Welding - Made Easy

WELDING
CLASSIFCATION:

Dr. G. R. C. PRADEEP

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Welding: It is the process of joining similar dissimilar
metals with / without application of heat, with / without
application of pressure and with / without addition of filler
material.
Weldability: It is the capacity of being welded into
inseparable joints having
specified properties such as
definite weld strength, proper structure etc. Weldability
depends on : (1) Melting point (2) Thermal conductivity (3)
Thermal expansion (4) Surface condition (5) Change in
Micro structure etc.
These characteristics may be controlled / corrected by
proper shielding atmosphere, proper fluxing material, proper
filler material, proper welding procedure, proper heat
treatment before and after deposition.
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Metallurgy of Weld

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Metallurgically there are 3 distinct zones in a welded part
namely. In the weld, the metal solidities from the liquid
state. Hence fusion welds are considered as castings and
the crystalline structure will usually be columnar
(Dendritic). The metallurgical changes are due to the
heating and subsequent cooling of the weld and the heat
affected zone of the parent materials. A random grain
growth take place in the melt boundary. Within the heat
affected zone, the grains become coarse due to heat input
and a partial recrystallization takes place. With increasing
distance from the melt boundary, the grains become liner
until the heat unaffected zone with original grains is
reached.
Note: Further discussion on HAZ is given in the next slides
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Grain-Coarsened-HAZ:
The peak temperatures reached in the grain-coarsened-HAZ
region range extends from much above the upper critical
transformation temperature to just below the solidus
temperature (2000 to 2700oF). The microstructure is austenite
(for the most part). Any carbides, which constitute the main
obstacle to growth of the austenite grains, dissolve resulting
in coarse grains of austenite and the likelihood of martensite
can be considered. It depends on the carbon content of steel.
Grain-Refinement-HAZ:
This
region
comprises
temperature from just above the lower critical transformation
temperature and up to 200oF higher. Austenite is still
produced and the likelihood of martensite can be considered.
It depends on the carbon content of steel.
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Intercritical-HAZ: The temperatures in this region include
the intercritical ranges, between the lower and upper critical
temperatures. Some austenite is produced in this partially
transformed range, such that very high potential for
martensite transformation exists. In medium and high
carbon steels, this austenite can contain large amounts of
carbon which has a higher tendency to produce
martensite on cooling.
Subcritical-HAZ: The subcritical-HAZ includes the
tempered area of the Fe-Fe3C phase diagram(since the heat
of welding may be sufficient for further tempering). There
are no phase transformations which take place in this area
since the lower critical transformation temperature is not
exceeded.
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Types of Welds & Welded joints:
The different types of joints are Lap, Butt, Corner, etc.
Butt Joints require edge preparation like V, U, J, Bevel.
V – Joints are easier to make but amount of metal to be
filled increases with thickness. Hence other preparations are
preferred for higher thicknesses.
Double preparation is done for still higher thicknesses.

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EDGE PREPERATIONS:

U

V

J

BEVEL
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Gas Welding:
It is the process of generating the heat required for melting
the joint by burning a combustible gas with air/oxygen in a
concentrated flame at high temp. It can weld most
common materials.
Fuel Gases for welding operations:
Commercial fuel gases have one common property: they all
require oxygen to support combustion. To be suitable for
welding operations, a fuel gas, when burned with air/oxygen,
must have the following:
1. High flame temperature
2. High rate of flame propagation
3. Adequate heat content
4. Minimum chemical reaction of the flame with base and
filler metals
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Among the commercially available fuel gases hydrocarbon
gases such as propane, butane, LPG, natural gas are NOT
suitable for welding ferrous materials due to their oxidizing
characteristics and are suitable for heating, bending, cutting.
MAPP gas is a liquefied petroleum gas mixed with
methylacetylene-propadiene (acetylene + propane) and has a
heat value a little less than acetylene and suitable for welding
and cutting. Hydrogen also produces low-temperature flame
and is best for aluminium. Hydrogen flame is non-luminous,
commonly used for underwater welding and cutting.
Acetylene most closely meets all the above requirements
Acetylene is also a hydrocarbon gas and when it reaches its
kindling temperature; the bond breaks and releases energy. In
other hydrocarbons, the breaking of the bonds between the
carbon atoms absorbs energy.
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(a) Oxy-Acetylene Welding:
This is suitable for joining metal sheets and plates having
thickness of 2 to 50 mm. Additional metal called filler
metal is added to the weld in the form of welding rods
whose composition is same as the part being welded. Oxygen
is stored at a pressure of 14 MPa. Acetylene decomposes in to
carbon and hydrogen if stored as a gas and increases the
pressure which may cause explosion. Hence Acetylene
cylinders are packed with porous material (balsa wood,
charcoal, corn pith, or portland cement) that is saturated with
acetone to allow the safe storage of acetylene. These porous
filler materials help in the prevention of high-pressure gas
pockets forming in the cylinder. Acetone is a liquid capable
of absorbing 25 times its own volume of acetylene gas at
normal pressure without changing the nature of the gas.
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Video
1,2
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Chemistry of Oxy Acetylene Process
The most common fuel used in welding is acetylene. It has
a two stage reaction;
(1) The first stage primary reaction involves the
acetylene disassociating in the presence of oxygen to
produce heat, carbon monoxide, and hydrogen gas.
2C2H2 + 2O2 = 4CO + 2H2 + Heat ---------- (1)
(2) A secondary reaction follows where the carbon
monoxide and hydrogen combine with more oxygen to
produce carbon dioxide and water vapour.
4CO + 2H2 + 3O2 = 4CO2 + 2H2O + Heat--------- (2)
When you combine equations (1) and (2) you will notice
that about 5 parts of oxygen is necessary to consume 2
parts of acetylene
2C2H2 + 5O2 = 4CO2 + 2H2O + Heat ----------- (3)
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Hence it can be seen that 2.5 volumes of oxygen is required
for consuming of 1 volume of acetylene. In the first reaction
35.6% of total heat is generated at the inner cone by burning
one volume of Oxygen and one volume of acetylene supplied
from the cylinders. The remaining 1.5 volumes of oxygen is
supplied from
atmosphere.
Types of Flames:
1. Neutral Flame: When oxygen and acetylene are supplied
in nearly equal volumes, this is produced having a max.
temperature of 3200oC. This is desired in most welding
operations. It has sharp brilliant Inner cone and outer cone
faintly luminous with bluish colour. Used for most welding
applications for many metals like Mild steel, Stainless steel,
Cast Iron, Copper, Aluminium etc.
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Carburizing Flame:
There is excess of acetylene. This has 3 zones, sharp inner
cone, intermediate whitish cone, bluish outer cone. The
length of the intermediate cone is an indication of the
proportion of excess acetylene. If little excess of acetylene
is used it is called reducing condition and is used for welding
High carbon steel, Ni, non-ferrous Alloys, low alloy steel etc.
If more excess of acetylene is used it is called carburizing
condition and is used for low carbon steels for carburizing
heat treatment purpose.

