welding

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welding,

process for joining separate pieces of metal in a continuous metallic bond. Cold-pressure welding is accomplished by the application of high pressure at room temperature; forge welding (forging) is done by means of hammering, with the addition of heat. In most processes in common use, the metal at the points to be joined is melted; additional molten metal is added as a filler, and the bond is allowed to cool. In the Thomson process, resistance to an electric current, passed through the sections to be joined, causes them to melt. Other notable methods include the thermitethermite
[from Thermit, a trade name], mixture of powdered or granular aluminum metal and powdered iron oxide. When ignited it gives off large amounts of heat. In wartime it has been used in incendiary bombs. A method for welding using thermite (invented by Dr.
..... Click the link for more information.
 process, oxyacetylene, electric arc, oxyhydrogen, and the atomic hydrogen flame. In this last-named method, molecules of hydrogen gas passing through an electric arc are broken up into atoms of hydrogen by absorbing energy; when outside the arc, the atoms reunite into molecules, yielding in the process enough heat to weld the material. Another process, the argon-arc method, is widely used with metals such as stainless steel, aluminum, magnesium, and titanium, which require an inert atmosphere for successful welding. The use of argon prevents slag from forming in the weld and greatly increases the speed of the welding.

Bibliography

See A. C. Davies, The Science and Practice of Welding (6th ed. 1972); J. E. Brumbaugh, Welder's Guide (3d ed. 1983).

Welding

 

a process in which solid materials are joined by the action of interatomic forces. This action results in a local coalescence or mutual plastic deformation of the parts being joined. Welding can be performed on objects made of metal or of nonmetallic materials, such as glass, ceramics, and plastics. It is possible to build up metallic, welded face layers of different thicknesses and different composition by means of appropriate adjustment of the welding conditions. Under certain conditions, special equipment makes it possible to use welding for processes essentially the opposite of joining, for instance, the flame or thermal cutting of metals.

Historical survey. Very simple welding techniques were known as early as 8,000–7,000 B.C. Most of the welded objects were made of copper; they were preheated and then pressed together. Forge welding was used to join objects made of copper, bronze, lead, and the noble metals. The parts to be joined were first molded and preheated, and then a previously melted metal was poured into the joint. Articles made of iron and iron alloys were obtained by heating the parts to a welding temperature in a forge and by subsequent hammering. Until the end of the 19th century only these two methods were in use.

The discovery of the arc discharge by V. V. Petrov in 1802 provided the impetus for new metal-joining techniques. The first practical methods of arc welding were proposed by N. N. Benardos in 1882 and N. G. Slavianov in 1890. During the early 20th century arc welding gradually became the preferred industrial method of joining metals. The first attempts to use combustible gases mixed with oxygen for metal welding and cutting also date from the early 20th century. The first oxygen-acetylene welding torch was designed by the French engineer E. Fouche, who obtained a German patent for his torch in 1903. This method was presumably known in Russia by 1905 and was widely used by 1911. The arc-welding process was further improved, and several versions of the technique appeared, including submerged arc welding and gas-shielded-arc welding. During the second half of the 20th century, other sources of energy came into use, including plasmas, electron, photon, and laser beams, explosives, and ultrasound.

Classification. Modern methods of welding metals can be classified into two large groups: fusion welding, or liquid-state welding, and pressure welding, or solid-state welding. In fusion welding, the molten metal of the parts being joined is combined into one mass spontaneously, without applying any external force, as a result of melting and wetting in the weld zone and of mutual coalescence of the material. In pressure welding, substantial pressure is applied to join the parts without melting. The division between these two groups is not always distinct. It is possible, for instance, to weld by first fusing the parts partially and then pressing the parts together, as is done in resistance welding.

In the above classification, each group includes several methods. Fusion welding includes arc, plasma, electroslag, gas, and electron-beam welding. Pressure welding includes forge, cold, ultrasonic, friction, and explosion welding. Other characteristics can also be used as criteria for classification. For example, the following types of welding can be distinguished according to the type of energy used: electrical welding (arc, resistance, electroslag, plasma, and induction welding), mechanical welding (friction, cold, and ultrasonic welding), chemical welding (gas and Thermit welding), and beam-energy welding (photon-beam, electron-beam, and laser welding).

