Choosing Alloy

Aluminium Alloy and Its Importance

Pure aluminium is soft and ductile and most commercial uses require greater strength than pure aluminium affords. So strength is achieved by the addition of other elements to produce alloys. Further strengthening is possible by means which classify the alloys roughly into two categories, non-heat-treatable and heat treatable.

NON-HEAT TREATABLE ALLOYS

The strength of alloys in this group depends upon the hardening effect of elements such as manganese, silicon, iron and magnesium. Since these alloys are work-hardenable, strength levels are controlled by various degrees of cold working, denoted by the ' H ' series of tempers. Alloys containing appreciable amounts of magnesium when supplied in strain-hardened tempers are usually given a final elevated-temperature treatment called stabilizing to ensure stability of properties

HEAT TREATABLE ALLOYS

The strength of alloy in this group is enhanced by the addition of alloying elements such as copper, magnesium, zinc and silicon. Since this elements show increasing solid solubility in aluminium with increasing temperature, thermal treatments imparts pronounced strengthening. The first step, called solution heat treatment, is an elevated - temperature process designed to put the soluble element or elements in solid solution. This is followed by rapid quenching, usually in water, which momentarily 'freezes' the structure and for a short time renders the alloy very workable. It is at this stage that some fabricators retain this more workable structure by storing the alloys at below freezing temperatures until they are ready to form them. At room or elevated temperature the alloys are not stable after quenching, and precipitation of the constituents from the super - saturated 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, particularly those containing magnesium and silicon or magnesium and zinc, continue to age - harden for long periods of time at room temperature. By heating for a controlled time at slightly elevated temperatures, further strengthening is possible and properties are stabilized. This process is called artificial ageing or precipitation hardening. By the proper combination of solution heat treatment, quenching, cold working and artificial ageing, the highest strengths are obtained.