ME 487
Welding Processes
Prof. Kannatey-Asibu , Jr.
The University of Michigan, Ann Arbor
Welding
Welding – Introduction
•Welding is one means for joining parts
•Major joining categories
ØMechanical Fastening
=Uses physical devices to hold parts together
ve.g. bolts, rivets
ØAdhesive bonding
=Uses non-metallic materials to hold parts
together
ve.g. glueing
ØWelding
=Uses heat and/or pressure to produce localized
coalescence of material
2000 # 2
Welding
Welding – Introduction
•Welding requires
ØHeat and/or pressure, with or without
ØFiller metal, shielding gas, or flux
Fig. 2.6, Metallurgy of Welding, by Lancaster, George 2000 # 3
Allen & Unwin
Welding
Welding – Categories
•Fusion welding processes
= Where a portion of the pieces to be joined actually melts
ØArc welding
ØResistance welding
ØGas, laser, electron beam welding
•Non-fusion welding processes
= No melting of the pieces to be joined
ØBrazing and soldering
ØSolid state welding
2000 # 4
Welding
Welding – Categories – Arc Welding
•Uses an electric arc to heat metals along the
joint between them to their melting point
ØMolten portions of the metals fuse together on
solidification to form a joint
•The arc is formed between the electrode and
workpieces
ØBy passing current through them
2000 # 5
Welding
Welding – Categories – Arc Welding
•Either the electrode is moved with the
workpiece stationary or vice versa
Fig. 1.31, The Procedure Handbook of Arc Welding, 2000 # 6
by Lincoln Electric
Welding
Welding – Arc Welding – Basic Concepts
•The arc is a gaseous medium existing in the
ionized state
ØPlasma
= Consists of ions and electrons
•Current is conducted by electrons moving from
the cathode to the anode
ØWorkpiece can be positive or negative depending
on its connection to the power source
2000 # 7
Welding
Welding – Arc Welding – Basic Concepts
•The electrode may be one of two forms
ØConsumable
= Electrode melts and fills the joint
vSupplied as a rod or wire and is similar in
composition to the base materials (workpiece)
ØNon-consumable
= Electrode only used to form the arc
vCarbon or tungsten rod usually used
vNot melted
= Joint filled with separate filler rod which is melted by arc
2000 # 8
Welding
Welding – Arc Welding – Basic Concepts
•Welding with consumable electrodes tend to
result in higher thermal efficiencies
Ø75-80%
•than with non-consumable electrodes
Ø50-60%
2000 # 9
Welding
Welding – Arc Welding – Basic Concepts
•Weld metal is protected from oxidation while it
is hot using
ØFlux, or
ØGas shield
•The flux or shield covers the arc and molten
pool with a blanket of
ØGas
ØVapor
ØSlag
2000 # 10
Welding
Welding – Arc Welding – Electrode Polarity
•Arc welding may be done with either
ØDirect current (dc), or
ØAlternating current (ac)
2000 # 11
Welding
Welding – Arc Welding – Direct Current
•Negative electrode with positive workpiece is
referred to as
ØStraight polarity
= Electrons then flow from electrode to workpiece
= More heat is generated at workpiece
vBecause electrons hit the workpiece at high
speeds
vThus preferable for thicker pieces
2000 # 12
Welding
Welding – Arc Welding – Direct Current
•Positive electrode with negative workpiece is
referred to as
ØReverse polarity
= Electrons flow from work to electrode
= More heat is generated at electrode
vThus preferable for thinner pieces
= Action of the arc cathode spots tends to remove oxide
films from
vAl and Mg
»The fast moving cathode spots on the
workpiece surface break up the oxide
»Using ac will reduce amount of heat
generated at the electrode
2000 # 13
Welding
Welding – Arc Welding – Alternating Current
•Polarity keeps changing
ØArc is extinguished several times a second
ØMakes arc striking difficult
= Can be eased by using a high frequency ac source
