|Coding Aluminum Laminates
In most countries, four-digit numerical systems are used to identify aluminum alloys. The first digit indicates the alloy group and the last two digits identify the aluminum alloy or indicate the purity of the aluminum. The second digit indicates modifications made to the original alloy or impurity limits.
Aluminum and its alloys are commonly grouped in series. The general characteristics of each series are described below:
Aluminum of 99.00% or higher purity has many applications, especially in the electrical and chemical fields. These aluminum grades are characterized by excellent corrosion resistance, high thermal and electrical conductivity, low mechanical properties, and excellent workability. Moderate increases in strength can be obtained by case hardening. Iron and silicon are the main impurities.
It contains 99.00% aluminum and is used in many applications, especially in the electrical and chemical fields. This high degree of purity in aluminum gives it excellent corrosion resistance, high thermal and electrical conductivity and, although it has low mechanical properties, it has very good malleability. The presence of iron and silicon are the most frequent impurities. The most common field of these alloys are decorative applications, luxury packaging (for cosmetics, perfumes, etc.) and for the manufacture of electrochemical capacitors, among others.
Copper is the main alloying element in this series, generally with magnesium as a secondary addition. These alloys require solution heat treatment to obtain optimum properties. In solubilized condition these alloys show mechanical properties similar to and sometimes superior to low carbon steels. In some cases, precipitation heat treatment (aging) is used to further increase the mechanical properties. This treatment increases yield stress, with consequent elongation losses; its effect on tensile strength is not as great. The 2xxx series alloys do not have as good corrosion resistance as most other aluminum alloys, and under certain conditions may be subject to intergranular corrosion.
Copper is the main element present in this type of alloy (Al-Cu), which is generally mixed with magnesium, but as a secondary addition. The 2xxx series requires solution heat treatment to obtain optimum properties; in fact, under soluble conditions, this type of alloy shows mechanical properties similar and, in some cases, superior to those of carbon steel. On the other hand, when these alloys are subjected to precipitation treatments (aging), creep resistance increases with a consequent loss of elongation and increased brittleness.
Manganese is the main alloying element in the 3xxx series. These alloys are generally not heat treatable, but have 20% higher strength than the 1xxx series alloys. Because only a limited percentage of manganese (up to about 1.5%) can be effectively added to aluminum, manganese is an important element in a few alloys.
Manganese between 1% to 5% is the main alloying element in the 3xxx series. These alloys are generally non-heat treatable, but have 20% higher strength than the 1xxx series alloys; because only a limited percentage of manganese – up to about 1.5% – can be effectively added to aluminum, it is an important element in a few alloys. They are generally used to manufacture cladding panels in the construction and culinary industries, among others.3xxx Series. Manganese between 1% to 5% is the main alloying element in the 3xxx series. These alloys are generally non-heat treatable, but have 20% higher strength than the 1xxx series alloys; because only a limited percentage of manganese – up to about 1.5% – can be effectively added to aluminum, it is an important element in a few alloys. They are generally used to manufacture cladding panels in the construction and culinary industries, among others.
The main alloying element in this series is silicon, which can be added in sufficient quantities (up to 12%) because of a reduction in the melting range without producing brittleness. For this reason, aluminum-silicon alloys are used in welding wires where the melting range is lower than that of the base metal. Most of the alloys in this series are non-heat treatable, but when used in welding heat treatable alloys, they absorb some of the alloy constituents of the latter and thus respond to heat treatment to a limited extent. Alloys containing appreciable amounts of silicon turn a dark gray to charcoal color when anodic oxide finishes are applied and are therefore in demand for architectural applications. Alloy 4032 has a low coefficient of thermal expansion and high wear resistance, making it suitable for the production of forged engine pistons.
The main alloying element in this series is silicon, which can be added in sufficient quantities (up to 12%). Silicon reduces the brittleness that occurs during melting, this characteristic is important for the manufacture of welding wires, where the melting temperature is also lower than that of the base metal. Aluminum and silicon alloys are not heat treatable, but when used in welding alloys, which do have some type of heat treatment, they absorb some of these components and can change their mechanical properties. Alloys containing significant amounts of silicon usually turn a dark gray to charcoal color when anodic oxide finishes are applied, which is why they are in high demand by industry for architectural applications. Alloy 4032, which belongs to this series, has a low coefficient of thermal expansion and high wear resistance, two characteristics that make it suitable for the manufacture of forged engine pistons.
