8. resistance spot welding, resistance projection welding ...mercury.kau.ac.kr/welding/welding...

8.

Resistance Spot Welding,

Resistance Projection Welding

and Resistance Seam Welding

8. Resistance Spot-, Resistance Projection- and Resistance Seam Welding 105

2005

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 con-

sequence of Joule resistance heating caused by current flow through an electrical conductor,

Figure 8.2. In spot and pro-

jection welding, the plates

to be welded in overlap.

Current supply is carried

out through spherical or

flat electrodes, respec-

tively. In roller seam weld-

ing, two driven roller elec-

trodes are applied. The

plates to be welded are

mainly overlapped. The

heat input rate Qinput is

generated by resistance

heating in a current-

carrying conductor, Figure

8.3. However, only the ef-

fective heat quantity Qeff is

instrumental in the forma-

tion of the weld nugget.

Qeff is composed of the

input heat minus the dissi-

pation heat. The heat loss

arises from the heat dissi-

pation into the electrodes

and the plates and also

from thermal radiation.

© ISF 2002

Classification of WeldingAccording to DIN 1910

br-er8-01e.cdr

resistance spotwelding

roller seam welding

projectionwelding

resistance buttwelding

flash buttwelding

resistance pressurewelding

induction pressurewelding

Conduction pressurewelding

pressurewelding

cold pressurewelding

friction welding

fusionwelding

welding

Figure 8.1

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4 loaded area

roller seam welding

workpiece usually in general overlap

driven roller electrode spot rows (stitch weld,

roller spots)

�

�

�

1

3

4

2

1

5 projection

projection welding

workpieces with elevations (concentration of electicity)

workpieces overlap pad electrode several joints in a single weld weld nugget joint

�

�

�

�

�

1 3

5

2

1

spot welding

�

�

�

workpieces overlap electrode weld nugget

1 electrode force2 elektrodes3 production part

1 32

1

Figure 8.2


2005

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% in-

creases the heat input rate by 105%, the reduction of the welding current down to 90% de-

creases 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-

tance height is greatly de-

pendent on the applied

electrode force. The higher

this force is set, the larger

are the conductive cross-

sections at the contact

points and smaller the re-

sistances. The contact sur-

faces, which are rapidly

increasing at the start of

welding, effect a rapid re-

duction of interface resis-

tances.

With the formation of the

weld nugget the interface

resistances between the

plates disappear. During

the progress of the weld

the material resistance in-

creases from a low value

(surrounding temperatures)

to a maximum value above

the melting temperature.

br-er8-04e.cdr

theoretical contact area100% metallic conduction contact

proportion at room temperature

proportion after firstmilliseconds welding time

low electrode forcehigh resistance

high electrode forcelow resistance

surface resistance is collapsed,a3 is highly extended

A1: area out-of-contactA2: contact area with high resistanceA3: contact area completely conductive

welding time

5 10 periods

resis

tance

mOhm

total resistance

sum of contact resistances

sum of material resistance

Figure 8.4

© ISF 2002br-er8-03e.cdr

Heat Balance in Spot Welding

Qeff

Q2

Q2

Q4

Q4

Q4

Fel

Fel

Q4

Q3

Q3

Q = Q - Qeff input 1l

Q = Q + Q + Q1 2 3 4

R(t) = R (t) + R (t)material c

Q = C I (t) R(t) dtinput

2

t=tS

t=0

F :

Q :

Q :

I:

Q :

Q :

Q :

Q :

R(t):

R (t):

R (t):

el

eff

input

1

2

3

4

material

c

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

Figure 8.3


2005

Figure 8.5 shows dia-

grammatically the different

resistances during the spot

welding process with act-

ing electrode force, but

without welding current.

Weld nugget formation

must therefore start in the

joining zone because of

the existing high contact

resistance there.

Figure 8.6 shows directly

cooled electrodes for resis-

tance welding. The coolant

is normally water. In the

cooling tube, the cooling

water is transported to the

electrode base. The dia-

gram shows the tempera-

ture distribution in the elec-

trodes and in the plates.

The maximum temperature

is reached in the centre of

the weld nugget and de-

creases strongly in the

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. For-

mation of the weld nugget in the joining zone of both workpieces.

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electrode force resistance rate

~_

R1

R3

R6

R7

R4

R2

0 100 200

R4

R7

R5

R6

R3

R [µOhm]

R5

Figure 8.5


Electrode Cooling

cooling tube

cooling drill-hole

slope

10 -

20

2-5

6-8

°C

Figure 8.6


2005

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. De-

pendent on the welding task different force and current programs can be set in the weld-

ing 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 post-

weld 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.


Time Sequence ofResistance Spot Welding

Fel Iw

electrode force Fel

welding current Iw

time t

top electrode

workpiece

weld nuggetlower electrode

tpre tw th

insufficiently melted weld nugget totally melted weld nugget

Figure 8.7


Functions of Pre- and Postwelding

set-up time

postweld-holding time

- 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

- holding time of workpiece during cooling of molten metal - prevention of pore formation in the welding nugget - prevention of lifting the electrode under voltage

The postweld-holding time hasinfluence on the weld point hardeningwithin certain limits.

