Lec 8 – Physics of Welding Arc

Lec 8 – Physics of Welding Arc


hello i welcome you all in this presentation
related with the subject joining technologies for the metals and this presentation is mainly
based on the shielded metal arc welding process and the basic fundamental principles of the
arc welding in general so we will try to first of all understand that how the heat is generated
and how it affects the quality of the weld joint in general so here you know for – for establishing welding
arc – arc for welding purpose what is important that the – the between electrode and the base
metals to be joined stable arc is established and for this purpose normally the electrode
and work piece are connected to the suitable power source say the work electrode is connected
to the negative terminal and work piece is connected to the positive terminal and with the suitable power supply when the
arc is strike the current flow takes place but we know initially when there is no arc
the gap between the electrode and work piece is full of the atmospheric gases and the gases
or not or non-electrical conducting so this gap must be made electrical conducting and
for that purpose only we have to use certain arc initiation methods and this arc initiation methods – arc initiation
methods basically rely on developing the charged particle column between the electrode and
work piece so that it can be made electrical conducting and these two methods – there are
two methods which are extremely commonly used one is touch start method and another is field
start method in case of the touch start method electrode is brought in contact with the work
piece for striking the arc or for initiating the arc and as soon as the touching takes place due
to the short circuit high flow of the current results in the excessive heat generation at
the contact points so increase in temperature at the contact points of the electrode and
work piece and once this heating takes place due to the short circuiting in this very short
period maybe sometimes partial melting – partial melting evaporation means metal vapor generation
all this takes place in a very short period of time and once this short circuiting is over the
electrode is taken away from the work piece so gap is created this gap is full of air
so here once the gap is created the open circuit voltage means the arc voltage is – is the
potential difference between the electrode and work piece is established and in this
situation what happens that due to the heating thermal ionization or thermal emission of
the – thermal emission of the electrons from the electrode tip and work piece side at the same time ionization of the – ionization
of the metal vapors takes place – metal vapors takes place which were generated due to the
during the short circuiting period and this so as a result of thermal ionic emission electrons
are produced and ionization of the metal vapors produces positively charged particles and
negatively charged particles in form of electrons we had that connected the work piece and the
electrode to the power supply with a particular kind of the polarity so here as per their priorities the electron
anion will start moving towards the cathode that is the electrode and electron and electrons
will start moving towards the anode that is the work piece so this how the flow of charged
particles movement of charged particles in the arc gap starts and once sufficient charged
particle density is available in the gap between the electrode and work piece it becomes electrically
conducting and once the gap is has become electrically
conducting the flow of current it starts so this is how the arc initiation takes place
in case of the touch start method and in case of the field start method high strength electromagnetic
field is established between the work piece and the electrode and so this high-strength
electromagnetic field helps emission of the electrons from the surface of the cathode
and this in turn produces the charged particles in the gap in this case no direct contact between the
– between the electrode and work which is needed but the development of high strength
electromagnetic fields helps in emission of the electrons from the cathode and thats how
the gap is made the full of the charged particles and the gap is made conducting so the important
thing for having a stable arc is that the gap between the – the there is a sufficient
potential difference between the electrode and work piece and the gap is made conducting in the event
when the gap is deficient with this charged particles in charged particle density is less
than the reduced electrical conductivity will be leading to the pass leading to the extinction
of the arc means arc may get extinguished in that situation and if it is really full
of the charged particle means the charged particle density is high in the gap then it
will be leading to the very good electrical conductivity and the flow of current through
the arc will be very smooth so that will keep on so that flow current
through the arc that is the welding current and that arc voltage potential difference
means that is the difference between the – between the electrode tip and the work piece this
is the arc voltage product of these two gives us the power of arc and this power of arc
helps in generating the heat desired for the melting of the work piece so they are too
methods for initiating the arc and these are the conditions which are necessary
