Lecture: 1

Introduction: Joining
This chapter presents the fundamental approaches used in manufacturing namely
casting, forming, welding and machining. Further, common methods of developing
joint and selection of suitable methods have been described. Applications,
advantages and limitations of welding as a fabrication technique have also been
covered.
Keywords: Manufacturing process, selection of joint, welding vs. manufacturing
processes, selection of welding process, advantages, application and limitation of
welding processes
1.1 Introduction
The manufacturing technology primarily involves sizing, shaping and imparting
desired combination of the properties to the material so that the component or
engineering system being produced to perform indented function for design life. A
wide range of manufacturing processes have been developed in order to produce
the engineering components ranging from simple to complex geometries using
materials of different physical, chemical, mechanical and dimensional properties.
There are four chief manufacturing processes i.e. casting, forming, machining and
welding. Selection of suitable manufacturing process for a produce/component is
dictated by complexity of geometry of the component, number of units to be
produced, properties of the materials (physical, chemical, mechanical and
dimensional properties) to be processed and economics. Based on the approach
used for obtaining desired size and shape by different manufacturing processes;
these can be termed as positive, negative and or zero processes.
 Casting: zero process
 Forming: zero process
 Machining: negative process
 Joining (welding): positive process
Casting and forming are categorized as zero processes as they involve only shifting
of metal in controlled (using heat and pressure singly or in combination) way from
one region to another to get the required size and shape of product. Machining is
considered as a negative process because unwanted material from the stock is
removed in the form of small chips during machining for the shaping and sizing of a
product purpose. During manufacturing, it is frequently required to join the simple
shape components to get desired product. Since simple shape components are
brought together by joining in order to obtain desired shape of end useable product
therefore joining is categorized as a positive process. Schematic diagrams of few
typical manufacturing processes are shown in Fig. 1.1.

a) b)

c) Machining

d) Joining
Fig. 1.1 Schematic diagram showing shaping approaches using different
manufacturing processes a) forming, b) casting, c) machining and d) joining
1.2 Selection of Joint
The fabrication of engineering systems frequently needs joining of simple
components and parts. Three types of joining methods namely mechanical joining
(nuts & bolts, clamps, rivets), adhesive joining (epoxy resins, fevicol), welding
(welding, brazing and soldering) are commonly used for manufacturing variety of
engineering product/component. Each type of joint offers different load carrying
capacity, reliability, compatibility in joining of similar or dissimilar materials besides
their fitness for use in different environments and cost. It will be appropriate to
consider following aspects while selecting type of joints for an application:
a) type of joint required for an application is temporary or permanent
 b) Whether similar or dissimilar materials are to be joined in order to take
care of the compatibility aspect as metallurgical incompatibility can be
disastrous for performance of the joints
c) Physical, chemical metallurgical properties of materials to be joined
d) requirements of the service from the joint under special conditions of
temperature, corrosion, environment, and reliability
e) type and nature of loading conditions (static and dynamic loading under
tension, shear, compression, bending etc.)
f) economy or cost effectiveness is one most important factors influencing
the selection of joint for manufacturing an engineering component
1.3 Welding and its comparison with other manufacturing processes
Welding is one of the most commonly used fabrication techniques for manufacturing
engineering components for power, fertilizer, petro-chemical, automotive, food
processing, and many other sectors. Welding generally uses localized heating during
common fusion welding processes (shielded metal arc, submerged arc, gas metal
arc welding etc.) for melting the faying surfaces and filler metal. However, localized
and differential heating & cooling experienced by the metal during welding makes it
significantly different from other manufacturing techniques:
 Residual stresses are induced in welded components (development of tensile
residual stresses adversely affects the tensile and fatigue properties of work
piece)
 Simple shape components to be joined are partially melted
 Temperature of the base metal during welding in and around the weld varies
as function of time (weld thermal cycle)
 Chemical, metallurgical and mechanical properties of the weld are generally
anisotropic
 Reliability of weld joint is poor.
 Little amount of metal is wasted in the form of spatter, run in and run off
 Process capabilities of the welding in terms of dimensional accuracy,
precision and finish are poor.
 Weld joints for critical applications generally need post weld treatment such as
heat treatment or mechanical working to get desired properties or reline
residual stress.
  Problem related with ductile to brittle transition behaviour of steel is more
severe with weld joints under low temperature conditions.
1.4 Selection of welding process
A wide range of welding processes are available to choose. These were developed
over a long period of time. Each process differs in respect of their ability to apply
heat for fusion, protection of the weld pool and soundmen of welds joint the so
performance of the weld joint. However, selection of a particular process for
producing a weld joint is dictated by the size and shape of the component to be
manufactured, the metal system to be welded, availability of consumables and
machines, precision required and economy. Whatever process is selected for
developing weld joint it must be able to perform the intended function for designed
life. Welding processes with their field of applications are given below:
 Resistance welding: Automobile
 Thermite welding: Rail joints in railways
 Tungsten inert gas welding: Aerospace and nuclear reactors
 Submerged arc welding: Heavy engineering, ship building
 Gas metal arc welding: Joining of metals (stainless steel, aluminium and
magnesium) sensitive to atmospheric gases
1.5 Advantages and Limitation of Welding as a Fabrication Technique
Welding is mainly used for the production of comparatively simple shape
components. It is the process of joining the metallic components with or without
application of heat, pressure and filler metal. Application of welding in fabrication
offers many advantages, however; it suffers from few limitations also. Some of the
advantage and limitations are given below.
Advantages of welding are enlisted below:
1. Permanent joint is produced, which becomes an integral part of work piece.
2. Joints can be stronger than the base metal if good quality filler metal is used.
3. Economical method of joining.
4. It is not restricted to the factory environment.
Disadvantages of welding are enlisted also below:
1. Labour cost is high as only skilled welder can produce sound and quality weld
joint.
 2. It produces a permanent joint which in turn creates the problem in dissembling
if of sub-component required.
3. Hazardous fumes and vapours are generated during welding. This demands
proper ventilation of welding area.
4. Weld joint itself is considered as a discontinuity owing to variation in its
structure, composition and mechanical properties; therefore welding is not
commonly recommended for critical application where there is a danger of life.

