7. pressure welding

2003

7.

Pressure Welding

7. Pressure Welding 85

Figure 7.1 shows a survey of the pressure welding processes for joining of metals

in accordance with

DIN 1910.

In gas pressure

welding a distinc-

tion is made be-

tween open

square and

closed square

gas pressure

welding, Fig-

ure 7.2.

Both methods use efficient gas torches to bring the workpiece ends up to the welding

temperature. When the welding temperature is reached, both joining members are

butt-welded by the application of axial force when a flash edge forms. The long pre-

heating time leads to the formation of a coarse-grained structure in the joining area,

therefore the welds are of low toughness values. As the process is operated mains-

independently and

the process equip-

ment is low in

weight and also

easy to handle, the

main application

areas of gas pres-

sure welding are

the welding of rein-

forcement steels

and of pipes in the

building trade.

Classification of WeldingProcesses acc. to DIN 1910

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pressure welding

welding

fusion welding

conductive pressurewelding

gas pressurewelding

resistance pressurewelding friction welding

induction pressurewelding

resistance spotwelding

projectionwelding

roll seamwelding

pressure buttwelding

flash buttwelding

Figure 7.1

Open Square and Closed SquareGas Pressure Welding

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initial state: gap closed initial state: gap opened(for special cases)

stationary mobile

ring-shaped burner(sectional view)

closed gap

working cycles:2. welding by pressing1. heating

gas flame torch in the open gap

workpiece

1. heating

2. torch positioning

3. welding by rapid pressing

completed weld seam

pressure

Figure 7.2


In pressure butt welding, the input of the necessary welding heat is produced by

resistance heating. The necessary axial force is applied by copper clamping jaws

which also receive the current supply, Figure 7.3. The current circuit is closed over

the abutting surfaces of the two joining members where, by the increased interface

resistance, the highest heat generation is obtained. After the welding temperature -

which is lower than the melting temperature of the weld metal – is reached, upset

pressure is applied and the current circuit is opened. This produces a thick flash-free

upset seam which is typical for this method. In order to guarantee the uniform heating

of the abutting

faces, they must be

conformable in their

cross-sectional

sizes and shapes

with each other and

they must have

parallel faces.

As no molten metal

develops during

pressure upset butt

welding, the joining

surfaces must be

free from contami-

nations and from

oxides. Suitable for

welding are unal-

loyed and low-alloy

steels. The welding

of aluminium and

copper material is,

because of the ten-

dency towards oxi-

dation and good

conductivity, possi-

Process Principle ofPressure Butt Welding

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before upset forcehas been applied

water-cooled clampingchucks (Cu electrodes)

upset force

~_bulging at the endof the weld

Figure 7.3

Schematic Structure of a FlashButt Welding Equipment

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secondary side

fixed clamping chuck mobile clamping chuck

clamping force

steel chuck

copper shoe

a+bb2

a

welding transformer

primary sidea = flashing lengthb = upset loss

Figure 7.4


ble only up to a point. For the most

part, smaller cross-sections with sur-

faces of up to 100 mm² are welded.

Areas of applications are chain manu-

facturing and also extensions of wires

in a wire drawing shop.

A flash butt welding equipment is, in

its principal structure, similar to the

pressure butt welding device, Fig-

ure 7.4.

While in pressure upset butt welding

the joining members are always

strongly pressed together, in flash butt

welding only “fusing contact” is made

during the heating phase. During the

welding process, the workpiece ends

are progressively advanced towards each other until they make contact at several

points and the current circuit is over these contact bridges closed. As the local cur-

rent density at these points is high, the heating also develops very fast. The metal is

liquified and, partly, evaporated. The metal vapour pressure causes the liquified

metal to be thrown out of the gap. At the same time, the metal vapour is generating a

shielding gas atmosphere; that is to say, with the exception of pipe welds, welding

can be carried out without the use of shielding gas. The constant creation and de-

struction of the contact bridges causes the abutting faces to “burn”, starting from

the contact points, with heavy spray-type ejection. Along with the occurrence of

material loss, the parts are progressively advanced towards each other again. New

contact bridges are created again and again. When the entire abutting face is uni-

formly fused, the two workpiece ends are, through a high axial force, abruptly

pressed together and the welding current is switched off. This way, a narrow, sharp

and, in contrast to friction welding, irregular weld edge is produced during the upset-

ting progress, which, if necessary, can be easy mechanically removed while the weld

is still warm, Figure 7.5.

