Lec 9 – Shielded Metal Arc Welding

Lec 9 – Shielded Metal Arc Welding


i welcome you all in this ninth presentation
on the subject joining technology for metal and in the last lecture we have seen the technical
aspects related to the shielded metal arc welding process and in this presentation i
will talk about first a few basic fundamentals related to the arc welding processes and thereafter
i will go into the gas tungsten arc welding process and submerged arc welding process. the fundamental aspects which i want to take
up first they are related with the factors governing the melting rate in case of the
arc welding process and for this purpose what we have to see the one empirical equation
which have been developed like ai plus bl i square where there are two terms which in
combination determine the melting rate and what these two terms are all about. like say this is electrode this is the work
piece and arc is generated so due to the flow of current through the electrode i square
rt heating of the electrode core wire takes place in case of the consumable arc welding
processes and at the same time heat generated at the welding arc which is given by vi comprises
basically the three constituents one is heat generated in the cathode drop zone which is
obtained from the vc voltage drop in the cathode drop zone. and the welding current i the voltage drop
in the plasma zone that is vp into i and anode drop zone va into i the voltage drop in the
anode drop zone so basically this voltage drop is sum of the vc that is voltage drop
in the cathode drop zone vp and va voltage drop in plasma and anode drop zone so if we
are talking about the melting rate of the electrode then the vc into i plays a big role
in melting of the electrode. because the heat being generated in the plasma
zone and in the anode drop zone is affecting much the melting of the electrode they are
away from the electrode tip and these heats are like the anode drop zone heat is mainly
used for melting of the work piece and plasma heat either part of the heat is used for melting
of the base metal or the heat carried by the molten metal drops which are being transferred
through the plasma. but most of the melting is in the cathode
drop zone is attributed to the heat being generated in the cathode drop zone the melting
of the electrode is primarily governed by the heat generated in the cathode drop zone
so there are two constituents which are related with the heat generation and heat generation
will be affecting the melting rate of the electrode so one heat generation which is
coming from the cathode drop zone or anode drop zone as per the polarity of the polarity
being used. if the electrode is connected to the positive
terminal then the anode heat will be considered for the melting of the electrode and the heat
being generated in the cathode drop zone will be attributed to the melting of the base metal
so heat being generated due to the electrical reactions in the anode drop zone or cathode
drop zone as per the case this ai factor accounts for the anode or cathode drop zone heat. and i square rt is due to the flow of current
through the electrode so these two factors are clubbed together here the first factor
accounts for the melting rate due to the heat being generated in anode or cathode drop zone
and the second one is due to the electrical assistance heating that is i square rt which
is in simplified form written as like bl i square as an empirical equation so here what
is a b l and i. that is what we will try to talk about here
so here if we see a is the coefficient or the constant which accounts the voltage drop
in the anode drop zone or cathode drop zone the chemical composition and the factors all
those material and process related factors that are affecting the heat generation due
to the heat generation in anode drop zone or the cathode drop zone i is of course the
welding current in ampere. while b is about the coefficient for the material
resistance related factors like say electrical resistivity of the metal is the big factor
that is considered though high electrical conducting metals will have the low b value
while the low electrical conducting metals will have the high b value and l is the electrode
extension and i is the welding current so if we see this equation for small diameter
electrode large electrode extension and for high current values. so for these three situations a small diameter
will be resulting in the higher electrical resistance for the flow of current greater
electrode extension will be offering more resistance for the flow of current so higher
electrical resistance heating and similarly higher value will be causing more welding
due to the second factor so the second factor dominates under these set of conditions if
the conditions are just reverse where the diameter is large electrode extension is short
and the welding current is also low. in that case the role of the second factor
is somewhat reduced and we will see that the first vector is dominating or governing the
melting rate related thing but it does not mean that we can achieve any kind of melting
rate just by adjusting the welding current because there are number of factors that limit
the lower and higher level of the welding currents which can be used. for example in case of the shielded metal
arc welding process higher level of the current is limited by the situation or by the current
value at which thermal decomposing of the coating starts due to the flow of current
say this is electrode and this is the coating if you keep on increasing the current value
