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Welding

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   Welding is a fabrication process that joins materials, usually metals
   or thermoplastics, by causing coalescence. This is often done by
   melting the workpieces and adding a filler material to form a pool of
   molten material (the weld puddle) that cools to become a strong joint,
   but sometimes pressure is used in conjunction with heat, or by itself,
   to produce the weld. This is in contrast with soldering and brazing,
   which involve melting a lower-melting-point material between the
   workpieces to form a bond between them, without melting the workpieces.
   Arc welding
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   Arc welding

   Many different energy sources can be used for welding, including a gas
   flame, an electric arc, a laser, an electron beam, friction, and
   ultrasound. While often an industrial process, welding can be done in
   many different environments, including open air, underwater and in
   space. Regardless of location, however, welding remains dangerous, and
   precautions must be taken to avoid burns, electric shock, poisonous
   fumes, and overexposure to ultraviolet light.

   Until the end of the 19th century, the only welding process was forge
   welding, which blacksmiths had used for centuries to join metals by
   heating and pounding them. Arc welding and oxyfuel welding were among
   the first processes to develop late in the century, and resistance
   welding followed soon after. Welding technology advanced quickly during
   the early 20th century as World War I and World War II drove the demand
   for reliable and inexpensive joining methods. Following the wars,
   several modern welding techniques were developed, including manual
   methods like shielded metal arc welding, now one of the most popular
   welding methods, as well as semi-automatic and automatic processes such
   as gas metal arc welding, submerged arc welding and flux-cored arc
   welding. Developments continued with the invention of laser beam
   welding and electron beam welding in the latter half of the century.
   Today, the science continues to advance. Robot welding is becoming more
   commonplace in industrial settings, and researchers continue to develop
   new welding methods and gain greater understanding of weld quality and
   properties.

History

   The Iron Pillar in Delhi.
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   The Iron Pillar in Delhi.

   The history of joining metals goes back several millennia, with the
   earliest examples of welding from the Bronze Age and the Iron Age in
   Europe and the Middle East. Welding was used in the construction of the
   Iron pillar in Delhi, India, erected about 310 and weighing 5.4  metric
   tons. The Middle Ages brought advances in forge welding, in which
   blacksmiths pounded heated metal repeatedly until bonding occurred. In
   1540, Vannoccio Biringuccio published De la pirotechnia, which includes
   descriptions of the forging operation. Renaissance craftsmen were
   skilled in the process, and the industry continued to grow during the
   following centuries. Welding, however, was transformed during the 19th
   century—in 1800, Sir Humphry Davy discovered the electric arc, and
   advances in arc welding continued with the inventions of metal
   electrodes by a Russian, Nikolai Slavyanov, and an American, C.L.
   Coffin in the late 1800s, even as carbon arc welding, which used a
   carbon electrode, gained popularity. Around 1900, A. P. Strohmenger
   released a coated metal electrode in Britain, which gave a more stable
   arc, and in 1919, alternating current welding was invented by C.J.
   Holslag, but did not become popular for another decade.

   Resistance welding was also developed during the final decades of the
   19th century, with the first patents going to Elihu Thompson in 1885,
   who produced further advances over the next 15 years. Thermite welding
   was invented in 1893, and around that time, another process, oxyfuel
   welding, became well established. Acetylene was discovered in 1836 by
   Edmund Davy, but its use was not practical in welding until about 1900,
   when a suitable blowtorch was developed. At first, oxyfuel welding was
   one of the more popular welding methods due to its portability and
   relatively low cost. As the 20th century progressed, however, it fell
   out of favour for industrial applications. It was largely replaced with
   arc welding, as metal coverings (known as flux) for the electrode that
   stabilize the arc and shield the base material from impurities
   continued to be developed.

   World War I caused a major surge in the use of welding processes, with
   the various military powers attempting to determine which of the
   several new welding processes would be best. The British primarily used
   arc welding, even constructing a ship, the Fulagar, with an entirely
   welded hull. The Americans were more hesitant, but began to recognize
   the benefits of arc welding when the process allowed them to repair
   their ships quickly after a German attack in the New York Harbour at
   the beginning of the war. Arc welding was first applied to aircraft
   during the war as well, as some German airplane fuselages were
   constructed using the process.

