09 Mig Mag Welding 2006

CSWIP 3.1 Welding Inspection

TWI Training & Examination Services

Course Reference WIS 5Course Reference WIS 5

Gas Metal Arc Welding Gas Metal Arc Welding

Metal Inert GasMetal Inert Gas

Metal Active GasMetal Active Gas

The MIG/MAG welding process was initially developed in the

USA in the late 1940s for the welding of aluminum alloys.

The latest EN Welding Standards now refer the process by the

American term GMAW (Gas Metal Arc Welding)

• The process uses a continuously fed wire electrode

• The weld pool is protected by a separately supplied shielding gas

• The process is classified as a semi-automatic welding process but may be fully automated

• The wire electrode can be either bare/solid wire or flux cored hollow wire

Gas Metal Arc Welding

MIG/MAG - Principle of operation


PROCESS CHARACTERISTICSPROCESS CHARACTERISTICS

• Requires a constant voltage power source, gas supply, wire

feeder, welding torch/gun and ‘hose package’

• Wire is fed continuously through the conduit and is burnt-off

at a rate that maintains a constant arc length/arc voltage

• Wire feed speed is directly related to burn-off rate

• Wire burn-off rate is directly related to current

• When the welder holds the welding gun the process is said to be a semi-automatic process

• The process can be mechanised and also automated

• In Europe the process is usually called MIG or MAG

Power return Power return

cablecable

Transformer/ Transformer/

RectifierRectifier

Power cable & Power cable &

hose hose

assemblyassembly

Liner for wireLiner for wire

Welding gun Welding gun

assemblyassembly

External wire External wire

feed unitfeed unit

Power control Power control

panelpanel

Internal wire Internal wire

feed systemfeed system

15kg wire spool15kg wire spool

Equipment for MIG/MAG

Plain top rollerPlain top roller

Half grooved Half grooved

bottom rollerbottom roller

Wire guideWire guide

Internal wire Internal wire

drive systemdrive system

MIG/MAG wire drive system


Types of wire drive systems:

4 roll wire drive4 roll wire drive2 roll wire drive2 roll wire drive


Types of drive rolls

••recommended for steel recommended for steel

wireswires

••recommended for softer recommended for softer

wires (aluminium)wires (aluminium)


Close wound stainless steel

spring wire liner

Teflon liner

MIG/MAG welding gun types

Goose neck type

Push-pull type

Welding gun bodyWelding gun body

Contact tipsContact tips

Spot welding Spot welding

spacerspacer

Gas diffuserGas diffuser

Nozzles or Nozzles or

shroudsshrouds

Welding gun assembly Welding gun assembly

(less nozzle)(less nozzle)

On/Off switchOn/Off switch

Hose Hose

portport

MIG/MAG welding gun assembly

Spatter Spatter

protectionprotection

MIG/MAG welding gun assembly

The Push-Pull gun

HandleHandle

Gas Gas

nozzlenozzle

Contact Contact

tiptip

Gas Gas

diffuserdiffuser

TriggerTrigger WFS remote WFS remote

control control

potentiometerpotentiometer

Union nutUnion nut

ARC CHARACTERISTICS

Volts

Amps

OCV

Constant Voltage Characteristic

Small change in voltage = large change in amperage

The self adjusting arc.

Large arc gap

Small arc gap

O.C.V. Arc Voltage O.C.V. Arc Voltage

Virtually no Change. Virtually no Change.

VoltageVoltage

Flat or Constant Voltage Characteristic Used With Flat or Constant Voltage Characteristic Used With

MIG/MAG, ESW & SAW < 1000 ampsMIG/MAG, ESW & SAW < 1000 amps

100100 200200 300300

3333

3232

3131

Large Current ChangeLarge Current Change

Small Voltage Small Voltage

Change. Change.

AmperageAmperage

Flat or Constant Voltage Characteristic

Wire feed speed:Wire feed speed:

Increasing the wire feed speed automatically increases the current in the wire

Voltage:

The voltage is the most important setting in the spray transfer mode, as it controls the arc length. In dip transfer it controlsthe rise in current

Current: Current:

The current is automatically increased as the wire feed is increased. Current mainly affects penetration

MAG Welding Variable Parameters

Inductance:

• Applicable to MIG/MAG process in dip transfer mode.

• The electrode is fed slowly through the arc until it touches the

weld pool, at this point the output from the power supply is

short circuited and a very high current flows through the

electrode. If this was allowed to continue, the wire would melt

and eject excessive amounts of spatter.

• The inclusion or the choke in the welding circuit controls the

rate at which the current rises so that the electrode tip is

melted uniformly without excessive spatter


Shielding Gases: Shielding Gases:

The gasses used in MIG/MAG welding can be either 100% CO2

or Argon + CO2 mixes.

• 100% CO2: Can not sustain true spray transfer, but gives very good penetration. The arc is unstable which produces

a lot of spatter and a coarse weld profile.

• Argon + CO2 mixes: Argon can sustain spray transfer above 24 volts, and gives a very stable arc with a reduction in spatter. Argon being a cooler gas produces less penetration than CO2. Argon in normally mixed with CO2 at a mixture of between 5-25%



MODES OF METAL TRANSFERMODES OF METAL TRANSFER

The current and voltage settings determine the way molten The current and voltage settings determine the way molten

droplets of weld metal transfer from the tip of the wire to the droplets of weld metal transfer from the tip of the wire to the

weld poolweld pool

There are 3 principle modes of droplet transfer, namelyThere are 3 principle modes of droplet transfer, namely

••dip transfer (shortdip transfer (short--circuiting)circuiting)

••spray transferspray transfer

••pulsed transferpulsed transfer

Dip TransferDip Transfer: (Voltage < 22) / (Amperage < 200)

• Thin materials positional welding

Globular Transfer: Between Dip & Spray Transfer

• Limited commercial, Used only in some mechanised MAG process using CO2 shielding gas

Spray Transfer: (Voltage > 27) / (Amperage > 220)

• Thicker materials, limited to flat welding positions, high deposition

Pulse Transfer: Both spray and dip transfer in

• one mode of operation, frequency range 50-300 pulses/second

• Positional welding and root runs


MIG/MAG - metal transfer modes

Current/voltage conditions

CurrentCurrent

VoltageVoltage

Dip transferDip transfer

Spray Spray

transfertransferGlobular Globular

transfertransfer

Electrode diameter = 1,2 mmElectrode diameter = 1,2 mm

WFS = 3,2 m/minWFS = 3,2 m/min

Current = 145 ACurrent = 145 A

Voltage = 18Voltage = 18--20V20V

Electrode diameter = 1,2 mmElectrode diameter = 1,2 mm


Current = 295 ACurrent = 295 A

Voltage = 28VVoltage = 28V


Dip TransferDip Transfer

• Dip transfer occurs when current & voltage settings are low

(typically < ~ 200amps & ~ 22volts)

• There is just enough energy to give an arc and cause fusion at the tip of the wire

• A droplet grows to a size larger than the wire diameter and eventually extinguishes the arc - causing a short-circuit

• The short circuit causes the current rises very quickly giving energy to violently ‘pinch-off’ the droplet

• This is akin to ‘blowing a fuse’ and causes spatter

• When the droplet detaches, the arc is re-established and the current

falls

• This cycle occurs at up to ~ 200 times per second

MIG/MAG-methods of metal transfer

Dip transferDip transfer

• Transfer occur due to short circuits between wire and weld pool, high level of spatter, need inductance control to limit current raise

• Can use pure CO2 or Ar- CO2

mixtures as shielding gas

• Metal transfer occur when arc is extinguished

• Requires low welding current/arc voltage, a low heat input process. Resulting in low residual stress and distortion

• Used for thin materials and all position welds


The pinch effect

CSA

CurrentPforcePinch

2

=


Dip TransferDip Transfer

TransferTransfer--mode advantagesmode advantages

•• The low energy conditions allow welding in all positionsThe low energy conditions allow welding in all positions

•• It can be used for putting in the root run on singleIt can be used for putting in the root run on single--sided sided

weldswelds

•• It can be used for welding thin materialsIt can be used for welding thin materials

TransferTransfer--mode disadvantagesmode disadvantages

•• It frequently gives lack of fusion and may not be allowed in It frequently gives lack of fusion and may not be allowed in

semisemi--automatic mode for highautomatic mode for high--integrity applicationsintegrity applications

•• It tends to give spatterIt tends to give spatter

(this can be reduced/controlled by having an (this can be reduced/controlled by having an ‘‘inductanceinductance’’

control on the power source)control on the power source)


Globular transfer Globular transfer

• Transfer occur due to gravity or short circuits between drops and weld pool

• Requires CO2 shielding gas

• Metal transfer occur in large drops (diameter larger than that of electrode) hence severe spatter

• Requires high welding current/arc voltage, a high heat input process. Resulting in high residual stress and distortion

• Non desired mode of transfer!


Spray TransferSpray Transfer

When current & voltage are raised together higher energy is

available for fusion (typically > ~ 25 volts & ~ 250 amps)

This causes a fine droplets of weld metal to be ‘sprayed’ from the tip of the wire into the weld pool

Transfer-mode advantages

• High energy gives good fusion

• High rates of weld metal deposition are given

• These characteristics make it suitable for welding thicker

joints

• Transfer-mode disadvantages

• It cannot be used for positional welding


Spray transfer Spray transfer

• Transfer occur due to pinch effect NO contact between wire and weld pool!

• Requires argon-rich shielding gas

• Metal transfer occur in small droplets, a large volume weld pool

• Requires high welding current/arc voltage, a high heat input process. Resulting in high residual stress and distortion

• Used for thick materials and flat/horizontal position welds


Pulsed transferPulsed transfer

• Controlled metal transfer, one droplet per pulse,

• No transfer between droplet and weld pool!

• Requires special power sources

• Metal transfer occur in small droplets (diameter equal to

that of electrode)

• Requires moderate welding current/arc voltage, a reduced heat input . Resulting in smaller residual stress and distortion compared to spray transfer

• Pulse frequency controls the volume of weld pool, used for root runs and out of position welds


Pulsed transfer

• Controlled metal transfer � one droplet per pulse � NO transfer during

background current!

• Requires special power sources

• Metal transfer occur in small droplets

(diameter equal to that of electrode)

• Requires moderate welding current/arc voltage �reduced heat input � smaller residual stress and

distortions compared to spray transfer

• Pulse frequency controls the volume of weld pool � used

for root runs and out of position welds


Pulse current parameters

Time (sec)Time (sec)

Current (A)Current (A)

Peak Peak

currentcurrent

Background Background

currentcurrent

Transition Transition

currentcurrent

Average current Average current

(ammeter reading)(ammeter reading)


Pulsed TransferPulsed Transfer

CurrentCurrent IIpp = peak current= peak current (spray transfer)(spray transfer)

IIbb = background current= background current (continuous arc but little or no fusion)(continuous arc but little or no fusion)

IIpp

IIbb

TTpp TTbb

TimeTimeTTpp = peak time= peak time

TTbb = background time= background time


Set-up for dip transfer Set-up for spray transfer

Electrode Electrode

extension extension

1919--25 mm25 mm

Contact tip Contact tip

recessed recessed

(3(3--5 mm)5 mm)

Contact tip Contact tip

extension extension

(0(0--3,2 mm)3,2 mm)

Electrode Electrode

extension extension

66--13 mm13 mm

MIG / MAG - self-regulating arc

Stable conditionStable condition

LL19 mm19 mm

Sudden change in gun positionSudden change in gun position

25 mm25 mmLL’’

Arc length L = 6,4 mmArc length L = 6,4 mm

Arc voltage = 24VArc voltage = 24V

Welding current = 250AWelding current = 250A


Melt off rate = 6,4 m/minMelt off rate = 6,4 m/min

Arc length LArc length L’’ = 12,7 mm= 12,7 mm






Vo

lta

ge

(V

)V

olt

ag

e (

V)

MIG/MAG - self-regulating arc

Sudden change in gun positionSudden change in gun position

25 mm25 mmLL’’