then high i square or heating of the core wire can start damaging the coating much earlier
than it should. because of this we will not be getting the
benefit of the coatings over the core wire in case of the smaw while in case of the tungsten
gas welding processes it will be damaging the electrode due to the excessive i square
rt heating so electrode life is reduced if the excessive current is used so now we will
see another important aspect related to the arc welding processes and that is about the
polarity. like the electrode and work piece can be connected
either to the positive or negative terminal or in case of ac of course these two will
keep on changing so when the positive terminal is connected to the electrode and negative
to the work piece in this case what we say the dc reverse polarity where your work piece
is negative and the electrode is positive. while the reverse polarity or state polarity
is one when the electrode is connected to the negative terminal of the power source
and work piece is connected to the positive terminal of the power source so which type
of polarity is to be used that is what we will try to talk about so this is called dc
state polarity or electrode negative dcen or this is electrode positive dcep or the
reverse polarity this is dcen or state polarity. so what are the changes associated with these
change in polarities during the welding especially in case of ac since the polarity keeps on
changing so the distribution means whatever factors are associated with the ac they will
be equally distributed in either side so what is the effect of polarity polarity affects three things in very big
way one is the heat generation or heat distribution during the welding like
in case of welding arc second is it affects the stability of the arc especially in cases
where electrode is expected to you mean the electrons for the good arc stability so the
change in polarity can affect the stability of the arc in those cases and another is cleaning
action which is offered by the welding arc when you are using a particular kind of the
polarity. so effect of polarity these are the three
main effects so which type of polarity is used and these three aspects will be available
this choice will be available with regard to the cleaning action or arc stability or
heat generation only when the dc is used over the ac. so what is the heat generation related aspect
generation or distribution related aspect this is very important like when the state
polarity is used like work piece is connected to the positive and the electrode in the negative
in this case electrons are emitted by the electrode and they will keep on moving towards
the work piece and they will striking the work piece and impact will be generating lot
of heat and so in this case since large number of the electrons will be impacting with the
work piece continuously. so which will be generating large amount of
heat which accounts to the two-third of the heat being generated in the anode side so
anode will be anode or work piece will be heated will be getting much more amount of
heat as compared to the electrode while the electrode in this case when ion current is
very limited about 1-2 percents so here the ions will be moving very slowly towards the
cathode and they will be impacting at a very low velocity with electrode. so heat generation in the electrode side or
the cathode side is very limited it is about one-third of the total arc heat so limited
heat generation will be causing the low melting rate in case of the consumable arc welding
process or it will be leading to the much better life of the electrode for the gtaw
process so this is about the heat generation if you want less heat in the work piece side
then work piece will be connected to the negative terminal and electrode will be connected to
the positive terminal. while in the other case if we want more amount
of the heat then of course electrode work piece will be connected to the positive terminal. so to take this advantage in case of the tungsten
arc welding to have much life of the tungsten electrode invariably the tungsten arc welding
process uses the dcen polarity so that heat generation in the electrode side is very limited
and we get much better it leads to the much lower amount of thermal damage to the electrode
and which in turn results in much better electrode life. if in other processes like submerged arc welding
process if you want higher deposition rate so in that case for submerged arc welding
process we want more heat is generated in the electrode side so for that purpose we
will be using the reverse polarity where electrode will be connected to the positive terminal
and work piece will be connected to the negative terminal so the higher amount of the heat
generation in the electrode side will be leading to the higher melting rate. so saw invariable in saw and for the gmaw
gas metal arc welding process dcrp or dcen both are same or dcep electrode positive or
the reverse polarity both are same and this kind of polarity is used to have the advantage
of the higher melting rate so this kind of facility s available only dc when we can select
the type of polarity as per the requirement of the heat either in the electrode side or
in the work piece side whether we want more heat or less heat in the work piece side. if you want less heat in the wok piece side
like for the welding of thin sheets in that case we will be choosing the reverse polarity
because in the case of reverse polarity heat generation in the work piece side is somewhat
lesser now we will see another factor with the effect of the polarity apart from the
