Background

Aluminium is the world’s most abundant metal and is the third most common element comprising 8% of the earth’s crust.

The versatility of aluminium makes it the most widely used metal after steel.

Production of Aluminium

Aluminium is derived from the mineral bauxite. Bauxite is converted to aluminium oxide (alumina) via the Bayer Process. The alumina is then converted to aluminium metal using electrolytic cells and the Hall-Heroult Process.

Annual Demand of Aluminium

Worldwide demand for aluminium is around 29 million tons per year. About 22 million tons is new aluminium and 7 million tons is recycled aluminium scrap. The use of recycled aluminium is economically and environmentally compelling. It takes 14,000 kWh to produce 1 tonne of new aluminium. Conversely it takes only 5% of this to remelt and recycle one tonne of aluminium. There is no difference in quality between virgin and recycled aluminium alloys.

Applications of Aluminium

Pure aluminium is soft, ductile, corrosion resistant and has a high electrical conductivity. It is widely used for foil and conductor cables, but alloying with other elements is necessary to provide the higher strengths needed for other applications. Aluminium is one of the lightest engineering metals, having a strength to weight ratio superior to steel.

By utilising various combinations of its advantageous properties such as strength, lightness, corrosion resistance, recyclability and formability, aluminium is being employed in an ever-increasing number of applications. This array of products ranges from structural materials through to thin packaging foils.

Alloy Designations

Aluminium is most commonly alloyed with copper, zinc, magnesium, silicon, manganese and lithium. Small additions of chromium, titanium, zirconium, lead, bismuth and nickel are also made and iron is invariably present in small quantities.

There are over 300 wrought alloys with 50 in common use. They are normally identified by a four figure system which originated in the USA and is now universally accepted. Table 1 describes the system for wrought alloys. Cast alloys have similar designations and use a five digit system.

For unalloyed wrought aluminium alloys designated 1XXX, the last two digits represent the purity of the metal. They are the equivalent to the last two digits after the decimal point when aluminium purity is expressed to the nearest 0.01 percent. The second digit indicates modifications in impurity limits. If the second digit is zero, it indicates unalloyed aluminium having natural impurity limits and 1 through 9, indicate individual impurities or alloying elements.

For the 2XXX to 8XXX groups, the last two digits identify different aluminium alloys in the group. The second digit indicates alloy modifications. A second digit of zero indicates the original alloy and integers 1 to 9 indicate consecutive alloy modifications.

Physical Properties of Aluminium

Density of Aluminium

Aluminium has a density around one third that of steel or copper making it one of the lightest commercially available metals. The resultant high strength to weight ratio makes it an important structural material allowing increased payloads or fuel savings for transport industries in particular.

Strength of Aluminium

Pure aluminium doesn’t have a high tensile strength. However, the addition of alloying elements like manganese, silicon, copper and magnesium can increase the strength properties of aluminium and produce an alloy with properties tailored to particular applications.

Aluminium is well suited to cold environments. It has the advantage over steel in that its’ tensile strength increases with decreasing temperature while retaining its toughness. Steel on the other hand becomes brittle at low temperatures.

Corrosion Resistance of Aluminium

When exposed to air, a layer of aluminium oxide forms almost instantaneously on the surface of aluminium. This layer has excellent resistance to corrosion. It is fairly resistant to most acids but less resistant to alkalis.

Thermal Conductivity of Aluminium

The thermal conductivity of aluminium is about three times greater than that of steel. This makes aluminium an important material for both cooling and heating applications such as heat-exchangers. Combined with it being non-toxic this property means aluminium is used extensively in cooking utensils and kitchenware.

Electrical Conductivity of Aluminium

Along with copper, aluminium has an electrical conductivity high enough for use as an electrical conductor. Although the conductivity of the commonly used conducting alloy (1350) is only around 62% of annealed copper, it is only one third the weight and can therefore conduct twice as much electricity when compared with copper of the same weight.

Reflectivity of Aluminium

From UV to infra-red, aluminium is an excellent reflector of radiant energy. Visible light reflectivity of around 80% means it is widely used in light fixtures. The same properties of reflectivity makes aluminium ideal as an insulating material to protect against the sun’s rays in summer, while insulating against heat loss in winter.

Mechanical Properties of Aluminium

Aluminium can be severely deformed without failure. This allows aluminium to be formed by rolling, extruding, drawing, machining and other mechanical processes. It can also be cast to a high tolerance.

Alloying, cold working and heat-treating can all be utilised to tailor the properties of aluminium.

The tensile strength of pure aluminium is around 90 MPa but this can be increased to over 690 MPa for some heat-treatable alloys.

Aluminium Standards

The old BS1470 standard has been replaced by nine EN standards. The EN standards are given in table 4.

