8.
Resistance Spot Welding,
Resistance Projection Welding
and Resistance Seam Welding
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
105
Figure 8.1 shows an extract from the classification of the welding methods according to DIN
1910 with a detailed account of the conductive resistance pressure welding.
In the case of resistance pressure welding, the heating occurs at the welding point as a consequence of Joule resistance heating caused by current flow through an electrical conductor,
Figure 8.2. In spot and projection welding, the plates
welding
to be welded in overlap.
pressure
welding
fusion
welding
cold pressure
welding
resistance pressure
welding
induction pressure
welding
Conduction pressure
welding
Current supply is carried
out through spherical or
friction welding
flat
electrodes,
respec-
tively. In roller seam welding, two driven roller elec-
resistance spot
welding
projection
welding
roller seam
welding
resistance butt
welding
flash butt
welding
© ISF 2002
br-er8-01e.cdr
trodes are applied. The
plates to be welded are
mainly
Classification of Welding
According to DIN 1910
overlapped.
The
heat input rate Qinput is
generated by resistance
Figure 8.1
heating
in
a
current-
carrying conductor, Figure
spot welding
roller seam welding
projection welding
workpieces overlap
electrode
weld nugget
workpiece usually in general
overlap
driven roller electrode
spot rows (stitch weld,
roller spots)
workpieces with elevations
(concentration of electicity)
workpieces overlap
pad electrode
several joints in a single weld
weld nugget joint
1
3
2
2
1
3
3
5
1
1
1 electrode force
2 elektrodes
3 production part
4 loaded area
instrumental in the formaQeff is composed of the
input heat minus the dissi-
4
1
fective heat quantity Qeff is
tion of the weld nugget.
1
2
8.3. However, only the ef-
pation heat. The heat loss
arises from the heat dissi-
5 projection
pation into the electrodes
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and the plates and also
from thermal radiation.
Figure 8.2
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
106
The resistance during resistance heating is composed of the contact resistances on the two
plates and of their material resistance. The reduction of the electrode force down to 90% increases the heat input rate by 105%, the reduction of the welding current down to 90% decreases the heat rate to 80% and a welding time reduction to 90% decreases the heat rate to
92%.
The time progression of the resistance is shown in Figure 8.4. The contact resistance is
composed of the interface resistances between the electrode and the plate (electrode/plate)
and between the plates
(plate/plate).
The
resis-
Fel:
Qeff:
Qinput:
I:
Q1:
Q2 :
Q3 :
Q4:
R(t):
Rmaterial(t):
Rc(t):
tance height is greatly dependent on the applied
electrode force. The higher
this force is set, the larger
are the conductive cross-
electrode force
effective heat
total heat input
current (time dependence)
heat losses
losses into the electrodes
losses into the sheet metal
losses by heat radiation
total resistance
material resistance
contact resistance
Fel
Q4
Q3
Qeff = Qinput - Q1l
Q3
Q4
Q2
2
Qinput = C
I (t) R(t) dt
t=0
points and smaller the re-
Qeff
Q4
t=tS
sections at the contact
Q4
Q2
Fel
Q1 = Q2 + Q3 + Q4
sistances. The contact sur-
R(t) = Rmaterial(t) + Rc(t)
© ISF 2002
br-er8-03e.cdr
faces, which are rapidly
Heat Balance in Spot Welding
increasing at the start of
welding, effect a rapid re-
Figure 8.3
duction of interface resistances.
theoretical contact area
100% metallic conduction contact
mOhm
proportion at room temperature
With the formation of the
resistances
between
the
resistance
weld nugget the interface
total resistance
low electrode force
high resistance
sum of material resistance
high electrode force
low resistance
plates disappear. During
proportion after first
milliseconds welding time
sum of contact resistances
the progress of the weld
5
10
welding time
the material resistance in-
periods
surface resistance is
collapsed,
a3 is highly extended
A1: area out-of-contact
A2: contact area with high resistance
A3: contact area completely conductive
creases from a low value
br-er8-04e.cdr
(surrounding temperatures)
to a maximum value above
the melting temperature.
Figure 8.4
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
Figure
8.5
shows
diaelectrode force
grammatically the different
resistance rate
resistances during the spot
R1
welding process with acting electrode force, but
R3
R3
R6
R6
_
~
R7
without
Weld
welding
nugget
107
current.
