Lecture 12: Physics of welding-4 (Arc Stability and Arc Blow)

Lecture 12: Physics of welding-4 (Arc Stability and Arc Blow)


See, in last class I had completed the arc
power in details. I was also discussing about different categories
of arc i.e. welding arc and different modes of welding arc, in details, in the last class. And today I will start new topics that are
actually called Arc Stability and Arc Blow. Today, I will try to cover this arc stability
and arc blow as well as the forces affecting the metal transfer. So, first of all, we should know what is arc
stability and arc blow. Before going to discuss about arc stability
and arc blow, first of all, we should know what is the function of current? Because what happens, to know the details
about arc stability and arc blow, first of all, we should know about current flow through
the arc. In case of an arc welding technique, what
happens, let this be the electrode. I should explain it and then we will go in
details about arc stability and arc blow. Here what do we observe? We observe that the arc cross section, is
gradually increasing. Let this is generally, cathode and this is,
generally, anode. So, what do we observe? We observe that in the case of arc, this is
the arc. So, what we observe is that this arc is generated
from cathode, means, the electrode and the arc cross-section is continuously varying
from cathode space region towards the anode space region, i.e, from cathode to anode region,
generally, arc structure cross section increases. So, what happens, if the arc structure cross
section increases? Then what happens? There is a chance of concentration of the
electron density. Corresponding to the electron density, generally,
the current density is affected because electron density is responsible for current density. So, if the cross section increases, the current
density, magnetic field strength and pressure, all will be decreased. Because, depending upon the electron density,
that means, lesser the cross-sectional area, more will be the concentration of the electrons,
more will be the current density. That is what, I have already discussed in
details, why this current density increases. So here, generally, due to this varying cross
section, that means, increasing cross section, the current density, magnetic field strength
and pressure, all decrease from the cathode drop region. Generally, that means, from this region, towards
the arc column and towards the work piece region. Because arc cross section increases rapidly
in the arc column. Due to this rapid variation of arc column,
generally, current density, magnetic field strength and pressure, all decrease from cathode
drop region towards arc column. That idea we get. So, as this current density, magnetic field
strength and pressure decreases from cathode region towards the anode region. That is why, generally, what happens, there
is generative varying magnetic field region. That means, due to this current density, what
happens? What we know that through a conducting medium
if current is flowing, that means, if electron or current is flowing through the conducting
medium, around this medium, generally, what happens is that, there develops a magnetic
field. So, let the current is flowing through this
direction. So, here generally, the magnetic field develops. That we know, due to this current flow, generally,
magnetic field develops around the electrode or you can say, around that conducting medium. So, here also, through the arc, there is continuous
flow of current and that is why, through the arc also, there will be generation of some
magnetic field. This magnetic field extends higher near the
cathode region and it is gradually decreasing towards anode region and it is comparatively
very less in anode drop region. Actually, that is what we are getting from
here. So, due to this variation of this magnetic
flux distribution around the arc from cathode region to work piece region, here generally,
develops some radial pressure gradient and axial pressure gradient, due to this variation
of this magnetic flux distribution through that. Here develops some pressure gradient, which
is in radial direction as well as it is in axial direction. That means, along the direction of current
flow. So, because there is an axial direction why
this special gradient is creating? This axial is created because this magnetic
flux density is not uniformly distributed over the arc length, It is generally varying
from cathode region to anode region. Due to that things what happens? There develops both radial pressure gradient
as well as axial pressure gradient. Due to this radial pressure gradient generally,
this radial pressure gradient constricts the arc, whatever the radial pressure gradient,
it is creating along the arc that radial pressure gradient is generally responsible for constricting
the arc. That means, it generally prevent the arc spreading. That means, it tries to compress the arc to
some confined space and this happens due to this radial pressure gradient. So, it generally compresses the arc or you
can say that it, generally, constricts the arc and what happens due to this constriction
of arc? Here generally, there is a rise in temperature. So, the radial pressure gradient constricts
the arc and rises the temperature of the arc discharge. So, radial pressure is responsible for this