ØEquipment cost generally less
2000 # 14
Welding
Welding – Arc Welding – Arc Starting
•Arc does not start automatically once power
source is turned on
ØIonized path required to conduct current across
the gap
ØProvided by
= Touching electrode to the workpiece and rapidly
withdrawing
= Providing high-frequency spark discharge by
supplying a high initial voltage of high frequency
2000 # 15
Welding
Welding – Arc Welding – Basic Terminology
•Arc length
ØDistance from tip of electrode to workpiece
surface
ØIncreasing arc length
= Increases voltage
= Reduces penetration
= Causes spatter
= May extinguish arc
Arc Voltage, Ea
Ea = Eo + ml
2000 # 16
Arc Length, l
Welding
Welding – Arc Welding – Basic Terminology
•Arc Blow
ØPhenomenon that causes the arc to deflect from
its normal direction
ØCaused by
= External magnetic field
= Unsymmetrical current flow
2000 # 17
Welding
Welding – Arc Welding – Metal Transfer
•Major forms
ØGlobular transfer
= Common with processes that use CO2 shielding gas
= Relatively large molten drops are propelled across
the arc
= Often results in considerable spatter
ØSpray transfer
= Occurs with Ar and He shielding gases
= Usually for GMAW
= Small droplets are transferred axially
= Several hundred droplets per second
= Process is stable, directional and almost spatter
free 2000 # 18
Welding
Welding – Arc Welding – Metal Transfer
•Major forms
ØShort-circuiting transfer
= Drop touches the weld pool before it is transferred
= Occurs at over 50 per second
= Relatively lower currents are used
vMakes it good for thin workpieces
2000 # 19
Welding
Welding – Arc Welding – Processes
•Major forms
ØShielded metal arc welding (SMAW)
ØGas metal arc welding (GMAW)
ØGas tungsten arc welding (GTAW)
ØSubmerged arc welding (SAW)
ØPlasma arc welding (PAW)
2000 # 20
Welding
Welding – Arc Welding – SMAW
•Shielded metal arc welding (SMAW)
•Also known as
ØManual arc, or
ØStick electrode
Fig. 5.1, The Procedure Handbook of Arc 2000 # 21
Welding, by Lincoln Electric
Welding
Welding – Arc Welding – SMAW
•Process is consumable
•Electrode consists of
ØCore wire covered with coating (baked flux)
ØGenerally in rod form
•Exposed end of electrode connected to power
source
•Workpiece is grounded
•Arc is established by striking
ØRapidly touching other exposed end of electrode
to workpiece and withdrawing
2000 # 22
Welding
Welding – Arc Welding – SMAW
•Power source may be ac or dc
•In either case, it has
ØConstant current or drooping characteristics
•System components
ØPower source
ØCables
ØElectrode holder
ØElectrode
ØWorkpiece
2000 # 23
Welding
Welding – Arc Welding – SMAW – Electrode
•Specified by a coding scheme consisting of
ØThe letter E and 4 or 5 digits
= e.g. E6012
ØFirst 2 or 3 digits indicate tensile strength of filler
metal (core wire metal)
= In 1000 psi
= e.g. E6012
vStrength = 60,000 psi
ØNext digit indicates welding position
= 1 - All positions
= 2 - Horizontal and flat
= 3 - Flat only
2000 # 24
Welding
Welding – Arc Welding – SMAW – Electrode
•Common welding positions
ØHorizontal
ØFlat
ØVertical Fig. 2.6, Metallurgy of Welding, Lancaster, George
Allen & Unwin
ØOverhead
2000 # 25
Welding
Welding – Arc Welding – SMAW – Electrode
•Core wire sizes may vary from
Ø3/32” to ¼”
•Process generally used for
Ø1/8” to ¾” thick plates
•Currents vary from
Ø50-300 amps
= Larger electrodes are used for higher currents
•Voltages vary from
Ø17-45 volts
2000 # 26
Welding
Welding – Arc Welding – SMAW – Electrode Coating
•Coating consists of flux material bound with
silicate material
Øe.g. Na and K
•Uses
ØMelted by arc to produce shielding gas
surrounding weld pool for protection
= Portions solidify to form a slag covering solidified
weld
ØProvides a means for establishing desirable arc
