4. TIG Welding and Plasma Arc Welding - hcmut.edu.vndantn/WT/WT1-c4.pdf · 4. TIG Welding and...

2003

4.

TIG Welding and

Plasma Arc Welding

4. TIG Welding and Plasma Arc Welding 43

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

arc welding processes, Figure 4.1. In all processes mentioned in Figure 4.1, the arc

burns between a

non- consumable

tungsten elec-

trode and the

workpiece or, in

plasma arc weld-

ing, between the

tungsten electrode

and a live copper

electrode inside

the torch. Exclu-

sively 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 oc-

curs over a length

of 10-4 mm.

A similarly high

voltage drop oc-

curs in the anode-

drop region, here,

however, over a

length of 0.5 mm.

© ISF 2002

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

br-er4-01e.cdr

Plasma arc welding with

semi-transferred arc

Plasma arc welding

with transferred arc

Plasma arc welding with

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

br-er4-02e.cdr

U

V20

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


The voltage drop on the remaining arc

length is comparatively low. Main en-

ergy conversion occurs accordingly in

the anode-drop and cathode-drop re-

gion.

Figure 4.3 shows the potential dis-

tribution by the example of a real TIG

arc under the influence of different

shielding gases. UA and UK have dif-

ferent values, the potential curve in

the arc is not exactly linear. There is

no discernible expansion of the cath-

ode-drop and anode-drop region

.

The electrical characteristics of the

arc differ, depending on the selected

shielding gas, Figure 4.4. As the ionisa-

tion potential of helium in comparison

with argon is higher, arc voltage must

necessarily be higher.

© 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

Argon60 A

Helium60 A

V

V

mm

mmARC

ARC

ARC

ARC

Figure 4.3

© ISF 2002br-er4-04e.cdr

arc

volta

ge

25

20

15

10

arc

leng

th

4

2

4

2

helium

argon

weld current

50 100 150 200 250 3500

mmV

A

Figure 4.4


The temperature

distribution of a

TIG arc is shown in

Figure 4.5.

In TIG welding just approximately 30%

of the input electrical energy may be

used for melting the base metal, Fig-

ure 4.6. Losses result from the arc ra-

diation and heat dissipation in the

workpiece and also from the heat con-

version in the tungsten electrode.

© ISF 2002

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

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TIG cathode

10 0

00 K

9 00

0 K

8 00

0 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


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 4.5

Figure 4.6


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.

© 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

© 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


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 alumin-

ium, alternating

current must be

used. For arc igni-

tion a high-

frequency high

voltage is super-

imposed and

causes ionisation

between electrode

and workpiece.

The central part of the torch for TIG welding is the tungsten electrode which is held

in a collet inside the torch body, Figure 4.11. The hose package contains the supply

lines for shielding gas and welding current. The shielding gas nozzle is more often

than not 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.

© ISF 2002

Chemical composition offiller rods and wires for TIG-welding

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Figure 4.9

© ISF 2002

Principle Structure of a TIG Welding Installation

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selector switch

high-frequency choke coil

filte

r ca

paci

tor

transformerSC: scattering core for adjusting the characteristic curve

mai

ns

high voltage impulse generator~

O_

O+rectifier

St

L1L2L3NPE

=~

Figure 4.10


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 weld-

ing always have a steeply drooping characteristic, Figure 4.12.

The non-contact

reignition of the

a.c. TIG arc after a

voltage zero cross-

over requires ioni-

sation of the elec-

trode-workpiece

gap by high-

frequent high

voltage pulses,

Figure 4.13.

© isf 2002br-er4-12e.cdr

current intensity

longer arc shorter arc

R and U rise R and Udrop

I drops I rises

volta

ge

U

arc length

long

shor

t

increasing

increasing

decreasing

decreasingi

Figure 4.12

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

br-er4-11e.cdr © ISF 2002

Construction of a Water-CooledT TIG Welding orch for

Figure 4.11

© ISF 2002

reignition of the arcby voltage impulses

++

- -

time

volta

ge

Reignition of the A.C. Arc Through Voltage Impulses

Tungsten

br-er4-13e.cdr

A.C.

Figure 4.13


When argon is used as a shielding gas, metals as, for example, aluminium and

magnesium, which besides having low melting points and also simultaneously form-

ing tight and hard-to-melt-off oxide skins, cannot be welded with a negative polarity

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 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 current flow are dependent on the temperature of the cathode.

