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7/29/2019 Chapter 4 - TIG Welding and Plasma Arc Welding
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4.
TIG Welding and
Plasma Arc Welding
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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-ShieldedArc Welding acc. to DIN ISO 857
br-er4-01e.cdr
Plasma arc
welding withsemi-transferred
arc
Plasmaarc welding
with transferredarc
Plasma arcwelding with
non-transferredarc
CO welding2 Mixed gaswelding
narrow-gapgas-shieldedarc welding
plasma metalarc welding
electrogaswelding
Metal inert-gaswelding
MIG
Metal active gasweldingMAG
Gas-shielded arc welding
Tungstenhydrogenwelding
Tungsten plasmawelding with
electrode
Tungsten inert-gas welding
TIG
Gas-shieldedmetal arc welding
GMAW
Gas-shieldedarc welding
tungsten
Figure 4.1
ISF 2002
Arc Potential Curve
br-er4-02e.cdr
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 4000C)
cathode spot (approx. 3600C)
arc column (4500-20000C)
arc length
arc potential curve(example)
Figure 4.2
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4. TIG Welding and Plasma Arc Welding 50
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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
arcvoltage
25
20
15
10
arclength
4
2
4
2
heliu
m
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)
br-er4-05e.cdr
TIG cathode
10
000
K
9
000
K
8
000
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
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4. TIG Welding and Plasma Arc Welding 51
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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)
br-er4-07e.cdr
tungsten electrode
electric contact
shielding gas
shielding gas nozzle
fillermetal
weld
arc
workpiece
weldingpowersource
Figure 4.7
ISF 2002br-er4-06e.cdr
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
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4. TIG Welding and Plasma Arc Welding 52
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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 Innert
Gas Welding Wire to EN 1668
br-er4-08e.cdr
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 -30C
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
br-er4-09e.cdr
Figure 4.9
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4. TIG Welding and Plasma Arc Welding 53
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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
br-er4-10e.cdr
selector switch
high-frequency choke coil
filter
capacitor
transformer
SC: scattering core for adjustingthe characteristic curve
mains
high voltageimpulse 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
br-er4-11e.cdr ISF 2002
Construction of a Water-CooledT TIG Weldingorch 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
short
increasing
increasing
decreasing
decreasingi
Figure 4.12
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4. TIG Welding and Plasma Arc Welding 54
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would cause thermal overloadof 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 flattenedelectrode tip is the result
isf 2002
Influence of the Half-Wave Componentsduring A.C. TIG Welding
br-er4-14e.cdr
smaller increasingheat load
of the electrode
cleaning effectlower stronger
electroniccontroled
powersource
w
ithoutfilter
capacitor
balancedhalf-wavecom
ponents
electrodepo
larity
time
time-
-
-
-
-
-
+
+
+
+
time- - -
+ +withfilter
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
voltage
Reignition of the A.C. ArcThrough Voltage Impulses
Tungsten
br-er4-13e.cdr
A.C.
Figure 4.13
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4. TIG Welding and Plasma Arc Welding 55
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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.
ISF 2002br-er4-16e.cdr
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
ISF 2002br-er4-15e.cdr
Figure 4.15
ISF 2002
Flow Chart of TIG Orbital Welding
br-er4-17e.cdr
preflow of theshielding gas
postflow of theshielding gas
movement inswitch-on position
current decay overlappulsingpreheatingrise ofcurrent
shieldinggas
orbitalmovement
weldingcurrent
360 00
Figure 4.17
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4. TIG Welding and Plasma Arc Welding 56
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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.
Plasma arc welding with
transferred arc is mainly
used for welding of joints.
isf 2002
Plasma Arc Welding
with Transferred Arc
br-er4-19e.cdr
Ignitiondevice
weldingpowersource
work piece
seam
plasma gas
plasma gas nozzle
transferredarc
shielding gas
contact tube
fillermaterial
shielding gas nozzle
tungstenelectrode
Figure 4.19
isf 2002
Plasma Arc Welding withNon-Transferred Arc
br-er4-18e.cdr
workpiece
surface weld
plasma gas
plasma gas nozzle
non-transferredarc
shielding gas nozzleshielding gas
contact tube tungsten electrode
Ignitiondevice
weldingpowersource
fillermaterial
Figure 4.18
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4. TIG Welding and Plasma Arc Welding 57
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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 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
br-er4-20e.cdr
surface weld
plasma gas
plasma gas nozzle
non-transferredarc
transferred arc
shielding gas
conveying gas andwelding filler (powder)
contact tube
workpiece
shielding gas nozzle
tungstenelectrode
weldingpowersource
ignitiondevice
Figure 4.20
ISF 2002br-er4-21e.cdr
Figure 4.21
ISF 2002br-er4-22e.cdr
Figure 4.22
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4. TIG Welding and Plasma Arc Welding 58
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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 Welding
with Different Shielding Gases
br-er4-23e.cdr
Arc shapes of shielding gases:
argon with 6,5% hydrogen
helium
50% argon, 50% helium
argon
plasma gas: argonarclength
Figure 4.23
ISF 2002br-er4-24e.cdr
plasma torch
weld (seam)
welding direction
keyhole
weldsurface
root
Figure 4.24
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4. TIG Welding and Plasma Arc Welding 59
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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 aDiaphragm Disk Made of CrNi
br-er4-25e.cdr
Figure 4.25
ISF 2002br-er4-26e.cdr
Figure 4.26