unit ii ppt mt i vol 1

8/3/2019 Unit II Ppt Mt i Vol 1

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UNITUNIT--22

JOINONG

PROCESSES


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WELDING DEFINITIONWELDING DEFINITION

The process of joining similar metals by

means of heat is calledWELDING.

Welding can be obtained with or withoutapplication of pressure and without addition

of filler metal, which is known as

electrode.

S.LOGANATHAN,LECT./MECH,MAHA COLLEGE OF ENGG.Salem. 3


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Classification of weldingClassification of welding

processprocess



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2.Plastic Welding orPressure Welding

The piece of metal to be

joined are heated to aplastic state and forced

together by external

pressure.

(Ex) Resistance welding

1.Fusion Welding or

Non-Pressure

Welding

The material at the

joint is heated to amolten state and

allowed to solidify.

(Ex) Gas welding, Arc

welding



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1.1. THERMIT WELDING1.1. THERMIT WELDING

y Welding the parts using liquid thermit

steel around the portions to be welded

is called as THERMIT WELDING.

y Welding principle:

The heat of the thermit reaction

used for welding plastic state and

mechanical pressure is applied to join

the parts.



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y Neither Arc is produced to heat parts

nor the flames.

y Thermit weld reaction is

8Al + 3Fe3O4 = 4Al2O3 +9Fe

y Thermite = mixture of Al and Fe3O4 fine

powders that produce an exothermic

reaction when ignited.



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y Filler metal obtained from liquid

metal.

y Process used for joining, but has more

in common with casting than welding.

y Two Types ofTW are

(a) Pressure welding process

(b) Non- Pressure welding process



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y (1) Thermit ignited; (2) crucible tapped, superheated metal

flows into mold; (3) metal solidifies to produce weld joint


Thermit Welding process


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Thermit Welding processThermit Welding process

y Parts to be welded is butted and

enclosed in mould.

y Heated iron slag is poured to the

mould.

y Them aluminum oxide is poured on

the parts to be welded.

y This will create the hating of partsand then pressure is applied on the

work piece to join.



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Applications of TWApplications of TW

yJoining of railroad rails.

y Repair of cracks in large steel

castings and forgings.y Steel rolling mills

y To weld non ferrous metals.



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1.2 Arc welding1.2 Arc welding

yArc welding is the process of joining

two metal pieces by melting their

edges by an electric arc.



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Arc welding processArc welding process

y The electrode and work piece arebrought nearer with a small air gap of

3mm approximately.y Then the current is passed through thework piece and the electrode toproduce an electric arc.

y Here electrical energy is converted intoheat energy



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y The electrode is melted by arc andelectrode is also melted hence the

work pieces become a single piece

without applying any external pressure.

y The temperature of arc produced isabout5000o to 6000o

y Electrode supplies additional fillermaterial in to the joints and depositedbetween the work pieces.



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y The depth to which the metal meltedand deposited is called Depth of Fusion.

y Electrode is coated with flux toprevent the reaction of molten metal

with the atmospheric air.

yAlso removes the impurities from themolten metal and forms slag whichdeposit over the weld to protect itfrom rapid cooling.



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y The molten metal is forced out of the

pool by the electric arc. Hence small

depression is formed in the parent

metal where the molten metal ispiled up. This is known as Arc

carter.

y The distance between the tip of thiselectrode and the bottom of the arc

crater is called Arc length



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ELECTRODE TYPESELECTRODE TYPES

1.Consumable Electrode

(a)Bare Electrode

(b)Light Coated Electrode

(c)Heavily Coated Electrode

2.Non-Consumable Electrode



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y Consumable consumed duringwelding process

Source of filler metal in arc welding

y Non-consumable not consumed

during welding process

Filler metal must be added separately if it

is added



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Consumable ElectrodeConsumable Electrode

Bare Electrode

y Dont have flux coating on its surface.

y Rarely used to weld wrought and cast

iron.

y It is used in submerged arc welding

and Inert gas welding processes.



