Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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Fig.

20.4 Diffusion soldering, (i) a thin tin foil is sandwiched between the silver-metallised

substrates; (ii) the tin foil melts and reacts with the silver

film,

forming Ag

Sn; (iii) Ag

Sn (m.

pt. 830

C) solidifies forming a heterogeneous bond; (iv) further heating homogenises the

bond forming a Ag/Sn solid solution (m. pt. about 900

C).

metallised* with silver and then separated by a thin layer of tin foil. The

assembly is then heated to 250

C so that the tin melts and begins to dissolve

the solid silver with which it reacts to form the intermetallic compound

Sn. Since Ag

Sn has a melting point of 480

C it begins to solidify, and

because the tin foil is thin compared with the silver layers it is soon used

up so that the joint solidifies completely. Continued heating at 250

C causes

the solid Ag

Sn and the remaining solid silver to diffuse into each other

more completely, so that Ag

Sn transforms to Ag

Sn (m.pt. 724°C) which

in turn transforms to a silver-tin solid solution with a melting range in

the region of 900

Indium foil may be used instead of tin in which case a lower treatment

temperature of 175°C can be used (m.pt. of indium 156.6

C). Here the

intermetallic compound AgIn

is first formed, transforming to Ag

In and

ultimately to a solid solution with a freezing range in the region of 900

Using diffusion soldering a joint can be made using lower temperatures

than those to which the device will be subjected in normal operation. Thus

expansion mismatch stresses can be kept very low, a useful feature for

example when joining a heat sink to a device.

Brazing

20.30 Metallurgically this process (17.80—Pt. 2) is similar to soldering in

that the filler metal melts and alloys with the solid work pieces. Brazing is

generally used where a tougher, stronger joint is required, provided that

* 'Metallising' involves coating some surface with metal by metal spraying, chemical or vacuum

deposition.

Fig.

20.3 The wave-soldering of printed circuit boards.

solder bath

substrate-

silver

coating

tin foil

silver

coating

substrate

the work will be neither melted nor otherwise damaged by the higher

temperatures involved in melting the brazing solder. Most ferrous and

non-ferrous alloys of sufficiently high melting point can be brazed. A tra-

ditional borax-type flux is used for solders which melt above 750

C, but

below that temperature borax does not fuse readily and a mixture of borax

and metallic fluorides is employed. Fluoride-base fluxes are also useful

when brazing refractory materials, whilst alkali-halide fluxes are necessary

for brazing aluminium.

Probably fifty-per-cent of industrial brazing is still carried out using a

hand-held gas torch but mass production employs a wide range of heating

methods in conjunction with pre-placed brazing alloy and flux. Stationary

gas torches and batch or conveyor-hearth furnaces have been long estab-

lished. Electric methods involve high-frequency induction and interface

heating (Fig. 20.5), whilst superalloys are generally brazed in a near

vacuum.

Fig.

20.5 The principles of resistance brazing by interface heating.

Ordinary brazing solder contains about 60Cu-40Zn. This would nor-

mally give a coarse Widmanstatten a + |3' structure, but during brazing

some of the zinc will volatilise and some may be absorbed by the work

pieces. The resultant joint will therefore be of a tough ductile a structure.

Higher-grade brazing compounds, or silver solders, contain up to 85%

silver and melt over lower temperature ranges. Special brazing alloys are

available for aluminium. Table 20.2 lists a few of the many brazing alloys

dealt with in BS 1845.

20.31 The brazing of silicon units in modern electronics can be carried

out using a brazing solder containing A1-12SL This is of eutectic compo-

sition (Fig. 17.2) and will also 'wet' silicon. If a lower brazing temperature

is required an aluminium-germanium brazing solder containing 53% ger-

manium is used and melts at 424°C. It alloys with silicon but has rather

poor fluidity which is improved by the addition of silver.

electrodes

brazing

alloy

work pieces

Table 20.2 Filler Alloys for Brazing

Composition (%)

BS 1845:

1977:

AL1

AG1

AG7

AG17

CP2

CZ3

CZ7A

CZ8

NK5

NK13

91.5

Rem.

