chapter 5 - gas shielded metal arc welding.pdf

8/11/2019 Chapter 5 - Gas Shielded Metal Arc Welding.pdf

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5. Gas-Shielded Metal Arc Welding 61

2005

The difference betweengas-shielded metal arc welding (GMA)and the gas tungsten arc

welding process is the consumable electrode. Essentially the process is classified as metal

inert gas welding (MIG)

and metal active gas

welding (MAG). Besides,

there are two more process

variants, the electrogas

and the narrow gap weld-

ing and also the gas-

shielded plasma metal arc

welding, a combination of

both plasma welding and

MIG welding, Figure 5.1.

In contrast to TIG welding,

where the electrode is

normally negative in order to avoid the melting

of the tungsten electrode, this effect is ex-

ploited in MIG welding, as the positive pole is

strongly heated than the negative pole, thus

improving the melting characteristics of the

feed wire.

Figure 5.2 shows the principle of a GMA weld-

ing installation. The welding power source is

assembled using the following assembly

groups: The transformer converts the mains

voltage to low voltage which is subsequently

rectified.

Apart from the torch cooling and the shielding

gas control, the process control is the most

important installation component. The process

control ensures that once set welding data areadhered to.

ISF 2002

gas-shielded arc

welding (SG)

Classification of Gas-ShieldedArc Welding Processes

br-er5-01e.cdr

gas-shielded metal-arcwelding (GMAW)

tungsten gas-shielded welding

metal inertgas welding

(MIG)

plasma jetplasma

arcwelding(WPSL)

plasmaarc

welding

(WPL)

Narrow-gap gas-shielded arc

welding (MSGE)

electrogaswelding(MSGG)

plasma gasmetal arcwelding

(MSGP)

gas mixturemetal-arcwelding

(GMMA)

gas metal-arc CO

welding

(MAGC)

2

hydrogentungsten arc

welding

(WHG)

plasmajet

welding

(WPS)

metalactive gaswelding

(MAG)

tungsteninert-gaswelding

(TIG)

tungstenplasmawelding

(WP)

consumable electrode non consumable electrode

Figure 5.1

wire feed unit

watercooling

shielding gascontrol device

control switch

cooling watercontrol

rectifier

transformer

welding power source

GMA Welding Installation

br-er5-02e.cdr ISF 2002

Figure 5.2


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2005

A selection of common welding installation variants is depicted in Figure 5.3, where the

universal device with a separate wire feed housing is the most frequently used variant in the

industry.

Figure 5.4 shows in detail a manually operated inert-gas shielded torchwith the common

swan-neck shape. A machine torch has no handle and its shape is straight or swan-necked.

The hose package contains the wire core and also supply lines for shielding gas, current and

cooling water, the latter for contact tube cooling. The current is transferred to the wire elec-trode over the contact tube. The shielding gas nozzle is shaped to ensure a steady gas flow

in the arc space, thus protecting arc and molten pool against the atmosphere.

A so-called Two-Wire-Drive wire feed system is of the most simple design,as shown in

Figure 5.5. The wire is pulled off a wire reel and fed into the hose package. The wire trans-

port roller, which shows different grooves depending on the used material, is driven by an

electric motor. The counterpressure roller generates the frictional force which is needed for

wire feeding.

ISF 2002br-er5-04e.cdr

Manual Gas-ShieldedArc Welding Torch

1 torch handle 2 torch neck 3 torch trigger 4 hose package 5 shielding gas nozzle 6 contact tube 7 contact tube fixture 8 insulator 9 wire core10 wire guide tube11 wire electrode12 shielding gas supply13 welding current supply

Figure 5.4


Types of Welding Installations

compact device universal device

mini-spool device push-pull device

10, 20 or 30m 5 to 10m

3 to 5m5, 10 or 20m

3 to 5m

Figure 5.3


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2005

More complicated but following the same operation principle is the Four-Wire-Drive,Fig-

ure 5.6. Here, the second pair of rollers guarantees higher feeding reliability by reducing the

risk of wheel slip. Another design among the wire feed drive systems is the planetary drive,

where the wire is fed in axial direction by the motor. A rectilinear rotation-free wire feed mo-

tion is the outcome of the

motor rotation and the an-

gular offset of the drive

rollers which are firmly

connected to the motor

shaft.

Figure 5.7 depicts the

metal transfer in the short

arc range. During the

burning phase of the arc,

material is molten and ac-


Wire Drives

4-roller drive

1 wire guide tube2 drive rollers3 counter pressure rollers4 wire guide tube

3 4 3

3

3

1

1

1

2 2

2

1 wire guide tube2 roller holding device3 drive rollers

planetary drive

direction ofrotation

Figure 5.6


Wire Feed System

1

2

4 2

F

65

1 wire reel

2 wire guide tube

5 wire feed roll with a V-groove for steel electrodes

6 wire feed roll with a rounded groove for aluminium

3 wire transport roll

4 counter pressure roll

4 4 3

Figure 5.5

ISF 2002

Short-Circuiting Arc Metal Transfer

br-er5-07e.cdr

1 ms

1 mm

time

time

weldingcurrent

weldingvoltage

Figure 5.7


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2005

cumulates at the electrode end. The voltage drops slowly while the arc shortens. Electrode

and workpiece make contact and a short-circuit occurs. In the short-circuit phase is the liquid

electrode material drawn as

result of surface tension into

the molten pool. The nar-

rowing liquid root and the

rising current lead to a very

high current density that

causes a sudden evapora-

tion of the remaining root.

The arc is reignited. The

short-arc technique is par-

ticularly suitable for out-of-

position and root passes

welding.

ISF 2002

Choke Effect

br-er5-08e.cdr

timetime

weldingcurrent

weldingcurrent

choke effectlow medium

Figure 5.8


Long Arc

weldingvoltage

weldingcurrent

time

time

Figure 5.9


Spray Arc

weldingvoltage

weldingcurrent

time

time

Figure 5.10


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The limitation of the rate of the current riseduring the short-circuit phase with a choke

leads to a pointed burn-offprocess which is smoother and clearly shows less spatter forma-

tion, Figures 5.8

In shielding gases with a

high CO2 proportion a

long arc is formed in the

upper power range, Figure

5.9. Material transfer is

undefined and occurs as

illustrated in Figures 5.13

and 5.14. Short-circuits

with very strong spatter

formation are caused by

the formation of very large

droplets at the electrode

end.

If the inert gas content of the shielding gas

exceeds 80%, a spray arcforms in the upper

power range, Figure 5.10. The spray arc is

characterised by a non-short-circuiting and

spray-like material transfer. For its high deposi-

tion rate the spray arc is used for welding filler

and cover passes in the flat position.

Connections between welding parameters,

shielding gas and arc typeare shown in Fig-

ure 5.11. When the shielding gas M23 is used,

the spray arc may already be produced with an

amperage of 260 A. With the decreasing argon

proportion the amperage has to be increased

in order to remain in the spray arc range. When

pure carbon dioxide is applied, the spray arc

ISF 2002

Welding Parameters in Dependence onthe Shielding Gas Mixture (SG 2, 1,2 mm)

br-er5-11e.cdr

weldingvoltage

150 200 250 300A

15

20

25

V

35

contact tube distance: approx. 15 mm

spray arc

long arc

short arccontact tube distance: approx. 19 mm

mixedcircuiting arc

C1

M21

M23

welding current

wire feed5,53,5 4,5 7,0 8,0 10,5m/min

shielding gas composition:C1: CO

M21: 82% Ar, 18% CO

M23: 92% Ar, 8% O

2

2

2

Figure 5.11


argon helium

argon

helium

temperature

thermalconductivity

hydrogen

nitrogen

CO2

CO282%Ar+18%CO2

Figure 5.12


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2005

cannot be produced. Figure 5.11 shows, moreover, that with the increasing CO2 content the

welding voltage must also be increased in order to achieve the same deposition rate.

