Parts are joined together by Fusion. Fusion is
brought about by a combination of heat and
pressure between parts being joined. In normal
welding processes very high temperatures and
little or no pressure is used.
Welding conditions
• Smooth joint surfaces that match each other
• Surfaces clean and free from oxides, grease and dirt.
• Metals to be joined have same microstructure
Welding conditions continued….
• The metals should be good quality (no internal
Welding Preparation
• Before starting a weld, the joint edges should be
carefully prepared.
• Beveling large edges
• Cleaning (Chemical/Mechanical)
Weld Joints
Welding Symbols
Weld defects
Welding Techniques
Joint Root
is that portion of a joint to be welded where the members
are closest to each other
• The joint root may be
either a point, line, or
an area
• The joint roots are
shown as shaded areas
in (A)-(D) and lines in
(E) (F)
Groove face, Root face and Root edge
• Groove face is “ that
surface of a member
included in the
• Root face (land) is
“that portion of the
groove face within
the joint root”
• Root edge is a root
face of zero width
Root opening and Bevel
• Root opening is
the separation
between the
work pieces at
the joint root
• Bevel
(chamfer) is an
angular edge
Butt Joint
A joint between two
members aligned
approximately in the
same plane
Lap Joint
A joint between two
overlapping members
T Joint
A joint between two
members located
approximately at right
angles to each other
Corner Joint
A joint between two
members located at
right angles to each
Edge Joint
A joint between the
edges of two or
more parallel or
nearly parallel
Welding Techniques
There are many different methods of welding. The difference
between them is outlined by two important features
• The way the metal is heated
• The way additional filler metal if any is fed into the weld
Types of Welding
• Electric Arc Welding
• Friction Welding
• Gas Welding
• Robotic Welding
• Resistance Welding
Electric Arc Welding
The heat for fusion is supplied by an electric arc
Arc is formed between electrode and work this melts
and fuses the joint edges
Types of Electric Arc Welding
Manual Metal Arc (MMA)
Metal Arc Gas Shielded (MAGS) MIG
Tungsten Arc Gas Shielded (TAGS) TIG
Submerged Arc Welding (SAW)
Manual Metal Arc (MMA)
• Most widely used of all
the arc welding processes
• Commonly called “stick”
repair work, structural steelwork,
Touch electrode against
work withdraw
electrode to establish
arc. Heat of arc melts
base metal, the
electrode’s metal core,
and any metal particles
in electrode’s covering.
Heat also melts,
vaporises, or breaks
down chemically
non-metallic substances in covering for arc shielding.
Mixing of molten base metal and filler metal from
electrode produces coalescence required to effect
• Used with many electrode types & sizes
• Used in all positions
• Used on great variety of materials
• Flexibility in operator control makes it the
most versatile of allwelding processes
• Low cost of equipment
• Rod becomes shorter & periodically needs replacing
• Slows production rate (% time welder welding)
The Electrode and Coating
Coating is a combination of chemicals
Cellulosic electrodes contain cellulose
Rutile electrodes titanium oxide (rutile)
Basic electrodes contain calcium
carbonate (limestone) and calcium fluoride
Function of Electrode Coating
• Produce gas to shield weld pool from
oxidisising effects of atmosphere
• Fluxing elements help weld pool to form
• Helps slag to form-removes impurities
• Slag slows down cooling preventing
• Can contain alloying elements or additional
filler metal
Equipment used in MMA
AC power source
Takes power directly from mains power
supply. It use a transformer to supply the
correct voltage to suit the welding
DC Generator
An electricity generator is driven by a
motor. The motor can be electric,
petrol or diesel. The generator
provides DC current for the arc
Metal Arc Gas Shielded (MAGS) MIG
MIG is similar to MMA in that heat
for welding is produced by forming
an arc between a metal electrode and
the workpiece
Sheet and Heavy plate, production
welding by robots on cars
MIG is similar to
MMA in that heat for
welding is produced
by forming an arc
between a metal
electrode and the
workpiece; the
electrode melts to
form the weld bead.
The main difference
is that the metal electrode is a small diameter wire fed from a
spool and a sheilding gas is used. As the wire is continuously
fed, the process is often referred to as semi-automatic welding.
Large gaps filled or bridged easily
Welding can be done in all positions
No slag removal required
High welding speeds
High weld quality
Less distortion of work piece
Equipmnt used in MAGS
Three major elements are :
Welding torch and accessories
Welding control & Wire feed motor
Power Source
Shielding Gas
Welding torch and accessories
The welding torch guides the wire and shielding gas to the
weld zone.
