Welding and Allied ProcessesWelding and Allied Processes

Dr. Dr. PulakPulak M. M. PandeyPandeyhttp://http://paniit.iitd.ac.in/~pmpandeypaniit.iitd.ac.in/~pmpandey

Manual Arc Welding

FundamentalsFundamentalsThe term The term joiningjoining refers to welding, brazing, soldering and refers to welding, brazing, soldering and adhesive 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 term The term assembly assembly usually refers to mechanical methods of usually refers to mechanical methods of fastening the parts together. Some of these methods allow easy fastening the parts together. Some of these methods allow easy disassembly, 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 partsare coalesced (joined together) at their contacting surfaces by are coalesced (joined together) at their contacting surfaces by a a suitable application of heat and/or pressure. Sometimes parts asuitable application of heat and/or pressure. Sometimes parts are re united together by application of pressure only without externaunited together by application of pressure only without external l heat. heat. In some welding process a In some welding process a filler filler material is added to facilitate material is added to facilitate coalescence. coalescence. Welding is most commonly associated with metallic parts but for Welding is most commonly associated with metallic parts but for plastics also it is used.plastics also it is used.

Types of welding processesTypes of welding processes

Solid state welding processesSolid state welding processesLiquid state welding processesLiquid state welding processesSolid / Liquid state bonding processesSolid / Liquid state bonding processes

SolidSolid--state Welding Processesstate Welding Processes

In solid state welding the surfaces to be joined are In 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 and Heating the surfaces without causing melting and applying normal pressureapplying normal pressureProviding relative motion between the two surfaces and Providing relative motion between the two surfaces and applying light normal pressureapplying light normal pressureApplying high pressure without heatingApplying high pressure without heating

In these processes the materials remain in solid In these processes the materials remain in solid state and welding is achieved through the state and welding is achieved through the application of heat and pressure, or high pressure application of heat and pressure, or high pressure onlyonly

Forge weldingForge weldingForge welding is the oldest method of welding in the category ofForge welding is the oldest method of welding in the category ofsolid state welding.solid state welding.Surfaces to be joined are heated till they are red hot and then Surfaces 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 the It 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, the welded pipes. In this process, the skulpskulp heated up to the required heated up to the required welding temperature is pulled through die which forces the two welding temperature is pulled through die which forces the two edges of the heated edges of the heated skulpskulp to contact under pressure and get to contact under pressure and get welded.welded.

Friction WeldingFriction WeldingIn this process the two surfaces to be welded are rotated relatiIn this process the two surfaces to be welded are rotated relative ve to each other under light normal pressure. When the interface to 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 reaches s the required welding temperature, sufficient normal pressure is the required welding temperature, sufficient normal pressure is applied and maintained until the two pieces get welded.applied and maintained until the two pieces get welded.

Explosion Welding Explosion Welding Welding is achieved in this process by very high contact preWelding is achieved in this process by very high contact pressure ssure developed by detonating a thin layer of explosive placed over ondeveloped by detonating a thin layer of explosive placed over one e of the pieces to be joined. The detonation imparts high kinetic of the pieces to be joined. The detonation imparts high kinetic energy to the piece which on striking the other piece causes plaenergy to the piece which on striking the other piece causes plastic stic deformation and squeezes the contaminated surface layers out of deformation and squeezes the contaminated surface layers out of the interface resulting in a high quality welded joint. No fillethe interface resulting in a high quality welded joint. No filler r material is used and no diffusion takes place. The nature of bonmaterial is used and no diffusion takes place. The nature of bond is d is metallurgical, in many cases combined with a mechanical metallurgical, in many cases combined with a mechanical interlocking that results from rippled or wavy interface betweeninterlocking that results from rippled or wavy interface between the the metals.metals.

Liquid State (Fusion) Welding ProcessesLiquid State (Fusion) Welding ProcessesArc WeldingArc WeldingResistance weldingResistance weldingOxyfuel gas weldingOxyfuel gas weldingOther processesOther processes

There are two inherent problems with fusion There are two inherent problems with fusion weldingwelding

Effect of Effect of localized heating and rapid cooling localized heating and rapid cooling on the on the microstructure and properties of the parent metals. microstructure and properties of the parent metals. Effect of residual stresses developed in the parent Effect of residual stresses developed in the parent metals due to restrained expansion or contraction. This metals due to restrained expansion or contraction. This effect the impact and fatigue life of effect the impact and fatigue life of weldmentweldment. .

