Que 1.7. Describe physics of welding in detail.
1. The physics of welding deals with complex physical phenomena associated
with welding including heat, electricity, magnetism, light etc.
2. Majority of welding processes require the application of heat which is
obtained through :
iii. Contact resistance,
iv. Electron beam,
v. Laser, and
vi. Exothermic reactions.
3. AC/DC electric power supply is used to generate heat for many welding
4. Magnetic fields set up due to the flow of current through the electrode
and the arc generate pinch effect and influence the welding arcs. Arc
blow, plasma streaming and metal transfer are some of the welding arc
features strongly influenced by the presence of magnetic fields.
Que 1.8. What is arc initiation ? How it can be achieved ?
1. Arc is initiated by providing a conducting path between the electrode
and the job or by ionizing the gap between the two.
2. This can be achieved in the following manners :
i. By momentarily touching the electrode with the job and taking it
away as shown in Fig. 1.8.1(i).
ii. By scratching the electrode with the job. Scratching is initiated a
little distance away from the point where the welding is to be started
and during scratching the electrode is brought to the proper place
of starting the welding as shown in Fig. 1.8.1(ii).
iii. In this case steel wool is kept pressed between the electrode and
the job. When the welding current is switched on, the steel wool
provides a conducting path for the arc to establish as shown in
Fig. 1.8.1(iii). This method can be used in atomic submerged arc
welding sets and automatic MIG welding machines.
iv. In automatic metal arc welding sets electric arc can also be initiated
with the help of a carbon rod. Suitable arc gap is kept between the
electrode and the workpiece, current is switched on, then the
electrode and job simultaneously are momentarily touched with a
carbon rod as shown in Fig. 1.8.1(iv).
v. In order to eliminate the chances of electrode contamination,
tungsten loss and tungsten transfer to the base metal the above
methods of initiating the arcs are avoided in welding processes
using tungsten or alloy tungsten electrodes. In such processes a
high frequency unit is inserted in the circuit, which superimposes
high frequency current, which jumps across the small gap between
the electrode and job, thereby establishing the arc.
Que 1.9. Discuss the phenomena of voltage distribution along the arc.
1. In the anode and cathode drop zones, a nonlinear voltage distribution is
prevalent along the arc length and high electric field strengths are
found in cathode and anode drop zones.
2. Voltage drop is very low across the cathode and anode drop zone and
voltage drop is very high across the arc column.
3. If V is the sum of cathode drop (Vc), column drop (Vp) and anode drop
V = Vc + Vp + Va
4. Approximate potential drop in the cathode and anode drop regions is of
the order of 10 volts and 12 to 1 volt respectively.
Que 1.10. Describe characteristic of arc with the help of diagram.
1. A welding arc is high current, low voltage, electric discharge operating
generally in the range of 10 to 2000 A and 10 – 50 volts are act as a load
resistor in a welding circuit.
2. In broad sense, welding arc consists of a mechanism for emitting electron
from cathode (–) which after passing temperature distribution through
ionized hot gas merge into arc column anode.
3. Basically welding arc is divided into five parts which are as follows :
i. Cathode spot,
ii. Cathode drop zone,
iii. Arc column,
iv. Anode drop zone, and
v. Anode spot.
4. Voltage drop is very low across the cathode and anode drop zone and
voltage drop is very high across the arc column.
Que 1.13. Write a short note on isotherms of arcs and arc blow.
A. Isotherms of Arcs :
1. Isotherms in the upper portion of Fig. 1.13.1 show the temperature
distribution for a weld made at 1.3 mm/sec with an energy input of
3940 J/mm for an initial plate temperature of 27 °C.
2. The isotherms in the lower portion of Fig. 1.12.1 show the temperature
distribution for a weld made at 2.5 mm/sec with an energy input of
3. The arc current, voltage and preheat temperature is same in both cases.
It can be observed from Fig. 1.13.1 :
i. The width of the heat affected zone reduces as the energy input
ii. As the speed increases, the ellipses crowd closer, increasing the arc
heat spreads the ellipses out.