Brings welding power to the wire also
Major components/parts of the torch are the contact tip,
shielding gas nozzle, gas diffuser, and the wire conduit
Welding control and wire feed motor
 Main
function is to pull
the wire from the spool
and feed it to the arc
 Controls wire feed speed
and regulates the starting
and stopping of wire feed
Welding power source
Positive (+) lead is connected
to the torch
Negative (-) lead is connected
to the work piece
Sheilding Gas
• Purpose of shielding gas is to
protect the weld area from the
contaminants in the atmosphere
• Gas can be Inert, Reactive, or
Mixtures of both
• Argon, Helium, and Carbon
Dioxide are the main three gases
used in MAGS
Tungsten Arc Gas Shielded (TAGS) TIG
TIG is similar to MMA in that
heat for welding is produced
by forming an arc between a
metal electrode and the
Used in joining magnesium and
Aluminium, stainless steels
for high quality welding
Thin sheet material
In the TIG process the arc
is formed between a
pointed tungsten
electrode and the work
piece in an inert
atmosphere of argon or
helium. The small intense
arc provided by the
pointed electrode is ideal
for high quality and
precision welding.
The electrode is not consumed during welding. When filler metal
is required, it must be added separately to the weldpool. There
are two currents one for starting the arc the other switched on
using a trigger or foot pedal, this is a high frequency current
to maintain the arc, this is generated by a separte unit.
• Superior quality welding
• Can be used in mechanised systems
• Used to weld aluminium and stainless
• Free of spatter
• Low distortion
Equipment used in TAGS
Power source
TIG must be operated with a
constant current power source either DC or AC
Electrodes for DC welding are normally pure
tungsten. In AC welding, as the electrode will be
operating at a much higher temperature, It should
be noted that because of the large amount of heat
generated at the electrode, it is difficult to
maintain a pointed tip and the end of the
electrode assumes a spherical or 'ball' profile.
Sheilding Gas
Shielding gas is selected according to the material being welded.
• Argon
• Argon + Hydrogen
• Argon/Helium
Helium is generally added to increase heat
input (increase welding speed or weld
penetration). Hydrogen will result in cleaner
looking welds and also increase heat input,
however, Hydrogen may promote porosity
or hydrogen cracking.
Submerged Arc Welding (SAW)
Similar to MIG welding, SAW
involves formation of an arc between
a continuously-fed bare wire
electrode and the workpiece
SAW welding taking place in the flat
position. Ideal for heavy workpieces
Carbon-manganese steels,low alloy
steels and stainless steels
Submerged Arc Welding (SAW)
The process uses a flux to generate protective gases and
slag, and to add alloying elements to the weld pool. A
shielding gas is not required. Prior to welding, a thin layer
of flux powder is placed on the work piece surface. The arc
moves along the joint line and as it does so, excess flux is
recycled via a hopper. Remaining fused slag layers can be
easily removed after welding. As the arc is completely
covered by the flux layer, heat loss is extremely low. There
is no visible arc light, welding is spatter-free and there is no
need for fume extraction.
Equipmnt used in SAW
SAW is normally operated with a single wire on either AC or
DC current. Common variants are:
• twin wire
• triple wire
• single wire with hot wire addition
• metal powdered flux addition
All contribute to improved productivity through a marked
increase in weld metal deposition rates and/or travel speeds.
Fluxes used in SAW are granular fusible minerals
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
Gas Welding (Oxy-acetylene)
A number of welding processes use a flame
produced by burning a mixture of fuel gas and
oxygen. The gas usually used is Acetylene but other
gases are also used.
Separate cylinders and
a hose pipe from each
cylinder transports the
gases to a torch.
Gas and fuel mix in
the torch
burns @ 3100°C.
During the welding heat from the flame is
concentrated on the joint edges until the metal
melts and starts to flow. When the molten metal
from both sides melts it starts to fuse, when the
metal cools down the two parts become
Permanently joined
Additional Filler
Metal is fed in by
hand into the weld
pool, at regular
intervals where it
becomes molten
and joins with the
parent metal.
The Oxy-acetylene welding Flame
Reducing or Carburizing
Excess acetylene (0.9:1)
(Alloy steels and
aluminium alloys)
Excess oxygen (1.5:1)
(Brasses, Bronzes, copper)
Equal acetylene & oxygen
(low carbon steel, mild
Inner Cone
Max. Temp.
Secondary Combustion
Equipment used in O-A welding
The oxygen and acetylene hose pipes
Gases used
Gas pressure Regulators
Flashback arrestor
Welding torch/Welding nozzle
Filler rods and fluxes
Gases used
Oxygen extracted from air and compressed into
cylinders at high pressure. Cylinder is black. Oil should
never be brought into contact and should not be used on
Acetylene (C2H2) is a fuel gas. Cannot be compressed
directly as explodes at high pressures. Cylinders are
packed with porous material which is filled with
acetone Acetone absorbs acetylene. Cylinder colour
coded maroon
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