Mechanical Engineer''''s Reference Book 2011 Part 16 pps

If two plates are placed in contact a close square butt joint they can be welded with full penetration by one run of weld metal deposited by a manual welding process from each side provi

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16/55

16.3.1.1 Types of joint

The types of joint used and their associated weld types are

described in detail in BS 499." The commonest joint types are

butt, T, corner and lap

Butt joints These are joints between parts that are generally

in line If two plates are placed in contact (a close square butt

joint) they can be welded with full penetration by one run of

weld metal deposited by a manual welding process from each

side provided that the plate thickness does not exceed approxi-

mately 8 mm (Figure 16.72) However, plate above 6 mm

thickness is generally bevelled and the Vee edge preparation

formed is filled by depositing a number of runs of weld metal

If high-current mechanized welding processes are used pene-

tration of the weld may be at least double the above dimen-

sions and for electron beam welding may be many times as

high

T joints and coiner joints The parts may be joined by fillet

welds or butt welds made by an arc welding process (Figure

16.73)

Lap joints These are commonly used for sheet metal up to

about 3 mm thick in which one sheet is overlapped by another

This type of joint is used for soldering, brazing, resistance spot

or seam welding, and for arc spot welding, plug welding, as

well as for adhesive bonding For material of 3 mm or thicker

(even up to 10 mm) lap joints are occasionally used and fillet

welds are deposited at the plate edges by arc welding

16.3.1.2 Welding processes

The various welding processes can be used to join the majority

of metallic materials, whether in cast or wrought form, in

thickness from 1 mm or less up to 1 m or more A simple

classificati'on of welding processes is shown in Figure 16.74

For a complete classification and for definitions of the pro-

cesses BS 499: Pt 1" should be referred to

A description of the welding processes is as follows

16.3.1.3 Manual metal arc welding

Manual mietal arc welding (referred to in the USA as shielded

metal arc welding) is the most widely used process and

accounts for approximately 50% of all the welding in the world

Figure 16.72 Edge preparation for butt welds (a) Square edge; (b)

single bevel

Fillet welds B u t t welds

Fillet weld B u t t weld

Figure 16.73 Examples of welded joinls

today With this process, welding is carried out with flux- coated electrodes which are connected via an electrode holder and length of cable to one terminal of a welding power source, such as an a.c transformer or a d.c generator (Figure 16.75) The other terminal of the power source is connected to the work piece via the earth return or the ground cable, so that when the end of the electrode is placed in contact with the work piece, electric current flows through the circuit By withdrawing the tip of the electrode to about 3 mm from the work piece an arc will be struck and current will continue to flow in the circuit and pass through the arc which is electrically conductive

If an arc is maintained between a rod-type electrode and plates to be welded together the tip of the rod becomes molten and so does a portion of the plates (the fusion zone) Gravity causes drops of molten metal to drip onto the plate and form a weld (Figure 16.76)

Apart from gravity, other forces caused by electromagnetic effects propel molten metal globules across the arc and these forces always transfer metal from the rod to the plate, whether a.c or d.c is used and whether the polarity is electrode positive or negative They will also transfer the metal against the force of gravity, so that vertical or overhead welding is possible

Electrodes These have core wire diameters from 2.4 mm to

10 mm and are 300-450 mm in length The deposition rate of weld metal, which governs the overall rate of welding, increases with the current and has a maximum value for each electrode length and diameter Exceeding the maximum current causes overheating of the electrode core wire by resistance heating, which can damage the electrode coating Welding currents vary from 60 A for the sniallest electrodes

up to 450 A for the largest The highest currents and deposition rates can only be used when welding downhand, Le in the flat position Vertical and overhead welding can be used with

electrodes having diameters up to 5 mm with maximum cur-

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Arc G a s Electron E lectro-slag Laser

welding

r - - l

I

metal-arc welding inert-gas ( M I G ) active-gas

Figure 16.74 Classification of principal welding processes

Electrode holder Power source and controls

Electrode

'7 l,lL 1

Work piece E a r t h return cable

Figure 16.75 Welding circuit for manual metal-arc welding

Direction of Globules of molten metal and slag

Molten weldpool

Figure 16.76 Manual metal-arc welding with covered electrode

droplets from oxygen and nitrogen in the air as they are transferred through the arc

To provide a slag which protects the hot, solidifying metal from oxidation The characteristics of the slag (e.g melting point, surface tension and viscosity) determine the shape of the weld bead and the suitability of the electrode for positional welding

To supply alloying elements to the weld metal; this means that an inexpensive rimming steel core wire can be used for many different weld metal compositions

The constituents of the flux covering are mixed together in dry powder form and then binding agents are added The flux paste is pressed into the form of slugs and loaded into machines which extrude the covering round electrode core wires as they pass at high speed through a die of appropriate size The electrodes are then dried as they pass through ovens and are stamped with identification marks before being packed

The classijication of flu coverings The development of flux coverings, consisting of mixtures of various minerals, has followed fairly well-defined lines with slight variations between different manufacturers and in different countries

Electrodes can be classified according to their coating types

and for a full description BS 639: 1986'l and the American standard AWS A5.1-81" should be consulted

Steel electrode types are designated by letters in BS 639 and the main characteristics of the different electrodes are as follows:

1 R (rutile): Rutile coverings containing a high proportion

of titanium dioxide in the form of the mineral rutile or ilmenite The electrodes are easy to use but produce weld metal having high hydrogen contents which can cause cracking of the weld or parent metal heat affected zone in heavily restrained joints

RR (rutile, heavy coated): The thick covering enables the electrodes to be used as contact electrodes which can be

3

4

2

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Welding, soldering and brazing 16/57 long life The development of d.c solid-state power sources has eliminated the moving parts and maintenance costs associated with rotary generators and has also reduced the capital costs Therefore the use of d.c welding is likely to increase in future, particularly in view of the fact that welding is easier because of the more stable arc and ability of d.c to weld non-ferrous alloys and to produce the highest quality welds in stainless steel

For use in repair work (for example, in the garage trade and for the DIY market) a number of Pow-power welding sets are available which operate from the single-phase 220/240 V supply These power sources can be used with electrodes up to

3.25 mm diameter and both a.c and d.c units are available For the home market, video instruction is available.14

Applications The main reasons for the popularity of manual metal arc welding are its versatity, simplicity of operation and the relatively low cost of equipment The process can be used with equal facility in a workshop or on-site To weld on a remote part of a structure it is possible, within reason, to lengthen the cables from the power source and when the limits

of the extended cables are reached the power sources are readily transported by crane or motor vehicle or by manhand- ling on level sites Movement of the power sources is facili- tated by their simplicity and robustness

The range of thicknesses welded varies from less than 2 mm

in the fabrication of sheet metal ventilation ducting to 75 mm and above in the production of nuclear containment pressure vessels These two examples are indicative of the wide range

of quality standards that may be required, from general sheet metal work up to the highest possible standards of radiogra- phic soundness and mechanical properties

Metals that are most commonly fabricated by manual metal arc welding are carbon and carbon manganese steels, low- alloy steels and stainless steels of both the corrosion- and heat-resisting types By selection of suitable electrodes described in various standards" I5-I8 the mechanical properties of the weld metal in respect of strength, ductility and toughness match those of the parent plate at ambient temperature and also at elevated or subzero temperatures as required References to American Welding Society (AWS) specifica- tions are included because of their worldwide use in the oil and petrochemical industries

Non-ferrous metals such as nickel, copper and aluminium and their alloys are welded much more extensively with the gas shielded processes, although nickel and nickel alloys are readily welded by the manual metal arc process and a wide range of electrodes are available.19-21 Some tin-bronze (copper-tin), and aluminium-bronze (copper-aluminium) electrodes are manufactured, but their main use is for repair work, particularly of castings (e.g marine propellers) These can also be used for welding pure copper, because the high conductivity of copper has prevented the successful production of a copper electrode Pure copper is generally used for its high thermal or electrical conductivity, and therefore the application of copper alloy electrodes is strictly limited to those circumstances where weld metal, having low thermal or electrical conductivity, is satisfactory Some non-ferrous electrodes based on nickel, nickel-iron or nickel-copper alloys are used for welding the cast irons.22

A wide range of electrodes is available for hard-surfacing components to increase their wear resistance under conditions

of abrasion, impact, heat or corrosion or various combinations

of these factors Electrodes for hard surfacing are manufactured from core wires of mild steel, carbon and alloy steels, stainless and heat-resisting steels, nickel-chromium and cobalt-tungsten-chromium alloys and are also made from steel tubes containing granules of refractory metal carbides such as tungsten and chromium carbides 23

held i n contact with the parent plate and dragged along

the joint at high welding speed Iron powder is often

added to the coating to increase the deposition rate, and

the RIR type electrodes are not suitable for welding in the

vertical and overhead position

B (bmic): A basic covering usually has a high content of

limestone (calcium carbonate) and fluorspar (calcium

fluoride) Basic covered electrodes are often referred to as

low hydrogen because they were developed to produce

weld metal having a low hydrogen content which reduces

any tendency to hydrogen-induced cracking This cov-

ering deconposes to give a gas shield containing a large

proportion of carbon dioxide These electrodes are used

extensively because of their ability to weld medium- and

high-tensile steels as well as high-sulphur (free-cutting)

steels without solidification cracking of the weld metal and

also because, by suitable drying treatment, the moisture

content of the flux covering can be reduced so that the

weld metal hydrogen content will be correspondingly low

This gives insurance against hydrogen-induced cracking of

both ithe weld metal and the heat-affected zone (HAZ)

Properly designed basic covered electrodes produce weld

metal which has the highest fracture toughness properties,

and they have the advantage over other types of elec-

trodes in that high fracture toughness is maintained in all

welding positions

B B (,basic high efficiency): These are similar to basic

covered electrodes but have iron powder added to the

coating so that the quantity of weld metal deposited is at

least 130% of the weight of the core wire The high

depmition rates make these electrodes unsuitable for

welding in the vertical and overhead positions

C (cellulosic): This designation indicztes a covering which

has a high content of cellulosic material These electrodes

operate at a high arc voltage, which gives a deep penetrat-

ing arc and rapid burn-off The covering forms a volumi-

nous ;gas shield, consisting chiefly of carbon monoxide and

hydrogen, and a small volume of slag which facilitates

work involving changes in welding position such as pipe

welding for which these electrodes are particularly suit-

able They can also he used for the fast vertical down

welding of vertical seams in storage tanks up to about

12 mm thick Because of the excellent penetration the

root does not require gouging before making a sealing run

on the reverse side In pipe welding the close control of

penetration is necessary because the deposition of a

sealing run on the inside is generally impossible

Power sources There are basically three types of power

source:

1 A.C generators

2 D.C rotary generators

3 D.C solid state

The choice of power source is described by John and Ellis.I3

A fourth type of power source of recent development is

based on an invertor which is useNd to convert the mains

frequency from 50 Hz to between 5 and 25 kHz Transformers

for currents operating at these high frequencies are much

Iighier than those used in conventional a.c generators In the

invertor type power source the a.c mains input is rectified to

give d.c which is then fed to an invertor which converts it back

to a high-frequency a.c The power is then reduced to the

welding voltage by a lightweight transformer and it is rectified

again to c1.c for welding

In Brhtin transformers have traditionally been the most

widely used type of power source for manual metal arc

welding because of their relative cheapness, reliability and

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16/58 Manufacturing methods

16.3.1.4 Gravity weldinE unmelted flux is collected and re-used The electrode wire, This is a semi-mechanized welding process which is used

principally in shipyards for fillet welding stiffeners to hori-

zontal plates Covered electrodes of the contact type, typically

600 mm long, are supported by an electrode holder which

slides down one arm of a tripod The other end of the

electrode is positioned in the corner of a T-joint to be welded

and when the curreht is switched on an arc is initiated and the

electrode moves along the joint line as the electrode holder

slides down the arm of the tripod under the force of gravity

One person can operate three gravity welding units simulta-

neously, thus trebling the rate of manual welding

16.3.1.5 Open-arc automatic welding

Although no longer used, it is appropriate to mention a

mechanized welding process which was employed extensively

in the 1950s up to the 1970s in ship building and bridge

building The engineer may find the process referred to in

periodic inspection reports of these types of welded structures

Mechanized welding with covered electrodes was carried

out with a continuous coiled electrode having a core wire from

4 to 8 mm diameter The core wire was wrapped helically with

two thin wires about 1 mm diameter, which anchored the

extruded flux in place and also acted as a means of conducting

electrical current from the jaws of the welding head to the core

wire

Automatic open-arc welding with continuous covered elec-

trodes has now been superseded by the submerged-arc pro-

cess

16.3.1.6 Submerged-arc welding

This is the most widely used mechanized welding process

(Figure 16.77) A bare wire (1.15-6.3 mm diameter but usually

3.25 or 4 rnm) is fed from a coil and an arc is maintained

between the end of the wire and the parent metal As the

electrode wire is melted, it is fed into the arc by a servo-

controlled motor which matches the wire feed rate to the

burn-off rate so that a constant arc length is maintained

The region of the joint is covered with a layer of granular

flux approximately 25 mm thick, fed from a hopper mounted

above the welding head The arc operates beneath this layer of

flux (hence the name ‘submerged arc’) Some of the flux melts

to provide a protective blanket over the weld pool and the

Wire feed nozzle 1 I

i

i Arc and molten pool hidden beneath flux

Work piece

Figure 16.77 Submerged-arc welding

welding head, wire drive assembly and flux hopper are mounted on a traverse system which moves along the work piece as the weld metal is deposited

The traverse system may consist of a carriage mounted on a boom or it may be a motorized tractor either on rails or running freely with manual adjustment to follow the weld seam Alternatively, the welding head can remain stationary while the work piece is moved This method is used for welding the circumferential seams of a pressure vessel while it

is rotated under the welding head

Electrode wires The electrode for submerged-arc welding is a bare wire in coil form usually copper coated Two types are available - solid wire or tubular wire The solid wire is widely used for general fabrication of mild and low-alloy steels, stainless steels and non-ferrous metals For welding mild and low-alloy steels it is either a low-carbon ultra-low-silicon steel

or a silicon-killed steel with manganese addition and sometimes low-alloy additions, the selection of either type depending upon the type of flux to be used with it (Le a flux with manganese or manganese and alloy additions or a neutral flux, respectively) The tubular wire (made by forming narrow strip into a tube) carries alloy powders which permit the economical production of a wider range of weld compositions than is possible by using the solid wire type Tubular wires are widely used for hard-facing Wire compositions for weldin carbon steel and medium tensile steel are listed in BS 4165

With coated manual electrodes, wire and coating are one unit so that such electrodes can be classified according to the type of coating and its effect on weld mechanical properties In

submerged-arc welding, any wire may be used with a number

of different fluxes with substantially different results in respect

of weld quality and mechanical properties Consequently, BS

4165 grades wire flux combinations according to the tensile and impact strengths obtained in the weld metal

A number of tubular wires are available, particularly for surfacing and hard-facing These contain alloy powders which produce weld metals consisting of low-alloy steels, martensitic and austenitic stainless steels, chromium and tungsten carbides, and various cobalt- and nickel-based heat- and corrosion-resistant alloys Some corrosion-resistant alloys, including stainless steel, are available in the form of coiled strips from 100 mm to 150 m m wide, 0.5 mm thick for high deposition rate surfacing by a submerged-arc welding process known as strip cladding

Fluxes Two main types of fluxes are available: fused and agglomerated Fused fluxes are manufactured by fusing together a mixture of finely ground minerals, followed by solidifying, crushing and sieving the particles to the required grain size Fused fluxes do not deteriorate during transporta- tion and storage and do not absorb moisture Agglomerated fluxes are manufactured by mixing finely ground raw materials with bonding agents such as sodium or potassium silicates

followed by baking to remove moisture This type of flux is

sensitive to moisture absorption and may require drying before use Agglomerated fluxes are more prone to mechanical damage which can cause segregation of some of the constituents