for emission of the for starting or starting the arc or initiating the welding arc and
these two methods are used invariably in most of the welding processes for example touch start method is commonly
used in processes like – like shielded metal arc welding process that is manual metal arc
welding process also in the gas tungsten arc welding process and like the gas metal arc
welding process and like submerged arc welding process electro slag welding process in all
these processes the touch start method but sometimes if it is felt that the contact
between the electrode and the work piece is not good from the soundness point of view
of the weld joints in that case like say the tungsten gets transferred at that time when
there is a contact between the work piece in case of the gtaw process so the tungsten
inclusion are formed – in tungsten inclusions are formed in case of the gtaw process if
the tungsten electrode tip is degraded and gets transferred to the weld pool so the it will be contaminating to the weld
with the tungsten inclusions and adversely affecting the mechanical properties and another
possibility is that when we dont want that direct contact between the electrode and the
work piece either not favorable due to the possibility of the contamination and inclusions
of the weld or its not practically possible to have the contact of the electrode with
the work piece like in the plasma welding so in those situations the high-strength electromagnetic
field is established for initiating the arc without so in that case the electrode is not
brought in contact with work piece for initiating the arc so plasma welding and all other automatic
gas tungsten arc welding where ever we find that the contact – contact is not suitable
or in automatic welding process where it is not feasible automatic welding where ever
automation has been implemented so we want that arc is initiated without direct contact
while feeding the work electro to the work piece for making the contact and initiating
the arc so these are the two ways by which arc is initiated as for as so this this basics
applicable for entire range of arc welding processes now we will see the things related with the
shielded metal arc welding process commonly abbreviated as shield smaw in case of the
smaw what we use – we use one electrode which is normally coated with a flux this plays
a very big role in case of the shielded metal arc welding processes here we have core wire
of the electrode and this is flux coating on the electrode flux coating in the shielded
metal arc welding process plays a very important role for success of the process and here in this case since the touch start
method is used and mostly the process is controlled manually so the power source we used with
this processes is of the constant current type means even the short circuit stage it
does not offer much higher much large current value like this is a kind of the v i – the
v i characteristic or the static characteristic of the constant current power source here
so the in short circuit stage the current is limited and also there is no major change in the value
of current with the fluctuation in the arc length which will be changing the voltage
so it despite of marginal change in the arc length the current does not change appreciably
so it maintains the heat it times the consistency in the heat being generated for uniform melting
and a good quality of the weld joint so it is preferred to have the constant current
type of the power source and mostly the type of current as far as this
process is concerned it can be ac or dc dc is very commonly used to because it can be
invariably used with all types of the electrodes like a rutile basic and is the cellulosic
type of the electrodes all these electrodes are extremely commonly extremely common in
the industry for the welding purpose and these can easily be used using the dc welding current for using the ac what is required that the
welding electrodes for ac needs the special care especially in terms of the low ionization
potential elements this low ionization potential elements must be present in the electrode
in sufficient quantity like sodium potassium and calcium so this low ionization potential elements
facilitate the easy emission of the electrons for striking the arc whenever current passes
through the zero level or the changes the direction or the voltage is zero so if this
is the current cycle and this is the voltage cycle in case of vs in case of the ac so its
important that when the current flow when using ac becomes zero at this stage to have
the gap conducting the sufficient charged particles has to be produced so that the gap can be maintained electrical
conducting and for that purpose low ionization potential elements or – or you see they are
very important for good arc stability thats why the ac should be used keeping in mind
the recommendation of the manufacturer for shielded metal arc welding electrodes so this
is about the power source and the type of current which is to be used now the role of the coating since if we see
here bare electrode is open at the top electrode at the top end is bare where the we dont have
the coating and the entire length otherwise it is coated this length maybe like say 250
to 400 mm and the diameter of the electrode is – is designated through the diameter of