characteristics and ease of welding
ØEnables alloying elements, deoxidizers, etc. to be
added to weldment
2000 # 27
Welding
Welding – Arc Welding – SMAW – Electrode Coating
•Types
ØCellulosic coatings
= Organic compound
= Generate gas with high H 2 content
ØRutile coatings
= TiO2 with some iron
= Less penetration than cellulosic
ØBasic coatings
= Mainly CaCO3
= Forms CO/CO2 shield
= Generates less gas than cellulosic or rutile
= Requires shorter arc length 2000 # 28
Welding
Welding – Arc Welding – GMAW
•Gas metal arc welding (GMAW)
•Also known as
ØMetal inert gas (MIG)
Fig. 1.3, Welding Handbook, Volume 1, by 2000 # 29
American Welding Society
Welding
Welding – Arc Welding – GMAW
Wire pool
Feed wheel Wire
feeder
Welding Torch
Welding Arc Filler wire
Solidified weld metal Welder
Power
Molten droplet Molten weld metal (pool)
2000 # 30
Welding
Welding – Arc Welding – GMAW
Fig. 5.12, The Procedure Handbook of Arc 2000 # 31
Welding, by Lincoln Electric
Welding
Welding – Arc Welding – GMAW
•Wire electrode fed continuously through motor-
driven rollers
•Consumable process
•Weld protected by shielding gas
ØArgon
ØHelium
= More expensive
ØArgon + Helium
= Inert gases applicable to almost all metals
ØCO2
= Makes arc difficult to control
= Tends to oxidize weld
= Much cheaper 2000 # 32
Welding
Welding – Arc Welding – GMAW
•Power source
ØConstant voltage or flat type
•Reverse polarity normally preferred
ØWith ac or straight polarity, metal transfer tends to
be unsteady
2000 # 33
Welding
Welding – Arc Welding – GMAW
•Wire may be
ØSolid
ØHollow with flux in core
= Flux cored arc welding
vExternal shielding gas is then optional
2000 # 34
Welding
Welding – Arc Welding – GTAW
•Gas tungsten arc welding (GTAW)
•Also known as
ØTungsten inert gas (TIG)
Fig. 5.15, The Procedure Handbook of Arc 2000 # 35
Welding, by Lincoln Electric
Welding
Welding – Arc Welding – GTAW
Fig. 5.16, The Procedure Handbook of Arc 2000 # 36
Welding, by Lincoln Electric
Welding
Welding – Arc Welding – GTAW
•Process is non-consumable
ØUses tungsten electrode
•Filler rod is added separately
•Shielding is by inert gas
ØAr, He (more expensive)
•DC straight polarity is preferred
ØWhy?
•For Al and Mg, AC is preferred
•Produces good welds
•Relatively slow and expensive 2000 # 37
Welding
Welding – Arc Welding – SAW
•Submerged arc welding (SAW)
•Similar to GMAW
•However, shielding is provided by granular flux
ØFed through a tubing that moves ahead of
electrode
2000 # 38
Welding
Welding – Arc Welding – SAW
•Flux completely surrounds arc and weld
ØProtects them from contamination
ØMay contain alloying elements
•Portion of flux melts
ØForms slag on the weld bead
•Low heat loss associated with process
ØResults in high deposition rates
ØAlso low cooling rates
ØGood for thick materials
= e.g. 1½” plate single pass
2000 # 39
Welding
Welding – Arc Welding – SAW
•d.c. and a.c. can be used
Øa.c. is more difficult to control
•No arc blow
•No spatter
2000 # 40
Welding
Welding – Arc Welding – PAW
•Plasma arc welding (PAW)
•Similar to GTAW
•Shielding gas
ØInert gases
Fig. 5.26, The Procedure Handbook 2000 # 41
of Arc Welding, by Lincoln Electric
Welding
Welding – Arc Welding – PAW
•Source of heating:
ØPlasma arc torch
= Consists of tungsten electrode surrounded by
nozzle
•Arc formed first contacts nozzle
ØConstricts the plasma arc
= Results in greater heat intensity
•Process may be
ØTransferred
= Current flow is between electrode and workpiece
ØNontransferred
= Current flow is between electrode and nozzle 2000 # 42
Welding
Welding – Arc Welding – PAW
•Gas shielding is provided by a second nozzle
ØSurrounds the original nozzle Fig. 5.27, The Procedure Handbook
of Arc Welding, by Lincoln Electric
•Faster than GTAW
2000 # 43
Welding