Arc WeldingArc WeldingIn In Electric Arc Welding Electric Arc Welding a sustained arc provides the heat required a sustained arc provides the heat required for melting the parent as well as filler material.for melting the parent as well as filler material.The The workpieceworkpiece and the electrode are connected to the two materials and the electrode are connected to the two materials of the power source. The arc is started by momentarily touching of the power source. The arc is started by momentarily touching the electrode on to the the electrode on to the workpieceworkpiece and then withdrawing it to a short and then withdrawing it to a short distance (a few mm) from the distance (a few mm) from the workpieceworkpiece..When the electrode and When the electrode and workpieceworkpiece are in contact, current flows and are in contact, current flows and when they are separated an arc is generated and the current when 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 andmoving 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 the Arc and power Source characteristics are given in the figurefigure..

About 70% of the heat liberated due to striking of electrons at About 70% of the heat liberated due to striking of electrons at anode raises the anode temperature to a very values (5,000 to anode raises the anode temperature to a very values (5,000 to 30,00030,000ooC). This heat melts the base metal as well as tip of the C). This heat melts the base metal as well as tip of the electrode in the area surrounding the arc. electrode in the area surrounding the arc. A weld is formed when when when the mixture of molten base A 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 a Since 70% heat is generated at anode a workpieceworkpiece connected to connected to anode will melt 50% faster as compared to if connected with anode will melt 50% faster as compared to if connected with cathode. This is why cathode. This is why workpieceworkpiece is usually made positive and is usually made positive and electrode 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, reversed When the work and electrode connections are reversed, reversed polarity is said to be employed.polarity is said to be employed.Both direct current (DC) and alternating currents (AC) are used Both direct current (DC) and alternating currents (AC) are used in arc welding. AC machines are less expensive to purchase and in arc welding. AC machines are less expensive to purchase and operate, 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 is DC equipment can be used on all metals with good results and is generally noted for better arc control. generally noted for better arc control.

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 surface e which on melting release gases like hydrogen or carbon dioxide twhich on melting release gases like hydrogen or carbon dioxide to o form 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 on nd on solidification forms a protective covering over the hot metal. Tsolidification forms a protective covering over the hot metal. This also his also slows 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 be e removed by light chipping. Electric arc welding of this type is removed by light chipping. Electric arc welding of this type is known known as as Shielded Metal Arc WeldingShielded Metal Arc Welding. More than 50% industrial arc welding . More than 50% industrial arc welding is done by this method. Limitation of this process is that only is done by this method. Limitation of this process is that only straight straight electrodes 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 is is 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 welding ding zone, or the weld area is covered by an inert gas. In zone, or the weld area is covered by an inert gas. In Submerged Arc Submerged Arc WeldingWelding the base electrode is shielded by granular flux supplied from the base electrode is shielded by granular flux supplied from a hopper, while is a hopper, while is Gas Metal Arc WeldingGas Metal Arc Welding shielding of the area is shielding 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.

Non consumable arc welding processes use tungsten electrodes Non 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) is Once such process, the Gas Tungsten Arc Welding (GTAW) is also called as also called as Tungsten Inert Gas (TIG)Tungsten Inert Gas (TIG) welding. welding. It uses tungsten alloy electrode and helium gas shield. Because It uses tungsten alloy electrode and helium gas shield. Because of inert gas atmosphere tungsten is not consumed. Filler of inert gas atmosphere tungsten is not consumed. Filler materials supplied by a separate rod or wire. materials supplied by a separate rod or wire.

Arc WeldingArc Welding

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Arc and Power Source Characteristics in Arc and Power Source Characteristics in Arc WeldingArc Welding

Arc Characteristics Power Source Characteristics

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Submerged Arc WeldingSubmerged Arc Welding

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Gas Metal Arc WeldingGas Metal Arc Welding

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Tungsten Inert Gas Welding Tungsten Inert Gas Welding (TIG)(TIG)

Resistance Spot WeldingResistance Spot Welding

Two opposing solid cylindrical electrodes are pressed against thTwo opposing solid cylindrical electrodes are pressed against the lap e 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 the Current 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 metal tal tend to squeeze out of the weld.tend to squeeze out of the weld.At high pressure, the resistance decreases and heat is less and At high pressure, the resistance decreases and heat is less and smaller weld formed provides lower weld strength. Thus, for a gismaller weld formed provides lower weld strength. Thus, for a given ven set of conditions, optimum electrode current and electrode pressset of conditions, optimum electrode current and electrode pressure ure are indicated.are indicated.The time duration of current flow should not be beyond certain The time duration of current flow should not be beyond certain critical, because the heat then has a chance to spread out and hcritical, because the heat then has a chance to spread out and harm arm workpieceworkpiece and electrode. and electrode. Optimum values of current, pressure and their application timingOptimum values of current, pressure and their application timing are are dependent on weld size and material. dependent on weld size and material.

