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Topic 1An introduction to materials welding andjoining
Objectives
At the completion of this topic, you should be able to:
(a) appreciate the importance of joining in industry;
(b) have some perception of the history of joining;
(c) appreciate the influence of joining on structural viability;
(d) know the basic principles of alternative joining processes;
(e) understand the feature of common joining processes; and
(f) understand how processes are classified
Scope
This subject provides the background and introductory information on common weldingprocesses. More detailed treatments of the processes are provided in other subjects withinthe course.
Module INT1 Introduction to Welding and Joining Processes
Topic 1 An Introduction to Materials Welding and Joining INT1.1 1 University of Wollongong 2001, Cranfield University 2002. All rights reserved.
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Recommended Reading
Houldcroft, P. and John, R. 1988, Welding and Cutting, Woodhead and Faulkner, ISBN 0-
85941-396-9.
Cary, H. B. 1979, Modern Welding Technology, Prentice Hall, 1979, ISBN 013-599290-7.
Norrish, J. 1992,Advanced Welding Processes,Adam Hilger, ISBN 0852743262-X.
Lancaster, J. F., Metallurgy of Welding,Allen & Unwin, ISBN 0-04-669011-5.
Welding Handbook, Welding Processes, vol. 2, 8th edn, American Welding Society, ISBN
0-87171-281-4.
Module INT1 Introduction to Welding and Joining Processes
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Sources of further information
There are many useful sources from which more specific information can be acquired on
welding processes and applications. Books which have been found to be particularly
useful in the preparation of these notes are listed above, but there are some hundreds of
publications dealing with particular aspects of welding technology.
When a technology is changing as rapidly as welding, the professional journals assume
greater importance than they might in other fields. The American Welding Journal
published by the American Welding Society is an extremely valuable source of material.
In addition, The Welding Institute (TWI) in the UK, WTIA in Australia and Edison Welding
Institute (EWI) in America maintain specialist library services and all have WWW sites on
the internet.
Some of the more important web sites are:
The Welding Institute - UK
http://www.twi.co.uk/home.html
Edison Welding Institute USA
http://www.ewi.org/home.html
Nederlands Institute of Welding - Holland
http://www.igr.nl/-henkbodt/nil/index-e.htm
WTIA-Australia
http://www.wtia.com.au
Module INT1 Introduction to Welding and Joining Processes
Topic 1 An Introduction to Materials Welding and Joining INT1.1 3 University of Wollongong 2001, Cranfield University 2002. All rights reserved.
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Notes
Introduction
Welding and joining are essential for the manufacture of a range of engineering
components which may vary from very large structures such as ships and bridges, to very
complex structures such as aircraft engines, or miniature components for microelectronic
applications.
The importance of joining
Joining of materials has been identified as a 'key enabling technology' which has a direct
impact on the wealth of nations1. It is also the common view of many engineers that joining
is:
- costly;
- hazardous; and
- difficult to control.
The high cost of joining operations is a consequence of the fact that many traditional
joining processes are labour intensive. Typically in steel construction the labour cost may
represent 70% to 80% of the fabrication cost. The hazards commonly associated withjoining processes arise from the use of high temperatures and pressures in many
processes, the need to use high pressure storage of gases which may be explosive,
flammable or asphyxiant, the use of high voltage electrical supplies, and the generation of
potentially hazardous fume, radiation and noise in some joining operations. The difficulty of
controlling joining processes is a result of the need to optimise a large number of
interrelated control variables. These issues will be further explored throughout this course
and in particular the effective control of weld quality will be discussed after examining
some of the history of fabrication.
History
Joining processes were developed by the earliest civilisations. These were often
mechanical joints or bindings, for example to attach stone heads to the wooden shafts of
work implements and weapons. More than 5000 years ago, brazing techniques were used
to fabricate decorative metalware and weapons and Homer's Illiad2 mentions a range of
decorative shields and swords which are fabricated by Haphaestos, the blacksmith of the
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Greek gods. For many years, however, the range of joining processes was restricted to
mechanical techniques, brazing, and forge welding.
With the industrial revolution in the West, there was a need for more sophisticated joining
processes, but the first steel ships and steam boilers were originally joined by riveting. The
development of higher temperature localised heat sources such as the oxy-acetylene
flame (around 1906) made fusion welding possible whilst resistance welding (1886)
offered a viable alternative to riveting for thin sections.
The arc welding processes stemmed from the discovery by Sir Humphrey Davy of the
electric arc in 1801 and the subsequent use of arc for joining with the carbon arc process
being invented in 1881 by Bernardos. Bernardos patented the carbon arc welding process
and formed a company 'Electro Hephaestos' to market the new invention in 1886.
The manual metal arc process which was the principal technique used for steel fabrication
for many years was however only developed and patented in 1907.
In the last 100 years, there has been a rapid development in joining technology, an
enormous number of new joining techniques have been developed and what was once
considered a 'black-art' is now a sophisticated manufacturing technology.