heat generation. another important factor is the arc stability
we know that electrode is expected to emit the electrons electrode if it is connected
to the positive terminal and the work piece connected to the electrode is connected negative
terminal and work piece connected to the positive terminal so in this case electrode is expected
to emit the electrons so the electrons will be emitted normally these are either coated
or these are designed to emit the electrons. so they will be emitting the electrons very
easily providing the gap charged gap between the electrode and work piece full of the charged
particles to make it electrically conducting so that very good stable arc can be produced
but if the polarity is reversed then in that case if the polarity is reversed means your
electrode is now positive and work piece is negative work piece may not be necessarily
of the low ionization potential elements it may not have very good electron emitting capability. so the poor emission of the electrons from
the work piece side during the welding will be reducing the charged particle density in
the gap between the electrode and work piece and which in turn will be increasing the resistance
for the flow of current and increased resistance for the flow of current may lead to the extinction
of the arc and unstable arc so the change in polarity especially in case of the work
piece where work piece is expected to emit the electrons and if it does not have very
good electron emitting capability then it will lead to the poor stability of the arc. so if the electrode is designed to have good
electron emission and then switching over to the like say changing the polarity from
the state polarity to the reverse polarity then this can cause the problem of the arc
stability due to the poor stability of the work piece while in general electrodes are
designed to have very good electron emission capabilities so making the dcen or state polarity
always has to have the good arc stability the cleaning action is another important aspect
in cleaning action which is exploited in case of the gas metal arc welding process and the
gas tungsten arc welding process what is that we know that electrons are emitted by the
cathode and under certain conditions like say 100-1000 ampere of current range sometimes
the cathode which will be emitting the electrons it emits the electrons from a particular place
and that place where from the electrons are emitted that is called cathode spot so the
location or the position in the electrode which emits the electrons is called as cathode
spot. three types of the cathode spots are normally
found like one is pointed in this case electrons are emitted from the tip of the conical shape
electrode and one other is normal where ball shape tip of the electrode emitting the electrons
from all entire area and another is mobile cathode spot mobile cathode spot will keep
on changing its position and it will be like moving at 5-10 meter per second over the surface
of the electrode. in this case basically the location of the
cathode spot keeps on changing so this concept is exploited from the cleaning action point
of view in the sense that if the work piece is made cathode and mobile cathode spot is
formed. like say this is tungsten electrode or any
other process electrode like say in this case if the aluminium or magnesium if they are
welded using the gtw process making the work piece negative and electrode positive so in
this case our work piece is expected to emit the electrons and if the mobile cathode spot
is formed the surface of the work piece so mobile cathode spot formation actually loosens
the oxides being formed. so this loosening of the oxides due to the
mobile cathode spot helps in easy separation from the molten metal and because of this
these oxides starts getting floated over the surface of the weld pool so they can be separated
easily so this is the advantage of using the polarity favorably so the use of the dcrp
is intentionally used sometimes to exploit this concept of the mobile cathode spot formation
especially in case of aluminium and magnesium welding. because the formation of the mobile cathode
spot helps to loosen the oxide layer and the loosened oxide layer starts floating over
the surface of the weld metal and thus it can be separated easily which in turn will
help to produce a cleaner metal. so if we see here like the process like gt
welding or gtw process where use of straight dce and polarity producing the lesser heat
and more heat in the work piece site while if the dcrp is used in that case we get the
much better cleaning action so these are the two contracting requirements we want that
less heat is generated in any case for the much better life of the electrode. at the same time when aluminium and magnesium
are welded then the cleaning action is also achieved but these two can be achieved using
the two different types of the polarities so in order to have a good balance between
the two instead of using one the ac is used so the polarity will keep on changing in one
half it will be dcen and another half it will be dcrp so it is common to use ac also in
case of the gtw process especially when welding the aluminium and magnesium. so that the advantage of the lower heat generation
in one half cycle and advantage of the cleaning action in the another half cycle can be achieved
so this is about the effect of the polarity. now coming to the effect of welding parameters
there are three welding parameters which are extremely important and determine the kind
