WELDINGWELDING

Welding is a metal joining method in Welding is a metal joining method in which the joining edges are heated which the joining edges are heated and fused together to form a and fused together to form a permanent ( homogeneous ) bond / permanent ( homogeneous ) bond / joint.joint.

ADVANTAGES OF WELDINGADVANTAGES OF WELDING

A Permanent pressure tight jointA Permanent pressure tight jointStrongest joint and any type of metal Strongest joint and any type of metal of any thickness can be joinedof any thickness can be joinedOccupies less spaceOccupies less spaceGives more economy of materialGives more economy of materialHas less weightHas less weightWithstand high temp. and pressure Withstand high temp. and pressure equal to joined materialequal to joined materialGives no colour change to jointsGives no colour change to jointsCan be done quicklyCan be done quickly

DEFINITION OF ARCDEFINITION OF ARC

Arc is an electric discharge between Arc is an electric discharge between two electrodes which takes place two electrodes which takes place through an electrically conducting hot through an electrically conducting hot ionised gas known as plasmaionised gas known as plasma

Arc used for welding is called welding Arc used for welding is called welding arc arc

Arc is between a thin rod/ wire and a Arc is between a thin rod/ wire and a plate , it is therefore a bell shapedplate , it is therefore a bell shaped

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Arc is a high current low voltage electric Arc is a high current low voltage electric discharge ( 10 – 200 A and 10-50 V) discharge ( 10 – 200 A and 10-50 V)Arc consists of a mechanism for emitting Arc consists of a mechanism for emitting electrons from the cathode which after electrons from the cathode which after passing through ionised hot gas merge passing through ionised hot gas merge into anodeinto anodeArc is divided into five parts ( the cathode Arc is divided into five parts ( the cathode spot, cathode drop zone , arc column, spot, cathode drop zone , arc column, anode drop zone, anode spot )anode drop zone, anode spot )

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Voltage drops across the arc column is Voltage drops across the arc column is more gradual while the voltage drop more gradual while the voltage drop across the cathode and anode drop zone across the cathode and anode drop zone are quite steepare quite steep

Arc voltage is a sum of the Cathode drop Arc voltage is a sum of the Cathode drop Vc , Column drop Vp and Anode drop Va Vc , Column drop Vp and Anode drop Va

Thus V = Vc + Vp + VaThus V = Vc + Vp + Va

TYPES OF WELDING ARCSTYPES OF WELDING ARCS

Two types of welding arcs Two types of welding arcs

STATIONERY / IMMOBILE / FIXED ARCSTATIONERY / IMMOBILE / FIXED ARC

MOVING / TRAVELLING / MOBILE ARCMOVING / TRAVELLING / MOBILE ARC

Fixed arcFixed arc

Fixed arc is formed between a non Fixed arc is formed between a non consumable electrode and a workpiececonsumable electrode and a workpieceArc may be used with or without fillerArc may be used with or without fillerA separate wire is introduced into the arc A separate wire is introduced into the arc column and thus melted to transfer into the column and thus melted to transfer into the weld pool weld pool Most of the heat going to the non Most of the heat going to the non consumable electrode remains unutilised consumable electrode remains unutilised (thermal efficiency between 45-60% ) (thermal efficiency between 45-60% )

Mobile ArcMobile Arc

Mobile arc is formed between a Mobile arc is formed between a consumable electrode and a work piececonsumable electrode and a work piece

An arc in which the molten metal from the An arc in which the molten metal from the tip of the electrode is transported through tip of the electrode is transported through it to become a part of the weld pool it to become a part of the weld pool

Mobile arc is a metal arcMobile arc is a metal arc

Thermal efficiency lies between 75 – 90 %Thermal efficiency lies between 75 – 90 %

ARC CHARACTERISTICSARC CHARACTERISTICS

METAL TRANSFER ACROSS THE ARCMETAL TRANSFER ACROSS THE ARC

Gravity forceGravity force

Gas expansion forceGas expansion force

Surface tensionSurface tension

Electromagnetic forceElectromagnetic force

GRAVITY FORCEGRAVITY FORCE

Molten globules formed at the arcing Molten globules formed at the arcing end of the electrode travel towards the end of the electrode travel towards the job in the molten pooljob in the molten pool

Gravitational force helps the transfer of Gravitational force helps the transfer of metal in flat or down-hand position and metal in flat or down-hand position and thus the deposition rate of weld metal thus the deposition rate of weld metal is increasedis increased

GAS EXPANSION FORCEGAS EXPANSION FORCE Flux coating on electrode melts due to the Flux coating on electrode melts due to the

arc heat , resulting in the :arc heat , resulting in the :

Production of carbon monoxide and Production of carbon monoxide and hydrogen mainlyhydrogen mainlyFormation of a sleeve of the flux at the Formation of a sleeve of the flux at the arcing end due to a little higher melting arcing end due to a little higher melting point of the flux coating than the core wire.point of the flux coating than the core wire.These gases expand and gain velocity. The These gases expand and gain velocity. The flux sleeve direct these gases to flow in flux sleeve direct these gases to flow in the direction of molten metal due to the direction of molten metal due to pushing effect and influence penetration.pushing effect and influence penetration.