1.6 Applications of welding

General applications
 The welding is widely used for fabrication of pressure vessels, bridges,
building structures, aircraft and space crafts, railway coaches and general
applications besides shipbuilding, automobile, electrical, electronic and
defense industries, laying of pipe lines and railway tracks and nuclear
installations.
 Specific components need welding for fabrication includes
1. Transport tankers for transporting oil, water, milk etc.
2. Welding of tubes and pipes, chains, LPG cylinders and other items.
3. Fabrication of Steel furniture, gates, doors and door frames, and body
4. Manufacturing white goods such as refrigerators, washing machines,
microwave ovens and many other items of general applications

The requirement of the welding for specific area of the industry is given in following
section.
Oil & Gas
1. Welding is used for joining of pipes, during laying of crude oil and gas
pipelines, construction of tankers for their storage and transportation. Offshore
structures, dockyards, loading and unloading cranes are also produced by
welding.
Nuclear Industry
2. Spheres for nuclear reactor, pipe line bends, joining of pipes carrying heavy
water require welding for safe and reliable operations.
Defense industry
 3. Tank body fabrication, joining of turret mounting to main body of tanks are
typical examples of applications of welding in defense industry.
Electronic industry
4. Electronic industry uses welding to limited extent e.g. joining leads of special
transistors but other joining processes such as brazing and soldering are
widely used.
5. Soldering is used for joining electronic components to printed circuit boards
(PCBs).
6. Robotic soldering is very common for joining of parts to printed circuit boards
of computers, television, communication equipment and other control
equipment etc.
Electrical Industry
7. Components of both hydro and steam power generation system, such as
penstocks, water control gates, condensers, electrical transmission towers
and distribution system equipment are fabricated by welding. Turbine blades
and cooling fins are also joined by welding.
Surface transport
8. Railway: Railway uses welding extensively for fabrication of coaches and
wagons, repair of wheel, laying of new railway tracks by mobile flash butt
welding machines and repair of cracked/damaged tracks by thermite welding.
9. Automobiles: Production of automobile components like chassis, body and its
structure, fuel tanks and joining of door hinges require welding.
Aerospace Industry
10. Aircraft and Spacecraft: Similar to ships, aircrafts were produced by riveting in
early days but with the introduction of jet engines welding is widely used for
aircraft structure and for joining of skin sheet to body.
11. Space vehicles which have to encounter frictional heat as well as low
temperatures require outer skin and other parts of special materials. These
materials are welded with full success for achieving safety and reliability.
Ship Industry
12. Ships were produced earlier by riveting. Welding found its place in ship
building around 1920 and presently all welded ships are widely used. Similarly
submarines are also produced by welding.
Construction industry
 13. Arc welding is used for construction of steel building structures leading to
considerable savings in steel and money.
14. In addition to building, huge structures such as steel towers also require
welding for fabrication.
References and books for further reading
1. Mikell P. Groover, Fundamentals of Modern Manufacturing: Materials,
Processes, and Systems, John Willey and Sons, (2010) USA
2. Richard Little, Welding and Welding Technology, McGraw Hill, (2001), 1st
edition
3. Welding handbook, American Welding Society, (1983), 7th edition, volume 1 &
2, USA
4. http://www.roymech.co.uk/Useful_Tables/Manufacturing/Welding.html
5. http://www.everlastgenerators.com/importance-of-welding-in-manufacturing-
industries.php
6. http://en.wikipedia.org/wiki/Metal_fabrication
 Lecture - 2
Classification of Welding Processes I
Welding is a process of joining metallic components with or without application of
heat, with or without pressure and with or without filler metal. A range of welding
processes have been developed so far using single or a combination above
factors namely heat, pressure and filler. Welding processes can be classified on
the basis of following techological criteria:
 Welding with or without filler material
 Source of energy for welding
 Arc and non-arc welding
 Fusion and pressure welding