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Oxidizing Flame:
There is excess oxygen. It has inner cone with purplish tinge
and outer cone. This is used for non-ferrous alloys. Such as
Cu-base and Zn-base alloys like Brass (Cu-Zn) and bronze
(Cu-Sn). The oxidizing atmosphere, in these cases, creates a
base metal oxide that protects the base metal. For example, in
welding brass, the zinc has a tendency to separate and fume
away. The formation of a covering copper oxide prevents the
zinc from dissipating.

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(b) Air Fuel Gas Welding:
This process uses a torch similar to a Bunsen burner and
operates on Bunsen burner principle. The air is drawn into
the torch as required and mixed with fuel gas. The gas is
then ejected and ignited, producing an air-fuel flame. The
common fuels used are natural gas, propane & Butane.
This type of welding has limited application because of low
temp. This is suitable for low melting point metals and
alloys such as lead etc.
c) Oxy Hydrogen Welding:This was once used for welding low temperature metals such
as Al, lead, Mg. The process is similar to oxygen –
acetylene system with the only difference being a special
regulator used in metering the hydrogen gas.
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Gas Welding procedures:
a) Leftward / Forward welding: The weld is made working
from right to left. This is found most advantageous on
plates up to about 3 mm.
b) Right ward / back ward welding: The weld is made
working from left to right. This method provides better
shielding against oxidation and slows down its cooling.
Hence the weld metal is denser, stronger and tougher.
Welding
speed is 20%
to 25% higher and fuel
consumption is 15% to 25% lower in this procedure
suitable for over 12mm thick plates.
c) Vertical welding:
This
is often advantageous for
thickness of 6mm and above. It does not require edge
preparation up to 15mm thickness. Here the operator
starts at the bottom and proceeds to the top.
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•

Leftward Welding

Rightward Welding

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Gas cutting / Oxygen cutting:
This used for cutting plates of large thickness and also when
cut is to be made along a specified contour. The equipment
is similar to that of Gas Welding, but with a different tip the
site of the hole depends on thickness to be cut. The metal is
heated to ignition / kindling temp. the Jet of Oxygen causes
rapid oxidization and blows away the oxide and molten
metal particles thus creating the cut (Kerf) (Kindling
Temperature – Kindling temperature is the lowest
temperature at which a substance bursts into flame)
Video
3

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Oxygen Lance cutting: It is the process by which holes are
pierced in heavy blocks of metal by a jet of oxygen passing
thro’ a consumable steel pipe (M.S) called lance creating
very high temperatures (45000C) due to reaction of oxygen
with hot metal. The pipe is packed with mixed metal wires of
iron, Al, Mg etc. Pure oxygen gas is passed through the pipe
from one end from an oxygen cylinder and regulator. The
other end of the pipe is preheated to its kindling temperature
with an oxy-acetylene torch. The wires in the pipe burns in
the oxygen coming down the pipe to produce enormous heat
and a liquid slag of iron oxides and other materials, which
dribbles and splashes out to longer distances depending on
oxygen flow rate. The flow of gas creates a combustionfriendly environment and the high-temperature flame
produced can easily cuts through steel, concrete (18500C)etc.
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Application: Also used for opening of tap holes in blast
furnace, making centering holes in heavy shafts, Cutting large
metal castings or frozen metal spills in foundries,
cutting concrete slabs and large steel beams in demolition and
renovation of buildings etc.

