Unit 2-JOINING PROCESSES

INTRODUCTION
What is Welding?
The process of joining similar
metals by the application of heat is
called “Welding”.

Classification of Welding process
According to source of energy employed
 Fusion welding
 Plastic welding

1.Fusion welding
The metal at the joint is heated to a
molten state and then it is allowed to
solidify.
Pressure is not applied so it is called
as non-pressure welding.

2.Plastic welding:
The metal parts are heated to a plastic state and
are pressed together to make joint. Hence
known as Pressure welding.
There is no filler materials required.

The Welding process can also be classified as:
(a) Autogeneous: no filler metal is added to the
joint interface.
(e.g. Electric resistance welding)
(b) Homogeneous: filler metal is added and is of
the same type as parental metal.
(c) Heterogeneous: filler metal is used but it is of
different type from the parental metal.
(e.g. Brazing, Soldering)

Types of Welding
1. Oxy-Fuel Gas Welding Processes
1. Air-acetylene welding
2. Oxy-acetylene welding
3. Oxy-hydrogen welding
4. Pressure gas welding

2. Arc Welding Processes
1. Carbon Arc Welding
2. Shielded Metal Arc Welding
3. Submerged Arc Welding
4. Gas Tungsten Arc Welding
5. Gas Metal Arc Welding
6. Plasma Arc Welding
7. Atomic Hydrogen Welding
8. Electro-slag Welding
9. Stud Arc Welding
10. Electro-gas Welding

3. Resistance Welding
1. Spot Welding
2. Seam Welding
3. Projection Welding
4. Resistance Butt Welding
5. Flash Butt Welding
6. Percussion Welding
7. High Frequency Resistance
Welding
8. High Frequency Induction Welding

4. Solid-State Welding Processes
1. Forge Welding
2. Cold Pressure Welding
3. Friction Welding
4. Explosive Welding
5. Diffusion Welding
6. Cold Pressure Welding
7. Thermo-compression Welding

5. Thermit Welding Processes
1. Thermit Welding
2. Pressure Thermit Welding
6. Radiant Energy Welding Processes
1. Laser Welding
2. Electron Beam Welding

(B) Allied Processes
1. Metal Joining or Metal Deposition Processes
1. Soldering
2. Brazing
3. Braze Welding
4. Adhesive Bonding
5. Metal Spraying
6. Surfacing

2. Thermal Cutting Processes
1. Gas Cutting
2. Arc Cutting

Gas Welding Equipments
1.Gas cylinders:
Two separate cylinders-Oxygen and Acetylene
Black colour for Oxygen cylinder-125 to 140
kg/cm².
Capacity is 6.23 m³.
Maroon colour for Acetylene-16 kg/cm².
Capacity is 7.6 m³.

2.Pressure Regulators
Regulates the working pressure of the gases.
Working pressure :
Oxygen is between 0.7 and 2.8 kg/cm².
Acetylene is between 0.07 and 1.03 kg/cm².
Work pressure depends upon the thickness of
the work pieces to be welded.

3.Pressure gauges:
Four gauges provided –two on oxygen
cylinder and acetylene cylinder.
O.C indicates the pressure inside the oxygen
cylinder which keeps lowering as the gas in
used up. O.L indicates the outgoing oxygen
pressure through the hose line.

4. Hoses:
 Both oxygen and acetylene gas can be
satisfactorily piped through the reinforce
flexible rubber hose.
The green hose is commonly used to carry
oxygen and red hose is used to carry
acetylene.

Welding torch:
 The welding torch has a mixing chamber in
which oxygen and acetylene will be mixed and the
mixture is ignited at the torch tip.
The pressure of oxygen and acetylene can be
equal.
To extinguish the flame, fuel gas should be turned
off first followed by the oxygen.
 Gas flow to the torch is controlled with the help
of two needle valves in the handle of the torch.

There are two basic types of gas
welding torches:
1. Positive pressure (also known as
medium or equal pressure), and
2. Low pressure or injector type
The positive pressure type welding
torch is the more common of the two
types of oxyacetylene, torches.

Welding tip:
 The welding tip is also called as nozzle.
 The oxyacetylene gas is ignited by a spark
lighter or a flame lighter and it burns with a
flame.

Other accessories and safety devices:
 Goggles protect the eyes of the welder from
spark and ultra violet rays while welding.
Leather gloves protect the hand from any
possible bone injuries.
Apron also be worn to protect the body.

Filler rod or Welding rod:
 Filler rod is the metal rod which is used in gas
welding to supply additional metal to make the
joint.
 Filler rods are generally made of low carbon
steel alloying elements such as chromium and
nickel may be added to the filler rod this will
increase the strength of the joint.
 The diameter of the filler rod ranges from
0.3 to 12 mm, it depends on the thickness of the
work pieces being welded.

Flux:
The purpose of using flux which is in a
powder or liquid form is to prevent
oxidation and remove the impurities.
Oxidation takes place, forming metal
oxides since molten metal comes in contact
with gases.
 The flux should have a melting point lower
than the metal being welded

The common fluxes used in the gas welding
are made of
 Sodium
 Potassium
 Lithium
 Borax
 Flux can be applied as paste, powder,
liquid, solid coating or gas.