   During the 1920s, major advances were made in welding technology,
   including the introduction of automatic welding in 1920, in which
   electrode wire was fed continuously. Shielding gas became a subject
   receiving much attention, as scientists attempted to protect welds from
   the effects of oxygen and nitrogen in the atmosphere. Porosity and
   brittleness were the primary problems, and the solutions that developed
   included the use of hydrogen, argon, and helium as welding atmospheres.
   During the following decade, further advances allowed for the welding
   of reactive metals like aluminium and magnesium. This, in conjunction
   with developments in automatic welding, alternating current, and fluxes
   fed a major expansion of arc welding during the 1930s and then during
   World War II.

   During the middle of the century, many new welding methods were
   invented. 1930 saw the release of stud welding, which soon became
   popular in shipbuilding and construction. Submerged arc welding was
   invented the same year, and continues to be popular today. Gas tungsten
   arc welding, after decades of development, was finally perfected in
   1941, and gas metal arc welding followed in 1948, allowing for fast
   welding of non- ferrous materials but requiring expensive shielding
   gases. Shielded metal arc welding was developed during the 1950s, using
   a consumable electrode and a carbon dioxide atmosphere as a shielding
   gas, and it quickly became the most popular metal arc welding process.
   In 1957, the flux-cored arc welding process debuted, in which the
   self-shielded wire electrode could be used with automatic equipment,
   resulting in greatly increased welding speeds, and that same year,
   plasma arc welding was invented. Electroslag welding was introduced in
   1958, and it was followed by its cousin, electrogas welding, in 1961.

   Other recent developments in welding include the 1958 breakthrough of
   electron beam welding, making deep and narrow welding possible through
   the concentrated heat source. Following the invention of the laser in
   1960, laser beam welding debuted several decades later, and has proved
   to be especially useful in high-speed, automated welding. Both of these
   processes, however, continue to be quite expensive due the high cost of
   the necessary equipment, and this has limited their applications.

Welding processes

Arc welding

   These processes use a welding power supply to create and maintain an
   electric arc between an electrode and the base material to melt metals
   at the welding point. They can use either direct (DC) or alternating
   (AC) current, and consumable or non-consumable electrodes. The welding
   region is sometimes protected by some type of inert or semi- inert gas,
   known as a shielding gas, and filler material is sometimes used as
   well.

Power supplies

   To supply the electrical energy necessary for arc welding processes, a
   number of different power supplies can be used. The most common
   classification is constant current power supplies and constant voltage
   power supplies. In arc welding, the voltage is directly related to the
   length of the arc, and the current is related to the amount of heat
   input. Constant current power supplies are most often used for manual
   welding processes such as gas tungsten arc welding and shielded metal
   arc welding, because they maintain a relatively constant current even
   as the voltage varies. This is important because in manual welding, it
   can be difficult to hold the electrode perfectly steady, and as a
   result, the arc length and thus voltage tend to fluctuate. Constant
   voltage power supplies hold the voltage constant and vary the current,
   and as a result, are most often used for automated welding processes
   such as gas metal arc welding, flux cored arc welding, and submerged
   arc welding. In these processes, arc length is kept constant, since any
   fluctuation in the distance between the wire and the base material is
   quickly rectified by a large change in current. For example, if the
   wire and the base material get too close, the current will rapidly
   increase, which in turn causes the heat to increase and the tip of the
   wire to melt, returning it to its original separation distance.

   The type of current used in arc welding also plays an important role in
   welding. Consumable electrode processes such as shielded metal arc
   welding and gas metal arc welding generally use direct current, but the
   electrode can be charged either positively or negatively. In welding,
   the positively charged anode will have a greater heat concentration,
   and as a result, changing the polarity of the electrode has an impact
   on weld properties. If the electrode is positively charged, it will
   melt more quickly, increasing weld penetration and welding speed.
   Alternatively, a negatively charged electrode results in more shallow
   welds. Nonconsumable electrode processes, such as gas tungsten arc
   welding, can use either type of direct current, as well as alternating
   current. However, with direct current, because the electrode only
   creates the arc and does not provide filler material, a positively
   charged electrode causes shallow welds, while a negatively charged
   electrode makes deeper welds. Alternating current rapidly moves between
   these two, resulting in medium-penetration welds. One disadvantage of
   AC, the fact that the arc must be re-ignited after every zero crossing,
   has been addressed with the invention of special power units that
   produce a square wave pattern instead of the normal sine wave, making
   rapid zero crossings possible and minimizing the effects of the
   problem.