Other elements

Al-86;

Si—10

Cd-19

P—6.5

Sn-0.3;

Si—0.3;

Mn-0.15

Ni-9.5; Si—0.3

Ni-rem.;

Si—3.5; B—1.8

Co—rem;

Cr—19; Ni—17;

Si—8;

W—4; B—0.8;

C—0.4

Freezing

Range CC)

535-595

620-640

780 (eutectic

alloy)

700-800

645-740

885-890

870-900

920-980

980-1070

1120-1150

Suitable for

brazing:

Aluminium and

some aluminium

alloys.

Copper and

copper-base

alloys;

mild-steel;

carbon steels and

alloy steels.

Copper and

copper-base

alloys.

Copper; all types

of steel; malleable

and wrought

iron;

nickel alloys; cobalt

alloys.

MiId-,

carbon-

and alloy steels;

stainless steels.

Nickel and nickel

alloys.

Welding

20.40 Some of the welding processes resemble both soldering and brazing

in so far as molten metal is applied to produce a joint between the two

pieces. However, in welding, the added metal is, more often than not, of

similar composition to the metals being joined, and a more positive state

of fusion exists at the metal surfaces. Thus, differences between the weld

metal and the pieces being joined are structural rather than compositional.

The method of production of a weld calls for rather different technique

to that employed in brazing and soldering. Since work pieces and filler

metal are of similar compositions the speed of working is particularly

important if fusion of the metal adjacent to the joint is to be avoided and

this calls for a high 'temperature gradient' in and around the weld. How-

ever, some welding processes rely on pressure to effect joining of the two

halves. In such cases no metal is added to form a joint, the weld metal

being provided by the two halves being joined. Thus we have both fusion

and solid-phase welding processes as indicated in Fig. 20.6.

Of recent years, welding has become one of the principal methods of

fabricating and repairing metal products. It is an economical means of

joining metals in almost all assembly processes and, assuming good welding

technique, produces a very dependable result. There are now over thirty

different welding methods available for specific applications so that it will

be possible to mention only the more important processes here. A fuller

description is given in Part 2.

Fusion Welding Processes

20.50 Either thermo-chemical sources, the electric arc or some form of

radiant energy can be used to melt the weld metal. In each case the equip-

ment must be capable of 'focusing' the high-energy heat source on to the

weld area.

20.51 Gas Methods In these processes the surfaces to be joined are

melted by the flame from a gas torch, the gases most commonly used

being suitable mixtures of oxygen and ethine (acetylene), which produce

temperatures up to 3000

C. Such a high temperature will quickly melt all

the ordinary metals and alloys, and is necessary in order to overcome the

tendency of sheet metals to conduct heat away from the joint so quickly

that fusion cannot occur. Moreover, rapid melting will reduce distortion,

overheating and oxidation of the surrounding metal.

In gas- and other fusion-welding processes a welding rod, or filler rod,

is used to supply the necessary metal for the weld. The rod is held close

to the work and melted by the flame, so that molten metal flows into the

prepared joint between the pieces being welded. Since the edges of the

pieces also fuse, a strong, metallurgically continuous joint is formed. Weld-

ing rods usually have a composition similar to that of the metals being

welded, although in some cases they are richer in those elements which

tend to volatilise during welding. Gas welding is now used mainly for

jobbing and repair work in mild steel and carbon steels. The term 'bronze

welding' refers to the joining of metals with high melting points, such as

steel, cast iron, nickel and copper, by the use of a copper-alloy filler rod.

In gas welding the flame may be made oxidising, reducing or neutral but

with those alloys which oxidise easily, such as those containing aluminium,

some form of flux will be necessary. This is normally supplied as a coating

to the filler rod. The flux should melt at a lower temperature than the

metal being welded and thus dissolves oxide films which form before fusion

of the metal begins. As the molten flux coats the surface it also protects

the work from excessive oxidation.

20.52 Thermit Welding now finds only limited application, chiefly in

the repair of large iron and steel castings, though it was the traditional

method for joining rails on site. A mould is constructed around the parts

to be joined, and above this is placed sufficient thermit powder. The parts

to be welded are preheated and the powder then fired.

