Aluminum alloys: classification, properties and applications

Aluminum alloys are metallic materials obtained by introducing one or more alloying elements into aluminum to achieve a combination of mechanical, physical and technological properties unavailable in the pure metal. Pure aluminum is characterized by high ductility and corrosion resistance, but its strength is low, which limits its use in load-bearing structures. Alloying with copper, magnesium, silicon, zinc, manganese and other elements allows strength characteristics to be multiplied many times over while preserving the main advantage of aluminum — its low density. Aluminum alloys occupy a key place in the aerospace, automotive, construction, shipbuilding and electrical engineering industries, where the combination of lightness, strength and corrosion resistance is a determining factor.

Classification of Aluminum Alloys

Aluminum alloys are classified according to several key criteria: manufacturing technology, ability to be strengthened by heat treatment, and alloying system. By manufacturing technology, wrought and cast alloys are distinguished. Wrought alloys are intended for pressure treatment: rolling, extrusion, forging, stamping and drawing. They are used to produce sheets, strips, profiles, rods, pipes and panels. Cast alloys are intended for producing shaped castings by sand casting, permanent mold casting, die casting or investment casting. Cast alloys have lower ductility but are characterized by high fluidity and the ability to fill complex molds, which allows parts of complex geometry to be manufactured. There are also powder alloys obtained by powder metallurgy methods, but their industrial application is limited.

By the ability to be strengthened by heat treatment, alloys are divided into non-heat-treatable and heat-treatable. Non-heat-treatable alloys (e.g., Al-Mn and Al-Mg systems) are strengthened only by work hardening (cold plastic deformation) and do not increase in strength upon heating and cooling. Heat-treatable alloys (Al-Cu, Al-Mg-Si, Al-Zn-Mg systems) undergo phase transformations during heat treatment, which allows their strength characteristics to be significantly increased through the precipitation of strengthening particles.

International Designation System for Wrought Alloys

The most widespread international classification of wrought aluminum alloys is the Aluminum Association system, in which each alloy is designated by a four-digit numerical code. The first digit indicates the main alloying system, the second indicates the modification of the original composition (0 — original, 1–9 — modifications), and the last two digits indicate the unique number of the alloy within the series.

1xxx series — technically pure aluminum (at least 99% Al). It has the highest corrosion resistance, electrical and thermal conductivity, and excellent ductility, but low strength. It is used for the production of foil, packaging, kitchen utensils, electrical conductors and heat exchangers.

2xxx series — alloys with copper. They are characterized by high strength, comparable to some steels, but are inferior to other series in corrosion resistance. They require heat treatment to achieve optimal properties. Classic representatives are duralumins, widely used in aircraft construction for skin, load-bearing elements and rivets.

3xxx series — alloys with manganese. They have moderate strength and high corrosion resistance, and are excellent for forming and deep drawing. The main application area is the production of beverage cans, automotive radiators and construction panels.

4xxx series — alloys with silicon. They are characterized by a reduced melting point and good weldability, and are used as filler material for welding and brazing aluminum structures.

5xxx series — alloys with magnesium. They combine high strength, excellent corrosion resistance in marine environments and good weldability. When the magnesium content is above 3.5%, there is a tendency to stress corrosion cracking. They are used in shipbuilding, automotive (body panels), and for the production of welded structures and tanks.

6xxx series — alloys with magnesium and silicon. They provide a balanced combination of medium to high strength, good corrosion resistance and excellent extrudability. They are the most in-demand in construction for the production of window, door and facade profiles, as well as in automotive and general engineering.

7xxx series — alloys with zinc (often with additions of magnesium and copper). They achieve the highest strength among all aluminum alloys but are inferior to other series in corrosion resistance. They are used in critical aircraft structures, highly loaded parts of sports equipment and landing gear components.

8xxx series — alloys with other elements (iron, nickel, lithium, etc.) that are not included in the previous categories. They are used for special applications, including aerospace technology and bearing alloys.

Cast Alloys

Cast aluminum alloys in Russia are classified according to GOST 1583-93, which divides them into five groups by main alloying element: aluminum-silicon-magnesium, aluminum-silicon-copper, aluminum-copper, aluminum-magnesium and alloys based on other systems. Silumins — alloys of the aluminum-silicon system (AK12, AK9, AK7 and their modifications) — are the most widespread. They have excellent fluidity, low shrinkage, good corrosion resistance and sufficient strength, making them suitable for die casting of housings, cylinder blocks, electrical engineering parts and household appliances. Alloys of the aluminum-copper system are characterized by high strength and heat resistance but are inferior to silumins in casting properties.

Properties and Heat Treatment

Aluminum alloys possess a set of valuable properties: low density (2.6–2.8 g/cm³), high specific strength, corrosion resistance, good electrical and thermal conductivity, non-magnetic nature, non-toxicity and ease of processing. Heat-treatable alloys are subjected to quenching and artificial or natural aging. Quenching consists of heating to the temperature at which the strengthening phases dissolve, rapid cooling to fix the supersaturated solid solution, and subsequent aging, during which finely dispersed particles precipitate from the supersaturated solution, blocking dislocation motion and increasing strength. Non-heat-treatable alloys are strengthened by work hardening — cold deformation, which increases the density of crystal lattice defects.

Applications

Aerospace industry — historically the main consumer of high-strength alloys of the 2xxx and 7xxx series. They are used to manufacture skin, load-bearing structure, spars, ribs and landing gear components. Duralumins make up a significant part of the airframe structure.

Automotive industry — alloys of the 5xxx and 6xxx series are used for body panels, frames, heat exchangers and structural elements. Cast silumins are used for cylinder blocks, cylinder heads, gearbox housings and wheel rims.

Construction and architecture — extruded profiles from 6xxx series alloys form the basis for window, door and facade systems, stained glass, translucent structures, roofing elements and bridge railings.

Shipbuilding — 5xxx series alloys, due to their high corrosion resistance in marine environments, are used for ship hulls, decks, superstructures and welded structures.

Packaging industry — alloys of the 1xxx and 3xxx series are used for the production of beverage cans, lids, foil and flexible packaging.

Electrical engineering — technical aluminum and 1xxx series alloys are used for the manufacture of wires, cables, busbars and current-carrying elements.

Safety and Storage

Aluminum alloys in the form of semi-finished products (sheets, profiles, rods, ingots, billets) do not present a hazard when standard storage and transportation rules are observed. They are non-toxic, non-flammable and recyclable. The main precautions are related to mechanical processing: sawing, milling and grinding generate fine chips and dust, which can be fire-hazardous, so effective ventilation and fire safety measures are required. Storage is carried out in closed warehouses or on open areas with hard surfacing that protect the metal from moisture and contamination.

Conclusion

Aluminum alloys are one of the key structural materials of modern industry. The variety of alloying systems and processing technologies allows a material with an optimal balance of strength, corrosion resistance, processability and cost to be selected for each specific task. Understanding the classification of alloys, their properties and applications is the basis for competent material selection and the organization of efficient supplies.