two. Whereas, whatever the axial pressure gradient
is generating, due to this variation of magnetic fields distribution, through the arc length
region, there develops some axial pressure gradient also. Due to this axial pressure gradient generally
what happens? This axial pressure gradient gives rise to
plasma streaming. So, this radial axial pressure gradient, generally,
gives rise to plasma streaming. This plasma streaming, generally, transfers
the material and heat from electrode to work piece. Due to this plasma streaming, whatever the
electrode melts or droplet is forming, that droplet is transferred from cathode region
towards the work piece region due to the streaming action. So, apart from these functions of axial pressure
gradient, it also helps to create some turbulence action over the molten pool. It also exerts some pressure on the molten
pool. Due to this turbulence action of this plasma
streaming and creating some pressure over the molten pool, generally, it helps to increase
penetration. That is, it increases the fusion depth also. That means whatever the axial pressure it
is developing, that axial pressure creates some pressure. Where? This is our molten pool. Some pressure is created over this molten
pool, due to this plasma streaming along this axial direction. It is creating some pressure on the molten
pool and due to this pressure; generally, it increases the fusion depth also. So, plasma streaming function is to transfer
the material from cathode to work piece as well as it also carries the heat from cathode
to work piece region. It creates some pressure over the molten pool
region of that work piece as well and due to this it increases the weld penetration. Now, here one thing we should keep it in mind,
that it is generally, both these effects; that means both this radial pressure gradient
as well as axial pressure gradient, is dependent on current flow. It has been seen that this radial pressure
gradient and axial pressure gradient is proportional to a square of the arc current. Now, we will go to the topic what is arc stability,
what is arc blow, because once we know the function of current follow through the arc,
then we will be able to understand this arc stability, arc blow, in details, in a very
easy way. Now, first of all we should know what arc
stability is. Arc stability means arc is said to be a stable
when it is uniform and steady. That means, if the arc is uniform in nature
and steady. If it does not vary with time then, what happens,
it will not vary with time. Then what happens, we can say, this arc is
uniform and steady and then this arc is said to be stable. Here one thing we should keep it in mind that
forgetting a good weld bead and defect free weld, our arc should be stable enough. That means, welding arc should be a stable
enough. If the welding arc is stable then only, we
can get a good weld bead as well as defect free weld nugget. That means, we can get defect free nugget
if our arc is be stable enough. Generally, this stability of the welding arc
is governed by so many factors, which can be mentioned below. First of all when we get a stable arc. That means, whatever the arc is developing
in between an electrode and the work piece, means, whatever the arc is developing, this
arc will be stable and generally the stability of this arc depends on so many factors. Like, first of all here we should know what
are the different factors governing this arc stability. Here the first thing we should know that the
first point of our arc stability is that there should be suitable matching of arc characteristics
and power source characteristics. That means, there will be a suitable matching
of arc characteristics and power source characteristics. Then what happens? That means, suitable matching of arc characteristics
and power source characteristic should be such that it does not extinguish its arc with
a small variation of arc length. That means, if there will be a suitable matching
of arc characteristics and power source characteristics then arc voltage should not extinguish the
arc, if there will be a small variation of arc length. That type of matching should be there. That means, suddenly it should not extinguish. Actually due to small variation, it should
not extinguish suddenly. This is only possible once a suitable matching
of arc and power source characteristics are there. So, that is the first point. That means, to be arc to be stable, first
thing is, its arc characteristics and power source characteristics should match properly. Then next point is, once arc characteristic
is proper, then what happens? Whatever the emission and ionization occur
in arc column region that should be also continuous and proper. If emission is continuous and proper through
the arc and ionization also continuous and proper in the arc region, then what happens? Our arc will be stable enough. It also depends on arc length. It has been observed that lower the arc length,
better the chance of arc stability, that means, better the arc stability. And higher the arc length, there is a chance
of lesser arc stability. That means, there is a chance to deflect the
arc and other things. That is why, we should keep the arc length
as minimum as possible. Then we can get better arc stability. It is also depends on electrode tip geometry,
especially, once we go for using non-consumable electrode like GTAW process. Generally, it depends on electrode tip geometry
