spot weld1643

15. - 17. 5. 2013, Brno, Czech Republic, EU

THE INFLUENCE OF RESISTANCE SPOT WELDING ON WELD JOINT QUALITY

AND SERVICE LIFE OF ELEKTRODES

Marie KOLAŘÍKOVÁ, Ladislav KOLAŘÍK

CTU in Prague, Prague, Czech Republic, EU

Abstract

The contribution deals with results of research of resistance spot welding of deep drawn sheets with surface

treatment used in automotive industry. Welded material was low carbon, high strength, deep drawn steel

HX180BD with hot-dip galvanic layer ZnMg. There were welded two of the same plates with thickness 0.8

mm with the same surface treatment. To obtain welding parameters area there was made optimalization

according to EN ISO 14327. The weld joint quality was evaluated by visual testing and mechanized peel test

according to EN ISO 14270. Spot joints welded by optimized parameters were compared with joints welded

by parameters commonly used in automotive industry for this material. There were made durability tests of

electrodes (according to EN ISO 8166) for optimized welding parameters and commonly used parameters.

The comparison of worn electrodes shown, that the optimized parameters result in increased service life, but

also to reducing the strength of resistance spot weld joints.

Keywords: Resistance Spot Welding; Weld Joint Quality; Service Life; Surface Treatment; Deep Drawn

Sheet

1. INTRODUCTION

Automotive industry is a typical sector using resistance spot welding. It is mainly used in production of car

bodies, where spot welds are main connecting element. Present trends lead to combining of materials of

different thicknesses, quality and surface treatment. Surface treatment is very important in terms of corrosion

protection. Therefore is necessary that welding does not damage or delete it. Coatings based on zinc are

used. This may leads to complications during welding. For this reason is constant development of materials

and its coatings, as well as optimization of welding parameters.

This paper deals with resistance welding of material HX180BD with hot-dip galvanic coating ZnMg. There

are presented results of optimization of welding parameters according to procedure described in EN

standards. Furthermore, there are evaluated the service live of welding electrodes. Both are compared with

actual parameters used in company ŠKODA Auto a.s.

2. WELDED MATERIAL

Bake hardening steel HX180BD is soft deep-drawn steel with controlled yield strength. Chemical

composition and base mechanical properties are in Tab. 1. The steel was hot dip galvanized by zinc alloy

ZnMg (100 g/mm2). Sheet thickness was 0.8 mm.

Table 1: Chemical composition and mechanical properties of base material

C Si Mn P S Al Nb Ti

0.003 0.04 0.17 0.008 0.006 0.032 0.007 0.0007

Tensile strength Rm [MPa] Yield strength Rp0,2 [MPa] Ductility A80 [%]

330 210 36.5


During resistance spot welding, in point of weld lens, is zinc layer melted earlier than base material (BM). On

the opposite surface (at the point of contact of electrode and BM) this layer must remain undamaged.

Compared to parameters for welding of uncoated plates, higher current and shorter time is used. Welding is

done in so called hard mode of welding (short time, high welding current, high electrode force). Short time

provides the shortest contact of Zn coating and contact cap. This leads to increase of service life of welding

electrodes, [1] [4]

3. USED EQUIPMENT

Equipment DALEX PMS 11-4 (Fig. 1a) was used for welding. Electrode caps 39D 1978-1 (Fig. 1b) were

used. Material of caps was CuCr1Zr according to DIN ISO 5182-A2/2. Caps were placed on the electrode

holders DALEX 2S30. Electrode diameter was 5 mm.

1a 1b

Fig. 1: Resistance welding machine DALEX PMS 11-4 (1a), electrode cap 39D 1978-1 - schema (1b)

4. EXPERIMENT

Optimization of welding process was made according to EN ISO 14327. Optimization is made either constant

electrode force or constant welding time. Recommended spacing of welds is according to EN ISO 14373.

