METAL JOINING PROCESS

RAMCO INSTITUTE OF TECHNOLOGY
Mr.M.LAKSHMANAN
Assistant Professor (Senior Grade)
Department of Mechanical Engineering

UNIT II
METAL JOINING PROCESSES

Syllabus
Operating principle, basic equipment, merits
and applications of: Fusion welding processes :
Gas welding - Types – Flame characteristics;
Manual metal arc welding – Gas Tungsten arc
welding - Gas metal arc welding – Submerged
arc welding – Electro slag welding; Operating
principle and applications of : Resistance
welding - Plasma arc welding – Thermit welding
– Electron beam welding – Friction welding and
Friction Stir Welding; Brazing and soldering;
Weld defects: types, causes and cure.

Welding
Welding is a process for joining metal/alloys
permanently by fusion with heat and with or
without pressure.

History
Welding was used in the construction of the Iron
pillar of Delhi, erected in Delhi, India about 310
AD and weighing 5.4 metric tons.

Classification of welding process
(i) Arc welding
• Metal arc
• Metal Inert Gas (MIG)
• Tungsten Inert Gas (TIG)
• Plasma arc
• Submerged arc
• Electro-slag

(ii) Gas Welding
• Oxy-acetylene
• Air-acetylene
• Oxy-hydrogen
(iii)Resistance Welding
• Butt
• Spot
• Seam
• Projection

(iv)Thermit Welding
(v)Solid State Welding
• Friction
• Friction stir welding
• Ultrasonic
• Diffusion
• Explosive

(vi)Radiant Welding
• Electron-beam
• Laser
(vii)Related Process
• Oxy-acetylene cutting
• Arc cutting
• Hard facing
• Brazing
• Soldering

Two Categories of Welding Processes
• Fusion welding - coalescence is
accomplished by melting the two parts to be
joined, in some cases adding filler metal to
the joint
Examples: arc welding, resistance spot
welding, oxyfuel gas welding
• Solid state welding - heat and/or pressure
are used to achieve coalescence, but no
melting of base metals occurs and no filler
metal is added
Examples: forge welding, diffusion welding,
friction welding

Arc welding
It is Electricity travels from electrode to base
metal to melt the metals at the welding point.
Arc welding is a process that is used to join
metal to metal by using electricity to create
enough heat to melt metal, and the melted
metals when cool result in a binding of the
metals.

It is a type of welding that uses a welding power
supply to create an electric arc between an
electrode and the base material to melt the
metals at the welding point.
They can use either direct (DC)
or alternating (AC) current, and consumable or
non-consumable electrodes.

A fusion welding process in which coalescence of
the metals is achieved by the heat from an
electric arc between an electrode and the work
1. Electric energy from the arc produces
temperatures ~ 10,000 F (5500 C), hot enough
to melt any metal.
2. Most AW processes add filler metal to increase
volume and strength of weld joint.

What is an Electric Arc?
An electric arc is a discharge of electric
current across a gap in a circuit .
• It is sustained by an ionized column of
gas (plasma) through which the current
flows
• To initiate the arc in Arc Welding,
electrode is brought into contact with
work and then quickly separated from it
by a short distance.

• Thermionic emission: Electrons and positive ions
from the electrode and the workpiece.
• Accelerated by the potential field between the
electrode and the work
• Produce heat when they convert their kinetic energy
by collision with the opposite charged element
• Electrons have much greater kinetic energy because
they can be accelerated to much higher velocities
under the influence of a given electric field.

Arc welding Equipment:
A welding generator (D.C.)or Transformer (A.C.)
• Two cables-one for work and one for electrode
• Electrode holder
• Electrode
• Protective shield
• Gloves
• Wire brush
• Chipping hammer
• Goggle

Alternating Current (AC) Welding
AC (alternating current)
Most common
180 Ampere or 225 Ampere between 220-240 volts
Can handle most agriculture and construction jobs
Alternating flow of electrons

Direct Current (DC) Welding
DC (direct current)
Can produce direct current of both straight (negative)
and reverse (positive) polarity.
Polarity is the direction in which the current flows
across the arc.
Makes a continuous flow of electrons

