Hello, welcome to the first lecture on the
online course on Analysis and Modelling of welding
My name is Gandham Phanikumar.
I am a faculty member of the department of
Metallurgical Materials Engineering - IIT,
Madras.
In this first lecture, we are going to introduce
the different welding processes, and then
look at how the heat input and other specific
aspects of the process will be differing across
the different welding processes.
Let us first look at under what over view
does the joining on welding processes come
under.
We have basically a large number of manufacturing
processes, which can be categorized broadly
as casting processes, which are the primary
manufacturing processes, such as ingot casting,
shape casting, powder metallurgy etcetera.
And then forming processes, where we a change
the shape by using processes such as forging,
extrusion, deep drawing, sheet metal forming
etcetera.
And then we have machining processes, which
are somewhat like material removal processes,
which include turning, boring, drilling, milling,
grinding etcetera; advanced processes such
as abrasive jet cutting, and water jet cutting
etcetera also fall under the machining processes.
And it is in this broad category that joining
processes also come; and they can be called
as fabrication processes, because they are
used to fabricate a large part from several
small parts.
So, the large number of joining processes
exits, and they can be categorized broadly
again into the following categories.
A mechanical fastening is a fabrication process,
where a different parts are joined together
mechanically without a metallurgical bond
between the two parts that are been joined.
We also have adhesive joining, where dissimilar
parts can be joined using epoxy and various
polymeric materials.
Brazing and soldering are joining processes
that are of importance to materials, where
the melting points are very different, and
it is the filler which will join these two
meeting parts or rather than melting of the
two meeting parts.
The difference between brazing and soldering
is basically definition of temperature 450
degrees centigrade is arbitrarily chosen as
a cutoff, so a process that works below 450
degree centigrade can be called as soldering;
the process that works above can be called
as brazing.
So, welding comes under this broad set of
processes.
So welding is one of the fusion processes
that come under this broad category.
And welding processes again can be categorized
by a different source of joining.
The first set is by arc welding, where there
is an electric arc that is used to join the
two materials.
I have given the acronyms here as per the
AWS - American Welding Society specification.
So there are several arc welding techniques
such as shielded metal arc welding, which
is also called as stick welding or manual
metal arc welding.
Then GMAW - Gas Metal Arc Welding which is
also referred to as MIG Welding - Metal Inert
Gas Welding, which is an automated process;
GTAW - which is also referred to as stick
welding, Gas Tungsten Arc Welding, and SAW
- which is Submerged Arc Welding; FCAW - Flux
Code Arc Welding; and then PAW - Plasma Arc
Welding.
So there are so many different varieties that
are possible using electric arc as a heat
source.
We have also processes that use a resistance
- electrical resistance to join the materials.
So we have a spot welding, seam welding projection,
welding etcetera.
The welding does not require actually the
materials to be completely molten, so solid
state welding also is possible.
So we have processes such as ultra sonic welding,
friction welding, explosive welding etcetera,
where the fusion is not explicitly caused.
And then we also have specialized processes,
where the melting is initiated by using a
beam of high energy.
So, we have LBW that is laser beam welding
and EBW that is electron beam welding as processes
that use in high energy beam to lead to the
welding process.
And these welding geometries can then be looked
at by looking at the type of joints.
So here are five joints and that are usually
refer to this is essentially to make the terminology
familiar to you as we use these names later
on.
So what is normally refer to as a butt joint
is essentially when the two meeting parts
are just opposed beside each other and then
the joint is made.
Lap joint is where the two meeting parts are
kept on top of each other and then the joint
is made.
A tee joint is where the meeting parts are
configured to look like a T.
And then, a corner joint where they are kept
at an angle to each other and then at the
edge, they are then joined; edge joint is
basically again a joint where the two meeting
parts are initially kept in the butt geometry,
but then are folded up so that they can be
joined along the edge rather than along the
plane.
So, we will also make all this similar with
some more joints, which can be understood
by looking at the extent of the weld that
is penetrated.
So there are what are called as a bead or
surface welds where a plate is welded by making
the welding torch move along the surface.
And these are generally used to join materials
that are of low thickness and there is not
much of preparation that is required to join
materials in this fashion.
For thicker materials, however, we will need
what is called as a groove weld and we will
talk about that in detail briefly.
A fillet weld is where we have them in the
lap geometry and at the junction between the
two plates, we have the joint been made using
a filler.
