Aluminum is a chemical element belonging to the 3rd period of the periodic table of elements and to group IIIA (of earth metals) with atomic number 13; it has an electronic configuration Ne 3s23p. Its symbol is Al and it is identified by the CAS number 7429-90-5. The name aluminum comes from the ancient name used to identify alum (aluminous potassium sulfate), which was alumen (from the Latin bitter salt).
Aluminum has only one naturally occurring isotope: aluminium-27 (27Al), which is not radioactive; however, radioactive isotopes with mass numbers between 24 and 30 can be prepared artificially.
Characteristics and properties
Aluminum can be recycled 100% and countless times without losing its original characteristics that remain unchanged indefinitely, even after several recycling steps, or when from being primary aluminum the metal acquires the definition of “recycling aluminum” or “secondary aluminum”.
With the same volume, aluminum weighs about 1/3 of copper and steel. Also for this reason, most of the means of transport of modern design are built using high percentages of aluminum: the shuttle is made of aluminum up to 90% and, on average, 80% of the weight of a transport aircraft is given by aluminum. The same can be said, in different percentages, for ships, yachts, high-speed trains, streetcars and subway cars, and automobiles: several car manufacturers make their chassis and bodies with 100% aluminum.
Aluminum oxidizes immediately in contact with air (it has in fact a great affinity with oxygen and forms a protective layer of oxide on the surface when exposed to air) creating a surface protection that makes it resistant to water and some chemicals. This characteristic makes it the most widely used metal in the transportation, building and construction industries.
Ductility and malleability
Aluminum is easily workable and suitable to undergo working processes both at high and low temperatures. For this reason, too, it is suitable for use in the manufacture of containers and packaging.
High electrical, thermal and sound conductivity
At equal weight it is a better conductor than copper. Aluminum cables conduct electrical current twice as much as copper cables of the same weight. Aluminum’s specific electrical conductivity makes it indispensable for electronics and electrical applications.
Aluminum allows the transmission of energy even over long distances, it is no coincidence that most high-voltage conductors are made of aluminum, but also the threaded base of light bulbs. This metal also boasts high thermal conductivity, which is why it is used in the construction of radiators and thermal containers, heat conditioning equipment and cooking containers for food. Finally, it is distinguished by its significant sound resonance, which is why it is used in the construction of instruments such as the violin, piano, etc.
Diffuses and reflects light reducing the dispersion of brightness from the light source and thus promoting energy saving.
Aluminum has no magnetic affinity, so this characteristic makes it ideal for shielding antennas and computer disks and allows its use in the construction of devices such as radios, radars and stereos.
Aluminum is obtained, almost exclusively, from bauxite, a red-brown earth composed essentially of aluminum oxide hydrates and oxides of iron, silicon and titanium. The metallurgy of aluminum can be divided into two phases: production of pure alumina by chemical means and electrolysis of the molten mixture of alumina and cryolite (sodium fluoride and aluminum) for the production of aluminum.
The process generally used for alumina production is the Bayer process, which is based on the reaction of bauxite with a concentrated solution of caustic soda, at relatively high pressure and temperature. The crushed and dried bauxite is reduced to powder; this is brought to the reaction conditions (temperature of 180÷200 °C and pressure of 150÷200 MPa = 15÷20 atm) in special autoclaves. In these conditions the aluminum solubilizes as sodium aluminate; the slags consist of iron, silicon, titanium oxides (“red sludge”).
The sodium aluminate solution, diluted and filtered, is stationed in decomposers for about 100 hours. During this time, the formation (triggered by the introduction of aluminum hydroxide crystals) of aluminum hydroxide occurs by hydrolysis reaction. The aluminum hydroxide is separated and “cooked” in rotary furnaces at about 1200 °C, the sodium hydroxide (caustic soda) is reintroduced into the production cycle. The alumina thus obtained has a purity of >99.5%. In order to obtain aluminium, it is necessary to electrolyze the previously produced alumina. The electrolysis process involves the dissolution of alumina in molten cryolite, in order to reduce the temperature of the electrolytic cell to acceptable values (about 970 °C). During electrolysis the metal is deposited at the bottom of the tank (cathode) and oxygen is developed at the anode (carbon electrode) which is consumed by combustion. The aluminum thus produced (called “first melt” aluminum) has a title of about 99.5%, with impurities consisting of iron and silicon. The molten metal can be cast immediately into cakes or sent to holding furnaces for the production of alloys.