also It has been observed that higher the included
angle, better the arc stability. What does it mean? Higher the included angle means, generally,
in case of tungsten electrode, included angle means, this angle is called included angle. Actually, alpha. Let this is the tungsten electrode. So this angle alpha is called included angle. It has been observed that higher the included
angle, greater the arc stability. That means, if the included angle is more,
chances of getting stable arc is more. That is, especially, this type of included
angle that we observe, in case of non-consumable electrode like plasma arc welding techniques,
GTAW techniques, this type of things we observe. Now, arc stability also depends on generally,
presence of dampness, oil, grease etc. on the surface of the work piece. These, generally, increase the arc instability. So, we should clean the work piece, that means,
dampness, oil, grease. Everything we should clean before doing the
welding operation i.e. before starting the welding arc. Generally, we should clean the work piece
before welding. Then only, we can get a good and stable arc. Apart from this thing, it is also dependent
on the welder. That means, it also depends on welder. Why because, if the welder is experienced
enough, then he can control this arc stability. So, what happens? Here, other things we should keep it in mind,
in case of limited practice of the welder also, there can be arc instability. That means, arc instability also depends on
limited practice of the welder. So, apart from this thing, arc stability highly
depends on polarity that you should know. Generally, why because, what happens, these
are whatever, the point I have told you, apart from this thing, it highly depends on polarity. Especially, the welding processes in which
the electrode is expected to emit free electron like a SMAW process, GTAW process, plasma
arc welding process, where the electrodes of these welding processes expected to emit
free electron, required for easy arc initiation and arc stability. For the requirement of that, means, for the
requirement of easy arc initiation and arc stability here, selection of polarity is highly
essential. That means, proper selection of polarity is
required i.e. selection of polarity affects the arc stability a lot, especially in this
welding process where generally electrode is expected to emit free electrons. Here generally, first of all, we should know,
what are the types of polarity that generally gives better arc stability. It has been observed that both consumable
as well as non-consumable types of electrode, generally DCEN, that means, electrode negative
polarity gives better arc stability. Generally, from both this consumable electrode
as well as non-consumable electrode generally, we get better arc stability, if we use an
electrode as negative but always this negative is not good though it is good for arc stability,
but sometimes DCEP is also required to be put there. That means, electrode positive terminal is
also sometimes required to be used. Because of this what happens? The electrode positive has some good effect
which, although does not help in the arc stability, but it has some other good effect also that
also we should know. That I will tell in details here. About electrode polarity here what I have
told? That to get a stable arc what is the thing
that is required? This electrode should be always kept negative. That means, DCEN direct current electrode
negative polarity gives more stable arc, a stable arc both in case of consumable electrode
and non-consumable electrode. Like in case of consumable electrode, generally
like in SMAW, that means, shielded manual metal arc welding using cover electrode this
cover element should have low ionization potential. If it is connected with DCEN, then generally,
it will provide a better and stable arc, better stable arc than that of DCEP. But here one thing you should keep in mind
as in case of consumable electrode, electrode is carrying current as well as it is transferring
the molten material. That means, drop of material is transferred
from electrode to work piece. That is why here generally we keep it in,
as the droplet is transferred from electrode to work piece. That is why, for better metal transfer, generally
DCEP is preferable though we are getting comparatively less stable arc. why? That I will discuss in detail in subsequent
slide. Here one thing we need to keep it in mind
that better stable arc we get in case of DCEN. But with DCEP, we generally get the smoother
metal transfer, in case of consumable electrode. Whereas, in case of non-consumable electrode,
welding like GTAW welding here, tungsten electrode is expected to emit electron for providing
a stable arc .Therefore, here DCEN is commonly used. Always keep it in mind, for of non-consumable
types of electrode like tungsten also, there we use DCEN which generally gives a stable
arc. But generally, for some material like reactive
material like aluminium, magnesium, titanium, for this type of material, once we go for
doing welding, there we should use DCEP because DCEP provides some good action on these types
of material although some exceptions are also there. So, what we get from here? That means, once we go for consumable electrode
as well as non-consumable electrode, generally direct current electrode once it connected
