Welding

JOINING PROCESSES
Lecture 14 & 15
Classification of Joining Processes
 Temporary – Mechanical joining by means of screws, nuts and bolts.
 Semi-Permanent – Rivets, Adhesive bonding by employing synthetic glues such as
epoxy resins
 Permanent – Welding, brazing and soldering
The choice of a particular fabrication method depends on a number of factors pertaining

to the joint such as
 Type of assembly – permanent, semi-permanent or temporary
 Materials being joined – steels, cast irons, aluminium, similar or dissimilar metals
 Economy achieved
 Type of service required – such as assembly subjected to heavy loading, impact
loading, high temperatures, etc.ff
 Welding is the process of joining two components for the desired purpose, can be
defined as the process of achieving complete coalescence (joining)of two or more
materials with the application of heat, with or without the application of pressure and
with or without the use of filler metal.
 Welding is a fabrication process used to join materials, usually metals or
thermoplastics, together. During welding, the work pieces to be joined are melted at
the joining interface and usually a filler material is added to form a pool of molten
material that solidifies to become a strong joint.
 In contrast, Soldering and Brazing do not involve melting the work piece but rather a
lower-melting-point material is melted between the work pieces to bond them
together.
 Soldering
 Produces coalescence of materials by heating to soldering temperature (below solidus of
base metal) in presence of filler metal with liquidus <450°C
 Brazing
 Same as soldering butcoalescence occurs at >450°C
Welding
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, oxyfuel gas welding
Solid state welding – Heat and/or pressure are used to achieve coalescence,
but no melting of base metals occurs while it is the plastic deformation of base
materials and no filler metal is added
 Examples: forge welding, diffusion welding, friction welding
Heat sources for fusion welding
• Fuel burning (Gas welding)
• Electric arc (MMAW, SAW, GTAW, GMAW,FCAW, PAW)
• Laser beam (LBW)
• Electron beam (EBW)
Applications:
Presently welding is widely being used in fabrication of pressure vessels, bridges,
building structures, aircraft and space crafts, railway coaches and general
applications. It is also being used in shipbuilding, automobile, electrical, electronic
and defense industries, laying of pipe lines and railway tracks and nuclear
installations etc.
General Applications:
Welding is vastly being used for construction of transport tankers for transporting
oil, water, milk and fabrication of welded tubes and pipes, chains, LPG cylinders and
other items. Steel furniture, gates, doors and door frames, body and other parts of
white goods items such as refrigerators, washing machines, microwave ovens and
many other items of general applications are fabricated by welding.
Selection of the welding process
• Availability of equipment
• Repetitiveness of the operation
• Quality requirements (base metal penetration, consistency, etc.)
• Location of work
• Materials to be joined
• Appearance of the finished product
• Size of the parts to be joined
• Time available for work
• Skill experience of workers
• Cost of materials
• Code or specification requirements
Microstructural zones
Welding Setup
Welding Arc
Energy can be obtained from an electrical or electromagnetic source in three distinct
ways:
(1) An electric arc;
(2) Resistance (I2R or Joule losses) to either the direct flow of current in a circuit or
currents induced in the workpiece; and
(3) High-intensity radiant energy or beams in which the kinetic energy of
particles in the irradiating field or beam is converted to heat by collisions with atoms
in the workpiece.
ARC: A luminous discharge of current that is formed when a strong current jumps a
gap in a circuit or between the two electrodes.
Welding Arc consist of a sustained electrical discharge through a high temperature,
conducting plasma, producing sufficient thermal energy as to be useful for the joining
of metals by fusion
Polarity
DCEN or Straight Polarity DCEP or Reverse Polarity AC

Types of Welded Joints
The weld joint is where two or more metal parts are joined by welding. The five basic
types of weld joints are the butt, corner, tee, lap, and edge.
 Butt Joint: it is used to join two members aligned in the same plane. This joint is
frequently used in plate, sheet metal, and pipe work.
 Corner and Tee Joints: these joints are used to join two members located at right
angles to each other. In cross section, the corner joint forms an L-shape, and the tee
joint has the shape of the letter T.
 Lap Joint: this joint is made by lapping one piece of metal over another. This is one
of the strongest types of joints available; however, for maximum joint efficiency, the
overlap should be atleast three times the thickness of the thinnest member of the
joint.
 Edge Joint: it is used to join the edges of two or more members lying in the same
plane. In most cases, one of the members is flanged, as seen in the figure. This type
is frequently used in sheet metal work for joining metals 1/4 inch or less in
thickness that are not subjected to heavy loads.
Welding Positions
Manual Metal Arc Welding (MMAW)
Submerged Arc Welding (SAW)
Advantages of SAW
The major advantages of the SAW or submerged arc welding
process are:
high quality metal weld.
extremely high speed and deposition rate
smooth, uniform finished weld with no spatter.
little or no smoke.
no arc flash, thus minimal need for protective clothing.
high utilization of electrode wire.
easy automation for high-operator factor.
normally, no involvement of manipulative skills.
Metal Inert Gas (MIG)
• Consumable electrode
• Inert gas shielding
• Good for welding Non Ferrous
Metals
• Fast process
• Out-of-position welding
• Semi-automatic or automatic
• For ferrous materials, CO2
shielding is adequate
Tungsten Inert Gas (TIG)
• Non-consumable electrode
• Inert gas shielding (Ar or He)
• Good for welding Non Ferrous
Metals
• Precise process – good for root
passes and thinner gauges
• Control over filler addition
• Manual or automatic
• Slow process
• Out-of-position welding difficult
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 at 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.
YouTube Links for the following Welding topics
 Different types of edge preparation for weld joints
https://www.youtube.com/watch?v=B-Hsmk_VBow
 Working animation of shielded metal arc welding process
https://www.youtube.com/watch?v=zwVrHfgcKhs
 How submerged arc welding works.
https://www.youtube.com/watch?v=lNMJA2f_Fpk
 Oxy-acetylene gas welding
https://www.youtube.com/watch?v=-SA4D098u-Q
 How Ultrasonic Welding process works
https://www.youtube.com/watch?v=TH5Pkj3ew7k
 How explosion welding process works
https://www.youtube.com/watch?v=d87dd5T2yYg
 How friction welding process works | Types of friction welding
https://www.youtube.com/watch?v=zhfjev_2c5I
SOLID STATE WELDING
Friction Welding
Friction Welding
Explosive Welding

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JOINING PROCESSES

The choice of a particular fabrication method depends on a number of factors pertaining

DCEN or Straight Polarity DCEP or Reverse Polarity AC

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