The_Physics_of_Electric_Arc_in_Welding_T.pdf

1
The Physics of Electric Arc
in Welding Technology
January 2015
Department of Engineering
Design and Production
* Contacts
Address: P.O. Box 14200, FI-00076 Aalto, Finland
Visiting address: Puumiehenkuja 3, Espoo
pedro.vilaca@aalto.fi ; Skype: fsweldone
Professor Pedro Vilaça *
Materials Joining and NDT
1
Engineering Materials
Department of Engineering Design
and Production
Summary
 Historical milestones of electric arc welding
 Fundaments of electric arc in welding: Plasma formation
 Electric arc: Main zones, Stability criteria, Energy and Efficiency
 Electric arc start techniques
 Influence of shielding gases in electric arc
 Case study: Activated TIG (”A-TIG”)
 Heat input (HI) formulation
 Arc blow phenomena
 Electric arc from different power sources
Contents

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At the end of the lecture the student should be able to
1. To explain the development of the major welding processes based on
electric arc power source
2. To reflect on the role of welding technology in the main historical events
of the XX century
3. To identify the different zones and properties of the electric arc
4. To establish the influence of shielding gases on stability and
temperature distribution on and electric arc
5. To formulate the HI efficiency and its hierarchy for different processes
6. How to avoid arc blow phenomena
Learning Outcomes
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Forging
Electric
Arc
SMAW
SAW
GTAW
Plasma
PAW
GMAW
EBW Laser
Coal electrode
1881
1904
1930
1940
1950
1950
1970
1958
FCAW
Sinergic
CMT
Electro-gas
Electro-slag …Thermite
CO2
Nd-YAG
Diodes
Fiber
Excimers
…
Vacuum
Patm
Disc
Micro
Conventional
Keyhole
Oxifuel
1920
1903
Bare electrode
1888 1970
1959
1961
1980
1990
Welding
Thermal Treat.
Brasing
Fusion Welding Technology
Historical Development

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 Electric Arc is one of the most versatille heat power source… Thus
supporting many of the most relevant welding processes
 (def. @ welding technology) Stable electrical breakdown under low
voltage between a cathode and an anode of a gas which produces
an ongoing plasma discharge (characterized by its ionization
potential and thermal conductivity), resulting from a current flowing
through normally nonconductive media such as air. Different from
arc discharge which may not be stable electrical breakdown. The
phenomenon was first described by Vasily V. Petrov, a Russian
scientist who discovered it in 1802
Fundaments of… Electric Arc
Some Basic Definitions
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 (def) The gas media enabling stable conduction of electrical current
 Plasma is known as the 4th physical state of material/media
aggregation (Solid  Liquid  Gas  Plasma)
 Contains: negative charged particles (mainly, electrons but also
anions), positive charged particles (cations) and neutral particles
 The plasma stability and properties is mainly critical for very high
(e.g. SAW) and very low (e.g. micro PAW) levels of electrical current
Plasma

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 The ignition and establishment
of the Plasma results from the
following stages:
1) Ionization of gas
2) Disruption (breackdown)  Streamer  Spark
Negative Streamer Positive Streamer
3) Electric
Discharge
transient stable
current
voltage
Plasma
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 About 200 years after the discovery of the Plasma phenomena, the
investigation of several Plasma physical features in atmospheric
general conditions (e.g. pressure) are still under development due to:
– diversity of phenomena
– difficulties in assessing experimentally zones with very high temperature and
electrical field gradients
– existing physical formulation/analytical laws are not directly applicable
 In the bidirectional energy flow within the Electric Arc, the low mass
of electrons enable them to react promptly to the electrical fields
when compared to the ions or neutral particles. Thus the electrons
are the main responsible for the energetic transference of energy
within the Electrical Arc
Plasma

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 Anode Fall Voltage
Zone
 Arc Column Zone
 Cathode Fall
Voltage Zone
cathode
Electric Arc
Distinct Zones
anode
Electrical
Spatial
charge
Anode
Fall
Voltage
Zone
Cathode
Fall
Voltage
Zone
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 Thermionic emission results from joule heating (resistance) of the cathode by
the imposed welding current until the electron energy at the cathode tip
exceeds the work function (energy required to strip off an electron).
 Thoria (ThO2), zirconia (ZrO2), or ceria (CeO2) are added to pure
tungsten in amounts up to 2.2 wt% ThO2, 0.4 wt% ZrO2, or 3.0 wt%CeO2
to lower the workfunction, which results in thermionic emission at lower
temperatures and avoids melting the cathode tip
 Nonthermionic emission, also called cold cathode, or Field emission
The “Procedure Handbook of Arc Welding - Lincoln Electric, 12 Edition
suggest 2 different alternative mechanisms to explain the CFV zone:
Electric Arc
Cathode Fall Voltage (CFV) Zone

