Aluminum alloys: selection and application

Alloys based on aluminum

Terms and Definitions

Aluminum alloy - base alloy of aluminum – aluminum alloy – this aluminum, which [1]:

• It contains one or more alloying elements, as well as some impurities;
• aluminum predominates by weight for each of the other chemical elements;
• aluminum content does not exceed 99,00 %.

alloying element:

• metallic or non-metallic element, which is controlled within certain upper and lower limits. It is introduced into an aluminum alloy to give it certain special properties [1].

Impurity:

• metallic or non-metallic element, which is present in the alloy, which is not controlled by the minimum content. The aluminum alloys, usually, the maximum concentration of impurities is controlled [1].

Doping in aluminum alloys

The most important alloying elements, which are used to convert the aluminum alloys with special properties - and wrought, and foundries (of course, in different quantities) – are:

• silicon (Si),
• magnesium (Mg),
• manganese (Mn),
• copper (Cu) and
• zinc (Zn).

The effect of, for example, copper content in aluminum alloy on its mechanical properties shown 1.

Figure 1- The effect of alloying an aluminum alloy with copper on mechanical properties [3]

Iron in aluminum alloys

Wrought aluminum alloys contain about 0,1 – 0,4 % (by mass) iron (Fe). Iron is usually regarded as an undesirable impurity. Its content depends on the quality of the original ore (bauxite) and the technology of electrolytic reduction. Sometimes doping with iron is used to obtain special properties of the material, for example, for the manufacture of aluminum foil.

Modifying alloys

In combination with the main alloying elements, other alloying elements are often used:

• bismuth (Bi)
• pine (B)
• chrome (Cr)
• lead (Pb)
• nickel (Ni)
• titanium (Ti) and
• zirconium (Zr).

These elements are typically used in small amounts, to 0,1 % by weight. At the same time, brown, lead and chromium can be up to 0,5 %, to give them specific properties, modify alloys to achieve special properties. Casting qualities can be special properties, workability, heat endurance, corrosion resistance, strength, etc..

Classification of aluminum alloys

Classification of aluminum alloys – aluminum alloys – produced by various criteria, including:

• Treatment method – casting and wrought
• mechanism of strengthening – thermally hardening and deformation hardening
• for major alloying elements

Two categories: casting and wrought

Two categories of aluminum alloys

• foundry
• wrought

A casting aluminum alloy – aluminum alloy, which is designed primarily for the production of castings.

Wrought aluminum alloy – aluminum alloy, which is intended primarily for the production of aluminum products hot and / or cold pressure treatment.

Wrought alloys

Wrought aluminum alloys are first cast into ingots (round or rectangular), and then treated with different technologies Forming – hot and cold – before giving them the desired shape:

• rolling - to obtain sheets and foil;
• extrusion - to obtain extruded profiles, pipes and rods;
• molding - to obtain more complex shapes from rolled or pressed semi-finished products;
• forging to produce complex shapes with high mechanical properties,
  and:
• drawing, punching, landing, hood, is rolled, distribution, flexible, etc.. P.

Popular wrought aluminum alloys 6xxx series, that apply for the production of extruded aluminum profiles, are presented below in Figure 7.

Figure 7 – Basic aluminum alloys 6xxx series

Casting alloys

Casting aluminum alloys in a molten state is poured directly into their final shape by one of various methods, such as, sand casting, casting molds or injection molding. When casting molds used complex. These alloys often have a high silicon content to improve their castability.

For these two categories of aluminum alloys, the classification for alloying alloys is similar, because the alloying elements in them are the same, but

but in different quantities, in cast alloys usually more.

The strength and other mechanical properties of aluminum alloys, like deformed, and casting, They are determined mainly by their chemical composition, t. it is. content of alloying elements in aluminum, and harmful impurities. However, these properties can be changed to achieve optimal combination through additional processing of alloys - or thermal deformation, or he, and other. As a result, the alloy changes its initial mechanical properties, and receives its final temper, which is delivered to the customer and. Hardening heat treatment is used as a casting, and to the deformed alloy, In this case they are called alloys, hardening heat treatment.

Two strengthening mechanism

Two classes of aluminum alloys:

• heat-treatable
• strain hardenable (cold work)

Thermally hardenable alloys

Thermally hardenable alloy – alloy, which can be hardened by appropriate heat treatment (Figures 2, 3 и 4).

Figure 2 – Hardening and aging hardening of aluminum alloys [2]

Figure 3 – Typical heat hardening by aging [4]

Figure 4 – thermal hardening effect on the mechanical properties of the alloy 7075 [4]

Cold-worked alloys

Work hardened alloy (“thermally neuprochnyaemy”, work-hardened) – alloy, which is hardened only by deformation processing (figures 5 и 6), instead of heat treatment.

