If we recall, we started with plate operations,
plate preparation, then plate cutting, followed
by plate and frame bending. Now, the question
of joining them. So, welding is nothing but
a technique of essentially joining. If you
go back pre world wars, it used to be primarily
riveted ships; ships used to be riveted. However
now, as we see, we have almost done away with
riveting as a method of joining. It is all
done by welding.
Riveting is something like this. If I look
in to the hull in the context of ship building,
it has to be done in this way. That means
one plate is coming and it is being riveted
to the other strake. It will have to go in
this fashion. Here it is riveted. So, what
happens?
Suppose two plates are to be riveted (not
welded), you have to shape the plate in this
way. That means this line remains flat one
line. That is internal surface suppose. When
I am joining both the plates, one plate is
overlapped. You drill a hole here and put
a rivet. Physically, rivet will look like
this. That means basically drill a hole, the
rivet originally will somewhat look like this.
So, this is put in the hole, from the other
side you hammer it, and make it flat; I mean
give this particular shape. So, they will
hold the plate together. That is the operation.
That means first you will have to bend the
plate at every joints. Suppose this is the
side shell plate, if you bend the plate all
along the length like this such that you can
overlap, this is referred to as lap joint.
[Not clearly audible] This joint is not water
proof?
No, it can be may be water proof. That means
there is a design of the joints. So, how many
rivets at what spacing, how compressed they
are? All those are the factors. Thereby, it
will be made water tight; there is no problem.
But you think of the involvement in the entire
process; how much of work involved? You have
to shape the plate that way. That means it
is additional work, to drill hole in both
the plates in unition so that they match.
Then, insert the rivet physically and do hammering.
So, this is the process of joining.
Now, when you do welding, then it becomes
much simpler. This is riveting . When you
do welding, then you have the plate like this
and the another plate; you just put plates
face to face as this and weld it. So, no additional
work; only the work of welding; you align
them and weld it. This riveting is no more
done. Now, all the jobs are through welding.
When we talk about welding, what welding is?
Essentially, welding is nothing but a method
of joining two pieces of metallic plates.
It can be non-metallic also, but here we will
talk about metallic plates only – steel-aluminum
welding. So, the entire process involves how
two pieces of plates put together by means
of welding makes it one integral plate. That
means the entire continuity is established
between the two plates along the welded joint;
along the welded line. So, how that can be
done?
That can be done by various processes. One
of the process could be you make the surface
so smooth and bring them so close that a metallic
bond is formed; a molecular bond is formed
and the plates get joined. That can be referred
to as solid-state welding. However, that is
not very feasible in case of ship building.
So, what would be the other method?
Other method would be generate enough heat
through friction and make the surface smooth
enough so that they come together close enough
to form that molecular bond. So, that can
be referred to as friction welding. There
can be another process, pass a current through
the joint such that the interface generates
sufficient heat to melt locally and then withdraw
the current. So, it is all solidifies. That
means by melting; by fusion. So, through fusion
what is being done? The metal is brought under
a liquid state wherein the molecules are free
to move around. Once the heat is removed,
they solidify and becomes one. The necessary
metallic bond is formed. So, that is what
is fusion welding.
Here we will talk about fusion welding. While
doing fusion welding, we need to have external
source of energy, which will in the form of
some heat or whatever. That will melt the
plate. That means raise the temperature of
the plate to the melting temperature.
One of the processes would be using electric
arc. That is how that particular welding process
can be referred to as electric arc welding.
As I said, other method of fusion welding
could be resistance welding. That means resistance
is electrical resistance. Through that it
generates enough heat and the fusion takes
place and the welding. There are other methods
also, maybe we will talk about those methods
later.
Now, concentrate on these that electric arc
welding is the primary fusion welding process,
which is most commonly used. That means here
in this case, the heat is obtained through
the electric arc. To melt the plate, heat
is obtained from the electric arc. There are
other methods of fusion welding like you can
think of using a so-called laser beam to melt
the plate and do the fusion welding,that will
be termed as laser welding. Use the resistance
of molten slag; slag is nothing but that we
get by burning the flux. That gives protection
to the molten metal. So, resistance of the
molten slag can be used which is referred
to as electro slag welding. That is also a
fusion welding; the heat is coming from that
slag - electro slag welding.
I can use the solar energy, concentrate it,
melt the plate and do the welding - solar
welding. I can use electron beam, focus it
to the point and that will generate intense
heat, I can do the welding - electron beam
welding. So, these are all explicit type of
welding processes. We will not go into those,
we will go up to the electro slag welding
and some more. We will talk about that later.
