4. TIG Welding and Plasma Arc Weldingmercury.kau.ac.kr/welding/Welding Technology I - Welding...

4.

TIG Welding and

Plasma Arc Welding

4. TIG Welding and Plasma Arc Welding 49

2005

TIG welding and plasma welding belong to the group of the gas-shielded tungsten arc weld-

ing processes, Figure 4.1. In the gas-shielded tungsten arc welding processes mentioned in

Figure 4.1, the arc burns between a non- consumable tungsten electrode and the work-

piece or, in plasma arc weld-

ing, between the tungsten

electrode and a live copper

electrode inside the torch.

Exclusively inert gases (Ar,

He) are used as shielding

gases.

The potential curve of the

ideal arc, as shown in Figure

4.2, can be divided into

three characteristic sectors:

1.cathode- drop region

2.arc

3. anode-drop region

In the cathode-drop region

almost 50% of the total

voltage drop occurs over a

length of 10-4 mm.

A similarly high voltage drop

occurs in the anode-drop

region, here, however, over

a length of 0.5 mm. The

voltage drop on the remain-

ing arc length is compara-

tively low. Main energy con-

version occurs accordingly

in the anode-drop and cathode-drop region.

Figure 4.3 shows the potential distribution by the example of a real TIG arc under the influ-

ence of different shielding gases. UA and UK have different values, the potential curve in the

© ISF 2002

Classification of Gas-Shielded Arc Welding acc. to DIN ISO 857

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Plasma arc welding with

semi-transferred arc

Plasma arc welding

with transferred arc


non-transferred arc

CO welding2 Mixed gas welding

narrow-gap gas-shielded arc welding

plasma metalarc welding

electrogas welding

Metal inert-gas welding

MIG

Metal active gas welding

MAG

Gas-shielded arc welding

Tungsten hydrogen welding

Tungsten plasma welding with

electrode

Tungsten inert-gas welding

TIG

Gas-shielded metal arc welding

GMAW

Gas-shielded arc welding

tungsten

Figure 4.1

© ISF 2002

Arc Potential Curve

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U

V

20

10

01 2 3 4 5

10-4

0,5

l

US

lmm

K

L

A

+-

A:

K:

L:

l:

anode spot (up to 4000°C)

cathode spot (approx. 3600°C)

arc column (4500-20000°C)

arc length

arc potential curve(example)

Figure 4.2


2005

arc is not exactly linear. There is no discernible expansion of the cathode-drop and anode-

drop region.

The electrical characteris-

tics of the arc differ, de-

pending on the selected

shielding gas, Figure 4.4. As

the ionisation potential of

helium in comparison with

argon is higher, arc voltage

must necessarily be higher.

The temperature distribu-

tion of a TIG arc is shown in

Figure 4.5.

© ISF 2002br-er4-04e.cdr

arc

voltag

e

25

20

15

10

arc

le

ngth

4

2

4

2

helium

argon

weld current

50 100 150 200 250 3500

mmV

A

Figure 4.4

© ISF 2002br-er4--03e.cdr

X

X

0

0

1

1

2

2

3

3

4

4

6

6

20

40

10

20

5

10

U

U

anode

anode

cathode

cathode

U = 6,5 VK

U = 6,5 VK

U = 3,5 VA

U = 6,1 VA

Argon

60 A

Helium

60 AV

V

mm

mm

ARC

ARC

ARC

ARC

Figure 4.3

© ISF 2002

Temperature Distribution in aTIG Arc (at I=100 A)

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TIG cathode

10

00

0 K

9 0

00 K

8 0

00 K

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

anodespot

weld pool

2

mm

4

6

8

2

mm

4

6

8 4 3 2 1 0 1 2 mm 4

Figure 4.5


2005

In TIG welding just approximately 30% of the

input electrical energy may be used for

melting the base metal, Figure 4.6. Losses

result from the arc radiation and heat dissipa-

tion in the workpiece and also from the heat

conversion in the tungsten electrode.

Figure 4.7 describes the process principle

of TIG welding.

Figure 4.8 explains by an example the code

for a TIG welding wire, as stipulated in the

drafts of the European Standardisations.

A table with the chemical compositions of the

filler materials is shown in Figure 4.9.

According to Figure 4.10, a conventional

TIG welding installation

consists of a transformer, a

set of rectifiers and a torch.

For most applications an

electrode with a negative

polarity is used. However,

for welding of aluminium,

alternating current must be

used. For arc ignition a

high-frequency high volt-

age is superimposed and

causes ionisation between

electrode and workpiece.

© isf 2002

Tungsten Inert Gas Welding (TIG)

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tungsten electrode

electric contact

shielding gas

shielding gas nozzle

filler metal

weld

arc

workpiece

welding powersource

Figure 4.7


melting of wire

welding direction

radiation

R.I2

P = U.I

thermal conductivity [W/m K]

fusion heat [kJ/kg]

specific heat [kJ/kg K]

Figure 4.6


2005

The central part of the

torch for TIG welding is

the tungsten electrode

which is held in a collet

inside the torch body, Fig-

ure 4.11. The hose pack-

age contains the supply

lines for shielding gas and

welding current. The

shielding gas nozzle is

mostly made of ceramic.