Straight/Reverse Polarity
Straight Polarity (DC-)
Work is positive and electrode is negative
Used to weld thinner metals
Shallow penetration
Reverse Polarity (DC+)
Work is negative and electrode is positive
Used to weld thicker metals
Deeper penetration

D.C. Arc welding vs A.C.Arc welding
D.C . Arc welding A.C. Arc welding
Suitable for both ferrous and non-
ferrous metals
Not suitable for non-ferrous metals
Efficiency is less Efficiency is more
Power consumption is more Power consumption is less
Cost of equipment is more Cost of equipment is less
Defects in welding due to arc blow
is occur
Quality of the weld is good

Electrodes
• All major manufacturers of welding electrodes
use the American Welding Society (AWS) code
of specifications.
• Each company makes basically the same
quality which is established by the AWS.
• Electrodes are classified according to type of
coating, composition of the weld metal and
operating characteristics.

GMAW – Gas Metal Arc Welding
SMAW – Shielded Metal Arc Welding
SAW – Submerged Arc Welding
Consumable Electrode
Non-Consumable Electrode
GTAW – Gas Tungsten Arc Welding
PAW – Plasma Arc Welding
Two Basic Types of AW Electrodes
Consumable – consumed during welding process
 Source of filler metal in arc welding
Nonconsumable – not consumed during welding
process
 Filler metal must be added separately if it is added

Consumable electrodes → Rods or wire.
• Welding rods → 225 to 450 mm long, < 10
mm dia.
• Welding rods → to be changed periodically
• Consumable weld wire → continuously fed
into the weld pool from spools → avoiding
the frequent interruptions .
Consumable Electrode

Non-consumable Electrodes
Made of tungsten (or carbon, rarely), which
resists melting by the arc
• Slow depletion → Analogous to wearing of a
cutting tool in machining
• Filler metal must be supplied by means of a
separate wire that is fed into the weld pool

Advantages
• Most efficient way to join metals
• Lowest-cost joining method
• Joins all commercial metals
• Provides design flexibility
Disadvantages
• Manually applied, therefore high labor cost.
• Not convenient for disassembly.
• More defects.

GAS WELDING
• It is a type of fusion, on pressure welding.
• The heat required to melt the metal parts is
supplied by a high temperature flame
obtained by a mixture of two gases.
• The gases are mixed in proper proportions in
a welding blow pipe called welding torch.
• The mixture of oxygen and acetylene gases is
extensively used for welding purposes.

GAS WELDING EQUIPMENT
• Gas Cylinders
Pressure
Oxygen – 125 kg/cm2
Acetylene – 16 kg/cm2
• Regulators
Working pressure of oxygen 1 kg/cm2
Working pressure of acetylene 0.15 kg/cm2
Working pressure varies depends upon the thickness of the
work pieces welded.
• Pressure Gauges
• Hoses
• Welding torch
• Check valve
• Non return valve

Procedure
TURNING ON:
Acetylene slowly turned on (quarter/half turn of the
needle valve) and ignited, producing a small flame.
At this stage, a small amount of soot/smoke is given
off the end of the flame.

• Acetylene increased and oxygen turned on slowly.
• Acetylene increased slowly and oxygen more
rapidly, to produce an intense, localised flame,
capable of precise welding.

Advantages
• Temperature of welding can be easily
controlled by adjusting the flame
• Maintenance cost is low
• Cost of equipment is less.
Disadvantages
• The process is slow
• Strength of the joints is less

Comparison between Arc welding Gas welding
Arc welding Gas welding
Heat is produced by an electric arc Heat is produced by the flame
The temperature of arc is about
4000°c
The temperature of flame is about
3200°c
Risk due to electric shocks Risk due to gas pressure
It is suitable for medium and thick
work
It is suitable for thin work

1. NEUTRAL FLAME
The neutral flame has a one-to-one ratio of
acetylene and oxygen. It obtains additional
oxygen from the air and provides complete
combustion. It is generally preferred for
welding. The neutral flame has a clear, well-
defined, or luminous cone indicating that
combustion is complete.

In the neutral flame, the temperature at the
inner cone tip is approximately 5850ºF
(3232ºC), while at the end of the outer
sheath or envelope the temperature drops
to approximately 2300ºF (1260ºC).
This variation within the flame permits some
temperature control when making a weld.