with negative terminal that means, DCEN polarity gives a stable arc for both cases, but sometimes
DCEP is required like in case of consumable electrode for a smoother metal transfer DCEP
is good. Similarly, once we go for non-consumable electrode
like GTAW (gas tungsten arc welding) process, there generally for reactive metal especially
aluminium, magnesium, and titanium. Once you go for doing these types of material
welding then what happened there, we they are generally DCEP is required because it
has some good action on these types of material, some lower quality we can get by using DCEP. But a stable arc will be generated by the
DCEN. Always keep this thing in mind. Now, we get the idea about arc stability. Now, we should know what is arc blow? It is the opposite thing that means, opposite
of whatever arc stability we goti.e. the arc blow is opposite thing of arc stability. Opposite thing here means there is disturbance
of arc that also we should know. What happens if there is a disturbance of
arc then what happens that we should know. What are the different categories of disturbance
of arc, what are the different categories of arc blow that means, blow of arc is there
that we should know. So, first of all the definition of arc blow. We can tell the unwanted deflection or the
wandering of welding arc, from its intended path is termed as arc blow or it is sometimes
called as arc bow because here, arc generally deflects from its intended path. So, its shape, can we like curve which generally
resemble with a bow. That is why sometimes it is also called arc
bow. Arc bow generally develops due to the flow
of current through the arc. This is the main reason of arc blow, always
keep this thing mind. When does this arc blow occur? So, this arc blow is the result of magnetic
disturbance, i.e, once the current is flowing there. I have already told you that there develops
a cell induced magnetic field. So, what happens? Once this magnetic field is disturbed that
means, this arc blow is the result of magnetic disturbance, which, generally, unbalances
the symmetry of self-induced magnetic field. Because there will be magnetic disturbances,
then what happens? Then the symmetry of the self-induced magnetic
field will not be there. So, if there is disturbance, that means, asymmetry
of magnetic field. Then the arc will not be able to retain its
original shape or original path. Then this arc will be shifted from its intended
path to another direction or it is generally changing its shape. Due to this change of path, generally what
happens? This arc blow occurs in the work piece. So, arc blow is the result of magnetic disturbances,
which unbalances the symmetry of self-induced magnetic field. So, generally due to this asymmetry of magnetic
field distribution, arc blow is occurring. This asymmetry of magnetic field distribution
is occurring through the arc to the electrode as well as to the work piece. Because of that what happens? Wherever there will be a flow of current,
there, a magnetic field will be created. So, what happens? If there is a magnetic disturbance ,then everywhere
these self-induced magnetic field will also get disturbed. Due to this disturbance generally what happens? This magnetic arc blow is occurring. Arc blow become severe once the welding is
being carried out in a confined corner on a heavy metal plate, especially once we use
DC current. Why? In DC current this magnetic arc blow is severe
that we should know. Generally what happens? In AC current that is why here, in next slide
I will show you why this is severe, in case of DC current compared to AC current. That also I will explain here. Generally, AC arc are less susceptible to
arc blow than DC arc because AC reverses direction. Generally, we know in every cycle of alternating
current, there is a zero crossing, that means, there is a positive flow as well as negative
flow of current. Due to this reverse direction, a reverse magnetic
field builds up, that means here, collapse and rebuild of magnetic field occurs as the
current reverses from positive to negative direction. Due to this collapse of magnetic field and
rebuilt of magnetic field here generally, magnetic field cannot build up to such a high
value so that it can create arc blow. Because of that, what happens? Due to this cyclic variation of current from
positive to negative, due to this alternating current here generally, magnetic field cannot
build up to such a high value which can generally produce some arc blow. That is why in case of AC current, arc blow
is rarely formed. Whereas, on the other hand in DC welding,
the magnetic field set up in work piece, continuously build up and the arc blow occurs. Here what happens? In DC current, there is no such reversing. So, here generally, continuous magnetic field
build up takes place. Due to this continuous magnetic field build
up, what happens? Here there is a huge chance of arc blow. Now, we will go for the different factors
generally affecting this arc blow. Generally magnetic field is produced in work
piece, so what are the factors affecting the arc blow that also we should know. The main factor which affects the arc blow,
is magnetic field distribution. Generally, this magnetic field is produced
in work piece adjacent to the welding arc, due to current flow through the arc that I
