Alloys of aluminium are light in weight compared to steel, brass, nickel and copper. These resist corrosion and have good electrical and thermal conductivities. These can readily accept a wide range of surface finishes and can be fabricated by all common processes. These lose part of their strength at elevated temperature.
At sub zero temperatures, however, their strength increases without loss of ductility and thus these are best suited for low-temperature applications.
Aluminium alloys can be classified according to the method of shaping them, viz. wrought products and cast aluminium alloys. These are further classed as to whether they respond to heat-treatment of strengthening-type or not.
Wrought Aluminium Alloys:
Strength greater than that of pure aluminium can be achieved by addition of other elements. Alloys can be strengthened by heat treating in some cases. Alloys which can’t be strengthened by heat treating are known as non-heat- treatable alloys. The initial strength of these alloys depends upon the hardening effects of elements such as Mn, Si, Fe and Mg, singly or in various combinations.
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These alloys are work- hardenable and further strengthening is achieved by various degrees of cold working. Alloys containing appreciable amount of magnesium when supplied in strain-hardened tempers are usually given a final elevated-temperature treatment called stabilising.
Heat-treatable alloys show increasing solid solubility in aluminium with increasing temperature and as such it is possible to subject them to thermal treatments which will impart pronounced strengthening.
In the first step (solution heat-treatment), the soluble element is put in solid solution at elevated temperature of about 490°C. Then it is rapidly quenched (usually in water) which momentarily freezes the structure and for a short time renders the alloy very workable.
Since alloy in this stage is very soft and ductile, it can be easily cold worked. It is at this stage that some fabricators retain this more workable structure by storing the alloys at below freezing temperature until they are ready to form them.
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At room or elevated temperatures the alloys are not stable after quenching, however, the precipitation of the constituents from the supersaturated solution begins. After a period of several days at room temperature, termed ageing or room-temperature precipitation, the alloy is considerably stronger.
Many alloys approach a stable condition at room temperature, but some alloys (containing Mg and Si, or Mg and Zn) continue to age-harden for long periods of time at room temperature. Thus depending on composition of alloy, some alloys start to become harder and stronger shortly after quenching (age-hardening). An interesting application is with Duralumin alloy containing 4% copper, whose rivets after solution treating are kept in a refrigerator.
After riveting, the become stronger due to age hardening at room temperature. Some alloys do not become stronger unless they are heated for a few hours after they have been solution treated. In such cases, by heating for a controlled time at slightly elevated temperature even further strengthening is possible and properties are stabilised (Artificial or precipitation hardening).
Casting Aluminium Alloy:
These alloys contain large quantities of Si or Mg, to produce a lower melting point alloy. Only a few alloys are suitable for die casting and sand casting. Some of these alloys naturally age after solution treatment, but others have to be given precipitation treatment.
Clad Aluminium Alloys:
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Heat treatable alloys containing copper or zinc as major alloying elements are found to be less resistant to corrosion attack than the majority of non-heat and treatable alloys. To increase corrosion resistance in sheet and plate form these are often clad (upto 2.5 to 5% of total thickness on either side) with high purity aluminium, a low magnesium-silicon alloy, or an alloy containing 1% zinc.
Cladding not only protects the composite due to its own inherently excellent corrosion resistance but also exerts a galvanic effect which further protects the core metal.
Duraluminium:
It is a very important alloy of aluminium.
Its composition is as given below:
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Copper … 3.5 to 4.5%,
Manganese … 0.4 to 0.7%
Magnesium … 0.4 to 0.7%,
Iron or Silicon … Not more than 0.7%,
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Aluminium … Rest.
This alloy has got high machinability. It can be heat treated to increase its tensile strength. By heat treatment tensile strength of the alloy can be raised up to 4300 kg/cm2 without affecting ductility of the alloy. Duraluminium is as strong as steel but has only about one-third of its weight. So it is used to fabricate parts of aircraft and automobiles.
Aluminium silicon alloys have excellent casting qualities and resistance to corrosion. The alloys are not hot- short and are easy to cast in thin or thick sections, but these are difficult to machine. Aluminium-magnesium alloys are superior to practically all other aluminium casting alloys with respect to resistance to corrosion and machinability. At the same time these have high mechanical strength and ductility. Aluminium alloys for pressure die-casting must possess considerable fluidity and be free from hot-shortness.
Classification of Aluminium Alloys:
Based on the sequences of basic treatments used to produce the various tempers, all forms of wrought and cast aluminium and aluminium alloys could be classified as:
(i) Fabricated:
Products which acquire some temper from shaping processes not having special control over the amount of strain-hardening or thermal treatment.
(ii) Annealed, Re-Crystallised (Wrought Products Only):
Wrought products subjected to softest temper.
(iii) Strain-Hardened (Wrought Products Only):
Products whose strength is increased by strain-hardening with or without supplementary thermal treatments to produce partial softening.
(iv) Solution Heat-Treated:
This refers to unstable temper possible to alloys which spontaneously age at room temperature after solution heat treatment.
(v) Thermally Treated to Produce Stable Tempers:
Products which are thermally treated with or without supplementary strain-hardening, to produce stable tempers.
Indian Standard Specifications on Aluminium and its Alloys:
Bureau of Indian Standards (BIS) have brought out following IS specification on aluminium metal and its alloys:
IS: 617 Casting for general engineering.
IS: 733 Wrought bars, rods and sections.
IS: 734 Wrought forging stock.
IS: 736 Wrought plates.
IS: 737 Wrought sheet and strip.
IS: 738 Wrought drawn tube.
IS: 739 Wrought wires.
IS: 740 Wrought rivet stock.
IS: 1284 Wrought bolt and screw stock.
IS : 6751 Castings and strips for bearings.
IS: 7793 Wrought I.C. engine pistons.
Effect of Alloying Elements on Aluminium:
Table below summarises the effect of various elements on aluminium:
Designation of Aluminium and its Alloys:
BIS have brought out IS : 6051 to describe the designation for aluminium and its alloys:
(a) Element Number Designations:
(b) Five Digit System:
