UTILISATION OF
ELECTRICAL POWER
PRATHYUSH M
Definition of Welding
➢It is the process of joining two pieces of metal or non-metal at faces rendered
plastic or liquid by the application of heat or pressure or both. Filler material may
be used to effect the union.
Welding Processes
All welding processes fall into two distinct categories :
1. Fusion Welding—It involves melting of the parent metal. Examples are:
(i) Carbon arc welding, metal arc welding, electron beam welding, electroslag
welding and electrogas welding which utilize electric energy and
(ii) Gas welding and thermit welding which utilize chemical energy for the melting
purpose.
2. Non-fusion Welding—It does not involve melting of the parent metal. Examples
are:
(i) Forge welding and gas non-fusion welding which use chemical energy.
(ii) Explosive welding, friction welding and ultrasonic welding etc., which use
mechanical energy.
(iii) Resistance welding which uses electrical energy.
Proper selection of the welding process depends on the
(a) kind of metals to be joined
(b) cost involved
(c) nature of products to be fabricated and
(d) production techniques adopted.
The principal welding processes have been tabulated in Fig.
Use of Electricity in Welding
➢Electricity is used in welding for generating heat at the point of welding in order
to melt the material which will subsequently fuse and form the actual weld joint.
➢There are many ways of producing this localised heat but the two most
common methods are as follows :
1. resistance welding—here current is passed through the inherent resistance
of the joint to be welded thereby generating the heat as per the equation I2 Rt/J
kilocalories.
2. arc welding—here electricity is conducted in the form of an arc which is
established between the two metallic surfaces
Formation of Electric Arc
➢An electric arc is formed whenever electric current is passed between two
metallic electrodes which are separated by a short distance from each other.
➢The arc is started by momentarily touching the positive electrode (anode) to the
negative metal (or plate) and then withdrawing it to about 3 to 6 mm from the
plate.
➢When electrode first touches the plate, a large short-circuit current flows and
as it is later withdrawn from the plate, current continues to flow in the form of a
spark across the air gap so formed.
➢Due to this spark (or discharge), the air in the gap becomes ionized i.e. is split
into negative electrons and positive ions.
➢Consequently, air becomes conducting and current is able to flow across the
gap in the form of an arc.
Effect of Arc Length
➢In metal arc welding, a fairly short arc length is necessary for getting good welds.
When arc length is long
1. large amount of heat is lost into the surrounding area thus preventing good
penetration and fusion;
2. arc flame is very unstable since effect of magnetic blow is increased. Hence,
arc flame will have a tendency to blow out;
3. air is able to reach the molten globule of metal as it passes from the electrode
to the weld and weld pool. It leads to the contamination of the weld due to
absorption of oxygen and nitrogen;
4. weld deposits have low strength, poor ductility, high porosity, poor fusion and
excessive spatter.
Arc Welding Machines
➢Welding is never done directly from the supply mains. Instead, special welding
machines are used which provided currents of various characteristics.
➢Use of such machines is essential for the following reasons :
1. To convert a.c. supply into d.c. supply when d.c. welding is desired.
2. To reduce the high supply voltage to a safer and suitable voltage for welding
purposes.
3. To provide high current necessary for arc welding without drawing a
corresponding high current from the supply mains.
4. To provide suitable voltage/current relationships necessary for arc welding at
minimum cost.
There are two general types of arc welding machines :
(a) d.c. welding machines
(i) motor-generator set
(ii) a.c. transformers with rectifiers
(b) a.c. welding machines
V-I Characteristics of Arc Welding DC Machines
➢It is found that during welding operation, large fluctuations in current and arc
voltage result from the mechanism of metal transfer and other factors.
➢The welding machine must compensate for such changes in arc voltage in order
to maintain an even arc column.
➢There are three major voltage/ current characteristics used in modern d.c.
welding machines which help in controlling these current fluctuations :
1. drooping arc voltage (DAV).
2. constant arc voltage (CAV).
3. rising arc voltage (RAV).
Carbon Arc Welding
➢Carbon arc welding differs from the more common shield metal arc welding in
that it uses non consumable carbon or graphic electrodes instead of the
consumable flux-coated electrodes.