SURFACE TENSIONSURFACE TENSION

It is the characteristic (force) of the It is the characteristic (force) of the base metal to attract and retain the base metal to attract and retain the molten metal in it. The short arc molten metal in it. The short arc promotes more surface tension effectpromotes more surface tension effect

ELECTROMAGNETIC FORCEELECTROMAGNETIC FORCE

Current passing through the electrode Current passing through the electrode forms magnetic lines of force in the forms magnetic lines of force in the form of concentric circlesform of concentric circles

This force exerts a pinch effect on the This force exerts a pinch effect on the molten metal globulesmolten metal globules

Globules is detached from the Globules is detached from the electrode and reaches the molten poolelectrode and reaches the molten pool

ELECTRON EMISSION MECHANISMELECTRON EMISSION MECHANISM

THERMIONIC EMISSIONTHERMIONIC EMISSION

AUTO ELECTRONIC EMISSIONAUTO ELECTRONIC EMISSION

PHOTO ELECTRONIC EMISSIONPHOTO ELECTRONIC EMISSION

SECONDARY EMISSIONSECONDARY EMISSION

THERMIONIC EMISSIONTHERMIONIC EMISSION

Liberation of electron from the heated Liberation of electron from the heated electrodeselectrodesDue to increase in temp., the kinetic Due to increase in temp., the kinetic energy of the free electrons increases to a energy of the free electrons increases to a point where they can escape from the point where they can escape from the surface of the negative electrode at the surface of the negative electrode at the cathode spot into the field free space cathode spot into the field free space outside in the face of attraction by the outside in the face of attraction by the positive ions left behind on the cathodepositive ions left behind on the cathode

AUTO ELECTRON EMISSIONAUTO ELECTRON EMISSION

Due to high voltage across the electrods , Due to high voltage across the electrods , air between them is ionised air between them is ionised

Electric discharge ensues with the Electric discharge ensues with the emission of electrons from the cathode emission of electrons from the cathode surface surface

PHOTO-ELECTRIC EMISSIONPHOTO-ELECTRIC EMISSION

Energy in the form of a beam of light falls Energy in the form of a beam of light falls on the cathode surface on the cathode surface

Kinetic enegy of electrons is increased Kinetic enegy of electrons is increased which results in their emission from the which results in their emission from the cathode into vacuum or another materialcathode into vacuum or another material

SECONDARY EMISSIONSECONDARY EMISSION

Due to increase in velocity of rapidly Due to increase in velocity of rapidly moving ions than the orbital velocities of moving ions than the orbital velocities of electrons in atoms of the materials of the electrons in atoms of the materials of the cathode which results in expulsion of cathode which results in expulsion of electronselectronsElectron emission is either thermionic or Electron emission is either thermionic or auto emission typeauto emission typeEmission of electron dependent upon the Emission of electron dependent upon the excitation/ work function of a materialexcitation/ work function of a material

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Work function is defined as the energy Work function is defined as the energy required,in ev/ joule , to get one electron required,in ev/ joule , to get one electron released from the surface of material to released from the surface of material to the surroundingthe surrounding

Ionisation potential is defined as the enegy Ionisation potential is defined as the enegy per unit charge in volts, required to per unit charge in volts, required to remove an electron from an atom to an remove an electron from an atom to an infinite distanceinfinite distance

CATHODE SPOTCATHODE SPOTIt is that part of the negative electrode It is that part of the negative electrode

wherefrom the electron are emittedwherefrom the electron are emitted

Three types of cathode spot mode:Three types of cathode spot mode:

Mobile cathode spot modeMobile cathode spot mode

Thermionic cathode spot modeThermionic cathode spot mode

Normal modeNormal mode

MOBILE CATHODE SPOT MODEMOBILE CATHODE SPOT MODE

One or more very small cathode spots One or more very small cathode spots appear appear at the cathode surfaceappear appear at the cathode surface

Speed of travel is 5-10 m/s.Speed of travel is 5-10 m/s.

E.g. on aluminium multiple spots which E.g. on aluminium multiple spots which generate complex series of branched generate complex series of branched tracks are observed while on copper the tracks are observed while on copper the trace left behind is commonly single trace left behind is commonly single without any branches without any branches

THERMIONIC CATHODE SPOT MODETHERMIONIC CATHODE SPOT MODE

It forms at the tip of a sharply pointed It forms at the tip of a sharply pointed tungston rod used with argon shieldingtungston rod used with argon shielding

It remains fixed in position It remains fixed in position

Current density of the order of 100 A/ Current density of the order of 100 A/ mm*mmmm*mm

NORMAL MODE CATHODE SPOTSNORMAL MODE CATHODE SPOTS

CATHODE DROP ZONECATHODE DROP ZONE

It is the gaseous region immediately It is the gaseous region immediately adjacent to the cathode in which a sharp adjacent to the cathode in which a sharp drop in voltage occurs.drop in voltage occurs.