Fluxes are classified as acid, neutral, or basic, the last being subdivided into semi-basic or highly basic The main characteristics of the fluxes are as follows:

1 Acid fluxes: High content of oxides such as silica or

alumina Suitable for high welding currents and fast travel speeds Resistant to porosity when welding rusty plate Low notch toughness Not suitable for multipass welding

of thick material

$5

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Welding, soldering and brazing 16/59 saving in quantity of weld metal used is considerable, with consequent increase in productivitỵ28

2

3

Neuiral fluxes: High content of calcium silicate or

alumina-rutilẹ Suitable for fairly high welding currents

and travel speeds and also for multipass welding

Bask fluxes: High content of chemically basic compounds

such as calcium oxidẹ magnesium oxide and calcium

fluoridẹ Highest weld metal quality in respect of rad-

iographic soundness and impact strength Lower welding

currents and travel speeds are suitable for multipass

welding of thick sections For further information on

fluxes referecces 25 and 26 should be consulted

Power sources Either d.c or ạc may be used D.C may be

supplied either by a motor generator or by a rectifier, either of

which can have flat or sloping current voltage characteristics

These are referred to as constant voltage or constant current

types Generators will deliver up to about 650 A continuous

output, rectifiers up to about 1200 A at 100% duty cyclẹ ẠC

is supplied by welding transformers which are designed to give

a drooping characteristic Transformer output may range up to

2000 A

Power sources are of rugged construction and are designed

for the 100% duty cyclẹ With the constant current type the

arc voltage determines the wire feed rate which varies to

maintain a constant arc length The constant voltage type of

power source produces a self-adjusting arc in which an

increase in arc length or arc voltage causes a decrease in

current ;and burn-off rate so that the original arc length is

rapidly obtained A decrease in arc length increases the

current and burn-off rate and this self-adjusting effect occurs

with a constant wire feed ratẹ

Application As the process operates with a continuous coil of

electrode wire, butt welds in the flat position requiring mul-

tiple runs to fi!! the joint can be made with minimal stops and

starts Thus circumferential joints in cylindrical bodies such as

pressure vessels pipes, etc can be made with one stop and

start per revolution of the work piecẹ this stop being necess-

ary to reset the position of the welding head Consequently,

the possiibility of stop and start defects is minimized: a most

important consideration when reliability in costing is required

Aithough most widely applied to welding of joints in mild

steel, lovv-alloy high-tensile steel, creep-resisting steels and, to

a lesser extent, stainiess steels, it is also widely used for

building- up work, either for reclamation or replacement of

defective parent metal or for hard-surfacing

Submerged-arc welding is suitable for welding material from

5 mm to 300 rnm and even thicker but plates less than about

10 mm t’hick are generally welded by the gas shielded or flux

cored arc welding process A semi-automatic variant of the

process is available in which the welder manually manipulates

a welding gun on which is mounted a small hopper containing

the flux Electrode wire is fed to the gun from a coil by a wire

feed unit This process which is used only to a limited extent,

Is sometimes referred to as ‘squirt’ welding

Other variations of the submerged-arc welding process are

mainly concerned with increasing deposition rates and there-

fore welding speed and productivitỵ These include:

1 Increasing the electrode extension or stick-out by up to

150 imm by using an insulated guide tubẹ The resistance

of thie wire increases the burn-off rate by the I’R heating

effect

Ađition of iron powder to the joint which increases the

weld volumẹ

The use of multiple wire techniques ir: which two or more

wires are used with two or three separate power sourcệ^'

Narrow gap welding of plates more than 100 mm thick in

which a parallel gap of 14-20 mm between the square

edges of the plates is used instead of a V or U groovẹ The

The joint is set up with a wide gap (approximately 25-36 mm, depending on plate thickness), the very large weld pool being contained in the joint by water-cooled copper shoes One, two or three wires are fed into the joint with or without a reciprocating motion to ensure uniform heat genera- tion, the plate thickness determining the number of wires One welding head with three wires can weld plate 450 mm thick The copper shoes rise up the joint to prevent spilling of metal and slag and these shoes form part of the welding head

A variation of the electro-slag process is known as consumable guide welding Here the welding head remains stationary and feeds one or more wires down a tube which melts into the pool The equipment is substantially cheaper than the conventional slag welding machines

The consumable guide process, while theoretically suitable for very thick sections, is more usually applied to metal up to, say, 50 mm because it is more manageable in these thicknesses The equipment needed is much simpler than for the conventional process consisting of a constant potential generator, rectifier or transformer with a flat characteristic and a wire feed unit These are also the essential ingredients of submerged-arc equipment and, by slight modification, they can be adapted for consumable guide welding

The ultra-slow cooling rate of an electro-slag or consumable guide weld minimizes hydrogen cracking susceptibility in the parent steel and weld metal but produces a large grain size weld This tends to give a comparatively poor notch impact strength as determined by conventional tests Consequently, for pressure vessel application, current codes require norma- lizing after welding

Fluxes Electro-slag welding fluxes produce complex silicate slags containing S O 2 , MnO, CaO, MgO and A1203 Calcium fluoride is ađed to increase electrical conductivity and Iower slag viscositỵ Slags based on CaF2-Ca0 have a strong desul- phurizing action which assists the welding of steels higher in carbon than 0.25% without solidification cracking in the weld metal Fluxes must be kept drỵ

Economics of the process On heavy steel plate, for pressure

vessels, boiler drums, etc the actual welding speed (rate of filling the joint) is about twice as fast in 40 mm plate, four times in 90 mm and eight times in 150 mm compared with

multi-run submerged arc welding Welding speed is 1-1.7 rn

per hour Plate-edge preparation by bevelling is also avoided

On this evidence it would seem highly attractive economicallỵ However, ‘setting up’ the machine and selecting the correct welding parameters is a a matter of experience or tests Therefore the ‘setting-up‘ time must also be considered and the cost of determining the correct procedure included in the cost estimatẹ

Thus the process gives full economic benefit on repetitive work where set parameters can be used based on experiencẹ

An example is the wide use of the process for the longitudinal seams of boiler drums in steel 125-150 mm thick On one-off applications involving plate thicker than 150 mm the process is

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16/60 Manufacturing methods

economical on each joint and, providing sufficient work of a

suitable type is available, the high capital cost is recovered

within a reasonable time

Mechanicnlproperties The weld metal mechanical properties

are determined to a considerable degree by the composition

and cleanliness of the parent steel However, proper selection

of wire and flux and suitable parent metal confer strength and

ductility equal to or better than the parent metal

In the as-welded condition degradation of notched impact

properties in the heat-affected zone results in values lower

than the weld metal Where notched impact requirements

must be met, it is necessary to normalize the completed joints

In general, electro-slag welding is an acceptable and economic

method of welding thick steel It finds application for ships'

hulls boiler drums, press frames, nuclear reactors, turbine

shafts, rolling mill housings and similar heavy fabrications

16.3.1.8 Gas-shielded metal arc welding

This general term covers a group of welding processes in which

no added flux is used The molten weld pool is protected by a

gas shield which is delivered to the welding gun through a

flexible tube at a controlled rate, either from a gas bottle or

from a bulk supply The shielding gas may be inert (e.g argon

or helium or mixtures of these gases) or it may be active (e.g

carbon dioxide (COJ or mixtures of C 0 2 with other gases

such as argon) Sometimes small additions of oxygen or

hydrogen are included in the shielding gas

The wide variety of shielding gases which may be com-

pletely inert or non-reactive or may be active (i.e slightly

oxidizing or reducing) has led to the use of the terms MIG

(metal inert gas) and MAG (metal active gas) to describe the

principal gas-shielded metal arc welding processes

MIG and M A G welding In MIG or MAG welding, referred

to in the USA as gas metal arc welding (GMAW) (Figure

16.78) a small-diameter (0.6-1.6 mm) wire is fed from a coil

by a wire feed unit which contains an electric motor, gearbox

and grooved drive rolls The wire is fed to a welding gun that

has a trigger which operates the wire feed drive, the current

and the flow of shielding gas

An arc is struck when the wire contacts the work and the arc

length depends on the voltage, which is preset by adjustment

of a knob on the power source Welding current is picked up

by the wire from a copper contact tube through which the wire

passes The distance between the contact tube where current

enters the wire and the end of the wire is usually a maximum

Flowmeter

Valve / Wire drive rolls

Figure 16.78 Metal inert-gas welding

of 25 mm, compared with 3 0 W 5 0 mm for a covered electrode Therefore overheading is not a problem, particularly as there is no flux coating Higher currents can be used than those normally employed for manual metal arc welding (e.g 120-450 A for 1.6 mm diameter wire) Therefore deposition rates are generally higher than for manual metal arc welding Another advantage of MIG/MAG welding is that the arc is automatically maintained at a length that depends on the arc voltage, which means that the welder has to move the welding gun only along the joint line holding the nozzle of the gun at approximately the same distance from the joint This is because the arc is self-adjusting because the voltage current characteristic of a MIGiMAG power source is flat or only slightly drooping

If the welding gun is moved away from the joint the arc length and the arc voltage increase slightly With a flat or slightly drooping characteristic a small increase in voltage will cause a large decrease in welding current and the wire will burn off at a lower rate The original arc length will be rapidly attained at which voltage the burn-off rate will once again match the wire feed speed Similar self-adjustment in the opposite sense will occur when the welding gun is moved towards the work piece

Filler wires The commonest filler wires are 1.0, 1.2, and 1.6 mm in diameter with 0.6 and 0.8 mm less frequently used Because of the wide range of current that can be used with each wire it is necessary to stock only one or two diameters This is in contrast with manual metal arc welding, where a number of different diameters of electrodes and possibly two

or more coating types may be required just for welding a single type of material such as carbon-manganese steel

However, a disadvantage of solid filler wires is the limited range of compositions available because it would be too expensive for a steel maker to produce small quantities of low-alloy or stainless steel wires Small batches of covered electrodes can readily be produced by introducing alloys in powder form through the coating This situation is reflected by the number of electrodes and filler wires for welding low-alloy and stainless steels listed in British Standards, which are:

Any deoxidizing elements such as silicon or aluminium required to refine or degas the weld pool are contained in the solid wire and compositions of wires available are listed in BS 2901: Parts 1-529 for ferritic steels, austenitic stainless steels, copper and copper alloys, aluminium and aluminium alloys and magnesium alloys and nickel and nickel alloys

Modes of metal transfer In MIG or MAG welding the

operating conditions in terms of current and voltage determine

the type of metal transfer which must be suitable for the application There are four modes of metal transfer

Short circuiting (dip transfer) This occurs when a low voltage and current are used which causes metal to be transferred from the end of the wire to the work piece by frequent short circuiting of the wire to the weld pool This technique produces low heat input and a small controllable weld pool essential for welding steel sheet in all positions and thicker steel sections in the vertical and overhead positions A disadvantage of the process is the production of spatter in the form of globules of metal expelled from the weld pool when each short circuit is broken Spatter particles which adhere to the work piece can be reduced by fine tuning of the inductance

of the power source

L o w alloy steels Stainless steels

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Welding, soidering and brazing 16/61 store the fixed parameters but microprocessor-controlled units are now available

A number of different electronic contro! systems have been developed for MIG/MAG welding in both the dip and spray transfer modes of operation, which overcome the setting-up difficulties mentioned above These setting-up difficulties were particularly acute with the pulsed arc mode of operation, but they have been overcome by the so-called synergic control technique used in conjunction with a transistorized power source With synergic control, precise independent regulation

of the p l s e shape, pulse current time, pulse frequency and background current is obtained The electronic power source can produce continuously variable 25-250 Hz pulse frequencies With variable-frequency pulsing the correlation between pulse energy, burn-off rate and arc characteristics can coincide with all electrode wire feed speeds to provide one metal drop transfer per pulse 30

With synergic control all the puke parameters are prepro- grammed for a wide range of wire feed speeds During welding the wire feed rate and pulse frequency automatically adjust together to produce one metal droplet transfer at a constant arc length The welder only needs to adjust one control -

average current

Modern power supplies have control systems based on microprocessor technology Memory chips in the control unit store process data and produce the optimum operating parameters if the user presses the appropriate switches to specify the types of filler wire and shielding gas In some power sources users can load their own operating programs into the control unit For further information references 31 and 32 should be consulted

As in manual metal arc welding, there are a number of

‘hobby’ sets on the market which can be used on the 13 A mains These have a limited number of current settings for use with 0.6 or 0.8 mm diameter wire and can be used to weld carbon steel, stainless steel and aluminium in thicknesses up to

6 mm Tuition by video is available.33

Applications MIG and MAG welding with the various modes of metal transfer can be used for applications similar to those fabricated by MMA welding In addition, they are more suitable for welding some of the non-ferrous alloys such as aluminium and copper alloys and are probably equally suitable for welding stainless steel and nickel alloys For sheet metal thicknesses which are welded by a single run of weld metal, MIG and MAG welding are generally up to 50% faster than MMA welding In thicker materials, MIG/MAG welding and MMA welding used with the same duty cycle, Le the proportion of arcing time to total time, will have approximately the

same overall welding rate, provided that full use is made of

positioners to enable a large proportion of the welding to be carried out in the flat or horizontal-vertical positions Claims made in the literature or in suppliers’ brochures about the superiority of one process over the other in respect of productivity and economic advantages should be treated with cau- tion, because they may only be valid for a specific application One great advantage of MIGiMAG welding over MMA welding is the ease with which the process can be mechanized either by fitting the welding gun to a traverse unit or by moving the work piece under a stationary gun either by linear motion or rotation

Robotic and automated MIGiMAG welding has advanced with the developments in microcomputers and electronic power sources, the latter providing very stable arcing conditions in spite of mains voltage fluctuations Automated MIG/ MAG welding utilizes seam-tracking devices which are necessary to compensate for inaccuracies of the component parts or distortion during welding

