- 1. 9/10/2012Casting,Casting Forming & Welding(ME31007) Jinu
PaulDept. of Mechanical Engineering 1 Lecture 5 06 Sept 2012,
Thursday 8.30-9.30 am Welding Processes- 2) A weldingArcldi 21
2. 9/10/2012 Arc welding (AW)- BasicconfigurationArc welding
TypesConsumable electrode SMAW, GMAW (MIG), Submerged arc
weldingNon consumableGTAW(TIG)Electrode 3 Arc Shielding in AW
process Accomplished by covering the electrode tip,p, arc, and
molten weld pool with a blanket of gas or flux, Common shielding
gases argon and helium, In the welding of ferrous metals with
certain AWprocesses, oxygen and carbon dioxide areused, usually in
combination with Ar and/or He,to produce an oxidizing atmosphere or
tocontrol weld shape 42 3. 9/10/2012Flux in AW process Flux is
usually formulated to serve severalfunctions: ( ) (1) To
remove/prevent oxidep (2) provide a protective atmosphere (3)
stabilize the arc, and (4) reduce spattering Flux delivery
techniques include (1) pouring granular flux onto the weld (2)
using a stick electrode coated with flux material in which the
coating melts during welding (3) using tubular electrodes in which
flux is contained in the core and released as the electrode is
consumed5Arc Welding- Consumable Electrodes Consumable electrodes
Rods or wire. Welding rods 225 to 450 mm long, < 10 mm dia.
Welding rods to be changed periodically reducing arc time of welder
Consumable weld wire continuously fed into the weld pool from
spools avoiding the frequent interruptions 63 4. 9/10/2012Arc
Welding- Non-consumable Electrodes Made of tungsten (or carbon
rarely), carbon, rarely)which resists melting by the arc Slow
depletion Analogous to wearingof a cutting tool in machining Filler
metal must be supplied by meansof a separate wire that is fed into
theweld pool 7AW-Type 1: Shielded metal arc welding (SMAW)
Shielding gas from electrode coating 84 5. 9/10/2012 Shielded metal
arc welding(SMAW) Consumable electrode consisting of a fillermetal
rod coated with chemicals that provideflux and shielding Currents
typically used in SMAW rangebetween 30 and 300 A at voltages from
15 to45 V. Usually performed manually Most common welding, 50 % of
industrialwelding uses SMAW 9SMAW: Electrode-coatingfunctions
Produces gases to shield weld from air g Adds alloying elements
De-oxidation Produces slag to protect & support weld Co o s
cooling a esControls coo g rates Stabilizes arc 10 5 6. 9/10/2012
Electrode coating in SMAW-constituents (optional) Shielding gas is
generated by either thedecomposition or dissociation of the
coating,C ll l i generates H2, CO H2O and CO2Cellulosic
tCO,dLimestone (CaCO3) generates CO2 and CaO slagRutile (TiO2) up
to 40% easy to ignite, gives slagdetachability, fine bead
appearance, generates O2 &H2 by hydrolysis Slag formers (flux):
SiO MnO2, FeO Al2O3 SiO, FeO.Al Arc stabilizers: Na2O, CaO, MgO,
TiO2 Deoxidizer: Graphite, Al, Wood flour Binder: sodium silicate,
K silicate Alloying elements: V, Co, Mo, Zr, Ni, Mn, W etc.
11SMAW-Adv & Applications It is preferred over oxyfuel welding
for thicker sectionsabove 5 mm because of its higher power density.
The equipment is portable and low cost, making SMAW highly
versatile and most widely used AW processes. Base metals include
steels, stainless steels, cast irons, and certain nonferrous alloys
126 7. 9/10/2012SMAW-Disadvantages Electrode length varies during
theggoperation Length affects the resistance heating ofthe
electrode, Current levels To be maintained withina safe range or
the coating will overheatand melt prematurely when starting a
newwelding stick13SMAW-Disadvantages Use of the consumable
electrode must periodically be changed reducesthe arc time Offers
limited shielding protectioncompared to inert gas shieldedprocesses
Some of the other AW processespovercome the limitations of welding
sticklength in SMAW by using a continuouslyfed wire electrode147 8.