The combined size of the cathode drop The combined size of the cathode drop zone and the anode drop zone is of the zone and the anode drop zone is of the order of 1/100 mm which is nearly equal to order of 1/100 mm which is nearly equal to the electron mean free paththe electron mean free path

ARC COLUMNARC COLUMN

Bright visible portion of the arc having high Bright visible portion of the arc having high temp. and low potential gradient temp. and low potential gradient

Temp. of the arc column depends upon Temp. of the arc column depends upon the gases present in it and the amount of the gases present in it and the amount of welding current flowing in the circuit.welding current flowing in the circuit.

At high temp. , all the molecular gases At high temp. , all the molecular gases present in the column get split into present in the column get split into electrons and ions . electrons and ions .

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No. of electrons and ions in any given volume of No. of electrons and ions in any given volume of the arc remains the same to keep the arc the arc remains the same to keep the arc electrically neutralelectrically neutralAverage ion is about one thousand times Average ion is about one thousand times heavier than an electron , so electrons are more heavier than an electron , so electrons are more mobile and carry most of the current across the mobile and carry most of the current across the columncolumnThe potential gradient is lower than that across The potential gradient is lower than that across the cathode drop zone or the anode drop zone the cathode drop zone or the anode drop zone in the column.in the column.

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Welding arc results in a bell shaped or Welding arc results in a bell shaped or cone shaped arc with apex of the cone at cone shaped arc with apex of the cone at or near the tip of the rod electrodeor near the tip of the rod electrode

Max. temp. is at the core of the columnMax. temp. is at the core of the column

Axial plasma velocity decreases as the Axial plasma velocity decreases as the arc periphery is approachedarc periphery is approached

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Contd…..Contd…..Gas flow lines and Plasma velocity lines in carbon arc weldingGas flow lines and Plasma velocity lines in carbon arc welding

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In a steady state the plasma jet has a In a steady state the plasma jet has a streamline flow with the flow velocity streamline flow with the flow velocity approximately proportional to the welding approximately proportional to the welding current current

When the flow is not symmetrical, it results When the flow is not symmetrical, it results in the setting up of magnetic forces which in the setting up of magnetic forces which deflect the arc column, it is known as deflect the arc column, it is known as arc blowarc blow

ARC STABILITYARC STABILITY

Arc is said to be stable if it is uniform and Arc is said to be stable if it is uniform and steadysteady

Stable arc will produce good weld bead Stable arc will produce good weld bead and a defect free weld nuggetand a defect free weld nugget

Unstable arc produce slag entrapment, Unstable arc produce slag entrapment, porosity, blow holes and lack of proper porosity, blow holes and lack of proper fusionfusion

FACTORS GOVERNING ARC STABILITYFACTORS GOVERNING ARC STABILITY

Suitable matching of arc and power source Suitable matching of arc and power source characteristiccharacteristicContinuous and proper emission of electrons Continuous and proper emission of electrons from the electrode and thermal ionization in from the electrode and thermal ionization in the arc columnthe arc columnPosition and movements of cathode & anode Position and movements of cathode & anode spotsspotsArc length and arc currentArc length and arc currentElectrode tip geometryElectrode tip geometrySteadiness of current in A.C welding as Steadiness of current in A.C welding as regards its magnitude and wave shaperegards its magnitude and wave shapePresence of dampness, oil, grease, etc. on Presence of dampness, oil, grease, etc. on the surface of work piecethe surface of work piece

ARC BLOWARC BLOWThe unwanted deflection or the wandering of a The unwanted deflection or the wandering of a welding arc from its intended path is termed as welding arc from its intended path is termed as arc blow arc blow Under arc blow, an arc may distort, deflect or Under arc blow, an arc may distort, deflect or rotaterotateAC arc are less susceptible to arc blow than DC AC arc are less susceptible to arc blow than DC arc because the alternating current reverses arc because the alternating current reverses direction which in turn reverses the magnetic direction which in turn reverses the magnetic field. The magnetic field builds up, collapses and field. The magnetic field builds up, collapses and rebuilds as current reverses from positive to rebuilds as current reverses from positive to negative. This phenomenon does not permit the negative. This phenomenon does not permit the magnetic field strength to build to a value so as magnetic field strength to build to a value so as to cause arc blow to cause arc blow