Neutral welding flames are commonly used to
weld:
• Mild steel
• Stainless steel
• Cast Iron
• Copper
• Aluminium

2. Carburizing Flame
• The carburizing flame has excess acetylene,
the inner cone has a feathery edge extending
beyond it. This white feather is called the
acetylene feather.
• If the acetylene feather is twice as long as the
inner cone it is known as a 2X flame, which is a
way of expressing the amount of excess
acetylene.
• The carburizing flame may add carbon to the
weld metal.

• This flame is obtained by first adjusting to
neutral and then slowly opening the
acetylene valve until an acetylene streamer
or "feather" is at the end of the inner cone.
• The length of this excess streamer indicates
the degree of flame carburization.

• This type of flare burns with a coarse
rushing sound. It has a temperature of
approximately 5700ºF (3149ºC) at the inner
cone tips.
• The steel, which is absorbing carbon from
the flame, gives off heat. This causes the
metal to boil.
• A carburizing flame is advantages for
welding high carbon steel and hard facing
such nonferrous alloys as nickel and Monel.

3. Oxidizing Flame
Oxidizing welding flames are produced when
slightly more than one volume of oxygen is
mixed with one volume of acetylene. To
obtain this type of flame, the torch should
first be adjusted to a neutral flame.
The flow of oxygen is then increased until the
inner cone is shortened to about one-tenth
of its original length. When the flame is
properly adjusted, the inner cone is pointed
and slightly purple.

• An oxidizing flame can also be recognized
by its distinct hissing sound. The
temperature of this flame is approximately
6300ºF (3482ºC) at the inner cone tip.
• When applied to steel, an oxidizing flame
causes the molten metal to foam and give off
sparks. This indicates that the excess oxygen
is combining with the steel and burning it.

• A slightly oxidizing flame is used in torch
brazing of steel and cast iron. A stronger
oxidizing flame is used in the welding of brass
or bronze.
• In most cases, the amount of excess oxygen
used in this flame must be determined by
observing the action of the flame on the
molten metal.
Oxidizing welding flames are commonly used to
weld these metals:
• zinc
• copper
• manganese steel
• cast iron

Gas Tungsten arc welding (GTA welding)/
Tungsten Inert Gas Welding (TIG Welding)

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. The weld area and electrode is
protected from oxidation or other
atmospheric contamination by
an inert shielding gas (argon or helium),

A constant-current welding power
supply produces electrical energy, which is
conducted across the arc through a column
of highly ionized gas and metal vapours
known as a ”plasma”.
This spark is a conductive path for the
welding current through the shielding gas
and allows the arc to be initiated while the
electrode and the workpiece are separated,
typically about 1.5 to 3 mm (0.06–0.12 in)
apart.

• In this welding an electric arc is produced
between a tungsten electrode and the work
piece.
• The inert gas from the cylinder passes
through the welding head around the
electrode.
• The inert gas surrounds the arc and protects
the weld from atmospheric effects.
• So welds are made without defects.

GTAW is most commonly used to weld thin
sections of stainless steel and non-ferrous
metals such as aluminium, magnesium,
and copper alloys.
Applications:
It is used extensively in the manufacture of
space vehicles, and is also frequently
employed to weld small-diameter, thin-wall
tubing such as those used in the bicycle
industry.

Advantages
 Welding speed is high
 No flux is required
 Both ferrous and non-ferrous metals can be
used
 Quality of the welding is good

Metal inert gas welding(MIG welding)

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.
Along with the wire electrode, a shielding
gas feeds through the welding gun, which
shields the process from contaminants in the
air.

The process can be semi-automatic or
automatic. A constant voltage, direct
current power source is most commonly
used with GMAW, but
constant current systems, as well
as alternating current, can be used.

If many workpieces are to be welded
continuously an electrode spool (in the
form of coil) is used. Consumable
electrode is continuously supplied from
this spool by a suitable feeding
mechanism. Commonly, servo
mechanisms are used for feeding long
electrodes.
Some wire feeders can reach feed rates as
high as 30.5 m/min but feed rates for
semiautomatic GMAW typically range
from 2 to 10 m/min.

