- Metals and alloys
- Aluminium: the metal
- Aluminium alloys
- Main properties
Aluminium is a metal of the 3rd group, with atomic number 13 and atomic weight 26.98.
It is one of the more diffused elements on the earth's surface, although it is not present in metallic form, but as complex oxides (bauxite).
Historical traces of its usage are present since about 300 AC, but only in 1886 an industrial process for the production of the metal was developed (Charles Hall and Paul Heroult).
Before 1866 the applications of aluminium, whose production cost was extremely high, were limited to the field of jewels and art objects; after that date the first industrial applications started, in the field of shipbuilding, construction and electrical applications.
The first commercial alloys, aimed at improving the mechanical properties of industrially pure aluminium, appeared between 1890 and the end of the century; in 1905 the mechanisms of solution treating and ageing of aluminium alloys was investigated, and in 1910 an alloy containing 4% copper and 0.5% magnesium, named from the production site "duralumin", was patented; this alloy, with a substantially similar chemistry, is produced and widely used in our days under the name of 2017.
Metals are chemical elements characterised by typical properties, like brightness (known as "metallic"), good electrical and thermal conductivity, formability. They seldom find applications in the modern technologies.
The addition to a metal (base metal) of quantities, in general small, of other metals (alloy metals), gives origin to metallic alloys (or plainly alloys), where with proper calibration of the additions of the alloy metals some properties of the base metal can be improved.
Metallic alloys are the basis for a wide number of components manufactured by the modern technologies.
The discovery, in pre-historical era, of the first alloys of copper with tin produced strong progresses for mankind, so that the corresponding period was called "the bronze age".
Changing type and quantity of metals additions to a base metal, alloys with properties also very different from each other can be obtained.
Aluminium is extracted out of bauxite, an ore present in nature, through the Hall-Herault process.
Bauxite is finely ground and washed with water to remove silicates and clay.
The remaining bauxite is dried in an oven, finely ground sodium carbonate and calcium oxide are added, then the mixture is processed in a converter, reduced under pressure and sent to a decanter, where several impurities are removed.
The mixture passes through filters, is cooled and additionally processed in a separator, then agglomerated and filtered, and at last is baked in a calcinating oven.
The resulting material is alumina, aluminium sesquioxide, that looks like a dusty material.
Alumina is then melted in dedicated cells (smelters), containing melted cryolite (the cryolite mineral is melted using electric power).
Alumina, in form of powder, is put into the cryolite bath, where it is melted and reduced to aluminium metal, that sets below the cryolite.
Liquid aluminium is pumped from the bottom of the cell and transferred to a crucible, and then is poured to obtain ingots, or transferred to processing ovens.
For the production of primary aluminium alloys the liquid metal can be transferred to the processing oven directly from the smelter, or the metal can be added in form of ingots previously cast and solidified.
In both cases the alloying elements (silicon, copper, magnesium, manganese, zinc, etc) are added to the molten metal in order to obtain alloys with defined chemistry, and thus defined properties.
The mix of molten metals is then cast into ingots of the right shape and size in view of the subsequent processing.
Traditionally the aluminium alloys are divided into:
- alloys for casting
- alloys for plastic deformation (wrought).
The following concepts apply to the latter
The aluminium alloys are nowadays identified by numerical codes, assigned by the Aluminum Association (AA), (e.g. 2024, 6082, 7075, etc.); such codes are now in general accepted by National and International Standards, in particular by the European Standards (EN).
The first digit of the code identifies the main alloying element added to aluminium (e.g.copper is the main addition in the 2xxx series alloys).
Every code refers to a well defined chemistry, that gives the alloy specific properties; for further details see section Alloys
Many alloys can improve their strength and hardness through suitable heat treatments; these alloys are commonly called heat treatable alloys.
In general all the aluminium alloys change their properties if subjected to thermal, mechanical or thermomechanical treatments.
Heat treatments are cycles of heating and cooling under controlled conditions, that produce changes in the structure and consequently in the properties of the alloy.
The mechanical treatments consist in plastic deformations, generally at room temperature (strain hardening) of the alloy; these too provide for changes in the properties of the alloy.