Plug weld is something like a spot weld where
a locally the melt zone is made to join the
two materials.
Little bit details about these; a bead or
a surface weld is used very often for a butt
welds; and the advantage of this geometry
is that there is no edge preparation that
is required; and this is used for thin sheets
of metal.
And this is also used for example, to build
up surfaces.
So if you want to repair a part, then you
can remove some material by gauging or any
other metal removal process and then the material
can be build up at the same location again
by making a bead on top of material.
We also can actually deposit different materials
on top of objects that require a different
property on the surface by using what is called
as a weld overlay.
So weld overlay is basically a process that
is very similar to welding, however, the joining
is not the objective of the process, but depositing
a different material usually a corrosion resistance
material or an abrasion resistance material
is deposited, and these are generally done
in a geometry that is called as surface weld.
Groove welds are once that are used for large
thickness joints.
So these are also usually made in butt geometry
and the large thickness also implies that
we will have the weld penetrating to the entire
thickness, and that would require a very detailed
edge preparation.
And very often the welding techniques may
not be able to join the entire thickness in
one go, so we may have to resort to what is
called as a multi-pass welding.
So how detailed can the joint preparation
or edge preparation be in a groove weld is
clear from this set of groove preparations.
The image has been taken from a Wikipedia,
but these joints are name are used in the
literature quite commonly; so we have what
are called square, closed square, single-bevel,
single-J, double-bevel, double-J, single-V,
single-U, double-V, double-U etcetera.
So we one can actually make a very detailed
edge preparation, so that we can achieve a
large thickness weld by making beats one after
other, and the number of beads depends upon
the thickness that we have to join.
So, there are situations, for example, in
nuclear industry where we may have to join
a several tens of centimeters of a thickness
plates and these could be joined for example,
in several dozens or even hundreds of beads
that are done one top of other.
Fillet welds are `what are used to join materials
when they are kept in lap geometry.
And tee, lap and corner all three can be joined
in this mode of fillets welds.
The advantage of this particular way of joining
is that there is no edge preparation; however,
there is a requirement that you need to have
filler.
Plug welds are basically replacements for
fastening process, fastening process be only
a mechanical joint; if you want to make that
fastening to be permanent in nature, then
we would normally resort to what are called
as a plug weld.
So holes are drilled on the sheets that are
on the top surface and then a weld bead is
made on the top, so that then the hole is
covered by the deposited material.
These also are used when there is no design
possibility to have an excess deposit and
plug welds can be also called as spot welds.
The welding itself can be done in different
directions, and there are names that are commonly
used in welding literature.
So you must familiarize us with this name
also.
And here I have given you a schematic that
shows the five different welding positions
in which welding is done.
Some welding positions are such that a certain
welding processes cannot be used.
I have shown on the left hand side, a vector
show in the gravity direction downwards, which
means that in this cube, the gravity is acting
in the downward direction.
The five directions along which the torch
can move are shown here.
We have the a most common way of joining which
is called as a flat geometry; in flat position,
essentially the welding torch is kept almost
vertically up and it is moved horizontally
on a plane and this geometry is most common.
A horizontal geometry is where we have the
torch moving horizontally, and the welding
torch is held not in the direction of gravity,
but at 90 degrees to it.
Vertically up and vertically down will also
require that the welding torch is held 90
degrees to the gravity direction, but it is
moved vertically up or down as oppose to horizontal
direction.
Overhead welding is a different geometry it
is where the torch is held exactly anti-parallel
to the gravity direction, and then it is moved
along the horizontal direction, which means
that the arc is going in the direction opposite
to the gravity.
So, the welding processes which we have looked
at till now fall under the broad category
what is called as a fusion welding.
And these fusion welding processes are also
then classified in different, different terminologies,
and we will make all these similar with the
terminologies here.
We will be able to classify them as consumable
and non-consumable electrode to basically
look at whether the electrode is used as filler
or not.
So we have processes such as TIG welding where
it to be a non-consumable electrode welding;
and then a process such as MIG welding would
be a consumable electrode welding.
The welding can also be done without any filler
at all in which case it is called as an autogenous
welding.
And if it is used with filler, you can say
that it is a welding with filler.
And this filler may match the material properties
of the base metal, in which case, you can
call it as a homogenous welding.