Fabricated structures - failures
Problems which may be directly attributed to welding and joining regularly arise in
production and service. Production failures may be rectified before the fabrication leaves
the manufacturer or alternatively the defective product may be scrapped. Both of these
options are costly and inefficient but may go unnoticed. Service failures are often more
dramatic and may be catastrophic in terms of human life and cost. Examples of welding
related failures are:
- the Liberty ships;
- the Kings Bridge; and
- the Alexander Kielland.
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The liberty ships
Figure 1.1 Ship failure
Courtesy of The Welding Institute, UK
These vessels were fabricated in the USA in the 1940s to rapidly build up the American fleet.
Some 2700 vessels were built and welding was used to replace the previous practice of riveting.
The use of welding was beneficial since it made high production rates and lower hull weight
possible. Of the 2700 ships, 400 developed cracking in the structure, 90 suffered serious
fracture, 20 suffered complete failure but, according to various reports less than 12 of the
vessels actually broke in two!
Analysis of the failures indicated that the plate used was of poor quality with ductile to brittle
transition occurring at temperatures above ambient, cracking normally started in the region of
sharp changes of section around hatch covers and often initiated at welds. The failures may
therefore be attributed to a poor appreciation of the effect of welding on materials and the need
to consider the weldability when choosing a material for a fabricated structure.
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The Kings Bridge
The Kings Bridge which spans the Yarra River Melbourne was built between 1957 and 1961
and opened to traffic in April 1961. On the 10 July 1962 whilst a low loader weighing 45 tons
was driving over the bridge a section of the structure collapsed causing a sag of around 300
mm. Subsequent examination revealed fractures of all four supporting girders in the span. The
fractures all started from the toes of welds which secured cover plates to the tension flanges of
the beams. The reasons for this cracking were reported 3 as
- high and variable carbon contents in the steel;
- low notch ductility in the material;
- inadequate preheat;
- poor weld sequence control; and
- poor electrode care.
The initial cracks originated in the heat affected zone after welding and before painting. The
cracks propagated from the flange and web of the beams during service.
The inexperience of the contractor with the material used, inadequate training and the failure
of post weld inspection were all highlighted in the enquiry.
The Alexander Kielland
The Alexander Kielland was a semi submersible offshore platform which was used as an
accommodation unit in the North Sea. The rig capsized in March 1980 and 123 of the 212
people on board were lost. The capsize was extremely rapid - taking only ten minutes from
initial failure to complete collapse. The failure analysis revealed that the cause of failure was
cracking propagating rapidly in a main sub sea brace. The cracks had however initiated from a
non load carrying sonar bracket attachment. The material of the flange which was welded to
the niobium treated steel brace was found to be of poor quality. There was evidence of both
lammellar tearing and hydrogen induced cold cracking and most of the cracks started in the
region of the 6 mm fillet weld, from root and toe regions. The failure may have resulted fromseveral different mechanisms but may again be attributed to poor control of welding and lack
of due consideration in the joining of what was obviously not considered to be an important
component
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Common factors
What all these examples share is the fact that catastrophic failure can be attributed
directly to the welding operation. In all cases the problems would have been avoided if
an appreciation of the underlying technology had been available and the correct
procedures had been adopted. Less dramatic failures occur all too often during
fabrication of welded structures. These failures may even be more costly as a whole
since they involve difficult and time consuming repair procedures.
These quality issues and structural failures may result from the complexity of welding
process control and the fact that it is very difficult to determine whether the properties of
a weld meet the expected requirements. Whilst the profile and size of a weld may be
measured and internal integrity may be checked using non-destructive testing it is often
difficult to assess potential defects and mechanical properties such as yield strength and
toughness without destructive testing. This problem is recognised in international quality
standards (ISO 9000 and ISO 3834) by nominating welding as a Special Process. Thisclassification calls for the application of process procedure control and monitoring. In
brief this entails proving the properties of a joint using a pre-production test plate which
is welded under the proposed production conditions and destructively tested. The
parameters used for a successful test plate are recorded and used in production.
Process monitoring of essential variables (e.g. of preheat, current, voltage etc.) provides
some assurance that production welds will achieve the same properties as the test plate.
This Welding Procedure Control is a major responsibility of the Welding Engineer or
Welding Coordinator and is discussed in detail in later topics of this course.
Fabricated structures - successes
Fortunately, the number of successful joining applications far outweigh the number of
failures. They range from electrical terminators on microchips to highly stressed nickel
superalloy components in aircraft engines (see Figure 1.2), a riveted structure such as
the Sydney Harbour Bridge (see Figure 1.3), oil rigs (see Figure 1.4) and large
earthmoving equipment such as walking draglines (see Figure 1.5).
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Figure 1.2 Rolls Royce Pegasus engine for the Harrier Jump Jet containingwelded nickel superalloy components
Courtesy of Rolls Royce
Figure 1.3 Sydney Harbour Bridge, a riveted structure which has been in servicesince 1932 with one of the latest welded aluminium ferries in the foreground
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Reproduced from the internet (72dpi) - no source quoted
Figure 1.4 North Sea production platform operating successfully in extremes oflow temperature and fatigue loading
Courtesy of Marathon Oil
Figure 1.5 Walking dragline weighing around 5000 tonnes. An example of a large,highly stressed steel structure
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Photography by J Norrish
Joining plays a key role in the economic fabrication of aircraft and motor vehicles as well
as common everyday objects such as cooking utensils and furniture. A wide range of
materials are also involved and joining of plastics and ceramics as well as most metals
and their alloys is widely practised.
