welding and allied processes.pdf

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Welding and Allied ProcessesWelding and Allied Processes

Dr.Dr. PulakPulakM.M. PandeyPandey

http://http://paniit.iitd.ac.in/~pmpandeypaniit.iitd.ac.in/~pmpandey

Manual Arc Welding


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FundamentalsFundamentals

The termThe termjoiningjoining refers to welding, brazing, soldering andrefers to welding, brazing, soldering andadhesive bonding. In these process a permanent joint between theadhesive bonding. In these process a permanent joint between theparts is formed and cannot be separated easily.parts is formed and cannot be separated easily.

The termThe term assemblyassembly usually refers to mechanical methods ofusually refers to mechanical methods offastening the parts together. Some of these methods allow easyfastening the parts together. Some of these methods allow easydisassembly, while others do not.disassembly, while others do not.

Welding is a material joining process in which two or more partsWelding is a material joining process in which two or more parts

are coalesced (joined together) at their contacting surfaces byare coalesced (joined together) at their contacting surfaces by aasuitable application of heat and/or pressure. Sometimes parts asuitable application of heat and/or pressure. Sometimes parts arereunited together by application of pressure only without externaunited together by application of pressure only without externallheat.heat.

In some welding process aIn some welding process afillerfiller material is added to facilitatematerial is added to facilitatecoalescence.coalescence.

Welding is most commonly associated with metallic parts but forWelding is most commonly associated with metallic parts but forplastics also it is used.plastics also it is used.


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Types of welding processesTypes of welding processes

Solid state welding processesSolid state welding processes

Liquid state welding processesLiquid state welding processes

Solid / Liquid state bonding processesSolid / Liquid state bonding processes


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SolidSolid--state Welding Processesstate Welding Processes

In solid state welding the surfaces to be joined areIn solid state welding the surfaces to be joined are

brought into close proximity by:brought into close proximity by:Heating the surfaces without causing melting andHeating the surfaces without causing melting andapplying normal pressureapplying normal pressure

Providing relative motion between the two surfaces andProviding relative motion between the two surfaces andapplying light normal pressureapplying light normal pressure

Applying high pressure without heatingApplying high pressure without heating

In these processes the materials remain in solidIn these processes the materials remain in solidstate and welding is achieved through thestate and welding is achieved through theapplication of heat and pressure, or high pressureapplication of heat and pressure, or high pressureonlyonly


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Forge weldingForge welding Forge welding is the oldest method of welding in the category ofForge welding is the oldest method of welding in the category of

solid state welding.solid state welding.

Surfaces to be joined are heated till they are red hot and thenSurfaces to be joined are heated till they are red hot and then

forced together by hammering.forced together by hammering.

It is a crude method of welding and quality depends upon theIt is a crude method of welding and quality depends upon the

skill of the welder.skill of the welder.

A modern version of this type of welding is manufacture of buttA modern version of this type of welding is manufacture of butt--

welded pipes. In this process, thewelded pipes. In this process, the skulpskulp heated up to the requiredheated up to the requiredwelding temperature is pulled through die which forces the twowelding temperature is pulled through die which forces the two

edges of the heatededges of the heated skulpskulp to contact under pressure and getto contact under pressure and get

welded.welded.


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Friction WeldingFriction Welding

In this process the two surfaces to be welded are rotated relatiIn this process the two surfaces to be welded are rotated relativeve

to each other under light normal pressure. When the interfaceto each other under light normal pressure. When the interface

temperature reaches due to frictional rubbing and when it reachetemperature reaches due to frictional rubbing and when it reachess

the required welding temperature, sufficient normal pressure isthe required welding temperature, sufficient normal pressure isapplied and maintained until the two pieces get welded.applied and maintained until the two pieces get welded.