have already told you. So, the main factor is magnetic field distribution. So, wherever there is disturbance of magnetic
field, there will be chances of arc blow. So that’s why, generally with multiple welding
heads, there are two different magnetic weld heads. Generally how it looks like, instead of single
arc. Generally, we use continuously 2-3 arcs. So, what happens? Along this first arc there will be a distribution
of magnetic field. Similarly in second arc also, there will be
a distribution of magnetic field. So, what happens? Due to this multiple welding head arc at one,
that means, arc at one head can affect the arc at other head, that means, due to this
distribution of magnetic field, one magnetic field can disturb the other magnetic field. Due to this disturbance there, arc blow can
occur. Apart from this thing, the magnetic field
producing the work piece around the arc connection, maintains to drive the arc away from the point
where this connection is made. So, due to this earth clamp also, there is
a chance of arc blow. Also ,what happens here? Generally current is flowing from arc, that
means, earth clamp to work piece. So, wherever there is a flow of current, so
there will be some magnetic field generated. This magnetic field can create arc blow. Once this arc is passing near the earth clamp
region, ok. So, these are the main factor which govern
the arc blow. That means, the main factor of arc blow is
magnetic field disturbance. This magnetic field disturbance can occur
in different way, that what I have told you. It can occur due to multiple welding head,
it can occur due to multiple welding head as well as it can occur due to this arc connection. So, what happens? These are the factors generally which affect
the arc blow. Now, we will go what are the different categories
of arc blow? Depending upon the above factors, this arc
blow is generally categorized into three different categories. So, what are the three different categories? A forward arc blow, at the starting end of
a weld, and a backward arc blow, at the finishing end of the weld. So, this is generally; that means, forward
and backward arc blow. A sideward deflection is another type of arc
blow and arc rotate is another type of arc blow. So, these are the three different categories
of arc blow that can be there. So, forward and backward arc blow can be there,
a sideward deflection or sideward arc blow can be there or arc rotation also is another
kind of arc blow. Now, I will discuss in details about this
forward, backward, sideward and arc rotation types of arc blow in details. So, first of all, forward arc blow. Generally forward arc blow takes place at
the starting end of the weld and backward arc blow takes place at the finishing end
of the weld. Why? Generally magnetic flux line gets crowded
near the starting and finishing end of the work piece, because they find an easier path
through the work piece than through the air. That we should always keep it in mind. Generally magnetic flux line passes through
the work piece rather than passing through the air. So, it is crowded i.e. instead of passing
through the air it passes through the work piece because it gets easier (lesser) resistance
in work piece rather than in air. That we know actually. That is why, the arc seek the path of least
resistance and deflect toward the weak flux side. Because of this what happens? Due to these types of magnetic flux distribution
generally concentration of magnetic flux is more at the starting end as well as at the
finishing end of a weld line. ,Because the concentration of magnetic line
will be more. So, due to this more concentration here, the
resistance will be more near the starting end as well as near the finishing end. So, what happens? Due to this more resistance generally arc
deflects away from that side. That means, arc deflects away from a starting
end side as well as arc deflects away from finishing end side, towards less concentration
flux distribution region. That I will show you, in details, by drawing
the thing. That means, this causes the forward arc blow,
at the starting end and backward arc blow, at the finishing end of the weld bead, in
the work piece. How that happens, that I will show now. Like here what happens? In case of welding, let this is a weld plate,
near the starting end. Let this is the arc, this is the electrode. Similarly, this is the starting end. Let this is the finishing end. So, what happens at the starting end as well
as in the finishing end? Generally, arc deflects like this. Why is this deflection taking place? So, what is happening? Here generally this concentration of this,
here you see one thing, you can observe that the concentration. This is the magnetic field concentration,
This is the welding direction actually. So, here we see the starting end. That means, near this position, actually,
this arc should be near to here, ok. Generally, here, the concentration of this
magnetic field line is more because this magnetic flux lines pass through the work piece instead
of passing through the air. It generally passes through the work piece. That is why, generally here, crowding of magnetic
field line is more than this other side. So, what happens that is why here, this side,
generally we get less resistance than this side. So, due to this less resistance, generally
arc seeks the path of least resistance direction. That is why, in the starting end generally,