Types of Gas welding
Three types:
 Oxy-acetylene welding
Oxy-hydrogen welding
Air-hydrogen welding

OXY-ACETYLENE WELDING:
Metal to be welded are melted by using gas
flame.
The flame is produced at the tip of a welding
torch.
The common gas employed for gas welding is
oxygen and acetylene.
Flame only will melt the metal so additional
metal to the weld is supplied by the filler rod.
A flux is used to remove impurities.

Metal 2mm to 50mm thick are welded by gas
welding.
The temperature of oxy-acetylene flame in its
hottest region is about 3200º C.
The gases O2 and C2H2 can be stored at high
pressure in separate steel cylinders.
There are two types of oxy-acetylene systems
employed.
High pressure system
Low-pressure system

In high pressure system both oxygen and
acetylene are supplied from high pressure
cylinders.
Oxygen is compressed to 120atm gauge
pressure.
But acetylene cannot be compressed more than
1.5atm like in the form of “dissolved acetylene”
The acetylene is dissolved in acetone under a
pressure of 16 to 22atm gauges.
At normal pressure, one liter of acetone is
dissolved about 25 liters of acetylene.

Pressure of acetylene in the cylinder through the rubber hose is 1 bar.
In high pressure system pressure of acetylene at the welding torch is from
0.66 to 1 bar.

In low pressure system, the acetylene is
produced at the place of welding by interaction
of calcium carbide and water in acetylene
generator.
The chemical reaction is
CaC2 + 2H2o => Ca (OH)2 + C2H2 + 127.3 KJ / mol.
The pressure of oxygen in welding torch is
(i) H.P. System ----- 0.1 to 3.5 bar
(ii) L.P System ----- 0.5 to 3.5 bar

Flame Characteristics
When acetylene burns with oxygen the reaction
can be given in the form
2C2H2 + 5O2 = 4CO2 + 2H2O
 Thus one volume of acetylene combines with
2.5 volume of oxygen.

The normal combustion has two flame zones
(i) An inner zone where the temperature will be
high and is governed by the primary reaction.
C2H2 + O2 = 2CO + H2 + 105 kCal
(ii) An outer zone where the carbon monoxide
(CO) formed by the above reaction will
combine with oxygen according to the
secondary reaction
2CO + O2 = 2C O2 + 68 kCal
2H2 + O2 = 2 H2O + 58 kCal

 Thus combustion takes place in two stages.
(i) Oxygen and acetylene (O2 and C2 H2) in
equal proportions by volume.
(ii) The carbon monoxide combines with oxygen
from the atmosphere and burns to form
carbon-di-oxide CO2

By varying the ratio Oxygen and acetylene,
the following three types of flames can be
obtained
(i) Neutral flame
(ii)Carburising flame
(iii)Oxidising flame

(i) Neutral flame:
 Obtained by supplying equal quantity of
oxygen and acetylene.
Neutral flame has two zones i.e., One sharp
bright inner cone and one bluish outer cone.
 The reaction of the inner cone is
C2H2 + O2 2CO + H2

The inner cone develops heat to melt the
metal.
The maximum temperature of neutral flame at
inner cone is 3200º C.
 The reaction of the outer cone are,
2CO + O2 2CO2
H2 + ½ O2 H2O
 Neutral flame is used to weld Steel, cast-iron,
copper, aluminium, etc.,
It has less Chemical effect on welded metal.

(ii) Carburising flame:
 It is also called as Reducing flame.
 Obtained by supplying more acetylene than the
oxygen.
This flame has three zones
1.Sharp inner cone.
2.White intermediate cone called “feather cone”
3. Bluish outer cone

 The theoretical mixture of carburising flame is
O2 : C2H2 = 0.85 to 0.95
 Carburising is used for welding very low carbon
steel, monel metal, alloy steels, nonferrous materials
etc.,

(iii) Oxidising flame:
Oxidising flame is obtained by supplying
more oxygen than acetylene.
 This flame has two zones
1. Smaller inner code
2. Outer cone

The theoretical mixture of oxidizing flame is
O2 : C2H2 = 1.15 to 1.05
 The inner cone is not sharply defined as that
have neutral or carburising flame.
Used for welding Brass and Bronze

Shielded Metal Arc Welding or
Manual Metal Arc Welding
Working Principle:
Heat for welding is produced through an
electric arc set up between a flux coated
electrode and the work piece.
The Electrical energy is converted into
Heat energy.

Working Procedure:
In this process, the heat is generated by an
electric arc between base metal and a
consumable electrode.
In this process electrode movement is
manually controlled hence it is termed as
manual metal arc welding.
This process can use both AC and DC.
However, AC can be unsuitable for certain
types of electrodes and base materials.

The electrode and work piece are brought
nearer with a small air gap of 3mm
approximately.
Then the current is passed through the work
piece and the electrode to produce an electric
arc.
The work piece is melted by the arc.
Electrode is also melted thus become a single
piece without applying any external pressure.
The temperature of arc is about 5000°C to
6000ºC.

