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metallurgy

metallurgy (mĕt´əlûr´jē), science and technology of metals and their alloys. Modern metallurgical research is concerned with the preparation of radioactive metals, with obtaining metals economically from low-grade ores, with obtaining and refining rare metals hitherto not used, and with the formulation of alloys. Powder metallurgy deals with the manufacture of ferrous and nonferrous parts by compacting elemental metal or alloy powders in a die. The resultant shapes are then heated in a controlled-atmosphere furnace to bond the particles so that the part will retain the shape at normal temperatures and pressures. Welding and soldering (see solder) are techniques for joining metals metallurgically. Extractive metallurgy is the study and practice of separating metals from their ores and refining them to produce a pure metal. This article discusses the extraction of metals in general terms, but methods for the treatment of ores are quite diverse; see also aluminum, copper, gold, iron, lead, nickel, silver, tin, and zinc for special procedures followed.

Concentration of the Ore

When an ore has a low percentage of the desired metal, a method of physical concentration must be used before the extraction process begins. In one such method, the ore is crushed and placed in a machine where, by shaking, the heavier particles containing the metal are separated from the lighter rock particles by gravity. Another method is the flotation process, used commonly for copper sulfide ores. In certain cases (as when gold, silver, or occasionally copper occur "free," i.e., uncombined chemically in sand or rock), mechanical or ore dressing methods alone are sufficient to obtain relatively pure metal. Waste material is washed away or separated by screening and gravity; the concentrated ore is then treated by various chemical processes.

Separation of the Metal

Processes for separating the metal from the impurities it is found with or the other elements with which it is combined depend upon the chemical nature of the ore to be treated and upon the properties of the metal to be extracted. Gold and silver are often removed from the impurities associated with them by treatment with mercury, in which they are soluble. Another method for the separation of gold and silver is the so-called cyanide process. The Parkes process, which is based on silver being soluble in molten zinc while lead is not, is used to free silver from lead ores. Since almost all the metals are found combined with other elements in nature, chemical reactions are required to set them free. These chemical processes are classified as pyrometallurgy, electrometallurgy, and hydrometallurgy.

Pyrometallurgy, or the use of heat for the treatment of an ore, includes smelting and roasting. If the ore is an oxide, it is heated with a reducing agent, such as carbon in the form of coke or coal; the oxygen of the ore combines with the carbon and is removed in carbon dioxide, a gas (see oxidation and reduction). The waste material in the ore is called gangue; it is removed by means of a substance called a flux which, when heated, combines with it to form a molten mass called slag. Being lighter than the metal, the slag floats on it and can be skimmed or drawn off. The flux used depends upon the chemical nature of the ore; limestone is usually employed with a siliceous gangue. A sulfide ore is commonly roasted, i.e., heated in air. The metal of the ore combines with oxygen of the air to form an oxide, and the sulfur of the ore also combines with oxygen to form sulfur dioxide, which, being a gas, passes off. The metallic oxide is then treated with a reducing agent. When a carbonate ore is heated, the oxide of the metal is formed, and carbon dioxide is given off; the oxide is then reduced.

Electrometallurgy includes the preparation of certain active metals, such as aluminum, calcium, barium, magnesium, potassium, and sodium, by electrolysis: a fused compound of the metal, commonly the chloride, is subjected to an electric current, the metal collecting at the cathode.

Hydrometallurgy, sometimes called leaching, involves the selective dissolution of metals from their ores. For example, certain copper oxide and carbonate ores are treated with dilute sulfuric acid, forming water-soluble copper sulfate. The metal is recovered by electrolysis of the solution. If the metal obtained from the ore still contains impurities, special refining processes are required.

Bibliography

See R. E. Reed-Hill et al., Physical Metallurgy Principles (1991); H. Chandler, Metallurgy for the Non-Metallurgist (1998); D. A. Brandt et al., Metallurgy Fundamentals (1999).

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metallography

met·al·log·ra·phy / ˌmetlˈägrəfē/ • n. the descriptive science of the structure and properties of metals. DERIVATIVES: met·al·log·ra·pher / -fər/ n. me·tal·lo·graph·ic / ˈmetl-əˈgrafik/ adj. me·tal·lo·graph·i·cal / ˌmetl-əˈgrafikəl/ adj. me·tal·lo·graph·i·cal·ly / ˌmetl-əˈgrafik(ə)lē/ adv.

