L2-Arc Welding Process

8/18/2019 L2-Arc Welding Process

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ManufacturingProcess I

LECTURE 2: ARC WELDING PROCESS


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ArcWelding

• Arc welding is a group of welding processes wherein coalescence is produced byheating with electric arc , mostly without the application of pre !re and with orwithout the use of "iller #etal depending upon the base plate thickness.

• In Electric Arc Welding a sustained arc provides the heat required for melting the parent as well as filler material.


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Carbon ArcWelding (CAW)

• The carbon arc welding (CAW) process is the oldest of all the arc weldingprocesses and is considered to be the beginning of arc welding. The WeldingSociety defines carbon arc welding as "an arc welding process whichproduces coalescence of metals by heating them with an arc between acarbon electrode and the work piece. !o shielding is used. ressure and

filler metal may or may not be used." #t has limited applications today$ but a %ariation or twin carbon arc welding is more popular.


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Joint Welded by ArcWelding


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• The workpiece and the electrode are connected to the two materials of the powersource. The arc is started by momentarily touching the electrode on to theworkpiece and then withdrawing it to a short distance a few mm! from theworkpiece.

• When the electrode and workpiece are in contact, current flows and when they are

separated an arc is generated and the current continues to flow.• The arc is generated by the electrons liberated form cathode and moving towards

anode. The arc changes electrical energy into heat and light.

Carbon Arc Welding:Principles of operation


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• About 70% of the heat liberated due to striking of electrons atanode raises the anode temperature to a ery high alues(!"### to $#"###oC)% &his heat melts the base metal as 'ellas tip of the electrode in the area surrounding the arc%

• A weld is formed when the mixture of molten base and electrodemetal solidi es in the weld area.

• Since 70% heat is generated at anode a workpiece connected to

anode 'ill melt !# faster as compared to if connected'ith cathode% &his is 'hy 'ork piece is usually madepositi e and electrode as negati e and is termed asstraight polarity%


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Ad antages• Heat input to the workpiece can be easily controlled bychanging the arc length.

• rocess can be easily mechani!ed.• rocess is simple and good welding skill can be ac"uired in

short time.• #otal welding cost is less as compared to other welding

processes.• $"uipment re"uired for carbon arc welding is simple and easilya ailable.

isad antages• #here are chances of carbon being transferred from electrode to

weld metal& thus causing a harder weld deposit in case offerrous materials.

• 'n the absence of proper electrode geometry and in con nedspaces arc blow results which gi es poor welds with blow holesand porosity.

• A separate ller metal is needed( which )when used* slowsdown the welding speed.

Applications

• +arbon arc welding process can be used for welding steel&aluminium& nickel& copper and a good number of other alloys.

Carbon ArcWelding


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*hielded metal arc 'elding (*MAW) & is a manual arc weldingprocess that uses a consumable electrode coated in ux to laythe weld. An electric current& in the form of either alternatingcurrent )A+* or direct current )/+* from a welding power supply&is used to form an electric arc between the electrode and themetals to be -oined.

*hielded Metal ArcWelding (*MAW)


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Arc te#perat!re and thus the arc heat can be increa ed $r decrea ed . Ahigh current arc and smaller arc length produces a very intense heat.

C$n !#a%le electr$de usually have a c$ating on its outer surface whichon melting release gases like h&dr$gen $r car%$n di$'ide to form a

protective covering around the molten pool.

The electrode coating also reacts to form lag which i a li(!id) lighterthan the #$lten #etal*

#he slag therefore rises to the surface and on solidi cationforms a protecti e co ering o er the hot metal.

#his also slows down the rate of cooling of the weld. #heslag layer can be remo ed by light chipping.

More than !# industrial arc 'elding is done by thismethod%



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Ad antages• #he e"uipment can be portable and the cost is fairly low.• elding can be carried out in any position with highest weld

"uality.• 1oints which because of their position are di2cult to be welded

by automatic welding machines are easily accomplished by uxshielded metal arc welding.

isad antages• 3ecause of the limited length of each electrode and brittle ux

coating on it mechani!ation is di2cult.• 'n welding long +oints )e.g.& in pressure essels*& as one

electrode nishes& the weld is to be progressed with the nextelectrode. 4nless properly cared& a defect )like slag inclusion orinsu2cient penetration* may occur at the place where weldingis restarted with the new electrode.