The electrode supplies additional filler
metal into the joints and is deposited
along the joint.
The depth to which the metal is melted
and deposited is called “Depth of
Fusion”
The electrode is kept at 70° inclination
for better depth of fusion.
Electrode used is generally coated with
Flux.

The flux removes impurities from the molten
metal and forms Slag.
Slag protects the weld seam from rapid
cooling.

A small depression is formed in the parent
metal since molten metal is forced out by the
electric arc.
This is known as “Arc carter”
The distance between the tip of the electrode and
bottom of the arc crater is called “Arc length”

Advantages of SMAW or MMAW:
1. Shielded Metal Arc Welding (SMAW)
can be carried out in any position with
highest weld quality.
2. Simplest of all the arc welding
processes.
3. This welding process finds
innumerable applications, because of the
availability of a wide variety of electrodes.
4. Big range of metals and their alloys
can be welded easily.

5. The process can be very well employed for
hard facing and metal resistance etc.
6. Joints (e.g., between nozzles and shell in a
pressure vessel) which because of their position
are difficult to be welded by automatic welding
machines can be easily accomplished by flux
shielded metal arc welding.
7. Welding equipment is portable and the cost
is fairly low.

Disadvantages of MMAW:
1. Due to flux coated electrodes, the chances of
slag entrapment and other related defects are
more as compared to MIG and TIG welding.
2. Duo to fumes and particles of slag, the arc
and metal transfer is not very clear and thus
welding control in this process is a bit difficult
as compared to MIG welding.
3. Due to limited length of each electrode and
brittle flux coating on it, mechanization is
difficult.

4.In welding long joints unless properly cared,
a defect (like slag inclusion or insufficient
penetration) may occur at the place where
welding is restarted with the new electrode
5. The process uses stick electrodes and thus it
is slower process as compared to MIG welding.

Applications of MMAW:
The process finds applications in
(a) Building and Bridge construction
(b) Automotive and aircraft industry, etc.
(c) Air receiver, tank, boiler and pressure vessel
fabrication
(d) Ship building
(e) Pipes and
(f) Penstock joining

Gas Tungsten arc welding
Working Principle:
Gas tungsten arc welding (GTAW), also
known as tungsten inert
gas (TIG) welding, is an arc
welding process that uses a non-
consumable tungsten electrode to produce
the weld.

Working procedure:
The work piece to be welded is placed on the
worktable.
Electrode holder in which the non-consumable
Tungsten electrode is fixed.
The non-consumable tungsten electrode and the work
piece are connected to the power supply (A.C or
D.C).
By supplying the electric power between the
electrode and the work piece, the inert gas from the
cylinder passes through the nozzle of the welding head
around the electrode.
Inert gas surrounds the arc and protects the weld
from effects thus defect free joints are made.
 Filler metal may or may not be used.

Tungsten electrode has high melting point
(3422°C) which will not be melted during
welding.
This process is used for the metals having
thickness less than 6.5mm.
The following inert gases are generally used
in TIG welding:
1. Argon
2. Helium
3. Argon-helium mixtures
4. Argon-hydrogen mixtures

Advantages of TIG welding:
Tungsten Inert Gas Welding produces
high quality welds.
The weld is automatically protected by
the inert gas during the welding process.
No slag is produced.
TIG Welding can be done in any
position.

Disadvantages of TIG welding:
Tungsten inert gas welding is a slow
process.
Highly skilled labour is needed.
Welder is exposed to huge intensities of
light.
TIG welding is more expensive when
compared to MIG welding.

Applications of Tungsten Inert Gas Welding:
TIG Welding is used for welding a variety of
metals. Some of them are:
Stainless steel
Alloy steel
Aluminium
Titanium
copper
magnesium
nickel alloys

Gas Metal Arc Welding
Working Principle:
Gas metal arc welding (GMAW), sometimes
referred to by its subtypes Metal Inert
Gas (MIG) welding or metal active gas (MAG)
welding, is a welding process in which an
electric arc forms between a
consumable wire electrode and the workpiece
metal(s), which heats the workpiece metal(s),
causing them to melt, and join.

Working Procedure:
The consumable electrode is in the form of a wire
reel which is fed at a constant rate, through the
feed rollers.
The welding torch is connected to the gas supply
cylinder which provides the necessary inert gas.
The electrode and the work-piece are connected
to the welding power supply.
The current from the welding machine is changed
by the rate of feeding of the electrode wire.
Normally DC arc welding machines are used for
GMAW with electrode positive (DCRP).

The DCRP increases the metal deposition rate
and also provides for a stable arc and smooth
electrode metal transfer.
The filler metal is transferred from the electrode
to the joint.
Depending on the current and voltage used for a
given electrode, the metal transfer is done in
different ways.
The current ranges from 100 to 400 A depending
upon the diameter of the wire.