The thermomechanical treatments consist in complex processing, including treatments of the two former types.
So, at the end of the day, the properties of an alloy come from:
- Chemical analysis (e.g. 2024)
- Temper, i.e. heat and mechanical treatments received by the alloy.
Aluminum Association defines temper by means of conventional codes (e.g. T651 = solution treated, quenched, stress relieved by stretching and then artificially aged in order to achieve peak strength).
Every technological product is then defined by AA through a complex code, including:
- the code of the alloy
- the code of temper
E.g.: 7075 -T651 = alloy 7075 solution treated, quenched, , stress relieved by stretching and then artificially aged in order to achieve peak strength.
Aluminium and aluminium alloys show, in comparison to other technical materials, specific and in some cases unique physical properties, that make them particularly appealing to designers, manufacturers and end users; among these:
Low weight: specific gravity is much lower than other metals and alloys. With a value of 2.7 g/cubic cm aluminium has a weight per unit volume of about one third of iron, steel, copper, bronze, brass.
Therefore it is easier to handle, cheaper to transport and is particular attractive for aerospace and transportation in general, for moving parts and for those items and structures, where saving weight is of importance.
- Strength: aluminium alloys can reach ultimate tensile stress up to over 560 Mpa; yield stress for the more resistent alloys is about 85% of uts; This allows to solve most problems in a number of applications.
Strength increases at low temperatures, without any phenomena of ductile to brittle transition.
- The strength to weight ratio is particularly high. Without the aluminium alloys the space missions and the development of wide commercial aircraft would not be possible; in other sectors the aluminium alloys provide appreciable benefits for structures where saving weight of the structure itself with the same strength allows to increase payloads (transportation equipment, bridges etc.).
- Corrosion resistance: aluminium shows a very good resistance to corrosion; it does not produce rust and is protected, in oxidising environment, by a thin layer of clear, stable, natural oxide.
Corrosion protection, specially for aluminium alloys, can be further improved by means of treatments of anodising, chemical conversion coating, painting.
- Thermal conductivity: at the same cost and weight aluminium transfers much more heat than any other metal, and therefore it is the best material for heat exchangers (2.37 W / cm °K at room temperature).
- Electrical conductivity is extremely high; conductivity per unit weight is about twice that of copper (37.7 x 10^6/m ohm at room temperature).
- Magnetic properties: aluminium and its alloys are amagnetic, and so particularly suitable for high voltage applications, electronics in general where strong magnetic fields are present, or near apparatus particularly sensitive to magnetic fields.
- Sparks: aluminium and its alloys do not produce sparks, and so they are perfectly suitable wherever explosion or fire dangers can occur.
- Impact strength: aluminium alloys show elevated impact strength and good capability of elastic deformation under load, with spring back to the original shape after impact or after removal of external load.
- Reflectivity: the reflectivity of aluminium is very high, and high reflectivity aluminium products can be used for manufacturing of good reflectors for light, infrared, radiofrequency.
- Fire resistance: products in aluminium and aluminium alloys do not burn and do not emit toxic fumes even at the most elevated temperatures.
- Strength at low temperature: mechanical strength increases with decreasing temperatures, without significant phenomena of brittle transition; therefore aluminium products are the ideal material for cryogenic applications, and anyway for applications at very low external temperatures.
- Recycling: aluminium base materials can be continuously recycled without losing the outstanding properties of the metal; aluminium scrap maintains elevated prices, therefore aluminium is particularly appealing as far as environmental impact is concerned.
- Ease of assembly: aluminium alloys parts can be assembled by means of mechanical fasteners (bolts, screws, solid and blind rivets); some alloys can be easily fusion and resistance welded and brazed.
- Kind of products: aluminium alloys are available in form of sheet, plate, rod and bar, shapes even very complicated in section, welded or seamless.
Manufacturing technologies determine some important properties of the products, such as dimensional tolerances, surface condition and, in many cases, mechanical properties.
- Sheet: flat product, thin gage, thickness between few tenths of mm and 6 mm, are identified through thickness, length and width.