Video
4

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Arc Welding:
It is a process of generating the heat required for melting the
joint by means of an electric arc. This is most widely used
than Gas welding because of the ease of use and high
production rates.
Principle of Arc:
An Arc is generated between two conductors of Electricity,
Cathode and Anode, when they are touched to establish the
flow of current and then separated by a small distance. An
arc is a sustained electric discharge through the ionized gas
column called plasma between the two electrodes. The
electrons liberated from the cathode strike the anode at high
velocity, generating large amount of heat (6000oC). About
65% to 75% of total heat is liberated at anode.
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It should be noted that Arc temperature depends upon the
energy density of the arc column.
With AC the cathode and anode change continuously and as
a result temp. across the arc would be more uniform
compared to a DC arc.
Straight Polarity / DCEN (Direct Current Electrode – ve) is
used for thick sheets. Here the W.P. is anode, thus more
heat is liberated at the anode which gives deeper penetration.
Reverse Polarity / DCEP (Direct current Electrode +ve) is
used for thin sheets. Here penetration is small.
In AC welding, the penetration obtained is medium.
DC welding is more expensive and is used for difficult
tasks.
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Electrodes:
The electrodes used can be consumable (same base material)
(or) Non-consumable (Tungsten, Carbon or Graphite). The
consumable electrode can be either coated (stick electrode)
or uncoated (bare electrode). The coatings serve a No. of
purposes.
1. To facilitate establishment and maintenance of arc
2. To produce shield gas around arc & weld pool
3. To provide formation of slag to reduce rapid cooling.
4. To introduce alloying elements not contained in core
wire.

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Video - 1
Shielded Metal Arc Welding: (SMAW)
Here a metal rod is used as electrode. The temp. is about
2400oc on -ve and 2600oc on +ve electrodes respectively.
This is called Shielded Metal Arc Welding (SMAW) when
stick (coated) electrodes are
used. This is a manual process
and used for general purpose
welding. A.C is the current
source. D.C also can be used.
This can be used for
thicknesses above 3mm.
The main disadvantages are
slow speed, slag inclusion,
moisture pick up by coatings,
wastage of electrode material etc.
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Flux – cored Arc Welding: (FCAW)
This is a variant of GMAW, where a consumable tubular
electrode wire containing flux at the centre is fed from a reel.
DC is used. It is limited to steel and some types of S.S.

Video
2

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Carbon Arc Welding:
Here, one or two rods of
carbon are used as –ve
electrodes and work is +ve.
The temp. is about 3200oc
on –ve and 3900oc on +ve
electrodes
respectively.
Here DC is always used as
fixed polarity is not
obtained with A.C. This is
used where no addition of
filler metal is required.
Used for welding sheet
steel, Al, Cu alloys like
Brass, Bronze etc.
Dr. G. R. C. PRADEEP

Video
3,4

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Atomic Hydrogen Welding: (AHW)
Here an arc is maintained between two non-consumable
tungsten electrode, while a stream of hydrogen gas under
pressure is passed through the arc
and around the
electrodes. As the molecules of H2 pass thro’ the arc, they
change into atomic state absorbing considerable amount of
energy. Just outside the arc, the atoms of H2 recombine into
molecules liberations large amount of heat and produces a
temp. of the order of 4000oC. This process removes all
oxygen and other gases which form oxides and impurities
and thus produces smooth, uniform, strong and ductile weld.
This is used for welding alloy steel, stainless steel and most
non-ferrous metals. This method is now obsolete after
development of MIG and TIG.
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Video
5

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Submerged Arc Welding: (SAW)
This is an automatic process developed for high quality butt
welds in steel plates like large container manufacturing,
bridges construction, ship building, penstocks, pressure
vessels, other structural applications etc. The arc is formed
under the layer of flux (Granular flux of coarse size) and is
not visible. The bare electrode is fed from a reel through a
gun/nozzle. Speeds up to 80 mm/s on thin plates and
deposition rates up to 45 Kg/hr on thick plates are possible.
Plate thicknesses up to 25 mm can be welded in a single pass
without edge preparation using DCEP. Deep penetration with
high quality weld is possible. gouge
Video
6
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Stud Arc Welding (SW)
It is a process for faster joining of the studs to the work
pieces such as M/C assemblies, motor assemblies, automobile
assemblies, structural assemblies etc. The equipment consists
of a Gun similar to GMAW torch which holds the stud to
weld. An are is initiated between the stud and the work
piece which melts the end of the stud and contact area of
work piece. The stud is pushed into the weld pool and
current is switched off simultaneously and thus the stud gets
welded to the work piece.
Video
7

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Gas Metal Arc Welding (GMAW) (or) Metal Inert Gas
(MIG) Welding:
This is a gas shielded, metal are welding process, where, the
consumable electrode wire is continuously fed from a reel
and the welding area is flooded with a inert gas which will
not combine with metal. The wire is often bare (or) very
lightly coated. This is advantageous
because of high welding speeds,
No flux requirement, welds many
metals The welding gun is either
Air cooled / Water cooled.
D.C is the current source and Video
mainly used for thick plates.
8