Keywords: Classification of welding process, autogenous weld, fusion vs.

pressure welding

2.1 Classification of welding processes on the basis of technical factors

2.1.1 Welding with or without filler material

A weld joint can be developed just by melting of edges (faying surfaces) of plates
or sheets to be welded especially when thickness is lesser than 5 mm thickness.
A weld joint developed by melting the fating surfaces and subsequently
solidification only (without using any filler metal) is called “autogenous weld”.
Thus, the composition of the autogenous weld metal corresponds to the base
metal only. However, autogenous weld can be crack sensitive when solidification
temperature range of the base metal to be welded is significantly high (750o -
100oC). Following are typical welding processes in which filler metal is generally
not used to produce a weld joint.
 Laser beam welding
 Electron beam welding
 Resistance welding,
 Friction stir welding
However, for welding of thick plates/sheets using any of the following processes
filler metal can be used as per needs according to thickness of plates.
Application of autogenous fusion weld in case of thick plates may result in
concave weld or under fill like discontinuity in weld joint. The composition of the
filler metal can be similar to that of base metal or different one accordingly weld
joints are categorized as homogeneous or heterogeneous weld, respecting.
In case of autogenous and homogeneous welds, solidification occurs directly by
growth mechanism without nucleation stage. This type of solidification is called
epitaxial solidification. The autogenous and homogeneous welds are considered
to be of lesser prone to the development of weld discontinuities than
heterogeneous weld because of a uniformity in composition and (b) if
solidification largely occurs at a constant temperature. Metal systems having
wider solidification temperature range show issues related with solidification
cracking and partial melting tendency. The solidification in heterogeneous welds
takes place in conventional manner in two stages i.e. nucleation and growth.
Following are few fusion welding processes where filler may or may not be used
for developing weld joints:
 Plasma arc welding
 Gas tungsten arc welding
 Gas welding
Some of the welding processes are inherently designed to produce a weld joint
by applying heat for melting base metal and filler metal both. These processes
are mostly used for welding of thick plates (usually > 5mm) with comparatively
higher deposition rate.
 Metal inert gas welding: (with filler)
 Submerged arc welding: (with filler)
 Flux cored arc welding: (with filler)
 Electro gas/slag welding: (with filler)
Comments on classification of welding processes based on with/without filler
The gas welding process was the only fusion welding process earlier using which
joining could be achieved with or without filler material. The gas welding
performed without filler material was termed as autogenous welding. However,
with the development of tungsten inert gas welding, electron beam, laser beam
and many other welding processes, such classification created confusion as
many processes were falling in both the categories.

2.1.2 Source of energy for welding

Almost all weld joints are produced by applying energy in one or other form to
develop atomic/metallic bond between metals being joined and the same is
achieved either by melting the faying surfaces using heat or applying pressure
either at room temperature or high temperature (0.5o to 0.9o Tm). Based on the
type of energy being used for creating metallic bonds between the components
to be welded, welding processes can be grouped as under:
 Chemical energy: Gas welding, explosive welding, thermite welding
 Mechanical energy: Friction welding, ultrasonic welding
 Electrical energy: Arc welding, resistance welding
 Radiation energy: Laser beam welding, electron beam welding
Comments on classification of welding processes based on source of energy
Energy in various forms such as chemical, electrical, light, sound, mechanical
energies etc. are used for developing weld joints. However, except chemical
energy all other forms of energies are generated from electrical energy for
welding. Hence, categorization of the welding processes based on the source of
energy criterion also does not justify classification properly.