And, in case the material that is used for
the filler is different from the base material,
so that the welding process is successfully
completed without any weld cracking etcetera,
then you would call that as a heterogeneous
welding processes.
So homogeneous and heterogeneous imply that
there is filler that is used which is either
same or different from the base material,
and that is also one more way of classifying
the fusion welding processes.
Fusion welding also means that the material
that is being joined is going to be molten;
and as you all know most of the metallic materials,
when there in the liquid state are highly
reactive and that would require that we have
a protection for the liquid metal, so that
it does not form oxides.
If oxides are formed, they will enter the
base metal during the solidification of the
weld pool, and then cause defects and cracks
later on.
So it is important to protect the liquid metal
during welding; and this can be done by either
a flux or an inert gas.
So, you could also classify fusion welding
processes as flux protected welding process
and inert gas protected welding process.
A fusion welding may not be successful for
a high thickness joint in one pass, so you
would also like to perhaps classify the fusion
welding processes as single pass and multi-pass.
Multi-pass actually would open up a more complications
to understand the thermal process, because
we would have multiple single passes that
are laid on top of each other, and the residual
heat would start playing a role with the welding
efficiency that would be implied for the further
process.
Some more terminology for us to make ourselves
familiar, so we would be referring to what
is called as a traverse rate; so what we mean
by traverse rate is basically the velocity
with which the welding torch or welding source
is moved, and it would be usually the units
same as velocity meters per second, but the
unit that is used in welding community will
be in millimeters per second, it would be
usually hundreds of millimeters per second.
And heat input is a specific term that is
used; it should not be confused with the English
meaning, which conveys that amount of heat
that is given, but it is actually a ratio
of the power that is being given by the welding
source to the base material to the velocity
at which the welding torch is moving.
So the power has units of joules per second
and velocity has units of meters per second,
so you would have heat input having the units
of joules per unit length of the weld that
is taking place.
This also means that a welding process, which
has a high-speed capability, would naturally
be of a low heat input.
And this may not be obvious from the word
heat input when we apply two processes such
as electron beam which are known to be low
heat input processes.
Rate of heat input or heat intensity is also
one term that we will use; it is basically
to show at what rate the heat will be arriving
from the heat source to the base material,
and it would be normally referred to in the
units of a power per unit area watt per meter
square.
And the area over which the rate of heat input
is being applied should also be normally known
so that we can integrate this particular quantity
to know how much of heat has been deposited
completely on the base metal over the duration
when the welding is taking place.
And how this heat is then distributed spatially
on the surface of the base metal is also important
and that is what we refer to as heat intensity
distribution.
Heat intensity distribution is often such
that it is a maximum value at the center of
the welding torch and goes down as you go
away from the welding torch.
However, this will not be the only variations
of heat intensity distribution, we can also
have other variation that are possible as
in for example, laser welding where a very
detailed set of lenses could give you any
distribution that you would wish.
We will come to that shortly in a next lecture.
So, these are the quantities that we will
be referring to again and again during the
course of this stock, so the technical meaning
of these terms and the rough values of these
quantities for a given welding process should
come to as naturally as we go along this course.
So, there is several welding process that
we are going to look at for an overview in
this first lecture.
And I have listed some of them.
This is not a compressive list of all the
processes that are important in the industry,
but it would give you a fairly large set of
processes that would cover, what would what
would be happening in the industry.
So these are the processes we are going to
look at shielded metal arc welding, gas metal
arc welding, tungsten arc welding, plasma
arc welding, submerged arc welding, electron
beam welding and laser beam welding.
So, before we proceed the further with an
over view of all these welding processes,
it is important for us to look at how the
electric arc is generated, and how it is sustained
and then how that is playing a role as a heat
sources.
So, here is the detail we have the geometry
on the right hand side in the schematic showing
you that there is an electrode and what is
coming out sharply below is the electrode,
which is usually tungsten in the case of non-consumable
electrode, and it is the same material as
filler that is used as a wire.
And workpiece is shown at the bottom.
Now it is a workpiece that has to be joined
during this welding process and I am showing
you between the electrode and workpiece what
appears to be an electric arc.
So there must be a polarity that must be applied
for the electrode and workpiece and usually
workpiece will be given a connection to the
earthing and electrode will be then given
a voltage either positive or negative has
polarity would require.
And the both electrode and workpiece have
to be conductors of electricity, so that upon
application of voltage and a small gap that
separates between the two then electric arc
can be struck.