Arc length LArc length L’’ = 12,7 mm= 12,7 mm






Vo

lta

ge

(V

)V

olt

ag

e (

V)

ReRe--established stable conditionestablished stable condition

25 mm25 mmLL

Arc length L = 6,4 mmArc length L = 6,4 mm





The effect of inductance

Controls the rate of current rise

Time (sec)

Current (A)

Short circuit

current

No inductance

Inductance added

Excessive current, high

spatter

Desired current for

good stability, low

spatter

The effect of inductance

Maximum inductanceMaximum inductance Minimum inductanceMinimum inductance

••reduced spatterreduced spatter

••Hotter arc Hotter arc �� more more

penetrationpenetration

••More fluid weld pool More fluid weld pool ��

flatter and smoother weldflatter and smoother weld

••Recommended on thicker Recommended on thicker

materials and stainless materials and stainless

steelssteels

••Colder arc Colder arc �� used only for used only for

arc stability when welding arc stability when welding

wide gapswide gaps

••ConvexConvex weld, more spatterweld, more spatter

••Improved weld pool controlImproved weld pool control

••Recommended on thin Recommended on thin

materialsmaterials

Terminating the arc

• Burnback time

– delayed current cut-off to prevent wire freeze in the

weld end crater

– depends on WFS (set as short as possible!)

Contact tipContact tip

WorkpieceWorkpiece

Burnback timeBurnback time 0,05 sec0,05 sec 0,10 sec0,10 sec 0,15 sec0,15 sec

14 mm 14 mm

8 mm 8 mm 3 mm 3 mm

Current Current -- 250A250A

Voltage Voltage -- 27V27V

WFS WFS -- 7,8 m/min7,8 m/min

Wire Wire diamdiam. . -- 1,2 mm1,2 mm

Shielding gas Shielding gas -- Ar+18%COAr+18%CO22

Insulating Insulating

slagslag

• Crater fill

MIG/MAG process variables

• Welding current

• Polarity

•Increasing welding current

•Increase in depth and width

•Increase in deposition rate


• Arc voltage

• Travel speed

•Increasing travel speed

•Reduced penetration and width, undercut

•Increasing arc voltage

•Reduced penetration, increased width

•Excessive voltage can cause porosity,

spatter and undercut

Penetration Deep Moderate ShallowPenetration Deep Moderate Shallow

Excess weld metal Maximum Moderate MinimumExcess weld metal Maximum Moderate Minimum

Undercut Severe Moderate MinimumUndercut Severe Moderate Minimum


Electrode orientation

• Electrode

extension

••Increased extensionIncreased extension


Types of Shielding GasTypes of Shielding Gas

MIG (Metal Inert Gas)MIG (Metal Inert Gas)

•• Inert Gas is required for Inert Gas is required for all nonall non--ferrous alloysferrous alloys (Al, Cu, Ni)(Al, Cu, Ni)

•• Most common inert gas is ArgonMost common inert gas is Argon

•• Argon + Helium used to give a Argon + Helium used to give a ‘‘hotterhotter’’ arc arc -- better for thicker better for thicker

joints and alloys with higher thermal conductivityjoints and alloys with higher thermal conductivity


Types of Shielding GasTypes of Shielding Gas

MAG (Metal Active Gas)

• Active gases used are Oxygen and Carbon Dioxide

• Argon with a small % of active gas is required for all steels (including stainless steels) to ensure a stable arc & good droplet

wetting into the weld pool

• Typical active gases areAr + 20% CO2 for C-Mn & low alloy steelsAr + 2% O2 for stainless steels100% CO2 can be used for C - steels

MIG/MAG – shielding gases

Type of materialType of material Shielding gasShielding gas

Carbon steelCarbon steel

Stainless steelStainless steel

AluminiumAluminium

COCO22 , Ar+(5, Ar+(5--20)%CO20)%CO22

Ar+2%OAr+2%O22

ArAr


Pulsed TransferPulsed Transfer

TransferTransfer--mode advantagesmode advantages

•• Good fusionGood fusion

•• Small weld pool allows allSmall weld pool allows all--position weldingposition welding

