1. Welding and its types
Welding is a process of joining similar metals by the application of heat with or
without the application of pressure and addition of filer material. It is broadly
classified into plastic and fusion welding.
Plastic Welding: The type of welding in which the pieces of metals to be joined are
heated to a plastic state and then forged together by external pressure is called
Fusion Welding: In fusion welding, the material at the joint is heated to a molten
state and allowed to solidify.
2. MIG Welding and
MIG welding is a gas shielded metal arc welding
process using the heat of an electric arc
between a continuously fed, consumable
electrode wire and the material to be welded.
Metal is transferred through a protected arc
column of inert gas to the work.
A wire of copper coated mild steel is fed
continuously from a reel through a gun with a
melting rate up to 5m/min.
Current through the wire ranges from 100 to
400 A depending upon the diameter of the
CO2 is principally used apart from argon or
argon-helium mixture as shielding gas.
The welding machine is a dc constant voltage
2.1 Development of GMAW and MIG Welding
• Sir Humphry Davis discovered the electric arc and metal electrodes
invented by N.G. Slavianoff and C. L. Coffin in the late 1800’s.
• Early predecessor of GMAW was invented by P. O. Nobel of General
Electric with no shielding gas.
• GMAW was further developed by Betelle Institute.
• Smaller diameter electrodes were developed.
• Started the use of argon for shielding.
• Use of carbon dioxide as a welding atmosphere was developed.
2.2 Advantages and disadvantages of MIG welding.
1. No flux required. 1. The arc is less stable.
2. Fast welding and deposition rates. 2. Generates more spatter.
3. Increased corrosion resistance. 3. Limited to short –circuit mode of metal
4. Easily automated welding. 4. Relatively high heat and light generation.
5. Suitable for all metals including aluminum and
5. Equipment is quite complex.
6. Least expensive and highly economic. 6. Not suitable for base metal contamination.
2.3 Tools and equipment used in MIG welding.
1. Power source: MIG welding uses a dc
constant voltage transformer.
2. Wire feeder: A wire feeder is required to feed
the electrode wire continuously and smoothly to the
3. Conduit and hoses: Conduit facilitates feeding of
wire to the torch and hoses supply the shielding gas.
4. Welding gun: it transfers the welding current to the
wire and provides the gas for shielding the arc and weld
5. Gas cylinders: They are used to store
shielding gases like CO2, argon, etc.
6. Safety equipment: They all the tools
for safety and precaution purpose like
helmet, goggles, apron, gloves and boots .
2.4 Welding defects
Cracks: Cracking occurs when the arc is struck but
the spot is not welded properly.
Distortion: This happens due to uneven shrinkage
of metal due to heating.
Gas inclusion: Gas inclusions is a wide variety of
defects that includes porosity, blow holes,
4. Undercut: This happen when the
weld reduces the cross-sectional
thickness of base metal.
5. Lack of fusion: Lack of fusion is the
poor adhesion of the weld bead to
the base metal.
6. Inclusions: Inclusions occur when
there is slag, dirt, flux or other
impurities in the weld.
2.5 Application of MIG welding
1. The most common application of MIG welding is automotive repair.
2. Special welding equipment can be used to weld pipes.
3. It can even be used to reinforce the surface of a worn out railroad
4. Because of its high economy and utility it is widely used in various