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Explosion WeldingExplosion Welding

Welding is achieved in this process by very high contact preWelding is achieved in this process by very high contact pressuressuredeveloped by detonating a thin layer of explosive placed over ondeveloped by detonating a thin layer of explosive placed over oneeof the pieces to be joined. The detonation imparts high kineticof the pieces to be joined. The detonation imparts high kineticenergy to the piece which on striking the other piece causes plaenergy to the piece which on striking the other piece causes plasticstic

deformation and squeezes the contaminated surface layers out ofdeformation and squeezes the contaminated surface layers out ofthe interface resulting in a high quality welded joint. No fillethe interface resulting in a high quality welded joint. No fillerrmaterial is used and no diffusion takes place. The nature of bonmaterial is used and no diffusion takes place. The nature of bond isd ismetallurgical, in many cases combined with a mechanicalmetallurgical, in many cases combined with a mechanicalinterlocking that results from rippled or wavy interface betweeninterlocking that results from rippled or wavy interface between thethemetals.metals.


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Liquid State (Fusion) Welding ProcessesLiquid State (Fusion) Welding Processes

Arc WeldingArc Welding

Resistance weldingResistance welding

Oxyfuel gas weldingOxyfuel gas welding Other processesOther processes

There are two inherent problems with fusionThere are two inherent problems with fusionweldingwelding

Effect ofEffect of localized heating and rapid coolinglocalized heating and rapid cooling on theon themicrostructure and properties of the parent metals.microstructure and properties of the parent metals.

Effect of residual stresses developed in the parentEffect of residual stresses developed in the parentmetals due to restrained expansion or contraction. Thismetals due to restrained expansion or contraction. Thiseffect the impact and fatigue life ofeffect the impact and fatigue life of weldmentweldment..


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Arc WeldingArc Welding InIn Electric Arc WeldingElectric Arc Welding a sustained arc provides the heat requireda sustained arc provides the heat required

for melting the parent as well as filler material.for melting the parent as well as filler material.

TheThe workpieceworkpiece and the electrode are connected to the two materialsand the electrode are connected to the two materials

of the power source. The arc is started by momentarily touchingof the power source. The arc is started by momentarily touching

the electrode on to thethe electrode on to the workpieceworkpiece and then withdrawing it to a shortand then withdrawing it to a short

distance (a few mm) from thedistance (a few mm) from the workpieceworkpiece..

When the electrode andWhen the electrode and workpieceworkpiece are in contact, current flows andare in contact, current flows andwhen they are separated an arc is generated and the currentwhen they are separated an arc is generated and the current

continues to flow.continues to flow.

The arc is generated by the electrons liberated form cathode andThe arc is generated by the electrons liberated form cathode and

moving towards anode.moving towards anode.

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

Arc and power Source characteristics are given in theArc and power Source characteristics are given in the figurefigure..


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About 70% of the heat liberated due to striking of electrons atAbout 70% of the heat liberated due to striking of electrons at

anode raises the anode temperature to a very values (5,000 toanode raises the anode temperature to a very values (5,000 to30,00030,000ooC). This heat melts the base metal as well as tip of theC). This heat melts the base metal as well as tip of theelectrode in the area surrounding the arc.electrode in the area surrounding the arc.

A weld is formed when when when the mixture of molten baseA weld is formed when when when the mixture of molten base

and electrode metal solidifies in the weld area.and electrode metal solidifies in the weld area.

Since 70% heat is generated at anode aSince 70% heat is generated at anode a workpieceworkpiece connected toconnected toanode will melt 50% faster as compared to if connected withanode will melt 50% faster as compared to if connected withcathode. This is whycathode. This is why workpieceworkpiece is usually made positive andis usually made positive andelectrode as negative and is termed as straight polarity.electrode as negative and is termed as straight polarity.

When the work and electrode connections are reversed, reversedWhen the work and electrode connections are reversed, reversedpolarity is said to be employed.polarity is said to be employed.

Both direct current (DC) and alternating currents (AC) are usedBoth direct current (DC) and alternating currents (AC) are usedin arc welding. AC machines are less expensive to purchase andin arc welding. AC machines are less expensive to purchase andoperate, but generally restricted to welding of ferrous metals.operate, but generally restricted to welding of ferrous metals.