the core wire – core wire – core wire in mm like say so its very common to have like 16
2 25 315 4 63 8 10 mm diameter electrodes so all these are indicating the diameter of
the core wire so electrode diameter means the diameter of the core wire and this is
the coating so the diameter of the electrode with the coating is if indicated by the letter
d and the diameter of the core wire is indicated through the letter small d in that case the
ratio of the core diameter of the electrode with the coating divided by the diameter of
the core wire this ratio gives us the idea about the coating factor coating factor is an indicator of the thickness
of the coating layer which has been applied over the surface of the core wire so this
coating factor value generally varies from 12 to 22 where in light coated electrode – light
quoted electrodes means the coating is very thin light coated electrodes they have this
value from 12 to 135 medium coated electrodes will have this value in the range of 14 to
17 and then heavily quoted or heavily coated
electrodes they will have value from 1 say greater than 17 to 1 22 this is the general
just to have the idea about the range so now it is very easy to understand the light coated
electrodes will have very thin layer of the coating and this thin layer of the coating
may not be enough to provide the sufficient shielding to the pool so – so for that we need to see that what
are the roles been performed if the coating is thin or if the coating is really very thick
so thickness and tennis of the coating will have the effect on the soundness of the weld
joint so far that how it will affect that is what we can see in respect of the – in
respect of the kind of role which are we – which are performed by the electrode the coatings
during the welding so as far as the role is concerned the bare
electrode we know bare electrode like of the iron or aluminium they are having higher ionization
potential so the electron emission capability is limited so they do not provide very smooth
and stable arc due to limited electrical limited connectivity of the gap between electron and
work piece so the arc is unstable mostly in case of the bare electrode and thats why the electrodes if they are coated
with the if they are coated then the coatings of the low ionization potential elements voting
with the low ionization potential elements will provide the arc stability so the first
role of the providing which is very important the purpose of providing the coating is first
to see that arc is stable so and this is achieved through the incorporation
of the low ionization potential elements like sodium potassium calcium in the coating these
elements have very low ionization potential and thats why and during in the welding conditions
during the welding they emit the electrons very easily and provide the conducting gap
between the electrode and work piece easily so the arc stability improves and the second purpose is to – is to provide
the de oxidizes de oxidizes say common like ferro manganese and ferro – ferro silicon
kind of things are added what they will be doing then they will so this whatever oxides
are being formed it will help to separate out the metal which has got oxidized and help
to save the metal work and then also to provide the – the flux for removing the impurities
– removing the impurities like the flux is designed in such a way that the ingredients
or constituents of the flux interact with the impurities and forms slag and this slag is designed to – slag is designed
to be lighter than the weld metal so whenever the weld metal the weld is brought weld pool
is formed the slag which is formed due to the interaction between the flux and the impurities
the slag becomes lighter and it starts floating over the surface of the metal at the same
time this the coatings are also provided for controlled alloying – controlled alloying
if it is required to the have a specific set of the properties in the weld metal and we want to incorporate some special elements
like chromium or molybdenum in the weld metal then these elements are incorporated with
the coating and when in the powder form and when this – when this will and then this will
be transferred to the weld pool so this is what is called controlled alloying so the
alloy – alloying elements which we want to incorporate in the weld metal they will be
added in the coatings so that during the welding this get transferred
after the fusion into the weld pool further this is also used for increasing the deposition
efficiency – increasing the deposition efficiency and how it is achieved like the iron powder
cell if the steel is being welded to increase the deposition efficiency iron powder is incorporated
with the coating and when this iron powder melts it gets transferred to the weld pool so coating efficiency because is calculated
from the weight of the core wire and when the iron powder is added with the flux it
helps to increase the weight which is being deposited in unit – unit time and thereby
it helps to increase the deposition efficiency or the deposition rate rather it is deposition
rate in unit time we are able to deposit more quantity because iron powder will be getting
transferred after melting into the weld pool and the iron powder which was incorporated
with the coating in that way it helps to increase the transfer of the transfer in that way it
helps to increase the deposition rate there are few more functions which are performed
