found application in load-bearing structural elements of aircraft.
  • Alloys of the 6xxx series (aluminum-magnesium-silicon) are widely used as profiles with moderate strength in construction, mechanical engineering and other areas of industry.
  • Alloys of the 7xxx series (aluminum-zinc-magnesium) provide high strength for critical structures and structures.
  • Special heat-hardenable alloys of the 8xxx series include an alloy 8001 (Al-1,1Ni-0,6Fe) for nuclear power plants for operation in water at high temperatures and high pressure and alloy 8011 (Al-0,75Fe-0,7Si), which is capable of deep drawing for fabrication, for example, bottle caps [1].

Casting alloys

International designation system for cast aluminum alloys

International classification and designation system of cast aluminum alloys is presented in the table 3.

Table 3 – International classification and
designation system for cast aluminum alloys [1]

Like wrought alloys, casting alloys are also subdivided into:

  • thermally unhardened and
  • thermally reinforcing.

When choosing a casting alloy, the following factors are considered:

  • type of casting technology
  • requirements for mechanical and other properties
  • cost of

Casting alloys, in which silicon is the main alloying element, are the most important for applications in various engineering fields. These alloys are characterized by high fluidity, since in terms of silicon content they are close to the eutectic chemical composition. These castings have high corrosion resistance combined with low thermal expansion and good weldability..

The number after the dot in the designation of the cast aluminum alloy

Numeral, which goes in the designation of the casting alloy after the point, indicates the shape of the product [3]:

  • The number zero (0) after the dot indicates the cast product itself (casting)
  • The number one (1) after the dot indicates the chemical composition limits for the ingot., which is used for the manufacture of cast products xxx.0.
  • The number two (2) after the dot indicates an ingot, which is used to manufacture the product xxx.0, but an ingot of a different chemical composition (usually narrower), than ingot xxx.1. Although not always, xxx.1 often indicates the chemical composition limits for the secondary alloy, while XXX.2 indicates the chemical composition limits for the primary alloy.

Important to remember, that the "0" after the dot indicates the casting, while "1" or "2" indicates the chemical composition of the ingots, which is required for the manufacture of this casting product (casting).

Since melting and melt processing can change the chemical composition of the alloy, which is prepared for making castings, chemical composition requirements xxx.1 or xxx.2 are always somewhat more stringent, than for the requirement for the chemical composition xxx.0 for the finished casting. According to the accepted agreement, ingots xxx.2 always have stricter chemical composition limits., than ingots xxx.1.

Not all alloys have both ingot shapes - xxx.1 and xxx.2. Many more traditional high pressure casting alloys will only have an xxx recycled ingot.1, and many alloys for casting high-quality castings have only ingot xxx.2.

Cast aluminum alloy designation letters

The numeric designation of the casting alloy may be preceded by a letter, for example, A360.0. The letters indicate some deviations from the originally developed alloy, possibly a version with a lower content of impurities or an additional controlled element.

Application of cast aluminum alloys

1xx series alloys

Alloys of the 1xx series are industrially used for casting rotors of electric rotors. Rotors are usually cast on vertical high pressure die casting machines, which are specially designed for this purpose. Due to the high electrical conductivity of commercially pure aluminum, it is used for the manufacture of electrically conductive busbars They are cast together with a steel sheath, which is pre-installed in the mold.

2xx series alloys

Series 2xx alloys include the highest strength aluminum cast alloys available today. Alloys of the 2xx series retain their strength better at elevated temperatures, than other cast aluminum alloys.

3xx series alloys

The 3xxx series alloys are the real workhorses of the aluminum foundry industry due to their high casting characteristics and good strength.. The most used of them are Al-Si-Cu alloys, which, with an increased copper content, are completely thermally hardened.

Alloy group 380 for a long time it was used in an overwhelming amount (over 85 %) for high pressure casting. These secondary (scrap aluminum) alloys have been specially developed for high pressure casting. They contain more silicon and iron, and also allow more impurities, than alloys, which are intended for casting by other methods. These alloys provide a good balance between low cost, moderate strength without the need for heat treatment and good casting characteristics. Magnesium in these alloys is usually very low controlled, to minimize the formation of oxides during very turbulent filling of the casting chamber. but it is believed, that the low content of magnesium (about 0,3 %) can significantly increase the hardness and machinability [3].

4xx series alloys

Alloys of the 4xx series are used then, when good casting properties are required, as well as higher corrosion resistance, than alloys of the 3xx series, which contain copper. They are used to make castings for seawater service., equipment for the food industry and components for the chemical industry, as well as such products, as the base of street lighting poles [3].

5xx series alloys

5xx series alloys have the highest corrosion resistance of all cast aluminum alloys. They also polish well to a mirror finish, well anodized with a pleasant natural aluminum look. Therefore, they are used for the manufacture of decorative castings., as well as cast products, which are used in the dairy and food industries, pipe fittings for marine and chemical equipment.

5xx alloys require more attention in preparation and casting, than low magnesium alloys, this is how they show high reactivity in the presence of oxygen, moisture in the atmosphere, lubricants, etc.. P.

Similar to 2xx alloys, 5xx alloys are characterized by low fluidity and tendency to hot cracking.. Therefore, conventional 5xx series alloys are rarely used for high pressure casting., although some special 5xx alloys are also used for high pressure casting [3].

7xx series alloys

7xx series alloys have good impact resistance and achieve a sufficiently high strength without the need for heat treatment. 7xx series alloys age gradually at room temperature, but gaining maximum strength during 20-30 days after casting. They are popular for making large machine parts., furniture, garden tool, parts of trucks and mining equipment.

Alloys of the 7xx series have the highest solidus temperature of all cast aluminum alloys., except for rotor alloys of the 1xx series. Therefore, they are suitable for joining parts by soldering [3].

8xx series alloys

8xx series alloys are used exclusively for casting bushings and support bearings. They have high compressive strength and unique lubricating properties under overheating conditions. The only ingredient in 8xx alloys is tin.. Tin is in the solidified casting in the form of small globules of pure tin. If there is no normal lubrication and overheating occurs in the bearing, then the tin phase melts at 231 ºС and seeps out of the overheated surface and provides emergency lubrication from liquid tin and thereby prevents catastrophic destruction of the system [3].

Sources:

  1. TALAT 1255 Metallurgical Background to Alloy Selection and Specifications for Wrought, Cast and Special Applications / M. H. Yacobs – 1999
  2. Aluminum and Aluminum Alloys /ed. J.R. Davis - ASM International, 1993
  3. Aluminum Cast Alloys: Enabling Tools for Improved Performance / D. Apelian – NADCA, 2009