• Chemical compositions – unchanged.
• Alloy numbering system – unchanged.
• Temper designations for heat treatable alloys now cover a wider range of special tempers. Up to four digits after the T have been introduced for non- standard applications (e.g. T6151).
• Temper designations for non heat treatable alloys – existing tempers are unchanged but tempers are now more comprehensively defined in terms of how they are created. Soft (O) temper is now H111 and an intermediate temper H112 has been introduced. For alloy 5251 tempers are now shown as H32/H34/H36/H38 (equivalent to H22/H24, etc). H19/H22 & H24 are now shown separately.
• Mechanical properties – remain similar to previous figures. 0.2% Proof Stress must now be quoted on test certificates.
• Tolerances have been tightened to various degrees.

Heat Treatment of Aluminium

A range of heat treatments can be applied to aluminium alloys:

• Homogenisation – the removal of segregation by heating after casting.
• Annealing – used after cold working to soften work-hardening alloys (1XXX, 3XXX and 5XXX).
• Precipitation or age hardening (alloys 2XXX, 6XXX and 7XXX).
• Solution heat treatment before ageing of precipitation hardening alloys.
• Stoving for the curing of coatings
• After heat treatment a suffix is added to the designation numbers.
• The suffix F means “as fabricated”.
• O means “annealed wrought products”.
• T means that it has been “heat treated”.
• W means the material has been solution heat treated.
• H refers to non heat treatable alloys that are “cold worked” or “strain hardened”.

The non-heat treatable alloys are those in the 3XXX, 4XXX and 5XXX groups.

Work Hardening of Aluminium

The non-heat treatable alloys can have their properties adjusted by cold working. Cold rolling is an example.

These adjusted properties depend upon the degree of cold work and whether working is followed by any annealing or stabilising thermal treatment.

Nomenclature to describe these treatments uses a letter, O, F or H followed by one or more numbers. As outlined in Table 6, the first number refers to the worked condition and the second number the degree of tempering.

DISCLAIMER

This Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn. In particular, the mechanical property requirements vary widely with temper, product and product dimensions. The information is based on our present knowledge and is given in good faith. However, no liability will be accepted by the Company is respect of any action taken by any third party in reliance thereon.

As the products detailed may be used for a wide variety of purposes and as the Company has no control over their use; the Company specifically excludes all conditions or warranties expressed or implied by statute or otherwise as to dimensions, properties and/or fitness for any particular purpose.

The metallic element aluminium is the third most plentiful element in the earth’s crust, comprising 8% of the planet’s soil and rocks (oxygen and silicon make up 47% and 28%, respectively).

In nature, aluminium is found only in chemical compounds with other elements such as sulphur, silicon, and oxygen. Pure, metallic aluminium can be economically produced only from aluminium oxide ore.

Metallic aluminium has many properties that make it useful in a wide range of applications. It is lightweight, strong, nonmagnetic, and nontoxic. It conducts heat and electricity and reflects heat and light. It is strong but easily workable, and it retains its strength under extreme cold without becoming brittle. The surface of aluminium quickly oxidises to form an invisible barrier to corrosion. Furthermore, aluminium can easily and economically be recycled into new products.

Background

Aluminium compounds have proven useful for thousands of years. Around 5000 B.C., Persian potters made their strongest vessels from clay that contained aluminium oxide. Ancient Egyptians and Babylonians used aluminium compounds in fabric dyes, cosmetics, and medicines. However, it was not until the early nineteenth century that aluminium was identified as an element and isolated as a pure metal. The difficulty of extracting aluminium from its natural compounds kept the metal rare for many years; half a century after its discovery, it was still as rare and valuable as silver.

In 1886, two 22-year-old scientists independently developed a smelting process that made the economical mass production of aluminium possible. Known as the Hall-Heroult process after its American and French inventors, the process is still the primary method of aluminium production today. The Bayer process for refining aluminium ore, developed in 1888 by an Austrian chemist, also contributed significantly to the economical mass production of aluminium.

In 1884, 125 lb (60 kg) of aluminium was produced in the United States, and it sold for about the same unit price as silver. In 1995, U.S. plants produced 7.8 billion lb (3.6 million metric tonnes) of aluminium, and the price of silver was seventy-five times as much as the price of aluminium.

Raw Materials

Aluminium compounds occur in all types of clay, but the ore that is most useful for producing pure aluminium is bauxite. Bauxite consists of 45-60% aluminium oxide, along with various impurities such as sand, iron, and other metals. Although some bauxite deposits are hard rock, most consist of relatively soft dirt that is easily dug from open-pit mines. Australia produces more than one-third of the world’s supply of bauxite. It takes about 4 lb (2 kg) of bauxite to produce 1 lb (0.5 kg) of aluminium metal.

Caustic soda (sodium hydroxide) is used to dissolve the aluminium compounds found in the bauxite, separating them from the impurities. Depending on the composition of the bauxite ore, relatively small amounts of other chemicals may be used in the extraction

of aluminium. Starch, lime, and sodium sulphide are some examples.

Cryolite, a chemical compound composed of sodium, aluminium, and fluorine, is used as the electrolyte (current-conducting medium) in the smelting operation. Naturally occurring cryolite was once mined in Greenland, but the compound is now produced synthetically for use in the production of aluminium. Aluminium fluoride is added to lower the melting point of the electrolyte solution.