The different thermal conductivity of the

shielding gases has a considerable influence

on the arc configuration and weld geometry,

Figure 5.12. Caused by the low thermal con-

ductivity of the argon the arc core becomes

very hot this results in a deep penetration in

the weld centre, the so-called argon finger-

type penetration. Weld reinforcement is

strongly pronounced. Application of CO2 and

helium leads, due to the better thermal conduc-

tivity of these shielding gases, to a wide and

deep penetration.

A recombination (endothermic break of the linkage in the arc space exothermal reaction

2CO + O2->2CO2in the workpiece proximity) intensifies this effect when CO2is used.

In argon, the current-carrying arc core is wider and envelops the wire electrode end, Figure

5.13. This generates electromagnetic forces which bring about the detachment of the liquid

electrode material. This so-called pinch effect causes a metal transfer in small drops, Fig-ure 5.14.


wire elektrodes

current-carryingarc core

argon carbon dioxide

Figure 5.14

Figure 5.13

ISF 2006

Influence of Shielding Gason Forces in the Arc Space

br-er5-13e.cdr

current-carrying

arc core

argon carbon dioxider

argon carbon dioxide

r

temperature

Fr

FaF

Fa

F

Fr


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2005

The pointed shape of the arc attachment in

carbon dioxide produces a reverse-direction

force component, i.e., the molten metal is

pushed up until gravity has overcome that

force component and material transfer in the

form of very coarse drops appear.

Besides the pinch effect, the inertia and the

gravitational force, other forces, shown in Fig-

ure 5.15, are active inside the arc space;

however these forces are of less importance.

If the welding voltage and the wire feed speed

are further increased, a rotating arc occurs

after an undefined transition zone, Figure

5.16. High-efficiency MAG welding has

been applied since the beginning of the nine-

ties; the deposition rate, when this process is

used, is twice the size as, in comparison, to spray arc welding. Apart from a multicomponent

gas with a helium proportion, also a high-rating power source and a precisely controlled wire

feed system for high wire feed speeds are necessary.

Figure 5.17 depicts the

deposition rates over the

wire feed speed, as achiev-

able with modern high-

efficiency MAG welding

processes.

During the transition from

the short to the spray arc

the drop frequency rate in-

creases erratically while thedrop volume decreases at


Forces in Arc Space

work piece

electrostaticforces

surfacetension S

accelerationdue to gravity

wire electrode

viscosity

droplets neckingdown

inertia

suction forces,plasma flowinduced

electromagneticforce F(pinch effect)

L

backlash forces f

of the evaporatingmaterial

r

Figure 5.15

Figure 5.16

Rotating Arc



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2005

the same degree. With an

increasing CO2-content,

this critical current

rangemoves up to higher

power ranges and is, with

inert gas constituents of

lower than 80%, hardly

achievable thereafter. This

effect facilitates the

pulsed-arc welding tech-

nique, Figure 5.18.

In pulsed-arc welding, a

change-over occurs be-

tween a low, subcriticalbackground current and a high, supercritical pulsed current. During

the background phasewhich corresponds with the short arc range, the arc length is ionised

Setting parameters:

- background current I- pulse voltage U

- impulse time t

- background time

t or frequency f with

f = 1 / ( t + t ), resp.

- wire feed speed v

G

P

P

G

G P

D

300 300

time

200

IG Im Ikrit

400 600

tG

tP

200 200

100 100

0 00

dropvolume

numberofdroplets 1/s 10 cm

-4 3

critical currentrange

A


Pulsed Arc

Figure 5.18


500

time

arcvoltage

150 5 10 20 300

50

100

150

200

250

300

350

400

5

10

15

20

25

35

A

V

weldingcurrent

ms

Um

Im

IEff

UEff

Figure 5.19

ISF 2002

Deposition Rate

br-er5-17e.cdr

conventionalGMA

0,8 mm

1,0 mm

1,2 mm

wire feed speed

deposition

rate

m/min

kg/hhigh performanceGMA welding

25

20

15

10

5

00 5 10 15 20 25 30 35 40 45

Figure 5.17


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2005

and wire electrode and work

surface are preheated. Dur-

ing the pulsed phase the

material is molten and, as in

spray arc welding, super-

seded by the magnetic

forces. Figure 5.20.