by this coatings which includes like in odd position welding or in vertical welding like this vertical or overhead welding conditions
where there is a heavy tendency of the molten metal – molten metal falling from the weld
pool so in this case it is desired that viscosity of the weld metal is high so the slag which
is slag is actually the flux is designed in such a way that the slag which is formed is
thick and viscous so its the falling tendency of the weld metal
can be reduced so basically the purpose is to purpose of the coating can also be to adjust
the – the viscosity and surface tension of the slag and molten metal so that the flux
falling tendency of the weld metal during the welding can be reduced so these are the
important functions which are performed by the fluxes- fluxes which are applied in the
coating there are different – there are different
types of the constituents which are added in the coatings and these are commonly used
as a low ionization potential elements like sodium potassium calcium there is a charcoal
which is commonly added and then sawdust is also used various hydrocarbons and commonly
used is a binder these are sodium or potassium silicates there are number of constituencies which are
added for performing the specific functions during the welding and thats what we can see
from this slide where in this list shows the different elements and which are added into
the flux for a performing the specific functions for example quartz is added for increasing
the current carrying capacity magnetite is added for refining the transfer of the molten
metal drops means that it helps in achieving them split transfer and the calcium carbonate
is helps in reducing the arc voltage and providing the inactive gases so that the arc can be
weld metal can be protected and the fluorspar for increasing the viscosity of the slag and
molten metal ferro manganese and ferro silicon are commonly
used as a de-oxidants and the cellulose for releasing the shielding gases and potassium
water silicate these are the bonding agents and the the rutile is also one of the common
element which is added in form of the titanium oxide for increasing the viscosity and the
since it has very good arc emission capabilities so it helps in re-striking of the arc so one of the important functions of this
flux is also is that when this coating – this coating when approaches to the arc in course
of the welding it is burnt out so burning of the coating produces burning of the coating
or competition of the coating material produces lot of inactive gases – inactive gases in
form of co2 and co so these gases are released due to the thermal decomposition of the coating
constituents and combustion so these gases from actually very loose cover
of the co of co2 around the welding arc and thereby it helps in protecting the weld metal
from the atmospheric contamination so this is very important role of – of providing the
coating in the shielded metal arc welding processes since the protection provided in
this process is not very good because it forms very loose cover so there is every possibility
that the atmospheric gases will be able to interact with the weld metal during the shielded
metal arc welding process and thats why are usually the concentration
of oxygen and nitrogen in the weld is on the higher side and normally say these are found
say 1 % to 08 % on side so the higher concentration of the oxygen and nitrogen in the weld metal
they tend to degrade the properties of the – of the weld joint and thats why these are
good for like say for shielded metal arc welding process is good for the general purpose welding
and wherever you see non-critical applications of the welding because the quality of the weld joint is not
that good and high due to the higher concentration of the gases only this process cannot be used
for the purpose of the welding of all the reactive metals like aluminium magnesium chromium
or the stainless steel because these metals easily react with the oxygen and form the
their oxides which are refractory ignitor nature and these oxides do not melt easily
therefore they interfere with a melting of the best material and so and these oxides also get mixed up
with a weld metal to form the inclusions so the problems are number there are many problems
associated with the welding of the reactive metals using shielded welding metal arc welding
process thats why they are not normally welded with a shielded metal arc welding process and more those process which provide more
effective protection to the weld pool are normally prepared for the welding purpose
of this reactive metals like gas metal arc welding process or gas tungsten arc welding
or the plasma welding process like those so this is now i will – i will conclude this
presentation in this presentation i have talked about that how the arc is established and how the arc is initiated what are the
conditions necessary for striking the arc and having this stable arc and what is the
role of the different coatings and the electrode in the shielded metal arc welding process
and what kind of metal systems which can be welded using this process thank you for your
attention in the next presentation i will talk about
the other welding process like submerged arc welding process and gas tungsten arc welding
process thank you for your attention

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