Methods

   Shielded metal arc welding
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   Shielded metal arc welding

   One of the most common types of arc welding is shielded metal arc
   welding (SMAW), which is also known as manual metal arc welding (MMA)
   or stick welding. Electric current is used to strike an arc between the
   base material and consumable electrode rod, which is made of steel and
   is covered with a flux that protects the weld area from oxidation and
   contamination by producing CO[2] gas during the welding process. The
   electrode core itself acts as filler material, making a separate filler
   unnecessary. The process is very versatile, requiring little operator
   training and inexpensive equipment. However, weld times are rather
   slow, since the consumable electrodes must be frequently replaced and
   because slag, the residue from the flux, must be chipped away after
   welding. Furthermore, the process is generally limited to welding
   ferrous materials, though speciality electrodes have made possible the
   welding of cast iron, nickel, aluminium, copper, and other metals. The
   versatility of the method makes it popular in a number of applications,
   including repair work and construction.

   Gas metal arc welding (GMAW), also known as metal inert gas (MIG)
   welding, is a semi-automatic or automatic welding process that uses a
   continuous wire feed as an electrode and an inert or semi-inert gas
   mixture to protect the weld from contamination. Since the electrode is
   continuous, welding speeds are greater for GMAW than for SMAW. However,
   because of the additional equipment, the process is less portable and
   versatile, but still useful for industrial applications. The process
   can be applied to a wide variety of metals, both ferrous and
   non-ferrous. A related process, flux-cored arc welding (FCAW), uses
   similar equipment but uses wire consisting of a steel electrode
   surrounding a powder fill material. This cored wire is more expensive
   than the standard solid wire and can generate fumes and/or slag, but it
   permits higher welding speed and greater metal penetration.
   Gas tungsten arc welding
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   Gas tungsten arc welding

   Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding,
   is a manual welding process that uses a nonconsumable electrode made of
   tungsten, an inert or semi-inert gas mixture, and a separate filler
   material. Especially useful for welding thin materials, this method is
   characterized by a stable arc and high quality welds, but it requires
   significant operator skill and can only be accomplished at relatively
   low speeds. It can be used on nearly all weldable metals, though it is
   most often applied to stainless steel and light metals. It is often
   used when quality welds are extremely important, such as in bicycle,
   aircraft and naval applications. A related process, plasma arc welding,
   also uses a tungsten electrode but uses plasma gas to make the arc. The
   arc is more concentrated than the GTAW arc, making transverse control
   more critical and thus generally restricting the technique to a
   mechanized process. Because of its stable current, the method can be
   used on a wider range of material thicknesses than can the GTAW
   process, and furthermore, it is much faster. It can be applied to all
   of the same materials as GTAW except magnesium, and automated welding
   of stainless steel is one important application of the process. A
   variation of the process is plasma cutting, an efficient steel cutting
   process.

   Submerged arc welding (SAW) is a high-productivity welding method in
   which the arc is struck beneath a covering layer of flux. This
   increases arc quality, since contaminants in the atmosphere are blocked
   by the flux. The slag that forms on the weld generally comes off by
   itself, and combined with the use of a continuous wire feed, the weld
   deposition rate is high. Working conditions are much improved over
   other arc welding processes, since the flux hides the arc and no smoke
   is produced. The process is commonly used in industry, especially for
   large products. Other arc welding processes include atomic hydrogen
   welding, carbon arc welding, electroslag welding, electrogas welding,
   and stud arc welding.
   Gas welding a steel armature
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   Gas welding a steel armature

Gas welding

   The most common gas welding process is oxyfuel welding, also known as
   oxyacetylene welding. It is one of the oldest and most versatile
   welding processes, but in recent years it has become less popular in
   industrial applications. It is still widely used for welding pipes and
   tubes, as well as repair work. The equipment is relatively inexpensive
   and simple, generally employing the combustion of acetylene in oxygen
   to produce a welding flame temperature of about 3100°C. The flame,
   since it is less concentrated than an electric arc, causes slower weld
   cooling, which can lead to greater residual stresses and weld
   distortion, though it eases the welding of high alloy steels. A similar
   process, generally called oxyfuel cutting, is used to cut metals. Other
   gas welding methods, such as air acetylene welding, oxygen hydrogen
   welding, and pressure gas welding are quite similar, generally
   differing only in the type of gases used. A water torch is sometimes
   used for precision welding of items such as jewelry. Gas welding is
   also used in plastic welding, though the heated substance is air, and
   the temperatures are much lower.