Metal Inert Gas Welding (MIG Welding) makes
use of the following components
• Consumable Electrode
• Inert Gas Supply
• Welding Head
• A.C or D.C Power Supply
• Electrode Feeding Mechanism

Advantages of MIG Welding
• Welding speed is high
• Possible to weld non ferrous metal like Al, Cu
etc.
• It is cheaper process.
• Consumable electrodes are easy to feed.
• No filler rod is needed.
• Welding is simple.
• Inert gas shield protects the weld automatically

Disadvantages of MIG Welding
• Improper welding may lead to the floating of
solid impurities over the liquid weld.
• If not handled properly, weld may become
porous.
• MIG Welding exposes welders to hazardous
gases.
• Workpieces and Electrodes should be kept clean
before welding.

TIG welding vs MIG welding
TIG welding MIG welding
It is manual welding process. It is semi automatic (or)
automatic process.
TIG welds are created with a
non-consumable electrode
MIG welds are created with a
consumable electrode
TIG welding is a more
complicated process
It is easy process.
Both ferrous and non-ferrous
metals can be used
Possible to weld non ferrous
metals like Al ,Cu
Quality of the welding is good It is poor quality
It is used to AC current It is used to DC current

Submerged arc welding (SAW)
• In submerged arc welding the arc is
produced between electrode and the work
piece.
• In this process the arc is completely
submerged in a granulated materials acting
as a flux.
• The arc is not visible in outside

SAW process variables
• Wire feed speed (main factor in welding
current control)
• Arc voltage
• Travel speed
• Contact tip to work (CTTW)
• Polarity and current type (AC or DC) and
variable balance AC current

Wire
• SAW is normally operated with a single
wire on either AC or DC current. Common
variants are:
• Twin wire
• Multiple wire (tandem or triple)
• Single wire with hot or cold wire addition
• Metal powder addition
• Tubular wire
A narrow gap process variant is also
established, which utilises a two or three
bead per layer deposition technique.

Flux
• Fluxes used in SAW are granular fusible minerals
containing oxides of manganese, silicon, titanium,
aluminium, calcium, zirconium, magnesium and other
compounds such as calcium fluoride.
• The flux is specially formulated to be compatible with
a given electrode wire type so that the combination of
flux and wire yields desired mechanical properties.
• All fluxes react with the weld pool to produce the
weld metal chemical composition and mechanical
properties.

It is common practice to refer to fluxes as 'active'
if they add manganese and silicon to the weld,
the amount of manganese and silicon added is
influenced by the arc voltage and the welding
current level.
The flux starts depositing on the joint to be
welded. Since the flux when cold is non-
conductor of electricity, the arc may be struck
either by touching the electrode with the work
piece or by placing steel wool between electrode
and job before switching on the welding current
or by using a high frequency unit.

Material applications
• Carbon steels (structural and vessel
construction)
• Low alloy steels
• Stainless steels
• Nickel-based alloys
• Surfacing applications (wear-facing and
corrosion resistant overlay of steels)

Advantages
• High deposition rates (over 45 kg/h have been reported).
• High operating factors in mechanized applications.
• Deep weld penetration.
• Sound welds are readily made (with good process design and
control).
• High speed welding of thin sheet steels up to 5 m/min is possible.
• Minimal welding fume or arc light is emitted.
• The process is suitable for both indoor and outdoor works.
• Welds produced are sound, uniform, ductile, corrosion resistant
and have good impact value.
• Single pass welds can be made in thick plates with normal
equipment.
• The arc is always covered under a blanket of flux, thus there is no
chance of spatter of weld.
• 50% to 90% of the flux is recoverable, recycled and reused.

Limitations
• Limited to ferrous (steel or stainless steels)
and some nickel-based alloys.
• Requires relatively troublesome flux handling
systems.
• Flux and slag residue can present a health and
safety concern.
• Requires inter-pass and post weld slag
removal.
• Limited to high thickness materials.

Electro-slag welding
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.
ESW is similar to electro gas welding, but the main
difference is the arc starts in a different location.
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 workpieces and the
filler metal are then melted using the electrical
resistance of the molten slag to cause coalescence.
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 600 A and 40-50 V,
higher currents are needed for thicker materials.
Because the arc is extinguished, this is not an arc
process.