Globular transfer (semi-shorting) This occurs when

somewhar higher currents and voltages are used than for dip

transfer welding of steel, but metall transfer still occurs by

short circuiting of the filler wire to the weld pool Because of

the large droplet size and the larger weld pool, this mode of

welding is not suitable for vertical or overhead welding The

production of spatter still occurs

Spray transfer Free flight of metal droplets occurs with no

short circuiting when the current and voltage are sufficiently

high This give maximum deposition rates and deep penetra-

tion welding suitable for flat-position welds in medium and

heavy steel plate and for horizontal-vertical fillet welding (e.g

between ;1 vertical and a horizontal plate) Spray transfer is

used for welding aluminium and aluminium alloys in all

positions because the spray transfer of droplets occurs at much

lower welding currents than with other metals Therefore

small wel’d beads can be deposited which solidify rapidly and

enable welding to be carried out in the vertical and overhead

positions

Pulsed transfer This was developed to produce spray transfer

at all current levels so that welding of all metal thicknesses in

all welding positions could be carried out without the forma-

tion of spatter In pulsed transfer the welding current is

switched from a high pulse current to a low background

current at a typical frequency of 50 Hz The background

current is, sufficient to sustain the arc but it is insufficient for

metal transfer The pulse current is set above !he critical level

to produce sufficient electromagnetic force with each pulse to

transfer one metal droplet from the tip of !the wire With the

first pulsed arc power supplies the pulse frequency had to be a

multiple iof mains frequency and setting up welding conditions

was difficult to the extent that it hindered the use of the

process in industry The average current, which depended on

the background current, the pulse current and the frequency,

had to produce a usable burn-off rate at a constant arc length

The process was also sensitive to electrode stick-out (the

electrode extension beyond the contact tube) which could

disrupt the balance between pulse energy and metal transfer,

causing arc extinction and spatter The full advantages of the

pulsed MIG process, including stable low mean current opera-

tion particularly when welding aluminium alloys or stainless

steel and positional welding capabilities of all metal

thicknesses, were made readily available with the develop-

ment of transistorized power sources referred to in the next

subsection

Power sources MIG and MAG welding are always carried

out with d.c and the principal types of power sources are

transformer rectifiers with constanl potential or controlled-

slope characteristics and motor generators which are used for

site work (e.g welding pipeline) Invertor type power sources,

described in the section on manual metal arc welding, are also

used for MIG arid MAG welding The main advantage of

invertors is the considerable decrease in size and weight

compareij with conventional transformer rectifiers

Electronic power control has had a considerable and bene-

ficial influence on MIG/MAG welding, enabling the process

parameters to be pre-programmed which eliminates the com-

piicated setting-up operation and enables ‘one-knob’ control

to he achieved Programmed control for both dip and spray

transfer was originally developed in the late 1960s The

relationship between wire feed speed and voltage for any filler

wire type and diameter could be programmed into the power

source and a single control could be used to vary mean current

continuously The equipment contained preset resistors to

Trang 9

Manufacturing methods

Seam-tracking devices contain contact-type sensors such as

probes or guide wheels or the non-contact types such as

electromagnetic, ultrasonic or video systems For further

information on robotic or automated welding references 34-36

should be consulted

16.3.1.9 Flux-cored arc welding

Flux-cored arc welding is similar in many respects to MIG/

MAG welding except that in one version of the process no

shielding gas is added In this case the gas shield originates

from the decomposition of minerals contained in the tubular

core electrode and this version of the process is sometimes

referred to as self-shielded welding

Cored electrodes in coiled form are manufactured from

steel strip which is first bent into a U-section as it passes

through forming rolls The U-shaped strip is then filled with a

metered quantity of flux and metal powders and the strip is

passed through dies to form it into a circular cross-section

from 0.9 to 3.2 mm diameter

Tubular cored electrodes Tubular cored electrodes may be

gas shielded with COP or Ar/C02 mixtures or they may be

self-shielded Cored electrodes are classified according to the

constituents contained in the core which influences the charac-

teristics of the electrode

For full descriptions of the different carbon and carbon-

manganese steel t pes, BS 7084: 1989,37 AWS A5.20-7938 and

and low-alloy steels can be welded with flux-cored wires

having matching strengths Stainless steel cored wires are

available for use either with or without shielding gas and many

different types of cored wires are used for hard-facing applica-

tions in which a coating is applied to a steel base to confer

resistance to wear, corrosion or heat

Application Flux-cored arc welding is used for applications

similar to manual metal arc or MIG/MAG welding and, like

MIG/MAG welding, the process can be mechanized Tubular

cored electrodes are available in a wider range of compositions

than solid wires because of the ease of introducing alloying

elements in powder form Flux-cored wires, particularly the

gas-shielded types, meet the mechanical property require-

ments of a range of applications and some grades give good

low-temperature impact properties The mechanical proper-

ties attainable with self-shielded cored wires is more limited,

with maximum weld metal strengths of 700 N mm-’

Self-shielded wires are particularly useful for site work

because unwieldy bottles of shielding gases are not required

Another advantage on-site is that there is no externally added

shielding gas which is susceptible to disruption by wind

Flux-cored wires can be used at higher maximum currents than

solid wires, resulting in high deposition rates

AWS A5.29-86 361 should be consulted Many higher-tensile

16.3.1.10 Gas-shielding tungsten arc (TIG) welding

In this process an arc is established between a tungsten

electrode and the parent metal, forming a weld pool into

which filler rod is fed, generally by hand (Figure 16.79)

Mechanized systems which feed the filler wire are available

and movement of the welding head along the joint line can

also be mechanized The tungsten electrode is non-

consumable and contamination of the weld pool by air is

prevented by an inert shielding gas such as argon, helium or

mixtures of these gases A high level of skill is required by the

welder, who can control penetration with great precision This

makes the process particularly suitable for the welding of thin

sections and for the deposition of root runs in pipe

Figure 16.79 Tungsten inert-gas welding

Electrodes and filler rods Pure tungsten electrodes can be used but improved arc initiation and stability are obtained by the use of electrodes containing additions of either thoria (thorium oxide) or zirconia (zirconium oxide) Thoriated electrodes are preferred for d.c welding and zirconiated electrodes are used for a.c Electrode diameters vary from 1.2

to 4.8 mm depending on the welding currents used, which can range from 75 to 450 A for thoriated electrodes and from 50 to

200 A for the zirconiated types

Filler rods which are specified in BS 2981: Parts 1-529 have diameters of 1.2-5.0 mm and are available in a wide range of

compositions suitable for welding carbon and low-alloy steels, stainless steels, copper and copper alloys, nickel and nickel alloys, aluminium and aluminium alloys, titanium and zirconium

Power sources An a.c or d.c power source with standard generators, rectifiers or transformers is used For stable operation the power source must have a ‘drooping characteristic’, so that when variations occur in voltage or arc length the current remains substantially constant When changes occur in the arc length when the welding torch is manually guided along the joint line the power input remains within +8% of the preset value

If the arc is initiated by touching the tungsten electrode onto the parent metal the electrode becomes contaminated and to avoid this, a high-frequency oscillator is incorporated into the power source Alternatively, a spark starter using a high- voltage coil similar to that in a car-ignition circuit can be used When the gas in the gap between the electrode and the parent plate is ionized by either the high frequency or the spark discharge the full welding current flows With d.c the high frequency is normally turned off automatically after arc initiation but with a.c it is operated continuously to maintain ionization of the arc path when the arc voltage passes through zero

Power sources are available for pulsed arc welding which enables a stable arc to be maintained at low currents down to

10 A In pulsed TIG welding the pulse frequency varies from

10 per second to 1 per second, and each pulse forms a molten pool which solidifies before the next pulse Pulsed TIG welding can be used to control penetration in thin sheet and in the root runs of pipes and positional welds in plate

Trang 10

Welding, soldering and brazing

metal at the sides of the hole is held in place by surface tension and the pressure of metal vapour in the hole

The keyholing welding technique can be used on carbon, low-alloy steels and stainless steels in thicknesses of 2.5-10 mm and in aluminium alloys up to 20 mm Welding speeds are generally 5G150% higher than those possible with TIG welding

A low-current version of the process is micro-plasma arc welding, which is used for precision welding of thin sheet from

0.025 to 1.5 mm thick at currents of 0.1-10 A The plasma arc

is much more stable than a TIG arc, which tends to wander from the joint line at low currents

Plasma cutfing If the current and gas flows are increased sufficiently the molten metal formed round the keyhole is ejected at the bottom of the hole and as the plasma torch is traversed along the work piece a cut is formed Plasma cutting

is especially suitable for cutting non-ferrous metals, such as aluminium, copper and nickel, and their alloys which are not easily cut by oxy-fuel gas flames Most non-ferrous metals are cut using nitrogen, nitrogen-hydrogen mixtures or argon- hydrogen mixtures as the plasma gas A secondary shielding gas delivered through a nozzle that encircles the plasma gas nozzle is selected according to the material being cut For mild steel and stainless steel it can be C 0 2 and for aluminium it is

an argon-hydrogen mixture Sometimes water is used instead

of the ancillary shielding gas and in another variety of the process water is injected round the end of the plasma gas nozzle, which has the effect of concentrating the plasma flame and allowing higher cutting speeds

Plasma cutting can be used for plate edge preparation (Le bevelling) and for shape cutting The process can be used manually or the torch can be mounted on mechanized cutting equipment identical to that used for oxy-fuel gas cutting For metal thicknesses up to 75 mm carbon steels can be cut faster

by plasma cutting than by oxy-fuel gas, and up to 25 mm thick the cutting speeds can be five times as fast

An important variation of the process is the use of compressed air for the plasma gas without the provision of any additional shielding gas The use of compressed air instead of water for cooling enables the torch to be of simplified construction

Small manual air plasma torches are available which find increasing applications in sheet metal cutting (e.g motor repair shops) For further information reference 40 should be consulted

Applications TIG welding is particularly suited to welding

light-gauge carbon, alloy and stainless steels and all non-

ferrous metals and alloys A clear, clean weld pool is formed

with precise control of heat input and the ability to weld with

or without filler metal in all positions makes the process

attractive *€or critical applications where exceptionally high

quality is #essential Examples are stainless steel piping for

nuclear applications and the wide range of piping composi-

tions used in chemical plant For such critical applications,

fully mechanized orbital welding equipment has been devel-

oped in which the welding torch and wire-feeding mechanism

rotates round the pipe joint Thin- and thick-section pipes can

be welded with a narrow gap joint preparation and in-situ

fabrication of nuclear and chemical plant is now possible

Other specialized TIG welding equipment is used for the

mechanized welding of tubes to tube plates

16.3.1.11

Plasma arc welding was developed from TIG welding by

placing a narrow orifice round the arc and supplying a small

flow of argon through the orifice (Figure 16.80) The con-

stricted arc dissociates the argon gas into positive and negat-

ively charged electrons to form a plasma When the plasma gas

flows away from the arc column it forms neutral atoms again

and gives up its energy in the form of heat

A low-current pilot arc is initiated between the tungsten

electrode and the water-cooled copper orifice The argon gas

flowing through the orifice is ionized and initiates the primary

arc between the tungsten ejectrode and the parent metal when

the currenlt is increased The arc and the weld zone are

shielded by a gas flowing through an outer nozzle The

shielding gas consists of argon, helium or gas mixtures of

argon with either hydrogen or helium

A normal tungsten arc has a temperature of approximately

11 000°C but the constricted arc of a plasma torch can reach

20 000°C The high-temperature ionized gas jet gives up its

energy when it contacts the parent metal and thus increases

the energy of the tungsten arc This produces a deep penetra-

tion weld with a high depth-to-width ratio with minimum

distortion of the parent metal The term 'keyhole' is used to

describe the shape of the hole formed in the parent metal

when a close square edge butt joint is welded As the torch is

moved along the joint, molten metal flows round the edges of

the hole and solidifies at the rear of the hole The molten

Plasma arc welding and cutting

, , , , , Work piece

Anode

Figure 16.80 Plasma arc welding

16.3.1.12 Gas welding and cutting

Gas welding is carried out by a f a m e produced by burning approximately equal volumes of oxygen and acetylene which are delivered at equal pressures from gas bottles to a welding torch The flame temperature is approximately 3100"C, which

is high enough to melt steel and other metals Filler metal, if required, is added by manually feeding a rod into the front edge of the weld pool while the torch is moved along the joint The products of combustion provide sufficient protection from the atmosphere when welding steel When welding other metals such as cast iron, stainless steel, aluminium alloys and copper alloys, fluxes are used to clean and protect the metal from oxidation

Equipment The welding torch has two knurled control knobs which regulate the flow rates of oxygen and acetylene so that a neutral or slightly oxidizing or reducing flame is obtained, depending on the application The torch has a screw-in nozzle from a set of nozzles having different diameter holes which produce the appropriate size of flame and therefore the

de

Trang 11

required heat input for the particular metal and thickness to be

welded The oxygen and fuel gas hoses are connected between

the welding torch and the gas bottles, the gases passing

through flashback arresters and pressure regulators Flash-

back arresters are safety devices that prevent a flame from

travelling back into the cylinders in the case of a backfire For

workshop use the gas bottles are generally mounted in pairs on

a trolley which can be moved to where it is required

Filler metal and fluxes Chemical compositions of filler metals

are specified in BS 1453: 1972 and include ferritic steels, cast

iron, austenitic stainless steels, copper and copper alloys and

aluminium alloys Ferritic steels do not require the use of a

flux but proprietary fluxes are available for other materials

Applications Gas welding is used mainly for repair and

maintenance work, particularly in the repair of car bodies and

agricultural implements, although it is slowly being replaced

by small TIG and MIG welding equipment Gas welding is

used, to a certain extent, for sheet metal work (Le heating

and ventilating ducting) and is still employed for making the

root runs in pipes, where it is particularly useful for bridging

gaps

Two applications where gas welding has distinct advantages

over other processes are in the welding and repair of grey iron

casting and in hard-facing with expensive alloys Grey iron

castings can be successfully welded by the use of high preheat-

ing temperatures of up to 600°C and gas welding with cast iron

filler rods The deposition of high-cost wear-resistant alloys

such as the cobaltxhromium tungsten types or those based on

chromium or tungsten carbides can he carried out with mini-

mum melting of the parent metal, so that dilution of the

deposited alloy and the consequent decrease in wear res-

istance is avoided Gas welding is also successfully applied in

jewellery manufacturing with miniature torches and small gas

bottles

Acetylene is the only fuel gas suitable for gas welding

because of its favourable flame characteristics of both high

temperature and high propagation rates Other fuel gases,

such as propane, propylene or natural gas, produce insuffi-

cient heat input for welding but are used for cutting, torch

brazing and soldering They are also used for flame straighten-

ing of distorted components and for preheating before welding

and post-heating after welding

Gas cutting Gas cutting, sometimes referred to as flame

cutting or oxygen cutting, involves an active exothermic

oxidation of the steel being cut when the material has been

preheated by an oxy-fuel gas flame to the ignition temperature

of around 900°C The equipment for gas cutting is the same as

for welding except that a special cutting nozzle is required

The nozzle has an outer ring of holes through which the

preheating gas mixture is delivered and a central hole through

which the oxygen jet flows The exothermic reaction of

oxidation of steel forms a fluid slag of iron oxide and after a

few seconds, depending on the metal thickness, the section is

pierced Iron oxide and molten metal are expelled from the

cut by the oxygen stream Movement of the cutting torch

across the work piece produces a continuous cutting action

and the torch can be operated manually or by a motorized

carriage Steel up to 300 mm thick can be cut by this process

Oxidation-resistant steels such as stainless steel may be cut

by specialized methods, including the introduction of iron

powder or other proprietary powders into the oxygen stream

These powders react with the refractory chromium oxides and

reduce their melting points and increase their fluidity, enab-

ling cutting to take place For further information reference 40

Numerically controlled cutting machines are available which use programs stored or punched on magnetic tape which send appropriate signals to the drive motors

16.3.1.13 Welding and cutting with power beams

Electron beam welding and laser welding utilize high-energy beams which are focused onto a spot of about 0.2 mm diameter on the work piece surface This intense heat source, which releases its kinetic energy when the beam hits the surface, is radically different from arc welding, in which the arc melts an area of about 5-20 mm in diameter, depending on the welding conditions

When the power density of an electron or a laser beam at the focused spot is 10 kW mm-’ or greater, energy is delivered at a faster rate than can be conducted away in the form

of heat in the work piece and the progressive vaporization of metal through the section thickness forms a hole If the beam

is then traversed along the work piece, molten metal flows around the sides of the hole and solidifies at the rear of the hole Molten metal at the sides of the hole is held in place by surface tension and the presence of metal vapour in the hole in the same manner as that described as the keyholding technique in Section 16.3.1.11

As with plasma welding, a deep penetration weld with a high depth-to-width ratio is formed with minimum distortion

of the parent metal Maximum penetration depths in steel are approximately 280 mm for electron beam welding and 12 mm for laser welding, although developments in the latter process are likely to increase this to 25 mm or more

Electron beam welding In this process a finely focused beam

of electrons passes from a cathode and travels through a hole

in an anode and is focused onto a spot on the work piece 0.2-1 mm diameter by means of a magnetic lens Deflection coils are used to cause the beam to move in a circular pattern

to increase the width of the weld, so that fusing two mating surfaces together in a close square butt joint is possible The cathode in the electron gun is maintained at a negative potential of 60-150 kV and the gun is contained in a vacuum

of 5 X lo-’ torr In the work chamber a pressure of 5 X 18-3 torr is suitable for welding most metals and for some applications a pressure of lo-’ torr or less is used with the advantage

of much shorter chamber excavation times, resulting in increased production rates

