Lec 10 – Gas Tungsten Arc Welding

Lec 10 – Gas Tungsten Arc Welding


hello i welcome you all in this presentation
related with the subject joining technologies for metals and in the earlier lectures i have
talked about the fundamentals of the arc welding processes and also other things related with
the shielded metal arc welding in this presentation i will talk about the
gas tungsten arc welding process this process is also known
as tungsten inert gas welding process so in abbreviation form
it is written as gtaw and tigw so there are few unique things related with this process
that it uses non-consumable tungsten electrode this is one and invariably only inert gases
are used for protection of the weld pool so these are the two very unique things. since the process does not use any consumable
electrode and only inert gases are used for protection purpose the commonly used inert
gases are argon and helium along with this another unique thing is arc length used in
this process is very short so combination of these three things results in very clean
weld where in oxygen and nitrogen content in the weld is very less and that is why the cleanest weld among the
all arc weld process is produced by the gas tungsten arc welding process process vice they are few components in this
process important things like the tig process has unique features like electrode power source
shielding gas and as far as electrode is concerned now the tungsten electrode is used tungsten
has ionization potential of about 4.4 electron volts so it is also very good electron metre
but when the high current is supplied during the welding it gets the heated and the life
is adversely affected because of the heating and the erosion during the operation. so frequently the pure tungsten electrodes
are coated with low ionization potential elements like thorium zirconium lanthanum and cerium
these have significantly lower ionization potential application of these elements in
coating helps in increasing current carrying capacity of the electrode means electrode
of the given diameter say 1.5 m size how much current it can carry? say in case of the pure
tungsten if it can carry 15- ampere then after coating this capacity to handle the current
increases it may handle 220-250 ampere. this ability to handle the current without
compromising with the life of the electrode improves when these are coated with the low
ionization potential elements and in term the life of the electrode used improves and
further the stability of the welding arc also increases because these low ionization potential
element provide the electrons very easily under the identical conditions and therefore
the stability of the arc improves due to the improved charged column density in the gap
between the electrode and the work piece. as far as the power source is concerned the
power source invariably the constant current sources is invariably used and for this purpose
as far as the type of current is concerned it can work with both dc and ac ac is normally
used when welding the non-ferrous metals to take the advantage of the cleaning action
which is offered by the mobile cathode spots at the same time reasonably good life of the
electrode while in case of the dc en is the normally the polarity which is used during
the welding in case of the tig process. if we use the dcrp or reverse polarity or
electrode positive polarity then in that case excessive heat generation in the electrode
side can adversely affect the life of the electrode and therefore it will require much
frequent replacements of the electrode or re-sharpening of the electrodes if you see
the electrodes which are normally used their size can vary from 0.3 mm in diameter to 0.8
mm in diameter and length can vary from 75 mm to 600 mm so actually these electrodes are used in this
form where as soon as they get degraded or they get damaged they are sharpened to have
the desired angle included angle at the tip so preparation of the tungsten electrode is
frequently needed when they lose their shape in order to have proper shape of the electrode
the power source as i have said it is constant current type of power source is invariably
used with the ac or dc type of the currents as per the capacity the gtw process power
sources can have range say of 3 to 200 ampere or 5 to 300 ampere current rating it is about
the maximum current which can be drawn from the power sources so normally the low capacity
power sources are air cooled means the welding torch as the air cooled while the high capacity
torches which will be handling much higher value of current and during the welding they
get heated excessively and to maintain the temperature of the electrode therefore cooling
of the torch is applied. so sometimes water cooled torches are used
as far as this gtw process is concerned so the torch is another member and they are also
categorised based on the kind of current that they can handle as i said 3 to 200 ampere
or 5 to 300 ampere or much higher so high capacity current capacity torches are water
cooled so another thing is about the shielding gas shielding gases are commonly used are
argon and the helium. we know that argon the low ionization potential
as compared to the helium helium offers much higher ionization potential so whenever we
work with the argon because of the low ionization potential it offers very good arc striking
or arc stability so arc strike or arc stability becomes very good means arc initiation and
arc stability becomes very good when it is used as compared to the helium it offers somewhat
shallow penetration. so it is good for the welding of somewhat
less thickness plate as compared to the helium and in addition to this what you will see
that the cost of the argon is much lower as compared to the helium so it is very economical
as compared to the helium on the other hand so all these factors are for the argon low
ionization potential offers very good arc stability and arc initiation shallow penetration
and economical because of the lower cost of the argon. on the other hand higher ionization potential
results in somewhat poor stability of arc this is one and very deep penetration is offered
or produced when helium is used so the logic behind this is that the arc characteristic
is affected when the helium is used which means like here we have the voltage and here
