gas metal arc welding (gmaw) - candu owners group library/20053423.pdf · 4 gas metal arc welding...
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Gas Shielded Welding Processes
Gas Metal Arc Welding(GMAW)
Lecture 4 p1
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4
Gas Metal Arc Welding
Process Fundamentals
• In GMAW the welding heat sour~ is an arc maintainedbetween a consumable wire electrode and the workpiece
• The weld is formed by melting and solidification of thejoint edges together with filler material transferred fromthe electrode
• An flow of inert gas shields the high-temperature arc andweld pool from reactions with the surroundingatmosphere
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GMAW Process Fundamentals
Electrode
Directionof Travel
•
/GaSNOZZle
GAS_~~~~,~/SHIELD
A~£P~BaseMetal~ Metal
Shielding Gas In
CONSUMABLEELECTRODE
Lecture 4 p:
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GMAW Metal Transfer Modes
• Short Circuiting or "Dip" Transfer
• Globular Transfer
• Spray Transfer
• Pulsed or Synergic Transfer
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ARCING PERIOD
GMAW Short Circuiting Transfer
II(.) ZE~Rn=O==~---~:::=====~------=~-_'- Time
zoE
II ~ I7ize'iiRno---rTTl'-.--.---.-i::=------
Lecture 4 p ~
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GMAW Globular Transfer ModeGAS NOZZLE
I(~Lp~ELECTRODE
r~-
( II I I I I
'"R (AI
R-Anode reactionP-Electromagnetic
"pinch" force
g{~(~I I I I
\.. jVIB)
p6
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GMAW Spray Transfer
Variation in volume and rate of drop transfer with welding current
300 15.10·'
.. • 20"- •§
,I ~ ~I ~
~
0.. 200 I 1/16 in. (1.6 mm) 10 .S E0 DROP MILD STEEL EII: I W 150 VOLUME Wa! I ELECTRODE. DCRP ::E ::Ew I ARGON-I% OXYGEN 3
~... 0
'" TRANSITIO~SHIELDING GAS >z 10
~1/4 in. (6.4 mm) ~ ~100 CURRENT I ARC LENGTH 5 a..... 0 0..
0 II: 0W c 5 ~
i0 0 0
0 100 200 300 400 600 600CURRENT, A
Lecture 4 p
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GMAW Spray Transfer
-
~---~- ---
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GMAW Pulsed TransferPULSE PEAK CURRENT
3 SPRAY TRANSFERCURRENT RANGE-,-------- - --------
PULSE 24 GLOBULAR .
« TRANSITION 5 TRANSFj;R.1-' CURRENT CURRENTz
1 RANGEUJa:a::J 1 2 3 4 5u BACKGROUND CURRENT
~U~~UTIME
• Pulse frequency and amplitude determine wire melting rate• "Synergic" control automatically gives the optimum pulse
conditions for a given wire feed rate
Lecture 4 p'
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4
GMAW Welding Procedures
• Process Variables- Welding current (electrode melting rate)- Polarity- Arc voltage (length)- Travel speed- Electrode extension- Electrode size- Shielding gas composition
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GMAW Electrode Melting Rate
C 800 20 C.-E .--700 - Ec -c E
600 -15 CLLILLI Wa.. 500 wen 0-
c 400 10 enLLI cLLI 300 WU. W
LLI I&.
0:: 200 5 W
~a::
100 -3:0
0 50 100 150 200 250 300 350 400 450
WELDING CURRENT (A)
Lecture 4 pl
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GMAW Torch Geometry
Nozzle
Contact Tube to Electrode ExtensionWork Distance
I \; \ Arc Length, ,
I \.---'-- 'rL------,
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GMAW Consumables
• Electrode composition is usually similar todesired weld metal composition with ad9itionaldeoxidizers e.g. Si, AI, Ti
• Electrodes are covered by AWS and otherspecifications- Carbon steel electrodes AWS A 5.18.
