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 pro jection 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 forma-
Qeff 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 weld nugget the interface resistances
between
the
total resistance e c n a t s i s e r
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 without Weld
welding nugget
107
R3
R3
R6 R6
_ ~
R7
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
is normally water. In the
8 6
cooling tube, the cooling
slope 0 5 2 - 2 0 1
water is transported to the 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 5->6
time th.
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 t pre, 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
welding current Iw
time t
tpre
tw
th
- 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
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
br-er8-07e.cdr
© 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, pro jection and roller seam welding machines. Differences are to be found merely in the type of electrode fittings and in the electrode shapes.
e c r o f e d o r t c e l e
Fel
t n e r r u c g n i d l e w
5
Iw
tw
th
tpre = pre-weld time tw = welding time th = holding time tpres = pressure time
1
9 tpre
time
10
tpres
2 3
6 e c r o f e d o r t c e l e
11
Fel
t n e r r u c g n i d l e w
12 4
Iw
5 7
8 7 5 8 time
e c r o f
e d o r t c e l e
t n e r r u c g n i d l e w
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
br-er8-10e.cdr
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 weld nugget cooling. In
two-sided single-shear single-spot welding
two-sided two-shear spot welding (stack welding)
one-sided single-spot welding with contact electrode
~
~
~
+
+
+
relation to the average cur-
+
+
rent values, peak loads ~
occur and, with that, increased
electrode
two-sided duplex spot welding
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
20
12
] 15 A10 k [ 5 t 0 n 0.00 0.02 0.04 0.07 e r r -5 u-10 c
] 10 A k [ 8
ing current is possible.
t 6 n e r r 4 u c 2
0.09 0.11 0.13 0.16
-15 -20
0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
welding time [s]
welding time
"conventional" direct current 18 16 ] A14 k12 [
[s]
impulse capacitor current 45 ] 40 A k [ 35 30 t 25 n e 20 r r u 15 c 10 5 0
10 t n 8 e r 6 r u 4 c 2 0 0.00
0.02 0.04
0.06 0.08 0.10 0.12
welding time
0.14 0.16
0.00
currents and shorter weld-
pacitor 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
precise control of the weld-
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 br-er8-14e.cdr
© ISF 2002
br-er8-15e.cdr
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
br-er8-16e.cdr
poor
© ISF 2002
br-er8-17e.cdr
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
e c r alteration o of f pressure o t n o i t a r plate e t l a
plate thickness
wear of electrodes
wear of cable
mains voltage fluctuation
a large number of possible secondary electrical impedance
Qeff
number of plates
spot welding. Welding cur-
t n e wear m p i u q e g n diversion i d l e heat w
Qeff = Qinput - Q losses
quality of plates
disturbances in resistance
plate surface
rent changes are caused by: shunt, electrode wear, cable wear, mains voltage variations, secondary im-
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 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.
Figure 8.22 shows a comparison between
materials
aluminium
weldability
the two methods lies in the definition of the
good weldability
sufficient weldability
satisfactory maximum content [%]
iron
very good
gold
satisfactory
cobalt
very good
copper
poor
magnesium
good
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
silver
very good
tantalum
very good
titanium
very good
tungsten
satisfactory
resistance spot and resistance projection welding. The fundamental difference between
alloying elements
weldable materials
influence of alloying elements (steel materials)
br-er8-21e.cdr
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-
spot welding
Figure 8.22
projection welding
embossed projection shape
elektrodes: diameter tip face electrode life
place where the nugget originates
up to 20 mm
> 20 mm
convex
flat
less
longer
mould pressed
elektrodes
projections
one
several
small
big
current distribution
no
yes
force distribution
no
yes
number of welding nuggets
pressed
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
br-er8-24e.cdr
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
counter dies.
mould plate
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 br-er8-26e.cdr
force distribution of a C-frame projection press welder with non-parallel positioning tables © ISF 2002
br-er8-27e.cdr
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
br-er8-30e_sw.cdr
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
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8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding
118
interrupted-current roller seam weld
overlap seal weld
continuous D.C. seal weld br-er8-29e.cdr
© ISF 2002
Weld Types for Roller Seam Welding
Figure 8.30
2005