Advantages of MIG Welding:
No Slag to chip as compared to SMAW and FCAW
The process can be used on thin materials with
relative ease if properly set. GTAW can also be used
on thin materials but in many cases such as Auto
Body, GMAW wins hands down.
Low Hydrogen weld deposit with all electrodes
High production factor since no slag is required to
be removed and uses a continuous electrode.
With the parameters properly set for the application,
anyone can weld after a very short amount of
practice.

Disadvantages of MIG welding:
Requires a Wire Feeder which is difficult to move
and can sometimes be a maintanence/repair burden.
Needs Shielding Gas so welding in windy conditions
can be difficult.
No slag system so out of position welds are
sometimes more difficult.
Increased chance of lack of fusion if parameters and
welding technique is not controlled.
The gun is difficult to get into tight places.
Is not suitable for windy conditions.

Applications of MIG welding:
MIG (metal inert gas) welding is the most
widely used of the arc welding processes,
suitable for everything from hobbies and small
fabrications or repairs, through to large
structures, shipbuilding and robotic welding.

Submerged arc welding
Working Principle:
Submerged arc welding is also called as
sub arc welding or hidden welding in
which welding is done by producing an
electric arc between the consumable bar
electrode and the work piece such that
the electrode is submerged or hidden under
the flux powder.

Working Procedure:
The flux starts depositing on the joint to be
welded.
Flux otherwise is an insulator but once it melts
due to heat of the arc, it becomes highly
conductive
The upper portion of the flux, in contact with
atmosphere, which is visible remains granular
(unchanged) and can be reused.

The lower, melted flux becomes slag, which is
waste material and must be removed after
welding.
The electrode at a predetermined speed is
continuously fed to the joint to be welded.
 In automatic welding a separate drive moves
either the welding head over the stationary job
or the job moves/rotates under the stationary
welding head.

The arc length is kept constant by using the
principle of a self-adjusting arc.
If the arc length decreases, arc voltage will
increase, arc current and therefore burn-off rate
will increase thereby causing the arc to
lengthen.
The reverse occurs if the arc length increases
more than the normal.
A backing plate of steel or copper may be
used to control penetration and to support
large amounts of molten metal associated
with the process.

Advantages of SAW:
High deposition rates (over 45 kg/h (100 lb/h)
have been reported).
High operating factors in mechanized
applications.
Deep weld penetration.
High speed welding of thin sheet steels up to 5
m/min (16 ft/min) is possible.
Minimal welding fume or arc light is emitted.
Practically no edge preparation is necessary
depending on joint configuration and required
penetration.
The process is suitable for both indoor and
outdoor works.

Disadvantages of SAW:
Limited to ferrous (steel or stainless steels)
and some nickel-based alloys.
Normally limited to long straight seams or
rotated pipes or vessels.
Requires relatively troublesome flux
handling systems.
Requires inter-pass and post weld slag
removal.

Applications of SAW:
Joining of pressure vessels such as boilers.
Many structural shapes, earth moving
equipment, pipes.
Railroad construction, locomotives and ship
building.
Repairing machine parts.

Electro Slag Welding
Working Principle:
The process in which the coalescence is
formed by molten slag and molten metal
pool remains shielded by the molten
slag.

Working Procedure:
Electroslag welding (ESW) is a highly productive,
single pass welding process for thick (greater than
25 mm up to about 300 mm) materials in a vertical or
close to vertical position.
An electric arc is initially struck by wire that is fed
into the desired weld location and then flux is added.
Additional flux is added until the molten slag,
reaching the tip of the electrode, extinguishes the arc.
The wire is then continually fed through a
consumable guide tube (can oscillate if desired) into
the surfaces of the metal work pieces and the filler
metal are then melted using the electrical resistance
of the molten slag to cause coalescence.

The wire and tube then move up along
the workpiece while a copper retaining
shoe that was put into place before
starting (can be water-cooled if desired) is
used to keep the weld between the plates
that are being welded.
Electroslag welding is used mainly to
join low carbon steel plates and/or
sections that are very thick.

This process uses a direct current (DC) voltage
usually ranging from about 600A and 40-50V,
higher currents are needed for thicker materials.
Because the arc is extinguished, this is not an
arc process.

Advanatages of Electroslag Welding:
Joint preparation is often much simpler
than for other welding processes.
Much thicker steels can be welded in
single pass and more economically.
Thicknesses up to 450 mm in plain and
alloy steels can be welded without difficulty
Electroslag welding gives extremely high
deposition rates.
Residual stresses and distortion produced
are low.

Disadvantages of ESW:
(i)Submerged arc welding is more
economical than electroslag welding for
joints below 60 mm.
(ii) In electroslag welding, there is some
tendency toward hot cracking and notch
sensitivity in the heat affected zone.

Applications of ESW:
Electro slag welding is used mainly in
heavy engineering industries.
This process has been widely used for
joining large castings and forgings to
produce very large composite structures.
It is useful for welding of mild, low and
high alloy steels and some titanium
alloys.

Resistance Welding
Working principle:
The parts to be joined are heated to plastic
state by their resistance to the flow of
electric current and mechanical pressure
is applied to complete the weld.

