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A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 0 3 6 location in the microstructure (solid solution, precipitate, dispersoid). The choice of alloy and condition is most often based on the trade-off between strength and conductivity. Figure 2 shows the general trade-off between strength and conductivity for solid-solu- tion, dispersion, and precipitation hard- ening. The optimal tradeoff is achieved by precipitation hardening, which is usually the most costly because of ei- ther the alloy additions or extra pro- cessing. Precipitation-hardening alloys exhibit increases in electrical conduc- tivity along with increased strength during the aging heat treatment, as ele- ments are removed from supersaturat- ed solid solution to form precipitates of intermetallic compounds. Copper and its alloys are also good conductors of heat, making them ide- al for heat-transfer applications, for example, radiators and heat exchang- ers. Changes in thermal conductivi- ty generally follow those in electrical conductivity in accordance with the Wiedemann-Franz relationship, which states that thermal conductivity is pro- portional to the product of electrical conductivity and temperature. CORROSION RESISTANCE Copper is a noble metal, but un- like gold and other precious metals, it can be attacked by common reagents and environments. Pure copper resists attack quite well under most corrosive conditions. Some copper alloys, how- ever, have limited usefulness in certain environments because of hydrogen em- brittlement or stress-corrosion crack- ing (SCC). Hydrogen embrittlement is ob- served when tough pitch coppers, which are alloys containing cuprous oxide, are exposed to a reducing at- mosphere. Most copper alloys are de- oxidized and thus are not subject to hydrogen embrittlement. Stress-corrosion cracking most commonly occurs in brass that is ex- posed to ammonia or amines. Brass- es containing more than 15% Zn are the most susceptible. Copper and most copper alloys that either do not contain zinc or are low in zinc content generally are not susceptible to SCC. Because SCC requires both tensile stress and a specif- ic chemical species to be present at the same time, removal of either the stress or the chemical species can prevent cracking. Annealing or stress relieving after forming alleviates SCC by reliev- ing residual stresses. Stress relieving is effective only if the parts are not subse- quently bent or strained in service; such operations reintroduce stresses and re- sensitize the parts to SCC. FABRICATION CHARACTERISTICS As stated previously, ease of fabri- cation is one of the properties of impor- tance for copper and its alloys. These materials are generally capable of being shaped to the required formand dimen- sions by any of the common forming or forging processes, and they are readily assembled by any of the various joining processes. A brief review of the fabrica- tion characteristics of copper and its al- loys is given subsequently. Workability. Copper and copper alloys are readily cast into cake (slabs of pure copper, generally 200 mm thick and up to 8.5 m long, or 8 in. by 28 ft), billet, rod, or plate—suitable for subse- quent hot or cold processing into plate, sheet, rod, wire, or tube—via all the standard rolling, drawing, extrusion, forging, machining, and joining meth- ods. Copper and copper alloy tubing can be made by the standard methods of piercing and tube drawing as well as by the continuous induction weld- ing of strip. Copper is hot worked over the temperature range 750 to 875°C (1400 to 1600°F), annealed between cold working steps over the tempera- ture range 375 to 650°C (700 to 1200°F), and is thermally stress relieved usually between 200 and 350°C (390 and 660°F). Copper and its alloys owe their excel- lent fabricability to the face-centered cubic crystal structure and the twelve available dislocation slip systems. Many of the applications of copper and its alloys take advantage of the work- hardening capability of the material, with the cold processing deformation of the final forming steps providing the re- quired strength/ductility for direct use or for subsequent forming of stamped components. Weldability. Copper and copper alloys are most frequently welded us- ing gas tungsten arc welding, especially for thin sections, because high local- ized heat input is important in materials with high thermal conductivity. In thick- er sections, gas metal arc welding is preferred. The weldability varies among the different alloys for a variety of rea- sons, including the occurrence of hot cracking in the leaded (free-machining) alloys and unsound welds in alloys con- taining copper oxide. Tin and zinc both reduce the weldability of copper alloys. Machinability. All copper alloys are machinable in the sense that they can be cut with standard machine tool- ing. High-speed steel suffices for all but the hardest alloys. Carbide tooling can be used but is rarely necessary, and while grinding may be required for a few alloys in very hard tempers, these are not conditions to be expected in high-speed production. For mass-pro- duced screw machine parts made from free-cutting brass or one of the other leaded copper alloys, high-speed steel is the standard tool material. Surface Finishes. For decorative parts, standard alloys in specific colors are readily available. Copper alloys can be polished and buffed to almost any desired texture and luster. They can be plated, coated with organic substances, or chemically colored to further extend the variety of available finishes. ~AM&P For more information: Harold T. Mi- chels, consultant, Manhasset, N.Y. 11030, cu.microbes@gmail.com, www. amcopper.com; retired senior vice pres- ident, Copper Development Associa- tion, www.copper.org . References 1. H.T. Michels and C.A. Michels, Cop- per Alloys—The New ‘Old’ Weapon in the Fight Against Infectious Disease. Curr. Trends Microbiol., Vol. 10, p 23-45, 2016.