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Light Coated Electrode:

yA coating layer of several tenths of a

millimeter and is 1 to 5% of

electrode weight.

y The mail purpose of light coating is

to increase arc stability called as

INOZING COATING.y It is used to weld non ferrous metals



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Heavily Coated Electrode:

yA heavy coating layer of several

tenths of a millimeter and is 1 to

3mm of thick.

y The weight of coating is from 15 t o 30

% of the electrode rod.



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NonNon--Consumable ElectrodeConsumable Electrode

y Non consumable electrodes are

made of carbon, graphite or tungsten

which do not consume during the

welding.

y The arc length remains constant and

hence it is stable.

y It is used in Atomic hydrogenwelding and TIG welding processes.



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Purpose of flux coatingPurpose of flux coating

y To give stability to the arc.

y To produce the gas shield around the

arc and molten metal.

y To provide the formation of slag so as

to protect the welding seam from

rapid cooling.

y Increase the deposition efficiency.



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1.2.1 Submerged arc welding1.2.1 Submerged arc welding

y Submerged arc welding is also

called as sub arc welding or hidden

arc welding.

yAn electrode is produced between

consumable bare electrode and the

work piece but the arc is completelysubmerged(hidden under the flux).



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Submerged arc welding processSubmerged arc welding process


y The metal electrode is fed continuously from thereel by moving head.

y The flux power is fed in front of the moving headand it is fed from hopper.

y

When the arc is produced in the welding zone atthe end of the electrode and the arc iscompletely covered by flux power.


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Submerged arc welding processSubmerged arc welding process

y The flux power used here is made up

of silica, metal oxide.

y The flux not only protect the weld

surface from atm. And also act as a

deoxidizer and scavenger.

y It is used specially for welding

carbon steels and alloys



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Applications of SAWApplications of SAW

y Steel fabrication of structural shapes

(e.g.,I-beams)

y Seams for large diameter pipes,

tanks, and pressure vessels

y Welded components for heavy

machinery

y Most steels (except hi C steel)

y Not good for nonferrous metals



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1.2.3. Plasma Arc welding1.2.3. Plasma Arc welding

y Tungsten electrode is contained in anozzle that focuses a high velocitystream of inert gas (argon) into arcregion to form a high velocity, intensely

hot plasma arc stream.

y Temperatures in PAW reach 28,000rC

(50

,000r

F), due to constriction of arc,producing a plasma jet of smalldiameter and very high energy density



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Plasma Arc welding processPlasma Arc welding process


rinciple:

y Plasma is high ionized gas. It is amixture of neutral atoms, positively

charged atoms and free elements.

y When this high temperature plasma ispassed through the orifice, the portionof the ionized gas increases and plasmaarc is formed.


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Working:

y When the high heat content plasma gas isforced through the torch orifice surroundedby ve tungsten electrode in the form of jet.

y The plasma cutting force imposes a swirl onthe orifice gas flow.


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y The arc is initiated in the beginningby supplying electrical energybetween nozzle and tungsten

electrode.y This will release high energy and

heat and is normally10000oC to30000oC

y Two types of plasma arc welding are1.Transfer type

2.Non-transfer type



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Applications of PAWApplications of PAWy Aerospace

y Welding titanium plates

y Welding Nickel Alloys


Advantages:Good arc stability and excellent weld qualityBetter penetration control than other AW processesHigh travel speedsCan be used to weld almost any metals

Disadvantages:High equipment costLarger torch size than other AW processes Tends torestrict access in some joints


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1.2.3.Atomic Hydrogen welding1.2.3.Atomic Hydrogen welding


Commonly used to bond two dissimilar

metals, e.g., to clad one metal on top of a

base metal over large areas

(1

) Setup in parallel configuration, and (2)during detonation of the explosive charge


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1.2.4.Metal Inert gas Welding1.2.4.Metal Inert gas Welding

y In this arc welding, the electric arc is

produced between a consumable

metal electrode and the work piece.

y During welding the arc an welding

zone are surrounded by an Inert

gas(Argon or Helium).

y The surrounding air protects theweld from atm.