(The distance from the plate edge to centre of weld shall not be shorter than 1.25 x d, where d is diameter of

weld. The distance between neighboring spot welds should not be shorter than 16 x t, where t is plate

thickness). [2] Three parameters were optimized: electrode force, welding time and welding current, 49 welds

were performed to optimize one parameter. The overall number of testing welds was 438. [4]

Quality of welds was verified by chisel and peel testing. The result of optimization of welding was

construction of diagrams „suitable area of welding parameters“. [2] From these diagrams were determined

optimized parameters. These parameters allow the production of spot welds without defects and with

adequate diameter of weld lens. Diagram must be created from at least four “growth curves”. These curves

must be created in four established welding times or four levels of electrode forces. [3]

Optimized parameters were compared with real parameters used in ŠKODA Auto a.s. for the same material

and thickness (electrode force = 26 N, welding time = 260 ms, welding current = 8 kA) in terms of electrode

service life. Assessment of service life of welding electrodes was made according to EN ISO 8166. The


service life of electrodes is defined as number of welds performed before treatment (cleaning, grinding) of

contact surface of electrodes. Electrode reaches its service time, if three consecutive welds have a diameter

of lens shorter than 3.5t-1/2

(according to mentioned standard). The quality of performed welds is verified by

peel and chisel testing or by metallography. Visual testing was done. Cracks, burned welds, spatters of BM,

baked electrodes or zinc was evaluated. Indentations depth of electrodes and lens diameter was also

evaluated. [5]

For optimized parameters (also for taken over parameters) were performed spot welds on plates - overall

192 welds according to EN. Another 8 welds (on separate plate) was tested by peel test. The size of weld

lens was 3 times measured on each weld. Arithmetic average was calculated form these values. To obtain

reference values (state of new electrode) the first 8 welds on first welded plate were evaluated. The total

number of welds was 2600 (for optimized parameters) and 1200 for taken over parameters. Then the welds

were evaluated as poor-quality and test of service life of electrodes was terminated.

The surface of welded plates was before welding free of grease, without residue of corrosion, coatings, burrs

or others defects. These defects can negatively affect the contact in the dividing plane or can require

excessive force for clamp plates. [2]

5. RESULTS

The resulting diagram of welding area at a constant electrode force is on Fig. 2. Optimal welding parameters

are: electrode force = 24 N, welding time = 240 ms, welding current = 7 kA.

Fig. 2: Diagram of welding area (WCR = welding current ratio)

We

ld t

ime t

s [m

s]

Weld current Is [kA]

□ – breach in dividing plane, ○ – particularly torned weld, ● – torned weld, ∆ - spatters

min Ø of weld = 3.5√t

Ø of weld = 5√t Optimal weld svar

Limit of spatters


In determining the service life of electrodes was monitored indentation depth of electrode, its contact

diameter and diameter of weld lens. The dependence of indentation depth of electrode on its diameter is in

Fig. 3. Indentation depth of electrode during first 50 to 100 welds dropped below 30% of thickness of welded

material. To stabilization occurred in 12% of thickness of welded material (Fig. 3) which is compliant with EN

ISO 14327. The contact diameter of electrodes increased during welding (Fig. 4). The increase of electrode

diameter is comparable for optimized parameters and for parameters used in ŠKODA Auto a.s. (see Table

2). The greater number of welds the lower rate of increase in diameter of electrode.

Fig. 3: Dependence of indentation depth of electrode on its diameter – optimized parameters

Macrostructures of quality welds made with used (assumed) parameters and with optimized parameters are

in Fig. 5. Microstructure of worn electrode cap is in Fig. 6.

Electrodes have reached its service life after welding 1800 spot welds – see Fig. 7. The limit diameter of

weld lens for plate with thickness 0.8 mm is 3.1 mm (according to EN ISO 8166).

Fig. 4: Macrostructure of electrode caps: new (left), after reaching the service live with parameters used in

ŠKODA Auto a.s. (middle), after reaching the service live with optimized parameters (right)

Dia

me

ter

of e

lectr

ode

[m

m]

∆ - indentation

depth of electrode

◊ - diameter of

electrode

Ø 5 mm Ø 9.4 mm Ø 9.8 mm

Inde

nta

tio

n d

ep

th o

f e

lectr

ode [%

]

Number of sample

0 400 800 1200 1600 2000 2400 2800

12

10

8

6

4

2

0

50

45

40

35

30

25

20

15

10

5

0


Table 2: Indentation depth of electrode, its contact diameter and weld lens diameter

Parameters from ŠKODA Auto a.s.