DC equipment can be used on all metals with good results and isDC equipment can be used on all metals with good results and is

generally noted for better arc control.generally noted for better arc control.


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The used can be either nonThe used can be either non--consumable or consumable.consumable or consumable.

Consumable electrodes usually have a coating on its outer surfacConsumable electrodes usually have a coating on its outer surfaceewhich on melting release gases like hydrogen or carbon dioxide twhich on melting release gases like hydrogen or carbon dioxide tooform a protective covering around the molten pool.form a protective covering around the molten pool.

The electrode coating also reacts to from slag which is a liquidThe electrode coating also reacts to from slag which is a liquid, lighter, lighter

than the molten metal. The slag therefore rises to the surface athan the molten metal. The slag therefore rises to the surface and onnd onsolidification forms a protective covering over the hot metal. Tsolidification forms a protective covering over the hot metal. This alsohis alsoslows down the rate of cooling of the weld. The slag layer can bslows down the rate of cooling of the weld. The slag layer can beeremoved by light chipping. Electric arc welding of this type isremoved by light chipping. Electric arc welding of this type is knownknown

asas Shielded Metal Arc WeldingShielded Metal Arc Welding. More than 50% industrial arc welding. More than 50% industrial arc weldingis done by this method. Limitation of this process is that onlyis done by this method. Limitation of this process is that only straightstraightelectrodes can be used as the coating is brittle.electrodes can be used as the coating is brittle.

For continuous arc welding operations, the consumables electrodeFor continuous arc welding operations, the consumables electrode isis

bare wire in the form of a coil and the flux us fed into the welbare wire in the form of a coil and the flux us fed into the weldingdingzone, or the weld area is covered by an inert gas. Inzone, or the weld area is covered by an inert gas. In Submerged ArcSubmerged ArcWeldingWelding the base electrode is shielded by granular flux supplied fromthe base electrode is shielded by granular flux supplied froma hopper, while isa hopper, while is Gas Metal Arc WeldingGas Metal Arc Welding shielding of the area isshielding of the area is

provided by an inert gas such as argon, helium, carbon dioxide ,provided by an inert gas such as argon, helium, carbon dioxide , etc.etc.


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Non consumable arc welding processes use tungsten electrodesNon consumable arc welding processes use tungsten electrodes

and shielding is provided by an inert gas around the weld area.and shielding is provided by an inert gas around the weld area.Once such process, the Gas Tungsten Arc Welding (GTAW) isOnce such process, the Gas Tungsten Arc Welding (GTAW) is

also called asalso called as Tungsten Inert Gas (TIG)Tungsten Inert Gas (TIG) welding.welding.

It uses tungsten alloy electrode and helium gas shield. BecauseIt uses tungsten alloy electrode and helium gas shield. Becauseof inert gas atmosphere tungsten is not consumed. Fillerof inert gas atmosphere tungsten is not consumed. Filler

materials supplied by a separate rod or wire.materials supplied by a separate rod or wire.


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Arc WeldingArc Welding

Back


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Back


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Submerged Arc WeldingSubmerged Arc Welding

Back


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Gas Metal Arc WeldingGas Metal Arc Welding

Back


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Tungsten Inert Gas WeldingTungsten Inert Gas Welding

(TIG)(TIG)


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Resistance Spot WeldingResistance Spot Welding


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Two opposing solid cylindrical electrodes are pressed against thTwo opposing solid cylindrical electrodes are pressed against the lape lap

joint and two metallic sheets to be welded.joint and two metallic sheets to be welded. Current ranges 3,000 to 40,000 Ampere depending on theCurrent ranges 3,000 to 40,000 Ampere depending on the

requirement causes a weld nugget of size varying from 6 to 10 mmrequirement causes a weld nugget of size varying from 6 to 10 mmdiameter to be formed at the metallic interface.diameter to be formed at the metallic interface.