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Cathode Fall
Zone
Electric Arc
Power dissipated in the Fall Voltage Zones
Anode Fall Zone
 
e
c
c
q
kTI
2
5
U
I
H 

 
e
a
a
q
kTI
IU
H
2
5


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Electric Arc
Arc Column Zone
The formation of plasma based on colision processes follows the
Saha equation that allows to determine the level of ionization of a
gas column:
Where:
ne, ni , n0 – Density of particles (electrons, positive ions and neutral
particles per unit of volume) ; Important: ne  ni
Vi – Ionization potential of a neutral particle
Zi, Z0 – Partition function of ions and neutral particles
h – Planck constant de, 6.6256x10-34J.s
me – Mass of steady electron, 9.1091x10-31Kg
k – Boltzmann constant , 1.3805x10-23J.K-1
 








kT
V
h
Z
kT
m
Z
n
n
n i
e
i
i
e
exp
2
2
3
0
2
3
0


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 The fall voltage zones have very small dimensions
(e.g. length < 0.01 mm) and high voltage gradients reaching
10
9
V/m at Cathode Fall Voltage zone versus 10
3
V/m at Arc Column
 High gradients in thermal field, varying from relatively low values at
the electrodes surface to very high values at Arc Column
 Expansion/Contraction of the plasma zone promoting a non-cylindrical
shape for the Electric Arc
 Voltage drop at vicinity of the Cathode zone is higher than the voltage
drop at the vicinity of the Anode zone, mainly for low values of current
Electric Arc
Characterization of Distinct Zones
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AC (Alternating current)
 Less stable electric arc then in DC. Need to restart arc every half
cycle
 Less stable metal transfer, (for welding processes using fuse
electrode), due to change of polarity
 Currents over 1000 A DC tend to create “arc blow” problems. AC is
most commonly used for high-current applications, for applications
where arc blow may be a problem, and in multiwire applications
 DCEP (Direct current with a positively charged electrode)
 DCEN (Direct current with a negatively charged electrode)
Electric Arc
Type of Current and Polarity

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Stability of the anode and cathode spots at the electrode and workpiece
Stability of the consumable electrode (when applicable) to the weld
pool in regular and axial drops, with no spatter
Weld pool in the workpieces should move smoothly, and maintain a
fixed position relative to the electrode, i.e. electric arc should always
cover the same area of weld pool, which in the case of high-speed
welding or small weld pools is particularly critical, since in these cases
the electric arc tends to have an erratic nature
Voltage and current should be stable and controllable
Electric arc should not extinguish easily (e.g.: due to arc blow effects)
Electric Arc
Stability Criterions
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 
a
p
c
total U
U
U
I
P 


1
1
1 n
c
I
B
A
U 

I
A
Up 2














c
3
3
c
3
3
I
I
para
I
I
para
3
3
n
a
n
c
a
I
B
A
U
I
B
A
U
Electric Arc
Total Electric Power Dissipated
3
1
tipically
: A
A
Note 

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EA
at
Suply
Power
Dissipated
Power
Electric Arc
Energetic Efficiency Curves
SMAW
GMAW
SAW
GTAW
EA current
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Short Circuit
Electric Arc
Arc Start Techniques – Short Circuit
Lift-arc
Contact
 Electrode touch workpiece
 With/without sacrificial plate
 Contact Short Circuit: Technique applied to welding processes with
consumable/melting electrodes (e.g.: SMAW ; SAW ; GMAW)
 Power source control
 Electrode quasi-touch
workpiece

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High Frequency
Note: Arc start cycle 0.5x10-3
s
 Power sources for GTAW and PAW processes include a high-frequency arc starting
device that impresses a high radio frequency (RF) voltage on the electrode. This
energy "jumps the gap" from the electrode to the workpiece, ionizing the shielding gas,
and permits establishment of an arc. Thus, the electrode need not touch the workpiece
Electric Arc
Arc Start Techniques – High Frequency
 Technique applied to welding processes with non
consumable thermoionic electrodes (e.g.: GTAW)
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T
k
qe
e
T
A
J 2



A = 6x105 A/m2K2 (metallic materials)
T – Superficial Temperature [K]
 – Thermoionic work function of electrode
surface [V]
qe = 1.6021x10-19C
k – Boltzmann Cte, 1.38065x10-23J.K-1
Electric Arc
Current Density for Thermoionic Electrodes