Fusion welding. Manual arc welding—one of the simplest welding methods—is based on the use of an electric arc. One pole of the power source is connected to a holder by means of a flexible cable; the other pole is connected to the object being welded. A carbon or metal electrode is inserted into the holder. If the electrode is touched briefly to the object, an arc is formed. The arc melts the base metal and the electrode rod (if the rod is made of metal), thus forming a molten pool, or weld puddle. Upon solidification, this pool becomes the weld. For a steel electrode, the temperature of the welding arc ranges from 6,000° to 10,000°C. Power supplied to the arc from specially designed units has a current of 100–350 amperes at 25–40 volts.

In arc welding, the oxygen and nitrogen of the atmosphere interact with the molten metal, forming oxides and nitrides that lower the strength and plasticity of the welded joint. There are internal and external methods of protecting the weld zone, such as introducing various substances into the electrode material or covering the electrodes (internal protection) or introducing inert gases or carbon dioxide into the weld zone or covering the weld zone with a flux (external protection). In the absence of external protection, the welding arc is called an unshielded arc; if external protection is used, it is called either a shielded arc or a submerged arc. Arc welding with an unshielded arc and consumable, covered electrodes is the most important method in actual practice. The high quality of the weld produced makes it possible to use this method for the fabrication of critical load-bearing parts.

One of the most important problems in welding engineering is the mechanization and automation of arc welding. Semiautomatic arc welding is often the most appropriate method for fabricating parts having complex shapes. In such a process, the feed of the electrode wire into the holder of a semiautomatic welder is mechanized. The arc may also be protected by a flux. The method, called submerged arc welding, was proposed in the late 19th century by N. G. Slavianov, who used powdered glass as a flux. An industrial version of the process was developed and introduced during the 1940’s under the direction of Academician O. E. Paton. Submerged arc welding has found a substantial number of industrial applications because it is easily automated, is quite efficient, can be used in producing various types of welded joints, and yields a high-quality weld. During the welding process, the arc is covered by a layer of flux. The layer protects the operator’s eyes from radiation, but it also makes it difficult to observe the formation of the weld.

Mechanized welding methods make use of gas-shielded arc welding, a technique introduced by N. N. Benardos at the end of the 19th century. The welding is done with a welding torch or in gas-filled chambers. Gases are fed to the arc continuously, thus ensuring a high-quality joint. Both inert and active gases can be used; the best results arc obtained using helium or argon. Helium is only used for special, critical work because of its high cost. A more widely used technique is automatic or semiautomatic welding with nonconsumable tungsten electrodes or consumable steel electrodes in argon or a mixture of argon with other gases. This method is appropriate for joining parts that usually are comparatively thin and that are made of aluminum, magnesium, or their alloys, all types of steel, high-temperature alloys, titanium and its alloys, nickel and copper alloys, niobium, zirconium, tantalum, and some other metals. Carbon dioxide welding is a very economical method that produces high-quality welds. Industrial applications of this technique were developed in the 1950’s by the Central Scientific Research Institute of Technology and Machine Building under the direction of K. V. Liubavskii. The method uses an electrode wire and is suitable for joining steel parts 1–30 mm thick.

Electric methods of fusion welding include electroslag welding. As in arc welding, this process begins with the formation of an arc and uses consumable electrodes. The process is then continued without an arc discharge, and a substantial amount of slag covers the weld puddle. The heat dissipated by the passage of electric current through the slag serves to heat the metal. This method was developed at the E. O. Paton Institute of Electric Welding and has been used commercially in industry since the late 1950’s. Electroslag welding with a single electrode is feasible for metals up to 200 mm thick; the use of several electrodes operated simultaneously makes it possible to weld metals up to 2,000 mm thick. The method is appropriate and economically advantageous for base metals with a thickness greater than 30 mm. Electroslag welding can be used for repair work and for hard facing where the required thickness of the surface layer is substantial. It is also being used in the manufacture of steam boilers, press beds, rolling mills, metal structural members, and other products.

