Tungsten Arc Gas Shielded Welding

1
Tungsten Arc Gas Shielded Welding
Roy Chadwick
2
Tungsten Arc Gas shielded Welding
Basic Principles When Tungsten Arc Gas
Shielded (TAGS ) welding, heat is generated by an
electric arc formed between a Tungsten electrode
and the metal being welded. The Tungsten
electrode is non consumable, i.e. when used
correctly it does not melt and become part of the
joint. Additional metal may be added using a
filler rod. The welding arc, electrode and weld
pool are shielded by an inert gas, usually argon,
which issues from a nozzle surrounding the
electrode.
SHIELDING GAS The main purpose of the shielding
gas is to protect the weld and hot electrode from
the atmosphere in order to prevent oxidation.
TAGS
3
Five Advantages over Manual Metal Arc welding
1. The arc is more concentrated giving deeper
penetration. 2.Control of penetration is better,
allowing the welder to achieve a more consistent
root bead profile. 3. The welder decided where
and when to deposit additional filler wire. 4. A
wider range of metals and alloys can be welded.
E.g.. Aluminium,Titanium, Copper and Nickel
alloys. 5. No flux is required and there is no
slag to control.
TAGS
4
EQUIPMENT

• Direct current power sources
• The power source for TAGS welding should give a
  drooping characteristic.
• The d.c welding supplies may be supplied from a
  d.c. generator, a transformer rectifier or an
  inverter set as shown.
• D.C. Supplies at the arc are required when
  welding
• Carbon steels.
• Stainless steels.
• Copper and its alloys.
• Nickel and its alloys.
• Titanium alloys.
• Hardsurfacing with cobalt based alloys

TAGS
5
Power Sources

• Alternating Current
• A.C. may be supplied from
• An a.c. transformer set.
• A transformer rectifier as shown,which is capable
  of supplying both a.c. or d.c.
• An inverter which can supply both a.c. and d.c.
• A.C. Supplies at the arc are required when
  welding
• Aluminium its alloys.
• Magnesium And its alloys
• Aluminium bronze.
• Cast iron.

TAGS
6
Striking the arc
Scratch Start When using d.c.it is possible to
strike the arc by touching the electrode onto the
work and withdrawing it to form an arc. This can
give unsatisfactory results as the electrode
point can be damaged or contaminated and tungsten
inclusions may be left in the weld. If this is
the only method provided the arc may be initiated
on a block of carbon and then transferred to the
work.
Lift Arc Some welding sets are provided with a
lift arc facility. With this technique the
electrode is brought into contact with the work
and current switched on, this is a very low value
of current until the electrode is lifted from the
work when the current rises initiating the arc.
this method reduces the risk of electrode
contamination and damage.
Rest the ceramic nozzle on the work, rock
forwards allowing the tungsten to contact the
work and rock back to initiate the arc.
TAGS
7
Striking the arc
Scratch Start
Scratch Start When using d.c.it is possible to
strike the arc by touching the electrode onto the
work and withdrawing it to form an arc. This can
give unsatisfactory results as the electrode
point can be damaged or contaminated and tungsten
inclusions may be left in the weld. If this is
the only method provided the arc may be initiated
on a block of carbon and then transferred to the
work.
DOUBLE TWICE CLICK TO ANIMATE
TAGS
8
Striking the arc
Lift Arc Technique
With this method the power source supplies a very
low current when the electrode is brought into
contact with the work (typically one amp.) When
the electrode is lifted from the work the current
rises to its set value. The initial low amperage
prevents the tungsten from sticking to the work
preventing damage to the electrode tip.
DOUBLE TWICE CLICK TO ANIMATE
TAGS
9
HIGH FREQUENCY START

• This is the preferred method of arc starting when
  using d.c. and is essential when using a.c.
• The power source is fitted with a high frequency
  spark generator. To initiate the arc the
  electrode is brought to within about 5mm of the
  work and the foot-switch/trigger is pressed, a
  high voltage high frequency spark jumps the gap
  between the tungsten and the work ionising the
  arc gap and the welding current flows across the
  gap forming the arc.
• A three position switch is provided.
• Continuous When using a.c. the H.F. is
  continuous to maintain arc stability.
• Off permits touch starts only sometimes used
  when working near sensitive electronic equipment.
• Start onlyWhen d.c. is being used the high
  frequency spark switches off when the arc is
  established.