Resistance welding

   Resistance welding involves the generation of heat by passing current
   through the resistance caused by the contact between two or more metal
   surfaces. Small pools of molten metal are formed at the weld area as
   high current (1000–100,000 A) is passed through the metal. In general,
   resistance welding methods are efficient and cause little pollution,
   but their applications are somewhat limited and the equipment cost can
   be high.
   Spot welder
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   Spot welder

   Spot welding is a popular resistance welding method used to join
   overlapping metal sheets of up to 3 mm thick. Two electrodes are
   simultaneously used to clamp the metal sheets together and to pass
   current through the sheets. The advantages of the method include
   efficient energy use, limited workpiece deformation, high production
   rates, easy automation, and no required filler materials. Weld strength
   is significantly lower than with other welding methods, making the
   process suitable for only certain applications. It is used extensively
   in the automotive industry—ordinary cars can have several thousand spot
   welds. A specialized process, called shot welding, can be used to spot
   weld stainless steel.

   Like spot welding, seam welding relies on two electrodes to apply
   pressure and current to join metal sheets. However, instead of pointed
   electrodes, wheel-shaped electrodes roll along and often feed the
   workpiece, making it possible to make long continuous welds. In the
   past, this process was used in the manufacture of beverage cans, but
   now its uses are more limited. Other resistance welding methods include
   flash welding, projection welding, and upset welding.

Energy beam welding

   Energy beam welding methods, namely laser beam welding and electron
   beam welding, are relatively new processes that have become quite
   popular in high production applications. The two processes are quite
   similar, differing most notably in their source of power. Laser beam
   welding employs a highly focused laser beam, while electron beam
   welding is done in a vacuum and uses an electron beam. Both have a very
   high energy density, making deep weld penetration possible and
   minimizing the size of the weld area. Both processes are extremely
   fast, and are easily automated, making them highly productive. The
   primary disadvantages are their very high equipment costs (though these
   are decreasing) and a susceptibility to thermal cracking. Developments
   in this area include laser-hybrid welding, which uses principles from
   both laser beam welding and arc welding for even better weld
   properties.

Solid-state welding

   Like the first welding process, forge welding, some modern welding
   methods do not involve the melting of the materials being joined. One
   of the most popular, ultrasonic welding, is used to connect thin sheets
   or wires made of metal or thermoplastic by vibrating them at high
   frequency and under high pressure. The equipment and methods involved
   are similar to that of resistance welding, but instead of electric
   current, vibration provides energy input. Welding metals with this
   process does not involve melting the materials; instead, the weld is
   formed by introducing mechanical vibrations horizontally under
   pressure. When welding plastics, the materials should have similar
   melting temperatures, and the vibrations are introduced vertically.
   Ultrasonic welding is commonly used for making electrical connections
   out of aluminium or copper, and it is also a very common polymer
   welding process.

   Another common process, explosion welding, involves the joining of
   materials by pushing them together under extremely high pressure. The
   energy from the impact plasticizes the materials, forming a weld, even
   though only a limited amount of heat is generated. The process is
   commonly used for welding dissimilar materials, such as the welding of
   aluminium with steel in ship hulls or compound plates. Other
   solid-state welding processes include co-extrusion welding, cold
   welding, diffusion welding, friction welding (including friction stir
   welding), high frequency welding, hot pressure welding, induction
   welding, and roll welding.

Geometry

   Common welding joint types – (1) Square butt joint, (2) Single-V
   preparation joint, (3) Lap joint, (4) T-joint.
   Enlarge
   Common welding joint types – (1) Square butt joint, (2) Single-V
   preparation joint, (3) Lap joint, (4) T-joint.