Summary
Welding and joining processes are essential in any industrial economy. The incorrect
application of the technology may lead to unacceptable health and safety hazards as
well as costly and sometimes catastrophic failure of structures. The effective use of
joining technology relies on a good background knowledge of the underlying principles
as well as the adoption of the correct control measures. With adequate knowledge and
preparation even the most complex fabrications may be produced successfully.
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Joining processes
The basic joining processes may be subdivided into: - welding
- mechanical joining
- adhesive bonding
- brazing and soldering.
A large number of joining techniques are now available and in recent years signif icant
developments have taken place, particularly in the adhesive bonding and welding
areas. Existing welding processes have been improved and new methods of joining
have been introduced. The proliferation of techniques which has resulted makes
process selection difficult and this may limit their effective exploitation.
The following section introduces some of the basic concepts which need to be
considered and highlight some of the features of traditional welding methods.
Classification of welding processes
Several alternative definitions are used to describe a weld, for example:
A union between two pieces of metal rendered plastic or liquid by heat or pressure or
both. A filler metal with a melting temperature of the same order of that of the parentmetal may or may not be used.
or alternatively:
A localised coalescence of metals or non-metals produced either by heating the
materials to the welding temperature, with or without the application of pressure, or
by the application of pressure alone, with or without the use of a filler metals.
The important principle to note is that a metallic bond is formed across the
interfaces between the parent metal and the weld. In other words the material has a
continuous atomic structure across the weld with atoms arranged in a crystal lattice
the same as that found in the bulk material.
Based on these definitions welding processes may be classified into those which rely
on the application of pressure and those which use elevated temperatures to achieve
the bond.
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Various charts illustrating the derivation of welding processes on this basis have been
published. Many of the 40 or so processes referred to in these classifications are of little
industrial importance but a small number of them are used extensively. Some of the
most important processes are shown in Figure 1.6.
A brief description of the most common processes, their applications and limitations is
given below.
Figure 1.6 Simplified classification of important welding processes
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Figure 1.7 Oxy-acetylene welding
The low temperature of the flame (about 3000C), compared with an arc, means that a
relatively large amount of heat is transferred to the workpiece, and this may result in high
levels of distortion and thermal damage. Torch nozzles of varying size are used to
optimise the size of the flame for specific applications.
The oxy-acetylene heat source is very versatile, being capable of heating, soldering,
brazing and cutting in addition to welding, and its independence from electricity makes it a
convenient process for field situations. However, it is relatively slow, and the low flame
temperature makes it unsuitable for the welding of large or thick components.
The safety aspects of the process also merit consideration. The gases used are stored in
steel bottles, which are bulky and heavy to move. The oxygen is stored at a maximum
pressure of 200bar, so the dangers of damage to a bottle or pressure reducing regulator
are serious. Acetylene becomes unstable if compressed to high pressures, so it is stored
dissolved in acetone at the lower pressure of about 18bar. Because the gas is stored in
dissolved form, these cylinders should never be used except in an upright position.
The principal features of the oxy-acetylene process are:
- extreme versatility;
- independence from electrical supply;
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- low flame temperature, hence high heat input; and
- potential explosive and flammability hazards.
GasFuel gas /
Oxygen ratio
Heat content
MJ/m3
Flame temperature
C
Acetylene 2.5 55 3087
Propane 5.0 104 2526
Natural Gas 2.0 37 2538
Hydrogen 0.5 12 2660
Thermit welding (chemical heat source)
Thermit welding utilises the heat generated by an exothermic reaction between a metal
material.
Iron and copper oxides are the most common compounds that may be reduced in this way
and the following reactions are possible:
3Fe304 + 8AI => 4AI203 + 9Fe + 3350 kJ (Temperature > 3000C)
3CuO + 2AI => 3Cu +Al203+ 1210 kJ Equation 1.3
In practice the oxide/aluminium powder mixture is placed in a crucible above the parts to
be joined, the mixture is ignited and the molten metal formed is allowed to flow into the
joint gap. A mould retains the molten metal until it has solidified.
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Figure 1.8 Thermit welding of rail
The process has the following features: - it is simple;
- it is a `single shot' process;
- it is easily/best performed outside; and
- it can be used to join complex sections.
The limitations are:
- the need for a suitable exothermic reaction; and
- the need for a purpose designed mould.
The main application of the process are for joining steel rails, reinforcing bars and heavy
copper conductors.
Electroslag welding (electrical resistance heating)
Electroslag welding utilises the heat generated by the electrical resistance heating of a
molten slag bath to melt both the parent material and the continuously fed filler wire. The
process is normally carried out with the joint axis vertical and water cooled copper shoes
are used to retain the molten slag and the weld pool prior to solidification
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Figure 1.9 Electroslag welding
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The process features are:
- high heat input
- high productivity; and
- heavy sections may be welded vertically.