Figure 8.8


2005

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.

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 arrange-

ment also plates can be welded where one of the two plates has a non-conductive surface

(as, for example, plastics).


Schematic Assembly of Spot Welding Machine

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 arm10 electrical power supply cable11 water cooled electrode holder12 electrode

1

2

3

4

5

6

7

8

9

10

11 12

Figure 8.10 Figure 8.9

br-er8-09e.cdr © ISF 2006

Course of Force and Current

t = pre-weld time

t = welding time

t = holding time

t = pressure time

pre

w

h

pres

tpres

ele

ctr

od

e f

orc

e

we

ldin

g c

urr

en

t

5 Iw

tpre tw th

Fel

time

ele

ctr

od

e f

orc

e

we

ldin

g c

urr

en

t

57 8

Iw

Fel

time

ele

ctr

od

e f

orc

e

weld

ing

cu

rre

nt

52

1

7 864

3

Iw

Fel

time

1 - initial force2 - welding pressure force3 - post pressure force4 - preheating current5 - welding current6 - postheating current7 - ascending current8 - descending current


2005

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

relation to the average cur-

rent values, peak loads

occur and, with that, in-

creased electrode wear.

These extreme peak loads

do not occur with direct

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 cur-

rent-on period and a more

precise control of the weld-

ing current is possible.

In order to realise higher

currents and shorter weld-

ing times, the impulse ca-

pacitor resistance welding

technique is applied. The

rectified primary current is

stored in capacitors and,

through a high-voltage

transformer, converted to


Variants of Spot Welding

~~

two-sided single-shearsingle-spot welding

one-sided duplex spot weldingwith conductive base

two-sided two-shearspot welding (stack welding)

~

one-sided single-spot weldingwith contact electrode

two-sided duplex spotwelding

~

+

~

+

~

+

one-sided multi-spot weldingwith conductive base

~

+ +

Figur 8.11

Figur 8.12


Current Types

impulse capacitor current

45

40

35

30

25

20

15

10

5

0

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

alternating current

welding time [s]

20

15

10

5

0

-5

-10

-15

-20

0.00 0.02 0.04 0.07 0.09 0.11 0.13 0.16

curr

en

t [k

A]

12

10

8

6

4

2

0

medium frequency direct current

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

welding time [s]

curr

ent [k

A]

"conventional" direct current

18

16

14

12

10

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 [s]

curr

ent [k

A]

curr

ent [k

A]


2005

high welding currents. The

advantages of this tech-

nique are low heat input

and high reproducibility.

Because of the high energy

density, materials with

good conductivity can be

welded and also multiple-

projection welds can be

carried out. A disadvantage

of this method is, apart

from the high equipment

costs, the difficult regula-

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.

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Electrode Materials

requirements

- 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

Cu - ETP

Cu Cdl

Cu Crl

Cu Crl Zr

Cu CO2 Be

Cu Ni2 Si

Cu Ni1 P

Cu Be2 Co Ni

Cu Ag6

CuAl10NiFe5Ni5

ISO 5182

A

1

2

3

4

1

2

1

2

1

2

1

2

3

4

Group Type No.

Key

W75 Cu

W78 Cu

WC70 Cu

Mo

W

W65 Ag

ISO 5182

B

10

11

12

13

14

15

Group Type No.

Key

Figure 8.15

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3-phase direct current

static-inverter direct current

capacitor impulse discharge

single-phasealternating current

Figure 8.13

Figure 8.14


Electrodes,Electrode Caps and Holders

electrodes

electrode caps

electrode holders

form A form B

form Fform E form G

form Dform C


2005

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 resis-

tance, 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 at-

tached 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.


Accessibility for SpotWelding Electrode

poor good

Figure 8.16


Contact Area forSpot Welding Electrodes

poor good

Figure 8.17


2005

In order to avoid the displacement of the elec-

trodes, 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

order to guarantee the unimpeded electrode

approach to the working point, Figure 8.17.

Dependent on the joining job, the process

variation, or the resistance welding method, a

so-called “shunt current/effect” may be no-

ticed. This current component, as a rule, does

not contribute to the formation of the weld

nugget; under certain circumstances it might

even prevent a reliable welding process. In

the example, shown in Figure 8.18, the shunt

current leads to undesired fusing contacts

and, because of the lacking electrode force at

this point, also to damages to the plate sur-

face.

If unsuitable welding parameters have been

set, weld spatter formation may occur, Fig-

ure 8.19. Liquid molten metal forms on the

plate surface or in the joining zone. The rea-

sons 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

the plate thickness or too small an edge dis-

tance of the welding point.

Figure 8.18

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Shunting

spot welding

roller seam welding

indirect weldingone side

A

coppercurrent path

shunt connection current

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Welding Spatter

Welding spatter:Discharge of molten material between two steelsheets or from the surface of steel sheets.