And this arc should then be ionizing the gas
that is present in between the electrode and
the workpiece, which usually will be argon
or helium as you may choose and the ionized
gas would then start moving the energy from
the electrode onto the workpiece.
So, it is important that the arc is stabilized
so that the welding process can continue during
the entire fabrication requirement.
And sustained energy discharged from the electrode
onto the workpiece is possible when we look
at what constitutes the environment that surrounds
the arc and we will look at that shortly.
So what kind of gases can be used to make
the arc in an arc welding?
So here I am just showing you some requirements
for the gas, the gas generally for a given
metal that you want to join could be a either
inert or active; and usually we would choose
the gas which is inert, because we do not
want it to react with the base material and
from compounds that are not desirable.
However, there are situations where an active
gas may also be applied.
And the most important role of the gas is
to shield the liquid metal from getting affected
by the environment around and to give the
stability of the arc.
And the kind of gases that are generally available
in engineering environment are listed here;
carbon dioxide, oxygen, nitrogen, hydrogen,
argon and helium.
And I have listed the parameter called as
ionization potential on the right hand side
in electron volts.
So, what it implies is that this is the kind
of energy that is required to ionize the gas,
and that gives you a hint about how much of
voltage is required to strike an arc and sustain
it.
And you could see that argon and helium at
the bottom are giving a bold, because they
are the once there are commonly used in welding
process.
As you can see that argon have a lower ionization
potential, and it is also having a slightly
higher density which means that the shroud
gas shroud will stay put near the arc without
getting diffused away very fast, which makes
argon as the gas of choice for use in welding
processes.
So, there are some characteristics of the
arc that we need to be familiar with so that
we can understand why the voltage and current
condensations that are chosen for welding
are like that.
So we are seeing in this a plot, a red curve
that is going from top to bottom, and this
curve is also called as the drooping characteristic
of the power source.
Essentially there is a voltage roughly between
60 and 80 volts that would be called as open
circuit voltage that is available for the
welding source.
And the voltage current characteristic is
such that it is drooping down that is going
down as the current is increased.
And how the arc will behave is given the roughly
u shaped or a tick shaped curve that is shown
in green color.
On the left hand side that is at low voltage
and low current corner the arc is not stable;
and therefore, those parameters should not
be used for welding.
So in the linear portion of this curve, we
can start exploring the parameters for the
welding purpose; and as you can see that for
the same current, you would need a higher
voltage to sustain arc over a longer length.
It also means that during the welding process,
if the arc length is varying because of any
reason such as a manual operation or because
of surface undulations on the sample surface
it also means that the voltage at which we
have to do the welding also will vary.
And the drooping characteristic of the power
source as well as the arc characteristic intersect
at points.
So those are the combinations which are basically
the appropriate voltage and current choices,
so that the welding can take place.
And I have highlighted the voltage ranges
in blue background to showing is a normal
operating range, which turns out to be between
15 and 30 volts for most of the arc welding
sources, and the current values that would
come out would the then between 100 and 300
amps.
So, the electrode can then be given different
polarity for the purpose of welding and we
normally have a terminology that would also
describe this, we call what is called as a
direct current straight polarity - DCSP when
the electrode is negatively charged.
And this is used whenever deeper penetration
is required.
DCSP is also called as DCEN that is electrode
negative.
There is a second type of polarity that is
referred to as direct current reverse polarity
DCEP electrode positive.
And you also can use alternating current that
is change the polarity from positive to negative
at a particular frequency.
So the electrode polarities are given in three
different manners.
To understand how these polarities are to
be chosen, we will go to a schematic on what
happens when we change the polarities.
So then the electrode negative polarity what
happens is that because electrode is negative
the negatively charged electrons are going
away from the electrode into the workpiece.
And these electrons are accelerated very high
in the arc and they are then stopped by the
workpiece, which converts the kinetic energy
of the electrons into heat that will be generated
in the workpiece which is basically the major
source of energy release because of the arc
on the surface of the workpiece.
And the positively charged ions, which basically
are argon ions, when argon is the gas that
is causing the arc; these argon ions are then
moving in the opposite direction going towards
the electrode in the case of DCEN.
And you can see that there is a change in
the direction when we go to the electrode
positive geometry and what would happen in
that situation are now analogues.
