Welding Arihant

8/6/2019 Welding Arihant

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BY:

ARIHANT SHARMA

C - 15


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Welding is a fabrication process that joins materials, usually metalsor thermoplastics , by causing coalescence . This is often done bymelting the workpieces and adding a filler material to form a pool ofmolten material (the weld puddle) that cools to become a strong joint, with pressure sometimes used in conjunction with heat , or byitself, to produce the weld. This is in contrast with soldering, whichinvolves melting a lower-melting-point material between theworkpieces to form a bond between them, without melting the

workpieces.


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The history of joining metals goes back several millennia, with theearliest examples of welding from the Bronze Age and the Iron Agein Europe and the Middle East . Welding was used in the constructionof the Iron pillar in Delhi, India , erected about 310 and weighing

5.4 metric tons .[1] The Middle Ages brought advances in forgewelding, in which blacksmiths pounded heated metal repeatedly untilbonding occurred. Welding, however, was transformed during the19th centuryin 1800,the electric arc welding was discovered, andadvances in arc welding continued with the inventions of metalelectrodes by a Russian and an American researcher. In the late1800s carbon arc welding , which used a carbon electrode, gainedpopularity. Around 1900, A. P. Strohmenger released a coated metalelectrode in Britain, which gave a more stable arc, and in 1919,alternating current welding was invented, but did not become popularfor another decade.


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ThermiteThermite welding was invented in 1893, and around that time,welding was invented in 1893, and around that time,another process,another process, oxyfuel weldingoxyfuel welding, became well established., became well established.AcetyleneAcetylene was discovered

in 1836 bywas discovered in 1836 by Edmund DavyEdmund Davy, but its use, but its usewas not practical in welding until about 1

900, when a suitablewas not practical in welding until about 1900, when a suitableblowtorchblowtorch was developed. At first, oxyfuel welding was one ofwas developed. At first, oxyfuel welding was one ofthe more popular welding methods due to its portability andthe more popular welding methods due to its portability andrelatively low cost. As the 20th century progressed, however,relatively low cost. As the 20th century progressed, however,it fell out of favor for industrial applications. It was largelyit fell out of favor for industrial applications. It was largelyreplaced with arc welding, as metal coverings (known asreplaced with arc welding, as metal coverings (known as fluxflux))

for the electrode that stabilize the arc and shield the basefor the electrode that stabilize the arc and shield the basematerial from impurities continued to be developed.material from impurities continued to be developed. Other Other recent developments in welding include the 1958 breakthrough of recent developments in welding include the 1958 breakthrough of electron beam weldingelectron beam welding , making deep and narrow welding possible, making deep and narrow welding possiblethrough the concentrated heat source. Following the invention of through the concentrated heat source. Following the invention of thethe laser laser in 1960,in 1960, laser beam weldinglaser beam welding debuted several decadesdebuted several decadeslater, and has proved to be especially useful in highlater, and has proved to be especially useful in high- -speed,speed,automated welding. Both of these processes, however, continue toautomated welding. Both of these processes, however, continue tobe quite expensive due the high cost of the necessary equipment,be quite expensive due the high cost of the necessary equipment,and this has limited their applicationsand this has limited their applications


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These processes use a welding power supplyto create and maintain anelectric arc between an electrode and the base material to melt metals atthe welding point. They can use either direct (DC) or alternating (AC)current, and consumable or non-consumable electrodes . The welding regionis sometimes protected by some type of inert or semi- inert gas , known as ashielding gas, and filler material is sometimes used as well . To supply theelectrical energy necessary for arc welding processes, a number ofdifferent power supplies can be used. The most common classification isconstant current power supplies and constant voltage power supplies. Thetype of current used in arc welding also plays an important role in welding.

Consumable electrode processes such as shielded metal arc welding and gasmetal arc welding generally use direct current, but the electrode can becharged either positively or negatively.If the electrode is positively charged, the base metal will be hotter,increasing weld penetration and welding speed. Alternatively, a negativelycharged electrode results in more shallow welds


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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 consumableelectrode rod, which is made of steel and is covered with a flux that protects theweld area from oxidation and contamination by producing CO2 gas during thewelding process. The electrode core itself acts as filler material, making a separatefiller unnecessary.Gas metal arc welding (GMAW), also known as metal inert gas or MIG welding, is asemi-automatic or automatic process that uses a continuous wire feed as anelectrode and an inert or semi-inert gas mixture to protect the weld fromcontamination. As with SMAW, reasonable operator proficiency can be achievedwith modest training. Since the electrode is continuous, welding speeds are greater for GMAW than for SMAW. Also, the smaller arc size compared to the shieldedmetal arc welding process makes it easier to make out-of-position welds (e.g.,overhead joints, as would be welded underneath a structure).


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The most common gas welding process is oxyfuel welding, also knownas oxyacetylene welding. It is one of the oldest and most versatilewelding processes, but in recent years it has become less popular in

industrial applications. It is still widely used for welding pipes andtubes, as well as repair work. It is also frequently well-suited, andfavored, for fabricating some types of metal-based artwork.Oxyfuel equipment is versatile, lending itself not only to some sortsof iron or steel welding but also to brazing, braze-welding, metalheating (for bending and forming), and also oxyfuel cutting. Theequipment is relatively inexpensive and simple, generally employingthe combustion of acetylene in oxygen to produce a welding flametemperature of about 3100 C. The flame, since it is lessconcentrated than an electric arc, causes slower weld cooling, whichcan lead to greater residual stresses and weld distortion, though iteases the welding of high alloy steels


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Resistance welding involves the generation of heat by passingcurrent through the resistance caused by the contact betweentwo or more metal surfaces. Small pools of molten metal areformed at the weld area as high current (1000100,000 A) ispassed through the metal. In general, resistance welding methodsare efficient and cause little pollution, but their applications aresomewhat limited and the equipment cost can be high.


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Energy beam welding methods, namely laser beam welding andelectron beam welding , are relatively new processes that havebecome quite popular in high production applications. The twoprocesses are quite similar, differing most notably in their sourceof power. Laser beam welding employs a highly focused laser beam,while electron beam welding is done in a vacuum and uses anelectron beam. Both have a very high energy density, making deepweld penetration possible and minimizing the size of the weld area.Both processes are extremely fast, and are easily automated,making them highly productive.


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Like the first welding process, forge welding , some modernwelding methods do not involve the melting of the materials being

joined. One of the most popular, ultrasonic welding, is used toconnect thin sheets or wires made of metal or thermoplastic byvibrating them at high frequency and under high pressure. Theequipment and methods involved are similar to that of resistancewelding, but instead of electric current, vibration provides energyinput. Welding metals with this process does not involve meltingthe materials; instead, the weld is formed by introducingmechanical vibrations horizontally under pressure.


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Welding Arihant

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