Welding is a process that joins materials by causing fusion and filling the joint with a filler material. There are several advantages to welding including lighter structures, maximum strength in joints, easy alterations, pleasing appearance, and strength equal to the parent material. Spot welding uses two electrodes to locally fuse materials and is commonly used in automotive and aircraft industries to join sheet metal. MIG welding uses an inert gas shield to prevent contamination and is often used for carbon/alloy steels, stainless steel, aluminum and other metals due to its high welding speed and economy. Common welding defects include lack of penetration, undercut, slag inclusion, porosity, cracks, spatter, and distortion.

1. WELDING

2. DEFINATION OF WELDING
A welded join is a parmanent joint which is
obtained by the fusion of the edges of the two
parts to be joined together, with or without
the application of pressure and a filler
material.

3. Advantages of welding
• The wlded structures are usually lighter than
riveted structures. This is due to the reason, that
in welding, gussets or other connecting
components are not used.
• The welded joints provide maximum efficiency
(may be 100%) which is not possible in case of
riveted joints.
• Alterations and additions can be easily made in
the existing structures.
• As the welded structure is smooth in appearance,
therefore it looks pleasing.

4. Cont. Advantages of welding
• In welded connections, the tension members are
not weakened as in the case of riveted joints.
• A welded joint has a great strength. Often a
welded joint has the strength of the parent metal
itself.
• Sometimes, the members are of such a shape (i.e.
Circular steel pipes) that they afford difficulty for
riveting. But they can be easily welded.

5. Cont. Advantages of welding
• The welding provides very rigid joints. This is
in line with the modern trend of providing
rigid frames.
• It is possible to weld any part of a structure at
any point. But riveting requires enough
clearance.
• The process of welding takes less time than
the riveting.

6. Disadvantages of Welding
• Since there is an uneven heating and cooling
during fabrication, therefore the members may
get distorted or additional stresses may develop.
• It requires a highly skilled labour and supervision.
• Since no provision is kept for expansion and
contraction in the frame, therefore there is a
possibility of cracks developing in it.
• The inspection of welding work is more difficult
than riveting work.

7. WELDING
Plastic
Under pressure, with out
addition of filler material.
Fusion
No pressure is required -but
addition of filler material
Table showing different welding processes.

8. Forged welding Resistance welding
Thermit welding with pressure
1)Lap welding
2) Butt welding
3) W-Welding
4)T-Welding
1) Spot Welding
2) Projection Welding
3) Seam Welding
4) Butt Welding
Upset
Flash
5)Percussion Welding
Heat Generated By
PLASTIC WELDING
Black smith fire Electric current Chemical reaction

9. Heat Generated By
GAS
1) Oxy-acetylene
Welding
2) Air Acetylene
3) Oxy-Other fuel
• High Pressure
• Low Pressure
1) Metal Arc Welding
2) Carbon Arc Welding
3) Tungsten Arc Welding
4) Argon Arc Welding
5) Submerged Arc Welding
Chemical Reaction
Thermit Welding
Without Pressure
FUSION WELDING

10. Classification of welding processes
Plastic welding
• Metals are joined together by applying pressure
• After bringing it to plastic state
Fusion welding
• Metals are joined together after bringing them to the
molten state and allowing them to solidify.

11. Plastic welding types
• Forge welding
• Resistance welding
• Thermit welding

12. Fusion welding types
• Metal arc welding
• Carbon arc welding
• Submerged arc welding
• Argon arc welding
• Automatic hydrogen arc welding
• Gas welding
• TIG welding
• MIG welding
• Plasma arc welding
• Electro slag arc welding and Electro gas welding

13. Spot Welding
• In this process overlapping sheets are joined by local fusion
at one or more spots, by the concentration of current
flowing between two electrodes.
• It essentially consists of two electrodes, out of which one is
fixed.
• The other electrode is fixed to a rocker arm (to provide
mechanical advantage) for transmitting the mechanical
force from a pneumatic cylinder.
• The other possibility is that of a pneumatic or hydraulic
cylinder being directly connected to the electrode without
any rocker arm. For welding large assemblies such as car
bodies, portable spot welding machines are used.

14. Spot welding

15. Spot Welding

16. Procedure of Spot Welding
• The electrode holders and the pneumatic
pressurizing system are present in the form of a
portable assembly which is taken to the place,
where the spot is to be made.
• The electric current, compressed air and the
cooling water needed for the electrodes is
supplied through cables and hoses from the main
welding machine to the portable unit.
• In spot welding, a satisfactory weld is obtained
when a proper current density is maintained.

17. Procedure of Spot Welding
• The current density depends on the contact area
between the electrode and the work-piece.
• With the continuous use, if the tip becomes upset
and- the contact area increases, the current
density will be lowered and consequently the
weld is obtained over a large area.
• This would not be able to melt the metal and
hence there would be no proper fusion.
• A resistance welding schedule is the sequence of
events that normally take place in each of the
welds.

18. Procedure of Spot Welding
1. The squeeze time is the time required for the electrodes to
align and clamp the two work-pieces together under them
and provide the necessary electrical contact.
2. The weld time is the time of the current flow through the
work-pieces till they are heated to the melting temperature.
3. The hold time is the time when the pressure is to be
maintained on the molten metal without the electric current.
During this time, the pieces are expected to be forged welded.
4. The off time is time during which, the pressure on the
electrode is taken off so that the plates can be positioned for
the next spot.