What happens when the electrode is positive
is that the electrons, which are having high
amount of kinetic energy, are then getting
absorbed in the electrode, which means that
the electrode is going to have more energy
generated in its surface.
And it may cause the electrode to heat up
fast and may even melt, which means that this
is not recommended polarity when you have
a non consumable electrode welding process.
However, for MIG welding where the electrode
is to be also molten and deposited then it
would be a recommended polarity.
And there is another reason why we would choose
the electrode positive polarity, and that
is evident from the direction in which the
argon ions are going to travel.
So, as you can see the positively charged
ions they are rushing towards the workpiece,
because the electrode is positively charged
and workpiece is at neutral.
And what this implies is that if the workpiece
is going to be having a surface layer, which
has a little bit of surface oxide then the
large argon ions that are going to bombard
the surface of the workpiece can break the
oxide layer on the surface and expose the
metallic surface for a smooth joining processes.
So, this also implies that if you are going
to join aluminum or stainless steel where
the surface generally as an oxide layer thin
oxide layer then DCEP polarity is suitable.
However, we normally have both of these alternating
for these kinds of materials.
So, we have AC where the positively charged
argon ions as well as the electrons which
are negatively charged are moving alternatively
towards the electrode and towards the workpiece,
which means that you would have the advantage
of both the surface cleaning namely the dissociation
of the surface oxide as well as enhanced energy
release on the surface of the workpiece by
the electrons that are coming with high kinetic
energy and getting stopped.
So often we have a choice of these three polarities
and we make a judicious pick of the combination
that is required for the particular material
that is been joined.
These values namely the voltage and current
need not to be kept constant during the welding
process.
As you can see in the AC polarity, the voltage
has to be going from positive to negative
at a particular frequency.
And you could then see that as usually a sine
wave.
You could also have a bias to that particular
sine wave, so that it can be made as an unbalanced
sine wave, and this also means that the electrode
will be having different amount of times spent
in the straight polarity and reverse polarity
reigns.
And this is tunable either going upwards or
downwards, so that whichever polarity you
desire most can be increased in the amount
of duration so that the unbalanced sine wave
can be made to work for the particular combination
that you are interested.
We also can have a square wave that is the
voltage can be kept constant at a particular
value and then we changed sharply and suddenly
to another value, and these variations can
then be repeated at the particular frequency.
So you could also have not only sine waves,
but also square waves.
Such temporal profiles of voltage would also
imply that the current also will have similar
temporal profiles.
You may have currents that are changed from
a high value to a low value at a particular
frequency, when you employ for example, a
square wave.
How these temporal profiles are useful in
designing low heat input welding processes
will be evident later on and we discuss further.
So, we must understand that not only the polarity
can be changed, but their temporal variations
can also be changed, which means that the
advances that are taking place in the welding
source equipment can then be used to design
a new welding processes by looking at all
these combinations electronically and automatically
configured.
So, before we end the first part of this module,
let me just also just summarize the different
characteristics of an arc welding process.
So we normally need to note down; what is
a voltage, current and the efficiency of the
heat transfer to the weld piece that will
be taking place.
And we also need to know what kind of a wave
form was used, was it a flat, or a square
wave, or a sine wave or unbalanced sine wave
etcetera; and in that case then, what would
be the frequency of that wave form etcetera.
And we also need to know whether there are
any pulsing effects in the current; in case
of a pulsing effect, what was the peak value,
what was the base value, at what frequency
is this switch over being made, and what is
the duration of both the peak and pulse values?
And the overall frequency of this process
will then determine the characteristics of
the arc welding process.
And at what rate is this arc moving along
the surface of the material that is being
joined is also important, so we need to note
down what would be the traverse rate at this
welding process.
And the welding torch may not move in a linear
path; and this has also implications.
So, we may have what is called as a magnetic
arc oscillation that may be placed in the
welding setup.
And in that case, then what would happen is
that the arc is not going in a linear manner,
it is going along a sine path.
And what would be the frequency of such a
path, what would be the amplitude as of such
a path is also important in understanding
how the welding process is going to take place.
So, as you can see that during the arc welding
process, the numbers of parameters that can
be changed and controlled are large; and all
of these are very important.
And how each of these will play a role in
the thermal processes that take place during
fusion welding using an arc welding process
will then be discussed as part of this course.
We will take a short break, and then will
come back to the second part of the talk.