Welding – Laser Beam Welding – LBW
•Source of heat
ØLaser beam
Fig. 5.11, Lasers – Principles and 2000 # 44
Applications, by Wilson & Hawkes
Welding
Welding – Laser Beam Welding – LBW
•Source of heat
ØLaser beam
= High intensity
= Coherent
= Monochromatic light
ØFocused to concentrate energy on a small area
= Using lens arrangement
ØMaterial at point of contact with workpiece
vaporizes
= Surrounded by molten pool
•LBW may be used with or without filler metal
2000 # 45
Welding
Welding – Laser Beam Welding – LBW
Fig. 5.15, Lasers – Principles and 2000 # 46
Applications, by Wilson & Hawkes
Welding
Welding – Laser Beam Welding – LBW
•Advantages
ØNarrow welds with deep penetration to be made
ØSmall heat affected zone (HAZ)
ØLow distortion
ØDoes not require vacuum
= c.f. electron beam welding
ØDoes not generate radiation
ØGood for dissimilar materials
2000 # 47
Welding
Welding – Laser Beam Welding – LBW
•Disadvantages
ØHigh cooling rates
= May produce brittle HAZ and weld bead
vThus preferable for low hardenability materials
vHighly hardenable materials may require pre
and/or post heating
ØRequires closely fitted joints
= Due to small beam area
ØRelatively expensive
2000 # 48
Welding
Welding – Electron Beam Welding – EBW
•Source of heat
ØElectron beam
= Kinetic energy of a large
number of electrons
2000 # 49
Welding
Welding – Electron Beam Welding – EBW
•Electrons are accelerated to high velocities
•Kinetic energy is converted to heat
ØAs electrons hit workpiece
•Electrons generated from filament
ØConnected to negative terminal of power source
= Cathode
•Electrons pass through annular anode
ØResults in acceleration of electrons
•Focused using electromagnetic attachment
2000 # 50
Welding
Welding – Electron Beam Welding – EBW
•Process normally takes place in vacuum
•Voltages
Ø20-200 kV
•Currents
ØOrder of milliamps
•Keyhole is formed as in laser welding
2000 # 51
Welding
Welding – Electron Beam Welding – EBW
•Advantages
ØNarrow welds with deep penetration to be made
ØSmall heat affected zone (HAZ)
ØLow distortion
ØGood for dissimilar materials
•Disadvantages
ØHigh cooling rates
= May produce brittle HAZ and weld bead
ØRequires closely fitted joints
= Due to small beam area
ØRelatively expensive
ØOften requires vacuum 2000 # 52
Welding
Welding – Gas Welding
•Heat source
ØBurning combustible gas with air or oxygen
= Acetylene (C 2H2) most common
vProduces higher temperatures than other
gases
•Flame has 2 sections
ØInner cone
= C2H2 + O2 2CO + H 2
ØOuter cone
= 2CO + O2 2CO2
= 2H2 + O2 2H2O
2000 # 53
Welding
Welding – Gas Welding
•Combustible gas and O2 fed from gas
cylinders to torch consisting of
ØRegulating valves
ØMixing chamber
ØNozzle
2000 # 54
Welding
Welding – Gas Welding – Flame Types
•Neutral flame
ØDoes not react with materials being welded
= Preferable for welding
•Oxidizing flame
ØHas highest temperature
ØOxidizes weld metal
ØMuch smaller inner cone
•Carburizing flame
ØHas reddish feather at tip of inner cone
ØReduces oxides
ØHas 3 regions 2000 # 55
Welding
Welding – Gas Welding
•Advantages
ØWidely used
= Because of low cost
ØGood for sheet metal
= Because of low heat intensity
•Disadvantages
ØSlow manual process
= Thus used mainly for repair work
2000 # 56
Welding
Welding – Solid State Welding
•Do not involve melting of any of the
components
•Often requires
ØClean surfaces
ØPressure
= To expose fresh metal
ØHeat
•Common processes
ØFriction welding
ØUltrasonic welding
ØDiffusion welding 2000 # 57
Welding
Welding – Solid State Welding – Friction Welding
•Applicable mainly to parts with rotational
symmetry
Fig. 12.34, Manufacturing Processes for Engineering
Materials, by S. Kalpakjian , Addison Wesley
2000 # 58
Welding
Welding – Solid State Welding – Friction Welding