High-vacuum welding High-vacuum welding, in which the work chamber is maintained at a pressure of to torr depending on the application, has the following features:

1 Maximum weld penetration and minimum weld width and shrinkage, enabling all thicknesses to be welded in a single pass

The highest purity weld metal is produced because of the absence of any contaminating gases such as oxygen or nitrogen

2

Trang 12

Welding, soldering and brazing 16/65

Pulsed laser welding is extensively used in the electronics industry where miniaturization requires very precise positioning of small welds Typical examples are encapsulation of microelectronic packages and the joining of fire wires by butt, lapped or cross-wire joints Spot welds can be made between overlapping sheets and seam welds formed by a series of overlapping spot welds Most metal can be welded, including steel, copper, nickel, aluminium, titanium, niobium, tanta- lum, and their alloys Solid-state lasers are also used for precision drilling of holes with very small diameters

The carbon dioxide laser In the carbon dioxide laser the lasing medium is a gaseous mixture of carbon dioxide, nitrogen and helium at a reduced pressure of 2-50 torr The output

of the carbon dioxide laser is in the mid-infrared (10.6 pm) and COz lasers are available with owers of up to 20 kW The high power density produced (10 W mm-' or more) forms a cavity or keyhole in the work piece which enables a deep penetration weld to be produced similar in appearance to an electron beam weld

Penetration is less than for an electron beam operating at the same power because of plasma gas formed in the keyhole The plasma gas escapes from the keyhole and interacts with the laser beam, restricting penetration to approximately

12 mm in mild steel Methods have been developed for overcoming this problem4' by the use of either a pulsed output from the laser with pulses of shorter time than that required to generate the plasma or a high-velocity jet of helium to disrupt the plasma above the weld Such developments are likely to increase the thicknesses that can be laser welded to 25 mm or more

Carbon dioxide lasers are used in production in some automotive and aerospace applications, particularly for titanium and nickel alloys The advantage of laser welding over elctron beam welding is that a vacuum is not required, which simplifies the welding operation Other materials that can be welded by the carbon dioxide laser are steels, copper alloys, zirconium and refractory metals, but aluminium alloys are not readily weldable with the C02 laser because of their reflectiv- ity of the laser beam

Laser cutting Cutting with the C 0 2 laser is carried out by a combination of melting and vaporization with an auxiliary jet

of gas to blow the molten metal from the cut Various gases are used for this purpose, including oxygen, compressed air, inert gases and carbon dioxide

Oxygen produces an exothermic reaction with ferrous metals which increases the efficiency of cutting The inert gases produce clean unoxidized surfaces - important features when cutting readily oxidized metals such as aluminium and titanium

a

3 A high1 vacuum allows a long distance to be maintained

between the gun and the work piece, which facilitates

observ,ition of the welding process

4 The pump downtime is lengthy, up to an hour or more

depending on the size of the chamber, which would lower

the production rate of small jobs with shallow welds but

would be insignificant when wends in plate 50 mm or

thicker are made with a single pass

Welding speeds for steel are shown In Table 16.4

Table 16.4 Penetration depth and welding rates

Weiding Plate thickness Welding speed

Medium-vacuum welding In medium-vacuum welding the

working chamber is maintained at a pressure generally within

the range of l W 3 to 10-' torr, although pressures of up to 25

torr are reported in the literature4' and correspondingly short

pump downtimes of a few seconds

Medium-vacuum welding with small working chambers is

used extensively for small repetitive work such as welding of

finish machined geartrains and similar high-volume mass-

production applications for the motor industry Many high-

precision semi-finished or fully machined components for

aircraft engines are also welded by the electron beam process

Medium-vacuum electron beam welding is not suitable for

welding reactive metals and alloys such as titanium and

zirconium, which require the high-vacuum process to obtain

sound welds

Out-of-vacuum electron beam welding Provided that the

gun-to-work distance is less than about 35 mrn to allow for the

greater dispersion of the electron beam compared with work-

ing in a vacuum, it is possible to weld many materials out of

vacuum With 60 kW non-vacuum equipment single-pass

welds can be made in metal thicknesses up to 25 mnn Metals

welded out of vacuum include carbon and low-alloy steels, and

copper and aluminium alloys Because of the presence of air,

which can 'cause contamination of the weld metal, it is usually

necessary to provide an inert shielding gas to cover the weld

zone

Laser welding and cutting Two types of lasers are used for

welding and cutting: the solid-state YAG (yttrium-

aluminium-garnet) and the carbon dioxide (COz) laser The

term 'laser' is an acronym for Light Amplification by Stimu-

lated Emission of Radiation and a laser is a device for

producing monochromatic (single-wavelength) light that is

coherent (i.e all the waves are in the same phase) A laser

beam can be transmitted over many metres and can be focused

to produce the high-energy density required for welding or

cutting

The solid-state YAG laser The solid-state laser is stimulated

to emit coherent radiation by means of the light from one or

more powerful flash tubes and the output is in the infrared

region aroiund 1.06 pm Both input and output are generally

pulsed and the power output is a maximum of 500 W

16.3.1.14 Resistance welding Spot, seam and projection welding Spot, seam and projection welding are carried out by electric resistance heating two overlapping metal parts which are pressed together by copper

or copper alloy electrodes Local melting occurs at the faying surfaces and an internal weld nugget is formed The welding cycle, comprising current, pressure and time, is readily con- troiled automatically, giving the following advantages:

1 Little skill is required in operation

2 Welding can readily be built into production lines

3 Welding can be associated with automatic loading, un- Ioading and transferring of components so that resistance welding is the simplest process for automation or robotic welding

Welding times are short so that output is high

N o filler wires or fluxes are used

4

5

Trang 13

6 Distortion is a minimum as heating is confined to a small

area

The high currents required for resistance welding are gen-

erally obtained from a single-phase a x transformer having a

primary winding of several hundred turns The secondary

winding consists of one or two turns of thick copper which may

be water cooled The voltage is stepped down to a value

between 4 and 20 V

Three-phase welding machines are available which are more

expensive but they have a better power factor A three-phase

transformer is sometimes used with a d.c rectifier to carry out

resistance welding with d.c For further information on power

sources reference 41 should be consulted

Spot welding Spot welding (Figure 16.81) is used for the

fabrication of sheet metal from 0.6 to 3 mm thick to produce

lap joints intermittently welded and therefore not pressure-

tight Typical applications are low-carbon steel components

for car bodies, cabinets and general sheet metalwork Spot

welding is also applicable to stainless steel, aluminium and

aluminium alloys, and copper alloys

Because of high electrical conductivity, pure aluminium is

difficult to spot weld and its softness results in heavy indenta-

tion by the electrodes The high electrical conductivity of

copper makes it unsuitable for spot welding Stainless and

heat-resisting steels are spot welded for aircraft and gas

turbine engines

Seam welding Seam welding (Figure 16.82) is similar in

principle to spot welding and uses rotating wheel electrodes

which roll the overlapping components between them The

welding current passes intermittently through the electrodes,

forming a series of welds that overlap one another The

electrode wheels rotate continuously and at least one is

power-driven to move the component along, in addition to

carrying the welding current

Pressure-tight seams can readily be made and the process is

faster than spot welding However, machines are heavier and

more costly than spot welding machines Seam welding is

primarily suited to making long straight welds, although

curved welds can be made (for example, welds which may

occur at the corners when joining two half pressings together

to form a fuel tank) Typical applications are pressure-tight

seams for oil drums and refrigerator parts

Projection welding In this process the current is concentrated

by the shape of the components themselves, small dimples

being formed on one of the sheets to be joined as shown

diagrammatically in Figure 16.83 The electrodes are of rela-

tively large area compared with spot welding electrodes and

can therefore be made of a hard material having a compara-

tively high electrical resistance, such as a copper-tungsten

alloy As several welds can be made at the same time, the

Transformer Copper alloy electrode

2

Sheets being welded

Figure 16.81 Resistance spot welding

Transformer

I

/

Driven electrode wheels

Figure 16.82 Resistance seam welding

I

Sheets being welded

\ \ \ \ \

Fixed electrode

Figure 16.83 Projection welding

welding machine has to be of high electrical capacity and must also be capable of applying the high total mechanical load to the components during welding Three projections welded simultaneously give the best results, and up to five projections are common Applications involving larger numbers of projections are known, but as the number increases it becomes increasingly difficult to ensure uniform and adequate welding

of all the projections The process can also be used for attaching studs, nuts and disks to flat plates

The main features of the process are as follows:

Welds are generally of better external appearance than spot welds due to the absence of indentations

T-joints may be made as well as lap joints

Machines are generally more costly than spot welding machines

The cost of preparing components is greater because of the necessity of forming projections

16.3.1.15 Resistance butt and flash welding Resistance butt welding The two ends of the parts to be joined are brought into contact and current is passed across

Trang 14

Welding, soldering and brazing 16/67

Weld

Transformer

Figure 16.84 Resistance butt welding

the joint while a moderate mechanical force is applied to the

components (Figure 16.84) As the joint heats by electrical

resistance the material softens and welding takes place The

finished joint shows a thickening or upsetting of the compo-

nents in the joint area, and for some distance away from the

joint Resistance butt welding is essentially a solid-state weld-

ing process in which no melting occurs

Recrystallization of the metal takes place across the faying

surfaces ,and the upsetting action has the effect of removing

oxides from the joint The process has two main applications:

1 Joining two components of the same cross-section end to

end (e.g wire, rod, bar or tubing) Used in rod and wire

mills for joining the ends of coils for continuous process-

ing

2 Continuous welding of longitudinal seams in pipe or

tubin.g formed from flat plate

The process is applicable to the welding of carbon, alloy and

stainless steel, aluminium alloys, copper alloys, nickel alloys,

and electrical resistance alloys

Resistance butt welding machines are generally designed to

weld a particular family of alloys because the current densities

and pressures as well as the rate of application of the pressure

differ widely between, for example, steels and aluminium

alloys The cross-sectional areas to be joined also determine

the current capacity of the welding transformer

The welding machine has two platens on which are mounted

clamping dies which grip the components One platen is

stationary and the other moves to produce the pressure

required

F h h welding Flash welding was developed from resistance

butt welding and the principle is shown in Figure 16.85 The

currrent is switched on before the ends of the components are

in contact, with the result that small volumes of metal melt

explosively at the points where contact is first made This

process c:ontinues as the two components are moved towards

one another, causing a large amount of flashing and removal

of metal while, at the same time, heating the areas in contact

and the material immediately behind Once suitable tempera-

ture conditions have been established, the two ends of the

compone:nts are forced together by a sudden increase in the

load applied and the current is switched off The effect of this

sudden load is to squeeze out from the joint all the overheated

and oxidized metal and to form a high-quality pressure weld

The fin or flash of upset metal surrounding the joint is

normally removed before the component is put into service

Typical applications of flash welding are the production of

continuous welded railway track, motor-car wheel rims

formed from flat steel stock and mitre joints in door and

t'

Transformer

Figure 16.85 Flash welding

window frames The materials welded are similar to those that can be resistance butt welded

16.3.1.16 Friction welding

Friction welding is a solid-state joining process, i.e there is no

fused metal involved The heat to make a forge weld is

produced by moving one component relative to a mating component under pressure The motion is usually rotational, although the linear relative motion can be used and is under

development A friction weld is similar in appearance to a

resistance butt or a flash weld with upset metal or flash which

is generally removed after welding by a machining operation Butt welds can be made in rods or tubes, and rods or tubes can be welded to plates One of the components is generally circular in cross-section although square rods can be joined by friction welding A wide range of sizes can be friction welded from small-diameter wires used in the electronics industry up

to 150 mm diameter aluminium bus bars

Typical applications are found where high production rates are required (e.g the motor industry) Examples are axles, drive shafts, steering shafts and valves where a high-alloy heat-resisting head is welded to a cheaper carbon steel shank Bar stock can be welded to plates to produce parts that would normally be forged

Most metals and alloys, with the notable exception of cast iron, can be friction welded Dissimilar metals can also be welded without the formation of a brittle zone that often occurs with arc welding (e.g aluminium can be welded to steel)

Friction surfacing is an important development in which a corrosion- or wear-resistant alloy rod is rotated rapidly under pressure against a surface to be clad with the alloy Under carefully controlled conditions a friction weld can be deposited over the surface and the weld metal is unchanged in composition because there is no dilution of the weld caused by melting

of the parent metal as occurs in surfacing by arc welding For further information on friction welding reference 41 should be consulted

16.3.2 Soldering and brazing

Brazing and soldering are carried out at temperatures below the melting points of the metals being joined Brazing filler metals have melting points above 450°C and solders melt below this temperature In both processes the molten filler metal flows by capillary action between closely fitted surfaces

of the parts to be joined

Unlike welding, soldering and brazing do not generally produce joints having mechanical properties matching those of

Trang 15

the parent metal Typical applications, such as car radiators

and electrical connections, require leak-tightness and elec-

trical conductivity, respectively If high strength or ductility is

required correct alloy selection and joint design are essential

and for further guidance references 42 and 43 should be

consulted

The low temperatures involved in soldering and brazing

have two beneficial effects compared with welding: (1) parent

metal properties are less affected, especially by soldering; (2)

residual stresses and distortion are lower For successful

joining, the following procedures are required:

1 For wetting of the present metal by the solder or brazing

alloy the surfaces must be chemically clean Degreasing

and mechanical cleaning or pickling may be necessary

During the joining operation a flux is generally used to

remove surface oxides and to prevent them from reform-

ing In some brazing processes a reducing or neutral

atmosphere or a vacuum is used

Heating of the parent metal must be carefully controlled

so that both the mating surfaces reach the melting temper-

ature of the filler metal before it is applied to the joint

When the filler metal is pre-placed in the joint the same

requirement applies because the molten filler metal will

not wet the surface unless that surface is hot enough

All traces of flux residues which might lead to corrosion

must be removed after soldering or brazing

2

3

4

16.3.2 I Soldering

Solders The normal range of soft solders are listed in BS 219:

1977@ which also indicates typical uses The commonest

solders are the tin-lead alloys, some of which have antimony

added These solders are used in various applications, depend-

ing on the composition, including manufacture of food-

handling equipment, tin-plated cans, domestic utensils, elec-

tronic assemblies, heat exchangers and general engineering

work Tin-antimony solder is used for higher service tempera-

tures (e.g above 100°C) and in applications for joining

stainless steels where lead contamination must be avoided

The alloy is also used for plumbing and refrigeration applica-

tions Tin-silver solder is used for fine instrument work and

food applications while tin-lead-silver solder is used for

service above 100°C and also below -60°C

Soldering methods Many sources of heat can be used for

soldering, including the following: heated iron, flame or torch,

hot-plate or oven, HF induction, electric resistance, hot gas,

as well as dip, wave or flow methods involving molten solder

baths For further information on both manual and mecha-

nized soldering techniques references 43, 45 and 46 should be

consulted

Manual soldering methods using a soldering iron or a torch

are fairly slow and require a reasonable degree of skill,

although repetitive work carried out by an experienced

operator can give high production rates Dip soldering is faster

and the electronics industry uses specially designed equipment

for wave soldering of printed circuit boards

16.3.2.2 Brazing

Brazing alloys are specified in BS 184547 under eight groups

based on aluminium silver, copper-phosphorus, copper,

copper-zinc, nickel palladium and gold The choice of braz-

ing alloy for a particular parent metal can be fairly wide and a

decision may be based on metallurgical considerations, includ-

ing corrosion resistance maximum temperature to which the

parent metal can be heated to avoid deterioration in proper-

ties or melting of previously applied brazing alloys For further

information references 42, 48 and 49 should be consulted A brief summary of the uses of the eight main groups of brazing alloys is given in Table 16.5

Brazing alloys are available in the form of wires, rods, powders and inserts of various shapes for pre-placing in the joint

