Homework of Materials Science

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Дата конвертации29.10.2012
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Umut Karahan


Homework of Materials Science

Copper and copper alloys constitute one of the major groups of commercial metals. They are widely used because of their excellent electrical and thermal conductivities, outstanding resistance to corrosion, ease to fabrication, and good strength and fatigue resistance. They are generally nonmagnetic. They can be plated, coated with organic substances, or chemical colored further extend the variety of available finishes.

Pure copper is used extensively for cables and wires, electrical contacts, and a wide variety of other parts that are required to pass electrical current. Coppers and certain brasses, bronzes, and cupronickels are used extensively for automobile radiators heat exchangers, home heating system, panels for absorbing solar energy, and various applications requiring rapid conduction of heat across or along a metal section. Because of their outstanding ability to resist corrosion, coppers brasses, some bronzes, and cupronickels are used for pipes, valves, and fitting systems carrying potable water, process water, or other aqueous fluids.

In all classes of copper alloys, certain alloy combinations for wrought products have counterparts among the cast alloys; this enables designers to make an initials alloy selections before deciding on the manufacturing process. Most wrought alloys are available in various cold-worked conditions, and room-temperature strengths and fatigue resistance of these alloys depend of amount of the cold work as well as the alloy content. Typical applications of cold-worked wrought alloys (cold-work tempers) include springs, fasteners, hardware, small gears, cams, electrical contacts, and components.

Certain types of parts, most notably plumbing fittings and valves, are produced by hot forging simply because no other fabrication process can produce the required shapes and properties economically. Copper alloys containing 1 to 6% Pb are free-machining grades. These alloys are widely used for machining parts, especially those produced in screw machines.

Although fewer alloys are produced now than in the 1930s, new alloys continue to be developed and introduced, in particular to meet the challenging requirements of the electronic industry.

Properties of Importance

Along with strength, fatigue resistance, and ability to take a good finish, the primary selection criteria for copper and copper alloys are:

  • Corrosion resistance

  • Electrical conductivity

  • Thermal conductivity

  • Color

  • Ease to fabrication

Corrosion Resistance. Copper is a noble metal but, unlike gold and other precious metals, can be attacked by common reagents and environments. Pure copper resists attack quite well under most corrosive conditions. Some copper alloys, however, sometimes limited usefulness in certain environments because of hydrogen embrittlement stress-corrosion cracking (SCC).

Hydrogen embrittlement is observed when tough pitch coppers, which are alloys containing cuprous oxide, are exposed to a reducing atmosphere. Most copper alloys are deoxidized and thus are not subject to hydrogen embrittlement.

Stress-corrosion cracking most commonly occurs in brass that exposed to ammonia or amines. Brasses containing more than 15% Zn are the most susceptible. Copper and most copper alloys that either do not contain zinc content generally are not susceptible to SCC. Because SCC requires both tensile stress and a specific chemical species to present at the same time, removal of either the stress or chemical species can prevent cracking. Annealing or stress relieving after forming alleviates SCC by residual stresses. Stress reliving is effective only if the parts are not subsequently bent or stained in service: such operations reintroduce stresses and resensitize the parts to SCC.

Dealloying is another form of corrosion that affects zinc-containing copper alloys. In dealloying the more active metal is selectively removed from an alloy, leaving behind a weak deposit of the more noble metal.

Copper-zinc alloys containing more than 15% Zn are susceptible to a dealloying process called dezincification. In the dezincification of brass, selective removal of zinc leaves porous and weak layer of copper and copper oxide. Corrosion of a similar nature continues beneath the primary corrosion layer, resulting in gradual replacement of sound brass by weak, porous copper. Unless arrested, dealloying eventually penetrates the metal the metal, weakening it structurally and allowing liquids or gases to leak through the porous mass in the remaining structure.

Electrical and thermal conductivity. Copper and its alloys are relatively good conductors of electricity and heat. In fact, copper is used for these purposes more often than any other metal. Alloying invariably decreases electrical conductivity and, to lesser extent, thermal conductivity. The amount of reduction due to alloying does not depend on conductivity or any other bulk property of alloying element, but only on the effect that the particular solute atoms have on the copper lattice. For this reason copper and high-cooper alloys are preferred over copper alloys containing more than a few percent total content when high electrical or thermal conductivity is required for application.

Color. Copper and certain copper alloys are used for decorative purposes alone, or when a particular color and finish is combined with a desirable mechanical or psychical property of the alloy.

Ease of Fabrication. Copper and its alloys are generally capable of being shaped to the required form and dimensions by any of the common fabricating processes. They are routinely rolled, extruded, forged and formed at elevated temperature. Copper alloys are readily stamped and formed into components. Most countries in the world employ copper alloys for coinage. There are casting alloys for all the generic families of coppers and copper alloys. Copper metals can be polished, textured, plated, or coated to provide wide variety of functional or decorative surfaces. Copper and copper alloys are readily assembled by any of the various mechanical or bonding processes commonly used to join metal components. Crimping, staking, riveting, and bolting are mechanical means of maintaining joint integrity. Soldering, brazing, and welding are most widely used processes for bonding copper metals. Selection of the best joining process is governed by service requirements, joint configuration, thickness of the components and alloy composition(s).

Mechanical Working

High-purity copper is very soft metal. It is softest in its undeformed single-crystal form and requires a shear stress only 3.9 MPa on {111} crystal planes for slip. Annealed tough pitch corner is almost as soft as high-purity copper, but many of the copper alloys are much harder and stiffer, even in annealed tempers.

Copper cans easily deformed cold. Once flow has been started, it takes little energy to continue, and thus extremely large charged in shape or reductions in section are possible in a single pass. The only limitation appears to be the ability to design and build the necessary tools. Very heavy reductions are possible, especially with continuous flow. Rolling reductions of more than % 90 in passes are used for rolling strip.

Copper and many of its alloys also respond well to sequential cold working. Tandem rolling and gang-die drawing are common. Some copper alloys work harden rapidly; therefore, the number of operations that can be performed before annealing to resoften is limited.

Cold working increases both tensile strength, but it has a more pronounced effect on the latter. For most coppers and copper alloys, tensile strength of hardest cold-worked temper is approximately twice the tensile strength of the annealed temper. For the same alloys, the yield strength of the hardest cold worked temper can be as much as five to six times that of the annealed temper.

Hardness as a measure of temper is inaccurate: The relation between hardness and strength is different for different alloys. Usually, hardness and strength for a given alloy can be correlated only over a rather narrow range of conditions. Also, the range of correlation is often different methods of hardness determinations.

Hot working. Not all shaping is confined cold deformation. Hot working is commonly used for alloys that remain ductile above recrystallization temperature. Hot working permits more extensive changes in shape than cold working, and thus a single operation often can replace a sequence of forming and annealing operations. To avoid preferred orientation and textures, and to achieve processing economy, copper and many copper alloys are hot worked to nearly finished size. Hot working reduces the as cast grain size from about 1 to 10mm to about 0.1 mm or less and yields a soft texture-free structure suitable for cold finishing.

Some hot working operations may produce strengths that of the annealed temper. However, property control by hot working is very difficult and is rarely attempted.
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