Gas Shielded Welding Processes

Gas Metal Arc Welding(GMAW)

Lecture 4 p1

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

p2

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GMAW Process Fundamentals

Electrode

Directionof Travel

•

/GaSNOZZle

GAS_~~~~,~/SHIELD

A~£P~BaseMetal~ Metal

Shielding Gas In

CONSUMABLEELECTRODE

Lecture 4 p:

GMAW Metal Transfer Modes

• Short Circuiting or "Dip" Transfer

• Globular Transfer

• Spray Transfer

• Pulsed or Synergic Transfer

p4

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I,

ARCING PERIOD

GMAW Short Circuiting Transfer

II(.) ZE~Rn=O==~---~:::=====~------=~-_'- Time

zoE

II ~ I7ize'iiRno---rTTl'-.--.---.-i::=------

Lecture 4 p ~

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

GMAW Spray Transfer

-

~---~- ---

p8

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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'

4

GMAW Welding Procedures

• Process Variables- Welding current (electrode melting rate)- Polarity- Arc voltage (length)- Travel speed- Electrode extension- Electrode size- Shielding gas composition

p10

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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

GMAW Torch Geometry

Nozzle

Contact Tube to Electrode ExtensionWork Distance

I \; \ Arc Length, ,

I \.---'-- 'rL------,

p12

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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

GMAW TlPical Welding ProceduresCarbon Steel

R 'Q ~~-pz ?? zzJl/fl ?? zzl T ~eM+F ~RA B C

p14

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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!

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

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

GMAW Mechanized Application

Field welds in oil/gas transmission pipelines

(

Weld preparationand bead sequence

P 18

.......__......_--......-----.....----------""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

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

p20

____...._ ......... ...._ .......... 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,

P21

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

p22

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Lecture 4

WELDING PROCESSES

GAS TUNGSTEN ARCWELDING

(GTAW)

p23

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.

p24

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GTAW: Process Fundamentals

Lecture 4

•DIRECTION OFWELDING

ARC

SHIELDINGGAS IN

I NON-CONSUMABLETUNGSTEN ELECTRODE

SOUDIFIEDWELD METAL

p25,

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.

P 26

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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

e4

GTAW: Effect of Polarity

----~-

p28

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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~

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

P 30

Lecture 4

GTAW: Gas backing

p3'

GTAW: Pipe internal purge (

GAS INLET PURGE GASOUTLET

!!::~~~~====~--BAFFLE

PIPE WELD JOINT

,4 P 32

Lecture 4

GTAW Equipment Schematic

TORCH ELECTRICALCONDUCTOR

POWERARC SOURCE

p3::

4

GTAWTorchPOWER CONDUCTOR ...

COOLING WATER IN/OUT ~1Sl '"

SHIELDING GAS INLET

/HANDLE

SHIELDING GAS OUTLET- ---.......

TUNGSTEN ELECTRODE:- _

p34

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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

Hot Wire GTAW (

GTAWTORCH

DCGTAW

~.:=-.. POWER

HIGH SPEED FILLERWIRE FEEDER

AC HOTWIRE

POWER

HEATEDWIRE

AUX.SHIELDING "-.~GAS •

CON~\TUBE---~

4 P36

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

4

Narrow-Gap GTAW

Special Narrow-Gap.v.::::.-..~Torch

Narrow-Gap Joint~.Lh4- Preparation

p38

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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