Under the heading of electric arc welding,
again we will have certain different types
of welding processes. We will see that later.
Now, let us see that from where we are getting
the heat, from the electrical power. That
means there is a something called power source
.
When we have electrical power source, we call
it electric arc welding. When the power source
is a laser beam, laser welding; like that.
So, all those welding electric arc welding,
resistance welding, electro slag welding,
submerged arc welding, gas metal welding are
basically electric welding. In some cases,
we are using electric arc; in some cases,
we are using the resistance of the metal interface
or of the flux; I mean slag burnt flux. In
all these cases, we use electrical power source.
So, we will look into what are the types of
power source.
First, let us look into the types of power
source. Generally, welding power sources are
low-voltage high-current power source. That
means the power, which we get, will be able
to deliver very high current. For example,
our domestic power source. What is the domestic
power source voltage? It is 220 volts approximately.
Probably for the wiring you have, it can deliver
to a maximum of nothing more than 15 amperes.
That is why generally we will see the highest
ampere rate socket we have is that of 15 ampere.
That means definitely you are drawing current
less than that. Which all equipment draw more
current? Geyser, iron, air conditioner; they
draw quite a heavy current when you switch
on, then it falls down little bit, but still
current. So, maximum is around 15 ampere and
voltage is 220 volts. So, accidentally if
you touch the terminals, you get really a
severe shock and that shock period extends,
one can collapse straight fatal; it can be
fatal.
Whereas, in welding power source it is just
the reverse. The open circuit voltage will
be of the order of maximum say around 80 to
100 volts maximum. Open circuit means when
you are not doing any welding, you just a
put a multimeter or a voltmeter across the
terminal and it will show 80 or 100 maximum.
That is the open circuit voltage. Depending
on the capacity of the machine, current could
be 100 ampere, 200 ampere, 300 ampere or 3000
ampere of that order. That means even in the
low power welding, ampere will be of the order
of 100, whereas as we saw in domestic power,
it is only 15. Voltage is 80, whereas the
ampere is 100.
Little high heat or high heat input welding
or if I say more powerful welding, it could
be of the ampere of 600, 700, 800 ampere.
It can even go up to 2000, 3000 ampere. So,
that is how it is basically a low-voltage
high-current power source.
[Not audible]
We will come to that. It is low-voltage high-current.
Maybe we can see how the heat is generated.
If we just look into it, the entire circuitry
is very simple. This is the power source.
So, what you have? You have one terminal connected
to the electrode and another terminal connected
to the job. Generally, this is the configuration.
This is electrode and this is the job so this
is the work piece or the job or the plate,
which is being welded.
This is the power source. Power source could
be either direct current or alternate current
power source. That means it could be either
DC or AC. If it is DC, then either of them
will be plus or minus. That also has a significance;
we will see that later. If it is AC, then
one is live and another is neutral. So, this
is the circuit.
Probably if I look in to the electrical circuit,
it would be something like this . 
Let us assume a DC power source. We have like
this – positive-negative terminal; there
is one resistance. This is 
R and R a; that is all. R is the resistance
in the entire circuitry. That means the internal
impedance of the machine and that of the resistance
of the cables. R a is the arc resistance;
resistance in the arc; the resistance here
.
If the voltage is V, then I have a current
I flowing in the circuit. Then, what would
be the heat generated? Heat generated would
be I square R a. There is a joule heating
taking place. That is how the current is important.
That is how we have high current because had
it been current of the order of 10 or 15 ampere,
then hardly any heat would have been there.
So, we talk about 100, 800, 1000 ampere so
that I can have a high current. Because R
a is the resistance of the arc column, I cannot
go on increasing. Resistance of the arc column
is very low. It is essentially the resistance
of the plasma column; ionized gas. That is
somewhat fixed because you will have a somewhat
size of the arc column. You cannot make it
very big; the instability of the arc will
come. You cannot make it very short; there
can be chances of short circuit. So, it will
be sufficiently small. I mean that will not
contribute much; what contributes is the current.
Thereby, we see that welding current becomes
one of the very important process parameter
in the entire thing. That is how the welding
current is the square of the current. So,
how much heat is generated will depend on
the amount of the current. So, that automatically
tells us that if you need to weld a thicker
steel plate, you need to have a higher current
because to melt more metal, more heat is needed.