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E.g.: Isothermals GTAW
 6000K, high concentration of easy to ionize gases
 20000K, inert gas atmosphere
Reference values:
Electric Arc
Temperature Distribution in Plasma
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Reference values:
Electric Arc

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GMAW of Aluminium
Influence of GTAW EA length
Different current GTAW
Electric Arc
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Electric Arc
Shielding Gases Properties
Gas Proprieties Density (15ºC ; 1atm) [kg/m3]
Hydrogen (H2) Reducer 0.085
Oxygen (O2)
Oxidant
1.35
Carbon Dioxide (CO2) 1.59
Helium (He)
Inert
0.169
Argon (Ar) 1.69

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Electric Arc
Dissociation and ionization of gases
components
Gases thermal conductivity
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 Good for non ferrous (e.g.: Al e Cu) and
reactive materiais (e.g.: Ti e Mg)
 Promote chemical oxidizing reducing
reactions
 Applicable to ferrous materials
Electric Arc

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 Arc Start / Arc Re-Start:
Less Ionization Energy
Faster and Easier
Process of Arc Start / Re-Start of Electric Arc
Electric Arc
Influence of Ionization Energy and Thermal Conductivity
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 Electric Arc Stability :
Less Ionization Energy
(consumes less energy demands less voltage for the same EA length)
+
Less Thermal Conductivity
(less loses of energy  Hotter Electric Arc Plasma)
Higher Stability of Electric Arc
(easy to maintain the plasma and thus… the stable electrical discharge )
Electric Arc

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 Shape + Penetration/Width of Electric Arc:
Electric Arc
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Electric Arc

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Electric Arc
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 Formation of Spatter
Electric Arc

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 Weld Bead Shape and Spatter
Electric Arc
Influence of Gases in Weldability of Structural Steels
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 Weld Bead Shape and Spatter
Electric Arc
Influence of Gases in Weldability of Stainless Steels

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Case Study
Activated TIG: “A-TIG” (1/14)
 Method of increasing the penetration capability of the arc in TIG welding
 Achieved through the application of a thin coating of activating flux
material onto the workpiece surface prior to welding
 Effect of flux is to constrict the arc which increases the current density at
the anode root and the arc force on the weld pool
 The consistency in quality, reduced need for edge preparation, reduced
distortion and the improved productivity could make the A-TIG welding
process more attractive than the conventional TIG, e.g., process in tube
welding
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Case Study
 Activating fluxes for TIG welding was first reported by the EO Paton
Institute of Electric Welding in the former Soviet Union in the 1950s
 More recently activating fluxes have become commercially available
from several sources
 These fluxes claim to be suitable for the welding of a range of materials,
including C-Mn steel, Cr-Mo steels, stainless steels and nickel-based
alloys
 The fluxes are generally available in the form of either an aerosol or as a
paste (powdered flux mixed with a suitable solvent) which is applied
onto the surface with a brush
 Activating fluxes can be applied in both manual or mechanised welding

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Case Study
Conventional TIG
Electric Arc Comparison (application to Stainless Steel)
A-TIG
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Case Study
Advantages A-TIG versus conventional TIG
 Increased productivity due to greater depth of penetration, i.e., up to 8mm
in stainless steel compared to 3mm for conventional TIG welding
 Increased productivity is derived through a reduction in welding time
and/or a reduction in the number of welding passes
 Reduced distortion, i.e., use of a square edge closed butt joint
preparation reduces weld shrinkage compared with a conventional
multipass V butt joint
 Problems of inconsistent weld penetration associated with cast-to-cast
material variations can be eliminated. E.g. deep penetration welds can be
made in low sulphur stainless steel (~0.002%), which would otherwise
show a shallow, wide weld bead in conventional TIG welding (see:
http://www.arcmachines.com/news/case-studies/effects-sulfur)

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Case Study
Proposed mechanisms of A-TIG welding
 Ability of flux to wet surface of the molten pool has an effect on
composition modifying the surface tension. Change in fluid flow is related
Thermal Coefficient of Surface Tension (TCST) of the molten pool:
 If the TCST is negative, the cooler peripheral regions of pool will
have a higher surface tension than the centre of the weld pool
and the flow will be outwards creating a wide shallow weld pool
 In materials with a positive gradient, this flow is reversed to the
centre of the weld pool and in the centre the molten material
flows down. This creates a narrower deeper weld pool for exactly
the same welding conditions
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Case Study
 Change in fluid flow is related Thermal Coefficient of Surface Tension
(TCST) of the molten pool:
TCST is negative
TCST is positive