• Shielding Gases- Various shielding gases are used depending on metal
being welded and desired transfer mode- Principally Ar, C02 and mixtures of Ar-G02, 02 or He- Several commercial "brand-name" compositions
Lecture 4 pt
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GMAW TlPical Welding ProceduresCarbon Steel
R 'Q ~~-pz ?? zzJl/fl ?? zzl T ~eM+F ~RA B C
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GMAW Welding Equipment
ELECTRODE SuPPLY
Lecture 4
Welding Gun
Workpiece(j) WOAK WoO(i) WA1eATOQU.I() WATERFAONQUN
@ GUNSWfTCH CIACUT<!> SHfElDlNG GAS TO GUN
@ CABLEA&SEMIM.Y
<i> 8HElDING OM FROM cvt.HJER
~ WELlllNO OOHTACl'OA CONTflOl
'" PCMEA CAaE® """"""'H'UTPOWER
Shielding~as
Supply
P 1!
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GMAW Welding Gun (
SHIELDING GAS PATH
INPUT WATER LINE
POWER CABlE!RETURN WATER
CONTROLSWITCH
Q
o
WATERCHAMBER~~~~'--
CONTACT TIP
GAS NOZZLE POWER BLOCK
WIRE CONOUIT
ELECTRODE
4 p16
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GMAW Applications
• Aluminum alloys, copper/bronze, magnesium.titanium, austenitic stainless steels, nickelalloys
• Sheet metal fabrication (short circuit mode)- eg automotive, appliance, light structures
• General structural fabrication (spray/pulsedmode)
• High production, mechanised and roboticwelding
Lecture 4 p17
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GMAW Mechanized Application
Field welds in oil/gas transmission pipelines
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Weld preparationand bead sequence
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.......__......_--......-----.....----------""J----------'-~~"'<.c.;.,;,0; Uk ~.U;6lJ .1Mf; Ii i!CI : .
GMAW: Mechanized Applications
.JUndelwalerloading station
Shielded Workstation
r.w,:=-Welding guns & seam trackef
Transfer Flask
Fuel cominer
~.,., A/o "X:.<'""''''i i ' ",52
"'X>Fuel Bundles '. j-,/, .,.- '-{".
Spent Fuel Bay
Cover seal weldedafter loading
Lecture 4 p19
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GMAW: Mechanized applications (
Shielded station wall
o Video mon~or
Welding guns mounted onautomatic seam trackers
ShieldingGas
I'"IIr~~+---f:+----"-+ ,-+1-----.!::,~J--' fl"----'
1...... 1 WireI I feeder
1 1I", 1
I" IL---liI---+ I Control"'-1 Panel1 ~--,
I lop IL-----1r----'IC"" \
L------ft----t-"...... 1l ~
TV Camera
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____...._ ......... ...._ .......... nl"il"" ~__~·"
Process ControlComponent Status Weld Cycle TIme
Lecture 4
SEAMTRACKER
TURNTABLE
waDINGEQUIPMENT
Weld overlap
Tracking
Drive In/Out
Idle
Motion
Idle
Welding Current
Shield gas flow
Idle
Cycle start, Cycle end,
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GMAW Capabilities & Limitations (
+ Applicable to range ofmetals andthicknesses
+ Higher productionrates than SMAW orGTAW
+ No flux or slagresidues
+ Adaptable to manualor mechanized/roboticapplications
- Complex equipmentand set up _
• Wire feeding canbe temperamental
- Less portable thanSMAW
- Gas shield sensitive toair currents
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Lecture 4
WELDING PROCESSES
GAS TUNGSTEN ARCWELDING
(GTAW)
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4
GTAW: Process Fundamentals
• In GTAW the welding heat source is an arcmaintained between a non-consumabletungsten electrode and the workpiece
• Inert gas shields the arc and weld zone fromatmospheric contamination
• Filler need not always be added (autogenouswelding)
• Filler if required is added to the weld pool in theform of wire or rod.