© ISF 2002br-er7-05e.cdr

Figure 7.5


In flash butt welding, a fundamental distinction is made between two different work-

ing techniques. During hot flash butt welding a preheating operation precedes the

actual flashing process, Figure 7.6. The preceding resistance heating is carried out

by “reversing”, i.e., by the changing short-circuiting and pressing of the joining sur-

faces and by the mechanical separation in the reversed motion. When the joint ends

are sufficiently heated, is the flashing process is initialised automatically and the

following process is similar to cold flash butt welding. In contrast to cold flash butt

welding, the advantage of hot flash butt welding is that, on one hand, sections of 20

times the size can be welded with the same machine efficiency and, on the other

hand, a smaller temperature drop and with that a lower cooling rate in the workpiece

can be obtained. This is of importance, especially with steels which because of their

chemical composition have a tendency to harden. The cooling rate may also be re-

duced by conductive reheating inside the machine. A smooth and clean surface is

not necessary with

hot flash butt weld-

ing. If the abutting

faces differ greatly

from the desired

plane-parallelism,

an additional flash-

ing process (a short

flashing period with

low speed and high

energy) may be car-

ried out first.

The welding area of the structure of a flash butt weld shows a zone which is re-

duced in carbon and other alloying elements, Figure 7.7. Moreover, all flash butt

welded joints have a pronounced coarse grain zone, whereby the toughness proper-

ties of the welded joint are lower than of the base metal. By the impact normalizing

effect in the machine successive to the actual welding process, can the toughness

properties be considerably increased. By one or several current impulses the weld

Figure 7.6

Flashing Travel, Upset Travel, Upset Forceand Welding Current in Timely Order

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hot flash welding cold flash welding

upset travel

flashing travel

upset force

amperage

time time

preheating flashing flashing


temperatures are increased by up to approximately 50° over the austeniting tempera-

ture of the metal.

Steels, aluminium, nickel and copper alloys can be welded economically with the

flash butt welding

process. Supported

by the axial force,

shrinkage in flash

butt welding is so

insignificant that

only very low resid-

ual stresses occur.

It is, therefore, pos-

sible to weld also

steels with a higher

carbon content.

Profiles of all kind are butt welded

with this method. The method is used

for large-scale manufacture and with

components of equal dimensions. The

weldable cross-sections may reach

dimensions of up to 120,000 mm².

Areas of application are the welding of

rails, the manufacture of car axles,

wheel rims and shafts, the welding of

chain links and also the manufacture

of tools and endless strips for pipe

production.

Friction welding is a pressure weld-

ing method where the necessary heat

for joining is produced by mechanical

friction. The friction is, as a rule, gen-

erated by a relative motion between a

Secondary StructureAlong a Flash Butt Weld

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heat affected zone

10 mm

0,1 mm

material: C60 E

weld coarse grain zone fine grain zone soft-annealing zone base metal

Figure 7.7


n

n

F friction force1

F upset force2

Figure 7.8


rotating and a stationary workpiece

while axial force is being applied at

the same time, Figure 7.8.

After the joint surfaces are adequately

heated, the relative motion is discon-

tinued and the friction force is in-

creased to upsetting force.

An even, lip-shaped bead is produced

which may be removed in the welding

machine by an additional accessory

unit. The bead is often considered as

the first quality criterion.

Figure 7.9 shows all phases of the

friction welding process. In most

cases this method is used for rota-

tion-symmetrical parts. It is, nowa-

days, also possible to accurately join

rectangular and polygonal cross-

sections.