The Manufacturing Process

Aluminium manufacture is accomplished in two phases: the Bayer process of refining the bauxite ore to obtain aluminium oxide, and the Hall-Heroult process of smelting the aluminium oxide to release pure aluminium.

The Bayer process

• 1 First, the bauxite ore is mechanically crushed. Then, the crushed ore is mixed with caustic soda and processed in a grinding mill to produce a slurry (a watery suspension) containing very fine particles of ore.
• 2 The slurry is pumped into a digester, a tank that functions like a pressure cooker. The slurry is heated to 230-520°F (110-270°C) under a pressure of 50 lb/in 2 (340 kPa). These conditions are maintained for a time ranging from half an hour to several hours. Additional caustic soda may be added to ensure that all aluminum-containing compounds are dissolved.
• 3 The hot slurry, which is now a sodium aluminate solution, passes through a series of flash tanks that reduce the pressure and recover heat that can be reused in the refining process.
• 4 The slurry is pumped into a settling tank. As the slurry rests in this tank, impurities that will not dissolve in the caustic soda settle to the bottom of the vessel. One manufacturer compares this process to fine sand settling to the bottom of a glass of sugar water; the sugar does not settle out because it is dissolved in the water, just as the aluminium in the settling tank remains dissolved in the caustic soda. The residue (called “red mud”) that accumulates in the bottom of the tank consists of fine sand, iron oxide, and oxides of trace elements like titanium.
• 5 After the impurities have settled out, the remaining liquid, which looks somewhat like coffee, is pumped through a series of cloth filters. Any fine particles of impurities that remain in the solution are trapped by the filters. This material is washed to recover alumina and caustic soda that can be reused.
• 6 The filtered liquid is pumped through a series of six-story-tall precipitation tanks. Seed crystals of alumina hydrate (alumina bonded to water molecules) are added through the top of each tank. The seed crystals grow as they settle through the liquid and dissolved alumina attaches to them.
• 7 The crystals precipitate (settle to the bottom of the tank) and are removed. After washing, they are transferred to a kiln for calcining (heating to release the water molecules that are chemically bonded to the alumina molecules). A screw conveyor moves a continuous stream of crystals into a rotating, cylindrical kiln that is tilted to allow gravity to move the material through it. A temperature of 2,000° F (1,100° C) drives off the water molecules, leaving anhydrous (waterless) alumina crystals. After leaving the kiln, the crystals pass through a cooler.

Casting is the original and most widely used method of forming aluminium into products. Technical advances have been made, but the principle remains the same:

Molten aluminium is poured into a mould to duplicate the desired pattern. The three most important methods are die casting, permanent mould casting and sand casting.

Die casting

The die casting process forces molten aluminium into a steel die (mould) under pressure. This manufacturing technique is normally used for high-volume production. Precisely formed aluminium parts requiring a minimum of machining and finishing can be produced through this casting method.

Permanent mould casting

Permanent mould casting involves moulds and cores of steel or other metal. Molten aluminium is usually poured into the mould, although a vacuum is sometimes applied. Permanent mould castings can be made stronger than either die or sand castings. Semi-permanent mould casting techniques are used when permanent cores would be impossible to remove from the finished part.

Sand casting

The most versatile method for producing aluminium products is sand casting. The process starts with a pattern that is a replica of the finished casting. Virtually any pattern can be pressed into a fine sand mixture to form the mould into which the aluminium is poured. The pattern is slightly larger than the part to be made, to allow for aluminium shrinkage during solidification and cooling. As compared to die and permanent mould casting, sand casting is slow process but usually more economical for small quantities, intricate designs or when a very large casting is required.

Casting Applications

Widespread use in the automotive industry and homes

The automotive industry is the largest market for aluminium casting. Cast products make up more than half of the aluminium used in cars. Cast aluminium transmission housings and pistons have been commonly used in cars and trucks since the early 1900s. Parts of small appliances, hand tools, lawnmowers and other machinery are produced from thousands of different unique aluminium casting shapes. The casting product most often used by consumers is cookware, the first aluminium product that was made available for everyday use.

Take-Away Facts

• Casting must include a part-removal design
  Casting moulds must be designed to accommodate each stage of the process. For part removal, a slight taper (known as draft) must be used on surfaces perpendicular to the parting line so the pattern can be removed from the mould.
• Casting parts with cavities
  To produce cavities within castings (such as for engine blocks and cylinder heads used in cars), negative forms are used to make cores. Casts of this nature are usually produced in sand moulds. Cores are inserted into the casting box after the pattern is removed.
• Casting for light weight and strength
  Aluminium’s properties of light weight and strength bring fundamental advantages when casting into parts. One common application of die cast aluminium is thin-walled enclosures with ribs and bosses on the interior to maximise strength.
• Casting in the early history of aluminium
  The first commercial aluminium products were castings such as decorative parts and cookware. Though produced through a centuries-old process, these products were considered new and unique.