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metallurgy

met·al·lur·gy / ˈmetlˌərjē/ • n. the branch of science and technology concerned with the properties of metals and their production and purification. DERIVATIVES: met·al·lur·gic / ˌmetlˈərjik/ adj. met·al·lur·gi·cal adj. met·al·lur·gi·cal·ly adv. met·al·lur·gist n.

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metallurgy

metallurgy Science and technology concerned with metals. Metallurgy includes the study of: methods of extraction of metals from their ores; physical and chemical properties of metals; alloy production, and the hardening, strengthening, corrosion-proofing and electroplating of metals. See also anodizing; galvanizing

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metallurgy

metallurgy •haji • algae • Angie •argy-bargy, Panaji •edgy, sedgy, solfeggi, veggie, wedgie •cagey, stagy •mangy, rangy •Fiji, gee-gee, squeegee •Murrumbidgee, ridgy, squidgy •dingy, fringy, mingy, stingy, whingy •cabbagy • prodigy • effigy • villagey •porridgy • strategy • cottagey •dodgy, podgy, splodgy, stodgy •pedagogy •Georgie, orgy •ogee • Fuji •bhaji, budgie, pudgy, sludgy, smudgy •bulgy •bungee, grungy, gungy, scungy, spongy •allergy, analogy, genealogy, hypallage, metallurgy, mineralogy, tetralogy •elegy •antilogy, trilogy •aetiology (US etiology), amphibology, anthology, anthropology, apology, archaeology (US archeology), astrology, biology, campanology, cardiology, chronology, climatology, cosmology, craniology, criminology, dermatology, ecology, embryology, entomology, epidemiology, etymology, geology, gynaecology (US gynecology), haematology (US hematology), hagiology, horology, hydrology, iconology, ideology, immunology, iridology, kidology, meteorology, methodology, musicology, mythology, necrology, neurology, numerology, oncology, ontology, ophthalmology, ornithology, parasitology, pathology, pharmacology, phraseology, phrenology, physiology, psychology, radiology, reflexology, scatology, Scientology, seismology, semiology, sociology, symbology, tautology, technology, terminology, theology, topology, toxicology, urology, zoology • eulogy • energy • synergy • apogee • liturgy • lethargy •burgee, clergy •zymurgy • dramaturgy

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Metallurgy

Metallurgy

Chemical or extractive metallurgy

Physical metallurgy

Mechanical working

Metallic coatings

Resources

Metallurgy is the science and technology of metals. As indicated in Table 1, the recorded history of metal working dates back over 6,000 years. Chemical or extractive metallurgy is concerned with the extraction of metals from ores and with the refining of metals. Physical metallurgy is concerned with the physical and mechanical properties of metals as affected by composition, mechanical working, and heat treatment.

Chemical or extractive metallurgy

Metalliferous ores that are taken directly from a mine are seldom suitable for metal smelting. These ores must first undergo removal or separation of waste matter to increase the concentration of the desired mineral. These processes include sorting, crushing and grinding, sizing, and separation by magnetics, electrical conductivity, specific gravity, etc.

Flotation is a widely used separation technique that takes advantage of the fact that some mineral components attract water (hydrophilicity) and others repel it (hydrophobicity). Finely divided air is introduced into a mixture of solid minerals and water. Air bubbles adhere to the hydrophobic particles, causing them to rise to the surface. These components are skimmed off. The hydrophilic components remain behind in the pulp. The sulfides of heavy metals are readily floatable, so flotation is an important method for concentrating copper, lead, and zinc ores.

Other methods of treating impure metals include magnetic separation and electrolytic refining. In magnetic separation, the magnetic components of an ore are separated from the nonmagnetic residual material. In electrolytic refining, the metal is cast into plates that serve as electrodes in electrolytic tanks. The electric current causes the metal to dissolve, and the pure metal is deposited at the electrode of opposite polarity to the plates.