Electro-slag welding
• This method is combination of both arc welding
and resistance welding because at starting, heat is
generated by establishes an arc between
electrode and base metal (as in arc welding).
• This heat leads to melt flux and create a molten
metal pool between the electrode and base
metal.
• Now the current flow through this molten metal
pool and heat is developed due to electric
resistance (as in resistance welding).
• Due to this reason, this is called combination of
arc welding and resistance welding.

Benefits of ESW
• Its high metal deposition rates(between 15 and 20 kg per hour
per electrode)
• Its ability to weld thick materials.
• Many welding processes require more than one pass for
welding thick workpieces, but often a single pass is sufficient
for electroslag welding.
• The process is also very efficient, since joint preparation and
materials handling are minimized while filler metal utilization
is high.
• The process is also safe and clean, with no arc flash and low
weld splatter or distortion.
• Electroslag welding easily lends itself to mechanization, thus
reducing the requirement for skilled manual welders.
• Welds on materials with a thickness of 25 to 75 mm

Disadvantages
• Not preferable for thin plates.
• Only suitable for butt welding
• Initial cost is high.

Plasma arc welding
Plasma arc welding (PAW) is an arc
welding process similar to gas tungsten arc
welding (GTAW). The electric arc is formed
between an electrode and the workpiece.
The key difference from GTAW is that in PAW, by
positioning the electrode within the body of the
torch, the plasma arc can be separated from
the shielding gas envelope.
The plasma is then forced through a fine-bore
copper nozzle which constricts the arc and the
plasma exits the orifice at high velocities and a
temperature approaching 28,000 °C or higher.

Principle
Plasma arc welding is a constricted arc process.
The arc is constricted with the help of a water-
cooled small diameter nozzle which squeezes the
arc, increases its pressure, temperature and heat
intensely and thus improves arc stability, arc
shape and heat transfer characteristics.
Plasma arc welding processes can be divided into
two basic types:
1. Non-Transferred Arc Process
2. Transferred Arc process

1. Non-Transferred Arc Process
• The arc is formed between the electrode(-) and the
water cooled constricting nozzle(+).
• Arc plasma comes out of the nozzle as a flame. The
arc is independent of the work piece and the work
piece does not form a part of the electrical circuit. Just
like 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 plasma arc 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).

2. Transferred Arc process
The arc is formed between the electrode(-) and the
work piece(+). In other words, arc is transferred from
the electrode to the work piece.
A transferred arc possesses high energy density and
plasma jet velocity. For this reason it is employed to
cut and melt metals. Besides carbon steels this process
can cut stainless steel and nonferrous metals where
an oxyacetylene torch does not succeed.
Transferred arc can also be used for welding at high
arc travel speeds. 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.
The temperature of a constricted plasma arc
may be of the order of 8000 - 25000°C.

• In this welding, the heat generated by an
ionized gas jet called plasma is used for
joining metal pieces together.
• Argon or hydrogen may be used as a plasma
gas.
• An inert gas is ionized by an electric arc.
• In plasma arc welding an arc is produced
between a tungsten electrode and a water-
cooled copper nozzle.

Advantages
• All metals can be welded in this process.
• Production rate will be high.
• Faster process.
Disadvantages
• Cost of equipment is high.
• Ultra violet radiation is produced by plasma.

Resistance welding (RW)/Electric
Resistance Welding (ERW)
Electric resistance welding (ERW) refers to a
group of welding processes such as spot and
seam welding that produce coalescence of faying
surfaces where heat to form the weld is generated
by the electrical resistance of material combined
with the time and the force used to hold the
materials together during welding.
Small pools of molten metal are formed at the point
of most electrical resistance (the connecting or
"faying" surfaces) as an electrical current (100–
100,000 A) is passed through the metal.

Resistance welding
• It is done by passing electric current through
two metal parts to be welded.
• There are two copper electrodes in the circuit.
• The metal parts are placed between the
electrodes
• When current is passed heat is generated.
• In general, resistance welding methods are
efficient and cause little pollution, but their
applications are limited to relatively thin
materials and the equipment cost can be high.