Seam WeldingSeam Welding

Automatic Spot Welding Operation Automatic Spot Welding Operation Performed by a RobotPerformed by a Robot

Oxyfuel WeldingOxyfuel WeldingOFW is the term to describe the group of fusion OFW is the term to describe the group of fusion operations that burn various fuels mixed with oxygen to operations that burn various fuels mixed with oxygen to perform welding. perform welding. The OFW processes employ several type of gases, which The OFW processes employ several type of gases, which is the primary distinction among the members of this is the primary distinction among the members of this group. group. The most important OFW process is oxyacetylene The most important OFW process is oxyacetylene welding. Filler materials are used to supply additional welding. Filler materials are used to supply additional material to the weld zone. Flux is often used to clean the material to the weld zone. Flux is often used to clean the surfaces and to retard oxidation by providing inert gas surfaces and to retard oxidation by providing inert gas shield around the weld area. It also helps in removing shield around the weld area. It also helps in removing oxide and other impurities. Borax, is the most common oxide and other impurities. Borax, is the most common flux, but sometimes other substances are added to flux, but sometimes other substances are added to improve its effectiveness. improve its effectiveness.

Salient points about oxyacetylene weldingSalient points about oxyacetylene welding

The heat is obtained by combustion of acetylene The heat is obtained by combustion of acetylene and oxygen. Here primary combustion occurring and oxygen. Here primary combustion occurring in 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 tip The maximum temperature temperature at the tip of inner cone reaches up to 3000of inner cone reaches up to 3000--35003500C. C. Therefore, most gas welding is performed by Therefore, most gas welding is performed by keeping this inner zone tip just above the metal to keeping this inner zone tip just above the metal to be welded so that maximum temperature is be welded so that maximum temperature is available for welding. available for welding.

HeatHCOOHC +++ 2222 2

HeatOHCOOHCO ++++ 2222 25.12

Temperature distribution along the flameTemperature distribution along the flame

A A neutral flameneutral flame is obtained when the ratio of is is obtained when the ratio of is oxygen and acetylene is 1. Most gas welding oxygen and acetylene is 1. Most gas welding operations are carried out by this flame. operations are carried out by this flame. An An oxidizing flameoxidizing flame is obtained when this ratio is more is obtained when this ratio is more than 1. This type of flame is not suitable for welding of than 1. This type of flame is not suitable for welding of steels since excess oxygen present reacts with carbon in steels since excess oxygen present reacts with carbon in steel and is generally used for welding of copper and its steel and is generally used for welding of copper and its alloys. alloys. When the ratio in mixture is less than 1 a When the ratio in mixture is less than 1 a carburizing carburizing flameflame is obtained. In this type of flame acetylene is obtained. In this type of flame acetylene decomposes into carbon and hydrogen and the flame decomposes into carbon and hydrogen and the flame temperature gets reduced. Joining operations such as temperature gets reduced. Joining operations such as brazing and soldering which require lower temperature brazing and soldering which require lower temperature generally use this flame.generally use this flame.

Flame CuttingFlame CuttingMetal is merely melted by the Metal is merely melted by the flame of the flame of the oxyfueloxyfuel gas torch gas torch and blown away to form a gap or and blown away to form a gap or kerfkerf..When ferrous metal is cut, When ferrous metal is cut, actually burning of iron takes actually burning of iron takes place according to one or more of place according to one or more of the following reactions. the following reactions.

QOFeOFeQOFeOFe

QFeoOFe

++++

++

322

432

23423

Because, these reactions cannot take place below 815Because, these reactions cannot take place below 815C C oxyfueloxyfuel flame flame is first used to raise the metal temperature where burning can bis first used to raise the metal temperature where burning can be e initiated. Then a stream of pure oxygen is added to the torch (oinitiated. Then a stream of pure oxygen is added to the torch (or the r the oxygen content of the oxygen content of the oxyfueloxyfuel mixture is increased) to oxidize the mixture is increased) to oxidize the iron. The liquid iron and iron oxides are then expelled from theiron. The liquid iron and iron oxides are then expelled from the joint joint 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,oxyfueloxyfuel produces a relatively large heat affected zone and thus produces a relatively large heat affected zone and thus associated 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 sufficient Theoretically heat generated due to burning of Fe is sufficient to to 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 continue inue cut. This is called Oxygen Lance Cutting. A cut. This is called Oxygen Lance Cutting. A workpieceworkpiece temperature of temperature of 12001200C 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.