Limitations of the process include:
- thermal damage in the heat affected zone;
- the need to weld in the vertical position; and
- minimum thickness around 25 mm.
The main applications of the process have been for ship, pressure vessel and building
construction.
Gas tungsten arc welding (arc heating)
The Gas tungsten Arc Welding process (GTAW) process is also known as Tungsten Inert
Gas (TIG) in most of Europe, Wulfram Inert Gas (WIG) in Germany and is still referred to
by the original trade names Argonarc or Heliarc welding in some countries. In the GTAW
process the heat generated by an arc which is maintained between the workpiece and a
non-consumable Tungsten electrode is used to fuse the joint area. The arc is sustained in
an inert gas which serves to protect the weld pool and the electrode from atmospheric
contamination.
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- it is conducted in a chemically inert atmosphere;
- the arc energy density is relatively high;
- the process is very controllable;
- joint quality is usually high; and
- deposition rates and joint completion rates are low.
The process may be applied to the joining of a wide range of engineering materials including
stainless steel, aluminium alloys and reactive metals such as titanium. These features of the
process lead to its widespread application in the aerospace, nuclear reprocessing and power
generation industries as well as in the fabrication of chemical process plant, food processing
and brewing equipment.
Plasma welding (arc heating)
Plasma welding uses the heat generated by a constricted arc to fuse the joint area, the arc is
formed between the tip of a non consumable electrode and either the workpiece or the
constricting nozzle.
Figure 1.11 Plasma arc welding
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A wide range of shielding and cutting gases are used depending on the mode of operation
and the application.
In the normal Transferred Arc Mode the arc is maintained between the electrode and the
workpiece. The electrode is usually the cathode and the workpiece is connected to the
positive side of the power supply. In this mode a high energy density is achieved and theprocess may be used effectively for welding and cutting.
The higher energy density can permit higher welding speeds, or operation in the
`keyholing' mode, in which a relatively narrow full penetration weld is produced. The
penetration characteristics of the process can be markedly altered by variations in the arc
current, and also the flow rate and composition of the plasma gas.
The non transferred arc system, as its name implies, operates with an arc between the
electrode and the constricting orifice. The flow of plasma gas through the orifice carries the
heat to the workpiece, and this may be supplemented with a low current arc to theworkpiece, operating in the ionised atmosphere created by the non transferred arc. This
technique is often used for the welding of thin film material, and components for the
electronics industry such as capacitor cans or transistor cases.
The features of the process depend on the operating mode and the current , but in
summary the plasma process has the following characteristics:
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- good low current arc stability;
- improved directionality compared with TIG;
- improved melting efficiency compared with TIG; and
- possibility of keyhole welding.
These features of the process make it suitable for a range of applications from the joining
of very thin materials, for the encapsulation of electronic components and sensors, to high
speed longitudinal welds on strip and pipe.
Shielded metal arc welding (SMAW) (arc heating)
The Shielded metal arc welding process (SMAW) is known as MMA, Manual Metal Arc
Welding in the countries of Europe and is still referred to as Stick welding in the fabrication
industry. SMAW has for many years been one of the most common techniques applied to
the fabrication of steels.
The process uses an arc as the heat source but shielding is provided by gases generated
by the decomposition of the electrode coating material and by the slag produced by the
melting of mineral constituents of the coating. In addition to heating and melting the parent
material the arc also melts the core of the electrode and thereby provides filler material for
the joint.
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Figure 1.12 Shielded metal arc welding
The electrode coating may also be used as a source of alloying elements and additional
filler material.
The flux and electrode chemistry may be formulated to deposit wear and corrosion
resistant layers for surface protection.
SMAW is normally operated as a manual process, although automated variants do exist,
such as firecracker and gravity welding. In skilled hands, the process is capable of
producing high integrity welds in all positions on materials of all thicknesses, although
preheat may be required on thicker materials. The productivity of the process is relativelylow, due to the time required to remove slag from the weld, and the necessity to change
electrodes frequently. This can be offset by the simplicity and cheapness of the equipment,
which can allow more welders to be utilised. For these reasons the process has been
traditionally used in structural steel fabrication, shipbuilding and heavy engineering as well
as for small batch production and maintenance.
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The composition of electrode core and flux materials is classified by various authorities,
including International Standards (ISO), the American Welding Society (AWS), the ASTM
and the BSI. Relevant standards are BS639 and AWS A5.1. Manufacturer's data sheets
can also be useful in selecting appropriate consumables for a specific application.
Significant features of the process are:
- the equipment is simple;
- a large range of consumables are available;
- the process is extremely portable;
- the operating efficiency is low; and
- it is labour intensive.
Submerged arc welding (SAW) (arc heating)
Submerged arc welding is a consumable electrode arc welding process in which the arc is
shielded by a molten slag and the arc atmosphere is generated by decomposition of
certain slag constituents.
Figure 1.13 Submerged arc welding
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The filler material is a continuously fed wire and very high melting and deposition rates are
achieved by using high currents (for example, 1000 amps) with relatively small diameter
wires (for example, 4 mm).
The flux is normally in powder or granular form, and automatic systems are available to
deposit a layer of flux ahead of the welding arc, and to recover unfused flux after the weld
has been made.