The current is switched on for a duration lasting 0.1 to 0.5 secThe current is switched on for a duration lasting 0.1 to 0.5 sec.. At low pressures, the resistance and heat are high and melted meAt low pressures, the resistance and heat are high and melted metaltal

tend to squeeze out of the weld.tend to squeeze out of the weld.

At high pressure, the resistance decreases and heat is less andAt high pressure, the resistance decreases and heat is less andsmaller weld formed provides lower weld strength. Thus, for a gismaller weld formed provides lower weld strength. Thus, for a givenvenset of conditions, optimum electrode current and electrode pressset of conditions, optimum electrode current and electrode pressureureare indicated.are indicated.

The time duration of current flow should not be beyond certainThe time duration of current flow should not be beyond certaincritical, because the heat then has a chance to spread out and hcritical, because the heat then has a chance to spread out and harmarmworkpieceworkpiece and electrode.and electrode.

Optimum values of current, pressure and their application timingOptimum values of current, pressure and their application timing areare

dependent on weld size and material.dependent on weld size and material.


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Seam WeldingSeam Welding


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Automatic Spot Welding OperationAutomatic Spot Welding Operation

Performed by a RobotPerformed by a Robot


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Oxyfuel WeldingOxyfuel Welding

OFW is the term to describe the group of fusionOFW is the term to describe the group of fusionoperations that burn various fuels mixed with oxygen tooperations that burn various fuels mixed with oxygen to

perform welding.perform welding.

The OFW processes employ several type of gases, whichThe OFW processes employ several type of gases, whichis the primary distinction among the members of thisis the primary distinction among the members of thisgroup.group.

The most important OFW process is oxyacetyleneThe most important OFW process is oxyacetylenewelding. Filler materials are used to supply additionalwelding. Filler materials are used to supply additionalmaterial to the weld zone. Flux is often used to clean thematerial to the weld zone. Flux is often used to clean the

surfaces and to retard oxidation by providing inert gassurfaces and to retard oxidation by providing inert gasshield around the weld area. It also helps in removingshield around the weld area. It also helps in removingoxide and other impurities. Borax, is the most commonoxide and other impurities. Borax, is the most commonflux, but sometimes other substances are added toflux, but sometimes other substances are added to

improve its effectiveness.improve its effectiveness.


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Salient points about oxyacetylene weldingSalient points about oxyacetylene welding

The heat is obtained by combustion of acetyleneThe heat is obtained by combustion of acetyleneand oxygen. Here primary combustion occurringand oxygen. Here primary combustion occurringin the inner zone gives:in the inner zone gives:

and the second reaction in the outer zone givesand the second reaction in the outer zone gives

The maximum temperature temperature at the tipThe maximum temperature temperature at the tipof inner cone reaches up to 3000of inner cone reaches up to 3000--35003500C.C.Therefore, most gas welding is performed byTherefore, most gas welding is performed by

keeping this inner zone tip just above the metal tokeeping this inner zone tip just above the metal tobe welded so that maximum temperature isbe welded so that maximum temperature isavailable for welding.available for welding.

HeatHCOOHC +++ 2222 2

HeatOHCOOHCO ++++ 2222 25.12


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Temperature distribution along the flameTemperature distribution along the flame


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AA neutral flameneutral flame is obtained when the ratio of isis obtained when the ratio of isoxygen and acetylene is 1. Most gas weldingoxygen and acetylene is 1. Most gas weldingoperations are carried out by this flame.operations are carried out by this flame.