The most common variant of friction

welding is friction welding with a con-

tinuous drive and friction welding with

a flywheel drive, Figure 7.10. In fric-

tion welding with continuous drive,

the clamped-on part to be joined is

kept at a constant nominal speed by a

drive, while the workpiece in the slid-

ing chuck is pressed with a defined

friction force. The nominal speed is

maintained until the demanded tem-

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Phases of Friction Welding Process

Figure 7.9


conventional friction welding

clutch

driving motor

brakeclamping tool clamping tool

workpiece pressure elementfor axial pressure

flywheel friction welding

clamping tool clamping tool

workpiecepressure elementfor axial pressure

flywheel

Figure 7.10


perature profile has been achieved. Then the motor is declutched and the relative

motion is neutralised by external braking. In general, the friction force is raised to up-

setting force after the rotation movement has been discontinued. During flywheel

friction welding, the inertia mass is raised to nominal speed, the drive motor is de-

clutched and the stationary workpiece is, with a defined axial force, pressed against

the rotating workpiece. Welding is finished when the total kinetic energy - stored in

the flywheel – has been consumed by the friction processes. This is the so-called

self-breaking effect of the system. The method is used when, based on metallurgical

processes, extremely short welding times may be realised. Further process variants

are radial friction welding, orbital friction welding, oscillation friction welding and fric-

tion surfacing.

However, these

process variants

are until today still

in the experimental

stage. Recently,

new developments

in the field of friction

stud welding –

studs on plates –

have been intro-

duced.

Figure 7.11 depicts the variation in time of the most important process parame-

ters in friction welding with continuous drive and flywheel friction welding. The occur-

ing moments’ maxima may be interpreted as follows: The first maximum, at the start

of the frictional contact, is explained by the formation of local fusion zones and their

shearing off in the lower temperature range.

The torque decreases as a result of the risen temperature - which again is a conse-

quence of the increased plasticity - and of the lowered deformation resistance. The

second maximum is generated during the braking phase which precedes the spindle

standstill. The second maximum is explained by the increased deformation resis-

tance at dropping temperatures. The temperature drop in the joining zone is ex-

Comparison of the Welding Processes forConventional and Flywheel Friction Welding

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conventional friction welding flywheel friction welding

number ofrevolutions

axialpressure

torque

20...100 Nmm-2

40...280

Nmm-2

time

friction welding time1...100s

friction welding time0,125...2s

braking0,1...0,5s

1800...

5400 min-1

900...

5400min-1

40...280

Nmm-2

Figure 7.11


plained by the lowered energy input – due to the rotation-speed decrease – and

also by the augmented radial displacement of the highly heated material into the weld

upset.

In friction welding

with a continuous

drive, the process

variation “com-

bined friction

welding” allows

the free and inde-

pendent from each

other selection of

the braking and

upsetting mo-

ments, Fig. 7.12.

In this case, the rotation-energy which has been stored in the drive motor, the spin-

dle and also in the clamping chuck, may be totally or partially converted by self-

breaking. Here, the breaking phase matches the breaking phase in flywheel welding.

The use of this process variant allows the welding structures to influence each other

in a positive way when many welding tasks are to be carried out. Moreover, the

torque range may

be accurately pre-

determined by the

microcontroller of

the braking initiator

which prevents the

slip-through of the

workpieces in the

clamping chuck.

The universal fric-

tion welding ma-

© ISF 2002

Combined Friction Welding

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time

number of revolutions

friction force

reduction

upset force

Figure 7.12

© ISF 2002

Types of Friction Welding Processes

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a) b)

c) d)

e) f)

P

P

P P

P

P

PP

P

Figure 7.13


chine is in its structure similar to a turning lathe, however, for the transmission of the

high axial forces, the machine structure must be considerably more rigid.

Basically, there are three types of friction welding: a) friction welding with a rotating

workpiece and a translational motion of the other workpiece; b) friction welding with

rotation and translational motion of one workpiece facing a stationary other work-

piece, c) rotation and translation of two workpieces against a stationary intermediate

piece. The remaining variations, shown in Figure 7.13, also find applications when

both workpieces have to rotate in opposite direction to each other. For example,

when a low diameter and, in connection with this, the low relative speeds demand the

necessary heat quantity.

