7. Pressure Welding - KAUmercury.kau.ac.kr/welding/Welding Technology I - Welding Processes... · manufacture and with components of equal dimensions. ... Friction welding is a pressure

7.

Pressure Welding

7. Pressure Welding 89

2005

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

cordance with DIN 1910.

In gas pressure welding a distinction is made between open square and closed square gas

pressure welding, Figure 7.2.

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

perature. 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 preheating 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 equipment is low in

weight and also easy to

handle, the main applica-

tion areas of gas pressure

welding are the welding of

reinforcement steels and of

pipes in the building trade.

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

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 pressing

1. 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


2005

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 uni-

form heating of the abutting

faces, they must be con-

formable 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 contaminations and from oxides. Suitable for welding are unalloyed and low-

alloy steels. The welding of

aluminium and copper ma-

terial is, because of the

tendency towards oxidation

and good conductivity,

possible only up to a point.

For the most part, smaller

cross-sections with sur-

faces of up to 100 mm² are

welded. Areas of applica-

tions are chain manufactur-

ing and also extensions of

wires in a wire drawing

shop.

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+b

b2

a

welding transformer

primary side

a = flashing lengthb = upset loss

Figure 7.4


2005

A flash butt welding equipment is, in its principal structure, similar to the pressure butt

welding device, Figure 7.4.

While in pressure upset butt welding the joining members are always strongly pressed to-

gether, 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 current 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 destruction 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 uniformly 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 upsetting pro-

gress, which, if necessary, can be easy me-

chanically removed while the weld is still

warm, Figure 7.5.

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

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 surfaces and by the mechani-

© ISF 2002br-er7-05e.cdr

Figure 7.5


2005

cal 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 effi-

ciency 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 re-

heating inside the machine.

A smooth and clean sur-

face is not necessary with

hot flash butt welding. If the

abutting faces differ greatly

from the desired plane-

parallelism, an additional

flashing process (a short

flashing period with low

speed and high energy)

may be carried out first.

The welding area of the

structure of a flash butt

weld shows a zone which

is reduced in carbon and

other alloying elements,

Figure 7.7. Moreover, all

flash butt welded joints

have a pronounced coarse

grain zone, whereby the

toughness properties of the

welded joint are lower than

of the base metal. By the

impact normalizing effect in

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

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


2005

the machine successive to the actual welding process, can the toughness properties be con-

siderably increased. By one or several current impulses the weld temperatures are increased

by up to approximately 50° over the austeniting temperature 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 insignifi-

cant that only very low residual stresses occur. It is, therefore, possible 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 welding method where the necessary heat for joining is pro-

duced by mechanical friction. The friction is, as a rule, generated by a relative motion be-


n

n

F friction force1

F upset force2

Figure 7.8

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Phases of Friction Welding Process

Figure 7.9


2005

tween a 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 discontinued and the

friction force is increased 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 of-

ten 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 rotation-symmetrical parts. It is,

nowadays, also possible to accurately join

rectangular and polygonal cross-sections.

The most common variant of friction weld-

ing is friction welding with a continuous drive and friction welding with a flywheel drive, Fig-

ure 7.10. In friction 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 sliding chuck is

pressed with a defined friction force. The nominal speed is maintained until the demanded

temperature profile has

been achieved. Then the

motor is declutched and

the relative motion is neu-

tralised by external braking.

In general, the friction force

is raised to upsetting force

after the rotation movement

has been discontinued.

During flywheel friction

welding, the inertia mass

is raised to nominal speed,


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

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


2005

the drive motor is declutched 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 proc-

esses, extremely short welding times may be realised. Further process variants are radial

friction welding, orbital friction welding, oscillation friction welding and friction surfacing. How-

ever, these process variants are until today still in the experimental stage. Recently, new de-

velopments in the field of friction stud welding – studs on plates – have been introduced.

Figure 7.11 depicts the variation in time of the most important process parameters in

friction welding with continuous drive and flywheel friction welding. The occuring moments’

maxima may be interpreted as follows: The first maximum, at the start of the frictional con-

tact, 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 consequence

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

resistance at dropping

temperatures. The tem-

perature drop in the joining

zone is explained by the

lowered energy input – due

to the rotation-speed de-

crease – and also by the

augmented radial dis-

placement of the highly

heated material into the

weld upset.