Table 16.5 Main uses of brazing alloys

Brazing alloy !2ia;wzu0 surtable for brazing

Aluminium Silver

h r c aluminium and some aluminium Most ferrous and non-ferrous metals and alloys except aluminium, magnesium and refractory metals and their alloys

Ferrous materials in a protective atmosphere or in a vacuum Stainless steel and other heat- and corrosion-resistant alloys in a

protective atmosphere or in a vacuum heat-resisting alloys for

high-temperature service Palladium and gold Metallized ceramics, copper, nickel

and ferrous alloys in a protective atmosphere or in a vacuum

alloys

Copper-phosphorus Copper and copper alloys Copper

Copper-zinc

Nickel Stainless steel and nickel-chromium

Heating methods A full description of the various heating methods and their advantages and disadvantages are described

in BS 1723: Part 2: 1986.4s The methods comprise hand-torch brazing, mechanized flame brazing, induction brazing, furnace brazing (including protective atmosphere), vacuum and open- furnace brazing, and immersion brazing, including flux bath, dip bath and salt-bath brazing Special processes referred to are infrared brazing and laser brazing Reference 48 also contains details of design and location methods for brazing

Applications Brazing is applicable to cast irons, steels, galva- nized steel, aluminium, copper, magnesium, nickel and their alloys, stainless and heat-resisting steels, titanium, zirconium, ceramics and refractory metals and to dissimilar metal joints

It is selected in preference to other joining processes for the following reasons:

1 Where heating must be restricted to avoid melting or

distortion of one of the parts to be joined

2 Where strength or dimensional accuracy of the assembly would be impaired by heating to a high temperature

3 Where a soft soldered joint would not be strong enough

4 Where parts cannot be welded because of the properties

of the parent materials involved (e.g ceramics, refractory metals)

Where a number of joints have to be made in a small complicated assembly

Where joints would be inaccessible to the welding processes

5

6

16.3.3 Productivity and welding economics

Productivity, in the broadest sense, is what determines profit Unfortunately, the cost of welding is one of the least- documented topics in the whole field of welded fabrication When a company decides to review its fabrication methods

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Welding, soldering and brazing 36/69

Table 16.6 Observed MMA welder activities (percentage breakdown

in time)

there is often a lack of understanding of the principles

involved and only limited knowledge of the alternatives avail-

able Reliance may be placed on equipment salespersons who

may be fair and have an excellent product to sell but, quite

naturally, may be biased in their approach

There is a lack of information compared with that readily

available on machine tools (e.g cutting speeds, depth of cut

which can enable a machine to be integrated into a manufac-

turing system to give a high utilization or duty cycle and

therefore maximize productivity) The same objective should

be applied to welding as part of the manufacturing system

The first task in reviewing fabrication methods is a simple

one and c'oncerns design In the case of arc welding, fabrica-

tiom should be designed to have the minimum amount of weld

metal thal will meet the service requirements of the compo-

nents in terms of mechanical properties," The total quantity

of weld metal in a fabrication is, of course? only a very small

proportion of the total weight (perhaps 2% or less) but,

nevertheless, it is sensibie to use the minimum amount necess-

ary This requires close attention to detail design, shopfloor

supervision and inspection to ensure that welded joints are not

overdesigned and weld metal is not being wasted

Sometimes partial-penetration butt joints will be adequate

and interimittent fillet welds can replace continuous ones

Fillet weld profiles should not be excessively convex because

any weld metal in excess of a mitre fillet does not contribute to

the strength of a joint

For butt joints the plates are usually cut with bevel angles of

30" to given an included angle of 60" when the plates are

placed together With a gap at the root of 1-3 mm this is the

optimum edge preparation to give full penetration and room

to manipu.late a covered electrode or MIG welding gun If the

included angle is increased to 70" the weld volume is increased

by 20%, which represents 20% waste If a fillet weld having a

size or leg length of 6 mm is adequate, then a leg length of

8 mm will result in 57% wasted weld metal because of the

amount of overwelding The increase in the cost of wasted

consumables may be small but the cost of depositing excess

weld met,al includes labour costs and overheads; which can

increase the overall cost by a considerable amount

Having ensured that the optimum welded detail design is

employed, there may be a desire to increase productivity still

further The next point to consider i s the deposition rate

which depends on the welding current whichever arc welding

process i s used As a simple example in manual metal arc

welding if the electrode size is increased from 4 mm to 5 mm

diameter and the current increased accordingly the cost of

welding can be reduced by approximately 25% A similar

increase in welding current has the same effect in other

processes such as MIG or submerged-arc welding The above

simple ex,ample relates to a particular job where the duty cycle

was 30% (the duty cycle being the ratio of arcing time to total

elapsed time for the welding operation) A welder is obviously

not welding for 100% of the time and typical activities for a

manual welder in various industries are shown in Table 16.6

Duty cycles may be considerably lower than those shown in

the table For example, in repair work where defects have to

be laboriously removed by mechanical means the welding

duty cycle may be Iess than 10% In contrast, the duty cycle of

a robot MIG welding parts of car bodies may have to be 70%

or higher to justify the capital investment

Returning to the manual welder who increases the deposi-

tion rate by changing to a larger electrode and increasing the

current, this gives a wort:hwhile cost saving provided that the

duty cyclic is at least approximately 20% If the welder only

deposits weld metal for 10% of the time, then even if the

deposition rate is doubled it will have only a slight effect on

productivity The duty cycle may be low because the welder

vessel pipework Lighi Heavy

to indicate how improvements in productivity may be accomplished with the equipment currently in use Costing methods for arc welding must take account of the costs of consumables, weight or volume of weld metal, deposition rates and duty cycles, labour costs and overheads and amorti- zation of equipment For further information on the calcula- tion of welding costs, references 51-53 should be consulted

A further important point to consider in economic surveys

of welding operations is the welding position Deposition rates can be maximized by welding in the flat position because a large molten pool of weld metal can be maintained, whereas in the vertical or overhead welding positions the size of the weid pool has to be restricted Therefore the provision of a welding manipulator or turning rolls can enable work to be rotated so that welding can be carried out in the flat position with maximum deposition rates In some cases, depending on the shape and size of component, the rate of welding can be doubled or trebled by suitable manipulation of the work piece When a welding engineer has considered all the elementary principles of welding engineering and has determined the possible increase in productivity at minimum cost, the next question to be asked is whether this is satisfactory for the company Management decisions are required on the rate of production required at present and in the foreseeable future, and any plans for the manufacture of different products New product lines may raise the question of what is technically required in respect of quality of welded joints (for example, static or fatigue strength, fracture toughness, corrosion or wear resistance) With this information, a choice can be made

on the most appropriate welding processes, equipment and consumables capable of achieving the quality and production rates required If a welding engineer is available he or she will obviously be expected to make recommendations on the choice of welding processes and equipment Otherwise, the company may have to contact outside sources such as equipment suppliers, national advisory bodies or a consultant In any of these circumstances it is advisable that the manager making investment decisions should have some basic knowledge of welding technology if only to be able to ensure that all the possible alternatives have been considered For further information on the choice of manual, mechanized or robotic welding processes coupled with economic considerations, references 36 and 54-56 should be consulted

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16.4 Adhesives

16.4.1 General comments

Adhesives can offer substantial economic advantages over

conventional methods of joining Indeed, it was for this reason

that the application of adhesives to metal fabrication, in

common with many other technological innovations, was

pioneered by the aircraft industry A realization of the advan-

tages of adhesive bonding over other methods of joining

materials has led to a rapid growth of the adhesives industry

This has been accompanied by advances in the science and

technology of adhesion and adhesives, with an emphasis on

high-performance synthetic products for the mechanical engi-

neering sector Adhesive bonding, either alone or in com-

bination with other methods of fastening, represents a key

enabling technology for the exploitation of new materials and

for the development of novel concepts and structural con-

figurations

The strength of an adhesive bonded joint is determined by

the strength of its weakest element; this is not necessarily the

adhesive! The correct choice of adhesive and an appreciation

of appropriate process procedures are necessary for the satis-

factory fabrication of bonded assemblies Load-bearing joints

require proper design Thus the engineer requires at least a

qualitatively correct overall picture of the various factors

influencing adhesion and controlling joint performance This

section seeks to present a balanced overview of these factors

16.4.2 Definitions and terms

16.4.2.1 Adhesives

An adhesive may be defined as any material which, when

applied to surfaces, can join them together and resist their

separation Thus adhesive is the general term and may include

cement, glue, gum, paste, etc There are several, largely

synthetic, generic groups of adhesives of value to the engineer

embracing numerous subgroups, individual formulations and

generic combinations

The adhesive is involved with wetting, adsorption and

interdiffusion reactions with the substrate surface before soli-

dification A schematic cross-section through a bonded inter-

face is given in Figure 16.86

16.4.2.2 Adhesion

Adhesives join materials primarily by attaching to their sur-

faces within a layer of molecular dimensions The term

'adhesion' refers to the attraction between substances where-

by, when they are brought into contact, work must be done in

order to separate them Thus adhesion is associated with

intermolecular forces acting across an interface, and involves a

consideration of surface energies and interfacial tensions

16.4.2.3 Adherends or substrates

The materials being joined are usually referred to as the

adherends or substrates

16.4.2.4 Adhesive bonding

Bonding means the uniting of similar or dissimilar adherend

surfaces with a relatively low modulus interlayer of adhesive

material The resultant properties of the composite made are a

function of the bonding, the materials involved and their

interaction by stress patterns

Figure 16.86 Elements of a metal adherendladhesive interface

16.4.2.5 Advantages and limitations of adhesive bonding

The main advantages and limitations of adhesive bonding as compared with welding or mechanical fastening are given in Table 16.7 The relative importance of individual items naturally depends upon the perspective of different users

16.4.2.6 Requirements for a satisfactory bonded joint

1 Selection of a suitable adhesive

2 Adequate preparation of the adherend surface

3 Appropriate design of the joint

4 Controlled fabrication of the joint itself

5 Post-bonding quality assurance

16.4.3 Adhesives

16.4.3.1 Introduction

Adhesives may be classified as either organic or inorganic materials in a number of different ways - for example, by origin, by method of bonding, by end-use or by chemical composition Some classifications employ more than one criterion, such as that offered in Table 16.8

Natural adhesives such as starch, animal glues and plant

resins have been used for centuries, and are still used widely

today for packaging and for joining wood Rubber-based adhesives were introduced in the shoe and tyre industries towards the end of the nineteenth century, but the birth of modern structural adhesives is generally dated from the early twentieth century with the introduction of phenol- formaldehyde resins Over the past four or five decades the natural adhesives have been improved, and there has been an intense development of synthetic adhesives to meet more technically demanding applications These synthetic adhesives include thermoplastic and thermosetting types Thermoplastic adhesives may be softened by heating and rehardened on cooling, and included in this group are relatively low- performance materials such as acrylic polymers, 'hot-melts' and products based upon polyvinyl acetates (PVAs) Thermo- setting materials represent thermally stable highly cross-linked

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Table 16.7 Advantages and limitations of adhesive bonding

Ability to join dissimilar materials

Ability lo joint thin sheet material efficiently

More uniform stress distribution in joints which imparts

enhanced fatigue resistance

Weight savings over mechanical fastening

Smooth external surfaces are obtained

Corrosion between dissimilar metals may be prevented

or reduced

Gluelint: acts as a sealing membrane

No need for naked flames or high-energy input during

joint fabrication

Capital and/or labour costs are often reduced

Surface pretreatments normally required, particularly with

a view to maximum joint strength and durability Fairly long curing times frequently involved Poor resistance to elevated temperature and fire Structural joints require proper design

Brittleness of some products, especially at low temperatures

Poor creep resistance of flexible products Poor creep resistance of all products at elevated temperatures

Toxicity and flammability problems with some adhesives Equipment and jigging costs may be high

Long-term durability, especially under severe service conditions, is often uncertain

(b) Vegetable Resin: gum arabic, tragacanth, colophony, Canada

Oils: linseed oil Waxes: carnauba wax Proteins: soya bean Polysaccharides: starch, dextrine Waxes: paraffin

Resins: copal, amber Other materials: silicates, phosphates, sulphur, magnesia, litharge, bitumen

Vinyl polymers and copolymers:

poly(viny1 acetate), poly(viny1 alcohol), poly(viny1 acetal)s, polystyrene Acrylic polymers:

polyacrylates, polymethacrylates, polyacrylamides, poly(cyanoacry1ate)s polyamides and saturated polyesters polyurethanes

Cellulose acetate, cellulose nitrate, cellulose

balsam, natural rubber

Cellulose derivatives:

acetate-butyrate, methyl cellulose, hydroxy ethyl cellulose, carboxy ethyl cellulose, and others Phenolic resins:

phenol-formaldehydes, resorcinol formaldehydes

Amino plastics:

Polyepoxides and derivatives:

urea-formaldehydes, melamine formaldehydes polyepoxides, epoxy-polyamide,

epoxy-bitumen, epoxy-polysulphide, etc Unsaturated polyesters

Polyaromatics:

Polybenzimidazole, polyimide, polybenzothiazole

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linked structures, and may be regarded as structural adhes-

ives Included in this group are epoxides and phenolics

16.4.3.2 Application and setting

During the bonding operation the adhesive must be applied to

the substrate in a fluid form, to wet the surface and penetrate

its surface irregularities without leaving air voids The adhes-

ive must then set by changing from a liquid to a solid The

solidification mechanisms used are:

1 Solvent evaporation

2

3 Chemical reaction

Mechanisms (1) and (2) generally apply to low-performance,

thermoplastic, materials while (3) applies to most thermosett-

ing engineering adhesives The rate of chemical reaction is

increased considerably by exposure to heat, and as a rule of

thumb is approximately doubled for every 8°C rise in tempera-

ture

Cooling from above melting point

16.4.3.3 Selection criteria

Lees5’ states that there exist twelve major family groups of

adhesives likely to be of value to the engineer This poses a

major problem in weighing up the pros and cons of different

systems for particular applications However, some computer-

ized processes do exist, and are excellent for newcomers to the

technology because they enable the user to gain valuable

insight into the criteria which limit selection Current UK

proprietary software includes CATS (Centre for Adhesive

Technology), EASeL (Design Council), PAL (Permabond

Adhesives) and STICK (Lucas)

The major considerations governing the process of adhesive

6 Production processes (method of application, speed of

cure, pretreatments, etc.)