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Inert gases used:
1. CO2 is used for steel,
2. Ar (or) Ar – He mixture is used for Al (or) Cu
3. Ar – O2 [1 to 5 % of Oxygen is added for better fluidity
and improved arc stability] (or) He – Ar mixture is used for
stainless steel
4. Pure Ar gas is used for Titanium
5. Ar – He mixture is used for Cu-Ni and high-Ni alloys.
Helium has higher thermal conductivity. So it gives higher
arc voltage for a given current and higher heat input.
However, helium being lighter (than argon and air) rises in
turbulent manner and tends to disperse into air. So higher
flow rate will be required in the case of helium shielding.
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Modes of metal transfer in GMAW Welding:
In GMAW, the filler metal is transferred from the electrode
to the joint. Depending on the current and voltage used,
different ways of transfer occurs.
1. Short circuit / Dip Transfer: Here the electrode tip melts
and forms a Globule of molten metal at tip. As the
electrode advances it touches the W.P. short circuit
occurs. The tip is pinched by electromagnetic forces and
transferred by surface tension into the weld pool. This is
used up to thicknesses of 5mm with small diameter wires (up
to 0.9 mm). Best for vertical welding and overhead welding.

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2)

1)

3)
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4)
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2. Globular / Drop Transfer: It occurs at higher currents than
the first. The melted tip forms a big size drop (twice the
wire dia) at tip which is pinched by electromagnetic forces
and pulled by gravity in to the weld pool. It causes
excessive spatter hence usually avoided mode of transfer. It
may sometimes cause short circuit also.
3. Spray Transfer: It occurs at higher currents than the
second. Here the molten metal is detached from tip by the
increased electromagnetic pull irrespective of gravity force.
It produces very little spatter & used for thick plates (>6 mm)
in flat and horizontal positions only. Wire diameters are more.
4. Pulsed Spray Transfer: The current is pulsed between spray
transfer range and nearer to globular range cyclically so that
it is suitable for all positions of welding. It is mainly used for
S.S as it reduces distortion and inter granular corrosion.
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Tungsten Inert Gas (TIG) welding:
This process was invented for welding Al as Al forms an
oxide layer immediately on exposing to atmosphere. DCEP
was used in welding Al as it causes peeling of oxide layer
(Cathode cleaning process). A.C. was later found to give
better result. Filler material can be used if required in TIG
welding by feeding as if in Gas welding. Pure tungsten is
used for DCEN for welding most of the metals. Thoriated
tungsten or Zirconated tungsten is used for A.C and DCEP
for welding Al and Mg alloys.
This process is being widely used
for thin sheets for precision
welding in nuclear, air craft, space craft,
chemical industries.
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Video
9

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Plasma Arc Welding: (PAW)
It is extension a TIG. Difference is constriction of arc
column. Plasma is a high-temp. ionized gas and occurs in
any electric are between two electrodes. The ionized gas
gets hotter by resistance heating from the current passing
through it. If the arc is constrained by an orifice, the
proportion of ionized gas increases and plasma are welding
is created. A non-consumable tungsten electrode with a
water-cooled nozzle is enveloped by a gas. The gas is
forced past an electric arc thro’ the constrained opening of
the nozzle. The gas passing thro’ the arc is dispersed and
temp. raises to the order of 11000oC to 14000oC.
Application is in electronic, instrumentation, aero space
industries. It can also weld Carbon steels, S.S, Cu, Brass, Al,
Ti, Monel, Inconel, Mo, Tantalum, Haste Alloys etc.
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Video
10
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A lower flow rate of the orifice inert gas is maintained, as
excessive flow rate may cause turbulence in the weld pool.
This flow rate is insufficient to shield the weld pool
effectively. Hence inert gas at higher flow rate is also passed
through outer gas nozzle to protect the weld pool.
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Arc Blow:
Due to fixed polarity in D.C. Welding, magnetic lines form
in the W.P. When welding at the centre of W.P. these lines are
equally distributed on both sides so Arc will be straight. But
while welding at the edges, the magnetic lines will try to pull
back the arc and it
will be deflected
towards the W.P., as
these lines will be
formed only in the
material. This
phenomenon is called
arc blow and causes
spatter and improper
bead geometry.
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Reducing the Arc blow:
• Keeping metal plates at entry and exit of the arc.
• Holding as short an arc as possible to help the arc force
counteract the arc blow.
• Reducing the welding current - which may require a
reduction in arc speed
• Changing the ground
positions.
• Inclining the electrode
with the work opposite
to the direction of arc
blow as shown:

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Arc cutting:
This is based on melting the metal by the heat of an
electric arc and blowing molten metal by a jet of air
supplied along the electrode and into the cut. This is used
for cutting small sections like pipes, angle channels,
separation of gating system from castings, etc.
Power sources in Arc Welding:
Selection of power source is mainly dependent on type
welding process. The open circuit voltage normally ranges
between 70-90 V and short circuit current ranges between
600-1000A in any welding transformer. Welding voltages and
welding currents are lower as compared to open circuit
voltage of the power source.
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a) Constant current type transformer (Non-Linear):
In manual arc welding since are length cannot be
controlled, the arc current is controlled by the transformer.
It has the drooping V-I characteristic curve as shown. It can
be observed that a major change in Arc voltage causes in
significant change in Arc current.
b) Constant Voltage Transformer (Linear):
It has a flat V-I characteristic with a slight droop. This is
used for continuous electrode wire welding like GMAW,
SAW and other automatic welding processes. It can be
observed that a major change in Arc current causes in
significant change in Arc voltage.