2.1.3 Arc or Non-arc welding

Metallic bond between the plates to be welded can be developed either by using
heat for complete melting of the faying surfaces then allowing it to solidify or by
apply pressure on the components to be joined for mechanical interlocking. All
those welding processes in which heat for melting the faying surfaces is provided
after establishing an arc either between the base plate and an electrode or
between electrode & nozzle are grouped under arc welding processes. Another
set of welding processes in which metallic bond is produced using pressure or
heat generated from sources other than arc namely chemical reactions or
frictional effect etc., are grouped as non-arc based welding processes. Welding
processes corresponding to each group are given below.
 Arc based welding processes
 Shielded Metal Arc Welding: Arc between base metal and covered
electrode
 Gas Tungsten Arc Welding: Arc between base metal and tungsten
electrode
 Plasma Arc Welding: Arc between base metal and tungsten electrode
 Gas Metal Arc Welding: Arc between base metal and consumable
electrode
 Flux Cored Arc Welding: Arc between base metal and consumable
electrode
 Submerged Arc Welding: Arc between base metal and consumable
electrode

 Non-arc based welding processes

 Resistance welding processes: uses electric resistance heating
 Gas welding: uses heat from exothermic chemical reactions
 Thermit welding: uses heat from exothermic chemical reactions
 Ultrasonic welding: uses both pressure and frictional heat
 Diffusion welding: uses electric resistance/induction heating to
facilitate diffusion
 Explosive welding: involves pressure
Comments on classification of welding processes based on arc or non arc based
process
Arc and non-arc welding processes classification leads to grouping of all the arc
welding processes in one class and all other processes in non-arc welding
processes. However, welding processes such as electro slag welding (ESW) and
flash butt welding were found difficult to classify in either of the two classes as
ESW process starts with arcing and subsequently on melting of sufficient amount
flux, the arc extinguishes and heat for melting of base metal is generated by
electrical resistance heating by flow of current through molten flux/metal. In flash
butt welding, tiny arcs i.e. sparks are established during initial stage of the
welding followed by pressing of components against each other. Therefore, such
classification is also found not perfect.

2.1.4 Pressure or Fusion welding

Welding processes in which heat is primarily applied for melting of the faying
surfaces are called fusion welding processes while other processes in which
pressure is primarily applied (with little or no application of heat for softening of
metal up to plastic state) for developing metallic bonds are termed as solid state
welding processes.
 Pressure welding
o Resistance welding processes (spot, seam, projection, flash
butt, arc stud welding)
o Ultrasonic welding
o Diffusion welding
o Explosive welding
 Fusion welding process
o Gas Welding
o Shielded Metal Arc Welding
o Gas Metal Arc Welding
o Gas Tungsten Arc Welding
o Submerged Arc Welding
o Electro Slag/Electro Gas Welding

Comments on classification of welding processes based on Fusion and pressure

welding
Fusion welding and pressure welding is most widely used classification as it
covers all processes in both the categories irrespective of heat source and
welding with or without filler material. In fusion welding, all those processes are
included in which molten metal solidifies freely while in pressure welding, molten
metal if any is retained in confined space (as in case of resistance spot welding
or arc stud welding) and solidifies under pressure or semisolid metal cools under
pressure. This type of classification poses no problems and therefore it is
considered as the best criterion.

References and books for further reading

 Metals Handbook-Welding, Brazing and Soldering, American Society for

Metals, 1993, 10th edition, Volume 6, USA.
 Richard Little, Welding and Welding Technology, McGraw Hill, 2001, 1st
edition.
 H Cary, Welding Technology, Prentice Hall, 1988, 2nd edition.
 S V Nadkarni, Modern Arc Welding Technology, Ador Welding Limited,
2010, New Delhi.
 R S Parmar, Welding process and technology, Khanna Publisher, New
Delhi
 Welding handbook, American Welding Society, 1987, 8th edition, volume
1 & 2, USA.
 http://www.substech.com/dokuwiki/doku.php?id=classification_of_welding
_processes
 http://www.newagepublishers.com/samplechapter/001469.pdf
 http://www.typesofwelding.net
 http://books.google.co.in/books?id=PSc4AAAAIAAJ&pg=PA1&lpg=PA1&d
q=classification+of+welding+processes&source=bl&ots=G9EbFzqzBa&sig
=T1EqGIMpChzzqwSZJJeuD9PlaKQ&sa=X&ei=qlsyUO_XCMnZrQfn8oH
oCQ&sqi=2&ved=0CCIQ6AEwBA#v=onepage&q=classification%20of%20
welding%20processes&f=false
 http://www.kobelcowelding.com/20100119/handbook2009.pdf
 http://me.emu.edu.tr/me364/ME364_combining_fusion.pdf
 Lecture: 3
Classification of Welding Processes II
Apart from technical factors, welding processes can also be classified on the
fundamental approaches used for deposition of materials for developing a joint.
This chapter presents the classification of welding processes as welding
processes and allied process used for developing a joint
Keywords: Welding and allied processes, approach of classification, cast weld,
resistance weld, fusion weld, solid state weld