19. Before spot welding one must make
sure that
(i) The job is clean, i.e., free from grease, dirt, paint, scale,
oxide etc.
(ii) Electrode tip surface is clean, since it has to conduct the
current into the work with as little loss as possible. Very fine
emery cloth may be used for routine cleaning.
(iii) Water is running through the electrodes in order to
(a) Avoid them from getting overheated and thus damaged,
(b) Cool the weld.
(iv) Proper welding current has been set on the current
selector switch.
(v) Proper time has been set on the weld-timer.

20. Spot welding electrodes
Spot welding electrodes are made of materials which
have
(1) Higher electrical and thermal resistivities, and
(2) Sufficient strength to withstand high pressure at
elevated temperatures.
Copper base alloys such as copper beryllium and
copper tungsten are commonly used materials for spot
welding electrodes. For achieving the desired current
density, It is important to have proper electrode shape
for which three main types of spot welding electrodes
are used which are pointed, domed and flat electrodes.

21. Applications of Spot Welding
1. It has applications in automobile and aircraft
industries.
2. The attachment of braces, brackets, pads or clips to
formed sheet-metal parts such as cases, covers or
trays is another application of spot welding.
3. Spot welding of two 12.5 mm thick steel plates has
been done satisfactorily as a replacement for riveting.
4. Many assemblies of two or more sheet metal
stampings that do not require gas tight or liquid tight
joints can be more economically joined by spot
welding than by mechanical methods.
5. Containers and boxes frequently are spot welded.

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Brazing
Brazing is similar to soldering but it gives much stronger
joint .
• Harder filler metal which melts at a higher Temperature than
that of solder is used in brazing.
• The filler metal used in brazing is called SPELTER.
• They have melting range from 850 to 9500 c.
• Borax is widely used flux.

23. Brazing

24. Projection Welding
• This process is a resistance welding process in which two or
more than two spot welds are made simultaneously by
making raised portions or projections on predetermined
locations on one of the work piece.
• These projections act to localize the heat of the welding
circuit.
• The pieces to be welded are held in position under pressure
being maintained by electrodes.
• The projected contact spot for welding should be
approximately equal to the weld metal thickness.
• The welding of a nut on the automotive chasis is an
example of projection welding.

25. Projection Welding

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• The arc and welding zone is shielded by inert gas to
prevent atmospheric contaminations.
• It is also referred as Gas metal arc welding (GMAW).
• Direct current reverse polarity (DCRP) is generally used in
MIG welding.
• DC straight polarity (DCSP) is sometimes used but AC is not
used in MIG due to unequal burn off rates of electrode wire.
MIG welding

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MIG welding
10

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Advantages
• The flux is not required hence no slag formation.
• High welding speed.
• Increased corrosion resistance.
• High economy.
• Easily automated welding.
• Welds all metals including aluminum and stainless
steel.

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Disadvantages
• Equipment cost is high.
• Not suitable for out door work as the strong wind may
break the shielding gas.

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Applications
• MIG welding is used for welding carbon and alloy steels ,
stainless steel , copper and aluminum alloys etc.
• With special techniques (pre heating and post heating), the
process can be employed for welding cast iron, titanium
and other refractory materials.
• MIG welding has been widely used in air-craft and automobile
industries .

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Most common defects in welding are :
i) Lack of penetration. ii) Undercut.
iii) Slag Inclusion . iv) Porosity
v) Weld crack vi) Spatter
viii) Warping and Distortionvii) Poor Fusion
Defects in welding

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Lack of weld metal actually entering the parent metal.
• Caused by too long an arc length.
Groove formed along the edges of welding bead, thereby reducing
the thickness of base metal.
• Cause due to excessive current , excessive speed and
wrong electrode position
Lack of penetration
Under cut

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Mixing of slag or other foreign matter actually inside the
weld metal.
• Caused due to inadequate slag removal between runs
or electrode replace
Hole in the weld metal caused by the mixing of gas.
• Caused due to wrong type of electrode.
Slag inclusion
Porosity

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Discontinuity in the weld zone.
• Generally results from stresses caused by expansion and
contraction.
Globules of metal expelled from an electrode and deposited on the
surface of the parent metal.
• Caused by damped welding conditions or incorrect
heat input.
Weld crack
Spatter

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Lack of thorough and complete mixing between filler metal and
base metal.
• Caused due to faulty technique.
Deviation of weldments from straightness or flatness.
• caused by use of improper welding fixtures
Poor fusion
Warping and distortion

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Welding defects
UNDER

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The following non-destructive tests are used for detecting
defects in the weldments.
i) Visual inspection ii) Radiography test
iii) Ultrasonic test iv) Magnetic test
v) Liquid penetration test
Testing and inspection

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This reveals the defects such as undercuts, surface blow holes, cracks
and dimensional error.
Radiography test is based on the different absorption of radiation (x-
ray or y-rays) passing through the work.
This method reveals porosity, cracks, poor fusion and slag inclusion.
Visual Inspection
Radiography Test

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Magnetic test reveals the surface defects due to dispersion of
magnetic flux at the defect.
This method is used to defect fine cracks and holes in the weld.
This test is used to reveal fine cracks.
The waves reflect from the defect and the opposite face produce the
electrical Impulse which are fed into a cathode ray oscilloscope to
study the defect.
Magnetic Test
Liquid penetrant Test
Ultrasonic Test
In this test the ultrasonic waves are passed through the work.

40. THANK YOU
By : S. Arun Kumar