9/10/2012 AW-Type 2: Gas metal arcwelding (GMAW) -MIG 15 Gas metal
arc welding-Features Consumable wire electrode is fedcontinuously
and automatically from a spoolthrough th welding gunth h theldi
Inert shielding gas : protects the arc and themolten or hot,
cooling weld metal from air.Also, provides desired arc
characteristicsthrough its effect on ionization No electrode
coating No flux or additional filler DCRP used (electrode +ve, work
ve)16 8 9. 9/10/2012AW-Type 3: Flux-Cored ArcWelding (FCAW) Flux
core FluxFl cored electrode d l t d Consumable electrode Consumable
wire electrode With/ Without shielding gas Core contents- alloying
elements,ll i l t shielding gas generators flux, etc.17AW-Type 4
Submerged Arc welding (SAW)The blanket of granular flux submerges
the weldingoperation, prevents sparks, spatter, and radiation18 9
10. 9/10/2012Submerged Arc welding Continuous, consumable bare wire
electrode Arc shielding provided by a cover of granular flux
Granular flux is introduced into the joint slightlyahead of the
weld arc by gravity from a hopper Unfused flux remaining after
welding can berecovered and reused efficiency of Energy transfer
from the electrode toworkpiece is very high- Low losses Welding is
restricted to flat and horizontal positions 19AW-Type 5: Gas
Tungsten ArcWelding (GTAW or TIG) 20 10 11. 9/10/2012GTAW- Features
Non-consumable tungsten electrode Inert gas for arc shielding With
or without filler rod Aluminium and stainless steel high-quality
welds, no weld spatter because no filler metal Little or no post
weld cleaning because no flux is used21 Arc welding Types-Summary
NameElectrodeElectro FillerShieldingFlux Remarkstypede
rodgascoatingShielded metal ConsumableYES NILProvided by Provided
byManual arc welding rod electrode electrode welding (SMAW) coating
coatingGas metal arcConsumable NILNIL YESNIL Automatewelding wired
welding(GMAW)-MIGFlux-Cored Arc Consumable
NILNILWith/withoutProvided by Manual/au Welding wireelectrode
coretomated (FCAW) electrode Submerged Consumable NILNILNILGranular
fluxManual/au Arc welding wiretomated (SAW)electrodeGas TungstenNon
NILWith/ YESNIL Automate Arc Welding consumable without d
welding(GTAW-TIG)2211 12. 9/10/2012 Lecture 5 06 Sept 2012,
Thursday 8.30-9.30 am Physics of Arc welding Arc-on-time in Arc
welding The proportion of hours worked that arcwelding is being
accomplished Arc time = Time arc is ON / Hours worked Arc ON
timeManual Welding ~ 20 %Machine, automatic, &robotic welding~
50 % 2412 13. 9/10/2012The electric arc Thermionic emission:
Electrons andpositive ions from the electrode andthe workpiece.p
Accelerated by the potential fieldbetween the electrode and the
work Produce heat when they converttheir kinetic energy by
collision with++ +the opposite charged element El tElectrons have
much greater kinetichh t ki tienergy because they can beaccelerated
to much highervelocities under the influence of agiven electric
field 25Polarity in Arc welding Consumable electrode Normally Anode
;work cathode ++ Non consumable+electrode NormallyCathode, Work
anode 2613 14. 9/10/2012Arc welding - Arc Types Steady (from a DC
power supply) Intermittent (due to occasional, irregular short
circuiting) Continuously unsteady (as the result of an AC powerp
supply) Pulsing (as the result of a pulsing DC power supply) Time
27 Pulsed DC in Arc welding The higher pulsing rates increase
Equivalentpuddle agitation a better grain steady DCmolecular
structure within the weld Pulsed DC High speed pulsing constricts
andfocuses the arc; Increases arc stability,penetration and travel
speeds Reduces arc blow (created byinfluence of magnetic field)g) A
smaller heat-affected zone 4 Variables: peak amperage,background
amperage, peak time andTimepulse rate28 14 15. 9/10/2012 Effect of
Magnetic Fields on Arcs Arc blow (deflection) A blArc blow arises
fi from ttwo b i conditionsbasic diti the change in direction of
current flow as it leaves the arc and entersthe workpiece to seek
ground Asymmetrical arrangement of magnetic material around the arc
The effects of magnetic fields on welding arcs isdetermined by the
Lorentz force, which isproportional to the cross-product of the
magneticfield (B) and the current flow density (J), B x J Arc blow
can be reduced by using AC or pulsed DC 29 Creation of arc
plasmaElectrons emittedElectrons emittedfrom cathode from
cathode-e-e Inert gas-e Metalic vapour -eSecondarySecondary
electronselectronsHigh temperature plasma Low temperature plasma15
16. 9/10/2012 The Arc Plasma Electrode Plasma, the ionized state of
a gas Comprises of a balance of negativeelectrons and positive ions
(bothcreated by thermionic emission froman electrode) and Plasma
Collisions between these electrons andatoms in the gaseous medium
secondary emission from gas ionisation of gaseous mediumWorkW k
Gaseous medium could be a self-generated (e.g. metal vapour)
orexternally supplied inert shielding gas31 The Arc Plasma The
establishment of a neutral plasma state attained at equilibrium
temperatures magnitude depend on the ionization potentialof gas
from which the plasma is produced(e.g., air, argon, helium) The
higher work function of the gaseousmedium Higher Arc temperature
E.g. Helium tighter bonding of outermostelectrons compared to Ar
Hotter arc32 16 17. 9/10/2012 The PlasmaFormation of a plasma is
governed by an extended conceptof the ideal gas law and law of mass
action ne ni 2Z i 2me kT 2/3n0 Z 0 h 3e Vi / kTwhere ne, ni, and no
are the number of electrons, ions, andneutral atoms per unit volume
(i.e., the particle density),Vi is the ionization potential of the
neutral atom,T is the absolute temperature (K),Zi and Z0 are
partition functions for ions and neutral particles,h is Plancks
constant (6.63 x 10-34 J/s),me is the mass of an electron (9.11 x
lO-31 kg),k is Boltzmanns constant (1.38 x 10-23J/K)33 Arc/Plasma
Temperature Factors affect the plasma temperature Constituents of
the particular plasma Its density Lowered by the presence of fine
metallic particles Lowered by convection/radiation heat lossPlasma
Inert gas, Metal particles, vapours,constituents Alkali metal
vapours, fine particles of molten flux (or slag)34 17 18.