TransferTransfer--mode disadvantagesmode disadvantages

•• More complex & expensive power sourceMore complex & expensive power source

•• Difficult to set parameters Difficult to set parameters -- requires power source requires power source

manufacturer to provide pulse programmes to suit wire type, manufacturer to provide pulse programmes to suit wire type,

dia. and type of gasdia. and type of gas


Types of Filler WireTypes of Filler Wire

•• Filler wires have similar composition to the base materialFiller wires have similar composition to the base material

•• Wires can be solid, flux cored or metal coredWires can be solid, flux cored or metal cored

•• Flux cored wires are designed to run in spray mode and thereforeFlux cored wires are designed to run in spray mode and therefore they they

give good fusiongive good fusion

•• Flux cored wires cannot be used for root runs on unbacked jointsFlux cored wires cannot be used for root runs on unbacked joints

•• The slag formed from flux cored wire enables welding to be done The slag formed from flux cored wire enables welding to be done in allin all--

positionspositions

•• Most flux cored wires have a folded seam that can allow moistureMost flux cored wires have a folded seam that can allow moisture to get to get

into the fluxinto the flux

•• Controlled storage & handling is required for Controlled storage & handling is required for ‘‘seamedseamed’’ wireswires

•• Metal cored wires have the same general characteristics as solidMetal cored wires have the same general characteristics as solid wires wires --

they can be operated in dip or spray modethey can be operated in dip or spray mode

•• Some flux cored wires do not require a gas shield (Some flux cored wires do not require a gas shield (InnershieldInnershield))

The welding equipment

A visual check should be made on the equipment to ensure it is in good

working order

The electrodes

The diameter, specification and the quality of the wire are essential for

inspection. The level of deoxidisation in the wire, single, double or triple

de-oxidised. The quality of the wire winding and the copper coating

should also be inspected to minimize wire feed problems.

Checks when MAG Welding

Wire liner

Check that the liner is the correct type and size for the wire being used.

Steel liners for steel and Teflon liners for aluminium.

Contact tips

Check the tip is the correct size for the wire being used and check the

amount of wear. Excessive wear will affect wire speed and electrical

current pick-up

Gas and gas flow-rates

Type of gas and the flow rate need to be checked to ensure they comply

with the WPS

Other welding variables

Check WFS, amps, volts and travel speed

Checks when MAG Welding

Most welding imperfections in MIG/MAG are caused by lack of

welder skill, or incorrect settings of the equipment

•Worn contact tips will cause poor power pick up, or transfer

•Bad power connections will cause a loss of voltage in the arc

•Silica inclusions (in Fe steels) due to poor inter-run cleaning

•Lack of fusion (primarily with dip transfer)

•Porosity (from loss of gas shield on site etc)

•Solidification problems (cracking, centerline pipes, crater pipes) especially on deep narrow welds

MIG/MAG typical defects

AdvantagesAdvantages DisadvantagesDisadvantages

�� High productivity

� Easily automated

� All positional (dip & pulse)

� Material thickness

range

� Continuous electrode

�� Lack of fusion (dip)

� Small range of consumables

� Protection on site

� Complex equipment

� Not so portable


QU 1. State the possible problems when using the dip

transfer mode in the MAG welding process

QU 2. State the application areas for the spray transfer mode when using the MAG welding process.

QU 3. What power source characteristic is required and

electrode polarity/current type for the MAG welding process

QU 4. State the main variables for the MAG welding process

QU 5. State the advantages and disadvantages of the MAG

welding process when compared to MMA

Questions

Metal Active Gas Welding

Flux Core Arc Welding

WELDING PROCESS

Flux cored arc welding

FCAW FCAW

methodsmethods

With gas With gas

shielding shielding --

““OutershieldOutershield””

Without gas Without gas


““InnershieldInnershield””

With metal With metal

powder powder --

““Metal coreMetal core””

“Outershield” - principle of operation

“Innershield” - principle of operation

Arc Characteristics

VoltsVolts

AmpsAmps

OCVOCV

Constant Voltage CharacteristicConstant Voltage Characteristic

Small change in voltage = Small change in voltage =

large change in amperagelarge change in amperage

The self The self

adjusting arc.adjusting arc.