Figure 5.19 shows an ex-

ample of pulsed arc real

current path and voltage

time curve. The formula for

mean current is:

=T

0

midt

T

1I

for energy per unit length of weld is:

=T

0

2eff dti

T

1I

By a sensible selection of welding parameters, the GMA welding technique allows a selection

of different arc types which

are distinguished by their

metal transfer way. Figure

5.21 shows the setting

range for a good welding

process in the field of con-

ventional GMA welding.

Figure 5.22 shows the ex-

tended setting range for the

high-efficiency MAGM weld-

ing process with a rotating

arc.

ISF 2002

Parameter Setting Rangein GMA Welding

br-er5-21e.cdr

optimal settinglower limitupper limit

working range welding current / arc voltage

400325

50

10

15

20

25

30

35

40

45

50 75 100 125 150 175 200 225 250 275 300 350 375

spray arc

transition arc

short arcshielding gas: 82%Ar, 18%CO2wire diameter: 1,2 mmwire type: SG 2

voltage

[v]

welding current

Figure 5.21

we

ldingcurrent

pulsed current intensity

Non-short-circuitingmetal tranfer range

backround currentintensity

time

Pulsed Metal Transfer

br-er5-20e.cdr isf 2002

Figure 5.20


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2005

Some typical applicationsof the different arc types are depicted in Figure 5.23. The rotating

arc, (not mentioned in the figure), is applied in just the same way as the spray arc, however,

it is not used for the welding of copper and aluminium.

The arc length within the

working range is linearly

dependenton the set weld-

ing voltage, Figure 5.24.

The weld seam shape is

considerably influenced by

the arc length. A long arc

produces a wide flat weld

seam and, in the case of

fillet welds, generally under-

cuts. A short arc produces a

narrow, banked weld bead.

On the other hand, the arc length is inversely proportional to the wire feed speed,Figure

5.25. This has influence on the current over the internal adjustment with a slightly dropping

power source characteristic. This again is of considerable importance for the deposition rate,

i.e., a low wire feed speed leads to a low deposition rate, the result is flat penetration and low

base metal fusion. At a constant weld speed and a high wire feed speed a deep penetration

can be obtained.

At equal arc lengths, the

current intensity is de-

pendent on the contact

tube distance, Figure 5.26.

With a large contact tube

distance, the wire stickout is

longer and is therefore

characterised by a higher

ohmic resistance whichleads to a decreased current

ISF 2002

Applications of Different Arc Types

br-er5-23e.cdr

arc types

applications

spray arc long arc short arc pulsed arc

MIG

MAG

M

MAGC

weldingme

thods

seamtype,positions

workpiecethickness

aluminiumcopper

aluminiumcopper

aluminium(s < 1,5 mm)

steel unalloyed, low-alloy, high-alloy steel unalloyed,low-alloy

steel unalloyed,low-alloy

steel unalloyed,low-alloy

steel unalloyed, low-alloy,high-alloy steel low-alloy,high-alloy

-

-

-

fillet welds or butt weldsat thin sheets, all positions

root layers of butt welds

all positions

inner passes and coverpasses of fillet or buttwelds in positionPC, PD, PE, PF, PG(out-of-position)

at medium-thick or thickcomponents,

fillet welds or innerpasses and coverpasses of thin andmedium-thickcomponents, allpositions

root layer welds onlyconditionally possible

fillet welds or innerpasses and coverpasses of butt weldsat medium-thick or thickcomponents in positionPA, PB