9/10/2012Arc temperatureArc Welding Arc constituents ~Arc
temperature Ktype (Theoretical values)Plasma Arc Pure plasma, no
metal50,000welding (PAW)transferGas tungsten Metal vapor from
30,000arc weldingnonconsumable electrodeActual values(with inertand
any molten metal limited byshielding gas) particles from
fillerlossesGMAW large concentrations of20,00020 000((convection, ,
metal ions and vapor and radiation, molten
dropletsdiffusion)SMAW/easily ionized materials6000Flux cored arc
such as alkali metalswelding(sodium, potassium) 35 Arc
TemperatureTemperature of the arc columns for various
gasesGasTemperature of arccolumn close to cathode (K) ~
Alkali-metal vapour (Na, K)4000 Alkaline Alk li earth vapour (C M )
th(Ca, Mg) 5000 Iron vapour6000 Argon (200 A)10,000-15,000 3618 19.
9/10/2012Arc TemperatureIsothermal Map of an pArgon-tungsten arc
Measured by spectral emission of excited/ionized atoms Normally is
in the range of 5000 to 30,000 K The actual temperature in an arc
is limited by heat loss(radiation, convection, conduction, and
diffusion), ratherthan by any theoretical limit 37 WeldingLecture 6
g 07Sept2012,Friday11.3012.30amArc Welding- Electrical features 38
3819 20. 9/10/2012 Arc: V-A Characteristics The total potential of
anarc first falls withincreasing current, andthen rises with
furtherincreases in current The initial decrease isattributed to a
growth ofthermally inducedelectron emission at thearc cathode and
thermalionization39Influence of Arc length Potential barrier
increaseswith the arc length (gap) Lengthening the arc exposes more
of the arccolumn to cool boundary More losses Higherdemand for
voltage40 20 21. 9/10/2012V-A Characteristics of an Arc Arc welding
low-voltage, high-current arcsbetween a nonconsumable or
consumableelectrode and a work piece Arc welding power source
static and dynamiccharacteristics Static volt-ampere
characteristics, (1) constant-current and (2)
constant-voltageconstant voltage Dynamic characteristics determined
bymeasuring very short-duration (~1 ms) transientvariations in
output voltage and current thatappear in the arc itself4141Constant
current power sources A change in arc length willcause
corresponding changeini arc voltage and smallltdllchange in
current. Electrode melting and metaldeposition rate remainconstant
with slight changesin arc length Greater tolerance to arclength
Variations Used for manual SMAW andGTAW4242 21 22.
9/10/2012Constant current power sources Used primarily with coated
electrodes Small change in amperage and arc power for
acorresponding relatively large change in arcvoltage or arclength
The curve of a constant current machine dropsdown-ward sharply
often called a drooper In welding with coated electrodes, the
amperageis set by the operator while the voltage isdesigned into
the unit The operator can vary the arc voltage byincreasing or
decreasing the arc length4343 Constant-Voltage Power Sources A
slight change in arc length causes a large change incurrent, so
melting rate changes rapidly in response. This has the effect of
self-regulation, increasing the meltingrate as arc length is
inadvertently shortened, and viceversa4444 22 23. 9/10/2012
Constant-Voltage Power Sources CV power supplies are attractive
forconstantly fed continuous electrode t tl f d ti l t dprocesses
such as GMAW, FCAW, orSMAW, to maintain near-constant arclength. 45
45 Combined Characteristic Sources Single power supplythat can
provide either pconstant-voltage orconstant-current Higher-voltage
portion Constant current Below a certainthreshold ltth h ld
voltage, ththecurve switches to aconstant voltage type 46 4623 24.
9/10/2012Combined Characteristic Sources Utility for a variety of
processes, and areactually a combination of the straight CV orCC
types Useful for SMAW to assist in starting A id electrode sticking
i th weld poolAvoid l t d ti ki in the ld lshould the welder to use
shorter arc length 47 4724