we have current so when helium is used if this is the characteristics for argon so use
of the helium changes the arc characteristic means under the identical conditions the arc
voltage improves. so for the same current we work with the higher
arc voltages so it generates excessive heat more heat generation when the helium is used
resulting in the higher temperature of the welding arc which in turn increases the depth
of the penetration during the welding but the cost of the helium is much higher as compared
to that of the argon so our economy gets disturbed because of this helium application. other shielding gases are very rarely used
like carbon-di-oxide is not used which because it will decompose in the arc environment provide
the oxygen the oxygen will contaminate the tungsten so it is not good to use other inactive
gases like co2. so now i will talk about the reason behind
why it offers the cleanest weld means with the minimum percentage of the oxygen and nitrogen
in the weld so here the schematic of the system with like this we have power source positive
and negative terminals so negative terminal is connected to the electrode say this is
electrode tungsten electrode and here is the work piece. so here we have torch having the nozzle and
through this our inert gases will be coming out so here this gap is generally 1-3 mm in
case of the gtw means arc length is 1-3 mm and the electrode is moved consistently over
the surface of the work piece since the electrode is non-consumable arc length is very short
and it is well protected by the inert gases all around so the possibility for entry of
the gases from the atmosphere into the weld pool is very limited and that is why it offers
the clean weld. so the constant and short arc length is one
reason behind reduced possibility for entry of the gases into the weld pool another is
non-consumable electrode in other processes electrode also contributes in entry of the
impurities into the weld pool because electrode tip melts and gets transferred into the weld
pool but here it is non-consumable so there is nothing like which will get transferred
into the weld pool as far as the impurities like gases are concerned. so non-consumable electrode is another reason
behind its limited means very less percentage of presence of very less gases in the weld
metal and another is it mostly relies only on the inert gases inert gas flow rate normally
it may vary 5-12 litre per minute this is the common use but it may go as high as 50
litre per minute so helium the flow rate of the helium is much higher may be 2-3 fold
of the argon because helium is lighter just after coming out of the nozzle it starts to
moving up. so if this is the nozzle this is electrode
when we supply argon argon being heavier than the air it starts to settle down all around
the weld pool and the welding arc so provides effective protection while in case of the
helium helium being lighter than the air so as soon as it comes out of the nozzle it starts
to move up so in order to have moving up means it will not be protecting the weld pool and
the arc effectively because immediately after coming out it starts moving up since it is
lighter than the atmospheric air. and because of this the flow rate requirement
for the helium is much higher than the argon and however once if the flow rate is sufficient
for given set of the conditions like arc length the speed of the welding then we will see
that the protection provided by the helium is quite good only thing is that the cost
of the helium is high the stability offered by the helium for the arc is poor because
of the low ionization because of the high ionization potential as compared to the argon. and because of these reasons in addition to
the effective in addition to the capability to produce clean weld in terms of the low
concentration of the oxygen and nitrogen the tig offers another unique feature or ability
among the arc welding process that is capability to melt with very low heat input so
the net heat input which is needed in case of the tig process this is very less as compared
to the other process. the reason for this is simple; the electrode
arc length is very short the diameter of the electrode is very small and the area over
which heat delivered by the welding arc is very short and power density of the tig is
much higher than the smaw shielded metal arc welding saw gmaw so as compared to these three
processes commonly used arc welding processes tig offers the higher power density. so the amount of heat required for fusion
of the given thickness of the plate is less so the heat input required by the tig process
is less so hnet is less due to the higher power density as compared to the other arc
welding process if the hnet is less so it facilitates the melting even with the lesser
heat input. and this goes in the favour that when heat
input is less the heat affected zone being formed on both the sides of the weld joint
is limited and the high cooling rate conditions resulting in the final grain structure in
the weld so these two combinations like reduced size of the heat affected zone and to increased
cooling rate leading to the refinement of the grain structure in the weld these two
features result in the much improved quality of the weld joint. so here lesser heat input and the cleaner
weld these are the two very positive sides related with the tig process and that is why
this process is mainly used for producing the quality weld joints for critical applications
which includes the development of the joints for nuclear applications and aero space and
air craft components because the quality of the joint is much better due to these two
reasons the cleaner weld reduced heat input helps in improving the size of heat affected
zone final grain structure and so the much better quality of the weld joint is formed. this is truly irrespective of the metal system
whether it is aluminium or the ferrous metal systems and that is why for all critical applications
tig welding is invariably preferred over the other arc welding processes like submersed
or shielded metal arc or gas metal arc. but the gas tungsten arc welding process suffers
with the one major limitation and that is since the process here like in case of the
welding of the thick plates it offers the two kinds of the problems one is normal gtw
process offers the maximum penetration around 2-2.5 mm or in very severe condition very