•One workpiece is held stationary, and the other
rotated
•The two are then pushed against each other
•Heat generated by friction softens materials at
interface
ØCoalesce under pressure
ØPart of softened material is squeezed out
•Actual welding time
ØFew seconds 2000 # 59
Welding
Welding – Solid State Welding – Friction Welding
•Principal process parameters
ØRotational speed
ØAxial force
ØWelding time
•Advantages
ØOverall heating effect does not result in
substantial influence on metallurgical properties of
the material
ØGood for dissimilar materials
2000 # 60
Welding
Welding – Solid State Welding – Ultrasonic Welding
•High frequency vibrator is pressed against the
two pieces in contact Fig. 12.33, Manufacturing Processes for Engineering
Materials, by S. Kalpakjian , Addison Wesley
2000 # 61
Welding
Welding – Solid State Welding – Ultrasonic Welding
•Application of pressure while the ultrasonic
transducer is vibrating produces a joint (weld)
between the workpieces
ØFriction between surfaces removes surface
contamination and oxide film, exposing nascent
(clean) material
ØWeld then forms under pressure at point of
contact (spot)
2000 # 62
Welding
Welding – Solid State Welding – Ultrasonic Welding
•Applications
ØDissimilar metals
ØThin sheets
ØPlastics
2000 # 63
Welding
Welding – Solid State Welding – Diffusion Welding
•Surfaces to be welded are
ØCleaned
ØBrought into contact under pressure
= Does not result in extensive (plastic) deformation
ØSubjected to high temperature for some time
= Lower than melting point
•Diffusion between the contacting surfaces
results in joining
ØThus slow process
2000 # 64
Welding
Welding – Solid State Welding – Diffusion Welding
•Sheet of metal may be placed between the two
surfaces
ØEnhances diffusion
•Process used mainly where
ØVery close tolerances are desired
ØMetallurgical changes in base material is to be
avoided
ØDimensional changes due to welding are
unacceptable
2000 # 65
Welding
Welding – Resistance Welding
•Generally used for lap joint of sheet metal
Fig. 29.12, Fundamentals of Modern Manufacturing,
by M. P. Groover, Prentice Hall
2000 # 66
Welding
Welding – Resistance Welding
•Two workpieces are overlapped under
ØPressure
= 10 kN (2000 lbf)
ØHigh current then passed through
= Usually a.c. – 15000 amps
ØFor some time
= About 0.5 secs.
•Both pressure and current applied through
ØElectrodes
2000 # 67
Welding
Welding – Resistance Welding
•Resistance at interface results in heat
generation
ØThus melting of workpieces in that locality
= Forms nugget
•Electrodes are shaped to concentrate heat
ØGenerally copper
= For good electrical and thermal conductivity
ØWater-cooled
2000 # 68
Welding
Welding – Resistance Welding
•Heat generated given by
H = kI2Rt
k = constant
I = current
R = resistance
t = time
2000 # 69
Welding
Welding – Resistance Welding
•Highly conductive materials are difficult to
resistance weld
Øe.g. Al, Cu, Ag
= High current is applied for a short time so that heat
does not have time to spread
•Workpiece and electrode surfaces have to be
clean
ØEspecially Al and Mg
•Equipment cost is relatively high
2000 # 70
Welding
Welding – Resistance Welding – Process Variables
•Time
ØToo long a welding time will heat and harm
electrodes and workpieces
Fig. 1.4, Welding Handbook, Volume 3, 7th 2000 # 71
Edition, by American Welding Society
Welding
Welding – Resistance Welding – Process Variables
•Current
ØToo much current produces indentations on
workpiece surface
ØToo much current also squeezes metal out of the
weld
Fig. 1.3, Welding Handbook, Volume 3, 7th 2000 # 72
Edition, by American Welding Society
Welding
Welding – Resistance Welding – Process Variables
•Pressure
ØToo much pressure produces poor weld
= Because resistance is low and heat less