TAGS
10
Striking the arc
HIGH FREQUENCY START
This is the preferred method of arc starting when
using d.c. and is essential when using a.c.
DOUBLE CLICK TO ANIMATE
TAGS
11
Front Panel A.C. D.C TAGS Power Source
TAGS
Post purge
Process selector
Soft start
H.F Start
Remote current control
12
Suppressor Unit
TAGS power sources which can give an a.c. output
are usually fitted with a suppressor unit. This
is only used when welding with a.c. The function
is to correct an imbalance of current flow across
the arc caused by the arc being between two
dissimilar metals e.g. Tungsten and Aluminium.
TAGS
13
Other features may include Pulsed current. The
Current is supplied in a series of high amperage
pulses, a low background current keeps the arc
ionised during the pulse off periods. Advantages
better control of penetration bead shape,
easier welding of thin materials, reduced
distortion.
TAGS
14
Slope In Slope Out
Slope in On striking the arc current builds up
from a low value to the set value, over a period
of time which can be pre-set with the control.
Reduces the risk of burn through when starting on
thin sheet.
Slope out At the end of the weld the current can
be set to decay to zero over a pre-set period of
time. This allows for crater fill and reduces the
risk of crater cracks.
TAGS
15
TAGS
Other Features of TAGS Power Sources
Shielding gas pre-flow and post-flow ( pre purge
and post-purge) The pre-flow purges the weld
area with shield gas prior to starting the arc,
this is fixed with most power sources. Post- flow
allows the shield gas to flow for a few seconds
after breaking the arc whilst the weld and
tungsten cools. May be adjustable, higher
amperages require a longer post purge. Remote
footswitch control When the footswitch is set to
remote the current may be increased or reduced by
applying more or less pressure to the footswitch.
This is useful for filling the crater at the end
of a weld or for controlling heat build up
especially when welding aluminium.
16
WELDING TORCHES
TORCH COMPONENTS
TAGS
N.B. The torch may be fitted with a trigger
switch or a footswitch may be used.
17
WELDING TORCHES
Air cooled
These are normally used for currents up to 150
amps.
Water cooled
For current above 150 amp. The coolant is pumped
first through the torch head taking heat from the
torch and tungsten electrode, the water then
returns through the welding current cable,
flowing between the plastic insulation and the
copper conductor thus cooling the cable as well
as the torch head.
Advantages
Higher currents can be used, thinner power cable
is more flexible, electrode can be used at higher
amperage due to improved cooling
TAGS
18
WATER COOLED FUSE
Whilst most modern water cooling systems will
prevent current from being switched on if the
coolant is not flowing. Some older systems have a
water cooled fuse fitted in line with the welding
current cable. The purpose of this fuse is to
prevent the torch from being damaged if the water
supply is interrupted or if the operator forgets
to turn it on.
TAGS
19
Gas Nozzles

• These are usually made from a heat resisting
  ceramic material.
• The choice of size depends on a number of factors
  such as
• Current value.
• Electrode diameter.
• Joint type.
• Gas flow rate.
• Material being welded.
• Welding position.