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Note:
1. Voltage required to generate arc at no load condition is
called Open Circuit Voltage (VOC )
2. Current required during arc generation is called Short
Circuit Current (ISC).
Duty Cycle:
Duty cycle is the ratio of arcing time to the weld cycle time
expressed as percentage. If arcing time is continuously 5
minutes then as per European standard it is 100% duty cycle
and 50% as per American standard. At 100% duty cycle
minimum current is to be drawn. The welding current which
can be drawn at a duty cycle can be evaluated from the
following equation:
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DR x IR2 = I2 x D100
Where

I
D100
IR
DR

= Current at 100% duty cycle
= 100 % Duty cycle
= Current at required duty cycle
= Required duty cycle

Duty cycle and associated currents are important as it
ensures that power source remains safe and its windings are
not getting damaged due to increase in temperature beyond
specified limit. The maximum current which can be drawn
from a power source depends upon its size of winding wire,
type of insulation and cooling system of the power source.

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Expressions:
1.For a linear power source characteristic, the arc voltage
is given by :
V = Voc – ((Voc / ISC) x I)
Where I = Arc current
2.For a stable arc, in a constant voltage transformer,
Varc = Vtransformer
3.For a stable arc, in a constant current transformer,
Iarc = Itransformer
3.For a linear power source, the Arc length – Voltage
characteristic is given by
V = a + bl
where l = Arc length,
a, b = constants.
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4. The equation of the line can also be written as
(V-V1) = {(V2-V1) / (I2-I1)} (I-I1)
5. Heat required for melting =
Volume melted x rate of melting
Volume melted = Area of Joint x welding speed
6. Net heat supplied = ηHT x V x I
ηHT = Heat transfer Efficiency
7. ηMelting = Heat Reqd. to melt the joint / Net heat supplied

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Resistance welding: (RW)
This is a fusion welding process where both heat and pressure
are applied on the joint, but no filler metal (or) flux is added.
The heat necessary for the melting of the joint is obtained by
the heating effect of the electrical resistance of the joint.
Here a low voltage (typically 1 V) and very high current
(typically 15000 A) is passed thro’ the joint for a very short
time (typically 0.25 sec.). This heats the Joint due to the
contact resistance at the joint and melts it. The pressure on
the Joint continuously maintained fuses the metal parts.
Electrodes:
Copper in alloyed form is used for making electrodes.
Cu - Cd Alloys  for non-ferrous materials like Al & Mg.
Cu – Cr Alloy  for mild steels and low alloy steels
Cu with Be & Co  for S.S., Tungsten steels.
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Note: The transformer in the machine converts low amperage,
240V shop line current in to high secondary amperage, low
voltage welding current, safe from electrical shock. Proper
earthing is also important. (Range: 1–25V, 1000–100,000 A)
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Heat Balance:
Proper fusion is obtained only when proper heat balance is
there. This can be provided by increasing or decreasing the
contact areas of the electrodes as follows for different
combinations.
1.Small contact area for thin sheet, big contact area for thick
sheet.
2. Large contact area is required for high electrical
conductivity and small contact area for low electrical
conductivity (Dissimilar metals)
3. Smaller contact area is required for higher thermal
conductivity and large contact area
for low thermal
conductivity (Dissimilar metals).

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Upset Butt welding: (UW)
The parts to be welded are clamped edge to edge in Copper
Jaws of welding M/c and brought together in Solid contact,
which forms a locality of high electric resistance. As the
current flows here, the joint gets heated us and the pressure
applied upsets the parts together.
This is used for non-ferrous
materials and is used for welding
bars, rods, wires, tubes, pipes etc.
Video
1

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Flash Butt welding: (FW)
Here the edges are brought together in light contact. A high
current starts a flashing action between the two surfaces and
continues as reached. The
upsetting action will cause melted
metal to flash out through the
joint and forms like a fin around
the joint. This is used for ferrous
Video
2
materials and is used for welding
bars, rods, wires, tubes, pipes etc.
This is not suitable for
materials like lead, Tin, Zinc,
Antimony, Bismuth etc.
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Percussion Welding:
Here one part is held
stationary, and other
part is held in a clamp
mounted on slide which
is backed up against pressure
Video
3
from a heavy spring. During
welding, the movable clamp released rapid carries the part
forward. When the distance between the parts is approx.
1.5mm, a sudden discharge of electrical energy is released,
causing intense Arc between the two surfaces. To complete
the weld it takes about 0.1 sec. No upset / flash occurs at the
weld. This is a automatic process and is limited to small
areas of 144 mm2 max. and is suitable for welding small
wires to electrical components.
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Spot Welding: (RSW)
This is employed to join overlapping strips, sheets or plates
at small areas. This is widely used in electronic, electrical, air
craft, automobile, home appliance industries for body
constructions.
Projection Welding: (RPW)
This is modification of spot welding. One (or) both of the
work pieces are embossed to produce projections. The
current and pressure employed on the embossing flattens out
this projection resulting in good welds at point of contact. By
this process fastening attachments like nuts, brackets handles
etc. can be welded to sheet metal in electrical, electronic,
domestic equipment.
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•