To increase the welding speed or deposition
rate, how much metal is being deposited again
will depend on the welding current. If more
the current, more heat is generated. So, more
metal can be melted. So, welding current becomes
one of the very important process variable;
one of the very important welding parameter.
We will see its effects later. So, that is
how the current is important.
We have been talking about this power source
, what is this power source? It can be either
DC or AC. Now, we will see that if it is a
DC power source, then this voltage or current
would be something like this straight with
time. If it is an AC one, then obviously it
will have a cyclic thing and we will be using
the so-called line power. Line power means
the power being generated; the industrial
power, which is being generated in the power
plants. So, we will use that power.
Obviously, where this so-called line or supplied
power is not available, you will have to have
your own generator or alternator to generate
power. That is a different issue. Again the
same thing if I use alternator, I generate
AC power; if I have a DC generator, I generate
DC power. So, either I use a DC generator
and directly use or I use an alternator, use
a rectifier and make it DC if I want. If use
the line power, I use a transformer. If I
want to do AC welding, I do AC and I use a
rectifier to get it to DC. So, either of these
two are used.
Now, which one to be used? Naturally the one
which will give me a consistent quality, a
good quality at the end of the job; I mean
rather the job should have a superior quality;
a job where I can control the parameters more
finely such that I get the desired joint quality.
Desired joint quality means the purpose of
welding is not barely to join two plates,
but to make them integrally one, means as
if it was only one; there are no two separate
plates. That means the joint should have the
same physical and mechanical parameters if
not superior compared to those of the original
parameters , original properties of the plates.
That means the welded joint should be free
from all kinds of defects 
and also should produce a microstructure and
mechanical properties that of the original
plate. That means have good property.
To achieve that what do you think from common
sense, which kind of power would give you
better result? DC or AC to start with? Generally
DC; direct current for the simple reason;
you see what happens with AC. As I see with
time, it increases, decreases, then at one
point of time it goes off; it is 0 here . The
power is 0; voltage or current in the circuit
is 0. Again, this is picking up, going to
maximum, decreasing and again becoming 0.
In other words, how many times the electric
arc gets extinguished? If it is a 50 cycle
power, 50 times or 100 times?
100 times.
I think 100 times per second. So, per second,
100 times it is going on and off. So, it is
happening very fast that we do not see. Also,
the heat I square R a is changing continuously.
It is becoming maximum, coming down, becoming
0, and again becoming maximum. So, it is a
kind of a fluctuating power. If it is a fluctuating
power, then the melting rate also will be
fluctuating. So, it will lead to a fluctuation
in the weld deposition. That means though
it is happening very fast 100 times; I mean
things are changing 100th of a second, effect
is not that grossly visible, but the effect
is there. Whereas,in this case, I have a very
consistent power supply and consistent heat
in DC. The melting rate, rate of heat generation,
rate of melting, everything is uniform over
time. So, I get a very consistency in deposition
in DC. So, if I use DC, I expect to get a
superior quality of weld deposit.
The weld deposit what is important. How I
am depositing the molten metal? From the fillar
metal, the electrode being melted and getting
deposited. Thereby, the parent metal is also
melting. Both are getting mixed up and getting
solidified, welding is being done. So, weld
deposit is also important. So, the quality
of the weld deposit will depend on the power
supply. So, a DC power supply will give you
a better weld deposit.
There are other aspects in general, one can
say that for steel welding, when we weld steel
material for ship building purpose, we will
use DC power supply. However, if you look
in the road side welding shops, you will find
people using AC power supply and also welding
steel. I am not saying that you cannot weld
steel with AC you can, but if you want a better
quality and superior strength, you should
go for DC. When we talk about the road side
welding, means where welding things, which
are of not that great quality requirement.
Say you are welding those window grills or
fencing or garden gate and all that. There
you do not have so stringent requirement,
but when you are welding for offshore platform
or a ship or a submarine, you have much more
stringent requirement. So, there we will be
using DC when you do weld for steel. Just
for information if have to weld aluminum alloys,
that logic supersedes some other logic. Thereby,
AC becomes a preferable power source for aluminum
welding. There are some other requirements
and some other things happen. So, AC power
is used for aluminum welding. We have all
these difficulties in the power, but that
helps. We will see that later if time permits.
However, for your information, steel is always
DC for that simple reason and aluminum is
AC power.
We have DC power source. DC power source means
what? That means we have a device, which can
deliver DC power. From where this DC power
is coming? Here I have the AC line supply
440 volts supply; the industrial power 440
volts.
