Apr. 29, 2024
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.
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Aluminium Alloys Explained
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.
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.
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.
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.
Table 1. Designations for wrought aluminium alloys.
Alloying Element Wrought None (99%+ Aluminium) 1XXX Copper 2XXX Manganese 3XXX Silicon 4XXX Magnesium 5XXX Magnesium + Silicon 6XXX Zinc 7XXX Lithium 8XXXFor 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.
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.
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.
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.
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.
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.
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.
Table 2. Properties for aluminium.
Property Value Atomic Number 13 Atomic Weight (g/mol) 26.98 Valency 3 Crystal Structure FCC Melting Point (°C) 660.2 Boiling Point (°C) 2480 Mean Specific Heat (0-100°C) (cal/g.°C) 0.219 Thermal Conductivity (0-100°C) (cal/cms. °C) 0.57 Co-Efficient of Linear Expansion (0-100°C) (x10-6/°C) 23.5 Electrical Resistivity at 20°C (Ω.cm) 2.69 Density (g/cm3) 2.6898 Modulus of Elasticity (GPa) 68.3 Poissons Ratio 0.34Aluminium 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.
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Further reading:The tensile strength of pure aluminium is around 90 MPa but this can be increased to over 690 MPa for some heat-treatable alloys.
Table 3. Mechanical properties of selected aluminium alloys.
Alloy Temper Proof Stress 0.20% (MPa) Tensile Strength (MPa) Shear Strength (MPa) Elongation A5 (%) Elongation A50 (%) Hardness Brinell HB Hardness Vickers HV Fatigue Endur. Limit (MPa) AA1050A H2 85 100 60 12 30 30 H4 105 115 70 10 9 35 36 70 H6 120 130 80 7 39 H8 140 150 85 6 5 43 44 100 H9 170 180 3 48 51 0 35 80 50 42 38 21 20 50 AA2011 T3 290 365 220 15 15 95 100 250 T4 270 350 210 18 18 90 95 250 T6 300 395 235 12 12 110 115 250 T8 315 420 250 13 12 115 120 250 AA3103 H2 115 135 80 11 11 40 40 H4 140 155 90 9 9 45 46 130 H6 160 175 100 8 6 50 50 H8 180 200 110 6 6 55 55 150 H9 210 240 125 4 3 65 70 0 45 105 70 29 25 29 29 100 AA5083 H2 240 330 185 17 16 90 95 280 H4 275 360 200 16 14 100 105 280 H6 305 380 210 10 9 105 110 H8 335 400 220 9 8 110 115 H9 370 420 230 5 5 115 120 0 145 300 175 23 22 70 75 250 AA5251 H2 165 210 125 14 14 60 65 H4 190 230 135 13 12 65 70 230 H6 215 255 145 9 8 70 75 H8 240 280 155 8 7 80 80 250 H9 270 310 165 5 4 90 90 0 80 180 115 26 25 45 46 200 AA5754 H2 185 245 150 15 14 70 75 H4 215 270 160 14 12 75 80 250 H6 245 290 170 10 9 80 85 H8 270 315 180 9 8 90 90 280 H9 300 340 190 5 4 95 100 0 100 215 140 25 24 55 55 220 AA6063 0 50 100 70 27 26 25 85 110 T1 90 150 95 26 24 45 45 150 T4 90 160 110 21 21 50 50 150 T5 175 215 135 14 13 60 65 150 T6 210 245 150 14 12 75 80 150 T8 240 260 155 9 80 85 AA6082 0 60 130 85 27 26 35 35 120 T1 170 260 155 24 24 70 75 200 T4 170 260 170 19 19 70 75 200 T5 275 325 195 11 11 90 95 210 T6 310 340 210 11 11 95 100 210 AA6262 T6 240 290 8 T9 330 360 3 AA7075 0 105 225 150 17 60 65 230 T6 505 570 350 10 10 150 160 300 T7 435 505 305 13 12 140 150 300The old BS1470 standard has been replaced by nine EN standards. The EN standards are given in table 4.
Table 4. EN standards for aluminium
Standard Scope EN485-1 Technical conditions for inspection and delivery EN485-2 Mechanical properties EN485-3 Tolerances for hot rolled material EN485-4 Tolerances for cold rolled material EN515 Temper designations EN573-1 Numerical alloy designation system EN573-2 Chemical symbol designation system EN573-3 Chemical compositions EN573-4 Product forms in different alloysThe EN standards differ from the old standard, BS1470 in the following areas:
A range of heat treatments can be applied to aluminium alloys:
The non-heat treatable alloys are those in the 3XXX, 4XXX and 5XXX groups.
Table 5. Heat treatment designations for aluminium and aluminium alloys.
Term Description T1 Cooled from an elevated temperature shaping process and naturally aged. T2 Cooled from an elevated temperature shaping process cold worked and naturally aged. T3 Solution heat-treated cold worked and naturally aged to a substantially. T4 Solution heat-treated and naturally aged to a substantially stable condition. T5 Cooled from an elevated temperature shaping process and then artificially aged. T6 Solution heat-treated and then artificially aged. T7 Solution heat-treated and overaged/stabilised.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.
Table 6. Non-Heat treatable alloy designations
Term Description H1X Work hardened H2X Work hardened and partially annealed H3X Work hardened and stabilized by low temperature treatment H4X Work hardened and stoved HX2 Quarter-hard – degree of working HX4 Half-hard – degree of working HX6 Three-quarter hard – degree of working HX8 Full-hard – degree of workingTable 7. Temper codes for plate
Code Description H112 Alloys that have some tempering from shaping but do not have special control over the amount of strain-hardening or thermal treatment. Some strength limits apply. H321 Strain hardened to an amount less than required for a controlled H32 temper. H323 A version of H32 that has been hardened to provide acceptable resistance to stress corrosion cracking. H343 A version of H34 that has been hardened to provide acceptable resistance to stress corrosion cracking. H115 Armour plate. H116 Special corrosion-resistant temper.
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.
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This information has been sourced, reviewed and adapted from materials provided by Aalco - Ferrous and Non-Ferrous Metals Stockist.
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