Working procedure:
There are two copper electrodes in a circuit of
low resistance.
The metal parts to be welded are placed
between the electrodes.
When current is passed through the electrodes
,the electrical resistance at the metal joints
becomes very high.
The metals are brought to red-hot plastic
condition.
Now mechanical pressure is applied to
complete the weld.

The heat generated in the weld may be
expressed by
Q= I²RT
Where, Q = heat
I = Current in amps
R = Resistance of the assembly
T = Time of current flow
The heat developed by the current is
proportional to the electric resistance of the
weld.

A.C with suitable transformer is used for the
power supply.
4 to 12 volts is used dependent on the
composition, area and thickness of the metal to
be welded.
The power supply ranges from 6 to 18KW per
cm³ area used.

Advantages of RW:
1.High speed welding
2.Easily automated
3. Suitable for high rate production
4.Economical
Disadvantages:
1.Initial equipment costs
2. Lower tensile and fatigue strengths
3. Lap joints add weight and material

Applications of Resistance welding:
Automotive / auto suppliers
Electrical / electronics
Aerospace / air plane
Train carriage / rail
Radiator / container
Domestic hardware
Medical instruments
Nuclear equipment
Food and drink
Other metal processing industries.

Types of resistance welding
1. Spot – welding
2. Seam welding
3. Butt – welding
4. Projection welding
5. Stud welding
6. Percussion Butt - welding

1. Spot – welding
 Spot welding is used for making lab joints.
 By using this method, metal sheets from
0.025 mm to 1.25 mm thickness can be
easily welded.
 The metal pieces are assembled and placed
between two copper electrodes and then
current is passed.
 Then the electrodes are pressed against the
metal pieces by mechanical or hydraulic
pressure.

 The electrode pressure can be in the range
of up to 2 KN.
 Electrodes are cooled with water during
operation to prevent overheating.
 Spot welding can be done on metal strips
upto 12 mm thick .
 It is used for fabricating all types of sheet
metal structure where mechanical strength
rather than water or air tightness is
required.

2. Butt – welding
This is one kind of resistance welding .
There are two types of butt – welding,
namely
(i) Upset butt welding
(ii) Flash butt welding.

(i) Upset butt welding
 For making upset welding, the edges of the
work piece should be cleaned perfectly and
flatten the parts to welded are clamped in
copper jaws.
 The jaws act as electrodes.
 Both the work pieces of edges are prepared
and butted together .
 There may be some gap between the parts,
but it should be such that no arcing takes
place.

 Then the jaws are bought together in a
solid contact when current flows through
the point of contact of jaws to form a
locality of high electric resistance.
 At this point, the applied pressure upsets
or forges the parts together.
 This process is mainly used fro welding
nonferrous materials of smaller cross
section such as bars, rods, wire, tube etc.,

(ii) Flash butt welding
 In this process, the parts to be welded are
clamped in copper jaws of the welding
machine.
 They act as electrodes.
 The jaws are water – cooled .
 They are connected to the heavy current
electric power supply.
 The work pieces are bought together in a slight
contact when current flows through the work
pieces, an electric arc or flash is produced.

 The ends reach fusing temperature and
power is switch off.
 Now, the ends are forced together by
applying mechanical force to complete the
weld.
 A major advantage of flash butt welding is
that many dissimilar metal with different
melting temperature can be flash welded.

3. Seam welding
 Seam welding is used to produce
continuous joint between two overlapping
pieces of sheet metal.
 The work pieces are placed between two
rotating wheel electrodes when electric
current is passed through the electrodes.
 High heat is produced on the work pieces
between the wheels.

 At the same time, pressure is applied to
complete the weld.
 The leak proof continuous seam is
achieved by supplying coolant to the
electrodes, finally, it speeds up the welding
process.
Applications:
 Seam welding is used to make leak proof
tanks, drums, radiators, household utensils,
automobile bodies etc.
 It is used for welding thin sheets.

4. Percussion welding
 The parts to be welded are clamped in
copper jaws of the welding machine.
 In which one clamp is fixed and another
one is movable.
 The movable clamp is backed up against
pressure from a heavy spring.
 The jaws act as electrodes.
 Heavy electric current is connected to the
work pieces.

 The movable clamp is released rapidly and
it moves forward at high velocity.
 When the two parts are approximately 1.6
mm apart, a sudden discharge of electrical
energy is released, causing an intense arc
between the two surfaces.
 The arc is extinguished by the percussion
blow of the two parts coming together with
sufficient force to complete in 0.1 second.
 The method is limited to small areas of
about 150 to 300 mm2.

5. Projection welding
 In this, a series of spots are welded at a
time.
 The metal pieces to be welded are placed
between two metal arms which act as
electrode.
 The work pieces are clamped between the
arms.
 When the A.C is supplied, the welding
current will be passed through these
projections.

 The heat is produced at the contact point of
the base metal because of electrical
resistance.
 Now, the work pieces are pressed together
by bringing down the upper electrode.
 The projections are made into flat under
pressure and the two pieces are joined
together by a strong weld at all points of
contact.
 Projection welding is used for joining thin
sheet metals of thickness up to 3mm.