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Case Study
 Spectroscopic analysis shows a decrease in intensity of argon
lines and an increase in intensity of alkali metals in the arc medium
 Arc constriction effect of flux is related to the evaporation of the
flux and its preferential ionisation
 Preferential ionisation of the alkali metals and its high
dissociation temperature are believed to be responsible for the arc
constriction
 Strong electromagnetic force from the constricted arc is believed
to reverse the flow pattern overcoming the effect of TCST in A-TIG
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Case Study

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Case Study
Transverse weld section of A-TIG and conventional TIG welds in
48mm OD, 4mmWT 304L stainless tube
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Case Study
Transverse weld sections of Conventional TIG and A-TIG welds in
29mm OD 1.6mm WT laser seam weld 304L tube

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Case Study
Transverse weld sections of A-TIG and conventional TIG welds in
6mm OD, 1.0 WT 304 L stainless tubes
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Case Study
Conventional TIG and A-TIG welds in 29mm OD 1.6mm WT laser seam
welded 304L tube showing a deflected weld bead in the conventional TIG

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Depends on :
• Chemical structure of plasma gas
• Temperature of Plasma during
EA discharge
• Pressure within EA
Electric Arc
Emission of Radiation
wavelength
short
long
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Electric Arc
Safety Rules are Mandatory
 Follow manufacturers recommendations
 Check all cables insulation
 Wear appropriate PPE
 Never touch electrical or welding wire
when the switch is on
 Never weld in wet locations or when wet
 Use pliers for hot metal
 Insure adequate ventilation
 Have machine repaired by competent
person
 Turn off and safety store welder when
done welding

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Electric Arc
Safety Rules are Mandatory… to Avoid Hazards
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v
v
HI
VI
P

 

Where:
 – welding process efficiency
V – voltage [V]
I – current [A]
P = V x I – Total electric power supply by the power source [watt]
v – welding travel speed [mm/min]
Electric Arc
Heat Input - Formulation

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qe – Power transfer by conduction to electrode
qp – Power lost by arc column via radiation and
convection
qw – Power transferred into workpieces
n – Portion of energy projected by the electric
arc radiation into workpieces
m – Portion of energy lost in workpieces by
conduction into remaining workpieces and
radiation to the exterior
 
VI
mq
q
n
q w
p
e 




1
1

Electric Arc
Heat Input – Efficiency Factor
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Electric Arc
Heat Input – Efficiency Factor
SAW
GTAW
SMAW
GMAW
Absorb
Power,
kW
Arc Power, kW
v
HI
VI



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Interaction between Electric field and Electromagnetic field promotes:
Arc Blow may affects EA stability and arc deflection. Furthermore:
Arc Blow
 Bad stability of the localization of CFV and AFV zones
 Plasma instability
 Irregular material transference from consumable electrodes
 When Arc blow can not be avoid, its effects may be controlled/reduced
to acceptable levels via external magnetic fields
Electric Arc
Arc Blow Phenomena
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The effect of external magnetic fields on electric arc are govern by
Lorentz Force
 The Lorentz force is the force on a point charge due to electromagnetic
fields. It is given by the following equation in terms of the electric and
magnetic fields
Electric Arc
Arc Blow Phenomena

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Arc Blow is only relevant for high current values (as a rule of thumb,
if the magnetic field strength is greater than ~ 50 gauss (50 x 10
-4
tesla)
arc blow may be experienced). This effect may become significant
typically under the following 3 conditions:
Condition 1:
Backward arc blow tends to occur when
welding in a direction towards the current
return connection, or earth connection, and
forward arc blow when welding in the other
direction.
Electric Arc
Arc Blow Phenomena
Workpiece
Ground
Clamp
Electrode
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Condition 2:
Asymmetric distribution of electromagnetic
field on the vicinity of the electrode tip, near
the ends of ferromagnetic workpiece materials
Electric Arc
Arc Blow Phenomena
Workpiece Electrode

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Condition 3:
Multiple Arc Welding. when the two arcs are
located close together, may cause magnetic
arc blow. When the arcs are of different
polarity (a), the magnetic fields combine to
blow arcs outward. If the arcs are of the same
polarity (b), magnetic fields oppose each other
and the arcs blow inward. With one arc
powered by DC current and the other by AC
current (c), little or no arc blow occurs
Electric Arc
Arc Blow Phenomena
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Electric Arc
Avoiding Arc Blow (1)
To solve:
Use of multiple earth connections may solve the problem altogether
Use tab extensions of ferromagnetic materials at the ends of the weld seam
Use as short an arc length as possible (lower arc voltage) and the lowest current that
is practical for the affected joint (possibly a smaller diameter electrode)
Use alternative welding procedures, e.g.: backstep

