01lecture01b introduction readings

7/31/2019 01Lecture01B Introduction Readings

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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.




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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


http://www.twi.co.uk/home.htmlhttp://www.igr.nl/-henkbodt/nil/index-e.htmhttp://www.igr.nl/-henkbodt/nil/index-e.htmhttp://www.twi.co.uk/home.html


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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

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