3.1 Classification of welding processes

There is another way of classifying welding and allied processes which is
commonly reported in literature. Various positive processes involving addition or
deposition of metal are first broadly grouped as welding process and allied
welding processes as under:
1. Welding processes
i. Cast weld processes
ii. Fusion weld processes
iii. Resistance weld processes
iv. Solid state weld processes
2. Allied welding processes
i. Metal depositing processes
ii. Soldering
iii. Brazing
iv. Adhesive bonding
v. Weld surfacing
vi. Metal spraying
This approach of classifying the welding process is primarily based on the way
metallic pieces are united together during welding such as
 Availability and solidification of molten weld metal between
components being joined are similar to that of casting: Cast weld
process.
  Fusion of faying surfaces for developing a weld: Fusion weld process
 Heating of metal only to plasticize then applying pressure to forge
them together: Resistance weld process
 Use pressure to produce a weld joint in solid state only: Solid state
weld process

3.2 Cast welding process

Those welding processes in which either molten weld metal is supplied from
external source or melted and solidified at very low rate during solidification like
castings. Following are two common welding processes that are grouped under
cast welding processes:

o Cast weld processes

 Thermite welding
 Electroslag welding
In case of thermite welding, weld metal is melted externally using exothermic
heat generated by chemical reactions and the melt is supplied between the
components to be joined while in electroslag welding weld metal is melted by
electrical resistance heating and then it is allowed to cool very slowly for
solidification similar to that of casting.

Comments on classification based on cast weld processes

This classification is true for thermite welding where like casting melt is supplied
from external source but in case of electroslag welding, weld metal obtained by
melting of both electrode and base metal and is not supplied from the external
source. Therefore, this classification is not perfect.

3.3 Fusion Weld Processes

Those welding processes in which faying surfaces of plates to be welded are
brought to the molten state by applying heat and cooling rate experienced by
weld metal in these processes are much higher than that of casting. The heat
required for melting can be produced using electric arc, plasma, laser and
electron beam and combustion of fuel gases. Probably this is un-disputed way of
classifying few welding processes. Common fusion welding processes are given
below:

o Fusion Weld Processes

 Carbon arc welding
 Shielded metal arc welding
 Submerged arc welding
 Gas metal arc welding
 Gas tungsten arc welding
 Plasma arc welding
 Electrogas welding
 Laser beam welding
 Electron beam welding
 Oxy-fuel gas welding
3.4Resistance welding processes

Welding processes in which heat required for softening or partial melting of base
metal is generated by electrical resistance heating followed by application of
pressure for developing a weld joint. However, flash butt welding begins with
sparks between components during welding instead of heat generation by
resistance heating.

o Resistance welding processes

 Spot welding
 Projection welding
 Seam welding
 High frequency resistance welding
 High frequency induction welding
 Resistance butt welding
 Flash butt welding
 Stud welding
 
3.5 Solid state welding process

Welding processes in which weld joint is developed mainly by application of

pressure and heat through various mechanism such as mechanical interacting,
large scale interfacial plastic deformation and diffusion etc.. Depending up on the
amount of heat generated during welding these are further categorized as under:

o Solid state welding process

 Low heat input processes
 Ultrasonic welding
 Cold pressure welding
 Explosion welding
 High heat input processes
 Friction welding
 Forge welding
 Diffusion welding

There are many ways to classify the welding processes however, fusion welding
and pressure welding criterion is the best and most accepted way to classify all
the welding processes. The flow chart is showing classification of welding and
allied processes for better understanding of nature of a specific process (Chart
3.1).
 

 
 

Chart 3.1 Classification of Welding and Allied Processes 

Welding and allied processes

Welding processes Allied processes

Cast weld Fusion weld Resistance Solid state Metal depositing

process process weld process weld process process

Thermit Carbon arc Spot Low heat High heat Soldering

input input
Shielded metal
Electroslag Projection Brazing
arc Ultrasonic Friction
Submerged Adhesive
Seam Cold
arc Forge bonding
pressure
H. F. Weld
Gas metal arc Diffusion
resistance Explosion surfacing
bonding
Gas tungsten Metal
H.F. induction
arc spraying
Resistance
Plasma arc
butt

