Que 1.19. Write short note on use of transformer, rectifier and generators in welding.
A. Use of Transformer :
1. Welding transformers are used in AC machines to change alternating
current from the power line into a low-voltage, high amperage current
in the secondary winding.
2. A combination of primary or secondary taps on the welding transformer
is commonly used to provide a macro adjustment of the welding current,
as well as adjustment of secondary voltage.
3. Transformer ratings for AC machines are expressed in
kilovolt-amperes for a specified duty cycle.
4. This duty cycle rating is a thermal rating, and indicates the amount of
energy that the transformer can deliver for a stated percentage of a
specific time period, usually one minute, without exceeding its
5. Since heating is a function of the welding current, this parameter gives
an indication of the thickness of the materials that can be welded.
B. Use of Rectifier :
1. The rectifier may consist of metal plates coated with a selenium compound
or silicon diodes, each unit having the special property of allowing the
current to flow in one direction only.
2. Rectifier units are designed to provide a choice of low voltage for MIG
and submerged welding or a high open circuit with drooping voltage
characteristics for TIG and flux shielded metal arc welding.
C. Use of Generators :
1. A DC welding generator produces direct current in either straight or
reverse polarity. The polarity selected for welding depends upon the
kind of electrode used and the material to be welded.
2. A DC generator is powered either by an electric motor or a diesel engine.
Diesel operated generator sets are suitable for out-door applications or
other areas where power is not available.
Que 2.1. What is a manual metal arc welding or shielded metal arc welding ?
1. Arc welding is a welding process wherein coalescence is produced by
heating with an electric arc.
2. Most of the electric arc welding is done by without pressure and with
or without use of filler metal depending upon the parent plate thickness.
3. Electric arc is formed when an electric current passes between two
electrodes separated by a short distance from each other.
4. In arc welding one electrode is the welding rod or wire while other is
the metal to be welded (workpiece).
5. Arc connected to the supply, one to the positive terminal and other to
the negative terminal.
6. Arc is started by momentarily touching the electrode on the plate and
then withdrawing it to about 3 to 4 mm from the plate.
7. When the electrode touches the plates, a current flows and as it is
withdrawn from the plate the current continues to flow in the form of
a spark across the very small gap first formed, this cause the air gap to
become ionised or made conducting and as a result the current is able
to flow across the gap.
8. Approximately 2/3rd of the heat is generated on positive pole and 1/3rd
on the negative pole.
Que 2.2. Give the advantages, disadvantages and applications of arc welding.
A. Advantages of Arc Welding :
1. Flux shielded manual arc (FSMA) welding is very simple as compared
to all arc welding processes.
2. Cost of equipment is very low.
3. Portable equipment is available.
4. Various types of metal and their alloys can be welded.
5. Welding can be carried out in any position i.e., horizontal, vertical,
overhead, flat or inclined.
6. High deposition and penetration can be achieved.
B. Disadvantages of Arc Welding :
1. FSMA welding is a slow process as compared to MIG welding.
2. Different materials cannot be welded together.
3. Due to flux coated electrode, chance of slag entrapment and other
defects are more as compared to MIG and TIG welding.
4. Metal transfer is not so clear as compared to MIG welding.
C. Applications of Arc Welding :
1. Ship building.
2. Pipes and penstocks joining.
3. Construction work.
4. Boiler, chemical and fertilizer industries etc.
Que 2.3. Explain the MIG welding or GMAW process and also
give advantages, disadvantages and applications of it.
A. MIG Welding :
1. It is an arc welding process wherein coalescence is produced by heating
the job with an electric arc established between a continuously fed
metal electrode and the job.
2. No flux is used but the arc and molten metal are shielded by an inert
gas, which may be argon, helium, etc.
B. Principle of Operation of MIG Welding :
1. Before igniting the arc, gas and water flow is checked.
2. Proper current and wire feed speed is set and the electrical connections
3. The arc is struck by any one of the two methods :
i. In the first method, current and shielding gas flow is switched on and
the electrode is scratched against the job as usual practice for striking
ii. In the second method, electrode is made to touch the job, is retracted
and then moved forward to carry out welding, but before striking the
arc, shielding gas, water and current is switched on.