6. Stud welding
 An electric arc is produced between the
stud and the flat surface of the work piece.
 The arc melts the end of the stud and the
stud is forced on the work metal surface.
 A special welding gun is used for welding
which consists of spring, solenoid, trigger,
timer etc.,
 The stud is placed in the welding gun.
 The front end of the gun is held against the
work surface.

 When the trigger of the gun is pressed,
welding current flows between the end of
the stud and the work surface.
 An arc is produced between the gap of the
stud and work.
 It melts the end of the stud.
 A molten pool is formed on the work
surface.
 Now, the melted end of the stud is pressed
on the molten pool of work and it gets
welded to the work.

Plasma Arc Welding
Working principle:
Plasma is high temperature ionized gas
which is a mixture of atoms, positively
charged atoms and free elements.
When this high temperature plasma is
passed through the orifice, the
proportion of the ionized gas increases
and plasma arc welding is formed.

Working Procedure:
The plasma is forced through a fine-bore
copper nozzle which constricts the arc and
the plasma exits the orifice at high velocities
(approaching the speed of sound) and a
temperature approaching 28,000 °C (50,000
°F) or higher.
The gas gets ionized after passage of electric
current through it and it becomes a
conductor of electricity.

In ionized state atoms break into electrons
(−) and ions (+) and the system contains a
mixture of ions, electrons and highly excited
atoms.
The degree of ionization may be between 1%
and greater than 100% i.e.; double and triple
degrees of ionization.
The energy of the plasma jet and thus the
temperature is dependent upon the electrical
power employed to create arc plasma.

A typical value of temperature obtained in a
plasma jet torch may be of the order of
28000 °C(50000 °F ) against about 5500 °C
(10000 °F) in ordinary electric welding arc.

Plasma arc welding process can be divided into
two basic types:
1.Non-transferred arc process
The arc is formed between the electrode(-)
and the water cooled constricting nozzle(+).
The arc is independent of the work piece
and the work piece does not form a part of the
electrical circuit. Just as an arc flame (as in
atomic hydrogen welding), it can be moved
from one place to another and can be better
controlled.

The non transferred arc plasma possesses
comparatively less energy density as
compared to a transferred arc plasma and it
is employed for welding and in applications
involving ceramics or metal plating
(spraying).
High density metal coatings can be produced
by this process.
A non-transferred arc is initiated by using a
high frequency unit in the circuit.

2. Transferred arc process:
The arc is formed between the electrode(-) and the
work piece(+).
 A transferred arc possesses high energy density and
plasma jet velocity.
For initiating a transferred arc, a current
limiting resistor is put in the circuit, which
permits a flow of about 50 amps, between the
nozzle and electrode and a pilot arc is
established between the electrode and the
nozzle.
As the pilot arc touches the job main current
starts flowing between electrode and job, thus
igniting the transferred arc.

Advantages of Plasma arc welding:
Penetration is uniform.
Arc stability is good.
Fully penetrated keyholes can be obtained.
High accuracy weld can be produced.
High speed weld can be obtained.

Disadvantages of Plasma arc welding:
Huge noise occurs during welding.
Chances of electric hazards may occur
during welding.
Limited to high thickness applications.
Ultraviolet radiations can affect human
body.
Gas consumption is high.

Applications of Plasma arc welding:
Used in Aero space applications.
Used for melting high melting point
metals.
Used for welding titanium plates.
Used in welding nickel alloys.
Used for tube mill applications.

Thermit welding
Working principle:
In thermit welding the heat is
produced by highly exothermic
reactions between metal oxides (usually
iron oxides) and a metal reducing agent
(usually aluminium but magnesium).
The chemical affinity of aluminium for
oxygen is the basis for the thermit
process.

Working procedure:
Thermit is a mixture of aluminium and iron
oxide in the ratio of 1:3.
This is placed in a furnace and it is ignited.
So the chemical reaction takes place
8AL+3Fe3O4= 4Al2O3+9Fe
Due to this ,liquid and slag are formedwhich
are used for welding.

The thermit welding process can be classified
into two types.
1.Pressure welding process
2.Non pressure welding process

1.Pressure welding process:
During the pressure welding process,the
parts to be welded are butted and
enclosed in a mould.
The mould can be removed easily after the
welding of metals.
First, the heated iron slag is poured to the
mould and then the aluminium oxide is
poured on the parts to be welded.

This will create the heating of parts and
then the pressure is applied on the workpiece
to join.
2.Non-Pressure welding:
The parts to be welded are lined up in
parallel and around the welding parts.
The wax pattern is formed in and around the
welding parts.
The sand is rammed around and the mould
is completed.

Mould is heated and wax is melted, it will
give a space between the joint.
Finally, the heated iron slag and
aluminium are poured into the mould
after solidification of liquid metal by which
joint is made.

Advantages of Thermit welding:
For welding new necks to rolling mill rolls
with pinions.
Used for welding large broken crankshafts.
Used for building up damaged wobblers.
For welding busted frames of machines.
For restore broken teeth on big gears.