Electrogas Flash butt

Laser beam

Electron beam

Oxy-fuel gas

References and books for further reading

 Metals Handbook-Welding, Brazing and Soldering, American Society for Metals,
1993, 10th edition, Volume 6, USA.
 Richard Little, Welding and Welding Technology, McGraw Hill, 2001, 1st edition.
 H Cary, Welding Technology, Prentice Hall, 1988, 2nd edition.
 S V Nadkarni, Modern Arc Welding Technology, Ador Welding Limited, 2010,
New Delhi.
 R S Parmar, Welding process and technology, Khanna Publisher, New Delhi
 Welding handbook, American Welding Society, 1987, 8th edition, volume 1 & 2,
USA.
 http://www.substech.com/dokuwiki/doku.php?id=classification_of_welding_proce
sses
 http://www.newagepublishers.com/samplechapter/001469.pdf
 http://www.typesofwelding.net
 http://books.google.co.in/books?id=PSc4AAAAIAAJ&pg=PA1&lpg=PA1&dq=clas
sification+of+welding+processes&source=bl&ots=G9EbFzqzBa&sig=T1EqGIMp
ChzzqwSZJJeuD9PlaKQ&sa=X&ei=qlsyUO_XCMnZrQfn8oHoCQ&sqi=2&ved=0
CCIQ6AEwBA#v=onepage&q=classification%20of%20welding%20processes&f=
false
 http://www.kobelcowelding.com/20100119/handbook2009.pdf
 http://me.emu.edu.tr/me364/ME364_combining_fusion.pdf
 Lecture: 4
Power density and welding process
In this chapter, energy density and temperature associated with different welding
processes have been presented. Further, the influence of energy density on the
performance parameters of the weld joints has also been described.
Keywords: Power density, temperature of heat source, heat input, distortion,
mechanical properties
4.1 Introduction
Fusion welding processes can be looked into on the basis of range of energy density
which they can apply for melting the faying surfaces of base metal for joining. Heat
required for fusion of faying surfaces of components being welded comes from
different sources in different fusion welding processes (gas, arc and high energy
beam). Each type of heat source has capability to supply heat at different energy
densities (kW/mm2). Even for a given arc power (arc current I X arc voltage V),
different welding processes provide heat at different energy densities due to the fact
that it is applied over different areas on the surface of base metal in case of different
processes. Energy density (kW/mm2) is directly governed by the area over which
heat is applied by a particular process besides welding parameters. Power density in
ascending order from gas welding to arc welding to energy beam based welding
processes is shown in table 4.1. Typical values of energy densities and approximate
maximum temperature generated during welding by different processes are shown in
Table 4.1.
Table 4.1 Heat intensity and maximum temperature related with different
welding processes

Sr. No. Welding process Heat density (W/cm2) Temperature (0C)

1 Gas welding 102 -103 2500-3500
2 Shielded meta arc welding 104 >6000
Gas metal arc welding 105 8000-10000
3 Plasma arc welding 106 15000-30000
4 Electron beam welding 107 -108 20,000-30000
8
5 Laser beam welding >10 >30,000
4.2 Effect of power density
Energy density associated with a particular welding process directly affects amount
of heat required to be supplied for fusion of the faying surfaces. An increase in power
density decreases the heat input required for melting and welding of work pieces
because it decreases time over which heat is to be applied during welding for
melting. The decrease in heat application time in turn lowers the amount of heat
dissipated away from the faying surfaces to the base metal so the most of the heat
applied on the faying surfaces is used for their fusion only. However, it is important to
note that heat required for melting the unit quantity of a given metal is constant and
is a property of material. Heat for melting comprises sensible heat and latent heat.
Latent heat for steel is 2 kJ/mm3.
Fusion welding processes are based on localized melting using high-density heat
energy. To ensure melting of base metal in short time it is necessary that energy
density of welding process is high enough (Fig. 4.1). Time to melt the base metal is
found inversely proportional to the power density of heat source i.e. power of (arc or
flame) / area of work piece over which it is applied (W/cm2). Lower the energy
density of heat source greater will be the heat input needed for fusion of faying
surface welding as a large amount of heat is dissipated to colder base material of
work piece away from the faying surface by thermal conduction (Fig. 4.2).

