The fundamentals of welding arc, mechanisms of electron emission, different zones in welding arc, electrical aspects related with welding arc, arc forces. and their significance in welding.

1. Physics of Welding Arc
By : Dr. Lalit Yadav
Department of Mechanical Engg.
Sir Padampat Singhania University, Udaipur

2. Introduction
• A welding arc is an electric discharge that develops primarily due to
flow of current from cathode to anode.
• Flow of current through the gap between electrode and work
piece needs column of charged particles for having reasonably good
electrical conductivity.
• These charged particles are generated by various mechanisms such as
thermal emission, field emission secondary emission etc.

3. Emission of Free electrons
• Free electrons and charged particles are needed between the electrode and
work for initiating the arc and their maintenance. Ease of emitting electrons
by a material assessed on the basis of two parameters work function and
ionization potential.
1. Thermo-ionic emission
• Increase in temperature of metal increases the kinetic energy of free
electrons and as it goes beyond certain limit, electrons are ejected from the
metal surface.
2. Field emission:
• High potential difference (107 V/cm) between the work piece and electrode
is established for the field emission purpose.
3. Secondary emission
• High velocity electrons moving from cathode to anode in the arc gap collide
with other gaseous molecules

4. Zones in Arc Gap
• On establishing the welding arc, drop in arc voltage is observed across
the arc gap.
• However, rate of drop in arc voltage varies with distance from the
electrode tip to the weld pool (Fig.1).
• Generally, five different zones are observed in the arc gap namely
cathode spot, cathode drop zone, plasma, anode drop zone and
anode spot (Fig. 2).

5. Fig.1 Potential drop as function of distance form the
cathode to anode
Fig. 2 Zones in arc gap of a welding arc

6. Electrical Fundamentals of Welding Arc
• The welding arc acts as impedance for flow of current like an electric conductor.
• The impedance of arc is usually found a function of temperature and becomes
inversely proportional to the density of charge particles and their mobility.
Product of potential difference across the arc (V) and
current (I) gives the power of the arc indicating the heat
generation per unit time.
Arc voltage (V) is taken as sum of potential drop across the
cathode drop region (Vc), potential drop across the plasma
region (Vp), and potential drop across the anode drop
region (Va) as shown in Fig.3.
Power of the arc (P) = (Vc+ Vp+ Va) x I
Potential drop in different zones is expressed in terms
of volt (V), welding current in ampere (A) and power
of arc P is in watt (W).
Fig. 3 Three different zone in which voltage
drop takes place

7. • Arc Initiation
• There are two most commonly used methods to initiate an electric
arc in welding processes namely touch start and field start. For eg.
automatic welding operations & TIG welding.
• Touch Start
• In this method, the electrode is brought in contact with the work
piece and then pulled apart to create a very small gap

8. Fig. 4 Schematic diagram showing mechanism of arc initiation by touch start method a) when circuit closed
by touching electrode with work piece b) emission of electrode on putting them apart

9. • Field Start
• In this method, high strength electric field (107 V) is applied between
electrode and work piece so that electrons are released from cathode
electro-magnetic field emission (Fig. 5).
Fig. 5 Schematic diagram showing the field-start method of arc initiation

10. Maintenance of Arc-Low Ionization Potential Elements
6

11. Arc Characteristic
Variation in charged particle density in arc zones associated different arc welding processes such as SMAW,
GMAW and GTAW is attributed to appreciable difference in arc voltage vs. arc length relationship (Fig. 5).
7

12. Temperature of the Arc
Fig. 8 Schematic diagram showing typical temperature distribution in the arc
Temperatures in anode and cathode drop
zones are generally lower than the
plasma region due to cooling effect of
electrode/work piece. Temperature of arc
can vary from 5000-30,000K depending
upon the current voltage shielding gas
and plasma gas. For example, in case of
SMAW, temperature of arc is about
6000K while that for TIG/MIG welding arc
it is found in range of 20000-25000K.

13. Arc Forces and Their significance on Welding
• All the forces acting in arc zone are termed as arc forces. In respect of
welding, influence of these forces on resisting or facilitating the
detachment of molten metal drop hanging at the electrode tip is
important which in turn affect the mode of metal transfer and weld
metal disposition efficiencies.