Suppose this is the 440 volts supply; 440
volts, 3-phase supply. There will not be only
two wires, but there will be four wires. We
are putting 440 volts, 3 phase in this machine
and we are getting positive and negative;
two terminals; DC. Here it will be open circuit
voltage, OCV of about 80 volts and current
that can go up to say 1200 ampere as the capacity
of the machine. That means it can deliver
up to 1200 ampere. What is this machine then?
This is a rectifier.
[Not audible]
No, just a rectifier. Transformer means internally
you have a sort of current transformer wherein
you bring down the voltage but boost up the
current. Then, rectify it and you have a DC
output. So, now, if I measure the output here,
then you get the open circuit voltage.
If I measure the power in the arc here, then
I get arc voltage and arc current. Arc current
is nothing but the current in the circuit,
but the arc voltage is the voltage drop in
the arc. So, more important parameters are
essentially arc voltage and arc current; not
OCV, the open circuit voltage.
How much drop is there in the arc? That is
important because that is generating the heat.
That means if arc voltage is V a and the arc
current is I; arc current is nothing but the
welding current in the circuit. So, once again
heat is I into V a; voltage drop at the arc.
If we measure this voltage and current output
at the arc, then we get a certain characteristic,
certain distribution of that. Depending on
those characteristics we have – if we plot
output voltage versus output current; here
output means the voltage in the arc and the
current in the circuit. This is referred to
as volt-ampere characteristics. They are basically
static characteristics and are easily measurable,
where I put a shunt in the arc to measure
the voltage drop; then, I put an ammeter in
the circuit to get the current. Like we do
welding in our lab, we can measure it very
easily with simple devices. So, that is what
is the static characteristics.
Another thing is referred to as dynamic characteristics.
What is that? They are transient variations
in the output voltage and current. This relation
of output voltage and output current is somewhat
fixed for a given machine and a given power
source.
If I refer to this R ; I have written as a
welding power source, it will have static
characteristics as well as dynamic characteristics.
Static characteristics are the relation between
the output voltage and output current; how
it is behaving. The dynamic characteristics
are the transient variations in this output
current and voltage. These transient variations
are very highly transient; means what? They
are not constant; they occur for a very short
period; they keep changing. They occur in
a period in the order of 0.001 second; very
short period.
When these dynamic phenomena are visible?
They are visible when you initiate the arc.
Just start the arc. At that point, whatever
the variations will take place in the output
current and voltage will depend on the dynamic
characteristics of the power supply of that
power of that machine. This is because here
it is a combination it is a power supply;
I am giving a 3-phase power and getting DC
power. Many ways I can get that. So, depending
on the characteristics, type and make of that;
I mean how you have designed it, what all
facilities you have incorporated, you will
have certain type of static characteristics
and certain type of dynamic characteristics.
We will come to static characteristics little
more in detail. As far as dynamic characteristics
are concerned just a few words: Because they
are of very transient in nature and they occur
only during the striking of an arc; means
when you are just initiating the welding and
then it continues. So, dynamic characteristics
play a role at the time of initiation of the
arc; that is one. During rapid changes in
arc length; for some reason, the arc length
is fluctuating. Then also, dynamic characteristics
will play a role. During metal transfer across
arc; the metal is getting transferred; that
is happening continuously because electrode
tip is melting and getting transferred.
[Not audible]
Which? What?
Direction of metal transfer.
No, direction of metal transfer means metal
getting transferred from the electrode to
the job. We will come to that little more
later.
Anyway, that is how… We see that dynamic
characteristics are the one which does play
a role, but not very significant as such because
it is primarily when you are just starting
the welding, then you do not have. Again,
if you do not fluctuate the arc length; if
that can be very well controlled; there is
not much of problem. Dynamic characteristics
may play a role only in metal transfer. So,
that is how we see that generally the power
supplies have certain static characteristics.
They do not take care; the ordinary power
supplies do not take care much about the dynamic
characteristics. Only very elaborate sort
of circuitry and expensive machines will have
a very good dynamic characteristics. That
essentially means what? Just at the time of
striking the arc, what you are doing? You
are short circuiting it. So, there will be
sharp change in the current and voltage. To
take care of that sharp change in a very short
time; they will be in a much fraction of a
second. So, the machine should be capable
of withstanding all those fluctuations. So,
that is how dynamic…
In fact, if a machine has a good dynamic,
power supply with a good dynamic characteristics,
will give us improvement in the uniformity
of metal transfer. That means we will be able
to achieve even higher quality of metal deposition.