Fig. 4.1 Effect of energy density and time on energy input

 increasing
Increasingthermal
damagedamage
to workprices

Heat input to workpiece

to workpiece

Gas
welding

Arc
Increasing penetration,
welding
welding speed, weld
quality and equipment
High energy cost
beam welding

Power density of heat source

Fig. 4.2 Effect of power density of heat source on heat input required for welding
[Kou S, 2003]

4.3 Need of optimum power density of welding process

As stated, low power density processes need higher heat input than high power
density processes. Neither too low nor too high heat input is considered good for
developing a sound weld joint. As low heat input can lead to lack of penetration and
poor fusion of faying surfaces during welding while excessive heat input may cause
damage to the base metal in terms of distortion, softening of HAZ and reduced
mechanical properties (Fig. 4.3). High heat input has been reported to lower the
tensile strength of many aluminium alloys of commercial importance due to thermal
softening of HAZ and development of undesirable metallurgical properties of the
weldment (Fig. 4.4). Moreover, use of high power density offers many advantages
such as deep penetration, high welding speed and improved quality of welding joints.
Welding process (where melting is required) should have power density
approximately 10(W/mm2). Vaporization of metal takes place at about 10,000W/mm2
power-density. Processes (electron and laser beam) with such high energy density
are used in controlled removal of metal for shaping of difficult to machine metals.
Welding processes with power density in ascending order are shown in Fig. 4.5.
 8

Distortion (degree) GTAW

6

2 EBW

10 20 30 40
Thickness (mm)

Fig. 4.3 Effect of welding process on angular distortion of weld joint as a function of
plate thickness[Kou S, 2003]

Al-Mg-Si
Tensile strength

Al-Cu-Mg

Al-Mg-Si

Heat input

Fig. 4.4 Schematic diagram showing effect of heat input on tensile strength of
aluminium alloy weld joints (magnfication of micrograph in figure is 200 X) [Kou S,
2003]
 LBW
EBW

PAW
GMAW
SMAW
GW

Fig. 4.5 Power densities of different welding processes

References and books for further reading

 Welding handbook, American Welding Society, 1983, 7th edition, volume 1 &
2, USA.
 Sindo Kou, Welding metallurgy, John Willey, 2003, 2nd edition, USA.
 S V Nadkarni, Modern Arc Welding Technology, Ador Welding Limited, 2010,
New Delhi.
 http://www6.conestogac.on.ca/~ffulkerson/MANU1060_files/solutions_ch31.pdf
 http://eagar.mit.edu/EagarPapers/Eagar061.pdf
 Lecture 5
Physics of Welding Arc I
This chapter presents fundamentals of welding arc, mechanisms of electron
emission, different zones in welding arc, electrical aspects related with welding arc
and their significance in welding.
Keywords: Welding arc, electron emission, thermo-ionic emission, field emission,
cathode and anode spot, arc power

5.1 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. Density of charged
particles in gap governs the electrical conductivity of gaseous column. In an electric
arc, electrons released from cathode (due to electric field or thermo-ionic emission)
are accelerated towards the anode because of potential difference between work
piece and electrode. These high velocity electrons moving from cathode toward
anode collide with gaseous molecules and decompose them into charged particles
i.e. electrons and ions. These charged particles move towards electrode and work
piece as per polarity and form a part of welding current. Ion current becomes only
about 1% of electron current as ions become heavier than the electrons so they
move slowly. Eventually electrons merge into anode. Arc gap between electrode and
work piece acts as pure resistance load. Heat generated in a welding arc depends
on arc voltage and welding current.
5.2 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.
Emission of electrons from the cathode metal depends on the work function. The
work function is the energy (ev or J) required to get one electron released from the
surface of material. Ionization potential is another measure of ability of a metal to
emit the electrons and is defined as energy/unit charge (v) required for removing an
electron from an atom. Ionization potential is found different for different metal. For
example, Ca, K, and Na have very low ionization potential (2.1-2.3ev), while that for
Al and Fe is on the higher side with values of 4 and 4.5 ev respectively. Common
mechanisms through which free electrons are emitted during arc welding are
described below:
5.2.1 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. This
mechanism of emission of electron due to heating of metal is called thermo ionic
emission. The temperature at which thermo-ionic emission takes place, most of the
metals melt. Hence, refractory materials like tungsten and carbon, having high
melting point exhibit thermo ionic electron emission tendency.
5.2.2 Field emission:
In this approach, free electrons are pulled out of the metal surface by developing
high strength electro-magnetic field. High potential difference (107 V/cm) between the
work piece and electrode is established for the field emission purpose.
5.2.3 Secondary emission
High velocity electrons moving from cathode to anode in the arc gap collide with
other gaseous molecules. This collision results in decomposition of gaseuous
molecules into atoms and charged particles (electrons and ions).
5.3 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. 5.1). Generally, five different zones are observed in the arc gap
namely cathode spot, cathode drop zone, plasma, anode drop zone and anode spot
(Fig. 5.2).
5.3.1 Cathode spot
This is a region of cathode wherefrom electrons are emitted. Three types of cathode
spots are generally found namely mobile, pointed, and normal. There can be one or
more than one cathode spots moving at high speed ranging from 5-10 m/sec. Mobile
cathode spot is usually produced at current density 100-1000 A/mm2. Mobile
cathode spot is generally found during the welding of aluminium and magnesium.
This type of cathode spot loosens the oxide layer on reactive metal like aluminium,
Mg and stainless steel. Therefore, mobile cathode spot helps in cleaning action
when reverse polarity is used i.e. work piece is cathode. Pointed cathode spot is
formed at a point only mostly in case of tungsten inert gas welding at about
100A/mm2. Pointed tungsten electrode forms the pointed cathode-spot. Ball shaped
tip of coated steel electrode forms normal cathode spot.
5.3.2 Cathode drop region:
This region is very close to the cathode and a very sharp drop of voltage takes place
in this zone due to cooling effect of cathode. Voltage drop in this region directly
affects the heat generation near the cathode which in turn governs melting rate of
the electrode in case of the consumable arc welding process with straight polarity
(electrode is cathode).
5.3.3 Plasma:
Plasma is the region between electrode and work where mostly flow of charged
particles namely free electrons and positive ions takes place. In this region, uniform
voltage drop takes place. Heat generated in this region has minor effect on melting
of the work piece and electrode.
5.3.4 Anode drop region:
Like cathode drop region, anode drop region is also very close to the anode and a
very sharp drop in voltage takes place in this region due to cooling effect of the
anode. Voltage drop in this region affects the heat generation near the anode & so
melting of anode. In case of direct current electrode negative (DCEN), voltage drop
in this zone affects melting of the work piece.
5.3.5 Anode spot:
Anode spot is the region of a anode where electrons get merged and their impact
generates heat for melting. However, no fixed anode spot is generally noticed on the
anode like cathode spot.
cathode
drop
Potential drop in
Potential drop (V)