7 Service conditions (exposure to heat, moisture, stress)

In general, it will be appreciated that flexible or thin sheet

materials should be bonded with relatively flexible adhesives

with a high strain to failure, whereas thick sections should be

united with a relatively stiff adhesive in order to make a

4 Acrylic (including anaerobic, cyanoacrylate and tough-

ened acrylic and tape variants)

Commercial formulations are, in general, complex and so-

phisticated blends of many components Fillers, toughening

agents, plasticizers, diluents, surfactants and anti-oxidants are

among the components which may be added to a ‘basic’

formulation

Epoxy Known to most engineers and designers, epoxies represent a group of very strong adhesives The adhesive comprises an epoxide resin and one of many reactive hardeners Numerous cold, warm and hot-cured formulations are available to meet particular requirements, and the adhesive can be supplied in paste, film, powder or liquid form Al- though often supplied as two components, ‘single-part’ formulations are also available in which the latent hardener is released for reaction polymerization at temperatures above 100°C These adhesives may also be toughened by the inclusion of a dispersedmbbery phase to improve peel and impact strength

Uses are frequently on large components, and where gap-filling, high strength, creep, moisture and heat resistance are required Many applications in civil engineering (using heavily filled cold-cure formulations), metal plate fabrication, structural engineering, vehicle and general product assembly

Polyurethane The polyurethane adhesives represent a group

of very versatile cold- or warm-cure adhesives, and many formulations are possible The very active reacting group of the resin is an isocyanate, which is combined with hardeners often similar in nature to those used in epoxies They may be supplied in paste or liquid form, either as two components or

as ‘single-part’ formulations; moisture is generally the catalyst

in the latter case Unlike epoxies, these materials can harden extremely quickly However, polyurethanes are generally weaker and more susceptible to moisture attack than other structural adhesives, so that environmental operating conditions must be considered carefully

Uses are frequently in thin-film form for bonding large sandwich panel assemblies andor where plastics and timber represent adherend materials Many applications in vehicle, general product assembly, car repairs, etc., where gap-filling and relatively light-duty connections prevail

Phenoliciresorcinolic This group represents phenol- formaldehyde and resorcinol-formaldehyde resins, numbered among the earliest structural adhesives to be developed They are rarely found outside the aircraft or timber and laminating industries Although good adhesives, they are difficult to use because water is liberated as a by-product of the curing reaction Heated presses or autoclaves are required to hold the components being bonded together under high pressures, while excess water escapes as steam They possess outstanding environmental durability

Uses are in thin-film form for bonding timber components, laminated assemblies and large metal components in the aircraft industry Their continued use stems from their ability

to maintain structural integrity under severe environmental conditions

Acrylic This group comprises several important cold-curing subgroups, all of which depend upon the activity of the acrylic group for polymerization reactions The structure of the acrylic backbone is adjusted according to the preferred curing mechanism Generally, they are used in thin-film form:

1 Anaerobic: Sometimes known as sealants and locking compounds, these may be supplied in liquid or paste form They are single-part adhesives that set characteristically in the presence of metal ions and the absence of atmospheric oxygen; under solely anaerobic conditions the rate of cure

is very slow This cure mechanism is very useful in assembly since hardening of the adhesive occurs only when the joint is closed Various levels of ‘strength’ are available, together with toughened variants which enable them to function as true adhesives

Trang 20

incorporation of rubber toughening into adhesive polymers has meant that reasonably high strength can now be obtained

in addition to toughness, at least in acrylates and epoxides A

comparison of the stresshain characteristics of different adhesives is given in Figure 16.88, showing the range of properties available

The modulus of polymers arises from the forces required to move molecules with respect to each other Adhesives therefore possess a relatively low modulus which decreases with increasing temperature For epoxies, Young's modulus E is typically between 1 and 6 G N I ~ - ~ , while the modulus of acrylics and polyurethanes may be a tenth, or even a hun- dredth, of such values Variations in temperature can trans- form materials which are tough and strong at 20°C to ones which are soft and weak at 100°C The glass transition temperature, Tgr denotes a marked change in the mechanical properties of a polymer Above Ts the material will be rubbery, and below, glass-like and stiff (see Figure 16.89) The

T i s of sealants are around -140"C, whereas those of room- temperature curing adhesives are around 40-50°C By warm-

or heat-curing adhesives their T 's are increased, and epoxides cured between 150°C and 200'C will have T i s in excess of 120°C

Organic polymers all absorb moisture and one effect is to plasticize the adhesive itself This modifies its response to mechanical deformation in a manner analogous to a lowering

of the T g Such effects are, however, reversible, depending upon proximity to moisture - in liquid or vapour form Thus water and heat have similar effects and this is illustrated in Figure 16.90 for the case of a cold-cure epoxy formulation Finally, adhesive polymers are visco-elastic and the time- dependent component of polymer response is of great importance to the use of adhesives required to sustain either permanent or transient loads; this can be a major determinant

of the fatigue life of a bonded joint Thus low-frequency load cycling and, to a lesser extent, elevated temperatures are very important considerations in adhesive selection

Uses are locking, sealing and retaining closely fitting metal

parts - particularly co-axial assemblies High-viscosity

variants are used as gasketing materials

Cyanoacrylate: These highly reactive single-component

adhesives harden only in thin-film form because surface

moisture is used to catalyse them Traces of (slightly

basic) moisture are found on most surfaces, and this slight

alkallinity is sufficient to cause polymerization within

seconds Acidic surfaces tend to inhibit curing Moisture

resistance is poor, and there is no gap-filling capability

Uses are bonding of small plastic and rubber parts, and

even human tissue

Toughened acrylic: These represent very versatile two-

comlponent adhesives for use in thin-film form The resin

is applied to one surface and an initiator to the other

When the joint is closed the initiator diffuses quickly

throltagh the resin and polymerization is complete within

miniites - although several hours may be required to

achieve full cure These adhesives bond extremely well to

most substrates, and the inclusion of a dispersed rubbery

phase confers excellent peel and impact resistance Re-

cent acrylic derivatives, developed to fill gaps, are true

two-part systems similar in nature to epoxy adhesives

Uses are bonding of sheet metal, coated metal, plastic

parts and some rubbers Reasonably large structures can

be bonded with confidence, especially where surface

treatment is minimal

16.4.3.5 Mechanical characteristics

In general, the properties of adhesive polymers are deter-

mined by their internal stmctuze, although the blend of

componi-nts in many commercial formulations prevents simple

chemico-physical relationships being drawn The types of

adhesives of greatest interest to the mechanical engineer range

from ductile polyurethane and acrylic formulations through

the stiffer epoxides to some stiff (and brittle) anaerobic

adhesives Some stiffer materials still are based upon imide

chemistry, and are used in advanced weapons and aerospace

structures The basic property trade-offs for single- hase

thermosetting adhesives have been described by Bolger' (e.g

Figure 16.87) and it can be seen that high strength is obtained

for the price of reduced ductility and toughness However, the

$

Silicones, Heat-resistant grades

urethanes,

non-structural Flexible General

adhesives grades purpose I

Figure 16.87 Properly trade-offs for single-phase thermosetting

adhesives (Reproduced from reference 58, copyright Marcel Dekker)

16.4.3.6 Summary of adhesive considerations

There are a number of sources listing the factors involved in adhesive selection and the performance properties to con-

~ i d e r ~ " ~ Reference 62 offers a checklist of considerations within the epoxy group, upon which Table 16.9 is based The choice is clearly a matter of swings and roundabouts

16.4.4 Adhesion and surface pretreatment

16.4.1.1 Concepts

simple:

1 Intimate contact between adhesive and silbstrate

2 Absence of weak layers or contamination at the interface

Adhesives join materials primarily by attaching to their sur-

faces within a layer of molecular dimensions, i.e of the order

of 0 1 4 5 nm In joints involving metallic substrates, the adhesive sticks to the surface oxide layer and not to the solid itself Being liquid, adhesives flow over and into the surface irregularities of a solid, corning into contact with it and, as a result, interact with its atomic forces The adhesive then solidifies to form the joint

Interfacial contact Adhesive bonding involves a liquid 'wetting' a solid surface, which implies the formation of a thin film

of liquid spreading uniformly without breaking into droplets

Adhesion, or how adhesives stick

The basic requirements for good adhesion are very

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Figure 16.88 Typical stress-strain characteristics of adhesives used for structural and mechanical engineering assembly (Based upon reference

57, copyright Permabond Adhesives Limited.) A I - 1 Stiff, heat-resisting, brittle epoxy; B1 - 1 tough, stiff, head-cured, single-part epoxy; C1 - 1

tough, cold-cured, two-part-epoxy; D1 - 1 stiff, cold-cured, polyurethane; E l - 1 tough, cold-cured ductile acrylic

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Adhesives 16/75 Table 16.9 Checklist of epoxy adhesive selection considerations

epoxy

Deformation characteristics

Creep Use adhesive with Tg well above service temperature;

best resistance with unmodified epoxides

(Figure 16.91); the water break-free test is the simplest

qualitative visual method of assessment Since adhesion in-

volves intimate contact of two surfaces, it is convenient to

think in terms of the free energies of the surfaces involved

Just as liquids have surface tensions or surface energy, so do

solid surfaces by virtue of the fact that they are surfaces

However, surface tension of solids tends to go unnoticed

because solids are usually too rigid lo be visibly distorted by

the interatomic, rather than intermolecular, forces holding

them together

Z i ~ m a n ~ ~ introduced the useful distinction between high-

and low-energy surfaces Most plastics and liquids have

surface-free energies below 100 mJ in-*, with organic adhes-

ives such as epoxides having low surface-free ener-

gies - usually <50 mJ m-’ Hard solids such as metals and

metal oxides, when atomically clean, have high surface-free

energies typically in excess of 500 MJ m-’ Some important

values of surface-free energies are collected in Table 16.10

An energetic surface will make a wetting liquid spread on it,

rather than remain as a discrete drop, so that adhesives should

readily spread and wet the oxide layers of metals There will,

however, be a problem with bonding plastics such as poly-

ethylene and polypropylene whose surface-free energies are

less than those of most organic adhesives However, the

problem with high-energy surfaces is that atmospheric conta-

In summary, the ideal conditions for establishing interfacial contact are that the:

Joint should be closed carefully to assist air displacement

Mechanisms of adhesion Once interfacial contact has been established, the adhesive cures in order to be able to transmit stress There is some debate regarding the basic nature of the forces then acting across interfaces which prevent them from separating under an applied load The four main theories of adhesion which have been proposed are: mechanical interlocking, adsorption, diffusion and electrostatic attraction ‘=’

However, the adsorption mechanism is generally favoured, with mechanical keying also playing an important role Whichever mechanism or combination of mechanisms are operating, the important surface characteristics are:

1 Surface energy

2 Surface chemistry

3 Surface micro- and macro-morphology The adsorption theory proposes that adhesive macromol- ecules are physically adsorbed onto the substrate surface because of the forces acting between the atoms in the two surfaces In effect, the polar nature of the adhesive molecules acts like a weak magnet and they are attracted toward polar adherend surfaces The most common interfacial forces are van der Waals’ forces, referred to as secondary bonds, although hydrogen bonding and primary bonding are involved

in some cases

Mechanical keying or interloclting of the adhesive into the irregularities or pores of the substrate underlies the instinctive procedure of roughening surfaces to improve adhesion On porous or fibrous materials, the adhesive certainly flows into

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Table 16.10 Values of surface-free energies

energy, y, in vacuo

(mJ m-’)

E (G Nm-*)

High-energy surfaces

C (diamond)

F e 2 0 3 (ferric oxide or red haematite)

Al’O, (alumina or aluminium oxide)

S i 0 2 (silica or silicon dioxide)

i

72.2

58 51.6

a Large polar component, y~

the structure of the material and is keyed in place, The

penetration of adhesive into the microstructure of high-alloy

metal oxide layers is important, and this micromechanical

depends upon the nature of the adherends (e.g Table 16.11), and some effects are summarized in Table 16.12 Surface pretreatment generally involves:

interlocking at a molecula; scale aids the retention of adhesion

under severe environmental (e.g Figure 2, 1, Removal of weak surface ,avers Cleaning -

i L no\

3 Re-cleaning

LU Y L )

16.4.4.2 Surface pretreatment

Adhesives are quite often blamed for ‘not sticking’, but the

general source of the trouble lies with the surface preparation

Surface pretreatments, while greatly affecting bond durability,

generally have less effect on initial strength Inadequate

surface pretreatment is usually the main cause of durability

problems and of joints failing in service.66 While adhesives are

available which can absorb oilv films and a certain amount of

It can be appreciated from Table 16.12 that surfaces can be made to be very much more ‘receptive’ towards adhesives in terms of altering the adherend’s surface chemistry, energy and morphology A detailed literature exists giving practical information on pretreatment procedures for a wide range of substrate^,^',",^^,^'-^^ while a number of surface analytical and optical techniques have been devised for examining surfaces before and after treatment.65@ Electron microscopy has been found to be particularly valuable (e.g Figures 16.92 and 16.93)

The main methods of surface pretreatment fall into four groups:

surface contamination, some form of pretreatment is generally

recommended The degree of surface pretreatment required

Adhesive macromolecular chains

I

Macrotopography

Figure 16.92 Schematic topography of solid surfaces

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Adhesives 16/77 Solvent degreasing - wiping, brushing, dipping or vapour degreasing in organic solvents such as acetone or trichlo- roethylene Wiping with MEK or IPA may be suitable for plastics and rubbers Alkaline cleaners and/or detergent solutions can also be used for metallic substrates Mechanical abrasion - action of wire brushes, sand and emery papers, abrasive pads, gritblasting, etc Dust must

be removed prior to bonding The effects on adhesion of such (obvious) roughening are often complex

Chemical treatments - numerous chemical treatment and rinsing procedures exist Very complex surface changes are brought about in terms of cleaning, altering surface chemistry and morphology (e.g Figure 16.93)

Physical methods - use of ionic bombardment or flaming action to clean and oxidize surfaces Useful on low-energy thermoplastic materials

Table 16:i 1 Pretreatment requirements

Material Suitability for Pretreatment

Table 16.12 Effects of surface pretreatments

Treatment Possible effects on surface Substrate

Solvent (etch Remove weak boundary layer M, P

Weaken surface: region by P plasticization

Increase surface roughness P Mechanical Remove weak boundary layer M, P

Increase surface roughness M, P Chemical1 Remove weak boundary layer M, P

Increase, or decrease, surface M, P roughness

Alter surface chemistry with M, P consequent changes in the rate and degree of wetting Physical (e.g Remove weak boundary layer M, P

flame, plasma, Weaken surface region by P

corona discharge) plasticization

Alter surface chemistry M, P

Substrate abbreviations: M, metal; P, plastic

Chromic-sulphuric Phosphoric acid

16.4.5 Joint design

Load-bearing bonded joints must be designed properly It is, for instance, not sufficient simply to substitute adhesive bonding for welding, bolting or riveting The adhesive, which can

be likened to plastic material, represents a iow-modulus interlayer and is likely to be the weakest link in a joint, unless very weak or thin-sheet adherends are present Design considerations generally involve the geometry of the bond, selection of an adhesive, knowledge of the properties of the adhesive and adherend, and analysis of the stresses to which the joint will be subjected

Adhesive bonded joints should be designed to provide, as far as possible:

1 A large bond area

Self-jigging features to locate and hold the assembly during curing

It is important to design an assembly correctly to incorporate adhesive bonding Adhesives generally have good shear strengths but poor peel and cleavage properties, and the joint design should be arranged to eliminate these weak modes

of loading - for example, by the use of additional mechanical support (Figures 16.94 and 16.95) Bonds are therefore designed to place the adhesive in shear or, ideally, compression! Other common engineering joints and joint designs are depicted in Figure 16.96

16.4.5.1 Joint behaviour

The lapped joint is one of the most commonly occurring joints

in practice, and is therefore both the most studied and the configuration most often used for testing adhesives 61,62

However, lapped joints, however fastened, support most of

the applied load as stress concentrations at the two ends of the joint As shown in Figure 16.94(a), the problem is that the

loads are not linear, and this causes the joint to rotate In consequence, the adhesive layer is subjected to shear and tearing stresses at the ends of the joint; the adherends, too, are subjected to shearing, stretching and bending Double-lap

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Figure 16.94 Elastic stress distributions in various kinds of bonded joints (a) Lap shear: (b) butt-tensile and scarf; (c) cleavage; (d) peel

joints, while eliminating gross joint rotation, simply represent

a back-to-back arrangement of two single-lap joints and

significant tearing stresses still exist

The stress peaks arising in most forms of bonded joints are

at a minimum when stiff adherends and/or flexible adhesives

are involved (e.g acrylics), but can become very large when

more flexible substrates or rigid adhesives (e.g stiff epoxides)

are present (Figure 16.97) The factors influencing these peaks

The stress concentration in elastic theory can be described in

simple terms by the coefficient G,e/Eht It may therefore be

Plastic strain capacity of adhesive

appreciated that lap joint strength depends generally on joint

width, and not overlap length (Figure 16.98) However, on the

positive side, real bonded joints are inevitably formed with spew fillets of adhesive at their extremities and this generally has the effect of reducing stress concentrations at the end of the overlap length.62