A survey of possible joint shapes achievable with friction welding is given in Fig-

ure 7.14. The specimen preparation of the joining members should, if possible, be

carried out in a way that the heat input and the heat dissipation is equal for both

members. Depend-

ing on the combina-

tion of materials can

this provision facili-

tate the joining task

considerably. The

abutting surfaces

should be smooth,

angular and of

equal dimensions.

A simple saw cut is,

for many applica-

tions, sufficient.

The method of heat generation causes a comparatively low joining temperature –

lower than the melting temperature of the metals. This is the main reason why friction

welding is the suitable method for metals and material combinations which are diffi-

cult to weld. It is also possible to weld material combinations (e.g. Cu/Al or Al/steel)

which cannot be joined using other welding processes otherwise only with increased

expenditure. Figure 7.15 shows a survey of possible material combinations. Many

© ISF 2002

Joint Types Obtainedby Friction Welding

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beforewelding

afterwelding

beforewelding

afterwelding

8. pipe with plate, without preparation

7. round material with plate, without preparation

6. pipe with plate

5. round material with plate

g/d 0,25...0,3»

4. pipe with pipe1. a)round stock with round stock

b) round stock with round stock, chamfered

2. a)round stock with round stock (different cross-sections, partially machined)

b) round stock with round stock (different cross-sections, bevelled)

3. round stock with piped=0,75D

dD

0,75d

dd D

d 0,6D»

1..2°

(1/6)d

d

g

Figure 7.14


Secondary StructureAlong a Friction Weld

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10 µm

1 mm

10 mm

metal: S235JRp = 30 N/mmt = 6 st = 250 N/mmn = 1500 U/min

2

2

St

structures on parallels with a 5 mm distance from the sample axis

base metal heat affected zone transition heat affected zone - weld metal

weld metal

Figure 7.16

combinations have, however, not yet been tested on their suitability to friction weld-

ing. Metallurgical reasons which may reduce the friction weldability are:

1. the quantity and distribution of

non-metal inclusions,

2. formation of low-melting or in-

termetallic phases,

3. embrittlement by gas absorp-

tion (as a rule, the costly, inert

gas shielding can be dis-

pensed with, even when weld-

ing titanium),

4. softening of hardened or pre-

cipitataly-hardened materials

and

5. hardening caused by too high

a cooling rate.

By the adjustment of the welding pa-

rameters in respect toweld joints, can

in most cases joints with good mechano-technological properties be obtained.

The secondary structure along the friction-welded joint is depicted in Figure 7.16. An

extremely fine-

grained structure

(forge structure)

develops in the join-

ing zone region.

This structure which

is typical of a fric-

tion-welded joint is

characterised by

high strength and

toughness proper-

ties.


aluminiumaluminium alloysaluminium, sinteredleadcast iron (GGG, GT)hard metal, sinteredcobaltcoppermagnesiumbrassmolybdenumnickelnickel alloysniobiumsilversteel, unalloyedsteel, alloyed steel, high alloyedsteel, austenticcast steelfree cutting steelstellitetantalumtitaniumvanadiumtungstencirkon

cirk

ontu

ngst

enva

nadi

umtit

aniu

mta

ntal

umst

ellit

efr

ee c

uttin

g st

eel

cast

ste

elst

eel,

aust

entic

stee

l, hi

gh a

lloye

dst

eel,

allo

yed

stee

l, un

allo

yed

silv

erni

obiu

mni

ckel

allo

ysni

ckel

mol

ybde

num

bras

sm

agne

sium

copp

erco

balt

hard

met

al, s

inte

red

cast

iron

(GG

G, G

T)

lead

alum

iniu

m, s

inte

red

alum

iniu

m a

lloys

alum

iniu

m

friction weldable

restricted friction weldable

not friction weldable

not tested

Figure 7.15


Figure 7.17 shows a comparison between a flash butt-welded and a friction-

welded cardan shaft. The two welds are distinguished by the size of their heat af-

fected zone and the development of the weld upset. While in friction welding a regu-

lar, lip-shaped upset is produced, the weld flash formation in flash butt welding is nar-

rower and sharper and also considerably more irregular. Besides, the heat affected

zone during friction welding is substantially smaller than during flash butt welding.