RPW

Video
4,5,6,7

RSW
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Video – 8,9
Seam Welding: (RSEW)
This is a method of making a continuous joint between two
overlapping pieces of steel metal. The work is placed
between wheels which serve as conductors for producing
continuous welds. Used for pressure tight / leak proof fuel
tanks in automobiles, seam welded tubes, drums, small
containers etc.

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Expressions:
1. Heat required for melting = Vol melted x rate of melting
= mL + mCp (Tm –Ta)
Where m = mass of metal melted = (vol melted x ρ)
L = Latent heat of fusion of the metal
Cp = Sp. Heat of metal
Tm = Melting temp. of metal
Ta = Ambient temp.
ρ = Density of metal
2. Net heat supplied = I2 RT = V2T / R (Since V = IR)
Where I
=
Current (Amp)
R
=
Resistance (Ω)
T
=
Time for welding (sec)
3.Melting η =
Heat Reqd to melt the joint / Net heat supplied
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Video - 1
Thermit Welding: (TW)
This is used for the welding of very thick plates, like ship
hulls, broken large castings, rail sections etc. Thermit is a
mixture of finely divided Al (1 part) and Iron oxide (3
parts). The Process is based on the chemical reaction where
Oxygen leaves Iron oxide and combines with Al, producing
Al. oxide and superheated thermit
steel. [8Al + 3Fe3O4  4 Al2O3 + 9 Fe]
The temperature is around 3000oC.
A wax pattern is first shaped around
the parts to be welded. A sand mould
is prepared around it. Pre heating is
done and wax is drained out. The
thermit mixture is poured in to the mould and then pressure
is applied after welding temp. is reached.
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•

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Electro slag welding : (ESW)
This is developed to weld very large plates (200 mm section)
without any edge preparation. Here a consumable electrode
is used for filling the gap between the two heavy plates. The
heat required for melting the plates and electrode is obtained
initially by means of an arc so that the flux will form the
molten slag. Then further heating is obtained by the
resistance heating of slag itself. For effective welding,
vertical welding is done to maintain a continuous slag pool,
which is contained in the gap with the help of water cooled
copper dam plates which move along with the weld.
Appln: Frames of heavy presses, rolling mills, Locomotives
etc.
Video
2
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•

BRIDGE GIRDER

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Electron beam Welding: (EBW)
Here a focused beam of electrons are accelerated towards
the anode from the electron gun which forms the cathode.
This is done with the help of a electro magnetic lens. The
material in the path on the beam gets melted. Larger
penetrations are possible here. No filler material / flux is
needed here. Here the welding zone is narrow and hence weld
distortions are eliminated. (0.25mm – 1mm dia beam can be
possible).
Appln: Specially suitable for welding dissimilar metals and
super alloys, turbine and air craft engine parts where
distortion is unacceptable, Air plane, automobile, farm
equipment etc.
Video
3

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Laser Beam Welding (LBW)
Here a laser beam is directed on to the joint to be welded.
Narrow. Heat Zones (0.05 mm to 0.1mm wide) are possible
here and hence very small wires used in electronic devices
can be welded. This is called Micro welding. They can also
be used for joining multi layer materials with differing
thermal properties. It can weld dissimilar metals and difficult
to weld metals like, Cu, Ni, S.S, Ti, Columbium etc. Widely
used in Aerospace and electronic industries.
The lasers used for welding are:
Solid–state lasers like Ruby - Neodymium (Nd); Nd - Glass;
YAG (Yttrium - Aluminium – Garnet) etc.
The chief gas laser is CO2 laser.
Video
4

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Forge Welding:
This is a oldest method. The ends to be joined are heated to
a temperature slightly below the solidus temperature and
pressure is applied so that a fusion joint is obtained. The
force can be applied by machines / continuously rotating
rolls / manually.
Video
5

ART METAL
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HORSE SHOE
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Friction Welding (FRW)
One of the parts to be joined is axially aligned and pressed
tightly against another part and rotated at a high speed (3000
rpm). The friction between the parts rises the temperature of
both ends. The rotation is stopped abruptly and pressure on
fixed part is increased so that joining takes place. Even
dissimilar metals can be joined. This
process is limited to parts with
rotational symmetry.
Video
6

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Friction welded parts in production applications span over
wide products for aerospace, agricultural, automotive,
defense, marine and oil industries.
Right from tong holds to critical aircraft engine components
are friction welded. Automotive parts like gears, engine
valves, axle tubes, driveline components, strut rods, shock
absorbers are friction welded.
Hydraulic piston rods, track rollers, gears , bushings, axles
and similar parts are commonly friction welded for
agricultural equipment.