• #he process uses stick electrodes and thus it is slo'er ascompared to 56 welding.

Applications• Almost all the commonly employed metals and their alloys can

be welded by this process.• S5A is used both as a fabrication process and formaintenance and repair -obs.



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"or continuous arc welding operations, the consumables electrode is barewire in the form of a coil and the flu# is fed into the welding $one, or theweld area is covered by an inert gas. In %ubmerged Arc Welding the baseelectrode is shielded by granular flu# consisting of silica, lime, manganeseo#ide, calcium fluoride and other compounds supplied from a hopper.

*ubmergedArc Welding


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Ad antages• 3ecause of high heat concentration& considerably higher speedscan be used.

• elding is carried out without sparks& smoke& ash or spatter.• 't can be used for welding in exposed area.

isad antages•

Since the operator cannot see the welding being carried out& hecannot -udge accurately the progress of welding.• #he process is limited to weld in at positionsApplications• Automoti e& a iation& ship,building and nuclear power industry.• 8ebuilding of worn out part.• 9or welding metals like mild steel.


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as 5etal Arc elding is fre"uently referred to as 5' welding. MI, welding isa commonly used high deposition rate welding process. ire is continuouslyfed from a spool. 5' welding is therefore referred to as a semiautomaticwelding process.Principle of Operation&efore igniting the arc$ gas and water flow ischecked. roper current and wire feed speed isset and the electrical connections are ensured.The arc is struck by any one of the two methods. In the first method current and shielding gasflow is switched on and the electrode isscratched against the 'ob as usual practice forstriking the arc. #n the second method $electrode is made to touch the 'ob$ is retractedand then mo%ed forward to carry out welding but. before striking the arc$ shielding gas$ waterand current is switched on. About * mm lengthof the electrode is pro'ected from the torch before striking the arc. +uring welding$ torchremains about , - mm the 'ob and length iskept between .* to mm. Arc length ismaintained constant by using the principles ofself ad'usted arc$ and self controlled arc in

semi automatic (manually operated) andautomatic welding sets respecti%ely.

/as 0etal Arc Welding (/0AW) or0etal #nert /as (0#/) Welding

,as Metal Arc Welding (,MAW)or MI, Welding


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Ad antages3ecause of continuously fed electrode& 5' welding process is much faster ascompared to #'

#hick and thin& both types of workpieces can be welded e;ecti ely


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*hielding ,as #he shielding gas& forms the arc plasma& stabili!es the arc on the metalbeing welded& shields the arc and molten weld pool& and allows smoothtransfer of metal from the weld wire to the molten weld pool. #here arethree primary metal transfer modes>

Spray transfer

lobular transferShort circuiting transfer #he primary shielding gasses used are>

ArgonArgon , 6 to ?% @xygenArgon , to B?% +@B

ArgonCHelium +@B is also used in its pure form in some 5' weldingprocesses. Howe er& in some applications the presence of +@B in theshielding gas may ad ersely a;ect the mechanical properties of theweld.


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&ungsten Inert ,as (&I,) Welding

Heating the -ob with anelectric arc struckbetween a tungstenelectrode and the -ob A shielding gas is usedto a oid atmosphericcontamination of themolten weld pool. )


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Ad antages=o ux is used& hence there is no danger of ux entrapment when welding

refrigerator and air conditioner components+lear isibility of the arc and the -ob& the operator can exercise a better control

on the welding process #' welding is ery much suitable for high "uality welding of thin materials )as

thin as 0.6B? mm*'t is a ery good process for welding nonferrous metals )aluminium etc.* and

stainless steelisad antages5' welding is a much faster process as compared to #' welding& since. #'

welding re"uires a separate ller rod #ungsten if it transfers to molten weld pool can contaminate the same.

#ungsten inclusion is hard and brittle9iller rod end if it by chance comes out of the inert gas shield can cause weld

metal contamination$"uipment costs are higher than that for ux shielded metal arc weldingApplications

elding aluminium& magnesium& copper& nickel and their alloys& carbon& alloyor stainless steels& inconel& high temperature and hard surfacing alloys like!irconium& titanium etc.

elding sheet metal and thinner sectionsrecision welding in atomic energy& aircraft& chemical and instrument

industries

&ungsten Inert ,as (&I,) Welding

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L2-Arc Welding Process