Welding DefectsWelding DefectsCracks (Cracks (figurefigure)) This causes significant reduction in the strength of This causes significant reduction in the strength of weldmentweldment. .

Welding cracks are caused by Welding cracks are caused by embrittlementembrittlement or low ductility or low ductility of the weld and/or base metal combined with high restraint of 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 particles metallic solid material particles

entrapped in the weld metal. The most common form is slag entrapped in the weld metal. The most common form is slag inclusion 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

Various forms of Welding Cracks

Several form incomplete fusion

Solid / Liquid State BondingSolid / Liquid State BondingLow temperature joining methods are used when Low temperature joining methods are used when the metal to be joined cannot withstand high the 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, or or dissimilar metals are to be joined, or weldabilityweldability of material is poor.of material is poor.In these methods, the gap between the metal In these methods, the gap between the metal pieces to be joined is filled with molten filler pieces to be joined is filled with molten filler material after heating the base metal. Melting material after heating the base metal. Melting point of filler material is much lower than base point of filler material is much lower than base metals.metals.The bonding is not due to melting of parent metal The bonding is not due to melting of parent metal and fusion.and fusion.

Filler material is drawn into the gap between the Filler material is drawn into the gap between the metal pieces to be joined by capillary action and the metal pieces to be joined by capillary action and the bond formation is initiated when the molten filer bond formation is initiated when the molten filer metal comes under intimate contact with the solid metal comes under intimate contact with the solid surface as in solid state welding. surface as in solid state welding. The nature of bond formed is much complex here, The nature of bond formed is much complex here, and invariably there is some degree of and invariably there is some degree of intersolubilityintersolubilitybetween filler and base metals.between filler and base metals.This interThis inter--diffusion at the base metal surface and diffusion at the base metal surface and resulting alloy has a strength which is very close to resulting alloy has a strength which is very close to that the base metal.that the base metal.

For a good joint strength the For a good joint strength the liquid filler metal; must flow into liquid filler metal; must flow into the gap between the metal pieces the gap between the metal pieces to be joined and cover the entire to be joined and cover the entire surface area, without gaps or blow surface area, without gaps or blow holes. The following usually holes. The following usually insures good bonding:insures good bonding: Clean base metal surfacesClean base metal surfaces Maintain optimum gapMaintain optimum gap Heat the joining area above melting Heat the joining area above melting

temperature of the filler materialtemperature of the filler material Use fluxes for welding of base metal Use fluxes for welding of base metal

surfaces.surfaces.Joint strength is sensitive to the Joint strength is sensitive to the gap and there exists an optimum gap and there exists an optimum gap for a filler material.gap for a filler material.

BrazingBrazingBrazing methods

(a) Torch and filler rods

(b) Ring of filler metal at entrance of gap

(c) Foil of filler metal between flat part surfaces

In brazing the joint is made by heating the In brazing the joint is made by heating the base metal red hot and filling the gap with base metal red hot and filling the gap with molten metal whose melting temperature is molten metal whose melting temperature is typically above 450typically above 450C but below melting C but below melting temperature o base metal. The filler metals temperature 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 they Ag alloys are used for brazing because they form alloy with iron and have good form alloy with iron and have good strength.strength.Various brazing joints are shown in Various brazing joints are shown in figurefigure..

(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

Common braze metal familiesCommon braze metal families

SolderingSoldering

Soldering is very similar to brazing except that Soldering is very similar to brazing except that filler material is usually a leadfiller material is usually a lead--tin based alloy tin based alloy which has much lower strength and melting which has much lower strength and melting temperature around 250temperature around 250C. C. In this process less alloying action between base In this process less alloying action between base metal and filler material as compared to brazing metal 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.

Joints in SolderingJoints in Soldering

(a) Flat lock seam (b) Bolted or riveted joint (c) Copper pipe fitting (d) Crimping of cylindrical lap joint.

(a) Crimped lead wire PC board

(b) Plated through hole on PC board to maximize solder contact area

(c) Hooked wire on flat terminals

(d) Twisted wires

Some common solder alloy Some common solder alloy compositionscompositions

Comparison of welding processesComparison of welding processes