Thermit powder consists of an intimate mixture of powdered aluminium

and iron(III) oxide in molecular proportions. The iron(III) oxide is reduced

by aluminium to metallic iron:

Fig. 20.6 A

classification

of the

important

welding

processes.

WELDING

PROCESSES

SOLID-PHASE

WELDING

electrical

resistance

un-shielded

arc

radiant

sou

rce

FUSION

WELDING

electric

arc

thermo-chemical

source

gas

welding

flux-shielded

arc

gas-shielded

arc

laser

beam

electron

beam

thermit

welding

bronze

welding

acetylene

smith

welding

ultra-sonic

welding

cole:

welding

friction

welding

explosion

welding

diffusion

welding

projection

flash

butt

seam

spot

carbon

arc

spark

discharge

stud

welding

MIGTIG

plasma-

arc

electro-

slag

submerged

arc

metallic

arc

The heat of the reaction is so intense that the iron formed melts and

runs down into the prepared mould, combining with the preheated surfaces

and producing a weld. Variations of the process can be used to join non-

ferrous cables on site.

20.53 Electric-arc Methods In the earliest of these processes an elec-

tric arc was struck between two carbon electrodes or between a carbon

electrode and the work

itself.

Filler metal was supplied to the arc separ-

ately. In modern processes the carbon electrode has been dispensed with

and the filler rod itself becomes the electrode.

20.53.1.

The metallic arc process is by far the most popular method

(Fig.20.8(i)) of fusion welding. Basically it is similar to the carbon arc, but

the electrode is a metal rod of suitable composition which serves also as

the filler rod. The end of this rod melts and deposits on to the joint, whilst,

at the same time, the heat generated melts the edges of the work, producing

a continuous weld. The filler rod is usually coated with a suitable flux which

not only supplies the weld but also assists in stabilising the arc. Although

used chiefly for steel, the metallic-arc method also finds application in

welding many of the non-ferrous alloys.

20.53.2. The submerged-arc process is essentially an automatic form of

metallic-arc welding, which can be used in the straight-line joining of

metals. A bare consumable electrode is used and powdered flux is fed into

the prepared joint so that, on melting, it envelops the melting end of the

electrode and so covers the arc. The weld area is thus coated by protective

slag, allowing a smooth, clean weld surface to be produced. Very high

welding currents can be used making very deep welds possible and the

process is used in shipbuilding, structural engineering and for pressure

vessels as well as in general engineering for mild and alloy steels.

20.53.3.

Electro-slag welding is used to join plates generally more than

50 mm thick by a vertical 'run' (Fig. 20.7). A considerable pool of molten

weld metal is maintained between the two work pieces by two copper

'dams'

which travel upwards as the weld metal solidifies. One or more bare

steel electrodes are fed into the slag bath which floats on top of the weld

pool. The slag is sufficiently conductive to give rise to resistance heating

and the electrodes melt providing the weld metal.

20.53.4. Gas-shielded arc processes are widely used for all materials

Fig.

20.7 The principles of electro-slag welding.

filler

wire

water cooled

copper shoe*

weld

Fig.

20.8 Electric-arc welding processes.

(i) The metallic arc—here molten flux affords protection of the weld region, assisted to a

small extent by a gas shield provided by burning carbonaceous matter in the flux coating,

(ii) The metallic inert gas-arc (MIG) in which a very effective gas blanket is obtained by a

flow of argon or helium around the weld region, (iii) The tungsten inert-gas arc (TIG)—here

a separate filler rod is used, the tungsten electrode being non-consumable.

particularly those in which oxidation is troublesome. In these processes a

bare electrode protrudes from the end of a tube from which also issues an

inert gas (argon, helium or carbon dioxide). The weld region is thus

blanketed by a non-oxidising atmosphere making the use of flux unnecess-

ary (Fig. 20.8(ii)). Carbon dioxide can act as an oxidising atmosphere

when used as a gas shield at high temperatures. Nevertheless this can be

overcome by using filler rods containing deoxidants such as manganese

and silicon. It is a much cheaper gas than argon and during recent years

the CO2 process has become the most popular semi-automatic method for

welding steel.