fillet welds or innerpasses and coverpasses of butt weldsat medium-thick or thickcomponents in positionPA, PB

welding of root layers inposition PA

Figure 5.23

Setting Range or Welding Parametersin Dependence on Arc Type

br-er5-22e.cdr Quelle: Linde, ISF2002

10

20

30

50

V

voltage

high-efficiencyspray arc

rotatingarc

transition zones

short arc

high-efficiency short arc

100 200 300 400 600A

filler metal: SG2 -1,2 mmshielding gas: Ar/He/CO /O -65/26,5/8/0,52 2

welding current

spray arc

Figure 5.22


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2005

intensity. For the adjustment of the contact

tube distance, as a thumb rule, ten to twelve

times the size of the wire diameter should be

considered.

The torch position has considerable influ-

ence on weld formation and weldingproc-

ess, Figure 5.27. When welding with the torch

pointed in forward direction of the weld, a part

of the weld pool is moved in front of the arc.

This results in process instability. However, it

ha s the advantage of a flat smooth weld sur-

face with good gap bridging. When welding

with the torch pointed in reversing direction of

the weld, the weld process is more stable and

the penetration deeper, as base metal fusion


Welding Voltage

weld appearancebutt weld

weld appearancefillet weld

operating point:welding voltage:arc length:

highlong

mediummedium

lowshort

arc length:longmediumshort

U

v , ID

ALAMAK

AL AM AK

Figure 5.24


Wire Feed Speed

operating point:

wire feed speed:

arc length:

welding current:

deposition efficiency:

low

long

low

low

AL

medium

medium

medium

medium

high

short

high

high

AM AK

weld appearance:

arc length:

long

medium

short

v , ID

U

ALAM

AK

Figure 5.25


Contact Tube-to-Work Distance

lk1 lk2 lk3

wire electrode:

shielding gas:

arc voltage:

wire feed speed:

welding speed:

1,2 mm diameter

82% Ar + 18% CO

29 V

8,8 m/min

58 cm/min

2

contacttube-to-work

distancelk

mm

current

30

20

10

0200 250 A300 350

3

2

1

operating rule:

l = 10 to 12 dk D

Figure 5.26


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2005

by the arc is better, although the weld bead

surface is irregular and banked.

Figure 5.28 shows a selection of different ap-

plication areas for the GMA technique and the

appropriate shielding gases.

The welding current may be produced by dif-

ferent welding power sources. In d.c.welding

the transformer must be equipped with down-

stream rectifier assemblies, Figure 5.29. An

additional ripple-filter choke suppresses the

residual ripple of the rectified current and has

also a process-stabilising effect.

With the development of efficient transistors

the design of transistor analogue power

sources became possible, Figure 5.29. The

operating principle of a transistor analogue

power source follows the principle of an audio frequency amplifier which amplifies a low-level

to a high level input signal, possibly distortion-free. The transistor power source is, as con-

ventional power sources, also equipped with a three-phase transformer, with generally only

one secondary tap. The secondary voltage is rectified by silicon diodes into full wave opera-

tion, smoothed by capacitors

and fed to the arc through a

transistor cascade. The

welding voltage is steplessly

adjustable until no-load volt-

age is reached. The differ-

ence between source volt-

age and welding voltage

reduces at the transistor

cascade and produces a

comparatively high straypower which, in general,


Torch Position

penetration:

gapbridging:

arcstability:

spatter formation:

weld width:

weld appearance:

shallow average

average

average

average

average

average

bad

bad

good

good

low

smooth rippled

narrowwide

deep

strong

advance direction

Figure 5.27

ISF 2002

Fields of Application ofDifferent Shielding Gases

br-er5-28e.cdr

Argon4

.6

Argon4

.8

Helium

4.6

Ar/He-m

ixture

Ar+5%

H

or7,5%

H

99%Ar

+1%

O

or

97%Ar

+3%

O

97,5%

Ar+2,5%

CO

83%Ar

+15%

He+2%

CO

90%Ar

+5%

O

+5%

CO

80%Ar

+5%

O

+15%

CO

92%Ar

+8%

O

88%Ar

+12%

O

82%Ar

+18%

CO

92%Ar

+8%

CO

forming

gas(N-H-mixture)