- Threaded sheet: flat product, thin gage, thickness between 1 and 6 mm, upper surface has threads (raised pattern),are identified through thickness, thickness of raised pattern, length and width.
- Coiled sheet: flat product, thin gage, in form of coils, thickness between few tenths of mm and 6 mm, are identified through thickness, length and width.
- Plate: : flat product, medium to thick gage, thickness between 6 and 500 mm, are identified through thickness, length and width; grain flow, when applicable, is an important parameter; manufactured through direct chill casting (cast plates) or direct chill casting and subsequent hot rolling (rolled plates).
- Round bar: long product, circular cross section, defined by diameter, usually between few mm and some hundreds of mm, and by length of bar.
- Rectangular bar: long product, rectangular cross section, defined by length and width of section, usually between few mm and some hundreds of mm, and by length of bar.
- Square bar: long product, square cross section, defined by width of section, usually between few mm and some hundreds of mm, and by length of bar.
- Hexagon: long product, hexagonal cross section, defined by width of section (wrench size), usually between few mm and some tens of mm, and by length of bar.
- Tube: hollow bar, circular cross section, defined by external diameter, usually between few mm and some hundreds of mm, by inner diameter and by length of bar. Can also be defined by OD, wall thickness and length of bar.
- Square tube: hollow bar, square or rectangular cross section, defined by outer section length and width, wall thickness and by length of bar. Instead than by wall thickness can also be defined by inner section length and width.
- Shape: long product, L, T, U shaped cross section, defined by external dimensions of cross section, wall thickness and by length of bar.
- Special shape: long product, complex shaped cross section, usually defined by customer's drawing, defined by cross section and by length of bar.
Sheets are manufactured by cold rolling.
Plates are manufactured by casting (cast plates) or hot rolling (rolled plates).
Bars, tubes and shapes are manufactured by extrusion (extruded bars) or drawing (drawn bars); the two technologies produce different dimensional tolerances and surface conditions.
Prices can fluctuate, due to balance of offer and demand and to other factors.
It should be reminded that the price of metal (London Metal Exchange) is only one component of the cost of products; additionally, manufacturing, handling and transportation costs shall be considered.
The price of the metal is fixed day by day at the London Metal Exchange (LME) in US$ per ton, and refers to ingots of commercially pure aluminium. This price is recorded on the main economical newspapers.
Besides the LME price one shall consider a "country" extra price typical of each country or geographical area, determined on the basis of the local market situation, an "alloy" extra price to switch from industrially pure aluminium of LME to the aluminium alloy slabs or billets which are the starting material for cast, rolled or extruded products, and whose price is influenced by size of slab or billet and alloy chemistry.
At last, the transformation cost (cost of rolling, extrusion, drawing) shall be added.
The price that the final customer pays includes all the above mentioned costs, plus the cost of service (handling, scraps if any, packing, transportation).
From this standpoint the important thing is not that a product could be recycled, but that it is actually recycled.
Recycling is important for many reasons.
Storing of too big quantities of solid waste became a problem in developed countries; as much as possible they should be reworked for new usage.
The cost of recycled metal is lower than that of ore and electric power required to produce new metal.
Besides that, saving ore and electric power has an intrinsic value in view of environmental protection.
Recycling of aluminium allows saving 95% of the electric power required to produce new metal from ore; additionally, since 4 kg of bauxite are requested to produce 1 kg of metal, recycling 1 kg of metal allows saving 4 kg ore.
Experience showed that aluminium is one of the materials for which a recycling program is working since years, due to the high value of scrap, to the general attitude of people towards recycling of aluminium and to the strong attitude of industry towards recycling.
TAll aluminium products can be recycled, from packaging foil to car components, doors and windows, housekeeping tools; the maximum benefits is reached in case of heavy structures, like railway material, trucks and trailers. Well known is the case history of recycling, after more than 25 years service, of some hundreds railway coaches; in this case the value of the recycled metal reached 95% of the value of the metal used originally for the manufacturing of coaches.
We shall consider that there is no limit to the times aluminium can be recycled without losing its characteristic properties; no quality loss shall be expected using recycled aluminium, provided that the chemical analysis of the products conforms to the requirements of the applicable specifications.