Pure aluminum has poor mechanical properties and oxidizes very easily, forming a compact oxide film. The oxide is highly stable and does not attack the underlying metal, acting as a passivating agent. The possible presence of impurities of various kinds affects the formation of the compact oxide film, resulting in a greater susceptibility to deep oxidation. Aluminum has, however, interesting characteristics technological characteristics (malleability, ductility, deep-drawing) that are realized with a remarkable ease of cold and hot plastic processing; particularly remarkable is the preservation of these characteristics also for aluminum alloys.
The limited mechanical properties of primary smelted aluminum can be significantly increased by the addition of alloying elements. The main aluminium alloying elements are: copper, silicon, manganese, magnesium, zinc; they can be added singularly to form binary alloys but more often they are added in “groups” to form more complex alloys. In order to modify or improve the physical or technological characteristics, corrective elements (iron, titanium, nickel) are added.
The various alloying elements can be added directly to the molten aluminum or through the formation of a binary alloy at the highest concentration of the binder (parent alloy) that is introduced into the molten aluminum. Let’s examine, briefly, the characteristics that give the following alloying elements to the aluminum alloy:
- Copper (Cu): forms heat-treatable (quenchable) alloys. As the percentage increases, mechanical strength and hardness increase; mechanical strength remains satisfactory up to temperatures of about 100 °C. Even if in small percentages it heavily influences, in a negative way, the corrosion resistance. In foundry alloys it is present in percentages between 4% and 10%; in plastic processing alloys it does not exceed 6%.
- Silicon (Si): it does not produce hardenable alloys. It increases mechanical strength and hardness without significantly reducing alloy ductility. It does not significantly affect corrosion resistance; it greatly increases castability characteristics and is therefore used to form foundry alloys. The percentages vary between 2 and 15%.
- Magnesium (Mg): greatly increases the corrosion resistance, allowing to exceed (in some environments) the characteristics of pure aluminum. It increases the mechanical characteristics if present in a maximum percentage of 10%. It affects negatively the melting, increasing the oxidability of aluminum alloys. In plastic processing alloys it is present in percentages between 1 and 5%; in foundry alloys between 3 and 10%.
- Zinc (Zn): increases mechanical strength and hardness but lowers heat resistance and corrosion resistance. The addition of about 3% copper, in an alloy with 10-12% zinc, minimizes these defects. Zinc greatly influences the hardenability qualities of light alloys, allowing Al-Zn-Cu-Mg alloys to achieve strengths comparable to steels.
- Manganese (Mn): it contrasts the undesired effects of iron and increases resistance to corrosion.
As can be seen from the above, it is the presence of some alloying elements that makes aluminum alloys “temperable”, with a substantial increase in mechanical properties.
Classification of aluminum alloys
Light alloys are, in the first instance, classified according to the technological transformation to which they are destined: plastic processing or foundry. Other classification criteria are added to this subdivision: according to the chemical composition and according to the attitude to the treatment of hardening and tempering.
According to the chemical classification, alloys are subdivided on the basis of the main binder; we will have, therefore, the following groups of alloys:
- Al-Cu alloys (and derivatives: Al-Cu-Si, Al-Cu-Mg, etc.): characterized by good mechanical strength, maintained up to about 100 °C, and quenchable.
- Al-Zn alloys (and derivatives: Al-Zn-Mg-Cu): characterized by good cold resistance and workability, quenchable.
- Al-Si alloys (and derivatives): characterized by good castability.
- Al-Mg alloys (and derivatives): characterized by good corrosion resistance and aptitude for plastic and tool working.
- Al-Sn alloys: characterized by antifriction properties.
- Al-Mn-Ni alloys: characterized by high characteristics at high temperatures.
- Al-Mn and Al-Mn-Mg alloys: characterized by good properties and workability for plastic deformation.
- plastic deformation.