The ores or concentrates of heavy metals such as copper, lead, zinc, and nickel (but not iron and tin) consist for the most part as sulfides of those metals. Removal of the sulfur is accomplished by a process

Table 1. History of Metallurgy. (Thomson Gale.)
History of metallurgy
Date Technology developed
Prior to 4000 BCGold, copper, and meteoritic iron used occasionally without melting. Hammered into shape. Copper first annealed about 4000 BC.
4000 to 3000 BCReduction of oxidized ones of copper and lead. Bronzes produced by intentionally mixing copper and tin ores (about 3500 BC.) Permanent molding of stone and metal. Soldering with copper-gold and lead-tin alloys.
3000 to 2000 BCMost jewelry techniques known before 2500 BC.
2000 to 1000 BCBellows used in furnaces by 1800 BC. Wrought iron important by 1600 BC. Steel produced by carburization in hearth.
1000 to 1 BCCast iron known in China. Iron and steel welded into composite tools and weapons. Stamping of coins by 700 BC.
1 to 1000 ADZinc smelted in China and India.
1380Blast furnaces used to carburize and melt iron.
1440Type metals for printing. The earliest type metals were tin-based. These were later displaced by lead-antimony in the 1600s.
1509First cast iron cannon produced.
1627Brass known to have been produced from copper and metallic zinc.
1718Tables of metal affinities published.
1783Phlogistron theory of metals disproved by Lavoisier. Phlogistron was hypothetical substance thought to be a volatile constituent of all combustible substances released as flame in combustion.
1841Objects shaped by powder metallurgy.
1875Bessemer process for making steel developed.
1882Manganese steel developed.
1886Electrolytic aluminum produced.
1892Carbonyl nickel process developed.
1898Heat treatment of high speed steels, i.e., alloy steels that remain hard and tough at red heat.

called roasting. Roasting is a heat treatment carried out in an oxidizing atmosphere that produces a metal oxide and sulfur dioxide gas, which is usually processed to sulfuric acid. Arsenic and antimony are also removed by roasting. Roasting produces a powder, which may be agglomerated by sintering.

Physical metallurgy

Casting

INGOT CASTING. Steel and nonferrous-metal ingots that will be further worked are usually cast into ingot molds made of cast iron. In 1875, Sir Henry Bessemer patented a method of continuous casting in which a metal would be cast between two water-cooled rollers and pulled out in the form of a single plate. If it had been practical, this method would have had the advantage of introducing no intermediate stages between the molten metal and the semifinished product. It was not until shortly before World War II that a modification of this technique proved feasible with aluminum. It was later used to cast copper, and is still under development as a tool for casting iron and steel.

MOLDS. Most metallic objects begin their history by being cast in a mold. Mold casting consists of introducing molten metal into a cavity or mold having the desired form and allowing it to solidify. The molding material affects the ease and cost of making the mold, the permanency of the mold, the rate of production, the rate of cooling of the molten metal, the surface roughness, the dimensional tolerances, and the mechanical strength of the molded piece.

Casting techniques that use a mold only once include the following:

Sand-mold casting, which is the oldest process known and is still used for the largest tonnage of castings. A pattern, slightly larger than the desired part to allow for shrinkage, is placed in a flask and molding sand is rammed around it. The pattern is then removed, and the mold is prepared for pouring. The sand used may include bonding agents such as fire clay, bentonite, cereal or liquid binders, and moisture to promote cohesion. Dry sand molds are dried thoroughly before pouring; green sand molds are poured without drying. The type of sand grain, binder, and moisture used depends on the desired results.

Shell mold casting, which uses molds that are thin shells of sand bonded with a thermosetting phenolic resin. The shell is removed from the pattern and baked at 300400°F (147202°C) to completely set the resins. Finally the shells are assembled to complete the mold.

Plaster of paris casting, which gives better surface finishes, dimensional accuracy, finer detail, and a more solid structure than sand castings, but it is more expensive. The plaster mold is made by mixing plaster of paris with water, then pouring it around the pattern and allowing it to partially set. The pattern is then removed. Separate parts of the mold assembly are baked separately to complete setting and to drive off moisture.

Precision casting, which differs from sand-mold casting in that the mold consists of a single part. Precision molds are used in the casting of metals and alloys that are difficult to machine. (Cast metals usually require little or no finishing treatment.) Such castings are frequently used in precision engineering, clock-making, and the manufacture of metal ornaments.

In lost wax casting, the most widely used precision casting method, a model is made of the desired product. The model is used to produce a permanent plaster or glue mold. Wax parts are then made from the mold. The casting mold is produced by pouring a specially bonded sand around the wax pattern and allowing it to harden. The mold is inverted, placed in an oven and baked. The baking hardens the mold and melts the wax, which escapes.

When a large number of parts is needed or when better surface or dimensional control is required, a permanent metal mold may be used. Semipermanent molds consist of metal and sand molds. Metal molds, however, are unsuitable for large castings or for alloys having high melting temperatures. Permanent casting techniques include the following:

Chill casting, which is used to obtain more uniform cooling rates. Thick sections can be made to solidify by chilling them with a metal mold or with pieces of metal close to the section. Thin pieces can be preheated or made from material having poor thermal conductivity.