this arc deflects in this direction. Whereas at the finishing end here also, similar
thing occurs. That means, this flux lying density here also,
occur like this. That means, concentration of this flux. So, something we can observe in this side
also. Here we see that flux line density is more
compared to this side. This is density flux line. Here density of flux line is more, magnetic
flux line is also more and here generally density is less. So, what happens? Due to this generally, arc deflect toward
the less dense region, means where the density is less, i.e, the density of the flux line
is less. So, in that direction the resistance also
will be less. If the density of the flux line is more there,
the resistance will be more. Due to this, generally the arc seeks the path
of less resistance and deflects towards the weak flux or we can say less flux density
side, it deflects. Due to that what happens? In case of arc welding technique at the starting
end and at the finishing end, we get arc blow. So, this is called forward arc blow at the
starting end. Whatever the arc blow is occurring, this is
called forward arc blow and this is generally called backward arc blow. So, why it is occurring that we understand. This has occurred due to the crowding of magnetic
flux line, near the starting end and finishing end of the weld line, these two different
types of arc blow occur. So, if we see the side view of
this things, this middle region we will get a stable a steady arc generally. Here this middle region that means apart from
a start and end region, in between we get a stable arc or you can say non-deflected
arc. So, generally, if we see the side view of
these thing, at the starting end, generally, we get an arc, which looks like this, at the
finishing end we get an arc, which looks like this, at the at the middle region generally
we get arc, which looks like this. So, what happens here? Here at starting end arc is deflected, at
finishing end arc is also deflected, but in between generally arc is not deflected. So, this is forward arc blow and this is generally
called backward arc blow, this is the direction of welding, ok. So, we get an idea about forward and backward
arc blow. Now, we will we will see what is sideward
arc blow. Generally, sideward arc blow depends on work
piece earth clamp. That means, we know that generally, in case
of welding, there is an earth clamp connection. One connection coming from earth to the work
piece and another terminal is connected to, what is called electrode that we know. So, what happens here? This sideward arc blow generally occurs due
to work piece earth clamp. What happens is that as the arc come near
the earth clamp. It deflects sideward in a direction and from
the clamp perhaps because of the magnetic flux, enacted in the workplace, by the earth
clamp. This magnetic flux generally is produced by
the flow of current from clamped work piece. Whatever the magnetic field here it is producing
or that is produced because there is a flow of current from earth clamp to work piece. So, wherever there will be a flow of current,
there will be magnetic field distribution. Due to this magnetic field distribution generally
what happens? When the arc come near to the this earth clamp
position, generally, it deflects sideward in a direction away from this earth clamp. It is also noted that as the arc process the
earth connection it has a tendency to come to the original line of travel. That means, arc has a tendency to come to
the original line of travel. This is how its looks like, that I am showing
here little bit. Then it will be clearer to you. Let this is a weld plate, this is a work piece. Now, in this work piece welding is required
to do. Let us see what happens. Here the earth connection is there. Let us here, put the earth clamp, these earth
clamp connected with this cable. Generally, this is this is generally called
ground cable. This is called earth clamp. So, generally what happens? Due to the flow of current from earth clamp
to work piece, here generally, develops some magnetic field, because there is a continuous
flow of current from earth clamp to work piece. So, what happens? Due to this, once this welding come near this
region it deflects, like this. So, how the welding looks like? Here at the weld bead looks like here (fig),
this is the weld bead. So, what happens on the welding? Welding arc coming to so, this is generally
deflected weld bead actually. So, what we observe here? Due to this earth clamp, generally here, arc
shifted to its intended path. That means, it is not going as straight line. Instead of going straight, it deviates. That means, arc is deflected away from the
earth clamp direction sideward, ok. Due to this generally here, the arc weld bead
we get, is deflected sideward. But once it crosses this earth clamp position,
then generally,. it returns back to its original line. That what happens, if it is just cross this
position, it generally again it returns back to its original weld line. So, these types of arc blow are called sideward
arc blow, because here generally, arc is deflected sideward, due to the position of earth clamp. Now, we will see what the different effects
of arc blow are, and what the remedies of arc blow are. That also we should know and apart from this
thing, we should know what is arc rotation? Here generally, in the last two cases what
