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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
7. Pressure Welding 90
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
br-er7-03e.cdr
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
7. Pressure Welding 91
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
7. Pressure Welding 92
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
7. Pressure Welding 93
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-
© ISF 2002br-er7-08e.cdr
n
n
F friction force1
F upset force2
Figure 7.8
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Phases of Friction Welding Process
Figure 7.9
7. Pressure Welding 94
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,
© ISF 2002br-er7-10e.cdr
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
7. Pressure Welding 95
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
7. Pressure Welding 96
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
7. Pressure Welding 97
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.
© ISF 2002br-er7-15e.cdr
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
7. Pressure Welding 98
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
© ISF 2002br-er7-17e.cdr
flash butt welding
friction welding
Figure 7.17
7. Pressure Welding 99
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
© ISF 2002br-er7-19e_sw.cdr
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
© ISF 2002br-er7-20e_sw.cdr
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
7. Pressure Welding 100
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
© ISF 2002br-er7-22e.cdr
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
7. Pressure Welding 101
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
© ISF 2002br-er7-24e.cdr
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
7. Pressure Welding 102
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
© ISF 2002br-er7-26e_sw.cdr
Figure 7.26
7. Pressure Welding 103
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.
© ISF 2002br-er7-27e.cdr
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
br-er7-28e.cdr
tool collar
fixed backing
contoured pin
workpiece
Figure 7.28
Recommended