R4
formation
R5
R5
R7
R2
R4
must therefore start in the
0
100
200
joining zone because of
R [µOhm]
the existing high contact
br-er8-05e.cdr
resistance there.
Figure 8.6 shows directly
Figure 8.5
cooled electrodes for resistance welding. The coolant
cooling tube
cooling drill-hole
6-8
is normally water. In the
cooling tube, the cooling
water is transported to the
2-5
10 - 20
slope
electrode base. The diagram shows the temperature distribution in the elec°C
trodes and in the plates.
The maximum temperature
Electrode Cooling
the weld nugget and decreases
strongly in
the
© ISF 2002
br-er8-06e.cdr
is reached in the centre of
Figure 8.6
electrode direction.
Sequence of a resistance spot welding process, Figure 8.7:
1->2 Lowering of the top electrode
2->3 Application of the adjusted electrode force Set-up time tpre, sequence
3->4 Switching-on of the adjusted welding current for the period of the welding time tw. Formation of the weld nugget in the joining zone of both workpieces.
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
108
An example shows the macrosection of a weld nugget after the welding time has
ended.
4->5 Maintaining the electrode force for the period of the set post-weld holding
time th.
5->6 Switching-off the force generating system and lifting the electrodes off the workpiece.
The functions of the set-up time and the post-weld holding time are listed in Figure 8.8. Dependent on the welding task different force and current programs can be set in the welding machines, Figure 8.9. In the top the simplest possible welding program sequence is
shown: The application of the electrode force, the set-up time sequence tpre, the switching-on
of the welding current and the sequence of the welding time tw, the sequence of the postweld holding time th and the switching-off of the force generating system. The diagram in the
centre is almost identical to the one just described.
Merely in the welding current range, welding is carried out using an adjustable current rise (7)
and current decay (8). The diagram below depicts a more sophisticated current program. In
addition, welding is carried out with a variable electrode force (2) and with preheating (4) and
post-heating current (6). Dependent on the control system, the process can be influenced by
adjustment.
Fel
Iw
set-up time
electrode force Fel
- compressing the workpiece
- build-up of electrode force to preset
value
- setting-up of reproducible resistance
before welding
- electrode resting after bounce
- preventing resting of electrode on
workpiece under electricle voltage
welding current Iw
time t
tpre
tw
th
top electrode
postweld-holding time
- holding time of workpiece during
cooling of molten metal
- prevention of pore formation in the
welding nugget
- prevention of lifting the electrode
under voltage
workpiece
lower electrode
insufficiently melted weld nugget
weld nugget
The postweld-holding time has
influence on the weld point hardening
within certain limits.
totally melted weld nugget
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© ISF 2002
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Time Sequence of
Resistance Spot Welding
Figure 8.7
© ISF 2002
Functions of Pre- and Postwelding
Figure 8.8
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
109
A controlled variable may be, for instance, the electrode path, the resistance progress, the
welding current or the welding voltage.
Figure 8.10 shows the principle structure of a resistance spot welding machine. The
main components are: the machine frame, the welding transformer with secondary lines, the
electrode pressure system and the control system. This principle design applies to spot, projection and roller seam welding machines. Differences are to be found merely in the type of
Fel
welding current
electrode force
electrode fittings and in the electrode shapes.
5
Iw
tw
th
tpre = pre-weld time
tw = welding time
th = holding time
tpres = pressure time
1
9
tpre
time
10
tp res
2
3
11
Fel
welding current
electrode force
6
12
4
Iw
8
5
7
7
5
8
electrode force
welding current
time
1
2
5
7
4
Fel
3
Iw
6
8
1 - initial force
2 - welding pressure
force
3 - post pressure force
4 - preheating current
5 - welding current
6 - postheating current
7 - ascending current
8 - descending current
1 electrode force cylinder
2 pneumatic equipment
3 machine tool frame
4 welding transformer
5 power control unit
6 current conductor
7 lower arm
8 foot switch
9 top arm
10 electrical power supply cable
11 water cooled electrode holder
12 electrode
time
© ISF 2006
br-er8-09e. cdr
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Schematic Assembly of
Spot Welding Machine
Course of Force and Current
Figure 8.9
© ISF 2002
Figure 8.10
Figure 8.11 depicts the possible process variations of resistance spot welding. These are
distinguished by the number of plates to be welded and by the arrangement of the electrodes
or, respectively, of the transformers. It has to be noted that with a corresponding arrangement also plates can be welded where one of the two plates has a non-conductive surface
(as, for example, plastics).