Arc welding may also be conducted underwater in both fresh and salt water. The first practical method of welding underwater was developed in the USSR in 1932 at the Moscow Electromechanical Institute of Railroad Transportation Engineers under the direction of K. K. Khrenov. The arc burns stably underwater. The cooling effect of the water is compensated by a slight increase in arc voltage, and the arc melts the metal as easily as it does in air. The welding is done manually with the aid of a consumable steel electrode having a thick waterproof covering, whose thickness is up to 30 percent of the electrode thickness. The quality of the weld is somewhat lower than the quality of the welds made in air because the weld metal is insufficiently plastic.

During the 1970’s in the USSR, the E. O. Paton Institute of Electric Welding introduced a process for semiautomatic welding underwater. The process uses an electrode in the form of a thin steel tube filled with a powdered flux mixture. The electrode is continuously fed into the arc. Underwater welding can be conducted at depths up to 100 m. Such welding is used in ship repairs and in rescue operations.

A promising welding method is plasma welding, which uses a flame torch. The method essentially consists of an arc formed between a tungsten electrode and the workpiece and blown by a gas jet. The plasma formed can be used to heat metal to a high temperature. Another promising version of plasma welding uses a constricted arc. In this method, the gas column of the arc passes through a calibrated bore in the nozzle of the torch and is stretched into a thin jet. When the arc is constricted, its properties change. The arc voltage is increased significantly, and the temperature of the arc increases radically—up to 20,000°-30,000°C. Plasma welding is used industrially in joining refractory metals. In such work, automatic and semiautomatic equipment used for arc welding can be easily adapted to plasma welding by using a different torch. Plasma welding is used both for joining very thick metal parts, as in multilayer argon shielded welding, and for joining plates and wires of a thickness ranging from tens of microns to 1 mm, as in microwelding and welding using a needle arc. A plasma jet can also be used for other kinds of plasma shaping of metals, including cutting.

Another method of welding by fusion is gas welding. It uses the energy of a gas flame to join various metals of comparatively low thickness—up to 10 mm. Aflame of suitable temperature can be obtained by combustion of various fuels in oxygen; such mixtures include hydrogen-oxygen, gasoline-oxygen, and acetylene-oxygen. Oxygen-acetylene welding is widely used in industry. The principal difference between gas welding and arc welding is the more even and slower heating of metal in gas welding. This dictates the choice of gas welding for joining thin parts made of metals that require preheating before being welded, such as pig iron and some special steels, or that must be cooled slowly, such as tool steels. Gas welding is suitable for repair work, since it is comparatively simple and universal in its applications and the equipment is easily portable.

Pressure gas welding is also used industrially for welding steel tubes and rails. In this method, an oxygen-acetylene flame is used to heat the metal uniformly at the joint until a plastic state is reached; the metal is subsequently upset by pressing or forging.

Another promising welding method that does not require pressure is beam welding, which was introduced in the 1960’s. Electron-beam welding uses a focused stream of electrons. The workpiece is placed in a chamber in which a vacuum of 10-2–10-4 newton/m2 is maintained. The vacuum is necessary to provide a free flow of electrons and to preserve the concentrated electron beam. A controlled electron beam generated by a high-power source, an electron gun, is focused by magnetic and electrostatic fields and directed toward the workpiece. The concentration of energy at the focused spot may be as high as 109 watts/cm2. By shifting the beam along the line of the weld, seams of any configuration can be made at a high speed. The vacuum also inhibits the oxidation of metal in the weld. The electron beam is capable of melting and vaporizing practically all metals; it can be used not only for welding but also for cutting, drilling holes, and similar operations. The speed of electron-beam welding is 1.5–2 times greater than that of arc welding for identical operations. The disadvantages of electron-beam welding are the high cost of providing the vacuum and the high voltage required to produce a beam of sufficient power.