Because the high operating currents generate a large molten pool, the process is normally
operated in the downhand position, and automated welding equipment is invariably used.
The main applications of submerged arc welding are on thick section plain carbon and low
alloy steels and it has been used on power generation plant, nuclear containment, heavy
structural steelwork, offshore structures and shipbuilding. The process is also used for the
high speed welding of simple geometric seams in thinner sections -for example in the
fabrication of pressure containers for liquified petroleum gas. As with manual metal arc
welding, with suitable wire/flux combinations the process may also be used for surfacing,
depositing nickel and cobalt based alloys as well as high strength steels.
The significant features of the process are:
- high deposition rates;
- automatic operation;
- no visible arc radiation;
- flexible range of flux/wire combinations;
- difficult to use positionally; and
- best for thicknesses above 6 mm.
Gas metal arc welding (GMAW) (arc heating)
Gas metal arc welding is known as Metal Inert Gas (MIG) or Metal Active Gas (MAG)
welding in Europe. The terms Semi-Automatic or CO2Welding are sometimes used but
are less acceptable. GMAW uses the heat generated by an electric arc to fuse the joint
area. The arc is formed between the tip of a consumable, continuously fed filler wire and
the workpiece and the entire arc area is shielded by an inert gas.
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Figure 1.14 Gas metal arc welding
The consumable is normally between 0.6 and 2 mm diameter, and the inert gas may be
based on argon or helium, with additions of oxygen, CO2, or other gases according to the
materials being welded. For some steel welding applications, pure CO 2 is used, this
process variant being known as MAG (metal active gas welding).
Some of the more important features of the GMAW process are summarised below:
- low heat input;
- continuous operation;
- high deposition rate;
- no heavy slag;
- reduced post weld cleaning;
- low hydrogen; and
- reduced risk of cold cracking.
Depending on the operating mode of the process it may be used at low currents for:
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- thin sheet; or
- positional welding.
The process is used for joining plain carbon steel sheet from 0.5 to 2.0 mm thick in the
following applications;
Automobile bodies, exhaust systems, storage tanks, tubular steel furniture, heating and
ventilating ducts.
The process is also applied to positional welding of thicker plain carbon and low alloy
steels in the following areas:
- oil pipelines;
- marine structures; and
- earth moving equipment
At higher current high deposition rates may be obtained and the process is used for
downhand and horizontal vertical welds in a wide range of materials. Applications include:
earth moving equipment, structural steelwork (for example, I' beam pre-fabrication), weld
surfacing with nickel or chromium alloys, aluminium alloy cryogenic vessels and military
vehicles.
Depending on the operating mode of the process it may be used at low currents for thin
materials or positional welding, for example on car bodies or metal furniture. With steel
consumables and at currents sufficiently high to achieve acceptable fusion on thicker
materials, the problem of achieving efficient transfer of metal from the electrode to the
molten pool in positions other than downhand gave rise to significant process problems.
These gave GMAW the reputation of being a low quality process, prone to lack of fusion
and bead shape defects. In recent years, the introduction of improved performance power
supplies and a better understanding of the underlying physics of the process have helped
to alleviate these problems, although these innovations have yet to be fully accepted.
Flux cored arc welding (FCAW)
A significant variant on the GMAW process is flux cored arc welding (FCAW). This usesthe same equipment as GMAW, but instead of a solid metallic electrode, a tubular
consumable is used, in which a metallic sheath is wrapped around a core containing
powdered material. This material may include alloying elements, arc stabilisers, slag
forming agents and the like. Tubular consumables used for the welding of steel can be
more easily produced with varying amounts of alloying elements such as nickel for specific
purposes, while other ingredients may be compounded to improve deposition rate, bead
shape or fusion characteristics. Some alloys, such as the cobalt based hard facing
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Safety considerations and the need for a vacuum dictate that all EB welding operations are
automated. Accelerating voltages are normally in the range 50 to 150 kV, and it should be
noted that when voltages in excess of 60 kV are used, the system must be considered
radiologically dangerous and film dosimeters should be worn by operating staff. With the
complexities of high voltage supplies, vacuum systems, workpiece manipulation and the
like, electron beam welding systems are expensive, many costing hundreds of thousands
of dollars. For this reason, the range of application of the process is largely governed by its
economics. It is mainly employed in applications where its repeatability and controllability
override economic factors (aerospace, defence), or where its high welding speed can be
utilised to amortise high operating costs over a large throughput of components
(electronics, automotive).
Because electron beam welds are very narrow compared with their depth, distortion levels
are very low. This enables components to be finish machined and then welded together, a
capability which enabled innovative design compromises to be made on products such as
the Rolls Royce RB211 aero engine and the Borg Warner T5 automotive transmission.
Although principally used for welding, EB techniques have also been used for cutting,
drilling and surface heat treatment.
Features of the process include;
- very high energy density;
- confined heat source;
- high depth to width ratio of welds;
- normally requires a vacuum; and
- high equipment cost.
Applications of electron beam welding have traditionally included welding of aerospace
engine components and instrumentation but it may be used on a wide range of materials
when high precision and very deep penetration welds are required.