Of the various groups mentioned above, in the field of aeronautical construction, Al-Cu and Al-Zn alloys and their derivatives are mainly used. Depending on the attitude to the quenching and tempering treatment, there are alloys that can be quenched and tempered and alloys that cannot be quenched and tempered. The hardening and tempering treatment is carried out in two phases: structural hardening and aging. This treatment allows to considerably increase the mechanical properties of the alloy. In the case of non-quenched and tempered alloys, increases in mechanical properties can be achieved through work hardening.
Heat treatment of light aluminum alloys
Heat-treatable aluminum light alloys are those in which there are compounds of Al-Cu, Al-Si, Al-Mg, Al-Zn, which are the fundamental alloying elements of heat-treatable alloys. Heat treatments performed on light alloys are:
- Structural hardening (solution heat treatment)
- Aging or re-precipitation (precipitation heat treatment)
- Annealing of various types.
The first two, carried out in succession, constitute the treatment of hardening and tempering.
The material is heated and maintained at a suitable temperature (generally between 480 and 530 °C) for a sufficient time to solubilize all the alloying elements. Thus, a stable crystalline lattice is formed, typical of these temperatures. By rapidly cooling the material in water, this crystal structure is “locked in” even at room temperature.
Aging or re-precipitation
The conditions of solubilization of the alloying elements in aluminum are, in fact, unstable at room temperature and the various components tend to separate “re-precipitating”, to bring in conditions of stability. This phenomenon, called aging, occurs quite slowly at room temperature (several days); in this period the material has an evolving structure.
If the material is not placed in the furnace for artificial aging within 2 hours after quenching, it is necessary to wait at least 2 days before artificial aging.
Re-precipitation can be significantly slowed by storing materials in a refrigerator at temperatures below 0 °C.
Structural quenching can somewhat limit the plastic deformation that can be performed (due to the increase in hardness); however, the mechanical properties of the material can be increased by performing plastic deformation immediately after structural quenching, before reprecipitation begins. In this way, the effects of hardening and tempering are superimposed on those of plastic processing, with obvious benefits. As an example of the above, consider the rivets made of alloy 2024, which, after being hardened, are stored in a freezer at -5 °C until the time of installation, which must occur within 15-20 minutes after leaving the freezer. The rivets acquire 100% of their characteristics 4 days after installation (96 hours).
The annealing of aluminum alloys can be performed according to various methods, depending on the results to be obtained.
- Stress relieving annealing. It aims at eliminating or reducing the internal stresses created during casting in water, during the production cycle of semi-finished products in alloys for plastic processing. It is carried out by keeping the material at about 300 C for a suitable length of time; under these conditions, there are no variations in the crystalline structure of the material.
- Homogenization annealing. Eliminates the effects of previous heat treatments and tempering and restores to ideal conditions an alloy in which undesirable alterations in the crystalline structure have formed. Homogenization annealing is performed at rather high temperatures (about 540÷560 °C), close to the melting point, and for quite long times; cooling is slow, so as to obtain the crystalline structure as shown in the state diagram.
- Recrystallization annealing. Hot and, above all, cold plastic processing cause a work hardening of the material (with deformation of the crystalline structure) such as to hinder further processing. Recrystallization annealing allows to rearrange the crystalline structure of the material, completely eliminating the initial work hardening state.
- Heterogenization (or precipitation) annealing. It is a little widespread treatment, mainly applied to Al-Mg alloys that have undergone heating, even localized (during processing) at temperatures in the order of 400 °C. The result is the elimination of work hardening effects resulting from previous plastic processing.
- Stabilization annealing. Many light alloys, especially those that can be quenched and tempered, do not completely reabsorb the expansion that they endure due to the temperature increase to which they have been subjected during the quenching and tempering cycle. There remains a residual expansion of about 0.3% which, although small, is often not negligible. Stabilization annealing remedies this situation; it is performed by heating at 240-270 °C for a suitable time, followed by cooling in air. It is obvious that this treatment, if performed on alloys that have already been quenched and tempered, leads to a decrease in the mechanical properties of the material.
Plated materials must be annealed with special care in order to minimize the high temperature residence time, since some alloying elements tend to diffuse into the plated state, reducing its corrosion resistance.