Disadvantages of Thermit welding:
Low deposition rate with operating factor.
Its cannot weld low melting point.
Extremely high level of fume.
It has slag inclusion.

Applications of Thermit welding:
It is used in steel rolling mills.
It is used to weld non ferrous metals.
Pipes, Cables, Rails, Shafts are made in this
process.
Automobile parts are welded by this
process.

Electron Beam Welding
Working principle:
Electron beam welding is a radiant energy
welding process in which the work pieces
are joined by the heat obtained from a
concentrated beam composed primarily
of high-velocity electrons impinging on
the surface to be joined.

Working Procedure:
The system consists of an electronic gun and a
vacuum chamber inside which the work pieces
to be joined are placed. The electronic gun
emits and accelerates the beam of electrons
and focuses it on the work pieces.
When a tungsten filament is electrically
heated in vacuum to approximately 20000°C it
emits electrons. The electrons are then
accelerated towards the hollow anode by
establishing a high difference of voltage
potential between the tungsten filament and a
metal anode.

The electrons pass through the anode at
high speeds (approximately half the speed of
light), then collected into a concentrated beam
and further directed towards the work piece
with the help of magnetic forces resulting
from focusing and deflection coils.
The highly accelerated electrons hit the base
metal and penetrate slightly below the base
surface. The kinetic energy of the electrons is
converted into heat energy.

The succession of electrons striking at the
same place causes the work piece metal to
melt and fuse together.
It should be noted that, the greater the
kinetic energy of the electrons, the greater
is the amount of heat released.
Since electrons cannot travel well through
air, they are made to travel in vacuum which
is the reason for enclosing the electron gun
and the work piece in a vacuum chamber.

Advantages of Electron Beam Welding:
 Any metals, including zirconium, beryllium
or tungsten can be easily welded.
 High quality welds, as the operation is
carried in a vacuum.
 Concentrated beam minimizes distortion.
 Cooling rate is much higher.
 Heat affected zone is less.
 Shielding gas, flux or filler metal is not
required.

Disadvantages of Electron Beam Welding:
 High capital cost.
 Extensive joint preparation is required.
 Vacuum requirements tend to limit the
production rate.
 Size of the vacuum chamber restricts the size
of the work piece being welded.
 Not suitable for high carbon steels. Cracks
occur due to high cooling rates.

Applications of Electron Beam Welding:
Electron beam welding is mainly used in
electronic industries, automotive and
aircraft industries where the quality of weld
required forms the decisive factor.

Friction welding
Working principle:
It is a solid state welding process wherein
coalescence is formed by the heat which is
obtained from mechanically induced sliding
motion between rubbing surfaces.

Working procedure:
Traditionally, friction welding is carried out
by moving one component relative to the
other along a common interface, while
applying a compressive force across the joint.
The friction heating generated at the
interface softens both components, and when
they become plasticized the interface material
is extruded out of the edges of the joint so that
clean material from each component is left
along the original interface.

The relative motion is then stopped, and a
higher final compressive force may be
applied before the joint is allowed to cool.
The key to friction welding is that no
molten material is generated, the weld
being formed in the solid state.

Advantages:
The initial cost is low
Dissimilar metals can be welded.
It is a simple and fast process
The power consumption is less.
The distortion is less

Disadvantages:
It is only suitable for circular butt weld.
The welded work piece should be machined.
It is not suitable for flat, Angular weld.
Possibility of heavy flash out.

Applications:
1. It is used in super alloys.
2. It is used in produce axle shafts,valves and
gears.
3. It is used in production cutting tools like
tapers reamers drills.
4.It is used in refrigeration.
5.It is used for making simple forgings.

Friction Stir welding
Working principle:
Friction-stir welding (FSW) is a solid-state
joining process (the metal is not melted) that
uses a third body tool to join two facing
surfaces.
Heat is generated between the tool and
material which leads to a very soft region near
the FSW tool.

Working procedure:
A constantly rotated non consumable
cylindrical-shouldered tool with a profiled
probe is transversely fed at a constant rate
into a butt joint between two clamped pieces
of butted material.
The probe is slightly shorter than the weld
depth required, with the tool shoulder riding
atop the work surface.

Frictional heat is generated between the
wear-resistant welding components and the
work pieces.
This heat, along with that generated by the
mechanical mixing process and
the adiabatic heat within the material, cause
the stirred materials to soften without melting.
As the pin is moved forward, a special profile
on its leading face forces plasticised material
to the rear where clamping force assists in a
forged consolidation of the weld.

This process of the tool traversing along the
weld line in a plasticized tubular shaft of
metal results in severe solid state
deformation involving dynamic
recrystallization of the base material.

Advantages of FSW:
Good mechanical properties in the as-
welded condition
Improved safety due to the absence of toxic
fumes or the spatter of molten material.
No consumables.
Easily automated on simple milling
machines.
Can operate in all positions.
Generally good weld appearance.
Low environmental impact.

Disadvantages of FSW:
Exit hole left when tool is withdrawn.
Large down forces required with heavy-
duty clamping necessary to hold the plates
together.
Less flexible than manual and arc
processes (difficulties with thickness
variations and non-linear welds).
Often slower traverse rate than some
fusion welding techniques.

