< ALUMINIUM 2
SELECTING AN ALUMINIUM ALLOY
The selection of an appropriate aluminium alloy is dependent upon a number of variable factors. These can include the proposed manufacturing process (DIE CAST, SAND CAST etc), the end use of the sculpture (for instance, does the artwork have a structural function), and the desired surface finish (ANODISED, POLISHED, PAINTED etc). Unless otherwise directed, founders will select a preferred cast or wrought alloy by default.
Aluminium casts of art and design works are most commonly formed in a grade of HEAT TREATABLE silicon/magnesium alloy. This typically suggests the use of the LM25/ASTM A336 alloy, which is suitable for both investment and sand casting processes. LM6/ASTM 413 (silicon) and LM4/ASTM 319 (silicon/copper), are often used for aluminium die castings.
Fabricators working with wrought alloys such as sheet usually prefer to chose from one of the ISO 1,3,5 or 6XXX series alloys, according to the particular demands of a job. In all cases, particular care taken to ensure any welding operation does not critically weaken the structure of the design in the HEAT AFFECTED ZONE (HAZ). Most fabrications are likely to be constructed from non-heat treatable 3 or 5XXX series manganese and magnesium alloy, using material supplied by stockholders in a variety of hardnesses (soft, quarter hard, half hard, fully hard etc). High strength alloys containing copper and zinc alloying elements (2 or 7XXX series) are usually avoided due to the likelihood of cracking under thermal expansion and contraction stresses when welded. The choice of a welding filler rod may be specific to the grade of alloy intended for welding, though for most grades, a general purpose aluminium/silicon (4043A) filler rod is usually sufficient (5056A for aluminium/magnesium 5XXX alloys).
In common with the copper/zinc (brass) alloys, aluminium alloys are readily available in an exceptionally wide range of preformed wrought and extruded sections. Likewise, an excellent range of fastenings is available, from rivets to clips. These fixings can be variously used to secure non-welded joints. Aluminium artworks and designs are usually presented in a PAINTED, POLISHED or ANODISED state of finish, although some limited chemical PATINATION is possible (a water based solution of caustic soda and calcium chloride produces a black patina on aluminium). The painting of aluminium casts and fabrications with an artist’s water based acrylic paint is possible, providing the metal is first prepared by etching with a 5% lye solution prior to the application of a primer coat.
ZINC & MAGNESIUM >
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