Laser welding (power beam)
The term LASER is an acronym for Light Amplification by Stimulated Emission of
Radiation. The laser beam is an intense monochromatic photon source which may be
focussed to give very high energy densities. The beam is produced by stimulating a gas
mixture in a CO2 laser or activating a solid lasing material such as yttrium aluminium
garnet in the YAG laser.
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The laser may be used as an alternative heat source for fusion welding The focused power
density of the laser can reach 1010 or 1012 watts/m2 and welding is often carried out using
the keyhole' technique.
Laser welding can operate in the atmosphere, and it is also easier to direct the output of a
laser to multiple welding stations, improving utilisation. The most significant recent area of
innovation in laser technology is the development of solid state lasers capable of operating
at in excess of 4kW average output power. In a similar manner to EB, lasers are also used
for cutting, drilling, heat treatment and surface cladding. Again, process economics will
dictate the principle areas of application.
Significant features of laser welding are:
- very confined heat source at low power;
- deep penetration at high power;
- reduced distortion and thermal damage;
- out of vacuum technique; and
- high equipment cost.
These features have led to the application of lasers for microjoining of electronic
components but the process is also being applied to the fabrication of automotive
components and precision machine tool parts in heavy section steel.
Welding with pressure
In contrast to the fusion processes described above, pressure welding techniques, as their
name implies, rely on forging the components to be joined together, normally at an
elevated temperature. In general, little or no melting takes place in the joint region.
Although some of these processes have been in use for many years, they have returned to
prominence with the introduction of new types of engineering materials, the properties of
which are achieved by careful control of the thermal cycle during manufacture. Such
materials cannot normally be welded by conventional methods, and pressure techniquesare thus the only appropriate welding processes.
Cold pressure welding
If sufficient pressure is applied to the cleaned mating surfaces to cause substantial plastic
deformation the surface layers of the material are disrupted, metallic bonds form across
the interface and a cold pressure weld is formed6. The main characteristics of cold
pressure welding are:
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- the simplicity and low cost of the equipment;
- the avoidance of thermal damage to the material; and
- most suitable for low strength (soft) materials.
The pressure and deformation may be applied by rolling, indentation, butt welding, drawingor shear welding techniques. In general the more ductile materials are more easily welded.
The process has been used for electrical connections between small diameter copper and
aluminium conductors using butt and indentation techniques. Roll bonding is used to
produce bimetallic sheet such as Cu/AI for cooking utensils, AI/Zn for printing plates and
precious metal contact springs for electrical applications.
Resistance welding
The resistance welding processes are commonly classified as pressure welding processes
although they involve fusion at the interface of the material being joined. Resistance spot,
seam and projection welding rely on a similar mechanism. The material to be joined is
clamped between two electrodes and a high current is applied. Resistance heating at the
contact surfaces causes local melting and fusion. High currents (typically 10,000 amps)
are applied for short durations and pressure is applied to the electrodes prior to the
application of current and for a short time after the current has ceased to flow.
Figure 1.16 Resistance welding
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Accurate control of current amplitude, pressure and weld cycle time are required to ensure
consistent weld quality is achieved but some variation may occur due to changes in the
contact resistance of the material, electrode wear, magnetic losses or shunting of the
current through previously formed spots. These `unpredictable' variations in process
performance have led to the practice of increasing the number of welds from the design
requirement to give some measure of protection against poor individual weld quality.
Significant developments have however recently been made in resistance monitoring and
control, these allow more efficient use of the process.
Most industrial spot welding is carried out using automated or robotic systems, those
employed as part of automobile bodyshell construction lines are probably the most well
known.
Features of the basic resistance welding process include:
- the process requires relatively simple equipment;
- it is easily and normally automated; and
- once the welding parameters are established it should be possible to produce
repeatable welds for relatively long production runs.
The major applications of the process have been in the joining of sheet steel in the
automotive and white goods manufacturing industries.
Friction welding
In friction welding a high temperature is developed at the joint by the relative motion of the
contact surfaces. When the surfaces are softened a forging pressure is applied and the
relative motion is stopped. Material is extruded from the joint to form an upset, and a solid
phase bond is formed.
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Figure 1.17
The power required during the heating phase is high, and to improve system efficiency a
flywheel is sometimes used to store energy in the interval between welds which is
dissipated during this phase of joint production.
The process may be divided into several operating modes in terms of the means of
supplying the energy.
Continuous drive. In which the relative motion is generated by direct coupling to the
energy source. The drive maintains a constant speed during the heating phase.
Stored energy. In which the relative motion is supplied by a flywheel which is disconnected
from the drive during the heating phase.
Rotational motion is the most commonly used mainly for round components where angular
alignment of the two parts is not critical. If it is required to achieve a fixed relationship
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between the mating parts angular oscillation may be used and for non circular components
the linear and orbital techniques may be employed.
Features of the process include:
- one shot process for butt welding sections;
- suitable for dissimilar metals;
- short cycle time;
- most suited to circular sections; and
- robust and costly equipment may be required.