The following metals may be welded by
Resistance Welding:
• Low carbon steels - the widest application of
Resistance Welding
• Aluminium alloys
• Medium carbon steels, high carbon
steels and Alloy steels (may be welded, but
the weld is brittle)

Applications
Resistance Welding (RW) is used for joining
vehicle body parts, fuel tanks, domestic
radiators, pipes of gas oil and water pipelines,
wire ends, turbine blades and railway tracks.

Advantages of Resistance Welding:
• High welding rates
• Low fumes
• Cost effectiveness
• Easy automation
• No filler materials are required
• Low distortions

Disadvantages of Resistance Welding:
• High equipment cost
• Low strength of discontinuous welds
• Thickness of welded sheets is limited - up to
1/4” (6 mm)

Types/ Methods of Resistance welding
 Spot welding
 Seam welding
 Flash welding
 Butt welding

1. Spot Welding (RSW)
Spot Welding is a Resistance Welding
(RW) process, in which two or more overlapped
metal sheets are joined by spot welds.
The method uses pointed copper electrodes
providing passage of electric current. The
electrodes also transmit pressure required for
formation of strong weld.
Diameter of the weld spot is in the range
1/8”-1/2”(3-12mm).
Spot welding is widely used in automotive
industry for joining vehicle body parts.

Advantages
• High speed of welding
• Low cost
• Less skilled operator needed.

2. Seam Welding
Seam Welding is a Resistance Welding
(RW) process of continuous joining of
overlapping sheets by passing them between
two rotating electrode wheels.
Heat generated by the electric current flowing
through the contact area and pressure
provided by the wheels are sufficient to
produce a leak-tight weld.

• In this process a continuous type of spot
welding over two overlapping metal sheets or
plates.
• The seam welding equipment has two
rotating copper wheels.
• These wheels act as electrodes.
• Seam Welding is high speed and clean
process, which is used when continuous tight
weld is required (fuel tanks, drums, domestic
radiators)

3. Flash Welding (FW)
Flash Welding is a Resistance Welding
(RW) process, in which ends of rods (tubes,
sheets) are heated and fused by an arc struck
between them and then forged (brought into a
contact under a pressure) producing a weld.
The welded parts are held in electrode clamps,
one of which is stationary and the second is
movable.
Flash Welding method permits fast (about 1 min.)
joining of large and complex parts.
Welded part are often annealed for improvement
of Toughness of the weld.

Welded Materials:
Steels, Aluminium alloys, Copper
alloys, Magnesium alloys, Copper
alloys and Nickel alloys may be welded by
Flash Welding.
Application:
Thick pipes, ends of band saws, frames, aircraft
landing gears are produced by Flash Welding.

4. Resistance Butt Welding (BW)
Resistance Butt Welding is a Resistance Welding
(RW) process, in which ends of wires or rods are held
under a pressure and heated by an electric current
passing through the contact area and producing a
weld.
The process is similar to Flash Welding, however in Butt
Welding pressure and electric current are applied
simultaneously in contrast to Flash Welding where
electric current is followed by forging pressure
application.
Butt welding is used for welding small parts. The
process is highly productive and clean.
In contrast to Flash Welding, Butt Welding provides
joining with no loss of the welded materials.

Thermit welding
Exothermic welding, also known as exothermic
bonding, thermite welding (TW) and Thermit
welding, is a welding process that employs
molten metal to permanently join the conductors.
The process employs an exothermic reaction of
a thermit composition to heat the metal, and
requires no external source of heat or current.
The chemical reaction that produces the heat is
an aluminothermic reaction
between aluminum powder and a metal oxide.

• It is fusion welding process.
• In this process, the welding is done by
pouring superheated liquid steel around the
parts to be welded.
• Thermit is a mixture of finely divided
aluminum powder and iron oxide at the
ratio of 1:3 by weight.
Thermit welding

Application
• Thermit welding is limited to heavy joints
• It is used for welding very large parts such
as joining of rails,shafts,broken teeth of large
gears etc.