Manually operated torches

for TIG welding which are

used for high amperages

as well as machine torches for long duty cycles are water-cooled.

In order to keep the influence of torch distance variations on the current intensity and thus on

the penetration depth as low as possible, power sources used for TIG welding always have

a steeply dropping char-

acteristic, Figure 4.12.

The non-contact reigni-

tion of the A.C. TIG arc

after a voltage zero cross-

over requires ionisation of

the electrode-workpiece

gap by high-frequent

high voltage pulses, Fig-

ure 4.13.

© ISF 2002

Designation of a Tungsten InnertGas Welding Wire to EN 1668

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identification of filler rod as an individual product: W2

chemical composition table

rods and wires for tig-welding

minimum impact energy value 47 J at -30°C

minimum weld metal yield point: 460 N/mm2

identification letter for TIG-welding

W 46 3 W2

Figure 4.8

© ISF 2002

Chemical composition offiller rods and wires for TIG-welding

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Figure 4.9


2005

When argon is used as a

shielding gas, metals as,

for example, aluminium

and magnesium, which

have low melting points

and also simultaneously

forming tight and high melt-

ing oxide skins, cannot be

welded with a negative po-

larity electrode. With a

positive polarity, however,

a “cleaning effect” takes

place which is caused by

the impact of the positive

charged ions from the shielding gas atmosphere on the negative charged work surface, thus

destroying the oxide skin due to their large cross-section. However, as a positive polarity

© ISF 2002

Principle Structure of a TIG Welding Installation

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selector switch

high-frequency choke coilfilte

r capacito

r

transformer

SC: scattering core for adjusting the characteristic curve

main

s

high voltage impulse generator~

O_

O+

rectifier

St

L1L2L3NPE

=

~

Figure 4.10

torch capwith seal

handle of the torch

control switch

control cable

shieldinggas supply

cooling watersupply

cooling waterreturn withwelding currentcable

torch bodywith cooling device

electrode collet

colletcase

tungsten electrode

gas nozzle

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Construction of a Water-CooledT TIG Welding orch for

Figure 4.11

© isf 2002br-er4-12e.cdr

current intensity

longer arc shorter arc

R and U rise R and Udrop

I drops I rises

voltage

U

arc length

long

sh

ort

increasing

increasing

decreasing

decreasingi

Figure 4.12


2005

would cause thermal overload of the electrode, these materials are welded with alternating

current.

However, this has a disturbing side-effect. The electron emission and, consequently, the cur-

rent flow are dependent on the temperature of the cathode.

During the negative phase on the work surface the emission is, due to the lower melting tem-

perature substantially lower than during the negative phase on the tungsten electrode. As a

consequence, a positively connected electrode leads to lower welding current flows than this

would be the case with a negatively connected electrode, Figure 4.14. A filter capacitor in the

welding current circuit fil-

ters out the D.C. compo-

nent which results in equal

half-wave components.

With modern transistorised

power sources which use

alternating current (square

wave) for a faster zero

cross-over, is duration and

height of the phase com-

ponents adjustable. The

electrode thermal stress

and the cleaning effect

may be freely influenced.

Figure 4.15 shows that the

thermal electrode load

can be recognized from the

shape of the electrode tip.

While the normal-load

negative connected elec-

trode end has the shape of

a pointed cone (point angle

approx. 10°), a flattened

electrode tip is the result

© isf 2002

Influence of the Half-Wave Componentsduring A.C. TIG Welding

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smaller increasingheat load

of the electrode

cleaning effectlower stronger

ele

ctr

onic

con

tro

led

po

wer

sou

rce

witho

ut filte

rcapacito

r

bala

nce

d h

alf-w

ave c

om

po

nen

ts

ele

ctr

od

e p

ola

rity

time

time-

-

-

-

-

-

+

+

+

+

time- - -

+ +

with filt

er

capacitor

current

a

time

time

+ + +

- - -

+ + +

- - -0

0

time- - -

+ +

current

a

0

weld seam width

Figure 4.14

© ISF 2002

reignition of the arcby voltage impulses

++

- -

time

vo

ltage

Reignition of the A.C. Arc Through Voltage Impulses

Tungsten

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A.C.

Figure 4.13


2005

from a.c. welding (higher thermal load by positive half-waves).The tip of a thermally over-

loaded electrode is hemispherical and leads to a stronger spread of the arc and thus to wider

welds with lower penetration.

All fusion weldable materi-

als can be joined using the

TIG process; from the eco-

nomical point of view this

applies especially to plate

thickness of less than

5 mm. The method is,

moreover, predestined for

welding root passes

without backing support,

Figure 4.16.


