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Principles of arc welding   Arc welding is the welding process, in which heat is generated

by an electric arc struck between an electrode and the workpiece.

Electric arc is luminous electrical discharge between two

electrodes through ionized gas.

 Any arc welding method is based on an electric circuit consistingof the following parts:

Power supply (AC or DC);  Welding electrode;  Work piece;  Welding leads (electric cables) connecting the electrode and

 work piece to the power supply.

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Electric arc between the electrode and work piececloses the electric circuit. The arc temperature may reach 10000°F (5500°C), which is sufficient for fusionthe work piece edges and joining them.

 When a long join is required the arc is moved along

the joint line. The front edge of the weld pool meltsthe welded surfaces when the rear edge of the weldpool solidifies forming the joint.

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e a ner as e ng ,GMAW)  Metal Inert Gas Welding (Gas Metal Arc Welding) is the arc

 welding process, in which the weld is shielded by an external gas(argon, helium, CO2, argon + oxygen or other gas mixtures).

Consumable electrode wire, having chemical compositionsimilar to that of the parent material, is continuously fed from aspool to the arc zone. The arc heats and melts both the workpieces edges and the electrode wire. The fused electrode materialis supplied to the surfaces of the work pieces, fills the weld pooland forms joint.

Due to automatic feeding of the filling wire (electrode) theprocess is referred to as a semi-automatic. The operator controlsonly the torch positioning and speed.

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Advantages of Metal Inert Gas Welding (MIG,GMAW): 

Continuous weld may be produced (no interruptions);

High level of operators skill is not required; Slag removal is not required (no slag);

Disadvantages of Metal Inert Gas Welding (MIG,GMAW  ): 

Expensive and non-portable equipment is required; Outdoor application are limited because of effect of wind,

dispersing the shielding gas.

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ungs en ner as rc e ng(TIG, GTAW)  Tungsten Inert Gas Arc Welding (Gas Tungsten Arc

 Welding) is the welding process, in which heat isgenerated by an electric arc struck between a tungstennon-consumable electrode and the work piece.

The weld pool is shielded by an inert gas ( Argon, helium,Nitrogen) protecting the molten metal from atmosphericcontamination.

The heat produced by the arc melts the work pieces edges

and joins them. Filler rod may be used, if required.

Tungsten Inert Gas Arc Welding produces a high quality  weld of most of metals. Flux is not used in the process.

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 Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):   Weld composition is close to that of the parent metal; High quality weld structure 

Slag removal is not required (no slag); Thermal distortions of work pieces are minimal due to

concentration of heat in small zone.

Disadvantages of Tungsten Inert Gas Arc Welding (TIG,

GTAW):  Low welding rate; Relatively expensive; Requires high level of operators skill.

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e e e a rc e ng(SMAW)  Shielded metal arc welding (Stick welding, Manual metal arc

 welding) uses a metallic consumable electrode of a propercomposition for generating arc between itself and the parent workpiece. The molten electrode metal fills the weld gap and joins the workpieces.

This is the most popular welding process capable to produce a great variety of welds.

The electrodes are coated with a shielding f lux of a suitablecomposition. The f lux melts together with the electrode metallic core,

forming a gas and a slag, shielding the arc and the weld pool. The fluxcleans the metal surface, supplies some alloying elements to the weld,protects the molten metal from oxidation and stabilizes the arc.The slag is removed after Solidification.

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   Advantages of Shielded Metal Arc Welding (SMAW):  Simple, portable and inexpensive equipment;

 Wide variety of metals, welding positions and electrodes are

applicable; Suitable for outdoor applications.

Disadvantages of Shielded Metal Arc Welding (SMAW): 

The process is discontinuous due to limited length of theelectrodes;

 Weld may contain slag inclusions;

Fumes make difficult the process control.

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Submerged Arc Welding (SAW)  Submerged Arc Welding utilizes a bare consumable metallic

electrode producing an arc between itself and the work piece within a granular shielding flux applied around the weld.

The arc heats and melts both the work pieces edges and theelectrode wire. The molten electrode material is supplied to thesurfaces of the welded pieces, fills the weld pool and joins the

 work pieces.

Since the electrode is submerged into the flux, the arc is

invisible. The flux is partially melts and forms a slag protectingthe weld pool from oxidation and other atmosphericcontaminations.

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 Advantages of Submerged Arc Welding (SAW): 

 Very high welding rate;

The process is suitable for automation; High quality weld structure.

Disadvantages of Submerged Arc Welding (SAW): 

 Weld may contain slag inclusions;

Limited applications of the process - mostly for welding horizontally located plates.