Friction welding machines are fully mechanized and may well be integrated into

production lines. Loading and unloading equipment, turning attachments for the

preparation of the abutting surfaces and for upset removal and also storage units for

complete welding programs make these machines well adaptable to automation. The

machines may furthermore be

equipped with parameter supervisory

systems. During welding are parame-

ters: welding path, pressure, rotational

speed, and time are governed by the

desired value/actual value compari-

son. This allows an indirect quality

control. A further complement to the

retension of parameters is the torque

control, however this method is costly

and it cannot be used for all applica-

tions because of its structural dimen-

sions.

Friction welding machines are mainly

used in the series production and

industrial mass production.

Nevertheless, these machines are also always applied when metals and material

combinations which are difficult to weld have to be joined in a reliable and cost-

effective way. With the machines that are presently used in Germany, it is possible to

weld massive workpieces in the diameter range of 0.6 up to 250 mm For steel pipes,

the maximum weldable diameter is at present approximately 900 mm, the wall thick-

nesses are approx. 6 mm.


flash butt welding

friction welding

Figure 7.17


Figures 7.18 to 7.20 show a selection of

examples for the application of friction

welding.

Figure 7.21 shows a comparison of the

cost expenditure for the manufacture

of a cardan shaft, carried out by forg-

ing and by friction welding, respec-

tively.

It shows that the application of the fric-

tion welding method may save approx.

20% of the production costs. This com-

parison is, however, not an universally

valid statement as for each component a

profitability evaluation must be carried out

© ISF 2002br-er7-18e_sw.cdr

1 2

3 4

1,2 joint ring 3 loading device

4 unloading grippersmaterial combination:Cf53/ Ck45


1 cardan shaft, AIZn 4,5 Mg 1

2 cardan shaft, retracted tube

3 cardan shaft, flattening test specimen

4 crown wheel, 16MnCr5/ 42Cr4

5 bimetal valve, X45CrSi9-3/ NiCr20 TiAl

Figure 7.18 Figure 7.19


1 pump shaft2 shaft C22E/ E2953 press cylinder S185/9S 20K4 hydraulic cylinder S235J3G2/ C60E or S235JR/ C155 cylinder case S235JR/ S355J2G36 piston rod 42Cr47 connecting rod 100Cr6/ S235JR8 stud S235J2G3/ X5CrNi18-109 knotter hook 15CrNi6

Figure 7.20


separately. The comparison is just to show that, in many applications, considerable

savings can be made if the matter of the joining technology by “friction welding” could

be circulated to a wider audience of design and production engineers.

Figure 7.22 shows

in brief the impor-

tant advantages

and disadvan-

tages of friction

welding in com-

parison with the

competitive

method of flash

butt welding.

Pressure welding with magnetically

impelled arc, “Magnetarc Welding”,

is an arc pressure welding method for

the joining of closed structural tu-

bular shapes, Figure 7.23. The weld-

able wall thickness range is between

0.7 and 5 mm, the weldable diameter

range between 5 and 300 mm. In

“Magnetarc Welding” an arc burns be-

tween the joining surfaces and is ro-

tated by external magnetic forces. This

is achieved by a magnet coil system

that produces a magnetic field.

The combined action of this magnetic

field and the arc’s own magnetic field

Cost Comparison of Forging/ FrictionWelding in a Case of a Cardan Shaft

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forged piece

motor shaft € 20,-

€ 20,-

friction-welded piece

material costs shaftØ30 und 40 mm

€

€

€

€

7,50

4,25

3,-

14,75

2x friction weldsincl. upset removal

flange,forged

160

mm

Ø40

mm

Ø30 mm

friction welds

940 mm

Figure 7.21


Advantages and disadvantages of friction weldingin comparison with the competitive flash butt welding

advantages:

disadvantages:

- clean and well controllable bulging- low heat influence on joining members- better control of heat input- low phase seperation phenomena in the bond zone- hot forming causes permanent recovery and recrystallisation processes in the welding area thus forming a very fine-grained structure with good toughness and strength properties (forged structure)- low susceptibility to defects, extremely good reproducibility within a wide parameter range- frequently shorter welding times- more choice in the selection of weldable materials and material combinations

- torque-safe clamping necessary- machine-determined smaller maximum weldable cross-sections- susceptibility to non-metal inclusions- high expenditure requested because of high manufacturing tolerances- high capital investment for the machine

Figure 7.22


effects a tangential force to act upon the arc. The rotation of the arc heats and melts

the joint surfaces. After an adequate heating operation, the two workpiece members

are pressed and fused together. A regular weld upset develops which is normally not

removed. The welding operation takes place under shielding gas (mainly CO2).