Welding Circuit
The basic circuit is shown in Fig. and can be used with d.c. as well as a.c. supply.
When direct current is used, the electrode is mostly negative (DCSP). The process
is started by adjusting the amperage on the d.c. welder, turning welder ON and
bringing the electrode into contact with the workpiece. After the arc column starts,
electrode is withdrawn 25 – 40 mm away and the arc is maintained at this
distance. The arc can be extinguished by simply removing the electrode from the
workpiece completely. The only function of the carbon arc is to supply heat to the
base metal. This heat is used to melt the base metal or filler rod for obtaining
fusion weld Depending on the type and size of electrodes, maximum current
values range from 15 A to 600 A for single-electrode carbon arc welding.
Electrodes
These are made of either carbon or graphite, are usually 300 mm long and 2.5 –
12 mm in diameter. Graphite electrodes are harder, more brittle and last longer
than carbon electrodes. They can withstand higher current densities but their arc
column is harder to control. Though considered non-consumable, they do
disintegrate gradually due to vaporisation and oxidisation..
Applications
1. The joint designs that can be used with carbon arc welding are butt joints, bevel
joints, flange joints, lap joints and fillet joints.
2. This process is easily adaptable for automation particularly where amount of
weld deposit is large and materials to be fabricated are of simple geometrical
shapes such as water tanks.
3. It is suitable for welding galvanised sheets using copper-silicon manganese
alloy filler metal.
4. It is useful for welding thin high-nickel alloys.
5. Monel metal can be easily welded with this process by using a suitable coated
filler rod.
6. Stainless steel of thinner gauges is often welded by the carbon-arc process with
excellent results.
Advantages and Disadvantages
1. The main advantage of this process is that the temperature of the molten pool
can be easily controlled by simply varying the arc length.
2. It is easily adaptable to automation.
3. It can be easily adapted to inert gas shielding of the weld and
4. It can be used as an excellent heat source for brazing, braze welding and
soldering etc.
Its disadvantages are as under :
1. A separate filler rod has to be used if any filler material is required.
2. Since arc serves only as a heat source, it does not transfer any metal to help
reinforce the weld joint.
3. The major disadvantage of the carbon-arc process is that blow holes occur due
to magnetic arc blow especially when welding near edges of the workpiece.
Gas Shield Arc Welding
➢In this fusion process, welding is done with bare electrodes but weld zone is
shielded from the atmosphere by a gas which is piped to the arc column.
➢Shielding gases used are carbon dioxide, argon, helium, hydrogen and oxygen.
No flux is required.
➢ Different processes using shielding gas are as follows.
(a) Tungsten inert-gas (TIG) Process
➢In this process, non-consumable tungsten electrode is used and filler wire is fed
separately.
➢The weld zone is shielded from the atmosphere by the inert gas (argon or
helium) which is ducted directly to the weld zone where it surrounds the tungsten
and the arc column.
(b) Metal inert-gas (MIG) Process
➢ It is a refinement of the TIG process.
➢It uses a bare consumable (i.e. fusible) wire electrode which acts as the source
for the arc column as well as the supply for the filler material.
➢The weld zone is shielded by argon gas which is ducted directly to the electrode
point.
Resistance Welding
➢It is fundamentally a heat and squeeze process.
➢The term ‘resistance welding’ denotes a group of processes in which welding
heat is produced by the resistance offered to the passage of electric current
through the two metal pieces being welded.
➢These processes differ from the fusion processes in the sense that no extra
metal is added to the joint by means of a filler wire or electrode.
➢According to Joule’s law, heat produced electrically is given by H = I2Rt/J.
➢Obviously, amount of heat produced depends on.
(i) square of the current (ii) the time of current and (iii) the resistance
offered.
As seen, in simple resistance welding, high-amperage current is necessary for
adequate weld.
➢Usually, R is the contact resistance between the two metals being welded
together.
➢The current is passed for a suitable length of time controlled by a timer.
➢The various types of resistance welding processes may be divided into the
following four main groups :
(i) spot welding (ii) seam welding (iii) projection welding and (iv) butt
welding which could be further subdivided into flash welding, upset welding and
stud welding etc.