Electron beam welding (EBW)
• It is defined as a fusion welding process.
• It is produced by the heat obtained by the
electrons on the work piece with high
velocity.
• A tungsten filament is heated in a vacuum
chamber to emission temperature (200◦c)
by a high voltage current.
• An electron gun is placed.
• This gun may be moved vertically and
horizontally.

The workpieces melt and flow together as the kinetic energy
of the electrons is transformed into heat upon impact.
Values of power density in the crossover (focus) of the beam
can be as high as 104 – 106 W/mm2.
Shallow penetration depths in the order of hundredths of a
millimeter. This allows for a very high volumetric power
density, which can reach values of the order 105 –
107 W/mm3. Consequently, the temperature in this volume
increases extremely rapidly, 108 – 1010 K/s.

Application
• It is used to automobile and aero plane
parts.
• It is widely used for joining dissimilar
metals.
• It is also used for welding stainless steel,
titanium etc.
Advantages
• It is a very high speed process.
• Temperature can be easily controlled
• Welds are very clean

Friction welding (solid state welding)

Friction welding (FRW) is a solid-
state welding process that generates heat
through mechanical friction between workpieces
in relative motion to one another, with the
addition of a lateral force called "upset" to
plastically displace and fuse the materials.
Because no melting occurs, friction welding is
not a fusion welding process in the traditional
sense, but more of a forge welding technique.
Friction welding is used with metals
and thermoplastics in a wide variety of aviation
and automotive applications.

• It is a solid state welding.
• It is produced by heat obtained from
mechanically induced sliding motion between
rubbing surfaces.
• The work parts are held together under pressure
• The two components to be friction welded are
held in axial alignments.

The combination of fast joining times (on the
order of a few seconds), and direct heat input at
the weld interface, yields relatively small heat-
affected zones.
Friction welding techniques are generally melt-
free, which mitigates grain growth in engineered
materials, such as high-strength heat-treated
steels.
Another advantage is that the motion tends to
"clean" the surface between the materials being
welded, which means they can be joined with less
preparation.

Applications
• It is used to weld similar and dissimilar metals.
• Friction welding is used for aero engine shafts.
Gas turbine shafts, flanges to pipes etc.
Advantages
• It is a quick process
• Power consumption is less.
• Initial cost is less.
• Quality of weld is good.

Friction stir welding (solid state welding)

Friction stir welding (FSW)
Friction stir welding (FSW) is a solid-state
joining process that uses a non-consumable
tool to join two facing workpieces without
melting the workpiece material.
Heat is generated by friction between the
rotating tool and the workpiece material,
which leads to a softened region near the FSW
tool.
While the tool is traversed along the joint line, it
mechanically intermixes the two pieces of
metal, and forges the hot and softened metal by
the mechanical pressure, which is applied by
the tool, much like joining clay.

A rotating cylindrical tool with a profiled probe is
fed into a butt joint between two clamped
workpieces, until the shoulder, which has a
larger diameter than the pin, touches the
surface of the workpieces.
The probe is slightly shorter than the weld depth
required, with the tool shoulder riding atop the
work surface.
After a short dwell time, the tool is moved
forward along the joint line at the pre-set
welding speed.

FSW Process parameters:
Welding speed
Tool rotational speed
Axial forces

Advantages
• 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 — lower
setup costs and less training.
• Can operate in all positions (horizontal, vertical, etc.), as
there is no weld pool.
• Generally good weld appearance and minimal thickness
under/over-matching, thus reducing the need for
expensive machining after welding.
• Can use thinner materials with same joint strength.
• Low environmental impact.
• General performance and cost benefits from switching
from fusion to friction.

Disadvantages
• 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).
Applications:
shipbuilding and offshore, aerospace,
automotive, rolling stock for railways, general
fabrication, robotics and computers.

Brazing
Brazing is a metal-joining process in which two
or more metal items are joined together by
melting and flowing a filler metal into the
joint, the filler metal having a lower melting
point than the adjoining metal.

• It is the process of joining two similar or
dissimilar metals by fusible alloy called filler
rod.
• The filler rod having a melting temperature
of about 600◦C, blow melting point of the
work pieces.
• The work pieces are not melted in the
brazing process.