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Metal Inert gas WeldingMetal Inert gas Welding


y The electrode is fed continuously

through welding head because at the

time of welding the electrode is

melted by the arc deposited over thework piece.


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Applications of MIGApplications of MIG

y To Weld thick plates of Aluminum,

Stainless steel, Nickel and

Magnesium metal parts are welded

using MIG without weld defects.


Advantages:

No flux is required

High welding Speed

Greater efficiency


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1.2.5. Metal Arc Welding1.2.5. Metal Arc Weldingy Uses a consumable bare metal wire as

electrode with shielding by flooding arcwith a gas.

y Wire is fed continuously and automaticallyfrom a spool through the welding gun.



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Metal Arc WeldingMetal Arc Welding

y Shielding gases include argon and

helium for aluminum welding, and

CO2 for steel welding

y Bare electrode wire plus shielding

gases eliminate slag on weld bead

No need for manual grinding andcleaning of slag



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Applications of MAWApplications of MAW

y Higher deposition rates

y Eliminates problem of slag removaly Can be readily automated



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1.2.6. Electro slag welding1.2.6. Electro slag weldingy Principle

Electro slag is a welding process in

which the coalescence is formed by

molten slag and molten pool remainsshielded by the molten slag



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Electro slag weldingElectro slag welding

Working:

y The electric arc is struck betweenthe electrode and the work joined by

use of steel.

y Welding flux is added and themelted by use of heat flux added andfurther melted by use of heat fromthe arc.



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Electro slag weldingElectro slag welding


y This action is stopped until the

molten slag is formed and molten

slag remains between the electrode

and work.y The temperature of this slag remains

1600oC to 1900oC.

y The electric current passes fromelectrode to the work piece through

the slag.


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1.3 GAS WELDING1.3 GAS WELDING

Types of Gaswelding

y

Oxy-Acetylenewelding

y Oxy-hydrogenwelding

y Air-hydrogen

welding



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1.3.1Oxy1.3.1Oxy--Acetylene weldingAcetylene welding

yA welding process in which the

edges of the metals to be welded

are melted melted by gas flame.

y No pressure is applied duringwelding except pressure gas

welding.

y Commonly used gases areAcetylene,Hydrogen,propane and

Butane.



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OxyOxy--Acetylene weldingAcetylene welding


The flame is produced at the tip of a welding torch.

The welding heat is obtained by burning a mixture

of oxygen and consumable gas.

The gases are mixed in the required proportion in a

welding torch provides control for the welding

flame.


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Types of OxyOxy--Acetylene weldingAcetylene welding

1.High pressure System

y Both 02 and C2H2 are supplied fromHigh pressure cylinders.

y

O2 is compressed to 120atm gaugepressure.

y But C2H2cant be compressed more than1.5atm in the from of dissolved

Acetylene which dissolved in acetoneunder the pressure of16 to 22atmgauge pressure



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Types of OxyOxy--Acetylene weldingAcetylene welding

2. Low pressure System Acetylene

y C2H2 is produced at the place of

welding by interaction of calcium

carbide and water in acetylenegenerator.

y Chemical reaction isCaC2+2H2O Ca(OH)2+C2H2



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1.3.2 Oxy1.3.2 Oxy--hydrogen weldinghydrogen welding

y This is similar to that of welding

process process.

y O2 and H2 are mixed with the

required proportion for producingheat.

y It is used to weld low temperature

metals such as Al,Pb,Mg.



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1.3.3 Air1.3.3 Air--Acetylene weldingAcetylene welding

y This is similar to that of welding

process process.

yAtmospheric air and C2H2 are mixed

with the required proportion forproducing heat.

y It is used to weld low melting

temperature metals such as Pb andalloys.



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Flame CharacteristicsFlame Characteristics

y Neutral flame

y Carburising flame

y Oxidising flame



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y ELECTRODE MOVEMENT

DIRECTION

S.LOGANATHAN,LECT./MECH,MAHA

COLLEGE OF ENGG Salem 51

unit ii ppt mt i vol 1

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