high heated condition may be 3 mm in single pass so gtw single pass is not tried with
the thicker plates except some modifications in the process are made. so the limited penetration is one less penetration
available with the gtw process is one major limitation and when we are working with a
thicker plates then it requires very large number of the passes for completing the weld
joint these may be high like 10 pass 12 pass 20 passes depending upon the number of depending
over the size of the groove type of the groove which has been made. so increased number of the passes requirement
again lowers the productivity of the process so the process becomes slow. so in this case what we need the process may
work in case of thin plates the process may work without any filler like thin sheets can
directly be joined using the autogenous approach like plate of the 3 mm thickness these are
the two plates it can by application of the tig arc we can directly fuse them together
and after the solidification will be getting the weld joint. but if the plates have to be welded then we
need to prepare a proper groove and in that case the groove will go in like this and in
that case we have to apply filler so the process may work with filler or without filler if
thin sheets have to be welded then no filler of course is needed and autogeneous weld will
be made but if the large size grooves are to be filled then filler is to be applied. so the filler metal is applied separately
or externally during the process which further complicates the application of the process
but for the want of the quality for the requirement of the quality in any case people look for
the gas tungsten arc welding process now we will talk about another variant which has
been developed for more of the industrial application that is the gas metal arc welding
process. gas metal arc welding process was it takes
care of most of the negative aspects related with the gas tungsten arc welding how? because
this process uses consumable electrode and it works with much higher current levels two
and three the protection is also reasonably protection is good because it mostly uses
the helium or argon as the shielding gases. but for the less critical applications maybe
it can work with something like co2 or the mixture of other gases like argon modified
with oxygen or argon with carbon-di-oxide or helium and argon mixtures the various combinations
of the other gases also used but since this process is different from the gas tungsten
arc welding process in the sense that it uses consumable electrode and because of this and
it uses the gases like argon helium or argon with oxygen or argon with hydrogen or helium
argon mixture or co2 with argon. so various gases are used but this not only
limited with inert gases even inactive gases are also used in case of the gas metal arc
welding process. so the two things one arc length is fluctuating
in case of the gas metal arc welding why? because the electrode is consumed this is
one and another one the electrode itself when melts and the molten metal is transferred
into the pool it transfers some of the impurities into the weld and when the gap is fluctuating
the stability of arc is not good as that of the gtaw wherein we were using the non-consumable
electrode short-arc length inert gases. so now we can see here if the arc length is
fluctuating this is the nozzle inert gases are coming out and if the arc length is fluctuating
then the protection being provided by the shielding gases will not be that effective
and because of this the oxygen and nitrogen concentration in gmaw weld is somewhat greater
than what is offered by the gtaw process. so still this is clean or cleaner than like
say it is still less than 0.05 percent of the oxygen and 0.03 or 2 percent of the nitrogen
so it is still cleaner weld but not as clean as that is produced by the gtaw process so
because of the still the quality of the weld in terms of the oxygen and nitrogen content
in the weld is quite good the process is very commercially used very much commercially used
reason being that the process uses the consumable electrode so here you see the electrode wrapped in its
pool feds through the cables to the nozzle and then through the nozzle it comes out the
shielding gases will also be coming out and here the electrode develops the arc between
the work piece mostly in this case the power source connected to the positive terminal
of the power source is connected to the electrode and negative is connected to the work piece and the arc is established between the electrode
and the work piece so basically heat of arc melts both electrode as well as base so the
combination of both these two will be resulting in filling in of the groove much faster and
while providing the protection with the help of all suitable shielding gases like argon
helium or carbon-di-oxide co2 is mainly used with the ferrous metals only not with the
others because it also provides effective cleaning
however whenever co2 is used oxygen concentration in the weld is somewhat more as compared to
the case when helium and argon is used for development of the weld joint so this is process
effectively counters the negative side of the gtaw process by using the consumable electrode
and whenever the consumable electrode is used the productivity of the process is very good productivity of the let us say gmaw process
is much better as compared to what we have seen in case of the gtaw process and also
very clean weld is made and because of these two effective reasons the process is very
successfully used in the industry for making the weld joints for all critical and the semi-critical
applications so here this was the background of gmaw process and how it is different from
the gas tungsten arc welding process and why it is used in the industry so here now i will conclude this presentation
basically i have talked about the fundamentals of the gas tungsten arc welding process importance
of the various components and why this process is good or what kind of the positive is related
with the process and the negatives relate with the process are so in addition to that
how the gas metal arc welding process helps in overcoming the negatives of the gas tungsten
arc welding process thank you for your attention

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