Photon-beam welding does not have the disadvantages of electron-beam welding. Unlike an electron beam, a light beam can travel long distances through the air with no appreciable loss of energy; this means that no vacuum is necessary. A light beam can also pass through transparent materials, such as glass and quartz, virtually without attenuation. The weld zone can be kept germfree by passing the beam through a transparent enclosure. The beam is focused by a mirror and is concentrated by an optical system, such as a quartz lens. Approximately 15 kilowatts can be concentrated in the beam from an input power of 50 kilowatts. A light beam can be formed by an artificial light source or by a natural light source—the sun. The latter method is called solar welding and is used in areas with considerable solar radiation.

Laser radiation is also used for welding. Laser welding is an important part of laser technology.

Pressure welding. Pressure welding methods yield welded joints of a strength that is sometimes higher than that of the base metal. In addition, in most instances of pressure welding, no changes occur in the chemical composition of the metal, since the metal is either not heated or heated very slightly. This makes pressure welding irreplaceable in a number of industries, such as electrical engineering, electronics, and aerospace engineering.

Cold welding is carried out solely by pressure, without the application of heat. The pressure causes a substantial plastic deformation until a state is reached wherein the metal will flow. The deformation must have a certain minimum value characteristic of the given metal. A thorough finishing and cleaning of the surfaces to be joined must precede cold welding. This preparation is usually done mechanically, for example, by rotating wire brushes. Cold welding has almost universal application. It is suitable for joining many metal articles, such as wires, rods, strips, thin-walled pipes, and shells, and nonmetallic materials of sufficient plasticity, such as resins, plastics, and glass. Cold welding holds promise for use in space.

The mechanical energy of friction can also be used for welding. Friction welding is performed on machines similar in external appearance to machine lathes. The parts are held in chucks and pushed together until their butt ends are in contact. One of the parts is rotated by an electric motor. As a result of friction, the surface layers of the butt ends of the parts are heated and fused. Rotation is then stopped, and the parts are upset. Friction welding is highly efficient and economical. It is used, for instance, to join the cutting tip of a metal-cutting tool to a holder.

Ultrasonic welding is based on the use of mechanical vibrations with a frequency of 20 kilohertz. The vibrations are generated by a magnetostrictive transducer that converts electromagnetic oscillations into mechanical vibrations. A winding is wound around a core made of a magnetostrictive material. A high-frequency current, which generates longitudinal mechanical vibrations in the transducer core, is supplied to the winding from a power source. A metal tip connected to the core serves as the welding tool. If the tip is pressed with some force onto the parts being welded, a weld is produced within a few seconds at the location where pressure was applied by the tool. As a result of vibration of the core, the surfaces are cleaned and slightly heated, which promotes the formation of a strong welded joint. In electrical engineering, electronics, and the radio industry, ultrasonic welding is used to weld thin metals (from several millimeters to 1.5 mm thick) and some plastics.

In the early 1970’s, ultrasonic welding was introduced in medicine to connect, build up, and cut live tissues. The techniques were developed by a group of researchers at the N. E. Bauman Moscow Higher Technical School under the direction of G. A. Nikolaev in cooperation with medical workers. In the welding and building up of such bone tissues as fragments of crural bones and ribs, a conglomerate of a liquid cyanoacrylate monomer adhesive and solid additives, such as bone shavings and various fillers and reinforcers, is deposited at the site of the injury and then solidified by ultrasonic welding, which accelerates polymerization. Ultrasonic cutting, in which the welding tool is a saw, scalpel, or knife, has also proved an effective surgical technique. The technique reduces the duration of an operation, loss of blood, and the sensation of pain.

Resistance welding is one method of electrical welding. In this method, an electric current passes through the weld spot, which exhibits an ohmic resistance to the passage of the current. The heated parts usually fuse and are pressed together or upset. In the latter case, resistance welding can be classified as welding by pressure. Resistance welding is noted for its high degree of mechanization and automation and is used more and more widely in mass production and large-lot production, for example, to join parts of automobiles, aircraft, and electronic equipment. It is also used to butt-weld large-diameter pipes, rails, and similar items.

