COPPER/SILICON (SILICON BRONZE, C87300-87800, CS101, EVERDUR®, DURONZE®, HERCULOY®, CUSILMN)
In recent years copper/silicon alloys have been adopted as the casting alloy of choice by an increasing number of art foundries. This is especially so in the US, where EVERDUR (4%Si/1%Mn/Cu rem), has effectively replaced the so called US STANDARD BRONZE (a 90/7/3 GUNMETAL). Reasons for the popularity of silicon bronze include the alloy’s ability to flow well into complicated moulds, produce an excellent colour match in welded joints (with a matching filler rod) and omit an inherent lead hazard.
Quite distinct from traditional art foundry alloys, cast silicon ‘bronzes’ (C87300 & similar grades) contain no significant tin, zinc or lead additions (<1%), instead 3.25-4.5% silicon and up to 1.5% manganese make up the balance of this copper alloy (see also MATERIALS & WELD TESTING).
A reasonable range of preformed wrought sections are available in a variety of Imperial and A/F sizes (Cusilmn, BS CS101, and US standard ASTM B96/98 C65500 grades are typical). Commonly stocked sections include bar (square, round and hexagonal), as well as sheet and plate in various gauges. Wrought silicon bronze, like the cast alloy, can be worked or forged in either a COLD or HOT state. The PLASTIC behaviour of this material is quite unlike that of gunmetal – which fractures all too easily when hot worked. Silicon bronze also has an better TENSILE STRENGTH than gunmetal and is significantly more resistant to corrosive attack. As a result the use of silicon bronze may be preferable to gunmetal if the sculpture is to be situated in an adverse environment, including coastal locations and heavily polluted industrial areas.
The joining of both cast and wrought silicon bronze sections is normally carried out by TIG welding with a compatible C9 type copper/silicon/manganese filler rod (similar to AWS 5.7 Cu S type fillers). Alternative fusion and non-fusion techniques include OXY-GAS welding and brazing (the latter two done with the protection of a boric acid or borax based flux). As mentioned, an excellent cast alloy/weld alloy COLOUR match can be achieved in this alloy, especially advantageous where a POLISH or light PATINATION is intended on an extensively welded cast. Silicon bronze is one of the easier copper alloys to weld due to the material’s poor conductivity (thus concentrating input heat into the weld pool), also assisted by the inherent deoxidising effect of the silicon content.
One occasional problem encountered when welding silicon bronzes is that of stress cracking. Stress cracking in the area of a weld at best leaves unsightly hairline fractures in the cast’s fabric (these cracks can become more visually prominent after a patination is applied), at worst stress cracking significantly weakens a joint. Working with a good welding technique is one solution (see FINISHING); there is also a suggestion that the addition of a small quantity of zinc to the molten charge helps reduce the incidence of weld induced stress cracking in cast alloys.
The red-gold lustre of silicon bronze can make an attractive alternative to polished phosphor bronze or gunmetal alloys. In keeping with most other copper based alloys, silicon bronze readily accepts a good range of patina treatments and alternative finishes, though the alloy’s resistance to corrosive attack may inhibit the development of the less aggressive compounds (note, silicon bronze can WORK HARDEN during a progressive POLISHING process, increasing the difficulty of achieving a high level of finish).
Casts formed in silicon bronze are neither better nor worse than those formed in any other alloy, including GUNMETAL. Each alloy has it’s own merits, and some founders and artists simply prefer the working properties and/or finish offered by one casting alloy over another. Founders who do not normally stock silicon bronze ingot may impose a surcharge if this alloy is specified instead of their standard foundry alloy.
INFO: To view typical compositional analysis of silicon bronze see: MATERIALS TESTING
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