Advantages
Some of the advantages of resistance welding are as under :
1. Heat is localized where required 2. Welding action is rapid
3. No filler material is needed 4. Requires comparatively lesser skill
5. Is suitable for large quantity production
6. Both similar and dissimilar metals can be welded
7. Parent metal is not harmed 8. Difficult shapes and sections can be
welded.
➢Only disadvantages are with regard to high initial as well as maintenance cost.
➢It is a form of resistance welding in which the two surfaces are joined by spots
of fused metal caused by fused metal between suitable electrodes under
pressure.
Spot Welding
The process depends on two factors :
1. Resistance heating of small portions of the two work pieces to plastic state
and
2. Application of forging pressure for welding the two work pieces.
➢Heat produced is H = I2Rt/J.
➢The resistance R is made up of (i) resistance of the electrodes and metals
themselves
(ii) contact resistance between electrodes and work pieces and
(iii) contact resistance between the two work pieces.
➢Generally, contact resistance between the two work pieces is the greatest.
Seam Welding
➢The seam welder differs from ordinary spot welder only in respect of its
electrodes which are of disc or roller shape as shown in Fig. (a).
➢These copper wheels are power driven and rotate whilst gripping the work.
➢The current is so applied through the wheels that the weld spots either overlap
as in Fig. (b) or are made at regular intervals as in Fig. (c).
➢The continuous or overlapped seam weld is also called stitch weld whereas
the other is called roll weld.
➢Seam welding is confined to welding of thin materials ranging in thickness from
2 mm to 5 mm.
➢It is also restricted to metals having low hardenability rating such as hot-rolled
grades of low alloy steels.
➢Stitch welding is commonly used for long water-tight and gas-tight joints.
➢Roll welding is used for simple joints which are not water-tight or gas-tight.
➢Seam welds are usually tested by pillow test.
Butt Welding
➢In this case, the two workpieces are brought into contact end-to-end and the
butted ends are heated by passing a heavy current through the joint.
➢As in other forms of resistance welding, the weld heat is produced mainly by the
electrical resistance of the joint faces.
➢In this case, however, the electrodes are in the form of powerful vice clamps
which hold the work-pieces and also convey the forging pressure to the joint [Fig].
➢This process is useful where parts have to be joined end-to-end or edge-to-
edge. i.e. for welding pipes, wires and rods.
➢It is also employed for making continuous lengths of chain.
➢The basic electrical requirement in arc welding is that there should be
(a) coated electrodes
(b) high open-circuit voltage
(c) no arc blow
(d) d.c. power supply.
The major disadvantage of carbon arc welding is that
(a) there is occurrence of blow holes
(b) electrodes are consumed fast
(c) separate filler rod is needed
(d) bare electrodes are necessary.
Unlike TIG welding, MIG welding
(a) requires no flux
(b) uses consumable electrodes
(c) provides complete protection from atmospheric contamination
(d) requires no post-weld cleansing.
Spot welding process basically depends on
(a) Ohmic resistance
(b) generation of heat
(c) application of forging pressure
(d) both (b) and (c).
During resistance welding heat produced at the joint is proportional to
(a) I2R
(b) kVA
(c) current
(d) voltage
For arc welding, D.C. is produced by which of the following?
(a) motor-generator set
(b) regulator
(c) transformer
(d) none of the above
Which of the following is not an inert gas?
(a) argon
(b) carbon dioxide
(c) helium
(d) all of the above
Electronic components are joined by which of the following methods?
(a) brazing
(b) soldering
(c) seam welding
(d) spot welding
(e) none of the above
Resistance welding cannot be used for
(a) dielectrics
(b) ferrous materials
(c) non-ferrous metals
(d) any of the above
Electric arc welding process produces temperature upto
(a) 1000 oC
(b) 1500 oC
(c) 3500 oC
(d) 5550 oC
Increased heat due to shorter arc is harmful on account of
(a) under-cutting of base material
(b) burn through
(c) excessive porosity
(d) all of the above
For arc welding current range is usually
(a) 10 to 15 A
(b) 30 to 40 A
(c) 50 to 100 A
(d) 100 to 350 A