Large arc gapLarge arc gap

Small arc gapSmall arc gap

Insulated extension nozzleInsulated extension nozzle

Current carrying guild tube Current carrying guild tube

Flux cored hollow wire Flux cored hollow wire

Flux powderFlux powder

Arc shield composed of Arc shield composed of vaporized and slag forming vaporized and slag forming compounds compounds

Metal droplets covered Metal droplets covered

with thin slag coating with thin slag coating

Molten Molten weld weld poolpoolSolidified weld Solidified weld

metal and slagmetal and slag

Flux coreFlux core

Wire jointWire joint

Flux core Flux core wireswires

Flux Core Arc Welding (FCAW)

Flux cored arc welding

FCAW FCAW

methodsmethods

With gas With gas


““OutershieldOutershield””

Without gas Without gas


““InnershieldInnershield””

(114)(114)

With metal With metal

powder powder --

““Metal coreMetal core””

With active With active

gas shielding gas shielding

(136)(136)

With inert gas With inert gas

shielding (137)shielding (137)

FCAW - differences from MIG/MAG

• usually operates in DCEP but some “Innershield”wires operates in DCEN

• power sources need to be more powerful due to the higher currents

• doesn't work in deep transfer mode

• require knurled feed rolls

• “Innershield” wires use a different type of

welding gun


350 Amps self shielded welding gun

Courtesy of Lincoln Electric

Contact tip

Thread protector

Conductor tube

Trigger

Handle

Hand shield

24V insulated

switch lead

Welding

gun cable

Close wound stainless steel spring wire liner

(inside welding gun cable)


Self shielded electrode

nozzle

Travel Angle

75° 75°90°

Backhand (“drag”) technique

Advantages

• preferred method for flat or horizontal position

• slower progression of the weld

• deeper penetration

• weld stays hot longer � easy to remove dissolved gasses

Disadvantages

• produce a higher weld profile

• difficult to follow the weld joint

• can lead to burn-through on thin sheet plates

Forehand (“push”) technique

Advantages

• preferred method for vertical up or overhead position

• arc is directed towards the unwelded joint �preheat effect

• easy to follow the weld joint and control the

penetration

Disadvantages• produce a low weld profile, with coarser ripples

• fast weld progression � shallower depth of

penetration

• the amount of spatter can increase

FCAW advantages

• less sensitive to lack of fusion

• requires smaller included angle compared to MMA

• high productivity

• all positional

• smooth bead surface, less danger of undercut

• basic types produce excellent toughness properties

• good control of the weld pool in positional welding

especially with rutile wires

• seamless wires have no torsional strain � twist free

• ease of varying the alloying constituents

• no need for shielding gas

FCAW advantages

Deposition rate for carbon steel welding

FCAW disadvantages

• limited to steels and Ni-base alloys

• slag covering must be removed

• FCAW wire is more expensive on a weight basis than solid wires (exception: some high alloy steels)

• for gas shielded process, the gaseous shield may be affected by winds and drafts

• more smoke and fumes are generated compared

with MIG/MAG

• in case of Innershield wires, it might be necessary to

break the wire for restart (due to the high amount of insulating slag formed at the tip of the wire)

Advantages:Advantages:

1) Field or shop use1) Field or shop use

2) High productivity2) High productivity

3) All positional3) All positional

4) Slag supports and 4) Slag supports and

shapes the weld Beadshapes the weld Bead

5) No need for shielding 5) No need for shielding

gasgas

Disadvantages:Disadvantages:

1) High skill factor1) High skill factor

2) Slag inclusions2) Slag inclusions

3) Cored wire is 3) Cored wire is

ExpensiveExpensive

4) High level of fume 4) High level of fume

((InnershieldInnershield))

5) Limited to steels and 5) Limited to steels and

nickel alloysnickel alloys

FCAW advantages/disadvantages

09 Mig Mag Welding 2006

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