AnAn oxidizing flameoxidizing flame is obtained when this ratio is moreis obtained when this ratio is morethan 1. This type of flame is not suitable for welding ofthan 1. This type of flame is not suitable for welding ofsteels since excess oxygen present reacts with carbon insteels since excess oxygen present reacts with carbon insteel and is generally used for welding of copper and itssteel and is generally used for welding of copper and its

alloys.alloys. When the ratio in mixture is less than 1 aWhen the ratio in mixture is less than 1 a carburizingcarburizing

flameflame is obtained. In this type of flame acetyleneis obtained. In this type of flame acetylenedecomposes into carbon and hydrogen and the flamedecomposes into carbon and hydrogen and the flametemperature gets reduced. Joining operations such astemperature gets reduced. Joining operations such as

brazing and soldering which require lower temperaturebrazing and soldering which require lower temperaturegenerally use this flame.generally use this flame.


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Flame CuttingFlame Cutting Metal is merely melted by theMetal is merely melted by the

flame of theflame of the oxyfueloxyfuel gas torchgas torch

and blown away to form a gap orand blown away to form a gap or

kerfkerf.. When ferrous metal is cut,When ferrous metal is cut,

actually burning of iron takesactually burning of iron takes

place according to one or more ofplace according to one or more of

the following reactions.the following reactions.

QOFeOFeQOFeOFe

QFeoOFe

++

++

++

322

432

23423


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Because, these reactions cannot take place below 815Because, these reactions cannot take place below 815CC oxyfueloxyfuel flameflame

is first used to raise the metal temperature where burning can bis first used to raise the metal temperature where burning can beeinitiated. Then a stream of pure oxygen is added to the torch (oinitiated. Then a stream of pure oxygen is added to the torch (or ther theoxygen content of theoxygen content of the oxyfueloxyfuel mixture is increased) to oxidize themixture is increased) to oxidize theiron. The liquid iron and iron oxides are then expelled from theiron. The liquid iron and iron oxides are then expelled from thejointjoint

by the kinetic energy of the oxygen gas stream.by the kinetic energy of the oxygen gas stream. Low rate of heat input, and need of preheating ahead of the cut,Low rate of heat input, and need of preheating ahead of the cut,

oxyfueloxyfuelproduces a relatively large heat affected zone and thusproduces a relatively large heat affected zone and thusassociated distortion zone.associated distortion zone.

The process is suitable when edge finish or tolerance is not criThe process is suitable when edge finish or tolerance is not critical.tical. Theoretically heat generated due to burning of Fe is sufficientTheoretically heat generated due to burning of Fe is sufficient toto

continue cutting however due to losses additional heat supply iscontinue cutting however due to losses additional heat supply isneeded. If the work is already hot due from the other processes,needed. If the work is already hot due from the other processes,

supply of oxygen through a small diameter pipe is needed to contsupply of oxygen through a small diameter pipe is needed to continueinuecut. This is called Oxygen Lance Cutting. Acut. This is called Oxygen Lance Cutting. A workpieceworkpiece temperature oftemperature of12001200C is needed to sustain the cutting.C is needed to sustain the cutting.

Low carbon steel from 5 to 75 mm can be cut.Low carbon steel from 5 to 75 mm can be cut.


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Welding DefectsWelding Defects

Cracks (Cracks (figurefigure)) This causes significant reduction in the strength ofThis causes significant reduction in the strength of weldmentweldment..

Welding cracks are caused byWelding cracks are caused by embrittlementembrittlement or low ductilityor low ductilityof the weld and/or base metal combined with high restraintof the weld and/or base metal combined with high restraint

during contraction.during contraction.

CavitiesCavities

These includes porosity and shrinkage voids.These includes porosity and shrinkage voids.

Solid inclusionsSolid inclusions These are metallic or nonThese are metallic or non--metallic solid material particlesmetallic solid material particles

entrapped in the weld metal. The most common form is slagentrapped in the weld metal. The most common form is slaginclusion or metallic oxides.inclusion or metallic oxides.

Incomplete fusion (Incomplete fusion (figurefigure))

A similar defect is lack of penetration.A similar defect is lack of penetration.

