- 1. 9/20/2012Casting,Forming&WeldingCasting Forming &
Welding (ME31007)J u auJinuPaulDept.ofMechanicalEngineering 1 1
WeldingLecture 7 g13Sept2012,Thursday8.309.30am Arc
Welding-Electrical features 2 21
2. 9/20/2012Arc Electrical Features-impedance An electric
welding arc is an impedance (relatedto the resistance of a circuit,
but includingcontributions ft ib tifrom capacitance and i d t it d
inductanceas well) to the flow of electric current Specific
impedance at any point in an arc isinversely proportional to the
density of thecharge carriers and their inherent mobility. The
total impedance depends on the radial and ppaxial distribution of
charge carrier density The impedance of the plasma column is
afunction of temperature (except regions near thearc terminals) 3
3Arc-Electrical featuresElectrodePlasmaWork All electric arcs
consist of three regions the cathode fall space (or drop zone); the
plasma column fall space (or drop zone) the anode fall space (or
drop zone) 4 42 3. 9/20/2012Arc-Electrical features Electrical
power dissipated in each regions ofthe arc given by P = I(Ea + Ec +
Ep) where Ea isanode voltage, Ec is cathode voltage, and Ep
isplasma column voltage Intermediate regions Involved in expanding
orcontracting the cross section of the gaseousconductor to
accommodate each of these mainregions.i As a consequence, welding
arcs assume bell orcone shapes and elliptical or some
othernoncylindrical contour.55Arc Shape ElectrodeArc shape =
Interaction of(plasma + Arc + Ambience) Plasma Bell Shape Cone
EllipticalWork Cylindrical66 3 4. 9/20/2012 Arc Shape- Influencing
factors1. Shape of the arc terminals (i.e., pointed welding
electrode producing a narrow arc f d ifocused at d t the electrode
tip and flat work piece electrode, which causes the arc to
spread)2. Gravitational forces3. Magnetic forces (from both g (
internally generated and externally induced or applied sources)4.
Interactions between the plasma and ambience (shielding gas)777 Arc
Shape- Influencing factors N t Nature of electrode- Cf l t d
Consumable/non bl / consumable Electrode coating/gas generation
Shielding gas M Magnitude & polarity of current source it dl it
ft8 4 5. 9/20/2012 Arc Shape(c)CO2 shielded(a)GTA GMAwelding (b)SMA
Non Consumable consumableConsumable electrode +l t delectrode +
electrode + Gas Inert gas CO2 gas generation 9 9Arc radiation Arc
radiation amount and characterdepends on the atomic mass and
chemical composition of thegaseous medium, the temperature, and the
pressure. Spectral analysis shows line andcontinuum emissions due
to excitedand ionized states of atoms and ions Radiation UV,
visible, IR Energy loss due to radiation 10-20% Highly hazardous to
eyes, skin10105 6. 9/20/2012Metal transfer in Arc welding The
manner in which moltenfiller metal is transferred to theweld pool
profound effectson the performance of aconsumable electrode
arcwelding process These effects include Ease of welding in various
positions Extent of weld penetration;p Rate of filler deposition
and Heat input Stability of the weld pool Amount of spatter loss 11
Mode of Metal transfer-Influencing parameters Pressure generated by
the evolution of gasat the electrode ti (for flt th l t d tip (f
flux-coated or flt d flux-cored electrode processes) Electrostatic
attraction between theconsumable electrode and the workpiece
Gravity y Pinch effect caused near the tip of theconsumable
electrode by electromagneticfield forces spray 12 6 7. 9/20/2012
Mode of Metal transfer-Influencing parameters Explosive evaporation
of a necked regionformed between the molten drop and solidportions
of the electrode due to very highconducting current density
Electromagnetic action produced by divergenceof current in the arc
plasma around a drop. Friction effects of the plasma jet
(plasmafriction) Surface tension effects once the molten drop(or
electrode tip) contacts the molten weld pool13Metal transfer types
Free-flight transfer: Completedetachment of the molten metaldrop
from the consumableelectrode flight to the work pieceand weld pool,
without any directphysical contact Bridging transfer: molten
metaldrops are never completely free;rather they are always
attached totheth consumable electrode and thbl l t dd theworkpiece,
momentarily bridgingthe two from a material standpoint -and
electrically Slag-protected transfer147 8. 9/20/2012
Free-flight-Globular Transfer Low welding currents (50-170 A)
inppure argon, molten metal from a gsmall diameter solid steel
wireelectrode is transferred in the formof globules Drops diameter
larger than thewire Large drops detach by gravity Low rate of
globule formation,detachment, and transfer (< 1-10s-1) Globular
transfer down-handposition 15 Free flight: Globular-projected
Transfer As the welding current increaseswithin the range of
50-170A the50 170A,drops become progressivelysmaller,
electromagnetic forcesare having an increasing effect ondetachment
For DCEP, drop size 1/ weldingcurrent.current As welding current is
increased,the rate of drop transfer alsoincreases 16 8 9.