we have observed? Generally, arc is deflected either forward
direction, backward direction or side ward direction. Here instead of, not only arc is deflected
but here the arc is rotated. Also that means, here rotation as well as
arc deflection is taking place. Due to this generally what happens here, shape
of which, looks like wavy in nature. That means here, some sort of rotation as
well as deflection, types of shape of the weld bead is generally observed here. This arc rotation indicates that under certain
condition of arc blow, perhaps, the arc experiences magnetic field line parallel to the arc axis. Generally, what happens? Here what we have observed generally, magnetic
flux line is generally generated and in transverse direction, that means, from what we have observed,
generally magnetic fields line is generated in the periphery direction. Instead of magnetic flux line, which is generated
in transverse direction, here generally these types of things can happen. That means, perhaps our experienced magnetic
field line here ,is parallel to the arc axis. Along arc axis here, it is also magnetic field
line, can be generated parallel to that arc axis also. Due to this generally what happens here is,
these types of arc rotation as well as arc deflection occur. This is generally third category of arc blow. The reason of this thing I will discuss in
details once I will discuss the metal transfer actually that means, drop transfer in case
of welding process in detail. Generally, the reason of this arc rotation
and arc deflection is due to very high current. And what happens? Due to this high current generally, this high
heat is generated and due to this high heat generally, this electrode cannot return to
its original a state, a straight shape. So, here generally electrode itself deflect
and due to this high heat, this electrode tip generally become plasticized and soft
enough and what happens electrode tip becomes deflected shape. Due to this deflection of electrode generally,
what happens here? Due to this deflection of electrode here generally
creates asymmetry of magnetic field distribution. Due to this wavy nature of this electrode
tip as well as asymmetric distribution that means, asymmetric distribution of magnetic
flux, here generally it creates two different types of forces. What are these two different types of forces
here it creates? Here generally, one of the forces is the radial
force and another force is along its transverse direction that is called azimuthally. Generally this is called azimuthally forces,
which are developed generally in case of arc rotation. Due to
these two different forces, that means, asymmetric radial forces and azimuthally forces, generally,
what happens here? Here these types of arc rotation as well as
deflection are taking place. This I will discuss in details. Also we will discussing the metal transfer. There generally these types of phenomena I
will discuss in detail. Now, what are the different effects of arc
blow? Arc blow has significant effect on weld qualities. Due to the arc blow generally they create
different types of weld defect in a welded sample. Generally, what happens? This arc blow has a very tremendous effect
on quality of welding, because where there will be arc blow there will be general unstable
arc, this is the main thing here. So, once there will be unstable arc, there
will get poor weld bead appearance, there will get irregular and erratic weld deposition. Because, if the welding arc is itself unstable,
then generally all the these are all different defects which arise due to this effect of
arc blow. Then there can be undercut lack of fusion
due to this arc blow, because here welding arc is unstable in nature, there can be spatter
due to this arc blow, there can be uneven and weak welded joint. There can be slag entrapment and there can
be porosity. So, these are all defects which can arise
if there will be arc blow. Now, how to eliminate this arc blow or how
to minimize this arc blow? So, we should know, what the remedies for
arc blow are. It can be minimized by keeping the following
factors in view. Generally, whatever the factors I have showing
here; if you just follow this step, then we can minimize or eliminate the arc blow. Like, what I have told you? One of the causes of this arc blow, we know
that means, at a starting position and finishing position there is, what called air gap is. So, that is why this flux, generally instead
of passing through air, concentrate towards the work piece region. Due to this, generally concentration of arc
flux is there over this region. So, if you can provide a run on and runoff
plate, generally then what happens, this magnetic flux can pass, through this. What happens, then there we can get more or
less uniform types of magnetic flux distribution. So, that we can, at least minimize or we can,
to some extent eliminate the forward and backward arc blow. So, by providing some run on and run of plate
that means, by use of which generally, the magnetic flux distribution can be made uniform. We can make the magnetic flux distribution
more or less more uniform. Another region of this arc blow is due to
this earth clamp. So, changing the position of earth clamp and
welding, away from the arc connection. So, if we do this welding operation far away