Pressure die casting, which permits economical production of intricate castings at a rapid rate. In this process molten metal is forced into a mold under considerable pressure. The pressure is maintained until solidification is complete.

Centrifugal casting, which involves pouring a molten metal into a revolving mold. Centrifugal action forces the metal tightly against the mold. The metal solidifies with an outer surface that conforms to the molds shape and surface of revolution on the inside.

The metal for casting may come from reduced ore, from an open hearth or other remelting furnace, from electroreduction processing, or from remelting and alloying. To obtain a perfect casting, the liquid metal must completely fill every part of the mold before solidifying. Vacuum melting, although expensive, permits higher casting temperatures, better fluidity, and lower surface tension conditions.

As the metal solidifies, impurities that were soluble in the liquid metal become concentrated in the last parts to solidify. This would normally give rise to nonuniform impurity distributions throughout the cast piece. Reservoirs are therefore often incorporated into the casting process to trap the impurities.

Powder metallurgy

In powder metallurgy, articles are produced by agglomeration of fine metallic powder. This technique is used where other methods of shaping such as casting, forging, and machining are impractical. The materials used in powder metallurgy usually consist of a mixture of metallic and nonmetallic powders. The are cold pressed to initially adhere the particles. Then they are heated in compacts in a nonoxidizing atmosphere (sintering) to obtain final cohesion. In isostatic pressing, the powder is pressed in a closed flexible container of rubber or plastic under liquid pressure.

Mechanical working

Mechanical working of a metal is plastic deformation performed to change dimensions, properties, and/or surface conditions. Plastic deformation below the recrystallization temperature is called cold working. Plastic deformation above the recrystallization temperature, but below the melting or burning point, is called hot working.

Cold working produces a good surface finish, close dimensional tolerance, and does not result in weight loss during working. It produces considerable increase in strength and hardness, and reduces ductility. By repeatedly cycling a material through stages of annealing and cold working, a very strong material can be produced.

Hot working is a combination of working and annealing that involves deforming the metal above the recrystallization temperature. Ductility is restored during recrystallization, and grain growth during or immediately following recrystallization.

Effects of hot working a metal piece may include: densifying the metal; refining the grain structure; introducing homogeneity into the metal; introducing a preferential orientation into the metal.

Forging

One of the most important properties of metals is their malleability, i.e., their ability to be mechanically deformed by forging, rolling, extrusion, etc., without rupture and without significant resistance to deformation. If metals can be mechanically deformed when cold, the material is said to be ductile. In the course of such deformation, most metals undergo work hardening (strain hardening). Metals that undergo work hardening are processed at room temperature. Those that are first heated above certain temperatures to make them malleable are hot formed. Forging is an important hot-forming process. In the process, the metal flows in the direction of least resistance. The most important forged metals are steel and steel alloys.

Cold extrusion

In cold extrusion, the metal is made to flow while cold by the application of high pressure. The process is used with any cold workable material, e.g., tin, zinc, copper and its alloys, aluminum and its alloys, and low-carbon soft-annealed steels.

Hot extrusion

Hot extrusion is a hot-working process that makes use of the deformability of heated metallic materials to shape them. The process is sited for producing barlike and tubular objects. Most metals and alloys can be extruded.

Cutting and machining

Forging and extrusion do not involve the removal of metal by means of cutting tools. Many important shaping processes are based on cutting operations. Cutting tools are made of special steels (tool steels), hard metals, oxide ceramics, and diamond.

Welding

Welding is the joining of metals by the application of heat and/or pressure, with or without the addition of a filler metal. Welding is used to form joints and connections, or to protect components against corrosion or wear by the application of an armoring layer of a more resistant metal.

In pressure welding, the parts to be joined are locally heated at the place where the joint is to be formed. The parts are then pressed together in the plastic state so that they are joined. Usually no filler is employed. Cold pressure welding makes use of high pressure, without the aid of heat, to unite parts. Ultrasonic and explosion welding are variations of this technique.

In fusion welding, metals are heated to the temperature at which they melt, and are then joined without hammering or the application of pressure. Although the joint can be formed without using a filler material, a filler is usually employed. The source of heat may be gas, electricity, chemical reactions, etc. Gas welding uses a flame produced by burning acetylene in oxygen or sometimes another fuel gas. This is a widely employed method of welding iron, steel, cast iron, and copper. The flame is applied to the edges of the joint and to a wire of filler material, which is melted and runs into the joint.