9/20/2012Free-flight transfer modesIncreasing currentIndividual
drop formation and detachment sequence in (a) globulartransfer and
(b) projected and (c) streaming axial spray transfer17Free flight
-SprayTransfer/streaming transfer At current > critical level
Noindividual drops Tip of the consumable electrodebecomes pointed
cylindricalstream of liquid metal flows towardthe work piece in
line with theelectrode. Near its tip (nearest the workpiece),piece)
this cylinder disperses intomany very small droplets
Electromagnetic pinch effect The rate at which droplets
aretransferred is hundreds per second.189 10. 9/20/2012Free flight
-Spray Transfer:Features Axial spray transfer mode Excellent
stability,virtually free of spatter Droplets are actively propelled
away from theconsumable electrode and into the molten weldpool to
be captured by surface tension force. Droplets are transferred in
line with the electrode Filler metal can be directed exactly where
it isneeded.needed This is a great advantage when making verticalor
overhead welds, where the propelling forceoffsets the disruptive
effect of gravity 19 Bridging or short circuitingtransfer Large
dia. Electrodes too hightransition current to achieve axial
spraytransfer (e.g., 200-220A for 1-mm-diameter steel wire)
Bridgingtransfer Voltage is kept low (say 17-21 Vversus 24-28 V for
globular transferwith steel wires) The tip of the electrode
periodicallydipped into the molten weld pool. 2010 11.
9/20/2012Bridging or short-circuiting transfer Bridging Molten
metal transfer bya combination of surface tension
andelectromagnetic forces Low voltage reduces the rate atwhich the
electrode is melted In the presence of carbon dioxide(CO2) in
shielding gas, the moltendrop at the electrode tip is pp p
pushedupward repelled transfer. In thiscase, short-circuiting
captures thedrop before it detaches in anunfavorable manner 21
Short-circuiting transfer- Advantages Less fluid molten metal (due
to(less superheat) Less penetration (due to lowerwelding voltage
and lower netenergy input). Easy handling in all
positions,especially overhead and for theoverhead,joining of
thin-gauge materials. Minimum Spatter 2211 12. 9/20/2012 Sequence
of short-circuiting transfer(a) globule of molten metal builds up
on the end of the electrode; (b)globule contacts surface of weld
pool; (c) molten column pinches off todetach globule; and (d)
immediately after pinch-off, fine spatter may result 23
Short-circuiting transfer: I-V trace 24 12 13. 9/20/2012 Pulsed arc
or pulsed current transfer Steady current to maintain the arc and
aperiodic current pulse to a higher level Periodic pulses detaches
a drop andpropels it into the weld pool, Advantage of axial spray
transfer at alower average current, and, thus, lower netheat input.
The time period of pulses must be shortenough to suppress globular
transfer, butlong enough to ensure that transfer by thespray mode
will occur 25 PULSED-CURRENT TRANSFER 2613 14. 9/20/2012 Pulsed
current transfer This pulsed mode differs from the normal spraymode
in that Th molten metal transfer is interrupted between theThelt t
ltf i i t t db t thcurrent pulses The current to produce spray is
below the normaltransition current Pulse shape (i-e., wave form,
especially the rateof the rise and fall of current) and frequency
canbe varied over a wide range in modern, solid-gstate power
sources Rate of molten metal transfer can be adjusted tobe one drop
or a few drops per pulse (byadjusting the pulse duration)27
Classification of transfer modes28 14 15. 9/20/2012 Dominant forces
in transfer modes 29Effect of welding process parameterson transfer
modes-Summary Current at which transition from globular to spray
transfer begins depends on Composition of the consumable electrode,
Electrode diameter, Electrode extension, Composition of shielding
gas. Shielding Gas Effects Process Effects Operating Mode or
Polarity Effects 3015 16. 9/20/2012Welding ectureWeldingLecture
820Sept2012,Thursday8.30am9.30amWeldingProcessesResistanceweldingResistance
welding3131Resistance welding (RW) Generate heat through
theresistance to the flow of electriccurrent in parts being welded
tit b ild d The parts are usually an integralpart of the electrical
circuit Contact resistance heats thearea locally by I2R, melting
formation of a nugget C t t resistance must beContacti t tbhigher
at the point to be weldedthan anywhere else.3232 16 17.