of joint which is produced and these are like welding current voltage and welding speed
so what we have to see we know i directly affects the heat generation because the arc
welding is a high current and low voltage process so if the current is changed then
it affects the heat generation significantly. so change in heat generation affects the melting
rate as we have seen from the ai plus bli square equation this is one thing second it
affects the penetration depth in the base metal it is about depth up to which like this
is the plate you just apply the arc so how deep melting is taking place that determines
the depth of penetration so this depth in this case like it is a partial penetration
or it may be through thickness penetration. so depending upon the welding current we may
penetrate through the thickness or it may be the partial penetration it also effects
the cross sectional area of the weld being deposited increase in current in general increases
like if we here have the current and cross sectional area then in general it increases
linearly and then it starts getting flattened or the slope starts decreasing gradually so
these are the three important factors related to the welding and effect of the welding current. on the other hand if we see the voltage voltage
does not play big role in generation it contributes but effect is not that high as that of the
welding current but it certainly affects the arc stability if the arc voltage is not enough
that it will lower the stability of the arc we know that if we see that if the arc gap
is short then the arc voltage is very limited under identical conditions if the gap is increased
then arc voltage increases. so in this case the area over which heat is
applied is very limited as compared to the case when gap is increased so when the gap
is increased the area over which heat is distributed is much larger so it directly affects two
aspects one is width of the weld and another is depth of the weld so the width is narrow
when the arc gap is less means arc voltage is less and the depth is more. on the other hand due to increasing or spread
when the arc gap is increased arc voltage increases and so we get the much wider the
weld and the depth is shallow so the shallow depth and the wider bead is obtained the shallow
depth and this kind of geometry may be good especially when the gap between the plates
we welded is not very much even straight and in that case the good gap bridging capability
is provided by the wider arc site. so if the gap is narrow then the gap bridging
capability is limited if the arc voltage is increased so the increase in width of the
weld will provide much better gap bridging capability so this is how we can see the effect
of the welding arc arc voltage it affects the arc stability the gap bridging capability
and the bead geometry in terms of the width and the depth of the penetration and further
if we see effect of the welding speed. welding speed directly affects the amount
of heat being delivered to the base metal in per unit length so if the speed is very
very low then heat will be delivered continuously over a small area and it will result in the
large cross section so depth may be limited but it is very wide actually by reducing the
welding speed we can keep on feeding the heat over the molten metal this in turn does not
help much in achieving the deeper penetration. but it increases the width of the weld thereby
it increases the cross sectional area of the weld but as this will be making the
wider bead with less reinforcement this height of the bead is not much because molten metal
starts flowing sidewise on the other hand for an optimum speed we will see that penetration
is full and width is also limited and the distribution of the bead is also good so here
the reinforcement is limited here reinforcement is good and the penetration is also good. but if the speed is too high so this is in
increasing order of the speed further increase in the speed can decrease the heat delivery
significantly this can lead to the reduced depth of the penetration and very peaked weed
can be formed so high reinforcement very limited penetration and so both narrow and peaked
bead is produced when very high welding speed is used and you know when we are moving very
fast. then due to the high relative velocity with
the ambient gas is it can deflect the arc means arc blow can also take place sometimes
the undercut is also formed because just little melting takes place both the sides so here
near the tow of the weld undercut is formed which provides the easy size for the stress
concentration and can deteriorate the performance of the weld joint so now here i will conclude
this presentation. in this presentation i have talked about the
fundamental aspects related with arc welding and three fundamental aspects i have talked
the factors governing the melting rate and the effect of the polarity and we have seen
that how the welding parameters affect the weld characteristics and certainly the change
in the amount of heat being generated will be affecting the kind of solidification time
and cooling rate experienced by the weld metal and heat affected zone. and these changes in turn will be affecting
the structure and the mechanical properties of the weld joint so those things will be
looking into the greater details when we will talk about the welding metallurgy related
aspect so here now i conclude this presentation and in the next presentation i will talk about
the gas tungsten arc welding process and submerged arc welding process thank you for your attention.

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