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
110
Figure 8.12 shows the current types which are normally used for resistance welding.
Alternating current has the
~
simplest structure (Figure
8.13) and is most price ef-
~
~
fective, unavoidable are,
however,
the
disadvan-
tages of current zeros and
two-sided single-shear
single-spot welding
two-sided two-shear
spot welding (stack welding)
one-sided single-spot welding
with contact electrode
~
~
~
weld nugget cooling. In
+
+
+
relation to the average cur-
+
+
rent values, peak loads
~
two-sided duplex spot
welding
occur and, with that, increased
electrode
one-sided multi-spot welding
with conductive base
© ISF 2002
br-er8-11e.cdr
wear.
Variants of Spot Welding
These extreme peak loads
do not occur with direct
one-sided duplex spot welding
with conductive base
Figur 8.11
current.
The structural design of a d.c. supply unit is, however, more complicated and, therefore,
more expensive than an a.c. supply unit.
As conventional welding machines operate with a 50 Hz primary current supply, the welding
current can be controlled only in 20 ms units (1 period). When the inverter-direct current
technique or, respectively, the medium-frequency technique is used, a finer setting of the current-on period and a more
alternating current
medium frequency direct current
12
15
[kA]
[kA]
20
10
0
0.00 0.02 0.04 0.07 0.09 0.11 0.13 0.16
-5
current
current
5
-10
-15
-20
ing current is possible.
8
6
4
2
0
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
welding time [s]
welding time
[kA]
0.00
0.02 0.04
0.06 0.08 0.10 0.12
welding time
0.14 0.16
45
40
35
30
25
20
15
10
5
0
0.00
currents and shorter weldpacitor resistance welding
technique is applied. The
0.02 0.04
0.06 0.08 0.10
0.12 0.14 0.16
[s]
© ISF 2002
Current Types
In order to realise higher
ing times, the impulse ca-
welding time
[s]
br-er8-12e.cdr
Figur 8.12
[s]
impulse capacitor current
current
current
[kA]
"conventional" direct current
18
16
14
12
10
8
6
4
2
0
precise control of the weld-
10
rectified primary current is
stored in capacitors and,
through
a
high-voltage
transformer, converted to
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
111
high welding currents. The
single-phase
alternating current
advantages of this tech-
static-inverter direct current
nique are low heat input
and
high
reproducibility.
Because of the high energy
density,
materials
with
capacitor impulse discharge
3-phase direct current
good conductivity can be
welded and also multipleprojection welds can be
carried out. A disadvantage
br-er8-13e.cdr
of this method is, apart
from the high equipment
costs, the difficult regula-
Figure 8.13
tion of the welding current.
Electrodes for spot resistance welding have the property of transferring the electrode force
and the welding current. They are wearing parts and, therefore, easily replaceable.
requirements
electrodes
form A
form B
form C
form E
form F
form G
form D
- good electrical conductivity
- good thermal conductivity
- high high-temperature strength
- high temperature stability
- high softening temperature
- little tendency to alloying with workpiece material
- easy options in machining
ISO 5182
Group
Type
1
electrode caps
2
A
3
4
ISO 5182
Key
Group Type
No.
1
Cu - ETP
2
Cu Cdl
1
Cu Crl
2
Cu Crl Zr
1
Cu CO2 Be
2
Cu Ni2 Si
1
Cu Ni1 P
2
Cu Be2 Co Ni
3
Cu Ag6
4
CuAl10NiFe5Ni5
Key
No.
10
W75 Cu
11
W78 Cu
12
WC70 Cu
13
Mo
14
W
15
W65 Ag
B
electrode holders
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© ISF 2002
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Electrodes,
Electrode Caps and Holders
Figure 8.14
Electrode Materials
Figure 8.15
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
112
Depending on the shape and type of electrode, solid electrodes or electrode caps, must
be either remachined or recycled. Figure 8.14 depicts various types of electrodes, electrode
caps and holders.