 

Figure 5 – Effect of cold plastic processing – nagartovki – strength, hardness and ductility of aluminum alloys [2]

Figure 6 – Work-hardening curves (strain hardening)
thermally unhardened aluminum alloys [4]

Series and alloying system

• All aluminum alloys – and wrought , and casting – divided into the main alloying elements of the series.
• Each series aluminum alloys, wrought and cast, They include one, two or three different alloying system.
• The alloying system can include only the main alloying element (highlighted below in bold) or one or more additional alloying elements.

Series wrought alloys

• 2xxx - Al-Cu, Al-Cu-Mg, Al-Cu-Mg-Si, Al-Cu-At the
• 3xxx – Al-Mn
• 4xxx – Al-Si
• 5xxx – Al-Mg
• 6xxx – Al-Mg-Si
• 7xxx – Al-Zn, Al-Zn-Mg, Al-Zn-Mg-Cu
• 8xxx – Al-Fe, Al-Fe-Ni, Al-Li-Cu-Mg

Series cast alloys

• 2xx – Al-Cu, Al-Cu-Ni-Mg, Al-Cu-Si,
• 3xx – Al-Si-Cu, Al-Si-Cu-Mg, Al-Si-Mg
• 4xx – Al-Si
• 5xx – Al-Mg
• 7xx – Al-Zn
• 8xx – Al-Sn

Aluminum alloys in designs

Rating Strength Aluminum Alloys

Unalloyed aluminum has tensile strength of about 90 MPa. However, small additions of alloying elements, such, as copper, magnesium,, manganese, tobelts, zinc, no large amounts of certain other elements prepared aluminum alloys.

Aluminum alloys provide for, to obtain aluminum with special properties, for example, with higher mechanical properties (figures 8 и 9).

Figure 8 – Strength rating of wrought aluminum alloys [2]

Figure 9 – The influence of alloying elements on the tensile strength, hardness, impact sensitivity and ductility [5]

The choice of alloy

When selecting an aluminum alloy as a structural material, primary factor is to ensure the strength of the manufactured structural member thereof. However, the structural strength of various types of elements provide different properties of the same structural material.

for instance, strength "thick" column will depend mainly on the yield strength of the metal, whereas the strength of "thin" column will depend mainly on the modulus of elasticity of the material. Since the yield strength of aluminum alloys often comparable yield points of ordinary structural steel, then aluminum could well compete with them for a "thick" columns. On the other hand, because the elastic modulus of aluminum and its alloys is only somewhere from the third elasticity modulus steels, the aluminum can hardly compete with steel in the "thin" columns.

Strength, However, It is not the only characteristic of the work or product design. Such additional factors, as corrosion resistance, ease of processing (compressibility or weldability), stiffness (modulus of elasticity), plastic fracture (elongation), weight (density), fatigue strength, and the cost, must to some extent be considered when choosing the right construction material.

Economy aluminum construction

Often, the material cost is a critical factor. However, a comparison of aluminum alloys and steels based on the value of a unit mass or volume can be misleading, as they have different strengths, density and other properties.

If the material cost was the only factor and carbon steel can be used without protective anti-corrosion coating, always and everywhere be used once they. However, the choice of material taken into consideration other factors, such as the cost of operation and maintenance during the entire life of the structure. Moreover, in certain specific circumstances "rule" about, the aluminum element twice lighter steel is not always true. for instance, aluminum component may weigh considerably less, If the thickness of the steel member needs to be increased in view of its possible impact on reducing too aggressive corrosion throughout the service life.

If the required profiles with complex cross-sections, as, for example, in enclosing facade constructions, in such cases, the cost of the steel member is much more, than the value of its material. The thing is, that it must be machined for making this element of steel billets, subjected to cold forging or bending, a, may be, and apply welding. At the same time production of aluminum profiles the cost is only a fraction of the cost of "raw" aluminum.

Due to the high cost of stainless steel, they are used only, if the weight of the element or structure does not matter, but important appearance and weldability. Usually, when stainless steel is used instead of aluminum, the cause is often only one - limiting aluminum alloys Welding.

Aluminum alloys of the Eurocode 9

Aluminum alloys offer design engineers a wide range of materials. Each alloy has its own special characteristics, which serve to provide given properties. When corrosion resistance, high strength to weight ratio and ease of manufacture are important design parameters, whereas aluminum alloys merit serious consideration.

The tables 1 и 2 wrought aluminum alloys are presented, that Eurocode 9 recommends and permits for use in buildings and structures (see. more here).

Table 1 – Thermally neuprochnyaemye aluminum alloys by Eurocode 9

Table 2 – Heat-treated aluminum alloys according to Eurocode 9

1. Guidance GAG Guidance Document 001 Terms and Definitions Edition 2009-01 March 2009
2. The welding of aluminium and its alloys / Gene Mathers – Woodhead Publishing Ltd, 2002
3. Aluminum and Aluminium Alloys / ed. Davis – ASM International, 1996
4. Aluminum and Aluminum Alloys – Subject Guide – ASM International, 2015
5. TALAT 1501