In friction welding with a continuous drive, the process variation “combined friction weld-

ing” allows the free and independent from each other selection of the braking and upsetting

moments, Fig. 7.12.

© ISF 2002

Combined Friction Welding

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time

number of revolutions

friction force

reduction

upset force

Figure 7.12


2005

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

mined by the microcontroller of the braking initiator which prevents the slip-through of the

workpieces in the clamping chuck.

The universal friction weld-

ing machine is in its struc-

ture similar to a turning

lathe, however, for the

transmission of the high

axial forces, the machine

structure must be consid-

erably more rigid.

Basically, there are three

types of friction welding: a)

friction welding with a rotat-

ing workpiece and a trans-

lational motion of the other

workpiece; b) friction weld-

ing with rotation and trans-

lational motion of one

workpiece facing a station-

ary other workpiece, c)

rotation and translation of

two workpieces against a

stationary intermediate

piece. The remaining varia-

tions, shown in Fig-

ure 7.13, also find applica-

tions when both work-

pieces have to rotate in

© 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

© 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 pipe

d=0,75D

dD

0,75d

d

d D

d 0,6D»

1..2°

(1/6)d

d

g

Figure 7.14


2005

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 Figure 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. Depending on the

combination of materials can this provision facilitate the joining task considerably. The abut-

ting surfaces should be smooth, angular and of equal dimensions. A simple saw cut is, for

many applications, 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 difficult 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 combi-

nations. Many combinations have, however,

not yet been tested on their suitability to fric-

tion welding. Metallurgical reasons which may

reduce the friction weldability are:

1. the quantity and distribution of non-metal inclusions,

2. formation of low-melting or intermetallic phases,

3. embrittlement by gas absorption (as a rule, the costly, inert gas shielding can be dis-

pensed with, even when welding titanium),

4. softening of hardened or precipitataly-hardened materials and

5. hardening caused by too high a cooling rate.


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

cir

kon

tung

ste

nva

nadiu

mtita

niu

mta

nta

lum

ste

llite

free c

utt

ing

ste

el

ca

st ste

el

ste

el, a

uste

ntic

ste

el, h

igh a

lloyed

ste

el, a

lloyed

ste

el, u

nallo

yed

silv

er

nio

biu

mnic

kel allo

ys

nic

kel

moly

bde

num

bra

ss

mag

ne

siu

mco

pper

co

balt

hard

meta

l, s

inte

red

ca

st iron

(G

GG

, G

T)

lea

dalu

min

ium

, sin

tere

dalu

min

ium

allo

ys

alu

min

ium

friction weldable

restricted friction weldable

not friction weldable

not tested

Figure 7.15


2005

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

By the adjustment of the welding parameters 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 Fig-

ure 7.16. An extremely

fine-grained structure

(forge structure) develops

in the joining zone region.

This structure which is

typical of a friction-welded

joint is characterised by

high strength and tough-

ness properties.

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

dan shaft. The two welds are distinguished by the size of their heat affected zone and the

development of the weld upset. While in fric-

tion welding a regular, lip-shaped upset is pro-

duced, the weld flash formation in flash butt

welding is narrower and sharper and also con-

siderably more irregular. Besides, the heat

affected zone during friction welding is sub-

stantially smaller than during flash butt weld-

ing.

Friction welding machines are fully

mechanized and may well be integrated into

production lines. Loading and unloading

equipment, turning attachments for the prepa-

ration of the abutting surfaces and for upset

removal and also storage units for

complete welding programs make these ma-

chines well adaptable to automation. The ma-

chines may furthermore be equipped with


flash butt welding

friction welding

Figure 7.17


2005

parameter supervisory systems. During weld-

ing are parameters: welding path, pressure,

rotational speed, and time are governed by

the desired value/actual value comparison.

This allows an indirect quality control. A fur-

ther complement to the retension of parame-

ters is the torque control, however this method

is costly and it cannot be used for all applica-

tions because of its structural dimensions.

Friction welding machines are mainly used in

the series production and industrial mass

production.