Reason here is high welding current, (fig. 1) or too-small edge distance (fig. 2)

fig. 1 fig. 2

porosity in the joint causedby welding spatter

discharge of moltenmaterial at the joint plane

Figure 8.19


2005

Figure 8.20 shows a list of

a large number of possible

disturbances in resistance

spot welding. Welding cur-

rent changes are caused

by: shunt, electrode wear,

cable wear, mains voltage

variations, secondary im-

pedance.

Different welding condi-

tions are caused by weld-

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 suit-

ability 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

resistance spot and resistance projection

welding. The fundamental difference between

the two methods lies in the definition of the

current transition point.

Figure 8.20

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Qeff

Q = Q - Qeff input losses

welding current changes

modification of the unit

alt

era

tio

n t

o f

orc

e

weld

ing

equ

ipm

ent

shuntconnection

platethickness

alteration

of

pressure

wear

platediversion

heat

wear ofelectrodes

qualityof plates

wear ofcable

numberof plates

mains voltagefluctuation

platesurface

secondaryelectrical

impedance

edgedistance

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Weldable Materials

weldable materials

materials weld-ability

aluminium

iron

gold

cobalt

copper

magnesium

molybdenum

nickel

platinum

silver

tantalum

titanium

tungsten

satisfactory

very good

satisfactory

very good

poor

good

satisfactory

very good

very good

very good

very good

very good

satisfactory

alloyingelements

goodweldability

sufficientweldability

maximum content [%]

C

C + Cr

C + Mo

C + V

C + Mn

C + Ni

Si

Cu

P + S

C+Cr+Mo+V

0,25

0,35

0,50

0,40

1,40

3,00

0,40

0,60

0,10

0,60

0,40

1,60

0,70

0,60

1,60

4,00

1,00

0,60

0,10

1,60

influence ofalloying elements(steel materials)

Figure 8.21


2005

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

for quality criteria of the

weld.

Depending on the de-

mands on the joint strength

or on the projection rigidity,

different projection shapes

are applied. These are an-

nular, circular or longitudi-

nal projections. The weld-

ing projections are, accord-

br-er8-22e.cdr

before welding after welding

projection

elektrode

follow-updistance

Figure 8.22


Differences Between ResistanceSpot and Projection Welding

spotwelding

projectionwelding

elektrodes:

diameter

tip face

electrode life

up to 20 mm

convex

less

> 20 mm

flat

longer

elektrodes projections

place wherethe nuggetoriginates

no

no

yes

yes

problems:

current distribution

force distribution

number ofwelding nuggets

one several

follow-up distance small big

Figure 8.23

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Customary Projection Shapes

circular

longitudinal

annular

interrupted annular

circular

longitudinal

annular

spot

contact

line

contact

embossedprojection shape

solidprojection shape

naturalprojection shape

pressed

mould pressed

struck

machined

cut

pushed

crossed wires

bolt-pipe

Longitudinal wire-platecut

Circular

weld nut

pushed Annular

Figure 8.24


2005

ing to their size, adapted to the used plate

thickness and may, therefore, appear as dif-

ferent types in the workpiece: embossed pro-

jections, solid projections and natural projec-

tions. The survey is shown in Figure 8.24.

In Figure 8.25 the production of embossed

projections in different shapes is shown. The

shape is embossed onto the plate surface by

appropriate die plates, dies and, if necessary,

counter dies.

Various problems occur in projection weld-

ing caused by the welding of several joints in

a single working cycle. Due to different current

paths - when using direct current - and a cur-

rent displacement - when using alternating

current -, welding nuggets with differing quali-


Problem of Current DistributionDuring Projection Welding

direct current distribution intensity of current decreases from the center tothe outer area caused by the longer current path

alternating current intensity of current increases from the center tothe outer area caused by current displacement

distribution

Figure 8.26


Problems of Force DistributionDuring Projection Welding

force distribution of a C-frame projection presswelder during bending of machine tool frame

force distribution of a C-frame projection welder with non-parallel positioning tables

press

Figure 8.27


Production ofEmbossed Projection Shapes

embossed projection ring projection

longitudinal projection

l

mould plate plate

die

b

d1

d1

die

plate

mould plate

die

plate

mould plate

counter-die

Figure 8.25


2005

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 ex-

amples of application using

projection welding are de-

picted. In this example, the

shapes are of the em-

bossed type.

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 pro-

duced again current flow is

initiated. Dependent on the

electrode feed rate and on

the welding current fre-

quency, spot welds or seal

welds with overlapping

weld nuggets are pro-

duced. The application of

d.c. current also produces

seal welds. © ISF 2002br-er8-28e.cdr

Roller Seam Welding

lap joint lap joint withwire electrode

lap jointwith foil

squash seamweld

butt weldwith foil

beforewelding

afterwelding

Figure 8.29

© ISF 2002br-er8-30e_sw.cdr

Application of Projection WeldingExamples

Figure 8.28


2005


Weld Types for Roller Seam Welding

interrupted-current roller seam weld

overlap seal weld

continuous D.C. seal weld

Figure 8.30

8. resistance spot welding, resistance projection welding ...mercury.kau.ac.kr/welding/welding...

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