< LIGHT METALS
ALUMINIUM 1 (ALLOY, LM SERIES, 1XXX – 8XXX SERIES)
Aluminium is a silver-white metal in appearance. In it’s elemental form aluminium has little TENSILE STRENGTH, though it is exceptionally resistant to corrosion due to the hard ‘refractory oxide’ layer that forms over the metal’s surface in the presence of oxygen. Because of it’s inherently low tensile strength, aluminium is almost always alloyed with a variety of additional elements to improve the metal’s working properties (hence the common reference to all aluminium products simply as ALLOY especially in the US). The addition of selected elements into the parent metal can produce aluminium grades that offer double the strength to weight ratio of mild carbon steels.
Alloying the metal invariably divides the resulting aluminium products into one of two distinct groups – those that can be HEAT TREATED, and those that cannot. Heat treatment allows treated alloys to be metallurgically modified within special furnaces. This type of processing is usually carried out after welding or casting operations, at which point heat treatment effectively improves the tensile properties of a finished cast or fabrication which may have been weakened during shaping or construction. Alloys which cannot be successfully heat treated usually contain additions of magnesium and manganese. To complicate matters further, some alloys (usually those containing copper additions, of which the best known is DURALUMIUM® after the ‘Durener’ works in Germany), undergo a process known as AGE HARDENING. Through age hardening, the alloy gradually acquires it’s full structural strength during a period of storage – usually two weeks or so after treatment or working. As already mentioned in the introduction to this section, heat treatment is a rarely carried out on sculptural casts formed in aluminium or any other metal alloy.
The following listings identify the major alloying elements found within various grades of aluminium. Where applicable, a typical cast grade of the aluminium/alloying element is given. Cast grades of aluminium are identified by the ‘LM’ prefix (they may also be affixed by a supplemental letter code which denotes heat treatment and condition). For additional reference, the ASTM (US), and ISO (international) four digit designation is also given. The ISO four digit code is specifically applicable to wrought aluminium materials. The first digit of the ISO code identifies the principal alloying element in the aluminium, the following three digits provide specific additional information, including the heat treatability of the alloy. These last three digits are variable, and when not specifically stated in the text, they are represented by an ‘X’.
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