4. About 15 mm length of the electrode is projected from the torch before
striking the arc.
5. During welding, torch remains about 10-12 mm away from the job and
arc length is kept between 1.5 to 4 mm.
C. Advantages of MIG Welding :
1. Because of continuously fed electrode, MIG welding process is much
faster as compared to TIG welding.
2. It can produce joints with deep penetration.
3. Thick and thin, both types of workpieces can be welded effectively.
4. Large metal deposition rates are achieved by MIG welding process.
5. The process can be easily mechanised.
6. Higher arc travel speeds associated with MIG welding that reduce
D. Disadvantages of MIG Welding :
1. The process is slightly more complex as compared to TIG welding.
2. Welding equipment is more complex, more costly and less portable.
3. Since air drafts may disperse the shielding gas, MIG welding may not
work well in outdoor welding applications.
4. Weld metal cooling rates are higher than with the processes that deposit
slag over the weld metal.
E. Applications of MIG Welding :
1. For welding tool steel and dies.
2. For the manufacturing of refrigerator parts.
3. In aircraft, automobile, pressure vessel and ship building industries.
4. For welding of carbon, silicon, and low carbon alloy steels, stainless
steel, aluminium, nickel etc.
Que 2.4. Explain the TIG welding or gas tungsten arc welding
(GTAW) with their applications. And also give its advantages and
With suitable sketch explain the process of TIG welding. What are
its disadvantages ?
Using neat sketch, explain TIG welding process. State its
applications. What are the variants of TIG welding ?
A. TIG Welding :
1. It is an arc welding process wherein coalescence is produced by heating
the job with an electric arc struck between a tungsten electrode and
2. A shielding gas (argon, helium, nitrogen, etc.) is used to avoid
atmospheric contamination of the molten weld pool.
B. Principle of Operation of TIG Welding :
1. Welding current, water and inert gas supply are turned on.
2. The arc is struck either by touching the electrode with a scrap metal
tungsten piece or using high frequency unit.
3. In this arc is initially struck on a scrap metal piece and then broken by
increasing the arc length.
4. This procedure repeated twice or thrice that warms up the tungsten
5. The arc is then struck between the electrodes and precleaned job to be
6. This method avoids breaking electrode tip, job contamination and
7. TIG welding is also known as Gas Tungsten Arc Welding (GTAW).
8. Both the AC and DC power source can be used for GTAW.
9. Electrodes employed varies in diameter from 0.5 to 6.5 mm carrying
current from 5 A to 6.5 A.
10. GTAW is an all position welding and gives the highest quality weld
amongst commonly used arc welding processes.
C. Applications of TIG Welding :
1. Precision welding in atomic energy, aircraft, chemical and instrument
2. Welding aluminium, magnesium, copper, nickel and their alloys etc.
3. Rocket motor chamber fabrications in launch vehicles.
4. Welding of expansion bellows, transistor cases and can-sealing joints.
D. Advantages of TIG Welding :
1. No flux is used hence there is no chance of flux entrapment, when
welding refrigerator and air conditioner components.
2. Because of clear visibility of the arc and the job, the operator can
exercise a better control on the welding process.
3. This is very much suitable for high quality welding of thin materials.
4. It is a very good process for welding non ferrous metals and stainless
E. Disadvantages of TIG Welding :
1. Equipment costs are higher than that for flux shielded metal arc
2. Under same applications, MIG welding is faster as compared to TIG
F. Variants of TIG Welding :
1. Activated tungsten inert gas (A-TIG) welding and Flux bound tungsten
inert gas (FB-TIG) welding are two such variants that utilize suitable
activating flux to improve upon various characteristics of conventional
2. These processes are also called flux assisted TIG welding as they
mandatorily require a layer of activating flux on the components to be