Number of sample

8 200 400 600 800 1000 1200

Weld lens diameter [mm]

6.1 6.1 5.3 5.7 5.7 5.3 4.1

Electrode diameter [mm]

5 6 7.5 7.9 8.9 9 9.4

Indentation depth of electrode [mm]

0.44 0.43 0.12 0.26 0.21 0.22 0.20

Optimized parameters

Number of sample

8 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

Weld lens diameter [mm]

5.2 5.0 4.5 4.8 4.8 4.0 4.5 3.4 4.7 3.8 3.4 4.0 2.9 3.9

Electrode diameter [mm]

5 6.8 7.4 7.9 7.98 8.3 8.8 9.0 9.1 9.1 9.2 9.6 9.6 9.8

Indentation depth of electrode [mm]

0.37 0.15 0.09 0.07 0.08 0.08 0.13 0.14 0.09 0.10 0.10 0.11 0.07 0.07

Fig. 5: Macrostructure of spot welds

Fig. 6: Microstructure of worn electrode cap

Zinc layer baked on

electrode cap

Alloy Zn and CuCr1Zr

Base material of electrode cap (CuCr1Zr)

Weld produced with parameters used in ŠKODA Auto a.s.

Weld produced with optimized parameters


On surface of electrode cap was deposited Zn layer from welded plates (Fig. 6). With higher number of

welds the zinc layer was peel off. Loose zinc negatively affects the weld quality. It caused inclusions in weld

lens, spatter of BM and burn of welded material.

Fig. 7: Joints after peel test after reaching the service life of electrode caps for optimized parameters

6. CONCLUSION

Welding parameters were optimized (according to the procedure described in standard EN ISO 14327).

Optimized parameters are lower than parameters used in practice in ŠKODA Auto a.s. This is advantageous

in term of thermal effect, service life of electrodes and smaller deformations. On the other hand the lower

parameters may cause a higher sensitivity to impurities and inequality on surface of welded material. This

corresponds with real experience in automotive industry, where are used higher parameters for this type of

weld. Both tested parameters do not cause problems with corrosion of weld joints.

Electrode caps have higher service life using optimized welding parameters (1800 spot welds) than using

parameters used in ŠKODA Auto a.s. (1200 spot welds). For welds performed with optimized parameters is

lower deformation of welded plate (Fig. 5) and surface quality is better. However higher service life of

electrodes reflected in size of spot welds. From this result the lower strength can be deducted. The smaller

diameter of weld lens was detected but according standard it is still acceptable. Both used parameters lead

to approximately the same growth rate of electrode diameter. Used parameters do not affected thickness of

zinc layer diffused or baked on the electrodes.

It can be said, based on the verification and optimization of welding parameters according EN, that the way

of welding in ŠKODA Auto a.s. leads to quality of welded joints. Reduces service life is compensated by

regular maintenance of electrode surface.

ACKNOWLEDGEMENTS

The research was financed by the Czech Ministry of Education, Youth and Sport within the frame of

project SGS CVUT SGS13/187/OHK2/3T/12.

LITERATURE

[1] LIPA, M. Odporové zváranie. 1.vyd. Bratislava: WELDTECH, 1995, 81 s. ISBN 80-88734-13-4.

[2] ČSN EN ISO 14373. Odporové svařování - Postup pro bodové svařování nepovlakovaných a povlakovaných

nízkouhlíkových ocelí. Praha: Český normalizační institut, 2007, 20 s.

[3] ČSN EN ISO 14327. Odporové svařování - Způsoby určení diagramu oblasti svařování při odporovém bodovém,

výstupkovém a švovém svařování. Praha: Český normalizační institut, 2005, 20 s

[4] ROČKAI, R. Odporové svařování povrchově upravených plechů v automobilovém průmyslu, diplomová práce,

2012, 119 s.

[5] NERAD, K. Posouzení životnosti elektrod pro odporové svařování, Bakalářská práce, 2012, 58 s.

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