The process is commonly applied to circular sections particularly in steel but it may also be
applied to dissimilar metal joints such as aluminium to steel or even ceramic materials to
metals. Early applications of the process included the welding of automotive stub axles but
the process has also been applied to the fabrication of high quality aero engine parts7
duplex stainless steel pipe for offshore applications8 and nuclear components9.
Recent developments of the process include the joining of metal to ceramics10, the use of
the process for stud welding in normal ambient conditions and underwater, and the use of
the process for surfacing11.
The linear technique has recently been successfully demonstrated on titanium alloy welds
having a weld area of 250 mm2 using an oscillation frequency of 25 KHZ, 110N/mm2 axial
force and an oscillation amplitude of 2 mm12.
A major development in this area has been the introduction of Friction Stir Welding, a
process developed by The Welding Institute in the UK.
Diffusion bonding
In diffusion bonding the mating surfaces are cleaned and heated in an inert atmosphere.
Pressure is applied to the joint and local plastic deformation is followed by diffusion during
which the surface voids are gradually removed13. The components to be joined need to be
enclosed in a controlled atmosphere and the process of diffusion is time and temperaturedependant. In some cases an intermediate material is placed between the abutting
surfaces to form an interlayer.
Significant features of the process are:
- suitable for joining a wide range of materials;
- one shot process;
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- complex sections may be joined;
- vacuum or controlled atmosphere required; and
- prolonged cycle time.
The process can however be used for the joining of complex structures which requiremany simultaneous welds to be made.
Explosive welding
In explosive welding the force required to deform the interface is generated by an
explosive charge. In the most common application of the process a two flat plates are
joined to form a bimetallic structure. An explosive charge is used to force the upper or
`flier' plate onto the baseplate in such a way that a wave of plastic material at the interface
is extruded forward as the plates join.
Figure 1.18 Explosive welding
For large workpieces considerable force is involved and care is required to ensure the safe
operation of the process. Features of the process include:
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- one shot process short welding time;
- suitable for joining large surface areas;
- suitable for dissimilar thickness and metals joining; and
- careful preparation required for large workpieces.
The process may also be applied for welding heat exchanger tubes to tube plates or for
plugging redundant or damaged tubes.
Magnetically impelled arc butt welding (MIAB)
In MIAB welding a magnetic field generated by an electromagnet is used to move an arc
across the joint surfaces prior to the application of pressure14.
Figure 1.19 MIAB welding
Although the process produces a similar weld to friction welding it is possible to achieve
shorter cycle times and relative motion of the parts to be joined is avoided. Features of the
process are:
- one shot process;
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- suitable for butt welding complex sections; and
- shorter cycle time than friction welding.
The process has been applied fairly widely in the automotive industry for the fabrication of
axle cases and shock absorber housings in tube diameters from 10 to 300 mm and
thicknesses from 0.7 to 13 mm15.
Flash butt welding
In flash butt welding a high current arc is passed between and across the abutting edges,
resistive preheating occurs followed by a period of arcing or `flashing' between the ends of
the component. On application of the upset force the two heated ends are forged together
and molten metal and oxides are extruded out from the joint edges. The features of the
process are:
- only suitable for butt welds;
- rapid;
- little preparation required;
- expensive equipment; and
- excessive sparking/spatter.
The main applications of the process have been for rail welding, wheel rims and chain
links.
Other joining processes
Mechanical joining
Mechanical joining techniques include:
- bolting
- riveting
- crimping.
Many of these techniques enable temporary connections to be made and this is useful for
structures which need to be assembled and disassembled, for example friction grip bolts
are often used for site connections of large structural fabrications. Most of the methods
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use a lap type seam and this requires excess material in the joint area and may involve an
increase in weight and potential corrosion and stress intensification.
Mechanical joining is however useful if there is a risk of metallurgical damage to a material
from the thermal cycle experienced in many welding operations. This is one of the reasons for
the extensive use of mechanical fasteners in aircraft structures.
Stud welding
Stud welding encompasses a range of welding methods which are used to attach
secondary fixing devices or brackets. The welding processes used include; arc, friction,
cold pressure and resistance welding techniques.
The arc stud welding processes utilises an arc which is initiated between the stud and the
metal surface to which it is to be attached. Arc initiation is normally achieved by short
circuiting and then withdrawing the stud. The arc melts an area on each electrode and
after a short heating cycle the stud is pressed onto the surface, extinguishing the arc andforming a weld. The power supply may take the form of either a DC source similar to those
used for other arc processes or a stored energy (capacitor discharge) device. It is common
to use a ceramic sleeve around the end of the stud to protect the weld area and reduce arc
radiation. Secondary inert gas shielding may also be used for some materials.
In the case of resistance stud welding a projection welding technique is often employed.
Stud welding is used extensively in sheet metal fabrication but is also applied in heavy
structures for example in the fixing of cladding attachments or hangers for corrosion
protection systems.
Brazing and soldering
Brazing and soldering use a filler material which has a lower melting point than the parent
material to form a joint. For structural applications both methods require a lap or capillary
type joint design. Filler materials with melting points below 500C are normally used for
soldering whilst materials which melt at temperatures between 500C and the melting point
of the parent material are used for brazing. Soldering alloys are commonly based on the
lead/tin system whilst brazing materials are often copper based. The temperature
resistance of such joints is limited by the melting point of the filler and the mechanical
strength is a function of interface area.
