Q I am designing a cast architectural feature as part of a town centre display board. The public will have access to this structure. I have been offered LG3, leaded gunmetal CuSn7Pb4Zn2 for the project but I am concerned that no British or European Standard exists for this alloy. Advice please.
A This alloy has been widely used for some notable contemporary statues since it patinates easily and is capable of being deformed to remove any distortion produced on casting. However, since no standard exists for this alloy there might be a problem since the public has access to the structure so it is suggested that leaded gun metals LG2 or LG4 are used.
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Q I am at the design stage of an engineering project, part of which may include the use of a copper alloy casting. Since considerable machining is involved I am interested in machining data for cast copper alloys.
A Machining data for copper casting alloys are similar to those of their wrought counterparts. The most easily machinable are classified as Group 1 and include LG2 (CC491K) with a rating of 90% (this rating is based on a 100% value for free machining brass CZ121,CW609N). The more difficult alloys to machine are classified as Group 3 and include the aluminium bronzes such as AB2 (CC333G) with a rating of 20%. For further details see CDA publication Pub 44 Machining Brass, Copper and Copper Alloys  .
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Q What values of electrical conductivity can be obtained in high purity copper castings? Are they comparable to the values available for wrought alloys?
A Generally the values of International Annealed Copper Standard for conductivity (IACS) values are lower than those of the wrought alloys due to the presence of a small % of gas porosity and a small % of impurities such as iron. A value of 93% IACS is guaranteed but with a very low porosity % and very pure copper, values up to 102% IACS may be obtained. Reputable foundries carry out careful conductivity checks on the copper raw material used for casting, such as offcuts of busbars or cathode copper, to ensure that as high a conductivity as possible is obtained.
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Q Is a heat treatment required for nickel aluminium bronze castings?
A For optimum corrosion resistance, it is recommended that the Royal Navy heat treatment of 675oC plus or minus 250C for 2-6hrs followed by air-cooling is specified.
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Q Which type of brass and what process is used to make bathroom and kitchen taps? How is the bright shiny surface produced?
A Taps are die cast from a general composition of 60%copper, 40% zinc. This composition gives a fluid alloy with essential hot ductility. A typical die cast alloy is DCB3 (CuZn40Pb-CC754S). Taps may be plated with successive layers of copper (3-4 microns), nickel (10-12 microns) followed by about one micron of chromium to give a hard shiny surface. Remember that when taps are replaced they have value as a source of recycled copper alloy scrap. Note that in order for copper alloys to exhibit antimicrobial activity, they should not be plated or coated.
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Q We have purchased castings in AB2 (CC333G). The standard BSEN 1982:2008 suggests in Annex A, clause A6, a stress relief heat treatment, but gives no details. Do you have a recommendation?
A Heat to 450oC for one hour and air cool. This should be agreed between you and the manufacturer when ordering.
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Q Is it possible to apply metallic coatings to give Gunmetals additional corrosion resistance?
A Yes, Gunmetals may be coated with nickel, chromium, tin or silver. The choice of coating depends on the application.
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