   Welds can be geometrically prepared in many different ways. The five
   basic types of weld joints are the butt joint, lap joint, corner joint,
   edge joint, and T-joint. Other variations exist as well—for example,
   double-V preparation joints are characterized by the two pieces of
   material each tapering to a single centre point at one-half their
   height. Single-U and double-U preparation joints are also fairly
   common—instead of having straight edges like the single-V and double-V
   preparation joints, they are curved, forming the shape of a U. Lap
   joints are also commonly more than two pieces thick—depending on the
   process used and the thickness of the material, many pieces can be
   welded together in a lap joint geometry.

   Often, particular joint designs are used exclusively or almost
   exclusively by certain welding processes. For example, resistance spot
   welding, laser beam welding, and electron beam welding are most
   frequently performed on lap joints. However, some welding methods, like
   shielded metal arc welding, are extremely versatile and can weld
   virtually any type of joint. Additionally, some processes can be used
   to make multipass welds, in which one weld is allowed to cool, and then
   another weld is performed on top of it. This allows for the welding of
   thick sections arranged in a single-V preparation joint, for example.
   The cross-section of a welded butt joint, with the darkest gray
   representing the weld or fusion zone, the medium gray the heat-affected
   zone, and the lightest gray the base material.
   Enlarge
   The cross-section of a welded butt joint, with the darkest gray
   representing the weld or fusion zone, the medium gray the heat-affected
   zone, and the lightest gray the base material.

   After welding, a number of distinct regions can be identified in the
   weld area. The weld itself is called the fusion zone—more specifically,
   it is where the filler metal was laid during the welding process. The
   properties of the fusion zone depend primarily on the filler metal
   used, and its compatibility with the base materials. It is surrounded
   by the heat-affected zone, the area that had its microstructure and
   properties altered by the weld. These properties depend on the base
   material's behaviour when subjected to heat. The metal in this area is
   often weaker than both the base material and the fusion zone, and is
   also where residual stresses are found.

Quality

   Most often, the major metric used for judging the quality of a weld is
   its strength and the strength of the material around it. Many distinct
   factors influence this, including the welding method, the amount and
   concentration of heat input, the base material, the filler material,
   the flux material, the design of the joint, and the interactions
   between all these factors. To test the quality of a weld, either
   destructive or nondestructive testing methods are commonly used to
   verify that welds are defect-free, have acceptable levels of residual
   stresses and distortion, and have acceptable heat-affected zone (HAZ)
   properties. Welding codes and specifications exist to guide welders in
   proper welding technique and in how to judge the quality of welds.

Heat-affected zone

   The HAZ of a pipe weld, with the blue area being the metal most
   affected by the heat.
   Enlarge
   The HAZ of a pipe weld, with the blue area being the metal most
   affected by the heat.

   The effects of welding on the material surrounding the weld can be
   detrimental—depending on the materials used and the heat input of the
   welding process used, the HAZ can be of varying size and strength. The
   thermal diffusivity of the base material plays a large role—if the
   diffusivity is high, the material cooling rate is high and the HAZ is
   relatively small. Conversely, a low diffusivity leads to slower cooling
   and a larger HAZ. The amount of heat injected by the welding process
   plays an important role as well, as processes like oxyacetylene welding
   have an unconcentrated heat input and increase the size of the HAZ.
   Processes like laser beam welding give a highly concentrated, limited
   amount of heat, resulting in a small HAZ. Arc welding falls between
   these two extremes, with the individual processes varying somewhat in
   heat input. To calculate the heat input for arc welding procedures, the
   following formula can be used:

          Q = \left(\frac{V \times I \times 60}{S \times 1000} \right)
          \times \mathit{Efficiency}

   where Q = heat input ( kJ/ mm), V = voltage (V), I = current ( A), and
   S = welding speed (mm/min). The efficiency is dependent on the welding
   process used, with shielded metal arc welding having a value of 0.75,
   gas metal arc welding and submerged arc welding, 0.9, and gas tungsten
   arc welding, 0.8.