Since the strength of structural adhesive joints bonded with relatively stiff adhesives is so determined by the stress peaks induced under load, many design modifications have been introduced to improve stress distribution uniformity by adherend scarfing, tapering, stiffening, and so on (see Figure 16.96) Simple butt joints are often formed with adhesives, but better designs utilize co-axial and rebated concepts in order to subdue large stress peaks caused by adherend deformations (Figure 16.99)

Where very flexible adherends are concerned, flexible adhesives must be used in combination with a joint design that minimizes peel and cleavage stresses in the adhesive Combi-

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Laminatiion Laminated timber beams Sealed joints

Location of pressure seals, gaskets, '0' rings Side seams for cans other than tinplate

Sandwich construction Transformer and motor stator stacks

Built-up sheet layers for

Stiffener

Car bonnet and boot lids

Mobile-home walls Aircraft fuselage skins

Rotational shear joints

Transmission drums Shaft-to-tube unions

Insertions

Metal edge channels for car Corner mortise joints for metal window frames Bearings in housings Bearing Small electric motor

housing armatures on shafts

Shaft quarter lights

in rotor

-

Tubular furniture TubuOar frameworlts Tube to tube

Figure 16.915 Design concepts for bonded structures

Dissimilar metal joints Aluminium trim (especially to combat

galvanic corrosion) copper, etc

Diecastings t o steel,

Figure 16136 Common engineering joints and joint designs (a) The lap joint and its variants; (b) containment joints for plates, extrusions and pultrusions; (c) ways of minimizing peel in laps, doublers and stiffeners

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Figure 16.97 Bondline stress distributions in a lap joint under load

made with adhesives of low and high modulus

nations of mechanical fastening and adhesive bonding are also

popular to overcome some of the problems of adhesive

bonded-only joints Thus bolts and rivets can be used to

subdue peeling or cleavage forces in the adhesive Clinching

and patent mechanical locking arrangements are also popular

as is spot-welding through the adhesive These hybrid

assemblies are sometimes referred to by the terms ‘riv-bond’,

‘clinch-bond’ and ‘weld-bond’

16.4.5.2 Design approaches

Structural adhesive joints are generally designed to be loaded

in shear, so that most of the analytical tools so far developed

are confined to load transfer through the adhesive in shear

Simple analyses allow only for elastic behaviour, while more

sophisticated developments enable plastic behaviour (of both

the adhesive and adherends) to be modelled

All the theoretical models for predicting joint strength

Joint length or width (rnrn)

Figure 16.98 Effect of overlap length and width on single lap joint strength (‘The 1.6 mm thick steel adherends begin to yield as overlap length increases beyond, say, 20 mm)

sleeve; (c) box assembly

1 Adherend tensile modulus ( E ) and Poisson’s ratio (v)

2 Adhesive shear (G,) and tensile (E,) moduli, and Pois- son’s ratio (va)

3 Ultimate strength properties

Analyses which allow for adhesive non-linear behaviour require data on ductility such as:

4 Yield stress (strain) and ultimate stress (strain), in shear

or tension, or both

Some very sophisticated analyses require information on the adhesive’s coefficients of thermal, and even hygroscopic, expansion

The empirical approach to the design of simple overlap joints was to construct a correlation diagram between failure

load and the joint geometrical ratio, hll, for a particular set of

test conditions Mathematical treatments of joint analysis, employing differential equations to describe stress (strain) fields, have been developed from the early work of Volkersen

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Adhesives 16/81 and of Goland and Reissner Sophisticated solutions allowing

for gross adherend deformations have been produced by

Renton and Vinson, Ailman, Hart-Smith, and Grant These

approaches are documented by Adams and Wake6’ and have

been adapted and formulated into various software packages

(see Section 16.4.5.4)

It IS now accepted that non-linear analysis is the key to

predicting failure of structural adhesive bonded joints It is

also apparent that joint strength is determined largely by the

ultimate stress or the ultimate strain capability of the adhesive

in tension Finite element methods are recognized as being the

most useful techniques for analysing real joints, particularly

where large non-linearities and/or geometrical complexities

can be moldePled.6’

In recent years a fracture mechanics approach to joint

failure has been taken, at least for test method^.^^,'^.^^ The

fracture energy (strain energy release rate), G,, or the critical

stress-inteirmsity factor (adhesive fracture toughness), K,, in

tensile Mode I opening (see Figure 16.100) represents the

most critical situation in bonded joints G, or K , may be

related to a critical crack length in a bonded assembly, such

that cracks which are shorter are safe and cracks which are

longer are self-propagating and potentially catastrophic

Naturally, the use of rubber-toughened adhesives which inhi-

bit crack propagation is an attractive option

16.4.5.3 Test procedures

Bulk adhesive andlor bonded joint tests may be used for

reasons such as:

1 Comparing the mechanical properties of a group of adhes-

ives

2 As a quality check €or a batch of adhesive

3 Check.ing the effectiveness of surface pretreatment(s)

4 Measurement of environmental effects

5 Determining quantitative mechanical and physical data on

adhesives for structural design and analysis

6 Analysis of fracture mechanisms

A large number of parameters are involved in the fabrication

and testing of bulk adhesive specimens and adhesive joints;

these must be controlled if meaningful experimental data are

to be obtained Bulk adhesive tests are useful for determining

quantitative mechanical and physical properties Data from

joint tests usually reflect the mechanical properties of the

adhesive as well as the degree of adhesion and the effective-

ness of surface treatments Many standard test procedures are

listed by ,4STM, BSI,74 DIN and other official bodies, and

some joint test methods are listed in Table 16.13 Useful

commentaries on the relative merits of such methods, and an

interpretation of the data generated, are presented by many

Comparative data on adhesive performance is commonly

obtained from single-lap shear testing to BS 5350 and/or

ASTM 931002 Adhesive formulators generally quote data

from such tests, and an enormous data bank now exists

worldwide As discussed previously, the adhesive is subjected

to cleavage as well as to shear, in a ratio dependent upon

various joint stiffness factors Nevertheless, comparative pro-

perties of different bonding systems and surface treatments

may be evaluated over a range of enivironmental conditions

Cylindrical pin-and-collar assemblies are often used for testing

low-viscosity adhesives such as anaerobics Peel tests are

generally employed where flexible substrates are involved, or

where a test of adhesion is required

Quantkative data on adhesive mecbanical properties such as

moduli, Poisson’s ratio, stress and strain to failure, and glass

transition temperature are difficult to determine Bulk tests

authors, 61,62,65,75

Figure 16.100 Principal fracture modes

(e.g tensile dumb-bells, blocks for compression, strips for torsion pendulum and water sorption experiments) may be conducted on carefully cast specimens Suitable joint tests include torsional shear or thick adherend shear test methods using appropriate strain measurement It is usual to conduct

such tests over a range of temperatures A discussion of such

methods and procedures is given by the Engineering Sciences Data Unit in London.76

Tests for adhesion and surface treatment effects should subject the bonded interface to tensile stresses Lap shear, peel and cleavage (fracture energy) tests are therefore common These should be used in conjunction with the results of surface analytical studies to assess surface energy, chemistry and morphology

Fracture energy tests are usually conducted in tension (Mode I) and/or shear (Mode 11) Parallel or tapered cantilever beam configurations are common for investigating GIC and K I ~ A thin parallel cantilever beam geometry is used in the ‘Boeing Wedge Test’ derivative €or assessing surface treatments - where crack propagation is monitored along the adherend/adhesive interface, as opposed to extending cohes- ively within the adhesive layer Ironically, ‘toughened’ adhesives can be awkward to test!

Environmental testing Durability assessment represents a very important aspect of adhesive bonding technology Most test methods are required to provide accelerated ageing Since the major environmental factors influencing joint degradation are moisture, heat and applied stress, accelerated ageing methods seek to combine some or all of these factors Suitable test configurations involve small bonded areas and include simple lap joints (sometimes perforated to accelerate moisture accession), peel tests, tensile butt-joints and wedge cleavage specimens All such joints may be exposed to various natural

or aggressive laboratory environments, either unstressed or stressed Fatigue loading may even be included Obviously, it

is important to note, or monitor: the locus of failure; interfacial failure indicates an unstable surface A detailed treatment of failure mechanisms and of methods of testing and assessment is given by Kinloch 66

Test joints versus real joints Real joints do not consist of simple, separate, elastic materials with a clear mathematical geometry The adherend surface is often rough, and the thickness and properties of the primer (if applied) and adhesive layer are often difficult to determine There also remains some debate as to whether the in-bondline, or thin-film form, properties of the adhesive are the same as they are in bulk In service, the applied loads will generally be much lower than those applied in the laboratory

There remains a need for the development of appropriate, cheap, simple and quantitative test methods for general engineering applications of adhesives The test coupon approach

to design and quality control has been reviewed by many

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Table 16.13 Some standard test methods for adhesive-bonded joints

ASTM D2557-72 (83) ASTM D3163-73 (84) and

ASTM D905(86), D906 (82) D2339-82 and D3535-79 (84) }

ASTM D3983-81 (91) DIN E 54451-77

BS 5350: Part C11: 1979 and ASTM

BS 5350: Part C12: 1979 and ASTM D

BS 53.50: Part C13: 1980}

D903-49 (83)

1986-72 (83) ASTM D1781-76 (81)

ASTM D429-73

ASTM D4027-81 (91) ASTM D229-70 (81) ASTM D2182-72 (78)

BS 5350: Part C15: 1982

BS 6319: Part 4: 1984

BS.5350: Part G2: 1987 ASTM D 3983-81(91) DIN E 54451-77

Single- or double-lap joint test Basic metal-to-metal single lap joint test Double-lap joint test

Specifically for polymeric substrates Single-lap joint test for metal-to-metal joints at elevated temperatures

As above but at low temperatures Specifically for wooden joints Thick substrates used; shear modulus and strength of adhesive determined

Floating-roller test

90" peel test

180" peel test

'T' peel test for flexible-to-flexible assemblies

Climbing drum test for skin-sandwich

Rubber-to-metal bonding

Modified rail test See Torque strength Disk shear in compression Bond strength in compressive shear Slant shear test, loaded in compression, for resins used in construction; concrete substrates used

Collar and pin bonded with anaerobic adhesive and loaded in tension

See lap joints loaded in tension

Compact tension specimen

Parallel- or tapered double- cantilever-beam joint for determining the adhesive fracture

energy, GIC

Wedge cleavage test (for aluminium adherends)

Single-lap joint loaded in tension

Laminated assemblies

For determining pure-shear strength and shear modulus of structural adhesives (napkin-ring specimen)

Specifically for ultraviolet light-cured glass-metal joints

Anaerobic adhesives on threaded fasteners

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Adhesives 36/83

Impact rlesistance ASTM D3807-79(84)

ASTM D950-82

BS 5350: Part C4: 1986

BS 5350: Part C7: 1976 ASTM D1780-72(83) and ASTM D2294-69(80) 1

Plastics-to-plastics joints Block shear specimen

Various test geometries permitted Single-lap joint loaded in tension Single-lap joint, having long overlap, and loaded in compression

Subjected to stress, moisture and temperature; uses peel joint

As above, but uses single-lap shear joint loaded

in tension

As above but uses a wedge test Exposure to moisture and temperature Exposure to cyclic laboratory ageing conditions Exposure to artificial and natural light Exposure to chemical reagents Exposure to oxygen

Natural weathering Exposure to high-energy irradiation

authors.;” A common observation is that problems which are

likely to arise in the ‘real’ structure simply do not arise in the

test coupon

16.4.5.4

The use o f structural adhesives requires both essential choices

between the many types available, as well as a considered

decision on the design approach appropriate to structures

assembled with them A balance of material requirements and

an understanding of their interaction must be achieved for

optimum1 performance The simplest way forward currently

could involve the following steps:

The WQY forward in design

Adhesive selection and usage considerations from litera-

ture sources and/or proprietary software such as CATS

(Centre for Adhesive Technology), EASel (Design Coun-

cil), PAL (Permabond Adhesives), STICK (Lucas)

Structural design using stress analysis programs, e.g

BISEPS and CADEPT (Harwell Laboratory), CATS,

ESDU s ~ i t e s , ~ ~ , ’ ~ PAL, STICK, and software from

PERA International Appropriate factors of safety should

be included to allow for adhesive material uncertainties

and changes with response to environmental conditions

Experimental work to investigate joint strength and dura-

bility, particularly with respect to the range of likely

operating conditions

16.4.6 Fabrication and assembly

Education and training of the personnel involved is essential

They must possess a qualitatively correct overall picture of the

importance of the different stages of the bonding operation,

and of the health and safety considerations Control of the

working environment (temperature, humidity) may also be

important, particularly with regard to cleanliness

As indicated in Section 16.4.4, some form of surface pretreatment is recommended - particularly to enhance interfacial stability This is particularly important where cold- curing epoxy formulations are involved, or where joints will be subjected to hot moist and/or stressed conditions in service Some adhesives can absorb oily films, but solvent cleaning is very effective in improving joint quality Depending upon the substrate, mechanical abrasion, chemical etching or physical treatments may additionally be necessary Such procedures must be well defined

The time elapsed between surface pretreatment and application of the adhesive should be kept to a minimum in order to minimize subsequent contamination Priming of the surfaces involved can serve the purposes of protection and sealing (of porous surfaces), as well as providing a more reliable and reproducible surface ready for bonding Priming may be conducted with, what are essentially, dilute solutions of adhesives themselves, chromate/phosphate solutions, or with chemical coupling agents such as silanes.”

Adhesive mixing, dispensing and application must be controlled carefully The techniques involved should be considered alongside the component design, production rate and adhesive employed Single-component adhesives can be applied by spraying, brushing, roller-coating, or direct extru-

Surface preparation of the components Application of a primer (if used) Mixing, dispensing and application of the adhesive Curing or setting of the adhesive

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sion (for paste-like materials) Two-component systems must

be carefully measured and mixed before dispensing, either

manually or automatically In recent years there has been a

revolution in adhesive packaging so that weighing or measure-

ment is not required, minimizing errors and mess For high-

volume applications equipment is available for automatic

metering, mixing and dispensing, even by robots, of adhesives

with a wide range of viscosities and mixing ratios

Insufficient adhesive will result in joint performance and

durability problems, while a gross excess will hamper assembly

and necessitate clean-up and extra finishing operations Joints

should also be closed carefully to minimize the inclusion of air

With close-fitting parts over large bonded areas a means of air

escape should be provided A good joint design will assist the

positioning and location of the joint parts, but jigs or perma-

nent mechanical fixings may also be required to hold the

components while the adhesive cures Bondline thickness

control is required both to confer a uniform design thickness

as well as to prevent displacement of the adhesive under

pressure from clamps or jigs It is often desirable to leave

fillets of adhesive around the perimeter of a bonded joint, and

these can be tooled when the joint has been closed

Once the adhesive has been applied, the curing and harden-

ing process must take place This may be within a matter of

seconds for some acrylate adhesives or several hours for

cold-curing epoxy formulations The various curing systems

employed were discussed in Section 16.4.3, from which it is

clear that provision may sometimes have to be made for

moisture or solvent evaporation, conversely for providing

moisture for polyurethanes or, perhaps, ultraviolet light for

some acrylic formulations The time of reaction can be shor-

tened considerably by the application of heat, and a typical

time-temperature curing profile for a single-part epoxy is

depicted in Figure 16.101 Ovens, infrared lamps, induction-,

radiant- and resistance-heating systems are among the me-

thods for achieving faster curing rates

16.4.6.2 Health and safety

Safety aspects should partner the measures practised within a

quality system The history of adhesive bonding has shown

that accident and health problems associated with the techno-

logy are rare Commonsense precautions such as the use of

skin and eye protection are sufficient for many applications

Figure 16.101 Typical time-temperature cure profile for single-part

epoxy adhesive in an air circulating overn (Based upon reference 57,

copyright Permabond Adhesives Limited)