In the tungsten inert gas—or TIG—process (Fig. 20.8(iii)) the arc is

struck using a non-consumable tungsten electrode and an atmosphere of

argon or helium. Since filler metal is supplied from a separate rod this

makes it a two-handed process requiring high skill when manual welding

is employed. High quality welds are possible with most metals and the

process is widely used in the aircraft and chemical industries, being particu-

larly suitable for heavy-gauge stainless steel.

20.53.5. Plasma-arc welding is related to the TIG process in that argon

passes through an electric arc formed within a very narrow anular orifice

between the tungsten electrode and the water-cooled outer tube. The argon

becomes ionised—or forms 'plasma'—and as these fast-moving ions strike

the weld area to recombine with electrons, considerable heat is generated.

Very deep penetration can be obtained.

Metallic inert gas—or MIG—welding employs a consumable electrode

which is fed continuously from a coil within the hand-held 'gun'. Argon or

helium are used as shielding gases for non-ferrous metals, but for iron and

steel a mixture of argon and CO2 or pure CO2 are used. In the latter case

the process is termed CO2 welding, and the electrode wire will contain

deoxidisers such as silicon, manganese, aluminium or titanium to offset

the oxidising nature of CO2 at high temperatures.

FLUX

COATING

SLAG

GAS

SHIELD

CONSUMABLE

ELECTRODE

ARGON

GAS

SHIELD

(ARGON)

TUNGSTEN

ELECTRODE

ARGON

FILLER

ROD

Gas-shielded welding using argon or helium is the only satisfactory

method for welding aluminium alloys. An important feature, common to

all gas-shielded arc methods, is that, since atmospheric nitrogen is kept

away from the weld during fusion, nitrogen embrittlement is avoided.

Gas-shielded arc welding will no doubt displace much of the gas- and

metallic-arc welding now used, due to the superiority of the weld and the

increased speed of manipulation which is possible.

20.53.6. Unshielded arc

processes

are used for small scale work. Energy

is supplied by spark

discharge

from a capacitor bank. Thermocouple wires

are commonly joined in this way but the process is used for the mass

production fixing of connecting wires to small components in the elec-

tronics industries. In stud welding by capacitor discharge a small protrusion

is formed on the stud. When this is brought into contact with the work

piece and the circuit thus closed the capacitor bank discharges and resist-

ance heating melts the protrusion so that a weld is obtained.

20.54 Electron-beam Welding In this process, fusion is achieved by

focusing a beam of high-velocity electrons on the weld area. On striking

the metal the kinetic energy of the electrons is converted into heat energy

so that melting of the metal occurs. Welding must be carried out in a

vacuum chamber, since the presence of oxygen and nitrogen molecules

would lead to collisions with the electrons and their consequent scatter, as

well as a loss of kinetic energy. Moreover, since the tungsten filament

which emits the electrons is working at 2000

C, it would oxidise rapidly if

exposed to the atmosphere.

The process is particularly useful for joining refractory metals, such as

tungsten, molybdenum, niobium and tantalum, and also metals which oxi-

dise easily, such as titanium, beryllium and zirconium.

20.55 Laser Welding has been developed for the production mainly

of spot welds in materials up to 3 mm thick. Unlike electron-beam welding

a vacuum chamber is not required. Inert-gas shielding is provided by focus-

ing the laser beam through the aperture of a nozzle carrying argon (Fig.

20.9).

Very high temperatures and hence rapid welding can be achieved.

Since the beam can be focused accurately to a point the process is adaptable

to small-scale work for joining wires, foil and components in the electronics

industries.

Fig.

20.9 The principles of laser welding.

work

pieces

argon

laser

beam

mirror

Solid-phase Welding

These processes include the oldest of all welding operations, that practised

by the blacksmith at his anvil. The smith heats the two parts to be welded

to a temperature short of actual fusion, and then applies pressure in the

form of hammer blows to cause the surfaces to unite. The sand, which he

often uses as a flux, combines with the iron oxide produced by heating to

a high welding temperature, and forms a fusible slag which is scattered by

the hammer blows.