Applications of TIG Welding

materials:- steels, especially high-alloy steel- aluminium and aluminium alloys- copper and copper alloys- nickel and nickel alloys- titanium- circonium- tantalum

workpiece thickness: - 0,5 - 5,0 mm

weld types: - plain butt weld, V-type welds, flanged weld, fillet weld - all positions - surfacing

application examples: - tube to tube sheet welding - orbital welding - root welding

Figure 4.16

Electrode Shapesfor TIG Welding

overloaded electrode

electrode for D.C. welding(direct current)

electrode for A.C. welding(alternating current)

influence of the electrodeshape on penetration profile


Figure 4.15

© ISF 2002

Flow Chart of TIG Orbital Welding

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preflow of theshielding gas

postflow of theshielding gas

movement in switch-on position

current decay overlappulsingpreheatingrise ofcurrent

shieldinggas

orbitalmovement

welding current

360 00

Figure 4.17


2005

For circumferential welding of fixed pipes, the TIG orbital welding method is applied. The

welding torch moves orbitrally around the pipe, i.e., the pipe is welded in the positions flat,

vertical down, overhead,

vertical-up and also inter-

pass welding is applied.

Moreover, a defect-free

weld bead overlap must be

achieved. Orbital welding

installations are equipped

with process operational

controls which determine

the appropriate process

parameters, Figure 4.17.

In plasma arc welding

burns the arc between the

tungsten electrode (- pole) and the plasma gas nozzle (+ pole) and is called the “non-

transferred” arc, Figure 4.18. The non-transferred arc is mainly used for metal-spraying and

for the welding of metal-foil strips.

In plasma arc welding with transferred arc burns the arc between the tungsten electrode (-

pole) and the workpiece (+

pole) and is called the

“transferred arc”, Figure

4.19. The plasma gas con-

stricts the arc and leads to a

more concentrated heat in-

put than in TIG welding and

allows thus the exploitation

of the “keyhole effect”.


transferred arc is mainly

used for welding of joints.

© isf 2002

Plasma Arc Welding with Transferred Arc

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Ignitiondevice

weldingpowersource

work piece

seam

plasma gas

plasma gas nozzle

transferredarc

shielding gas

contact tube

fillermaterial


tungstenelectrode

Figure 4.19

© isf 2002

Plasma Arc Welding withNon-Transferred Arc

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workpiece

surface weld

plasma gas

plasma gas nozzle

non-transferredarc

shielding gas nozzleshielding gas

contact tube tungsten electrode

Ignitiondevice

weldingpowersource

fillermaterial

Figure 4.18


2005


semi-transferred arc is a

combination of the two

methods mentioned above.

This process variant is

used for microplasma

welding, plasma-arc pow-

der surfacing and weld-

joining of aluminium, Fig-

ure 4.20

The plasma welding

equipment includes, be-

sides the water-cooled welding torch, a gas supply for plasma gas (Ar) and shielding gas

(ArH2-mixture, Ar/He mixture or Ar); the gas supply is, in most cases, separated, Figure 4.21.

The copper anode and the additional focusing gas flow constrict the plasma arc which leads,

© ISF 2002

Plasma Arc Welding with Semi-Transferred Arc

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surface weld

plasma gas

plasma gas nozzle

non-transferredarc

transferred arc

shielding gas

conveying gas andwelding filler (powder)

contact tube

workpiece


tungstenelectrode

weldingpowersource

ignitiondevice

Figure 4.20


Figure 4.21


Figure 4.22


2005

in comparison with TIG

welding, to a more concen-

trated heat input and thus to

deeper penetration. An arc

that has been generated in

this way burns more stable

and is not easy to deflect,

as, for example, at work-

piece edges, Figure 4.21.

The TIG arc is cone shaped

or bell shaped, respec-

tively, and has an aperture

angle of 45°. The plasma

arc, in comparison, burns highly concentrated with almost parallel flanks, Figure 4.22.

The shielding gas used in plasma arc welding

exerts, due to its thermal conductivity, a deci-

sive influence onto the arc configuration.

The use of a mixture of argon with hydrogen

results in the often desired cylindrical arc

shape, Figure 4.23.

In plasma arc welding of plates thicker than

2.5mm the so-called “keyhole effect” is util-

ised, Figure 4.24. The plasma jet penetrates

the material, forming a weld keyhole. During

welding the plasma jet with the keyhole

moves along the joint edges. Behind the

plasma jet as result of the surface tension and

the vapour pressure in the keyhole, the liquid

metal flows back together and the weld bead

is created.

© ISF 2002

Arc Shapes in Microplasma Weldingwith Different Shielding Gases

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Arc shapes of shielding gases:

argon with 6,5% hydrogen

helium

50% argon, 50% helium

argon

plasma gas: argonarc

le

ngth

Figure 4.23


plasma torch

weld (seam)

welding direction

keyhole

weldsurface

root

Figure 4.24


2005

Very thin sheets and metal-foils can be welded using microplasma welding with amperages

between 0.05 and 50 A.

Figures 4.25 and 4.26 show

these application exam-

ples: The circumferential

weld in a diaphragm disk

with a wall thickness of

0.15mm and the joining of

fine metal grids made of Cr-

Ni steel.

© ISF 2002

Microplasma Welding of a Diaphragm Disk Made of CrNi

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Figure 4.25


Figure 4.26

4. TIG Welding and Plasma Arc Weldingmercury.kau.ac.kr/welding/Welding Technology I - Welding...

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