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Electro slag Welding (ESW)  Electro slag Welding is the welding process in which the heat is generated by 

an electric current passing between the consumable electrode (filler metal) andthe work piece through a molten slag, which covers the weld surface.

Prior to welding the gap between the two work pieces is filled with a welding

flux. Electro slag Welding is initiated by an arc between the electrode and the work piece (or starting plate). Heat, generated by the arc, melts the fluxingpowder and forms molten slag. The slag, having low electric conductivity, ismaintained in liquid state due to heat produced by the electric current.

The slag reaches a temperature of about 3500°F (1930°C). This temperature issufficient for melting the consumable electrode and work piece edges. Metaldroplets fall to the weld pool and join the work pieces.

Electro slag Welding is used mainly for steels.

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 Advantages of Electro slag Welding:  High deposition rate - up to 45 lbs/h (20 kg/h);

Low slag consumption (about 5% of the depositedmetal weight);

Low distortion;

Unlimited thickness of work piece.

Disadvantages of Electro slag welding:  Coarse grain structure of the weld;

Low toughness of the weld;

Only vertical position is possible.

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Resistance Welding (RW)  Resistance Welding is a welding process, in which work pieces are welded due to a

combination of a pressure applied to them and a localized heat generated by a highelectric current flowing through the contact area of the weld.

Heat produced by the current is sufficient for local melting of the work piece at thecontact point and formation of small weld pool (”nugget”). The molten metal is then

solidifies under a pressure and joins the pieces. Time of the process and values of thepressure and f lowing current, required for formation of reliable joint, are determined by dimensions of the electrodes and the work piece metal type.

 AC electric current (up to 100 000 A) is supplied through copper electrodes connected tothe secondary coil of a welding transformer.

The following metals may be welded by Resistance Welding: 

Low carbon steels - the widest application of Resistance Welding  Aluminum alloys  Medium carbon steels, high carbon steels and Alloy steels (may be welded, but the weld

is brittle)

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 Advantages of Resistance Welding:  High welding rates; Low fumes; Cost effectiveness;

Easy automation; No filler materials are required; Low distortions.

Disadvantages of Resistance Welding:  High equipment cost; Low strength of discontinuous welds; Thickness of welded sheets is limited - up to 1/4” (6 mm); 

Resistance Welding (RW) is used for joining vehicle body parts, fuel tanks, domesticradiators, pipes of gas oil and water pipelines, wire ends, turbine blades, railway tracks.

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The most popular methods of Resistance Welding are:

Spot Welding (RSW);

Flash Welding (FW); Resistance Butt Welding (UW) ;

Seam Welding (RSEW).

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Spot Welding (RSW)  Spot Welding is a Resistance Welding (RW) process, in

 which two or more overlapped metal sheets are joined by spot welds.

The method uses pointed copper electrodes providingpassage of electric current. The electrodes also transmitpressure required for formation of strong weld.Diameter of the weld spot is in the range 1/8” - 1/2” (3 - 12mm).

Spot welding is widely used in automotive industry for joining vehicle body parts.

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Flash Welding (FW)  Flash Welding is a Resistance Welding (RW) process, in which ends of rods

(tubes, sheets) are heated and fused by an arc struck between them and thenforged (brought into a contact under a pressure) producing a weld.

The welded parts are held in electrode clamps, one of which is stationary and

the second is movable. Flash Welding method permits fast (about 1 min.) joining of large and complex parts. Welded part are often annealed for improvement of Toughness of the weld.

Steels, Aluminum alloys, Copper alloys, Magnesium alloys, Copper alloys andNickel alloys may be welded by Flash Welding.

Thick pipes, ends of band saws, frames, aircraft landing gears are produced by Flash Welding.

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Resistance Butt Welding (UW)  Resistance Butt Welding is a Resistance Welding (RW) 

process, in which ends of wires or rods are held under apressure and heated by an electric current passing throughthe contact area and producing a weld.

The process is similar to Flash Welding, however in Butt Welding pressure and electric current are appliedsimultaneously in contrast to Flash Welding where electriccurrent is followed by forging pressure application.

Butt welding is used for welding small parts. The process ishighly productive and clean. In contrast to Flash Welding,Butt Welding provides joining with no loss of the weldedmaterials.

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Seam Welding (RSEW)  Seam Welding is a Resistance Welding (RW) process

of continuous joining of overlapping sheets by passingthem between two rotating electrode wheels. Heat

generated by the electric current flowing through thecontact area and pressure provided by the wheels aresufficient to produce a leak-tight weld.