Applications of FSW:
Shipbuilding and offshore.
Aerospace.
Automotive.
Rolling stock for railways.
Robotics.

Brazing
In brazing a nonferrous alloys is
introduced in a liquid state between the
pieces of metal to be joined and allowed
to solidify.
The filler metal, having a melting
temperture of more than 840°f but
lower than the melting temperature of the
parent metal is distributed between the
surfaces by capillary action.

Braze welding is similar to ordinary brazing,
metal is melted and deposited at the point
where the weld is to be made.
Alloys of copper, silver, and aluminium are the
most brazing filler metals.
The basic joint types in brazing are lap, butt
and scarf designs.
Joint clearance is important, because
sufficient space must be allowed for capillary
attraction to provide filler metal distribution.
When joining two pieces the lap or butt should
be three times the thickness of the smaller
piece.

Advantages of Brazing:
Virtually all metals can be joined by some type of
brazing metal.
The process is ideally suited for dissimilar metals.
The lower temperature than that needed for welding
(welding is discussed shortly) means the process is
quicker and more economical.
The low working temperature reduces problems
with distortion that can occur during welding, so
thinner and more complex assemblies can be joined
successfully;
Brazing is highly adaptable to automation and
performs well in mass production.

Applications of Brazing:
Brazing is used for fastening of pipe fittings,
tanks, carbide tips on tools, radiators, heat
exchangers, electrical parts, axles, etc.
It can join cast metals to wrought metals,
dissimilar metals and also porous metal
components.
It is used to join band saws, parts of bicycle
such as frame and rims.

Soldering
In soldering, two pieces of metal are joined
with another metal that is applied between
the two in a molten state.
In this process, a little alloying with the base
metal takes place and additional strength is
obtained by mechanical bonding.
Lead and tin alloys having a melting range
of 3500 to 700oF are principally used.
The strength of the joint is determined largely
by the adhesive quality of the alloy, being
joined.

The most important point in soldering is that both
parts of the joint to be made must be at the same
temperature.
The solder will flow evenly and make a good
electrical and mechanical joint only if both parts of the
joint are at an equal high temperature.
Even though it appears that there is a metal to metal
contact in a joint to be made, very often there exists a
film of oxide on the surface that insulates the two
parts.
For this reason it is no good applying the soldering
iron tip to one half of the joint only and expecting this
to heat the other half of the joint as well.

Advantages of soldering:
Solder provides a strong bond and good
electrical conductivity.
Disadvantages of soldering:
Solder contains Lead and other
hazardous substances.
Applications of Soldering:
Used for plumbing, mechanical assembly.
Used in electronics and metalwork from
flashing to jewellery.

Weld Defects
1. Lack of Penetration:
It is the failure of the filler metal to penetrate
into the joint. It is due to
(a) Inadequate de-slagging
(b) Incorrect edge penetration
(c) Incorrect welding technique.

2. Lack of Fusion :
Lack of fusion is the failure of the filler metal
to fuse with the parent metal. It is duo to
(a) Too fast a travel
(b) Incorrect welding technique
(c) Insufficient heat

3. Porosity:
It is a group of small holes throughout the
weld metal. It is caused by the trapping of
gasduring the welding process, due to
(a) Chemicals in the metal
(b) Dampness
(c) Too rapid cooling of the weld.

4. Slag Inclusion:
It is the entrapment of slag or other impurities in
the weld. It is caused by
(a) Slag from previous runs not being cleaned
away,
(b) Insufficient cleaning and preparation of the
base metal before welding commences.

5. Undercuts:
These are grooves or slots along the edges of the
weld caused by
(a) Too fast a travel
(b)Bad welding technique
(c) Too great a heat build-up.

6. Cracking:
It is the formation of cracks either in the weld
metal or in the parent metal. It is due to
(a) Unsuitable parent metals used in the weld
(b) Bad welding technique.

7. Poor Weld Bead Appearance:
If the width of weld bead deposited is not uniform
or straight, then the weld bead is termed as poor.
It is due to improper arc length, improper welding
technique, damaged electrode coating and poor
electrode and earthing connections.
It can be reduced by taking into considerations the
above factors.

8. Distortion:
Distortion is due to high cooling rate, small
diameter electrode, poor clamping and slow arc
travel speed.

9. Overlays:
These consist of metal that has flowed on to the
parent metal without fusing with it. The defect is
due to
(a) Contamination of the surface of the parent
metal
(b) Insufficient heat
10. Blowholes:
These are large holes in the weld caused by
(a) Gas being trapped, due to moisture.
(b) Contamination of either the filler or parent
metals.

11. Burn Through:
It is the collapse of the weld pool due to
(a) Too great a heat concentration
(b) Poor edge preparation.
12. Excessive Penetration:
It is where the weld metal protrudes through the
root of the weld. It is caused by
(a) Incorrect edge preparation
(b) Too big a heat concentration
(c) Too slow a travel.

Unit 2-JOINING PROCESSES

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Unit 2-JOINING PROCESSES