Common heat sources include air/fuel gas, oxy-fuel gas, electric furnace or electrical
induction heating. Soldering is commonly used for electrical and electronic assemblies as
well as joining copper water systems. Brazing may be applied to a wide range of materials
including joints between ceramics and metals and other dissimilar materials.
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Adhesive bonding
Adhesive bonding is similar to brazing and soldering in terms of its use of a non matching
joining material and lap joint configuration, however in this case an organic bonding
medium is used. Bonding is provided by polar and Van de Waals forces across the
adhesive/metal oxide/metal interface.
The use of adhesives avoids thermal damage and allows dissimilar materials (including
non metals) to be joined. The maximum operating temperature of most adhesives is
however quite low and again mechanical properties depend on the bonded area.
A very wide range of adhesives is available and careful selection is essential if the desired
service performance is to be achieved. Surface pre-treatment also has a major influence
on the strength of the bond.
Future developments
Some probable future developments have been mentioned in the main body of the text.
Although relatively few new processes were expected to emerge, there have been some
significant process developments recently. Friction stir welding offers great potential for
low distortion welding of aluminium and the high power diode laser and microwave plasma
jets are potential alternative heat sources for fusion welding. It seems likely that the next
few years will however see the continuing introduction of automated and robotic welding
systems, for reasons of productivity and consistency, and also to distance human staff
from hazardous environments. These systems will become much more flexible, and will
begin to incorporate control systems utilising artificial intelligence techniques to make them
more capable of responding to changes in the welding situation.
As the use of quality assurance systems becomes more widespread, the monitoring and
analysis of welding operations will become more sophisticated. On line analysis of welding
parameters will be used to provide data both for process control and also for the quality
assurance system.
Until recently, the vast majority of welding engineers had initially trained as metallurgists,
and most welding problems were discussed from a metallurgical or materials science
orientation. With the introduction of a wide range of electronically controlled power
supplies, the growing use of computer based control systems for automated and roboticwelding facilities, and the increasing implementation of data logging and analysis systems
for quality assurance and production control, it seems likely that a change of emphasis is
required. The fabrication industry in general will require many more of its staff to be
computer literate, and to be capable of making the increasingly complex decisions which
result from the wide range of engineering materials, fabrication techniques and quality
assurance systems which are currently being introduced into industry. If industry is to
remain competitive internationally, the rate of change in manufacturing and management
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methods will have to be faster over the next few years than at any time since the beginning
of the Industrial Revolution.
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Questions/tasks
(a) How is the heat required for fusion welding developed in oxy-fuel welding? List the
relative merits and limitations of three alternative fuel gases.
(b) What are the basic principles and main applications of resistance spot welding?
(c) What are the two main groups into which welding processes are classified? Give
three examples from each group.
(d) What is a weld? How does it differ from a brazed or adhesively bonded joint?
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References
1. John, R. and Jones, S. The Economic Relevance of Materials Joining Technology,
The Welding Institute.
2. Homer, The Illiad.
3. Report of the Royal Commission into the failure of Kings Bridge, 1963, Victoria, A.
C. Brooks, Melbourne.
4. American Welding Society, Welding Handbook, vol. 1, Welding Technology, 8th
edn, ISBN 0-87171-281-4.
5. British Standard BS 499, 1983, Part 1, Welding Terms and Symbols, Glossary of
welding, brazing and thermal cutting.
6. Bay, N. June, August, October 1986, Cold Welding, Parts 1-3, Metal Construction,18 (6, 8, & 10).
7. Benn, B. August, September 1988, Friction Welding of Butt Joints for High Duty
Applications, Welding and Fabrication.
8. Nicholas, E. D., Teale, R. A. 2-5 May 1988, Friction Welding of Duplex Stainless
Steel, Offshore Technology Conference, Houston, Texas.
9. Nicholas, E. D. January 1982, A friction welding application in the nuclear power
industry, The Welding Institute Research Bulletin, 23 (1).
10. Essa, A. A. & Bahrani, A. S. 19-22 March 1989, The Friction Joining of Ceramics to
Metals, Procedures International Conference on The Joining of Materials, JOM-4,
Helsingor, Denmark.
11. Thomas, W. M. et al. 1984, Feasibility studies into surfacing by friction welding,
TWIResearch Report 236, The Welding Institute Cambridge.
12. Nicholas, D., Watts, E. April 1990, Friction Welding-a sparkling success, Number 8,
The Welding Institute, Connect.
13. Bartle, P. M. March 1983, Diffusion Bonding-Principles andApplications, The
Welding Institute Research Bulletin, 24 (3).
14. Johnson, K. I. et al. November 1979, MIAB welding, Principles of the process,
Metal Construction, 11 (11).
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15. Edson, D. A. 13-15 September 1983,Application of MIAB Welding, Proceedings on
Conference Developments and Innovations for Improved Welding Production, The
Welding Institute, Birmingham, England