Seam Welding is high speed and clean process, whichis used when continuous tight weld is required (fueltanks, drums, domestic radiators).

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Thermit Welding (TW)  Thermit Welding is a welding process, utilizing heat generated by exothermic chemical

reaction between the components of the thermit (a mixture of a metal oxide andaluminum powder). The molten metal, produced by the reaction, acts as a filler material joining the work pieces after Solidification.

Thermit Welding is mainly used for joining steel parts, therefore common thermit is

composed from iron oxide (78%) and aluminum powder (22%).The proportion 78-22 is determined by the chemical reaction of combustion of aluminum:

8Al + Fe3O4 = 9Fe + 4Al2O3 

The combustion reaction products (iron and aluminum oxide) heat up to 4500°F(2500°C). Liquid iron fills the sand (or ceramic) mold built around the welded parts, the

slag (aluminum oxide), f loating up , is then removed from the weld surface.Thermit Welding is used for repair of steel casings and forgings, for joining railroad rails,steel wires and steel pipes, for joining large cast and forged parts.

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 Advantages of Thermit Welding:  No external power source is required (heat of chemical reaction

is utilized);

 Very large heavy section parts may be joined.

Disadvantages of Resistance Welding:  Only ferrous (steel, chromium, nickel) parts may be welded;

Slow welding rate;

High temperature process may cause distortions and changes inGrain structure in the weld region.

 Weld may contain gas (Hydrogen) and slag contaminations.

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Electron Beam Welding (EBW)  Electron Beam Welding is a welding process utilizing a heat generated by a

beam of high energy electrons. The electrons strike the work piece and theirkinetic energy converts into thermal energy heating the metal so that the edgesof work piece are fused and joined together forming a weld after Solidification.

The process is carried out in a vacuum chamber at a pressure of about 2*10-7

to2*10-6 psi (0.00013 to 0.0013 Pa). Such high vacuum is required in order toprevent loss of the electrons energy in collisions with air molecules.

The electrons are emitted by a cathode (electron gun). Due to a high voltage(about 150 kV) applied between the cathode and the anode the electrons areaccelerated up to 30% - 60% of the speed of light. Kinetic energy of theelectrons becomes sufficient for melting the targeted weld. Some of theelectrons energy transforms into X-ray irradiation.Electrons accelerated by electric field are then focused into a thin beam in thefocusing coil. Deflection coil moves the electron beam along the weld.

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Electron Beam is capable to weld work pieces withthickness from 0.0004” (0.01 mm) up to 6” (150 mm) of steel and up to 20” (500 mm) of aluminum. Electron

Beam Welding may be used for joining any metalsincluding metals, which are hardly wieldable by other welding methods: refractory metals (tungsten,molybdenum, niobium) and chemically active metals(titanium, zirconium, beryllium). Electron Beam Welding is also able to join dissimilar metals.

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 Advantages of Electron Beam Welding (EBW): 

Tight continuous weld;

Low distortion; Narrow weld and narrow heat affected zone;

Filler metal is not required.

Disadvantages of Electron Beam Welding (EBW): 

Expensive equipment;

High production expenses;

 X-ray irradiation.

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Laser Welding (LW)  Laser Welding (LW) is the welding process, in which heat is generated by a

high energy laser beam targeted on the work piece. The laser beam heats andmelts the work pieces edges, forming a joint.

Energy of narrow laser beam is highly concentrated: 108-1011 W/in2 (108-1010 

 W/cm2

), therefore diminutive weld pool forms very fast (for about 10-6

sec.).Solidification of the weld pool surrounded by the cold metal is as fast asmelting. Since the time when the molten metal is in contact with theatmosphere is short, no contamination occurs and therefore no shields (neutralgas, flux) are required.

The joint in Laser Welding (Laser Beam Welding) is formed either as asequence of overlapped spot welds or as a continuous weld.

Laser Welding is used in electronics, communication and aerospace industry,for manufacture of medical and scientific instruments, for joining miniaturecomponents.

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Advantages of Laser Welding:  Easily automated process; Controllable process parameters;  Very narrow weld may be obtained; High quality of the weld structure;  Very small heat affected zone; Dissimilar materials may be welded;  Very small delicate work pieces may be welded;  Vacuum is not required; Low distortion of work piece.

Disadvantages of Laser Welding:  Low welding speed; High cost equipment;  Weld depth is limited.