Reduced weld pool turbulence, means when you
are doing welding, the electrode will be just
facing the metal below; will be in a molten
state. It will be a molten pool of metal just
below the arc. Naturally, the heat will continuously
have a molten pool of metal, which will keep
moving along with the arc moving. So, there
will be heavy turbulence in that molten pool
of metal. Turbulence is because of the metal
droplets that are falling. So, that will cause
turbulence and spatter. Suppose you drop something
in a liquid, it splashes. That is 
called spatter.
Lot of spatter is bad; lot of metal is wasted.
That may give raise to certain defects also.
That means if you have lot of spatter along
the weld line, it indicates that you may have
a bad welding done. I mean defects could be
there because when the metal spatter; why?
Because lot of turbulence occurred and metal
got spattered; so, there can be entrapment
of gas and some porosity may have formed.
Since the transient variations are less, spattering
also would be less. You will have a uniformity
of metal transferred. However, all these are…
Essentially when you have that transient variation,
striking of the arc, extinguishing of the
arc means at the beginning, at the end, they
mainly what, So, transient variation or the
dynamic characteristics are not that greatly
important. More role is played by the static
characteristics.
Depending on the static characteristics, generally
we have two types of power supplies: one is
referred to as constant current and another
is referred to as constant potential or constant
voltage. So, this is referred to as CC and
this is referred to as CP or CV; constant
potential or constant voltage. These are based
on the characteristics the voltage current
output by plotting the relation between the
voltage current this output voltage versus
this output current .
What is this? In one case, we call it constant
current; in another case, we call it constant
voltage. If it is a constant current, then
we have power characteristics something like
this. This is the current and this is the
voltage. This is referred to as constant current
characteristics 
or a CC power supply; constant current power
supply or a DC power supply having constant
current characteristics.
Now, tell me why we were referring to as constant
current, I can see the current is changing;
not only changing, it is drooping. At times
people also call it as a drooping power source.
I mean if somebody tells you drooping power
source, you should not be surprised. That
means he is referring to a constant current
power source or a welding power source, which
has constant current characteristics because
in one power source, you can have all these
characteristics. Only thing is you will have
to change the settings. You change the settings
to drooping characteristics; you change the
settings to flat characteristics.
The other one is referred to as constant voltage.
[Noise – not audible]
Yes, we will come. Let us draw this CV or
CP and also referred to as flat. Why flat?
Here we see somewhat like this. Constant current
means because what is happening, we are checking
into the voltage current characteristics.
Voltage current characteristics mean when
I am doing the welding, I have certain arc
voltage and certain arc current. Instead of
arc current, it is generally referred to as
welding current; certain arc voltage and welding
current or whatever; voltage and current at
the arc. At that voltage, what is happening
to that current or the total power I into
V a that we are interested in. So, what we
see here?
For a given arc voltage, this is the arc current;
current in the arc. Now, what is liable to
change here in the process? Liable to change
is the electrode position. Suppose I am doing
a manual welding, what will happen? The weld
electrode is getting consumed. When I do a
manual welding, the movement of hand is somewhat
like this . It has a translatory motion in
this direction as well as in this z direction.
You will have to gradually come down. So,
how you are coming down, how much your hand
is shaking and all that will affect the arc.
Arc length will change; keep changing. If
the arc length changes, then the voltage drop
at the arc length will change the arc. That
means V a will keep fluctuating because small
minute changes in the arc length will have
a significant effect on V a. This is because
the current is very high; current is 150 ampere.
When you are doing manual welding current
will be of that order. It can be of that order
150, 200 ampere, 250 ampere.
From here, suppose the voltage changes up
to this much this is fluctuation of the voltage.
From here, it drops to this much or from there,
it increases this much. So, current change
is only this much.
Now, if I have another constant current power
supply wherein I have the circuitry such or
other or whatever, it gives this particular
characteristic. I make it such that I have
a more stiffer curve. This nature of the curve
is more stiff. That means this is machine
A; power source A and this is power source
B. Both are having…
Suppose you go to the market and buy a constant
current power source. The fellow in the shop
shows you two machines of same capacity. That
means it can deliver… Primarily, you will
look for how much ampere it can deliver. You
see that both are giving you 1200 ampere.
However, he is saying that machine B is much
more expensive than machine A, but both are
of the same capacity. Why expensive? Its volt-ampere
characteristics are different; it is better.