Transparent gas nozzles are available, these give
a better view of the arc when joint access is
difficult
20
Gas Lens
21
Gas lens
Purpose this focuses the gas stream, giving a
lammeller column of gas which permits Lower gas
flow rates. Better gas coverage. Allows more
electrode extension.
22
Tungsten Electrodes
Electrodes for TAGS welding are made from
Tungsten which melts at around 3400oC. Providing
that maximum current is not exceeded and that
correct polarity is observed when using d.c. the
electrode will not melt in the arc.
Types of Tungsten Electrode Pure Tungsten These
may be used for any application but the types
listed below give better results. Thoriated
Tungsten Recommended for d.c. welding. The
addition of 1 to 2 Thorium oxide maintains a
sharp point on the electrode and increases its
current carrying capacity. Zirconiated Tungsten
Recommended for a.c. welding. The addition of a
small percentage of Zirconium oxide improves arc
stability and reduces the risk of Tungsten
inclusions in the weld.
23
Thoriated TungstenElectrodes
24
Electrode sizes and current ranges
Electrodes are normally available in 100 mm or
150 mm lengths. The diameters range from 1.0 mm
to 4.8 mm . Although larger diameters are
available for a.c.welding with high currents. The
choice of electrode diameter will depend on the
type of current, polarity ( d.c. ), type of
electrode and shielding gas.
Current carrying capacities
N.B. Too low a current results in an unstable arc.
25
ELECTRODE POINT DIMENSIONS WHEN A.C. WELDING
Electrode should be pre-ground as shown.
Prior to welding the the arc should be run on a
piece of scrap material to produce a balled end,
this reduces the risk of tungsten inclusions at
the start of a weld.
26
ELECTRODE POINT DIMENSIONS WHEN D.C. WELDING
The point dimensions below are a general guide
only and may be modified to suit the job in hand.
27
CORRECT METHOD OF GRINDING ELECTRODES
Ensure grinding marks run lengthways along the
electrode taper
28
EFFECT OF INCORRECT GRINDING OF ELECTRODES
It is important to grind the electrode lengthways
so that the grind marks run along the length of
the taper and not across it otherwise stray
arcing may occur when welding in vee preparations.
29
ELECTRODE STICK-OUT
This will vary according to the type of joint,
e.g. vee butt preparations will need more
stick-out than square butt welds.
30
EFFECT OF ELECTRODE POLARITY
D.C. ELECTRODE POSITIVE
The electrode is at the hot side of the arc, it
will overheat and contaminate the weld pool.the
electrons are flowing from the work towards the
electrode. This condition gives a cathodic
cleaning effect.
31
EFFECT OF ELECTRODE POLARITY
D.C. ELECTRODE NEGATIVE
The electrode is at the cool side of the arc and
does not overheat. The electrons are flowing from
electrode to work. ( no cathodic cleaning takes
place)
32
EFFECT OF ELECTRODE POLARITY
ALTERNATING CURRENT
A.C. Changes its direction of flow at twice the
frequency of the supply I.e. 100 reversals per
second. Each time the electrode becomes positive
cathodic cleaning takes place. When the
electrode becomes negative it cools. This
condition is used when welding aluminium its
alloys.
DOUBLE TWICE CLICK TO ANIMATE
33
SHIELDING GASES
A
C
D
C
34
Shielding Gas Control Equipment
REGULATOR Purpose, to reduce cylinder pressure to
working pressure. FLOWMETER To control and
indicate the amount of gas flowing, in litres
per/minute. MAINTAINENCE Check for leaks with
soap water solution, check threads and fittings
for damage.
35
SHIELDING GAS FLOW RATES
Argon When welding carbon steels up to 3mm thick
a flow rate of 5 to 6 litres per minute is
sufficient. When vertical welding the flow rate
may be increased slightly as argon is heavier
than air and will leave the joint area more
quickly.
36
Filler Rods and Wires
Composition Filler rods for TAGS welding should
conform to BS En 440 1995 When welding low
carbon steel a filler rod designated G3Si2 should
be used. This rod contains sufficient deoxidisers
to prevent porosity. Gas welding rods should not
be used, as they do not contain sufficient
deoxidisers to prevent porosity. Filler rods,
which comply with BS 2901 grade A15, or 18 may
still be used.
37
FILLER ROD DIAMETER
Filler diameters can be selected from the
following range of sizes
The filler diameter to be used will depend upon
the material thickness, joint type, size of
fillet and current value.
mm 0.5 0.8 1.0 1.2 1.6 2.4 3.2
Handling and storage
DO NOT 1. Store wires in damp or dirty
conditions. 2. Handle them with greasy gloves or
hands. 3. Allow different rods to become mixed
up. DO 1. Ensure that wires are correctly
labelled and packaged. 2. Return unused rods to
their correct packet.
38
Gas Economiser
Combined economiser flowmeter
Economisers may be fitted to some older TAGS
systems. When the torch is hung on the hook the
weight of the torch closes a valve shutting off
the gas supply.
39
BACKING TECHNIQUES WHEN BUTT WELDING
Backing applied to the root of a butt joint may
be required to either provide support for the
root run or to prevent oxidation. The types of
backing techniques available are as
follows. Temporary backing The backing bar is
usually made from a different material to that
being welded and it is removed on completion of
the joint. When welding carbon steels or
stainless steels aluminium or copper may be used
these are good conductors of heat compared to
steels. Carbon steel or stainless steel may be
used for aluminium and copper.
40
BACKING TECHNIQUES WHEN BUTT WELDING
Permanent or Fusible Backing
The backing strip is often used when the joint is
accessible from one side only, it can also be
used as an aid to assembly. The backing strip is
made from the same material as that being welded
and is left in place after welding.
Root run deposited and backing strip becomes part
of joint
41
BACKING TECHNIQUES WHEN BUTT WELDING
Fusible inserts
When a controlled penetration bead is required
and where access to the back of the joint is
impossible a root insert may be used, known as an
EB insert. ( EB refers to the Electric Boat
Company ). Often used on pipe joints.
The insert is tacked first to one component of
the joint and then to the other all tacks on the
topside of the joint. The root and the insert are
then fused together without the addition of
filler rod. When fusing the root the weld pool
will be seen to rise slightly indicating complete
fusion of the insert.
42
Applying a Backing Gas
Some metals such as stainless steels, nickel
alloys and titanium require shielding gas to be
applied to the underside of a joint in order to
prevent oxidation.
43
Method of back-purging a pipe butt joint
A commonly used flow rate when applying a backing
gas is 4 litres/min.
The soft faced plugs are fitted when the joint is
aligned for tacking and pulled out on completion
of welding, the assembly is then used for the
next joint.
44
CLOSED SQUARE BUTT WELD IN 3MM THICK LOW CARBON
STEEL
1.Establish the arc at the right-hand end of the
joint with the electrode held at an angle of 70o
- 80o. 2.Immediately fusion to the root of the
joint is obtained add filler metal to prevent
excessive fusion of parent metal. 3.Commence
leftwards movement without weaving of
torch. 4.Co-ordinate addition of filler metal and
rate of travel so as to maintain fusion to the
root and build up the weld to a slightly convex
profile. 5. Add filler metal immediately if
excessive penetration seems imminent.
45
POSITIONAL WELDING
Only slight changes are necessary when carrying
out positional welding, e.g. a slight increase in
slope angle and the shielding gas flow-rate may
be increased to overcome the effects of gravity.
VERTICAL UPWARDS
Overhead