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Fine (unalloyed) aluminium is a silver-white metal in appearance with a low TENSILE STRENGTH, though it is fairly resistant to corrosion due to the hard ‘refractory oxide’ layer that forms over the metal’s surface in the presence of oxygen. The pure metal's inherently low tensile strength means aluminium is almost always alloyed with a variety of additional elements to improve the metal’s working properties (hence the common reference to all varieties of aluminium stock simply as ALLOY, especially in the US). The addition of alloying elements into the parent metal can potentially produce aluminium grades that offer double the strength to weight ratio of mild carbon steels.

The metallurgical consequences of alloying pure aluminium means that aluminium products are generally divided into one of two distinct groups – those that can be HEAT TREATED and those that cannot. Heat treatment allows suspectable alloys to be metallurgically modified within special ovens or furnaces. This metallurgical modification is usually carried out after welding or casting operations, with successful treatment effectively improving the tensile properties of the finished cast or fabrication (which may have been weakened during shaping and construction).

Alloys which cannot be successfully heat treated usually contain additions of magnesium and manganese. To complicate matters further, some alloys undergo a process known as AGE HARDENING (esp. those containing copper additions - of which the best known is DURALUMIUM® after the ‘Durener’ works in Germany). During age hardening the alloy gradually acquires it’s full structural strength after a period of storage – usually two weeks or so after treatment or working. Though as noted in the introduction to this section, heat treatment is a rarely carried out on sculptural casts formed in aluminium (or any other metal alloy for that matter).

The descriptions given the following page outline the major alloying elements for the common grades of aluminium. Where available, a typical cast grade of that aluminium/alloying alloy is noted. 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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