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Plasma Arc Welding (PAW)  Plasma Arc Welding is a welding process utilizing heat generated by a

constricted arc struck between a tungsten non-consumable electrode andeither the work piece (transferred arc process) or water cooled constrictingnozzle (non-transferred arc process).

Plasma is a gaseous mixture of positive ions, electrons and neutral gasmolecules.

Transferred arc process produces plasma jet of high energy density and may beused for high speed welding and cutting of Ceramics, steels, Aluminum alloys,Copper alloys, Titanium alloys, Nickel alloys.

 Non-transferred arc process produces plasma of relatively low energy density. Itis used for welding of various metals and for plasma spraying (coating). Sincethe work piece in non-transferred plasma arc welding is not a part of electriccircuit, the plasma arc torch may move from one work piece to other withoutextinguishing the arc.

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 Advantages of Plasma Arc Welding (PAW): 

Requires less operator skill due to good tolerance of arcto misalignments;

High welding rate; High penetrating capability (keyhole effect);

Disadvantages of Plasma Arc Welding (PAW): 

Expensive equipment;

High distortions and wide welds as a result of highheat input.

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Solid State Welding (SSW)  Solid State Welding is a welding process, in which

two work pieces are joined under a pressure providingan intimate contact between them and at a

temperature essentially below the melting point of theparent material. Bonding of the materials is a result of diffusion of their interface atoms.

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Advantages of Solid State Welding:   Weld (bonding) is free from microstructure defects (pores, non-

metallic inclusions, segregation of alloying elements); Mechanical properties of the weld are similar to those of the parent

metals; No consumable materials (filler material, fluxes, shielding gases) are

required; Dissimilar metals may be joined (steel - aluminum alloy , steel - copper

alloy ).

Disadvantages of Solid State Welding:  Thorough surface preparation is required (degreasing, oxides removal,

brushing/sanding); Expensive equipment.

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The following processes are related to Solid Statewelding: 

Forge Welding (FOW) 

Cold Welding (CW) 

Friction Welding (FRW) 

Explosive Welding (EXW) 

Diffusion Welding (DFW)  Ultrasonic Welding (USW) 

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 Advantages of Forge Welding:  Good quality weld may be obtained;

Parts of intricate shape may be welded;

No filler material is required.

Disadvantages of Forge Welding:  Only low carbon steel may be welded;

High level of the operators skill is required; Slow welding process;

 Weld may be contaminated by the coke used inheating furnace.

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Cold Welding (CW)  Cold Welding is a Solid State Welding process, in

 which two work pieces are joined together at roomtemperature and under a pressure, causing a

substantial deformation of the welded parts andproviding an intimate contact between the weldedsurfaces.

 As a result of the deformation, the oxide film coveringthe welded parts breaks up, and clean metal surfacesreveal. Intimate contact between these pure surfacesprovide a strong and defect less bonding.

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  Aluminum alloys, Copper alloys, low carbon steels, Nickelalloys, and other ductile metals may be welded by Cold

 Welding.

Cold Welding is widely used for manufacturing bi-metalsteel - aluminum alloy strips, for cladding of aluminumalloy strips by other aluminum alloys or pure aluminum

(Corrosion protection). Bi-metal strips are produced by Rolling technology. Presses are also used for Cold Welding.

Cold Welding may be easily automated.

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Friction Welding (FRW)  Friction Welding is a Solid State Welding process, in

 which two cylindrical parts are brought in contact by afriction pressure when one of them rotates. Friction

between the parts results in heating their ends. Forgepressure is then applied to the pieces providingformation of the joint.

Carbon steels, Alloy steels, Tool and die steels,Stainless steels, Aluminum alloys, Copper alloys,Magnesium alloys, Nickel alloys, Titanium alloys may be joined by Friction Welding.

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Explosive Welding (EXW)  Explosive Welding is a Solid State Welding process, in which welded parts

(plates) are metallurgically bonded as a result of oblique impact pressureexerted on them by a controlled detonation of an explosive charge.

One of the welded parts (base plate) is rested on an anvil, the second part (flyerplate) is located above the base plate with an angled or constant interfaceclearance.Explosive charge is placed on the flyer plate. Detonation starts at an edge of theplate and propagates at high velocity along the plate.The maximum detonation velocity is about 120% of the material sonic velocity.The slags (oxides, nitrides and other contaminants) are expelled by the jetcreated just ahead of the bonding front.

Most of the commercial metals and alloys may be bonded (welded) by Explosive Welding.

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Dissimilar metals may be joined by Explosive Welding: Copper to steel; Nickel to steel;  Aluminum to steel; Tungsten to steel; Titanium to steel; Copper to aluminum.