plasma zone

Anode
drop

Distance from cathode to anode

Fig. 5.1 Potential drop as function of distance form the cathode to anode
 Electrode

Cathode
spot

Cathode

Cathode drop zone

+
Flow +
+-
of ions - + Plasma (charged particles)
Flow of - -
+ -
electrones +
Anode - -
spot - + -+ Anode drop zone
Anode

Workpiece

Fig. 5.2 Zones in arc gap of a welding arc

5.4 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. Therefore,
distribution of charged particles in radial and axial direction in the arc affects the total
impedance of the arc. Three major regions have been noticed in arc gap that
accounts for total potential drop in the arc i.e. cathode drop region, plasma and
anode drop region. 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. 5.3.
Power of the arc (P) = (Vc+ Vp+ Va) I………………………(5.1)
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). Equation 5.1 suggests that the
distribution of heat in three zones namely cathode, anode and arc plasma can be
changed. Variation of arc length mainly affects plasma heat while shielding gas
influences the heat generation in the cathode and anode drop zones. Addition of low
ionization potential materials (namely potassium and sodium) reduces the arc
voltage because of increased ionization in arc gap so increased electrical
conductivity which in turn reduces the heat generation in plasma region. Heat
generation at the anode and cathode drop zones is primarily governed by type of
welding process and polarity associated with welding arc. In case of direct current
(DC) welding, when electrode is connected to the negative terminal and workpiece is
connected with positive terminal of the power source then it is termed as direct
current electrode negative polarity (DCEN) or straight polarity and when electrode is
connected to the positive terminal of the power source and workpiece is connected
with negative terminal then it is termed as direct current electrode positive polarity
(DCEP) or reverse polarity. TIG welding with argon as shielding gas shows 8-10 time
higher current carrying capacity (without melting) than DCEP. The submerged arc
welding with DCEP generates larger amount of heat at cathode than anode as
indicated by high melting rate of consumable electrode.
Increase in spacing between the electrode and work-piece generally increases the
potential of the arc because of increased losses of the charge carriers by radial
migration to cool boundary of the plasma. Increase in the length of the arc column
(by bulging) exposes more surface area of arc column to the low temperature
atmospheric gas which in turn imposes the requirement of more number of charge
carriers to maintain the flow of current. Therefore, these losses of charged particles
must be accommodated to stabilize the arc by increasing the applied voltage. The
most of the heat generated in consumable arc welding process goes to weld pool
which in turn results in higher thermal efficiencies. This is more evident from the fact
that the thermal efficiency of metal arc welding processes is found in range of 70-
80% whereas that for non-consumable arc welding processes is found in range of
40-60%.