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Diffusion Welding: (DFW)
Also called Diffusion bonding is the process of joining two
parts purely by diffusion, which can be achieved by
keeping the two pieces in intimate contact under pressure.
This does not necessarily need heat. But its temperature is
raised, the diffusion rate is increased. The joint is formed
without any filler metal and the microstructure and
composition at the interface are the same as those of the base
metals. Pressure is applied which will cause local plastic and
creep deformation at the temperature of operation. Bonding
will take place due to diffusion and will depend on
temperature, time and the pressure applied. An interlayer foil
or coating may be used to improve the bonding
characteristics. This process makes it possible to join metal
to metal as well as metal to ceramic also.
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Video
7

Appln: Used by gold smiths to
bond gold over copper, most
suitable for joining dissimilar
metals like Ti, Be, Zr, refractory
materials, composite materials etc.
Diffusion bonding with super
plastic forming is widely used in
aero space (Wing Structures).
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Explosion welding: (EXW)
Here, detonation of explosives is used to accelerate a part to
move towards the other plate at a fast rate, so that the
impact creates the joint. As the plate moves at high
velocity and meets the other plate with a massive impact,
very high stress waves (of order thousands of MPa) created
between the plates makes a clean joint. Application is for
cladding of metals for the purpose of corrosion prevention.
Used for joining of dissimilar metals like Titanium to steel,
Al. to steel, Al to Cu etc. Tantalum can be explosively
welded to steel though the welding point is higher than
vapourisation temperature of Steel.
Eg: Ship building, chemical Industry.
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Video
8,9

•

ALUMINIUM TO STEEL

TITANIUM TO COPPER

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Brazing:
Here a filler material also called spelter is used, whose
melting point is less than the melting point of parts to be
joined. The parts to be welded are cleaned properly Flux
(usually Borax) is applied and then filler material is placed in
between and the parts are heated which melts the filler
material and it flows into the space by capillary action. The
filler materials are copper-base alloys / silver base alloys.
Brass is more commonly used filler metal.
Eg: Small LPG cylinders, Hydraulic Fittings, Heat
Exchangers, Tube Manipulations, Machined Assemblies
Pressed Assemblies etc
Video
10
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•

BRAZING OF
WATCH ASSEMBLIES, CONNECTORS IN AUTOMOBILES

BRAZE WELDING OF COPPER TUBES, CYCLE FRAMES
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Bronze Welding / Braze Welding:
This process requires more heat than Brazing and Tin is
added in filler metal for better flowing of melted filler metal.
This process is intermediate between true welding and true
brazing. Here the parts are heated to a temp. of melting
point of the bronze – filling rod which contains 60% Cu and
40% Sn. During the operation, the edges of the parent metal
are heated by oxy-acetylene flame or some other suitable heat
source. Here the filler metal reaches the Joint without the
capillary action since the Joint gap is more. The filler metal
enters the joint by gravity.
Eg: Carbide inserts in tool shanks, carbide drill bits, repair
Video
works etc
11
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Soldering:
This is a method of joining metal parts by means of a
fusible alloy called solder, applied in the molten state. Fluxes
used in soldering are ammonium chloride, zinc chloride etc.
The solder is composed of Pb and Sn with a melting point of
150 to 350oC
Soft soldering: is used for sheet metal works that are not
subjected to excessive loads.
Hard Soldering: employed solders whose melting temp. is
higher than soft solders.
Soft solder
lead 37%, tin 63%
Medium solder
lead 50%, tin 50%
Plumber solder
lead 70%, tin 30%
Video
Electricians solder Lead 58% , tin 42%
12
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Note: (1) During brazing or soldering flux is used for:
Dissolving oxides from the surfaces to be joined, Reduce
surface tension of molten filler metal i.e. increasing its
wetting action or spreadability, Protect the surface from
oxidation during joining operation. (2) Any metal which has a
melting point of < 4500C cannot be used as filler material in
brazing or braze welding and can only be used in soldering.

SOLDERING OF COPPER TUBES,
SEAT BELT BRACKETS, STEEL VALVE
TO SiC PLATE
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Weld Defects:
The defects in the weld can be defined as irregularities in
the weld metal produced due to incorrect welding
parameters or wrong welding procedures or wrong
combination of filler metal and parent metal.
Weld defect may be in the form of variations from the
intended weld bead shape, size and desired quality.
Defects may be on the surface or inside the weld metal.
Certain defects such as cracks are never tolerated but other
defects may be acceptable within permissible limits.
Welding defects may result into the failure of components
under service condition, leading to serious accidents and
causing the loss of property and sometimes also life.

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1) Poor Fusion  Lack of
thorough and complete union
between the deposited and
present metal this appears as a
discontinuity in the weld zone.
Lack of fusion is because of
failure to raise the temperature of
base
metal or previously
deposited weld layer to melting
point during welding. Lack of
fusion can be avoided by
properly cleaning of surfaces to
be welded, selecting proper
current, proper welding technique
and correct size of electrode.
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2) Under cut 
This appears as a small
notch in the weld
interface. Main reasons
for undercutting are the
excessive
welding
currents,
long arc
lengths and fast travel
speeds.