Hard surfacing. Hard surfacing differs from other, more common, techniques of joining parts by welding. It is used to build up a layer of material on the surface of a part and thus increases the weight and dimensions of the part to some degree. Hard surfacing can be used to restore the dimensions of a part that have been reduced by wear and to face the surface layer of an item. Restoring dimensions by hard surfacing is very economical since the method is suitable for use on complex, expensive parts. Hard surfacing is widely used for repairs in transportation, agriculture, construction, mining, and other industries.

As a facing technique, hard surfacing is used to deposit a layer of material possessing special properties on the surface of a part, for example, a surface layer exhibiting a high degree of hardness or resistance to wear. Such facing is used both in repair work and in the manufacture of new articles. Filler materials with special properties, such as the wear-resistant alloy sormait, are available for this type of hard surfacing. Hard surfacing can be performed using various welding methods, including arc, gas, plasma, and electron-beam welding. The process can be mechanized and automated. Special equipment for hard surfacing, in which the basic operations are automated, is now being manufactured.

Thermal cutting. Cutting differs technologically from welding and works in the opposite fashion but uses similar cutting equipment, materials, and techniques. The terms “thermal cutting” and “flame cutting” denote those processes in which the metal in the cutting zone is heated to a high temperature. The metal flows away spontaneously, or it may be removed in the form of softened slags and oxides, or it may be ejected by mechanical action, for example, by a gas jet or an electrode.

Cutting can be performed by several methods. The most important and widely used is oxygen cutting, which is based on the ability of iron to burn in oxygen. Oxygen cutting is usually used to cut steel 5 to 100 mm thick; it can also be used to separate materials up to 2,000 mm thick and in operations similar to those conventionally performed with a cutting tool, such as planing, turning, and deburring. Conventional thermal cutting methods are not suitable for certain alloyed steels, pig iron, and nonferrous metals; instead, powder-cutting methods are used. Oxygen cutting is widely used in metallurgical and machine-building plants and in repair shops.

Arc cutting, using both carbon and metal electrodes, is used in installation and repair work, for instance, in ship construction. Arc-air cutting is used for the surface treatment and planing of metals. In this method the metal is blown away from the cut by an air jet, which substantially improves the quality of the cut.

Cutting can also be performed by a high-temperature plasma jet. Light beams, fluorine jets, and lasers hold promise as techniques for cutting metals and forming holes by burning.

The future. Future progress and development of welding and cutting methods depends on introduction and wider use of new techniques, such as the plasma, electron-beam, and laser methods, and on the development of improved equipment. It is possible that welding and cutting will be increasingly used for work underwater and in space. New trends in welding technology include the further mechanization and automation of basic welding operations and auxiliary operations performed prior to or subsequent to welding, for example, by manipulators, tilters, and robots. Current work on improving quality control includes the use of feedback systems capable of automatic regulation of automated welders.

REFERENCES

Spravochnik po svarke, vols. 1–4. Moscow, 1960–71.
Glizmanenko, D. L., and G. B. Evseev. Gazovaia svarka i rezka metallov, 2nd ed. Moscow, 1961.
Tekhnologiia elektricheskoi svarki plavleniem. Edited by B. E. Paton. Moscow-Kiev, 1962.
Bagrianskii, K. V., Z. A. Dobrotina, and K. K. Khrenov. Teoriia svarochnykh protsessov. Kharkov, 1968.
Khrenov, K. K. Svarka, rezka i paika metallov, 4th ed. Moscow, 1973.
Slovar’-spravochnik po svarke. Compiled by T. A. Kulik. Kiev, 1974.

K. K. KHRENOV

welding

[′weld·iŋ]
(geology)
Consolidation of sediments by pressure; water is squeezed out and cohering particles are brought within the limits of mutual molecular attraction.
(metallurgy)
Joining two metals by applying heat to melt and fuse them, with or without filler metal.