Soldering

Soldering is the process of joining metal parts by means of a molten filler metal (solder) whose melting point is lower than that of the metals to be joined. The metals to be joined are wetted by the solder without themselves being melted (as in the case of welding). Unlike the case of welding, two different metals can be joined by soldering.

There are two types of solders: soft and hard. Soft solders usually consist of a mixture of lead and tin; and the heat required to melt them is supplied by a soldering iron. Hard solders include brass (copper-zinc alloys) solders, silver solders, copper solders, nickel-silver solders, and solders for light alloys; the heat to melt them is usually supplied by a blow torch.

Metal forming

Sheet metal can be formed into a wide variety of hollow shapes and sections. The equipment required to work sheet metal ranges from simple hand tools to highly automated machinery. The process usually begins with basic shearing operations such as cutting, slitting, and perforating. This is followed by shaping operations, i.e., folding and bending.

Metallic coatings

Galvanizing

Zinc plays an important role in protecting iron and steel from corrosion. The process of applying the zinc coating is called galvanizing. In hot-dip galvanizing, the zinc coating is applied by dipping the object to be coated into a bath of molten zinc; the zinc combines with the iron to form a coating of iron-and-zinc crystals. Other galvanizing techniques include electro-galvanizing, metallizing, and sherardizing (forming intermetallic compounds of iron and zinc on a steel surface by heating in the presence of zinc dust below the dusts melting point).

Metallizing

Metallizing is a process for applying protective coatings to iron and steel. It consists of spraying particles of molten metal to the surface to be treated, and can be used with most common metals including aluminum, copper, lead, nickel, tin, zinc, and various alloys. Coatings of lead, aluminum, silver or stainless steel are sometimes used for protection against corrosion in the chemical and food industries. Steel or hard alloy coatings are used as wearing surfaces. In the electronics industries, metallic coatings are applied to nonmetallic materials to make them electrically conductive.

Electroplating

Electroplating is the process of producing a metallic coating on a surface by electrodeposition involving

KEY TERMS

Annealing Heating to and holding at a suitable temperature and then cooling at a suitable rate to obtain the desired mechanical, physical, or other properties.

Cold working Deforming metal plastically at a temperature lower than the recrystallization temperature.

Ductility The ability of a material to deform plastically without fracturing.

Fracture stress The maximum principal true stress at fracture.

Metal An opaque lustrous elemental chemical substance that is a good conductor of heat and electricity, and when polished a good reflector of light.

Metalliferous Containing or yielding metal.

Sintering The bonding of adjacent surfaces of particles in a mass of metal powders by heating.

Yield strength The stress at which a material exhibits a specified deviation from the proportionality of stress to strain.

an electric current. In electroplating, the coating material is deposited from an aqueous acid or alkaline solution (electrolyte) onto the metal surface to be coated. Such coatings may have protective and/or decorative functions.

See also Metal production.

Resources

BOOKS

Brandt, D.A. and J.C. Warner. Metallurgy Fundamentals. Chicago: Goodheart-Wilcox, 2004.

Hosford, W.F. Physical Metallurgy. Boca Raton, FL: CRC Press, 2005.

Randall Frost

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Metallurgy

Metallurgy

Metallurgy is the science and technology of metals. As indicated in Table 1, the recorded history of metal working goes back over 6,000 years.

Chemical or extractive metallurgy is concerned with the extraction of metals from ores and with the refining of metals. Physical metallurgy is concerned with the physical and mechanical properties of metals as affected by composition, mechanical working, and heat treatment.


Chemical or extractive metallurgy

Metalliferous ores that are taken directly from the mine are seldom suitable for metal smelting. These ores must first undergo removal or separation of waste matter to increase the concentration of the desired mineral. These processes include sorting, crushing and grinding, sizing, and separation by magnetics, electrical conductivity , specific gravity, etc.

