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COPPER/ALUMINIUM (ALUMINIUM BRONZE, AB1, AB2 & CA EN CC330-334G SERIES)
Copper/aluminium alloys typically contain 10% aluminium and 3% iron added to the copper base; 5% nickel is added to make the AB2 grade (US 63000). Formulated for components used in adverse environments and non-sparking tools, this alloy is extremely hard and highly resistant to corrosive attack. The alloy’s corrosion resistance is in part due to the tough OXIDE SURFACE that forms on the alloy in the presence of oxygen, a common feature of aluminium bearing alloys. Being a material resistant to chemical action, ALUMINIUM BRONZE is not a suitable alloy for patination.
From a technical point of view, successfully casting aluminium bronze can be very challenging for the founder and considerable thought must be given to the design of the running and feeder systems. The difficulties encountered in casting aluminium bronzes are in no small part due to the oxide skin and DROSS produced on the surface of the charge during the melt. This film can adversely affect the quality of a cast if it enters an investment or sand mould. Casting is also complicated by the tendency for the alloy to dramatically shrink, (or DRAW) as it cools in the refractory mould. To counter these difficulties foundries specialising in the casting of aluminium bronze works usually set very large diameter runners into their moulds and often employ a pouring technique known as the MEIGH PROCESS. This Meigh process involves physically attaching the refractory mould to a tilting furnace. The furnace gradually fills the mould cavity as it rotates around an axis, this controlled filling action avoids unduly disturbing the metal charge during the pour.
Aluminium bronzes polish to a golden lustre with a slight green tint, it is this finish that is the principal attraction of the material to artists. Most artworks formed in aluminium bronze are taken to a mirror polish, though executing this can be a time consuming process given the degree of comparative hardness in the alloy. The high strength qualities of aluminium bronze may be of some value if structural considerations are paramount, for example if the cast (or fabrication), also needs to function effectively as a load bearing element.
A limited amount of wrought aluminium bronze material is available from non-ferrous stockholders, distinguished from the cast ‘AB’ grades by a ‘CA’ prefix. Most of these sections are round or hexagonal bars, to BS 2874 CA103 or ASTM C63000 (ISO CuAl10Ni5Fe3) standards. Wrought alloys containing 9% or more aluminium can be forged or extruded, these sections may also be HARDENED and TEMPERED through heat treatment. Both wrought and cast material can be welded by the TIG process, in either AC or DC mode, using a matching C12 or C13 type filler rod. Unlike gunmetals, aluminium bronze can be hot worked (forged), and it’s exceptional strength makes it a useful material for fabricating fixings and fastenings that are likely to be structurally stressed. If a softer product is required, a small quantity of lead may be added to the melting charge, (though adding lead will make welding more difficult).
It is advisable to seek specific information from the founder on the practicality and suitability of creating a work in this alloy. Owing to the practical difficulties presented by working aluminium bronzes, any artwork specified in this alloy is likely to be priced at a premium rate.
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