Distortion and cracking

   Welding methods that involve the melting of metal at the site of the
   joint necessarily are prone to shrinkage as the heated metal cools.
   Shrinkage, in turn, can introduce residual stresses and both
   longitudinal and rotational distortion. Distortion can pose a major
   problem, since the final product is not the desired shape. To alleviate
   rotational distortion, the workpieces can be offset, so that the
   welding results in a correctly shaped piece. Other methods of limiting
   distortion, such as clamping the workpieces in place, cause the buildup
   of residual stress in the heat-affected zone of the base material.
   These stresses can reduce the strength of the base material, and can
   lead to catastrophic failure through cold cracking, as in the case of
   several of the Liberty ships. Cold cracking is limited to steels, and
   is associated with the formation of martensite as the weld cools. The
   cracking occurs in the heat-affected zone of the base material. To
   reduce the amount of distortion and residual stresses, the amount of
   heat input should be limited, and the welding sequence used should not
   be from one end directly to the other, but rather in segments. The
   other type of cracking, hot cracking or solidification cracking, can
   occur in all metals, and happens in the fusion zone of a weld. To
   diminish the probability of this type of cracking, excess material
   restraint should be avoided, and a proper filler material should be
   utilized.

Weldability

   The quality of a weld is also dependent on the combination of materials
   used for the base material and the filler material. Not all metals are
   suitable for welding, and not all filler metals work well with
   acceptable base materials.

Steels

   The weldability of steels is inversely proportional to a property known
   as the hardenability of the steel, which measures the ease of forming
   martensite during heat treatment. The hardenability of steel depends on
   its chemical composition, with greater quantities of carbon and other
   alloying elements resulting in a higher hardenability and thus a lower
   weldability. In order to be able to judge alloys made up of many
   distinct materials, a measure known as the equivalent carbon content is
   used to compare the relative weldabilities of different alloys by
   comparing their properties to a plain carbon steel. The effect on
   weldability of elements like chromium and vanadium, while not as great
   as carbon, is more significant than that of copper and nickel, for
   example. As the equivalent carbon content rises, the weldability of the
   alloy decreases. The disadvantage to using plain carbon and low-alloy
   steels is their lower strength—there is a trade-off between material
   strength and weldability. High strength, low-alloy steels were
   developed especially for welding applications during the 1970s, and
   these generally easy to weld materials have good strength, making them
   ideal for many welding applications.

   Stainless steels, because of their high chromium content, tend to
   behave differently with respect to weldability than other steels.
   Austenitic grades of stainless steels tend to be the most weldable, but
   they are especially susceptible to distortion due to their high
   coefficient of thermal expansion. Some alloys of this type are prone to
   cracking and reduced corrosion resistance as well. Hot cracking is
   possible if the amount of ferrite in the weld is not controlled—to
   alleviate the problem, an electrode is used that deposits a weld metal
   containing a small amount of ferrite. Other types of stainless steels,
   such as ferritic and martensitic stainless steels, are not as easily
   welded, and must often be preheated and welded with special electrodes.

Aluminium

   The weldability of aluminium alloys varies significantly, depending on
   the chemical composition of the alloy used. Aluminum alloys are
   susceptible to hot cracking, and to combat the problem, welders
   increase the welding speed to lower the heat input. Preheating reduces
   the temperature gradient across the weld zone and thus helps reduce hot
   cracking, but it can reduce the mechanical properties of the base
   material and should not be used when the base material is restrained.
   The design of the joint can be changed as well, and a more compatible
   filler alloy can be selected to decrease the likelihood of hot
   cracking. Aluminium alloys should also be cleaned prior to welding,
   with the goal of removing all oxides, oils, and loose particles from
   the surface to be welded. This is especially important because of an
   aluminium weld's susceptibility to porosity due to hydrogen and dross
   due to oxygen.

Unusual conditions

   Underwater welding
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   Underwater welding

   While many welding applications are done in controlled environments
   such as factories and repair shops, some welding processes are commonly
   used in a wide variety of conditions, such as open air, underwater, and
   vacuums (such as space). In open-air applications, such as construction
   and outdoors repair, shielded metal arc welding is the most common
   process. Processes that employ inert gases to protect the weld cannot
   be readily used in such situations, because unpredictable atmospheric
   movements can result in a faulty weld. Shielded metal arc welding is
   also often used in underwater welding in the construction and repair of
   ships, offshore platforms, and pipelines, but others, such as flux
   cored arc welding and gas tungsten arc welding, are also common.
   Welding in space is also possible—it was first attempted in 1969 by
   Russian cosmonauts, when they performed experiments to test shielded
   metal arc welding, plasma arc welding, and electron beam welding in a
   depressurized environment. Further testing of these methods was done in
   the following decades, and today researchers continue to develop
   methods for using other welding processes in space, such as laser beam
   welding, resistance welding, and friction welding. Advances in these
   areas could prove indispensable for projects like the construction of
   the International Space Station, which will likely rely heavily on
   welding for joining in space the parts that were manufactured on Earth.