B. Arc Blow :
1. The unwanted deflection or the wandering of a welding arc from this
intended path is termed as arc blow.
2. Arc blow is the result of magnetic disturbances which unbalance the
symmetry of the self-induced magnetic field around the electrode, arc
3. Under arc blow, an arc may distort, deflect or rotate.
4. Arc blow becomes severe when welding is carried out in confined spaces
and corners on heavy metal plates, using a DC power source.
5. AC arcs are less susceptible to arc blow than DC arcs, because the
alternating current reverses direction which in turn reverses the
6. The magnetic field builds up, collapses and rebuilds as current reverses
from positive to negative. This phenomenon does not permit the magnetic
field strength to build to a value so as to cause arc blow.
7 On the other hand in DC welding, the magnetic field set up in the
workpiece (adjacent to the arc) continuously builds up and the arc blow
Que 1.14. What are the factors affecting arc blow ? Also give its effects.
A. Factors Affecting Arc Blow :
1. Magnetic field produced in the workpiece adjacent to the welding arc,
due to the current flow through the arc.
2. Presence of busbars carrying large DC, in the neighbourhood of the
place where welding is being carried out.
3. With multiple welding heads, arc at one electrode may be affected by the
magnetic field of the arc at the other electrode.
4. The magnetic field produced in the workpiece around the earth
connection may tend to drive the arc away from the point where this
connection is made.
B. Effects of Arc Blow :
1. Poor weld bead appearance.
2. Irregular and erratic weld deposition.
3. Undercutting and lack of fusion.
5. Uneven and weak welded joint.
6. Slag entrapment and porosity.
Que 1.15. Explain in detail the mechanism and types of metal
transfer in various arc welding processes.
AKTU 2018-19, Marks 10
1. When electric arc is produced between the workpiece and consumable
electrode, electrode start melting in the form of spherical shape, hangs
towards the job and finally a drop down on the workpiece therefore
metal transfer from electrode to workpiece is define metal transfer
2. Metal is transferred in arc welding in three ways :
i. By dip transfer,
ii. By free drop (large drop) transfer, and
iii. By spray (small drop) transfer.
3. In dip transfer a globule of molten metal is formed at the end of the
electrode during arcing in the first stage.
4. Subsequently it enlarges, elongates, touches the molten pool and
separates from the electrode.
5. The process does not free the globules immediately from the electrode
after its formation and as such a temporary short circuit occurs.
6. The process repeats several times to complete welding.
7. In free drop transfer, a drop of molten metal which is slightly smaller in
diameter than the air gap flies off from the electrode end after temporarily
(but partial) short circuiting the electrode with the molten pool of metal
on the job.
8. In spray or small drop transfer, the transfer takes place in the form of
tiny droplets (much smaller in diameter as compared to arc length)
which make free flight from the electrode to the molten pool.
9. The transfer rate is steady and the final job will have better mechanical
Que 1.16. What are the various types of welding power sources ? Explain the working principle of a transformer.
A. Types of Welding Power Sources : Various types of power sources
are as follows :
1. AC transformer,
2. DC rectifier,
3. AC/DC transformer rectifier,
4. DC generator, and
B. Working Principle of a Transformer :
1. A transformer operates on the principle of mutual inductance, between
two (and sometimes more) inductively coupled coils.
2. It consists of two windings in close proximity.
3. The two windings are coupled by magnetic induction. (There is no
conductive connection between the windings).
4. One of the windings called primary is energized by a sinusoidal voltage.
The second winding, called secondary feeds the load.
5. The alternating current in the primary winding set up an alternating
flux in the core.
6. The secondary winding is linked by most of this flux (f) and emf is
induced in two windings.
7. The emf induced in the secondary winding drives a current through the
load connected to the winding.
8. Energy is transferred from the primary circuit to the secondary circuit
through the medium of the magnetic field.