Flotation is a widely used separation technique that takes advantage of the fact that some mineral components attract water (hydrophilicity) and others repel it

TABLE 1. HISTORY OF METALLURGY
Date Technology developed
Prior to 4000 b.c. Gold, copper, and meteoritic iron used occasionally without melting. Hammered into shape. Copper first annealed about 4000 b.c.
4000 to 3000 b.c. Reduction of oxidized ores of copper and lead. Bronzes produced by intentionally mixing copper and tin ores (about 3500 b.c.). Permanent molding of stone and metal. Soldering with copper-gold and lead-tin alloys.
3000 to 2000 b.c. Most jewelry techniques known before 2500 b.c.
2000 to 1000 b.c. Bellows used in furnaces by 1800 b.c. Wrought iron important by 1600 b.c. Steel produced by carburization in hearth.
1000 to 1 b.c. Cast iron known in China. Iron and steel welded into composite tools and weapons. Stamping of coins by 700 b.c.
1 to 1000 a.d. Zinc smelted in China and India.
1380 Blast furnaces used to carburize and melt iron.
≈ 1440 Type metals for printing. The earliest type metals were tin-based. These were later displaced by lead-antimony in the 1600s.
1509 First cast iron cannon produced.
1627 Brass known to have been produced from copper and metallic zinc.
1718 Tables of metal affinities published.
1783 Phlogistron theory of metals disproved by Lavoisier. Phlogistron was a hypothetical substance thought to be a volatile constituent of all combustible substances released as flame in combustion.
1841 Objects shaped by powder metallurgy.
1875 Bessemer process for making steel developed.
1882 Manganese steel developed.
1886 Electrolytic aluminum produced.
1892 Carbonyl nickel process developed.
1898 Heat treatment of high speed steels, i.e., alloy steels that remain hard and tough at red heat.

(hydrophobicity). Finely divided air is introduced into a mixture of solid minerals and water. Air bubbles adhere to the hydrophobic particles, causing them to rise to the surface. These components are skimmed off. The hydrophilic components remain behind in the pulp. The sulfides of heavy metals are readily floatable, so flotation is an important method for concentrating copper , lead , and zinc ores.

Other methods of treating impure metals include magnetic separation and electrolytic refining. In magnetic separation, the magnetic components of an ore are separated from the nonmagnetic residual material. In electrolytic refining, the metal is cast into plates that serve as electrodes in electrolytic tanks. The electric current causes the metal to dissolve, and the pure metal is deposited at the electrode of opposite polarity to the plates.

The ores or concentrates of heavy metals such as copper, lead, zinc, and nickel (but not iron and tin) consist for the most part as sulfides of those metals. Removal of the sulfur is accomplished by a process called roasting. Roasting is a heat treatment carried out in an oxidizing atmosphere that produces a metal oxide and sulfur dioxide gas, which is usually processed to sulfuric acid . Arsenic and antimony are also removed by roasting. Roasting produces a powder, which may be agglomerated by sintering.


Physical metallurgy

Casting

ingot casting.Steel and nonferrous-metal ingots that will be further worked are usually cast into ingot molds made of cast iron. In 1875, Sir Henry Bessemer patented a method of continuous casting in which a metal would be cast between two water-cooled rollers and pulled out in the form of a single plate. If it had been practicable, this method would have had the advantage of introducing no intermediate stages between the molten metal and the semifinished product. It was not until shortly before World War II that a modification of this technique proved feasible with aluminum . It was later used to cast copper, and is still under development as a tool for casting iron and steel.

molds. Most metallic objects begin their history by being cast in a mold . Mold casting consists of introducing molten metal into a cavity or mold having the desired form and allowing it to solidify. The molding material affects the ease and cost of making the mold, the permanency of the mold, the rate of production, the rate of cooling of the molten metal, the surface roughness, the dimensional tolerances, and the mechanical strength of the molded piece.

Casting techniques that use a mold only once include the following:

Sand-mold casting, which is the oldest process known and is still used for the largest tonnage of castings. A pattern, slightly larger than the desired part to allow for shrinkage, is placed in a flask and molding sand is rammed around it. The pattern is then removed, and the mold is prepared for pouring. The sand used may include bonding agents such as fire clay, bentonite, cereal or liquid binders, and moisture to promote cohesion. Dry sand molds are dried thoroughly before pouring; green sand molds are poured without drying. The type of sand grain, binder, and moisture used depends on the desired results.

Shell mold casting, which uses molds that are thin shells of sand bonded with a thermosetting phenolic resin. The shell is removed from the pattern and baked at 300–400°F (147–202°C) to completely set the resins . Finally the shells are assembled to complete the mold.