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GTAW: Process Fundamentals
Lecture 4
•DIRECTION OFWELDING
ARC
SHIELDINGGAS IN
I NON-CONSUMABLETUNGSTEN ELECTRODE
SOUDIFIEDWELD METAL
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GTAW: Process Variables
• Welding current- DC, pulsed DC, high frequency pulsation, AC, variable
polarity AC -
• Arc length (Voltage)
• Weld travel speed
• Oscillation
• Filler addition
• Shielding gas composition &flow rate- generally Ar or He or mixtures.
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GTAW: Current Polarity
• DCEP is mostly used- approx 70% of heat produced at anode (workpiece)
• DCEN used to disperse tenacious surfaceoxides when welding AI, Mg
• AC also used in AI welding- variable polarity pulsed AC
Lecture 4
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e4
GTAW: Effect of Polarity
----~-
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- .. _ - ._ _- ._ _--_.._------------------_._-----~..-,
GTAW: Arc Voltage
ARC LENGTH
- - 0.08 in. (2 mm)- 0.16 in. (4 mm) -
HELIUM, ~
'... !lo-"'~d".--
50 100 150 200 250 300 350 400
ARC CURRENT, A
TUNGSTEN ARC.ALUMINUM
30
w·C)<{ 20~0> 15ua:<{
10
00
> 25
Lecture 4 P 2~
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Pulsed GTAW (PGTAW) (
Peakcurrent
-
pulse duration 0.1 - 1sec
~ r--~oo
It.<".I'~"O.ILI "'...... Background
'--- '-- --l::-.......J-+-'>..--L.. current
TIME
,4
Pulsed DC advantages:,f Greater penetration for given average current,f Minimizes heat affected zone & distortion,f Improved capability to weld in all positions
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Lecture 4
GTAW: Gas backing
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GTAW: Pipe internal purge (
GAS INLET PURGE GASOUTLET
!!::~~~~====~--BAFFLE
PIPE WELD JOINT
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Lecture 4
GTAW Equipment Schematic
TORCH ELECTRICALCONDUCTOR
POWERARC SOURCE
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4
GTAWTorchPOWER CONDUCTOR ...
COOLING WATER IN/OUT ~1Sl '"
SHIELDING GAS INLET
/HANDLE
SHIELDING GAS OUTLET- ---.......
TUNGSTEN ELECTRODE:- _
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Mechanized GTAW Applications
• Mechanization of- weld head travel motions- wire feed (if required)- process controls: start/stop sequence, weld current profile,
shielding gas flow, etc
• Typical applications: Pipe, tube, tube-tubesheetwelding, longitudinal welds in formed tubes.
• Advantages of mechanization-Increased productivity- reduced weld defect rates- shorter joint completion times- reduced need for skilled labour
Lecture 4 p35
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Hot Wire GTAW (
GTAWTORCH
DCGTAW
~.:=-.. POWER
HIGH SPEED FILLERWIRE FEEDER
AC HOTWIRE
POWER
HEATEDWIRE
AUX.SHIELDING "-.~GAS •
CON~\TUBE---~
4 P36
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GTAW Deposition Rates
20 9HOT WIRE WITH
'C 18 OSCILLATION~,/ / . 8.c:0 16 7:::::;.W 14
," 6
~ 12,,, 5 ...
Z 10, .c, HOTWlRE
4~0 ,8 ,
I- " COLD WIRE 3(/) 6 I
\I
0, - 2a.. 4
w0 2 -- 1
0 02 4 6 8 10
ARC ENERGY (kW)
Lecture 4 P37
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4
Narrow-Gap GTAW
Special Narrow-Gap.v.::::.-..~Torch
Narrow-Gap Joint~.Lh4- Preparation
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GTAW Capabilities & Limitations
Lecture 4
+ Superior qualitywelds free from fluxresidues or spatter
+ Excellent control ofpenetration
+ Applicable to almostall metals
+ Adaptable to manualor precisionmechanizedapplications
- Low deposition rates- Higher welder_skill
required in manualprocesses
- Gas shieldingsensitive to aircurrents
p39