Safety issues

   Welding, without the proper precautions, can be a dangerous and
   unhealthy practice. However, with the use of new technology and proper
   protection, the risks of injury and death associated with welding can
   be greatly reduced. Because many common welding procedures involve an
   open electric arc or flame, the risk of burns is significant. To
   prevent them, welders wear personal protective equipment in the form of
   heavy leather gloves and protective long sleeve jackets to avoid
   exposure to extreme heat and flames. Additionally, the brightness of
   the weld area leads to a condition called arc eye in which ultraviolet
   light causes the inflammation of the cornea and can burn the retinas of
   the eyes. Goggles and welding helmets with dark face plates are worn to
   prevent this exposure, and in recent years, new helmet models have been
   produced that feature a face plate that self-darkens upon exposure to
   high amounts of UV light. To protect bystanders, transparent welding
   curtains often surround the welding area. These curtains, made of a
   polyvinyl chloride plastic film, shield nearby workers from exposure to
   the UV light from the electric arc, but should not be used to replace
   the filter glass used in helmets.

   Welders are also often exposed to dangerous gases and particulate
   matter. Processes like flux-cored arc welding and shielded metal arc
   welding produce smoke containing particles of various types of oxides,
   which in some cases can lead to medical conditions like metal fume
   fever. The size of the particles in question tends to influence the
   toxicity of the fumes, with smaller particles presenting a greater
   danger. Additionally, many processes produce fumes and various gases,
   most commonly carbon dioxide and ozone, that can prove dangerous if
   ventilation is inadequate. Furthermore, because the use of compressed
   gases and flames in many welding processes pose an explosion and fire
   risk, some common precautions include limiting the amount of oxygen in
   the air and keeping combustible materials away from the workplace.

Costs and trends

   As an industrial process, the cost of welding plays a crucial role in
   manufacturing decisions. Many different variables affect the total
   cost, including equipment cost, labor cost, material cost, and energy
   cost. Depending on the process, equipment cost can vary, from
   inexpensive for methods like shielded metal arc welding and oxyfuel
   welding, to extremely expensive for methods like laser beam welding and
   electron beam welding. Because of their high cost, they are only used
   in high production operations. Similarly, because automation and robots
   increase equipment costs, they are only implemented when high
   production is necessary. Labor cost depends on the deposition rate (the
   rate of welding), the hourly wage, and the total operation time,
   including both time welding and handling the part. The cost of
   materials includes the cost of the base and filler material, and the
   cost of shielding gases. Finally, energy cost depends on arc time and
   welding power demand.

   For manual welding methods, labor costs generally make up the vast
   majority of the total cost. As a result, many cost-savings measures are
   focused on minimizing the operation time. To do this, welding
   procedures with high deposition rates can be selected, and weld
   parameters can be fine-tuned to increase welding speed. Mechanization
   and automatization are often implemented to reduce labor costs, but
   this frequently increases the cost of equipment and creates additional
   setup time. Material costs tend to increase when special properties are
   necessary, and energy costs normally do not amount to more than several
   percent of the total welding cost.

   In recent years, in order to minimize labor costs in high production
   manufacturing, industrial welding has become increasingly more
   automated, most notably with the use of robots in resistance spot
   welding (especially in the automotive industry) and in arc welding. In
   robot welding, mechanized devices both hold the material and perform
   the weld, and at first, spot welding was its most common application.
   But robotic arc welding has been increasing in popularity as technology
   has advanced. Other key areas of research and development include the
   welding of dissimilar materials (such as steel and aluminium, for
   example) and new welding processes, such as friction stir, magnetic
   pulse, conductive heat seam, and laser-hybrid welding. Furthermore,
   progress is desired in making more specialized methods like laser beam
   welding practical for more applications, such as in the aerospace and
   automotive industries. Researchers also hope to better understand the
   often unpredictable properties of welds, especially microstructure,
   residual stresses, and a weld's tendency to crack or deform.
   Retrieved from " http://en.wikipedia.org/wiki/Welding"
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