The main alloying element in series alloys is magnesium. When used as an alloying element or with manganese, the result is a work-hardenable alloy of moderate to high strength.magnesium is considerably more effective than manganese as a hardener, approximately 0.8% Mg which is equal to 1.25% Mn, and can be added in larger quantities. The alloys of this series have good welding characteristics and good corrosion resistance in marine environments.
The main alloying element for this series is magnesium, but they rarely contain more than 5%, because their mechanical properties decrease when they are exposed for long periods of time to the influence of temperature. Magnesium is considerably more effective than manganese as a hardener and can be added in larger quantities. The alloys of this series have good welding characteristics and good corrosion resistance in marine environments.
The 6xxx series alloys contain silicon and magnesium in the proportions required for the formation of magnesium silicide (Mg2Si). This compound makes them heat-treatable. Although not as strong as most of the 2xxx and 7xxx series alloys, the 6xxx series alloys have good formability, weldability, machinability and corrosion resistance. Alloys of this group can be formed in temper T4 (solution heat treatment, but not precipitation heat treatment) and then strengthened by full precipitation heat treatment.)
The alloys of this type of series contain silicon and magnesium in the required proportions for the formation of magnesium silicide (Mg2Si), this compound makes them suitable for heat treatment. These alloys have a good resistance to corrosion caused by air, are very suitable for extrusion and hot forging processes; they also have good behavior to be worked in cold deformation processes and acquire an excellent texture in anodizing processes. Although these alloys are not as strong as most of the 2xxx and 7xxx series, they have good formability, weldability, machinability and corrosion resistance. In addition, alloys of this group can be formed by solution heat treatment and then strengthened by full precipitation heat treatment (T6). (For more information see article on heat treatments for aluminum). This group of alloys is widely used in structures, frames and windows, among others, at the service of the architectural industry: also for the elaboration of all types of lamps or housings for public lighting, where the same property of thermal conductivity helps to dissipate the heat generated by the lights and makes it the ideal material for this type of applications.
Zinc, in amounts of 1 to 8%, is the main alloying element in 7xxx series alloys, and when combined with a minor percentage of magnesium, results in heat-treatable alloys of moderate to very high strength, and other elements, such as copper and chromium, are usually added in small amounts. Small additions of scandium (Sc) also improve properties. 7xxx series alloys are used in airframe structures, mobile equipment, and parts subject to high working stresses.7xxx high strength alloys exhibit reduced stress corrosion cracking (SCC) resistance and are often used in a slightly over-aged temper to provide better combinations of mechanical strength, corrosion resistance and fracture toughness.
To this group belong aluminum alloys with zinc, in average amounts of 1% to 8% and when combined with a lower percentage of magnesium, can be heat treated to achieve a high level of strength. Usually, zirconium and titanium are added to this type of alloy in quantities not exceeding 0.25 to increase the hardenability of the alloy, such as 7474, a variant of 7075, which replaces the latter, especially when thicknesses above 80 mm are required. They are widely used in airframe structures, mobile equipment and parts subject to high working stresses in the aerospace industry. 7xxx alloys have reduced corrosion resistance and are often used slightly over-aged to provide better combinations of mechanical strength, corrosion resistance and fracture toughness. It is important to mention that this type of aluminum alloy is known in the Colombian industry as duralumin.
They are alloys with a wide range of chemical compositions. For example, improved performance at elevated temperatures is achieved through the use of dispersion-strengthened Al-Fe-Ce alloys (e.g. 8019) or Al-Fe-V-Si alloys (e.g. 8009, manufactured by powder metallurgy). Low density and higher stiffness can also be achieved in those alloys containing lithium (e.g. 8090). The latter alloy, which is precipitation hardenable, has replaced 2xxx and 7xxx series alloys in aerospace applications (e.g., helicopter components).
8xxx series. Iron is added in order to generate a grain refinement, which increases the alloy’s creep resistance. They have a good suitability for forming processes and are especially used in the manufacture of fins for heat exchangers and spiral-shaped tubes; the two most common alloys in this group are 8006 and 8011.