Explosive Welding is used for manufacturing clad tubes andpipes, pressure vessels, aerospace structures, heat exchangers, bi-metal sliding bearings, ship structures, weld transitions,corrosion resistant chemical process tanks.

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 Advantages of Explosive Welding  Large surfaces may be welded; High quality bonding: high strength, no distortions, no porosity, no

change of the metal microstructure;

Low cost and simple process; Surface preparation is not required.

Disadvantages of Explosive Welding:  Brittle materials (low ductility and low impact toughness) cannot be

processed;

Only simple shape parts may be bonded: plates, cylinders; Thickness of flyer plate is limited - less than 2.5” (63 mm);  Safety and security aspects of storage and using explosives.

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Ultrasonic Welding (USW)  Ultrasonic Welding is a Solid State Welding process, in which two

 work pieces are bonded as a result of a pressure exerted to the weldedparts combined with application of high frequency acoustic vibration(ultrasonic).Ultrasonic vibration causes friction between the parts, which results in

a closer contact between the two surfaces with simultaneous localheating of the contact area. Interatomic bonds, formed under theseconditions, provide strong joint.

Ultrasonic cycle takes about 1 sec. The frequency of acoustic vibrationsis in the range 20 to 70 KHz.Thickness of the welded parts is limited by the power of the ultrasonicgenerator.

Ultrasonic Welding is used mainly for bonding small work pieces inelectronics, for manufacturing communication devices, medical tools,

 watches, in automotive industry.

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Advantages of Ultrasonic Welding:  Dissimilar metals may be joined;  Very low deformation of the work pieces surfaces; High quality weld is obtained; The process may be integrated into automated production lines; Moderate operator skill level is enough.

Disadvantages of Ultrasonic Welding: 

Only small and thin parts may be welded;  Work pieces and equipment components may fatigue at the

reciprocating loads provided by ultrasonic vibration;  Work pieces may bond to the anvil.

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Diffusion Welding (DFW)  Diffusion Welding is a Solid State Welding process, in which

pressure applied to two work pieces with carefully cleanedsurfaces and at an elevated temperature below the melting pointof the metals. Bonding of the materials is a result of mutual

diffusion of their interface atoms.In order to keep the bonded surfaces clean from oxides and otherair contaminations, the process is often conducted in vacuum.No appreciable deformation of the work pieces occurs inDiffusion Welding.

Diffusion Welding is often referred more commonly as SolidState Welding (SSW).

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Diffusion Welding is able to bond dissimilar metals, whichare difficult to weld by other welding processes:

Steel to tungsten;

Steel to niobium;

Stainless steel to titanium;

Gold to copper alloys.

Diffusion Welding is used in aerospace and rocketry industries, electronics, nuclear applications,manufacturing composite materials.

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Advantages of Diffusion Welding:  Dissimilar materials may be welded (Metals, Ceramics, Graphite,

glass);  Welds of high quality are obtained (no pores, inclusions,

chemical segregation, distortions). No limitation in the work pieces thickness.

Disadvantages of Diffusion Welding:  Time consuming process with low productivity;  Very thorough surface preparation is required prior to welding

process; The mating surfaces must be precisely fitted to each other; Relatively high initial investments in equipment.

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Carbon Arc Welding  Carbon Arc Welding (CAW) is the welding process, in

 which heat is generated by an electric arc struck betweenan carbon electrode and the work piece. The arc heats andmelts the work pieces edges, forming a joint.

Carbon arc welding is the oldest welding process.

If required, filler rod may be used in Carbon Arc Welding.End of the rod is held in the arc zone. The molten rodmaterial is supplied to the weld pool.

Shields (neutral gas, flux) may be used for weld poolprotection depending on type of welded metal.

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 Advantages of Carbon Arc Welding: 

Low cost of equipment and welding operation;

High level of operator skill is not required; The process is easily automated;

Low distortion of work piece.

Disadvantages of Carbon Arc Welding: 

Unstable quality of the weld (porosity);

Carbon of electrode contaminates weld material withcarbides.

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Carbon Arc Welding has been replaced by Tungsten InertGas Arc Welding (TIG, GTAW) in many applications.

Modification of Carbone Arc Welding is Twin CarbonElectrode Arc Welding, utilizing arc struck between twocarbon electrodes.

 Work piece is not a part of  welding electric circuit in TwinCarbon Electrode Arc Welding, therefore the welding torchmay be moved from one work piece to other withoutextinguishing the arc.