Imperfect shape (Imperfect shape (figurefigure))

Miscellaneous defects like arc strike, excessive spatterMiscellaneous defects like arc strike, excessive spatter


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Various forms of Welding Cracks

Several form incomplete fusion


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Solid / Liquid State BondingSolid / Liquid State Bonding

Low temperature joining methods are used whenLow temperature joining methods are used when

the metal to be joined cannot withstand highthe metal to be joined cannot withstand high

temperature, or intricate sections are to be joined,temperature, or intricate sections are to be joined,or dissimilar metals are to be joined, oror dissimilar metals are to be joined, or

weldabilityweldability of material is poor.of material is poor.

In these methods, the gap between the metalIn these methods, the gap between the metalpieces to be joined is filled with molten fillerpieces to be joined is filled with molten filler

material after heating the base metal. Meltingmaterial after heating the base metal. Melting

point of filler material is much lower than basepoint of filler material is much lower than base

metals.metals.

The bonding is not due to melting of parent metalThe bonding is not due to melting of parent metal

and fusion.and fusion.


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For a good joint strength theFor a good joint strength theliquid filler metal; must flow intoliquid filler metal; must flow intothe gap between the metal piecesthe gap between the metal piecesto be joined and cover the entireto be joined and cover the entiresurface area, without gaps or blowsurface area, without gaps or blow

holes. The following usuallyholes. The following usuallyinsures good bonding:insures good bonding:

Clean base metal surfacesClean base metal surfaces

Maintain optimum gapMaintain optimum gap

Heat the joining area above meltingHeat the joining area above meltingtemperature of the filler materialtemperature of the filler material

Use fluxes for welding of base metalUse fluxes for welding of base metal

surfaces.surfaces. Joint strength is sensitive to theJoint strength is sensitive to the

gap and there exists an optimumgap and there exists an optimumgap for a filler material.gap for a filler material.


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BrazingBrazing

Brazing methods

(a) Torch and

filler rods(b) Ring of filler

metal at

entrance of

gap

(c) Foil of filler

metal between

flat part

surfaces


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In brazing the joint is made by heating theIn brazing the joint is made by heating thebase metal red hot and filling the gap withbase metal red hot and filling the gap with

molten metal whose melting temperature ismolten metal whose melting temperature is

typically above 450typically above 450C but below meltingC but below meltingtemperature o base metal. The filler metalstemperature o base metal. The filler metals

are generally copper alloys. Cuare generally copper alloys. Cu--Zn and CuZn and Cu--

Ag alloys are used for brazing because theyAg alloys are used for brazing because theyform alloy with iron and have goodform alloy with iron and have good

strength.strength.

Various brazing joints are shown inVarious brazing joints are shown in figurefigure..


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(a) Conventional butt

(b) Scarf joint

(c) Stepped joint

(d) Increased crossest ion

(a) Conventional Lap

(b) Cylindrical part

(c) Sandwiched part

(d) Use of sleeve


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Common braze metal familiesCommon braze metal families


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SolderingSoldering Soldering is very similar to brazing except thatSoldering is very similar to brazing except that

filler material is usually a leadfiller material is usually a lead--tin based alloytin based alloywhich has much lower strength and meltingwhich has much lower strength and melting

temperature around 250temperature around 250C.C.

In this process less alloying action between baseIn this process less alloying action between basemetal and filler material as compared to brazingmetal and filler material as compared to brazing

takes place hence the strength of joint is lesser.takes place hence the strength of joint is lesser.

It is carried out using electrical resistance heating.It is carried out using electrical resistance heating.


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Joints in SolderingJoints in Soldering

(a) Flat lock seam (b) Bolted or riveted joint (c) Copper

pipe fitting (d) Crimping of cylindrical lap joint.


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(a) Crimped leadwire PC board

(b) Plated through

hole on PCboard to

maximize

solder contact

area

(c) Hooked wire

on flat

terminals(d) Twisted wires

S ld llS ld ll


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Some common solder alloySome common solder alloy

compositionscompositions


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Comparison of welding processesComparison of welding processes


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