Plaster of paris casting, which gives better surface finishes, dimensional accuracy , finer detail, and a more solid structure than sand castings, but it is more expensive. The plaster mold is made by mixing plaster of paris with water, then pouring it around the pattern and allowing it to partially set. The pattern is then removed. Separate parts of the mold assembly are baked separately to complete setting and to drive off moisture.

Precision casting, which differs from sand-mold casting in that the mold consists of a single part. Precision molds are used in the casting of metals and alloys that are difficult to machine. (Cast metals usually require little or no finishing treatment.) Such castings are frequently used in precision engineering , clockmaking, and the manufacture of metal ornaments.

In lost wax casting, the most widely used precision casting method, a model is made of the desired product. The model is used to produce a permanent plaster or glue mold. Wax parts are then made from the mold. The casting mold is produced by pouring a specially bonded sand around the wax pattern and allowing it to harden. The mold is inverted, placed in an oven and baked. The baking hardens the mold and melts the wax, which escapes.

When a large number of parts is needed or when better surface or dimensional control is required, a permanent metal mold may be used. Semipermanent molds consist of metal and sand molds. Metal molds, however, are unsuitable for large castings or for alloys having high melting temperatures. Permanent casting techniques include the following:

Chill casting, which is used to obtain more uniform cooling rates. Thick sections can be made to solidify by chilling them with a metal mold or with pieces of metal close to the section. Thin pieces can be preheated or made from material having poor thermal conductivity.

Pressure die casting, which permits economical production of intricate castings at a rapid rate. In this process molten metal is forced into a mold under considerable pressure. The pressure is maintained until solidification is complete.

Centrifugal casting, which involves pouring a molten metal into a revolving mold. Centrifugal action forces the metal tightly against the mold. The metal solidifies with an outer surface that conforms to the mold's shape and surface of revolution on the inside.

The metal for casting may come from reduced ore, from an open hearth or other remelting furnace, from electroreduction processing, or from remelting and alloying. To obtain a perfect casting, the liquid metal must completely fill every part of the mold before solidifying. Vacuum melting, although expensive, permits higher casting temperatures, better fluidity, and lower surface tension conditions.

As the metal solidifies, impurities that were soluble in the liquid metal become concentrated in the last parts to solidify. This would normally give rise to non-uniform impurity distributions throughout the cast piece. Reservoirs are therefore often incorporated into the casting process to trap the impurities.


Powder metallurgy

In powder metallurgy, articles are produced by agglomeration of fine metallic powder. This technique is used where other methods of shaping such as casting, forging, and machining are impractical. The materials used in powder metallurgy usually consist of a mixture of metallic and nonmetallic powders. The are cold pressed to initially adhere the particles. Then they are heated in compacts in a nonoxidizing atmosphere (sintering) to obtain final cohesion. In isostatic pressing, the powder is pressed in a closed flexible container of rubber or plastic under liquid pressure.


Mechanical working

Mechanical working of a metal is plastic deformation performed to change dimensions, properties, and/or surface conditions. Plastic deformation below the recrystallization temperature is called cold working. Plastic deformation above the recrystallization temperature, but below the melting or burning point, is called hot working.

Cold working produces a good surface finish, close dimensional tolerance, and does not result in weight loss during working. It produces considerable increase in strength and hardness, and reduces ductility. By repeatedly cycling a material through stages of annealing and cold working, a very strong material can be produced.

Hot working is actually a combination of working and annealing that involves deforming the metal above the recrystallization temperature. Ductility is restored during recrystallization, and grain growth during or immediately following recrystallization.

Effects of hot working a metal piece may include: densifying the metal; refining the grain structure; introducing homogeneity into the metal; introducing a preferential orientation into the metal.


Forging

One of the most important properties of metals is their malleability, i.e., their ability to be mechanically deformed by forging, rolling, extrusion, etc., without rupture and without significant resistance to deformation. If metals can be mechanically deformed when cold, the material is said to be ductile. In the course of such deformation, most metals undergo work hardening (strain hardening). Metals that undergo work hardening are processed at room temperature. Those that are first heated above certain temperatures to make them malleable are hot formed. Forging is an important hot-forming process. In the process, the metal flows in the direction of least resistance. The most important forged metals are steel and steel alloys.


Cold extrusion

In cold extrusion, the metal is made to flow while cold by the application of high pressure. The process is used with any cold workable material, e.g., tin, zinc, copper and its alloys, aluminum and its alloys, and low-carbon soft-annealed steels.