9/20/2012Resistance welding 33 33Resistance welding 3417 18.
9/20/2012Resistance welding Pairs of water-cooled copper electrodes
Apply pressure To reduce the contact resistance at the
electrode-to-workpieceinterfacei t f Contain the molten metal in
the nugget To literally forge the work surfaces together in the
vicinity of the weld The principal process variables welding
current (several thousands to tens of thousands of amperes) welding
time (of the order of s) electrode force and electrode shape DC
power (provided from either single-phase or(id d f ith i l
hthree-phase AC line 440-480 V using
step-downtransformer/rectifiers) Usually used to join overlapping
sheets or plates aslap joints, which may have different
thicknesses3535Resistance welding-types Resistance spot welding ( S
)es sta ce e d g (RSW) Resistance seam welding (RSEW) Projection
welding (PW) Flash welding (FW) Upset welding (UW) Percussion
welding (PEW)363618 19. 9/20/2012Resistance spot welding (RSW)
Series of discrete nuggetsproduced by resistanceheating Nuggets
(welds) areusually produced directlyunder the electrodes, Not
necessarily if there isanother more favourablepath (shunt), for
thecurrent3737 Series resistance spot welding3838 19 20. 9/20/2012
Resistance welding cycle Squeeze Time: Time interval between timer
initiationand the first application of current needed to assurethat
electrodes contact the work and establish fullforce Weld time: The
time for which welding current isapplied (in single impulse
welding) to the work Hold Time: The time during which force is
maintainedon the work after the last impulse of welding currentends
to allow the weld nugget to solidify and developstrength. Off Time:
The time during which the electrodes are offthe work and the work
is moved to the next weldlocation for repetitive welding.
39Pressure-current cycle404020 21. 9/20/2012Pressure-current cycle
1. Off time: Parts inserted between open electrodes, 2. 2Squeeze
TiSTime:Electrodes close and fEl t dld force iisapplied, 3. Weld
time current is switched on, 4. Hold time: Current is turned off
but force ismaintained or increased (a reduced current issometimes
applied near the end of this step forstress relief in the weld
region), and 5. Off time: Electrodes are opened, and the
weldedassembly is removed.4141Enhanced welding cycle1.
Pre-compression force is used to set electrodes and work pieces
together2. Preheat is2 P h t i applied t reduce th li d to dthermal
gradients at ldi t t the start of weld time or to soften coatings3.
Forging force is used to consolidate weld nugget4. Quench and
temper times are used to produce desired weld properties in
hardenable steels;5. Post heat is used to refine weld nugget grain
size and improve strength6. Current decay is used to retard cooling
of aluminum alloys to help prevent cracking42 21 22.
9/20/2012Enhancedresistanceweldingcycle 43Nugget formation Nugget
formation and heat dissipation into the surrounding base metal and
electrodes during resistance spot welding 4422 23. 9/20/2012Optimum
current, time in RW Optimum current and weld time for maximum
shearstrength of the RW joint45Resistance seam welding Conventional
resistance seam Roll spotContinuousC tiwelding, in
whichweldingresistanceoverlapping spots seam are produced4646 23
24. 9/20/2012Resistance seam welding47 Mash seam weld4848 24 25.
9/20/2012 Projection welding (PW), Projections or dimples in
overlapping joint elements concentrate the current during welding,
focusing the weldenergy and helping to locate the weld more
precisely Contact points determined by the design of the parts tobe
joined may consist of projections, embossments, orlocalized
intersections of the parts 4949Flash welding(FW)1. Normally used
for butt joints the two surfacesy j to be joined are brought into
near contact 2. Electric current is applied to heat the surfaces to
the melting point the surfaces are forced together to form the weld
5050 25 26. 9/20/2012 Upset welding (UW) Upset welding (UW)
issimilar to flash welding H ti i UW iHeating in isaccomplished
entirely byelectrical resistance atthe contacting surfaces;no
arcing occurs. Not a fusion-weldingprocess Applications of UW
&FW- joining ends ofwire, pipes, tubes etc.5151Percussion
welding Resistance heating by the rapid release of electrical
energyfrom a storage device ( g , capacitor).g(e.g., p) Similar to
flash welding, except that the duration of the weldcycle is
extremely short, ~ 1 to 10 ms Very localized heating attractive for
electronicapplications in which the dimensions are very small5252
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