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Electrode
Workpiece
Current field in
Electric Arc
Induced magnetic field
Eddy current
Electric Arc
Avoiding Arc Blow (2)
To solve:
 Use AC…rather than DC. When welding
with AC, the induced “Eddy” currents,
generates an induced magnetic field
opposite to the original one, resulting in no
Arc Blow effect, even for very high currents
(above 1000A)
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V = V0 + mI, onde: m<0
Constant-Current Sources:
a family of "drooping" volt-ampere (V-A) curves
Electric Arc
Direct Current Power Source: constant-current
Current
Voltage
OCV
(Open Circuit Voltage)

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Electric Arc
Direct Current Power Source: constant-current
Relevant part of the constant current characteristic
curve is linear: V = V0 + mI
Then:
m
dV
dI 1

The variation of Power with the
voltage is:
V
m
I
dV
dI
V
I
VI
dV
d 1
)
( 



For constant power (and heat input, if travel speed is constant):
V
I
m


1
If operation parameters are: I1 ; V1, then the optimum slope, m is:
And the equation of the optimum
linear characteristic curve yields:
1
1
I
V
m 

I
I
V
V
V
1
1
0 

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Electric Arc
Direct Current Power Source: constant-voltage
Constant-Voltage Sources:
Power sources intended for gas-metal arc welding (GMAW) exhibit a
relatively flat V-A curve. The Self-Correcting or Semi-Automatic
characteristic of GMAW regulates the electrode burn-off rate
Working V-I
Current
Voltage

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Introduction to Transfer Modes
Designation of transference mode Sample of Welding Process
1. Free flight transfer
1.1 Globular
1.1.1 Globular stable
1.1.2 Repelled
1.2 Spray
1.2.1 Drop-spray
1.2.2 Rotational (non-directional jet)
1.2.3 Spray (directional jet)
MIG/MAG low current
MAG and MIG with DCEN
MIG/MAG pulse current
MIG/MAG medium and high current
MIG/MAG high current
2 Transference with liquid bridge
2.1 Short circuit
2.2 Continuous bridge
MIG/MAG low current
TIG with weld metal (filler metal)
3. Transference with solid protection
3.1 Guided within flux
3.2 Other modes (explosion)
SAW, Electroslag Welding
SMAW, FCAW
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Most Significant Metal Transference Modes in GMAW
 Short-circuit
 Globular
 Spray (Axissymmetric and rotational)
 Drop-spray (axial spray of drop by drop in pulse current)

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Control Factors Metal of Transference Modes in GMAW
 Shielding gas type (composition)
 Shielding gas flow rate
 Electrode wire type (composition)
 Diameter of electrode wire
 WFS (proportional to current)
 Static electric characteristic curve (voltage/arc length)
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Drop-Spray
Short-Circuit
Comparison Between Metal Transference Modes

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Literature supporting the achievement of the learning outcomes
1. J. F. Lancaster (1986) The Physics of Welding”, 2nd ed., Pergamon Press.
2. Robert W. Messler (2004) Principles of Welding – Processes Physics,
Chemistry, and Metallurgy, Jr. Wiley-VCH ed.
 Chapter 1: Introduction to the Process of Welding (pages 1–16)
 Chapter 5: Energy for Welding
 Closing Thoughts
Other references
 ASM Metals Handbook – Vol. 6 – Welding Brazing and Soldering. 1993. ASM
International.
 AWS Welding Handbook – Vol. 1 to 4 –9th ed. American Welding Society.
References
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References
Literature supporting the A-TIG
• Lucas W, Howse DS (1996) Activating flux - increasing the
performance and productivity of the TIG and plasma processes,
Welding and Metal Fabrication
• Gurevich SM et al. (1965) Improving the penetration of titanium alloys
when they are welded by argon tungsten arc process' Automatic
Welding
• Makara AM et al. (1968) High-tensile martensitic steels welded by
argon tungsten arc process using flux' Automatic Welding
• Voropai NM and Lebedeva (1989)Physical properties of welding
fluxes based on TiO, formed in melting activated wires' Automatic
Welding

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References
Literature supporting the A-TIG
• Heiple CR and Roper JR (1982) Mechanism for minor element effect
on GTA fusion zone geometry' Welding Journal
• Simonik AG (1976) The effect of contraction of the arc discharge
upon the introduction of electro-negative elements Welding
Production
• Ostrovskii OE et al. (1997) The effect of activating fluxes on the
penetration capability of the welding arc and the energy concentration
in the anode spot' Welding Production
• V Kumar, et al. (2009) Investigation of the A-TIG mechanism and the
productivity benefits in TIG welding. JOM 15 and 6th International
Conference on Education in Welding (ICEW 6)

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