from this earth clamp position, then what happens? There is less chance of sideward arc deflection
also. So, we should put our earth clamp far away
from this weld line. So, by doing this also and apart from this
thing, instead of one single earth clamp, if you provide opposite side another earth
clamp there, due to this what happens, due to this they balance out. That means, by the effect of two different
earth clamps, there is a chance of less deflection of the arc. So, the first point is, by changing the position
of the earth clamp and by welding away from the earth connection, we can eliminate or
minimize the arc blow. Then what are the other reasons? It is storing work piece away from the magnetic
source. that means, another source of arc deflection
is source of magnetic field. So, what happens? If we do the welding near a magnetic source
region, then what happens, there is a chance of more arc deflection. So, what happens? Once we do the welding operation away from
this magnetic source, like one of the magnetic sources, in case of welding operation is the
magnetic power source itself. So, we should keep the power source far away
from the welding region, then generally there is a less chance of what is called arc blow. Then employing ground connection more than
one, what I have told you; that means, ground connection means, and earth clamp more than
one. Using short arc, generally I already told
you, generally short arc gives a stable arc. So, once there is stable arc, in case of short
arc, the chances of deflection are also less. Then lowering the arc current also, if the
arc current is less, then definitely the magnetic field intensity and other things will be less. So, there is a less chance of arc deflection. Now, thus another point is using a smaller
diameter electrode. If the diameter of the electrode is smaller,
then what happens, there will be generally less chance of arc blow. Then decreasing the travel speed, that means,
whatever the welding speed, by which that welding is carried out, that travel speed,
by decreasing that thing also, we can minimize the arc blow. And the last point we can say a superimposing
and counteracting externally applied longitudinal magnetic field. By superimposing this thing also, we can minimize
the arc blow. So, these are the different factors generally
if we just keep this thing in mind then we can minimize the arc blow a lot. So, now we have seen about arc blow as well
as arc stability in details. Now, we will go to metal transfer. Here what was so what we should know, before
going to metal transfer first of all here what we should know? We should know what are the forces generally
affecting metal transfer that we should. So, before going to details about metal transfer
here one thing we should know before going to first of all we should know what are the
different forces affecting the metal transfer then only we can be able to understand the
metal transfer in details. In case of metal transfer, there are two different
types of forces affecting that metal transfer or drop transfer from electrode to work piece. This is what I am discussing here. This is for consumable types of electrode. That means, this electrode generally melts
and drop is formed and what happens, there is drop transfer from electrode to work piece. These types of things are called consumable
types of electrode. Generally, I am telling because metal transfer
is taking place in case of consumable electrode [FL]. So, generally there are mainly two types of
forces affecting the drop transfer. One is called aid metal transfer forces and
other one is called retard metal transfer forces. What is aid metal transfer forces and retard
metal transfer forces? Aid metal transfer forces are those forces,
which helps to detach the droplets from electrode and which help to transfer the drop from electrode
to work piece. Then generally the forces which help for detachment
of that droplet from electrode as well as transfer from electrode to work piece, that
force is called aid force. That means, which help for metal detachment
as well as transformation. Another force is called retard force. Retard force means the opposite of this thing. That means, which opposes this effect. That means, which oppose this metal detachment
as well as metal transfer. This force is called retard metal transfer
forces. In general, in case of metal transfer phenomena,
there we get different types of forces. What are the different forces? Like, there can be surface tension forces. In general, there can be viscosity forces,
there can be high velocity gas jet forces, there can be gravity force and there can be
another force, which is Lorentz forces. So, these 5-6 types of forces are, which we
observe, in case of drop transfer. Generally, some of these forces aids metal
transfer force, which helps the metal detachment and transfer and some of this force which
generally oppose that phenomena, that means, which oppose that detachment as well as transfer
of metal. So, which force is opposing and which force
is detaching? That I will be discussing in details about
these different forces in the next lecture. So, I will discuss about all these forces. Apart from this thing, I will discuss about
metal transfer actually which works, in case of consumable types of electrode so that I
will discuss in next lecture. 1

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