Concentration and duration of exposure to hazardous materials generally governs any risks

Legislation requires the adhesive supplier to label and classify products, as well as including standard risk phrases and safety procedures Production information sheets must also be supplied in accordance with the Health and Safety at Work Act Useful guidelines for users of adhesives are documented by the British Adhesives and Sealants Association Most engineering adhesives contain no organic solvents and therefore problems such as solvent abuse are not present However, solvents are sometimes used in considerable volume for surface degreasing processes as well as for cleaning-up operations Some surface treatment processes utilize acids and other chemicals, for which a hazard is presented both in handling the liquids as well as with disposal of the residues Mechanical abrasion techniques carry an obvious, albeit limited, risk

When comparing the risks inherent in other joining processes such as brazing or welding the use of adhesives can significantly improve safety

16.4.7 Quality control and non-destructive testing

16.4.7.1 Quality control

Test methods for the control of fabrication procedures and non-destructive testing (NDT) are basic requirements for the formation of structural adhesive joints There are two essential but different quality aspects to be considered These are the:

1 Adhesion between the adhesive and substrate

2 Cohesive strength and integrity of the cured adhesive layer

A measurement of potential adhesion only is possible, restrict-

ing (1) to an assessment of the adherend surface characteristics

prior to bonding through visual inspection, wettability tests, or surface potential difference techniques (for monitoring contamination) Appropriate post-bonding mechanical tests of adhesion include assessments of peel, tension and cleavage The quality of the cured adhesive layer can depend on many factors Adhesive materials may be assessed in the fresh uncured state (e.g by measurements of viscosity, sag, pot-life, etc.) as well as in their hardened state In the latter case mechanical measurements and NDT techniques may be used Quality variations in the cured adhesive layer may be due to the presence of air voids (trapped during joint assembly), local areas of uncured material, insufficient curing or cracking due

to shrinkage on cure The presence of such defects may or may not be important, depending on their extent, location and so

on In many joints such defects are unimportant unless around the joint perimeter and/or in regions of high stress transfer (e.g at the ends of a bonded overlap) The nature and significance of defects is reviewed by Adams and Wake.62

16.4.7.2 Non-destructive testing (NDT)

NDT represents a lar e and diverse field in which a number of review papers exist.'282383 The NDT methods available are essentially void detectors, although claims are made that information can additionally be obtained on the density and thickness of the adhesive, and even on the overall structural stiffness of a bonded assembly Numbered among the techniques are:

1

2 Ultrasonic methods

3 Acoustic emission Sonic methods, such as coin-tapping

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16/85

16.5.2 Problems inherent in casting technology

The problems inherent in casting technology may be classified under the headings ‘Shrinkage’, ‘Gas’ (originating from metal and mould), ‘Coarse structure and segregation’, ‘Inclusions’ (originating from metal and mould) and such defects as ‘Cold shuts’ where metal streams meet but do not unite, or dimensional errors which arise from movement of cores

4 Thermography

5 Radiography

6 Holography

Methods using ultrasound (21, heat (4) and X-rays (5) have

been developed quite successfully for monitoring density

variations and disbonds in bonded structures, particularly in

sandwich panels Advances in such techniques have been

spurred on mainly by aircraft industry requirements for pro-

duction control and regular inspection of airframe integrity

16.5 Casting and foundry practice

16.5.1 Introduction

Casting has significant advantages compared with other me-

thods of component manufacture Castings are generally

cheaper than components made in other ways The casting

process in one or other of its forms provides the designer with

an unrestricted choice of shape made in a single stage A

casting can usually be made much closer to the chosen design,

which provides savings in both material and finishing pro-

cesses compared with other methods of manufacture In

addition, the cast structure has the highest resistance to

deformation at elevated temperatures so that castings have

higher creep strengths than wrought and fabricated compo-

nents This advantage can be enhanced by modifying and

aligning solidification to produce highly creep-resistant struc-

tures suchi as bundles of crystals with one crystallographic axis

oriented lengthwise or single crystals (see Chapter 7, Section

7.4 non-ferrous metals) Cast metal may also have superior

wear resistance than the equivalent forged metal These

advantages have combined to ensure that casting has become

the most important process for the manufacture of compo-

nents in metals (and in some other materials)

Castings may be cast in one of a variety of ‘sand’ moulds

formed airound a pattern and classified as ‘sand castings’; in

one of a variety of moulds formed around a fusible wax

pattern and classified as ‘precision’ or ‘lost wax’ castings; in a

metal mould and classified as ‘die castings’; or they may be

formed by centrifugal force and classified as ‘centrifugal

castings’ ‘Splat’ casting produces material with the optimum

mechanical properties in the form of small flakes

Even when casting has not been adopted to generate the

shape of a component it may well have entered into the

process of manufacture at an earlier state having been used to

produce the stock for mechanical working, forging, rolling or

extrusion by a process such as ‘ingpt’, ‘billet’, ‘continuous’ or

’semi-continuous’ casting

The advantages of castings have, in the past, been offset by

significant disadvantages compared with wrought products

Castings are considered to be less ductile than the equivalent

wrought product, and they have a less consistent performance

in fatigue: and inferior integrity The difference in ductility

may be more apparent than real A forged or rolled compo-

nent may have a higher ductility than a casting in the direction

of forging or rolling but a significantly lower transverse

ductility This is a distinct advantage if the longitudinal

direction has to resist the principal stress but it is not neces-

sarily a sign of inferiority of the casting

However, some of the problems concerned with brittleness,

lack of integrity and an inferior and less consistent perfor-

mance under fatigue loading stem from fundamental difficul-

ties in the: casting process, These problems will be highlighted

and analysed and the way in which modern developments

ameliorate and eliminate them will be indicated in the

accounts of individual techniques

16.5.2.1 Shrinkage and contraction

Metals (with the exception of some alloys of antimony, tin and bismuth) contract in volume when they solidify and continue

to contract as they cool to room temperature If, therefore, a substantial body of metal is located alongside thinner sections which solidify earlier so that it cannot be fed from them that body will shrink by forming external sinks, internal shrinkage cavities or possibly interdendritic voids which may communi- cate through the walls The influence of shrinka e cavities on the UTS of A357 alloy is shown in Figure 16.10’2!4 If the body

is constrained excessively by the mould or by the rest of the casting while its metal is in a hot short condition it will crack (An alloy is ‘hot short’ when it comprises solid islands or dendrites surrounded by thin bands of molten metal and therefore has no strength or ductility.) If it has reached a temperature at which it is ductile, it will distort and perhaps fail to clean up during subsequent machining

Design of components to be cast should therefore, wherever possible, avoid the introduction of isolated relatively heavy sections Where it has not proved possible to achieve this, casting design should ensure either that such sections are heavily chilled and the flow of metal arranged so that they solidify at the same time as the rest of the casting, or that adequate arrangements are made to feed them by liquid metal channelled to them for this purpose Interdendritic shrinkage cavities may be present even in cast material which n a y be considered to have been fed satisfactorily

Cores should, as far as possible: be made weak enough to give against the contraction stresses imposed by the cast metal and should be removed from the casting as S Q Q ~ as possible after the metal has solidified

16.5.2.2 Gas

The effect of gas varies with the metal cast Hydrogen dissolved in molten aluminium may generate cavities of its own but generally increases the size of shrinkage cavities In steel, hydrogen causes a number of very dangerous effects described in Chapter 7, Section 7.3 (Corrosion) The requirement for good-quality castings demands its complete removal from both metals Fortunately, hydrogen is reasonably easy to remove from molten metal, in the case of aluminium by bubbling gas through the melt before casting Figure 16.103

2 6 0 4 , 1 I 2 I 8 I I ’

Pore area m m ’

Figure 16.102 Strength of fully heat treateed A357 alloy as a

function of maximum observed pore size on the fracture surface

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Figure 16.103 Spinning rotor argon lance system for degassing molten aluminium (courtesy of Foseco plc)

shows the arrangement of a spinning rotor argon lance system

which, it is claimed, will approximately halve the quantity of

hydrogen in 250 kg of molten aluminium in 10 min Hydrogen

is removed from steel by vacuum treatment of the metal in the

molten state (see Chapter 7, Section 7.3, Ferrous metals)

Oxygen is removed from molten steel by ‘killing’ with

silicon andor aluminium In the case of ingot casting of

unkilled or ‘rimming’ steel advantage is taken of the effect of

the bubbles of oxygen evolved when this steel solidifies to

neutralize the solidification contraction and eliminate the pipe

on the top surface It therefore becomes possible to forge and

roll the whole of the ingot, and not go to the expense of

discarding the top 30%, as would have to be done with killed

steel

in solute (in the case of steel this includes carbon and other alloying agents) and the result is a coarse segregrated struc-

ture The result of variation of cooling rate on tensile proper-

ties of A357 alloy is illustrated in Table 16.1484 and the effect

of structural variations on fracture toughness in Figure 16.104.@ The cooling rate should be the maximum consistent with good feeding and is compared diagrammatically for a selection of casting processes in Table 16.15

16.5.2.4 Inclusions

Inclusions can originate from oxide formed on the metal surface from slag, flux, from the melting crucible or from the mould and can have just as great an effect on metal properties

as pores (see Figure 16.10584) Clearly, all possible precautions must be taken during melting to provide as clean a metal

as possible In addition, weirs that trap inclusions should be

16.5.2.3 Coarse structure and segregation

Slow cooling of solidifying metal generates a columnar struc-

ture of coarse dendrites growing perpendicular to the cooled

surface which have (in an alloy with a margin between liquidus

and solidus) a lower solute content than the melt The space

between and in advance of the dendrites is therefore enriched

provided in the runner of the casting and filters which are now available for all types of cast metal can be inserted in the metal stream Filters not only remove entrained oxide and non- metallin but will, if strategically placed, eliminate turbulence, the flowing metal on the downstream side emerging as a

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Casting and foundry practice 16/87

smooth coherent flow (see Figure 16.106) Filters restrict metal flows, a filter 50 x 50 X 22 mm gives a flow rate of

6 kg s-’ of grey cast iron at 300 mm head Impingement

against the mould wall and consequent entrapment of mould material are avoided

Upward pouring, as in the low-pressure die casting and Cosworth processes, is a very good solution to this problem because it allows metal to be transferred upwards from below its surface without turbulence, thus minimizing inclusions Once the casting has been formed the molten metal head is maintained until all parts of the casting have been fed, thus maintaining a reservoir of hot metal as long as required With top-pouring methods, on the other hand, the top of the feeders may solidify as soon as the casting

stress (MPa) (MPa) (”/I

Figure 16.104 Fracture toughness of A357 alloy in castings solidified

over a range of cooling rates with and without Sr modification

Inclusion size, mm’

Figure l(i.105 Fracture strength of a porosity-free magnesium alloy

as a function of inclusion size

16.5.2.6 Core movement

Cores are normally of lower density than the surrounding metal and may float when submerged or be displaced by the metal flow Core prints must be accurate in dimensions and adequate in size Where necessary, core prints may be provided with steel bushes which fit in corresponding bushes in the mould

16.5.2.7 General

The problems described above have been long appreciated and techniques are being developed which will completely overcome them These techniques are not necessarily applicable to all metals or to every size and shape of casting, but where they can be applied they will produce castings with consistency in fatigue behaviour adequate for any requirement

The following sections describe the techniques used to produce castings and stock for mechanical working with particular emphasis on the most recent developments and most advanced processes which provide the highest quality products in the most efficient way Not every difficulty has been eliminated with every metal and alloy cast, but this account will indicate how castings which will perform fully as well as forgings (and, in some cases, better) can be produced

in an increasing proportion of materials

It is not possible in an account of this length to detail the very large number of variations in casting process which are and have been employed Where a selection has had to be made the process or variation described is, as far as possible, the most efficient, the most modern, and produces a product

Table 16.15 Spectrum of casting processes represented as a function of cooling rates

illustrating some alloys cast by the various techniques

Low-pressure die casting

Gravity die casting

Cosworth and Investment casting

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b

s

J

Figure 16.106 'Dypur' filter unit showing how a strategically placed

filter can eliminate turbulence in a molten metal stream (courtesy of

Foseco plc)

of the highest material quality, mechanical properties, dimen-

sional accuracy and surface finish

16.5.3 Ingot, billet and slab casting

16.5.3.1 Conventional casting of steel

For many years stock for mechanical working was made by top

pouring individual ingots or billets typical of which are the

square steel ingots shown in Figure 16.107.85 During this

period great advances have been made in metal treating and

handling techniques Cleaner steelmaking technology, better

deoxidation control and secondary steel making in the ladle

and the arc furnace have been introduced and furnaces have

been designed (for steel and also for aluminium) which yield

metal free from both surface and bottom oxides and slag

Hydrogen can be removed from molten steel by vacuum

treatment, and from molten aluminium as described above

New types of refractory stoppers, both vertical and sliding,

Figure 16.107 Segregation and bridging in typical steel ingots

have made it possible to start, stop and control the rate of flow

of the casting stream

However good the metal quality, however carefully the dimensions, shape and casting parameters of the ingot were chosen, and however well the ingot was fed (and however effective the measures taken to keep the top of the ingot hot) ingots formed from killed steel developed a pipe (which sometimes bridged) in which was located considerable segregation (see Figures 16.107 and 16.108@) There was also usually segregation in a band round the ingot where the columnar crystals which advanced from the surface gave way

to the equiaxed structure Segregation also developed if cracks appeared between the columnar crystals at the surface and material of a higher solute concentration bled out of the cracks (This formation of blebs enriched in impurity elements also occurred in aluminium ingots.) To avoid vertically orien- tated surface cracks the perimeter section of large ingot moulds consists of a series of arcs intersecting at cusps

In a top-poured ingot the metal has to fall to the base of the mould from a height at least equal to that of the top of the mould The resultant splashing caused surface defects which were so serious that stainless steel ingots, where surface quality is paramount, used to be bottom poured in spite of the increase in pipe and segregation which resulted The combined effects of pipe, segregation surface defects and cracking

of individually cast ingots resulted in a substantial proportion

of metal being discarded The introduction of continuous (and semi-continuous) casting proved so effective in overcoming these defects that this process is ra idly displacing individual casting in steel (see Figure 16.109 2 ) and other metals

16.5.3.2 Continuous casting of steel

Billets, blooms, slabs, thin slabs and strip may be continuously cast Continuous casting eliminates longitudinal segregation and for billet and bloom casting horizontal segregation may be reduced or eliminated by electromagnetic stimng of liquid steel in the mould (M-EMS), in the strand (S-EMS) and/or in the final stage of solidification (F-EMS) (Centreline segregation may, however, be a problem in slab where electromagnetic stirring is difficult to apply.) Defects in quality are therefore restricted to surface defects, including cracking, possible internal cracks and inclusions derived from non-

metallic particles carried in the molten metal

The practices and techniques devised to improve the efficiency of a continuous slab castin machine may be described with reference to Figure 16.110,' which shows a continuous slab casting machine with hot connection facility Re- oxidation of the steel stream from ladle to tundish and from tundish to mould is prevented by using a ladle shroud, a submerged entry nozzle, and by flooding with argon Flow from the tundish to the mould is controlled b replaceable stoppers The tundish design (Figure 16.1118;) has been improved to give longer residence time, to incorporate weirs and dams and to allow bubbling of inert gas through porous plugs located at the bottom to improve flotation and removal

of inclusions Tundish level control has been made automatic and the tundish can be heated for start-up

Flow control from ladle to tundish and from tundish to mould has been integrated and automated, and level in the mould controlled by a duplex electromagnetidradioactive system The metal in the mould is protected and the metal mould interface lubricated by a 50 mm layer of 'black powder'.88 This is the name given to a series of proprietary fluxes probably containing calcium silicate and fluoride and more than one variety of carbon The geometry of the particles and

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