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

ing temperature substantially lower than during the negative phase on the tungsten

electrode. As a consequence, a positively connected electrode leads to lower weld-

ing current flows than this would be the case with a negatively connected electrode,

Figure 4.14. A filter capacitor in the welding current circuit filters 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 components

adjustable. The

electrode thermal

stress and the

cleaning effect may

be freely influ-

enced.

© 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

elec

troni

c co

ntro

led

pow

er s

ourc

ew

ithou

t filt

erca

paci

tor

bala

nced

hal

f-wav

e co

mpo

nent

s

elec

trod

e po

larit

y

time

time-

-

-

-

-

-

+

+

+

+

time- - -

+ +

with

filte

rca

paci

tor

currenta

time

time

+ + +- - -

+ + +- - -

0

0

time- - -

+ +

currenta

0

weld seam width

Figure 4.14


Figure 4.15 shows that the thermal

electrode load can be recognized

from the shape of the electrode tip.

While the normal-load negative con-

nected electrode end has the shape

of a pointed cone (point angle approx.

10°), a flattened electrode tip is the

result from a.c. welding (higher ther-

mal load by positive half-waves).The

tip of a thermally overloaded electrode

is hemispherical and leads to a

stronger spread of the arc and thus to

wider welds with lower penetration.

All fusion weldable materials can be

joined using the TIG process; from

the economical point of view this ap-

plies especially to plate thickness of

less than 5 mm. The method is,

moreover, predestined for welding

root passes without backing sup-

port, 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


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


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 po-

sitions flat, vertical down, overhead, vertical-up and also interpass welding is applied.

Moreover, a de-

fect-free weld bead

overlap must be

achieved. Orbital

welding installa-

tions are equipped

with process op-

erational controls

which determine

the appropriate

process parame-

ters, 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 be-

tween the tungsten

electrode (-pole)

and the workpiece

(+ pole) and is

called the “trans-

ferred arc”, Figure

4.19. The plasma

gas constricts the

© ISF 2002

Flow Chart of TIG Orbital Welding

br-er4-17e.cdr

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

© 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

weldingpowersourcefiller

material

Figure 4.18


arc and leads to a

more concentrated

heat input than in

TIG welding and al-

lows thus the exploi-

tation of the “key-

hole effect”. Plasma

arc welding with

transferred arc is

mainly used for

welding of joints.

Plasma arc welding with semi-transferred arc is a combination of the two methods

mentioned above. This process variant is used for microplasma welding, plasma-arc

powder surfacing and weld-joining of aluminium, Figure 4.20

The plasma welding equipment includes, besides 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, in comparison with TIG welding, to a more concentrated heat input

and thus to deeper

penetration. An arc

that has been gen-

erated in this way

burns more stable

and is not easy to

deflect, as, for ex-

ample, at work-

piece edges, Fig-

ure 4.21.

© 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 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


The TIG arc is cone shaped or bell shaped, respectively, and has an aperture angle

of 45°. The plasma arc, in comparison, burns highly concentrated with almost paral-

lel flanks, Figure 4.22.

The shielding gas

used in plasma arc

welding exerts, due to

its thermal conductiv-

ity, a decisive influ-

ence onto the arc

configuration. The

use of a mixture of

argon with hydrogen

results in the often

desired cylindrical arc

shape, Figure 4.23.


Figure 4.21


Figure 4.22

© ISF 2002

Arc Shapes in Microplasma Weldingwith Different Shielding Gases

br-er4-23e.cdr

Arc shapes of shielding gases:

argon with 6,5% hydrogenhelium50% argon, 50% heliumargon

plasma gas: argonarc

leng

th

Figure 4.23


In plasma arc welding of plates thicker

than 2.5mm the so-called “keyhole

effect” is utilised, Figure 4.24. The

plasma jet penetrates the material,

forming a weld keyhole. During weld-

ing 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.

Very thin sheets

and metal-foils can

be welded using

microplasma

welding with am-

perages between

0.05 and 50 A.


plasma torch

weld (seam)

welding direction

keyhole

weldsurface

root

Figure 4.24

© ISF 2002

Microplasma Welding of a Diaphragm Disk Made of CrNi

br-er4-25e.cdr

Figure 4.25


Figures 4.25 and 4.26 show these application examples: 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.


Figure 4.26

4. TIG Welding and Plasma Arc Welding - hcmut.edu.vndantn/WT/WT1-c4.pdf · 4. TIG Welding and...

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