Dependent upon the electrode application, different alloyed electrode materials are used,
Figure 8.15. The added alloying elements influence the red hardness, the tempering resistance, the conductivity, the fusion temperature, the electrode alloying tendency, and, finally,
the machinability of the electrode material. When beryllium is used as an alloying element,
the admissible MAC values must be strictly adhered to during remachining or dressing of the
electrodes.
Already during the design phase of the components to be welded, importance must be attached to a good accessibility of the welding point. Moreover, the electrode force which is
imperative to the process must be applied in a way that no damage is done to the workpiece.
In the ideal case, the welding point is accessible from the top and from below, Figure 8.16.
poor
good
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poor
© ISF 2002
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Accessibility for Spot
Welding Electrode
Figure 8.16
good
© ISF 2002
Contact Area for
Spot Welding Electrodes
Figure 8.17
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
113
In order to avoid the displacement of the electrodes, the electrode working surface must
be flat. Also during the design phase space
must be provided for an adequately large
clearing zone around the working point, in
spot welding
order to guarantee the unimpeded electrode
A
approach to the working point, Figure 8.17.
shunt connection current
Dependent on the joining job, the process
copper
current path
indirect welding
one side
variation, or the resistance welding method, a
so-called “shunt current/effect” may be noticed. This current component, as a rule, does
not contribute to the formation of the weld
nugget; under certain circumstances it might
roller seam welding
br-er8-18e.cdr
even prevent a reliable welding process. In
Shunting
the example, shown in Figure 8.18, the shunt
current leads to undesired fusing contacts
Welding spatter:
Discharge of molten material between two steel
sheets or from the surface of steel sheets.
Figure 8.18
and, because of the lacking electrode force at
this point, also to damages to the plate surface.
If unsuitable welding parameters have been
fig. 1
set, weld spatter formation may occur, Fig-
fig. 2
Reason here is high welding current, (fig. 1) or
too-small edge distance (fig. 2)
ure 8.19. Liquid molten metal forms on the
plate surface or in the joining zone. The reasons for this kind of process disturbance are,
for example, too low an electrode force with
regard to the set welding current or welding
time, too high an energy input with regard to
porosity in the joint caused
by welding spatter
discharge of molten
material at the joint plane
br-er8-19e.cdr
the plate thickness or too small an edge dis-
Welding Spatter
tance of the welding point.
Figure 8.19
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
114
Figure 8.20 shows a list of
welding current changes
shunt
connection
wear of
electrodes
wear of
cable
mains voltage
fluctuation
a large number of possible
secondary
electrical
impedance
disturbances in resistance
Qeff = Qinput - Qlosses
wear
Qeff
diversion
heat
plate
plate
thickness
quality
of plates
number
of plates
welding equipment
alteration to force
spot welding. Welding curalteration
of
pressure
rent changes are caused
by: shunt, electrode wear,
cable wear, mains voltage
variations, secondary im-
plate
surface
edge
distance
pedance.
modification of the unit
br-er8-20e.cdr
Different welding conditions are caused by weldFigure 8.20
ing machine wear, different
heat
dissipation.
Non-
uniform conditions by alterations to components are: different plate thicknesses, plate quality,
number of plates, plate surfaces, edge distances. Electrode force changes are caused by:
pressure fluctuations and -changes, plate bouncing.
The resistance spot welding method allows
welding of a large number of materials. A list
of the various materials is shown in Figure
materials
aluminium
weldability
alloying
elements
good
weldability
sufficient
weldability
satisfactory
maximum content [%]
8.21. The alloying elements which are used
for steels have a varying influence on the suitability for resistance spot welding. The values
which are indicated in the table are valid only
when the stated element is the sole alloying
constituent of the steel material.
iron
very good
gold
satisfactory
C
0,25
0,40
C + Cr
0,35
1,60
C + Mo
0,50
0,70
C+V
0,40
0,60
C + Mn
1,40
1,60
molybdenum satisfactory
C + Ni
3,00
4,00
nickel
very good
Si
0,40
1,00
platinum
very good
Cu
0,60
0,60
P+S
0,10
0,10
C+Cr+Mo+V
0,60
1,60
cobalt
very good
copper
poor
magnesium
silver
tantalum
Figure 8.22 shows a comparison between
good
very good
very good
titanium
very good
tungsten
satisfactory
resistance spot and resistance projection
welding. The fundamental difference between
the two methods lies in the definition of the
weldable materials
influence of
alloying elements
(steel materials)
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current transition point.