ØToo little pressure squeezes metal out of weld
ØToo little pressure also causes indentation
= Because resistance and heat are high
2000 # 73
Welding
Welding – Resistance Welding – Spot Welding
•Electrode ends are reduced
ØTo confine current flow through a concentrated
area
Fig. 12.36, Manufacturing Processes for Engineering 2000 # 74
Materials, by S. Kalpakjian , Addison Wesley
Welding
Welding – Resistance Welding – Spot Welding
•In welding several pieces together, the current
spreads out within the materials
ØCurrent density is therefore less in the middle than
close to the electrodes
= May result in poorer welds in the middle
2000 # 75
Welding
Welding – Resistance Welding – Heat Balance
•When two pieces of different size are welded
together, the thicker material is heated more
ØDue to greater resistance path
•For uniform and good weld
ØElectrode tip on thinner material is reduced to
increase current density in thin piece
•For materials of different thermal conductivity
ØHigher conductivity material is made thicker
2000 # 76
Welding
Welding – Resistance Welding – Heat Balance
Fig. 1.12, Welding Handbook, Volume 3, 7th 2000 # 77
Edition, by American Welding Society
Welding
Welding – Resistance Welding – Shunting
•When a second weld is made close to a
previous one,
ØFirst weld may act as a shunt to by-pass current
= May result in poorer second weld
2000 # 78
Welding
Welding – Resistance Welding – Projection Welding
•Similar to spot welding
•Projections on one workpiece enable current to
be concentrated at spots to be welded
Fig. 29.17, Fundamentals of Modern Manufacturing, 2000 # 79
by M. P. Groover, Prentice Hall
Welding
Welding – Resistance Welding – Seam Welding
•Consists of a series of overlapping or short-
spaced spot welds
Fig. 12.38, Manufacturing Processes for Engineering 2000 # 80
Materials, by S. Kalpakjian , Addison Wesley
Welding
Welding – Resistance Welding – Seam Welding
•Workpieces are passed between rolling circular
electrodes
•Overlapping welds result from continuous
running of electrodes with uninterrupted current
•Higher currents are used because of shunting
effect
•Applications
ØMufflers
ØBarrels
ØTanks 2000 # 81
Welding
Welding – Brazing
•Joining process where filler metal is placed at
or between faying surfaces to be joined
•Filler rod used is usually non-ferrous metal
ØHas melting point below that of the workpieces
= However, above 450oC
•Molten metal flows by capillary action between
overlapping joint workpieces
ØWorkpieces are not melted
2000 # 82
Welding
Welding – Brazing
Fig. 30.4, Fundamentals of Modern Manufacturing, by 2000 # 83
M. P. Groover, Prentice Hall
Welding
Welding – Brazing
•Common filler metals
ØCopper alloys
ØSilver alloys
ØAluminum alloys
= Mainly for Al
•Joint formed by diffusion of brazing metal into
hot base metal
ØResults in surface alloying
•Clearance between workpieces must be small
ØFor good capillary action
= And good joint 2000 # 84
Welding
Welding – Brazing
•Work surfaces must be clean
ØFor effective bonding
•Flux is used to break down surface oxides
ØCommon flux
= Borax
•Gas torch may be used for heating
2000 # 85
Welding
Welding – Soldering
•Similar to brazing
= However, filler metal melts below 450oC
•Common filler metals
ØLead alloys
ØTin alloys
•Joints not as strong as brazed joints
•Flux types
ØZinc chloride
ØRosin
2000 # 86
Welding
Welding – Soldering
•Flux is generally applied first
•Joint is then heated to just above melting point
of solder
•Solder is then touched to and flows into the
heated joint
•Heat sources
ØSoldering iron
ØGas torch
2000 # 87
Welding
Welding – Soldering – Common Joint Designs
Fig. 12.51, Manufacturing Processes for Engineering 2000 # 88
Materials, by S. Kalpakjian , Addison Wesley