Why? Because here you see that for the same
change in voltage, the change in current is
hardly anything; very small. That means it
is approaching to the true constant current
condition. That means though my hand is not
very steady in welding; my hand is shaking
and the welding arc is changing, but the metal
deposition is constant. The heat generated
is more or less constant. V a is changing.
That change is more; probably from 20 volt
to 25 volts but current change is hardly anything.
That is why it is referred to as constant
current. That means for the fluctuation in
the arc voltage, the change in current is
less; much less. That is what is a constant
current characteristic.
A machine power supply with characteristics
similar to that of B here is more preferable.
Obviously, it shows that when I do a manual
welding, it is preferable to use a constant
current power source. If I do manual welding
using DC power supply, it is preferable to
use a constant current power source because
then the weld quality will be better. Because
even if there is a fluctuation because of
the welders’ fluctuation in the arc, the
metal deposition will remain fairly constant.
What happens in the constant voltage? What
you see here means that in constant current,
some change in voltage causes very little
change in current. Here a small change in
voltage causes very high change in current;
just the opposite. Constant voltage power
supply is sort of opposite to that of constant
current. There a certain change in the voltage,
you had a very small change in current. Here
the same change in voltage, you have a very
large change in current.
The nature of the constant current curve is
drooping down. That is why it is referred
to as drooping power source. Here the nature
is somewhat flat; horizontal. So, it is called
flat power source. However, I have drawn a
curve, which eventually goes and meets the
current axis. That means there is a negative
slope. Why this negative slope? Ideally, it
should have been horizontal; internal impedance
resistance, but you better call it impedance
because it is a combination of LC RLC circuit.
So, because of the internal impedance, there
will be a downward slope; a negative slope.
You have this particular phenomenon here that
for a small change in voltage, you have a
very high change in the current. So, what
does it give, why do I use this? In this,
what I see then? For 
some reason if the arc length changes, immediately
there will be a surge of very high amount
of current. If the arc level decreases little
bit, for some reason immediately there will
be a surge of current. What will happen is
instantaneously lot of heat gets generated
and the arc length will come back to the original
length. Hence, the arc length was here, which
corresponds to this voltage. For some reason,
the arc length has dropped. If I further go
on putting it down, then it may stub in the
weld pool and the welding will stop. That
means short circuit case will occur. Instead,
suppose it comes down to this level, immediately
there will be a surge of current. So, what
will happen, you will burn off the tip of
the electrode and restore the required arc
length. So, this constant voltage power supply
gives a kind of a self-regulation of the welding
arc. So, this is useful in the case of automated
welding. When I have a constant feeding rate
of the electrode, I can design a welding machine
where I can continuously feed the electrode.
In manual welding, I have a welding stick.
I weld it, it is over; I throw the but and
put a new stick and again I weld. If I have
a coil of welding wire and I have a mechanism
of continuously feeding, it I can continuously
weld. So, if there is a mechanism wherein
I can feed the filler metal, the welding electrode
as such; the filler metal that melts and gets
deposited, there it will be worthwhile to
use a constant voltage power source. This
is because feeding rate is constant and if
there is any undulation in the plate or somehow
the electrode is coming closer to the plate
for some reason little undulations, the welding
arc will become smaller. Immediately, there
will be high surge of current and it will
automatically change to the required arc voltage.
So, that is how the constant voltage for automatic
operation and constant current for manual
operation.
However, again there is a thing if a very
skilled welder, who is doing some repair welding;
in case of primarily repair welding means
some cracks, fractures, damages taken place,
you have cut it out and that portion you are
depositing metal to fill it up.
At some place, you need certain deposition;
some other place, you need a more deposition
and so and so forth. That amount of deposition
is varying. Then, what happens as I have told,
since deposition rate will depend on the current,
if you are doing manually; a repair welding
will always be manual, generally, we will
not put any automation in repair welding.
It will be manually done with a manual stick
electrode, which is referred to as MMAW, manual
metal arc welding; the general abbreviation.
There what one can do is one will have to
change the current continuously. That is difficult.
Instead, one can use a constant potential
power supply, a skilled welder and he plays
with the arc length. If you want more deposition,
he increases the arc length; he pulls off
the electrode. Those are in the fraction of
millimeters; very highly skilled. If he extends
a little bit, he gets much high current or
deposition. So, very skilled welder can use
this for specific cases.
We will stop here today. Tomorrow, we will
see metal transfer mechanism and welding parameter
and their effects.