Weldable Materials
Figure 8.21
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
115
The differences between both methods are illustrated in Figure 8.23. The short life of the
electrodes used for resistance spot welding is explained by the higher thermal load and the
larger pressing area caused by the smaller electrode contact areas. The term “electrode life”
stands for the number of welds that can be carried out with one pair of electrodes without
further rework and without
exceeding the tolerances
after welding
before welding
for quality criteria of the
weld.
Depending
on
the
defollow-up
distance
mands on the joint strength
or on the projection rigidity,
elektrode
different projection shapes
are applied. These are an-
projection
br-er8-22e.cdr
nular, circular or longitudinal projections. The welding projections are, accord-
Figure 8.22
spot
welding
projection
welding
up to 20 mm
> 20 mm
embossed
projection shape
elektrodes:
diameter
tip face
pressed
mould pressed
convex
flat
electrode life
less
longer
place where
the nugget
originates
elektrodes
projections
one
several
small
big
current distribution
no
yes
force distribution
no
yes
number of
welding nuggets
circular
longitudinal
annular
solid
projection shape
natural
projection shape
struck
machined
cut
pushed
circular
longitudinal
annular
interrupted annular
spot
contact
line
contact
Circular
follow-up distance
weld nut
problems:
br-er8-23e.cdr
© ISF 2002
Longitudinal
cut
Annular
pushed
wire-plate
bolt-pipe
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Differences Between Resistance
Spot and Projection Welding
Figure 8.23
crossed wires
Customary Projection Shapes
Figure 8.24
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
116
ing to their size, adapted to the used plate
thickness and may, therefore, appear as difdie
die
plate
ferent types in the workpiece: embossed proplate
jections, solid projections and natural projections. The survey is shown in Figure 8.24.
d1
mould plate
mould plate
counter-die
d1
In Figure 8.25 the production of embossed
projections in different shapes is shown. The
ring projection
embossed projection
shape is embossed onto the plate surface by
appropriate die plates, dies and, if necessary,
die
b
l
mould plate
counter dies.
plate
Various problems occur in projection welding caused by the welding of several joints in
a single working cycle. Due to different current
longitudinal projection
br-er8-25e.cdr
© ISF 2002
Production of
Embossed Projection Shapes
paths - when using direct current - and a current displacement - when using alternating
current -, welding nuggets with differing quali-
Figure 8.25
alternating current distribution
intensity of current increases from the center to
the outer area caused by current displacement
force distribution of a C-frame projection press
welder during bending of machine tool frame
direct current distribution
intensity of current decreases from the center to
the outer area caused by the longer current path
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force distribution of a C-frame projection press
welder with non-parallel positioning tables
© ISF 2002
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Problem of Current Distribution
During Projection Welding
Figure 8.26
© ISF 2002
Problems of Force Distribution
During Projection Welding
Figure 8.27
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
117
ties are produced when no preventive remedies are taken, Figure 8.26.
A varying force distribution,
as shown by the example
in Figure 8.27, also leads
to differing qualities of the
produced weld nuggets.
In Figure 8.28 several examples of application using
projection welding are de© ISF 2002
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Application Examples of Projection Welding
picted. In this example, the
shapes are of the embossed type.
Figure 8.28
Figures 8.29 and 8.30 show several process variations of roller seam welding. Seam welding
is actually a continuous spot welding process, but with the application of roller electrodes. In
contrast to resistance spot welding the electrodes remain in contact and turn continuously
after the first weld spot has been produced. At the points where a welding spot is to be produced again current flow is
lap joint
lap joint with
wire electrode
lap joint
with foil
squash seam
weld
butt weld
with foil
initiated. Dependent on the
electrode feed rate and on
the welding current frequency, spot welds or seal
before
welding
welds
with
weld
nuggets
overlapping
are
pro-
duced. The application of
d.c. current also produces
after
welding
seal welds.
© ISF 2002
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Roller Seam Welding
Figure 8.29
2005
8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
118
interrupted-current roller seam weld
overlap seal weld
continuous D.C. seal weld
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© ISF 2002
Weld Types for Roller Seam Welding
Figure 8.30
2005