Chapter 8 - Resistance Spot Welding, Resistance Projection Welding and Resistance Seam Welding

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

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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

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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

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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

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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

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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.

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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

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Time Sequence of Resistance Spot Welding

Figure 8.7

© ISF 2002

Functions of Pre- and Postwelding

Figure 8.8 2005


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


12 4

Iw

5 7

8 7 5 8 time

e c r o f

e d o r t c e l e


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

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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).

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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

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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]

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Figur 8.12

precise control of the weld-

rectified primary current is stored in capacitors and, through

a

high-voltage

transformer, converted to

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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

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Electrodes, Electrode Caps and Holders

Figure 8.14

Electrode Materials

Figure 8.15 2005


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

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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

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the plate thickness or too small an edge dis-

Welding Spatter

tance of the welding point. Figure 8.19 2005


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)

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current transition point.

Weldable Materials

Figure 8.21 2005


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:

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© 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


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

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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


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

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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

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Chapter 8 - Resistance Spot Welding, Resistance Projection Welding and Resistance Seam Welding

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