FACTORS AFFECTING ARC BLOWFACTORS AFFECTING ARC BLOW

Magnetic field produced in the work-piece Magnetic field produced in the work-piece adjacent to the welding arc, because of the adjacent to the welding arc, because of the current flow through the arccurrent flow through the arcPresence of bus bars carrying large direct Presence of bus bars carrying large direct currents, in the neighborhood of the place currents, in the neighborhood of the place where welding is being carried outwhere welding is being carried outWith multiple welding heads, arc at one With multiple welding heads, arc at one electrode may be affected by the magnetic field electrode may be affected by the magnetic field of the arc at the other electrodeof the arc at the other electrodeThe magnetic field produced in the work-piece The magnetic field produced in the work-piece around the earth connection may tend to drive around the earth connection may tend to drive the arc away from the point where this the arc away from the point where this connection is made connection is made

TYPES OF ARC BLOWTYPES OF ARC BLOW

FORWARD ARC BLOWFORWARD ARC BLOW

BACKWARD ARC BLOWBACKWARD ARC BLOW

SIDEWARD ARC BLOWSIDEWARD ARC BLOW

ARC ROTATIONARC ROTATION

EFFECT OF ARC BLOWEFFECT OF ARC BLOWIncreased arc blow results in an unstable Increased arc blow results in an unstable arcarc

Poor weld bead appearancePoor weld bead appearance

Irregular and erratic weld depositionIrregular and erratic weld deposition

Undercutting and lack of fusionUndercutting and lack of fusion

SpatterSpatter

Slag entrapmentSlag entrapment

Porosity Porosity

Uneven and weak welded joint Uneven and weak welded joint

Thermal Effects of Welding on Parent MetalThermal Effects of Welding on Parent Metal In all welding processes, except cold welding, In all welding processes, except cold welding, heating and cooling are essential and integral parts heating and cooling are essential and integral parts of the process. of the process. High degrees of superheat in the weld metal may High degrees of superheat in the weld metal may be obtained in many fusion welding processes. be obtained in many fusion welding processes. The maximum temperature exceeds various critical The maximum temperature exceeds various critical temperatures at which phase transformations temperatures at which phase transformations occur in the metals involved in welding. occur in the metals involved in welding. The extent and slope of the thermal gradient is The extent and slope of the thermal gradient is determined by the heat supplied by the welding determined by the heat supplied by the welding process per unit volume of metal per unit time and process per unit volume of metal per unit time and the thermal conductivity of the base metal parts. the thermal conductivity of the base metal parts. The study of welding metallurgy requires a The study of welding metallurgy requires a consideration of the following metallurgical consideration of the following metallurgical phenomena that play an important role in fusion phenomena that play an important role in fusion welding:welding:

Thermal Effects of Welding on Parent MetalThermal Effects of Welding on Parent Metal

(a) The parent metal and the electrode melt and (a) The parent metal and the electrode melt and then resolidify as an integrated mass under the then resolidify as an integrated mass under the equivalent of chill casting conditions. This equivalent of chill casting conditions. This causes redistribution of the micro-constituents causes redistribution of the micro-constituents and the alloying elements in the weld metal and the alloying elements in the weld metal zone. zone. (b) The (unmelted) parent metal is subjected to a (b) The (unmelted) parent metal is subjected to a complex heat treatment in the form of a complex heat treatment in the form of a temperature gradient extending from the melting temperature gradient extending from the melting range to room temperature and followed by a range to room temperature and followed by a cooling cycle induced by the neighbouring cold cooling cycle induced by the neighbouring cold metal and the atmosphere. The phase changes metal and the atmosphere. The phase changes or changes in structures that result due to or changes in structures that result due to welding depend on welding depend on

Thermal Effects of Welding on Parent MetalThermal Effects of Welding on Parent Metal

Welding process used (arc, gas, etc.)Welding process used (arc, gas, etc.)Filler rod composition and the composition of flux in case Filler rod composition and the composition of flux in case of coated electrodes.of coated electrodes.The conditions under which the weld is made, i.e., the The conditions under which the weld is made, i.e., the amount of oxygen and nitrogen present. amount of oxygen and nitrogen present. The chemical composition of base metal.The chemical composition of base metal.The temperature and the phase changes that take place The temperature and the phase changes that take place in and around the weld introduce volume changes which in and around the weld introduce volume changes which result in plastic flow, residual stresses and sometimes result in plastic flow, residual stresses and sometimes cracking too. If one looks at a macrostructure of a cracking too. If one looks at a macrostructure of a welded joint, he clearly visualises three distinct zones, welded joint, he clearly visualises three distinct zones, namely: namely: (a) Weld metal zone - Grain growth region(a) Weld metal zone - Grain growth region(b) Heat affected zone - Grain refined region, Transition (b) Heat affected zone - Grain refined region, Transition region.region.(c) (Unaffected) base metal or parent metal.(c) (Unaffected) base metal or parent metal.