The shielding gas’

function is not the

protection of the

weld from the sur-

rounding atmos-

phere but rather a

contribution to-

wards the stabilisa-

tion of the arc. The

reproducibility of

the arc ignition and

motion behaviour and the regularity of

the weld bead are therefore improved.

The prerequisite for the application of

a material in “Magnetarc Welding” is its

electrical conductivity and melting

behaviour. Figure 7.24 gives a survey

of the material combinations which are

nowadays already weldable under in-

dustrial conditions.

As reason is the symmetric heat input,

the subsequent upsetting of the liquid

phase and the cooling off under pres-

sure. The cracking sensitivity of the

welds is, in general, relatively low.

This has a positive effect, particulary

Diagrammatic Representationof Magnetic Arc Welding

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1. starting position

2. starting of welding

3. heating

4. completion of welding

a) both workpieces are brought into contactb) welding current and magnetic field are switched on

a) both workpieces are seperated until a defined gap width is reached (retracting movement) - the arc ignites

a) the arc rotatesb) the joint surfaces are melting

a) both workpieces are broght into contact again and upsetb) welding current and magnetic field are switched off

Figure 7.23


steel, unalloyed

steel, lowalloyed

free cutting steel

cast steel

malleable

stee

l, un

allo

yed

stee

l, lo

wal

loye

d

free

cut

ting

stee

l

cast

ste

el

mal

leab

lematerials

suitable formagnetic arcwelding

not tested

Figure 7.24


when steels with a high carbon content or machining steels are welded. The joining

faces of the workpieces must be free from contamination, such as rust or scale.

To obtain a defect-free weld, normally a simple saw cut is a sufficient preparation of

the abutting sur-

faces. If special

demands are put

on the dimensional

accuracy of the

joints, the prefabri-

cation tolerances

have to be ad-

justed accordingly.

This applies also to

friction welding.

Figures 7.25 and 7.26 show several

application examples of pressure

welding with magnetically impelled arc.

Figure 7.27 shows a summary of the

most important advantages and dis-

advantages of this method in com-

parison with the competitive methods

of friction welding and flash butt weld-

ing.

In friction-stir welding a cylindrical,

mandrel-like tool carries out rotating

self-movements between two plates

which are knocked and clamped onto

a fixed backing. The resulting friction

heat softens the base metal, although

© ISF 2002

Applications for Magnetic Arc Welding

br-er7-25e_sw.cdr

Figure 7.25


Figure 7.26


the melting point is not reached. The

plastified material is displaced by the

mandrel and transported behind the

tool where a longitudinal seam devel-

ops.

The advantages of this method which

is mainly used for welding of alumin-

ium alloys is the low thermal stress of

the component which allows joining

with a minimum of distortion and

shrinkage. Welding fumes do not de-

velop and the addition of filler metal or

shielding gases is not required.


Advantages and disadvantages of magnetic arc welding incomparison with flash butt welding and/ or friction welding

advantages:

- lower energy demands

- material savings through lower loss of length

- better dimensional accuracy in joining especially for small

wall thicknesses

- in comparison with friction welding less moving parts

(only axial movement of one joining member during upsetting)

- no restrictions to the free clamping length

- smaller and more regular welding edge

- no spatter formation

disadvantages:

- suitable for small wall thicknesses only

(maximum wall thickness: 4 - 5 mm)

- welding parameters must be kept within narrow limits

- only magnetizing steels are weldable without any difficulties

Figure 7.27

Friction-Stir Welding

br-er7-28e.cdr

tool collar

fixed backing

contoured pin

workpiece

Figure 7.28

7. pressure welding

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