14. Types of Arc forces
1. Gravity Force
• This is due to gravitational force acting on molten metal drop hanging at the
tip of electrode.
• Gravitational force depends on the volume of the drop and density of metal.
• In case of down hand welding, gravitational force helps in detachment/transfer
of molten metal drop from electrode tip (Fig.9 a).
Gravitational force (Fg)= ρ x Vx g
Where
ρ (kg/m)3 is the density of metal,
V is volume of drop (m3) and
g is gravitational constant (m/s2). Fig.9

15. 2. Surface Tension Force
• This force is experienced by drop of the liquid metal hanging at the
tip of electrode due to surface tension effect. Magnitude of the
surface tension force given in Equation
• Where σ is the surface tension coefficient, R is drop radius and Re is
the radius of electrode tip.
• An Increase in temperature of the molten weld metal reduces the
surface tension coefficient (σ)
• This force tends to resist the detachment of molten metal drop
from electrode tip and usually acts against gravitational force

16. 3. Force Due to Impact of Charge Carriers
• As per polarity charged particles (ions & electrons), move
towards anode or cathode and eventually impact/collide with
them.
• Force generated owing to impact of charged particles on to the
molten metal drop hanging at the tip of electrode tends to
hinder the detachment (Fig.c).
• Force due to impact of charged particles
Fm= m(dV/dt)
• Where m is the mass of charge particles, V is the velocity and t is
the time

17. 4. Force Due to Metal Vapours
• Molten metal evaporating from bottom of drop and weld pool
move in upward direction.
• Forces generated due to upward movement of metal vapours
act against the molten metal drop hanging at the tip of the
electrode. Thus, this force tends to hinder the detachment of
droplet (Fig.d).

18. 5. Force Due to Gas Eruption
• Gases present in molten metal such as oxygen, hydrogen etc. may
react with some of the elements (such as carbon) present in molten
metal drop and form gaseous molecules (carbon dioxide).
• The growth of these gases in molten metal drop as a function of time
ultimately leads to bursting of metal drops which in turn increases the
spattering and reduces the control over handling of molten weld
metal (Fig. e1-e4).

19. 6. Force Due to Electro Magnetic Field
• Flow of current through the arc gap develops the electromagnetic
field. Interaction of this electromagnetic field with that of charge
carriers produces a force which tends to pinch the drop hanging at
the tip of the electrode also called pinch force.
• The pinch force reduces the cross section for molten metal drop near
the tip of the electrode and thus helps in detachment of the droplet
from the electrode tip (Fig. f1-f2).

20. Force generation due to gas eruption and f1&f2)
electromagnetic pinch force

21. Molten Metal transfer
• Metal transfer refers to the transfer of molten metal from the tip of a
consumable electrode to the weld pool and is of great academic and
practical importance for consumable electrode welding processes as
it directly affects the control over the handling of molten metal, slag
and spattering.
• However, metal transfer is considered to be more of academic
importance for GMA and SA welding than practical need.
• Four common modes of metal transfer are generally observed in case
of consumable arc welding processes. These have been described in
the following sections.

22. 1. Short Circuit Transfer
• This kind of metal transfer takes place, when welding current is very
low but high enough to have stable arc and arc gap is small.
• Under these welding conditions, molten metal droplet grows slowly
at the tip of the electrode and then as soon as drop touches weld
pool, short-circuiting takes place.
• Due to narrow arc gap, molten drop does not attain a size big enough
to fall down on its own (by weight) under gravitational force.
• This increase in arc voltage (due to setting up of the arc-gap) re-
ignites arc and flow of current starts.
• This whole process is repeated at a rate varying from 20 to more than
200 times per second during the welding. Schematically variation in
welding current and arc voltage for short circuit metal transfer is
shown in Fig.10

23. Schematic of short circuiting metal transfer
Fig. 10

24. 2. Globular metal transfer
• Globular metal transfer takes place when
welding current is low (but higher than that
for short circuit transfer) and arc gap is
large enough so molten metal droplet can
grow slowly (at the tip of the electrode)
with melting of the electrode tip.
• The transfer of molten metal drop normally
occurs when it attains size larger than the
electrode diameter. No short-circuit takes
place in this mode of metal transfer.

25. 3. Spray Transfer
• This kind of metal transfer takes place when
welding current density is higher than that is
required for globular transfer.
• High welding current density results in high
melting rate and greater pinch force as both
melting rate and pinch force are directly
related with welding current and are found
proportional to square of welding current.
• The transfer of molten metal from electrode
tip appears similar to that of spray in line of
axis of the electrode (fig.).

26. 4. Dip Transfer
• Dip type of metal transfer is observed
when welding current is very low and feed
rate is high.
• Under these welding conditions, electrode
is short-circuited with weld pool, which
leads to the melting of electrode and
transfer of molten drop.
• Approach wise dip transfer is similar to
that of short circuit metal transfer and
many times two are used interchangeably.
• However, these two differ in respect of
welding conditions especially arc gap that
lead to these two types of metal transfers.