Brazing differs from welding in that it does not
involve melting the work pieces and
from soldering in using higher temperatures for a
similar process, while also requiring much more
closely fitted parts than when soldering.
The filler metal flows into the gap between close-
fitting parts by capillary action. The filler metal is
brought slightly above its melting (liquidus)
temperature while protected by a suitable
atmosphere, usually a flux.
It then flows over the base metal (known as
wetting) and is then cooled to join the work
pieces together.

• It is similar to soldering, except for the use of higher
temperatures. A major advantage of brazing is the
ability to join the same or different metals with
considerable strength.
• High-quality brazed joints require that parts be
closely fitted, and the base metals exceptionally clean
and free of oxides.
• In most cases, joint clearances of 0.03 to 0.08 mm are
recommended for the best capillary action and joint
strength.
• However, in some brazing operations it is not
uncommon to have joint clearances around 0.6 mm.
• Cleanliness of the brazing surfaces is also important,
as any contamination can cause poor wetting (flow).

Filler materials
• Aluminum-silicon
• Copper
• Copper-silver
• Copper-zinc (brass)
• Copper-tin (bronze)
• Gold-silver
• Nickel alloy
• Silver
• Amorphous brazing foil using nickel, iron,
copper, silicon, boron, phosphorus, etc.

Advantages
• Very thin metals can be joined.
• It is quicker process.
• Strength of the joints is high.

• Brazing - Brazing as a group of joining
processes that produce coalescence of
materials by heating them to the brazing
temperature and by using a filler metal
(solder) having a liquidus above 840°F
(450°C), and below the solidus of the base
metals.
• Soldering - Soldering has the same
definition as brazing except for the fact that
the filler metal used has a liquidus below
840°F (450°C) and below the solidus of the
base metals.

• Soldering is the process of joining two pieces
of metals by adding a fusible alloy called
solder.
• It is used as a filler rod.
• The work pieces are not melted in soldering.
• Solder is an alloy of tin and lead.
• It melts at low temperature in the range of
150-350◦c.

• Soldering is a process in which two or more
metal items are joined together by melting
and then flowing a filler metal into the joint
the filler metal having a relatively low
melting point.
• Soldering is used to form a permanent
connection between electronic components.
• The metal to be soldered is heated with a
soldering iron and then solder is melted into
the connection.

Application
• It is used to automotive radiators or tin cans
• Electrical Connections
• Joining thermally sensitive components
• Joining dissimilar metals
• Plumbing, Electronics, and Metalwork
from Flashing to Jewelry.

Welding Defects
• Lack of fusion
• Lack of penetration or Excess penetration
• Porosity
• Inclusions
• Cracking
• Undercut

Lack of fusion
Lack of fusion results from too little heat input
and / or too rapid traverse of the welding
torch (gas or electric).

• Excess penetration arises from to high a heat
input and / or too slow transverse of the
welding torch (gas or electric).
• Incomplete penetration happens when
your filler metal and base metal aren’t
joined properly, and the result is a gap.

Porosity:
Porosity occurs when gases are trapped in the
solidifying weld metal. These may arise from
damp consumables or metal or, from dirt,
particularly oil or grease, on the metal in the
vicinity of the weld. This can be avoided by
ensuring all consumables are stored in dry
conditions and work is carefully cleaned and
degreased prior to welding.

Slag Inclusions:
These can occur when several runs are made
along a V join when joining thick plate using
flux cored or flux coated rods and the slag
covering a run is not totally removed after
every run before the following run.

Craking:
This can occur due just to thermal shrinkage or
due to a combination of strain accompanying
phase change and thermal shrinkage.
In the case of welded stiff frames, a
combination of poor design and inappropriate
procedure may result in high residual stresses
and cracking.
• Hot Cracks
• Cold Cracks
• Crater Cracks

Undercutting:
Undercutting is an extremely common welding
defect. It happens when your base metal is
burned away at one of the toes of a weld.
When you weld more than one pass on a joint,
undercutting can occur between the passes
because the molten weld is already hot and
takes less heat to fill, yet you’re using the same
heat as if it were cold.

Inspection/Detection
• Visual Inspection
• Liquid Penetrant Inspection
• X-Ray Inspection
• Ultrasonic Inspection
• Magnetic Particle Inspection

METAL JOINING PROCESS

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