Electrical-resistance welding is similar in principle, and is used in the

following modifications:

20.60 Spot Welding, in which the parts to be joined overlap and are

firmly gripped between heavy metal electrodes (Fig. 20.10(i)). When the

current is passed, local heating of the sheets to a plastic condition occurs,

resulting in welding at a spot. This method is used mainly for the joining

of sheets and plates and frequently in the production of temporary joints.

It is a very widely used process, particularly when robot-controlled, in the

motor industry.

Fig.

20.10 The principles of resistance-pressure welding processes: (i) spot; (ii) seam;

(iii) butt. In each case the electrical supply source (E) is the secondary of a 'step-down'

transformer.

20.61 Seam Welding is similar in principle to spot welding but the

electrodes are in the form of wheels which drive the work 'sandwich'

forward (Fig. 20.10(ii)). The current is 'pulsed' to form a series of spot

welds which either overlap or are regularly spaced according to the timing

of motion and current pulsing. The pressure applied to the wheels is suf-

ficient to produce a weld at the heated interface.

20.62 Butt Welding is used to join lengths of rod and wire. The ends

are pressed together (Fig. 20.10(iii)) and an electric current passed through

the work so that the ends are heated to a plastic state due to the higher

electrical resistance existing at the point of contact. The pressure is suf-

ficient to form a weld.

20.63 Flash Welding is similar to butt welding in that ends of sheets

and tubes can be welded together. The process differs, however, in that

the ends are first heated by striking an arc between them. This not only

damp

electrodes

produces

the

necessary high temperature

but

also melts away

any

irregu-

larities

at the

ends, which

are

then brought into sudden contact under

pressure

that

sound weld

produced.

20.64 Projection Welding

similar

some respects

stud welding

except that here electrical resistance heating

and

pressure

are

supplied

the electrodes.

projection

punched into

one of the

work pieces

that point contact occurs when

the

work pieces

are

clamped between

the

electrodes. Localised heating occurs

that, under pressure,

the

projection

collapses

and a

weld

formed.

20.65 Ultrasonic Welding

is a

cold-joining process

which

bond

is produced between

the

work pieces

ultrasonic vibratory energy.

The

work

gripped between

anvil

and a

'sonotrode'

tip

which vibrates

laterally

at an

ultrasonic frequency. Surface films

oxide

are

ruptured

and welding then occurs between

the

slipping surfaces under

the

gripping

pressure. This method

used

for

joining wires, foils

and in

printed circuits

the

electronics industry.

20.66 Cold Welding

used

for lap

joints

sheet

and

butt joints

wire

and rod.

Sheets

are

placed together

and

then punched

that slip

occurs between their surfaces. This breaks

surface films

oxide

that

intimate contact

achieved between

the

metal surfaces, leading

weld-

ing. Only malleable metals like copper

and

aluminium

can be

welded

this

way.

20.67 Friction Welding

used

butt-weld rods

and

bars.

One bar

is held

in a

rotating chuck whilst

the

other

held

in a

stationary

but

pressure-loaded chuck

(Fig.

20.11).

As the

ends

of the

work pieces

are

brought together considerable friction between

the

interfaces causes

softening

and

consequent welding.

advantage

this process

that

dissimilar metals

can be

joined. Thus carbon steel

can be

welded success-

fully

high-speed steel, titanium, aluminium

and

zirconium.

It has

been

used successfully

in the

motor

car

industry

for

joining transmission

members, back axles

and

steering gear.

The underwater repair

cracked butt welds

off-shore structures

friction stitch welding

now

employed. Essentially this involves rotating

tapered 'filler plug'

in a

prepared tapered cavity until welding occurs.

Overlapping welds

are

made

/stitching' along

the

length

of the

defect.

20.68 Explosive Welding

useful

welding together sheets

of dis-

similar metals,

cladding

steel.

The

sheets

are

placed

at a

slight angle

to each other

and

high-explosive charge detonated above

the

upper sheet.

The surfaces virtually

'jet'

together

and

welding occurs.

Fig.

20.11 The principles of friction welding.

stationary chuckrotating chuck

pressure

brake

weld