Nevertheless, these machines are also al-

ways applied when metals and material com-

© 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 shaft

2 shaft C22E/ E295

3 press cylinder S185/9S 20K

4 hydraulic cylinder S235J3G2/ C60E or S235JR/ C15

5 cylinder case S235JR/ S355J2G3

6 piston rod 42Cr4

7 connecting rod 100Cr6/ S235JR

8 stud S235J2G3/ X5CrNi18-10

9 knotter hook 15CrNi6

Figure 7.20


2005

binations 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 work-

pieces in the diameter

range of 0.6 up to 250 mm

For steel pipes, the maxi-

mum weldable diameter is

at present approximately

900 mm, the wall thick-

nesses are approx. 6 mm.

Figures 7.18 to 7.20 show

a selection of examples for

the application of friction

welding.

Figure 7.21 shows a com-

parison of the cost expenditure for the manufacture of a cardan shaft, carried out by

forging and by friction welding, respectively.

It shows that the application of the friction

welding method may save approx. 20% of the

production costs. This comparison is, however,

not an universally valid statement as for each

component a profitability evaluation must be

carried out separately. The comparison is just

to show that, in many applications, consider-

able 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 important ad-

vantages and disadvantages of friction

welding in comparison with the competitive

method of flash butt welding.

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 m

m

Ø40 m

m

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


2005

Pressure welding with magnetically impelled arc, “Magnetarc Welding”, is an arc pres-

sure welding method for the joining of closed structural tubular shapes, Figure 7.23. The

weldable wall thickness

range is between 0.7 and

5 mm, the weldable diame-

ter range between 5 and

300 mm. In “Magnetarc

Welding” an arc burns be-

tween the joining surfaces

and is rotated 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 effects a tangen-

tial force to act upon the arc. The rotation of the arc heats and melts the joint surfaces. After

an adequate heating operation, the two work-

piece members are pressed and fused to-

gether. 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 protec-

tion of the weld from the surrounding atmos-

phere but rather a contribution towards the

stabilisation of the arc. The reproducibility of

the arc ignition and motion behaviour and the

regularity of the weld bead are therefore im-

proved.

The prerequisite for the application of a mate-

rial in “Magnetarc Welding” is its electrical

conductivity and melting behaviour. Fig-

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

ste

el, u

nallo

yed

ste

el, low

allo

yed

free c

uttin

g s

teel

cast ste

el

ma

llea

blematerials

suitable for

magnetic arc

welding

not tested

Figure 7.24


2005

ure 7.24 gives a survey of

the material combinations

which are nowadays al-

ready weldable under in-

dustrial conditions.

As reason is the symmetric

heat input, the subsequent

upsetting of the liquid

phase and the cooling off

under pressure. The

cracking sensitivity of the

welds is, in general, rela-

tively low. This has a posi-

tive effect, particulary 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 sim-

ple saw cut is a sufficient preparation of the

abutting surfaces. If special demands are put

on the dimensional accuracy of the joints, the

prefabrication tolerances have to be adjusted

accordingly. This applies also to friction weld-

ing.

Figures 7.25 and 7.26 show several applica-

tion examples of pressure welding with mag-

netically impelled arc.

Figure 7.27 shows a summary of the most

important advantages and disadvantages of

this method in comparison with the competi-

tive methods of friction welding and flash butt

welding.

© ISF 2002

Applications for Magnetic Arc Welding

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Figure 7.25


Figure 7.26


2005

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

the melting point is not reached. The plastified

material is displaced by the mandrel and

transported behind the tool where a longitudi-

nal seam develops.

The advantages of this method which is

mainly used for welding of aluminium alloys is

the low thermal stress of the component

which allows joining with a minimum of distor-

tion and shrinkage. Welding fumes do not de-

velop and the addition of filler metal or shield-

ing gases is not required.


Advantages and disadvantages of magnetic arc welding in

comparison 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

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tool collar

fixed backing

contoured pin

workpiece

Figure 7.28

Documents

7. Pressure Welding - KAUmercury.kau.ac.kr/welding/Welding Technology I - Welding Processes... · manufacture and with components of equal dimensions. ... Friction welding is a pressure