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3) Porosity  Porosity results when
the gases
are entrapped in the
solidifying weld metal. These gases
are generated from the flux or coating
constituents of the electrode
or
shielding gases used during welding or
from absorbed moisture in the coating.
Rust, dust, oil and grease present on the
surface of work pieces or on electrodes
are also source of gases during
welding. Porosity can also be
controlled if excessively high welding
currents, faster welding speeds and
long arc lengths are avoided, flux and
coated electrodes are properly baked.
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4)Slag Inclusion  These may be
in the form of slag or any other
nonmetallic material entrapped in
the weld metal as these may not
able to float on the surface of the
solidifying weld metal. However,
if the molten weld metal has high
viscosity or too low temperature
or cools rapidly then the slag may
not be released from the weld
pool and may cause inclusion.
Slag inclusion can be prevented if
all the slag from the previously
deposited bead is removed, low
welding current are avoided.
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5) Cracks  Cracks occur
when
localized
stresses
exceed the ultimate tensile
strength of material. These
stresses are developed due to
shrinkage
during
solidification of weld metal.
Cracks may be developed
due to poor ductility of base
metal, high sulphur /
phosphorous and carbon
contents, high arc travel
speeds i.e. fast
cooling
rates, high hydrogen contents
in the weld metal etc.
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6) Distortion 
Bending of components due to improper thermal expansions
and contractions. Hence proper clamping and preheating is to
be done to avoid distortion.

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7) Miscellaneous Defects 
Multiple arc strikes, spatter,
grinding & chipping marks,
misalignment of weld beads,
un removed slag, etc.

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Design Considerations:
The selection of a welded joint and a welding process
involves the following considerations:
1. The configuration of the component or structure to be
welded, their thickness and size.
2. The service requirements, such as type of loading and the
stress generated.
3. The location, accessibility and ease of welding.
4. The effects of distortion and appearance.
5. The costs involved in the edge preparation, the welding,
post – processing of weld including machining and finishing
operations, Heat treatment etc.
Design guide lines:
1. Product design should minimize the number of welds.
2. Components should fit properly before welding.
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3. Select designs that can avoid (or) minimize the need for
edge preparation.
4. Weld bead size should be kept to a minimum to conserve
weld metal.
5. Weld location should be selected so as not to interfere with
further processing of the part.
Note:
1.The correct sequence in ascending order of their weldability
for most common metals is : Al < Cu < CI < MS
2. Due to improper surface cleaning, hydrogen may enter in
to weld pool and get dissolved in the weld metal. During
cooling it diffuses in to HAZ developing cracks due to the
residual stresses assisted by hydrogen coalescence (growing
together). This is called hydrogen embrittlement.
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Welding of Cast Irons:
They are difficult to weld because of high carbon contents
and poor ductility.
Massive carbon deposits have a tendency to form in the areas
adjacent to the weld.
Thus a high carbon martensite tends to form in the HAZ
which has very brittle micro structure that may lead to cracks
during welding or after welding under load application.
CI is joined by Oxy Acetylene welding and SMAW. Proper
pre heating and post heat treatment may be required.

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Welding of Stainless Steel:
Stainless steel is difficult metal to weld because it contains
both Ni and Cr. The best method for welding stainless steel is
TIG welding. SMAW is also used but requires use of a heavily
coated electrode. Low current setting with fast travel speed is
preferred for stainless steel as certain stainless steels are
subjected to carbide precipitation.
Ferritic stainless steels are generally less weldable than
austenitic stainless steels and require both preheating and post
weld treatments. Welding ferritic stainless steels can be done
autogenously (or) with an austenitic stainless steel (or) using a
high nickel filler alloy (or) type 405 filler containing
low % Cr (11%), low % C(0.08%) and small % Al (0.2%). It
can be welded by TIG, MIG, SMAW, PAW.
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WELD DECAY
Weld decay is a form of intergranular corrosion, usually found
in corrosion-resistant alloys like stainless steels or certain
nickel-base alloys and occurs as the result of sensitization in
the HAZ during the welding operation. The corrosive attack is
restricted to the HAZ. Positive identification of this type of
corrosion usually requires microstructure examination under a
microscopy although sometimes it is possible to visually
recognize weld
decay if parallel
lines are already
formed in the HAZ
along the weld as
shown.
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In this case, the precipitation of chromium carbides is induced
by the welding operation when the HAZ experiences a
particular temperature range (550oC~850oC). The precipitation
of chromium carbides will consume the alloying element –
chromium, from a narrow band along the grain boundary and
this makes that zone anodic to the unaffected grains. The
chromium depleted (consumed) zone becomes the preferential
path for corrosion attack or crack propagation if under tensile
stress. Weld decay can be prevented through:
• Using low carbon (e.g. 304L, 316L) grade of S.S electrodes.
• Using stabilized electrode grades alloyed with Ti (type 321)
or Nb (type 347). Ti and Nb are strong carbide- formers.
They react with the carbon to form the corresponding
carbides thereby preventing chromium depletion.
• Use post-weld heat treatment (PWHT).
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