2

2

2 2

2

2

2

2

2

2

2

2

2

2

2

2

autoclaves, vessels, mixers, cylinderspanelling, window frames, gates, gridsstainless steel pipes, flanges, bendsspherical holders, bridges, vehicles, dump bodiesreactors, fuel rods, control devicesrocket, launch platforms, satellitesvalves, sliders, control systemsstator packages, transformer boxespassenger cars, trucksradiators, shock absorbers, exhaustscranes, conveyor roads, excavators (crawlers)shelves (chains), switch boxesbraces, railings, stock boxesmud guards, side parts, tops, engine bonnetsattachments to flame nozzles, blast pipes, rollersvessels, tanks, containers, pipe linesstanchions, stands, frames, cagesbeams, bracings, cranewaysharvester-threshers, tractors, narrows, ploughswaggons, locomotives, lorries

chemical-apparatus engineeringshopwindow constructionpipe productionaluminium-working industrynuclear engineeringaerospace engineeringfittings productionelectrical engineering industryautomotive industrymotor car accessoriesmaterials-handling technologysheet metal workingcraftsmotor car repairsteel productionboiler and tank constructionmachine engineeringstructural steel engineeringagricultural machine industryrail car production

industrial sections shieldinggases

application examples

Figure 5.28


14/16


15/16


2005

300 500 sclearly longer

than those of analogue

power sources.

Series regulator power

sources, the so-called in-

verter power sources, dif-

fer widely from the afore-

mentioned welding ma-

chines, Figure 5.31. The

alternating voltage coming

from the mains (50 Hz) is

initially rectified, smoothed

and converted into a me-

dium frequency alternating voltage (approx. 25-50 kHz) with the help of controllable transistor

and thyristor switches. The alternating voltage is then transformer reduced to welding voltage

levels and fed into the welding process through a secondary rectifier, where the alternating

voltage also shows switching frequency related ripples. The advantage of inverter power

sources is their low weight. A transformer that transforms voltage with frequency of 20 kHz,

has, compared with a 50 Hz transformer, considerably lower magnetic losses, that is to say,

its size may accordingly be smaller and its weight is just 10% of that of a 50 Hz transformer.

Reaction time and effi-

ciency factor are compa-

rable to the corresponding

values of switching-type

power sources.

All welding power sources

are fitted with a rating

plate, Figure 5.32. Here

the performance capability

and the properties of thepower source are listed.

ISF 2002

GMA Welding Power Source, Electronically

Controlled, Primary Chopped, Inverter

br-er5-31e.cdr

weldingcurrent

mains

supply

Uist

Iist

filter

reference inputvalues

signal processor(analog-to-digital)

currentpickup

transistorinverter

energystorage

3-phasebridgerectifier rectifier

U . . U1 n

mediumfrequency

transformer

Figure 5.31

ISF 2002

Rating Plate

br-er5-32e.cdr

Spower range

power capacity

in dependence

of current flow

power supply

manufacturer

rotary current welding rectifier

VDE 0542

type productionnumberswitchgearnumber

protectivesystem

DIN 40 050

F F

IP21

35A/13V - 220A/25V

220

25

60%

15380

26

6,6 0,72

220 17

10

100%

15 - 38 23

170

insulationsclass

coolingtype

~_

X

I2

U2

I1U1

U1

U1

U1

I1

I1

I1

U0 V

EDED

A

A A

AV

V

V

V

A

A A

A A

A

V V

welding

MIG/MAG

input

3~50Hz

kVA (DB) cos

min. and max. no-load voltage

Figure 5.32


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chapter 5 - gas shielded metal arc welding.pdf

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