Hot extrusion

Hot extrusion is a hot-working process that makes use of the deformability of heated metallic materials to shape them. The process is sited for producing barlike and tubular objects. Most metals and alloys can be extruded.


Cutting and machining

Forging and extrusion do not involve the removal of metal by means of cutting tools. Many important shaping processes are based on cutting operations. Cutting tools are made of special steels (tool steels), hard metals, oxide ceramics , and diamond .


Welding

Welding is the joining of metals by the application of heat and/or pressure, with or without the addition of a filler metal. Welding is used to form joints and connections, or to protect components against corrosion or wear by the application of an armoring layer of a more resistant metal.

In pressure welding, the parts to be joined are locally heated at the place where the joint is to be formed. The parts are then pressed together in the plastic state so that they are joined. Usually no filler is employed. Cold pressure welding makes use of high pressure, without the aid of heat, to unite parts. Ultrasonic and explosion welding are variations of this technique.

In fusion welding, metals are heated to the temperature at which they melt, and are then joined without hammering or the application of pressure. Although the joint can be formed without using a filler material, a filler is usually employed. The source of heat may be gas, electricity , chemical reactions , etc. Gas welding uses a flame produced by burning acetylene in oxygen or sometimes another fuel gas. This is a widely employed method of welding iron, steel, cast iron, and copper. The flame is applied to the edges of the joint and to a wire of filler material, which is melted and runs into the joint.


Soldering

Soldering is the process of joining metal parts by means of a molten filler metal (solder) whose melting point is lower than that of the metals to be joined. The metals to be joined are wetted by the solder without themselves being melted (as in the case of welding). Unlike the case of welding, two different metals can be joined by soldering.

There are two types of solders: soft and hard. Soft solders usually consist of a mixture of lead and tin; and the heat required to melt them is supplied by a soldering iron. Hard solders include brass (copper-zinc alloys) solders, silver solders, copper solders, nickel-silver solders, and solders for light alloys; the heat to melt them is usually supplied by a blow torch.


Metal forming

Sheet metal can be formed into a wide variety of hollow shapes and sections. The equipment required to work sheet metal ranges from simple hand tools to highly automated machinery. The process usually begins with basic shearing operations such as cutting, slitting, and perforating. This is followed by shaping operations, i.e., folding and bending.


Metallic coatings

Galvanizing

Zinc plays an important role in protecting iron and steel from corrosion. The process of applying the zinc coating is called galvanizing. In hot-dip galvanizing, the zinc coating is applied by dipping the object to be coated into a bath of molten zinc; the zinc combines with the iron to form a coating of iron-and-zinc crystals. Other galvanizing techniques include electrogalvanizing, metallizing, and sherardizing (forming intermetallic compounds of iron and zinc on a steel surface by heating in the presence of zinc dust below the dust's melting point).

Metallizing

Metallizing is a process for applying protective coatings to iron and steel. It consists of spraying particles of molten metal to the surface to be treated, and can be used with most common metals including aluminum, copper, lead, nickel, tin, zinc, and various alloys. Coatings of lead, aluminum, silver or stainless steel are sometimes used for protection against corrosion in the chemical and food industries. Steel or hard alloy coatings are used as wearing surfaces. In the electronics industries, metallic coatings are applied to nonmetallic materials to make them electrically conductive.


Electroplating

Electroplating is the process of producing a metallic coating on a surface by electrodeposition involving an electric current. In electroplating, the coating material is deposited from an aqueous acid or alkaline solution (electrolyte ) onto the metal surface to be coated. Such coatings may have protective and/or decorative functions.

See also Metal production.


Resources

books

Lyman, Taylor. Metals Handbook. Metals Park, OH: American Society for Metals, 1961.

Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin Company, 1998.

Smith, Charles O. The Science of Engineering Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc, 1969.


Randall Frost

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Annealing

—Heating to and holding at a suitable temperature and then cooling at a suitable rate to obtain the desired mechanical, physical, or other properties.

Cold working

—Deforming metal plastically at a temperature lower than the recrystallization temperature.

Ductility

—The ability of a material to deform plastically without fracturing.

Fracture stress

—The maximum principal true stress at fracture.

Metal

—An opaque lustrous elemental chemical substance that is a good conductor of heat and electricity, and when polished a good reflector of light.

Metalliferous

—Containing or yielding metal.

Sintering

—The bonding of adjacent surfaces of particles in a mass of metal powders by heating.

Yield strength

—The stress at which a material exhibits a specified deviation from the proportionality of stress to strain.

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