NEW METHOD FOR MEASURING THE ELECTRICAL CONTACT RESISTANCEELECTRICAL CONTACT RESISTANCE

DURING RESISTANCE SPOT WELDING

Institute for Materials Science and WeldingMatthew Galler1,2, Wolfgang Ernst1,2, Rudolf Vallant1,2, Norbert Enzinger1,2

Institute for Materials Science and WeldingGraz University of Technology1

JOIN, Network of Excellence for Joining, Austria2

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BackgroundBackground

M J t W ldi i th W ld 2008M. Janota, Welding in the World, 2008

• Resistance Spot Welding remains the most commonly used joining process for automobile applications

• Demand for research in new high strength materials, and coated material is still very high

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y g

Presentation Outline1. Introduction: resistance spot welding (RSW)

– Importance of electrical contact resistance (Rce)Experimentally measure the Rce during welding– Experimentally measure the Rce during welding

2. Definition of new approach– Modified RSW process

• Single sheet welding, one period current application

3. Simulation procedure• Rce and bulk resistance curves altered• Rce and bulk resistance curves altered

4. Simulation results• FEM program SYSWELD: Uncoated DP600 steel

5. Comparison to experimental data6. Conclusions and Outlook

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– Rce was successfully measured at electrode sheet interface for the first time during RSW

Resistance Spot Welding

F 3 kN

Bulk electrode resistance

Electrical contact resistance (Rce) electrode/sheet

• Force ~ 3 kN

• Current ~ 7-9 kA

• Time ~ 0.2 s

Bulk sheet resistance

Electrical contact resistance (Rce) sheet/sheet

Bulk sheet resistance

H = I2RtotalElectrical contact resistance (Rce) electrode/sheet

Bulk electrode resistance

• Importance of Rce curve for simulation

• Never successfully measured during welding difficulty to separate Rce and bulk material resistance contributions

Overall research goalOverall research goal

• Experimentally measure Rce during welding from total resistance measurement in the welding electrodes

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the welding electrodes

• Problem: very difficult with conventional RSW New approach must adopted

Definition of New ApproachSingle sheet welding, one period of AC application, low welding currentWhy single sheet instead of two sheets?Why single sheet instead of two sheets?

• Eliminate Rce at the steel/steel interface to simplify the analysis

Why one period instead of 10?

V

Why one period instead of 10?

Why low effective welding current?

• Create a minimum amount of bulk material resistance heatingresistance heating

How can simulation help?• Goal of the simulation validate the new approachGoal of the simulation validate the new approach• Find the welding current ranges in which the total resistance measurement in the electrode represents mainly the Rce contribution

N i ll i l R d B lk i ib i h l i

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• Numerically isolate Rce and Bulk resistance contribution to the total resistance measurement

•

Simulation Procedure

15

20

-5

0

5

10

d cu

rrent

[kA]

0 2 4 6 8 10 12 14 16 18 20

-20

-15

-10

Wel

dTime [ms]

• 1 mm uncoated DP600 steel with 3.5kN welding force

• Real time current inputs 0.5 - 9.5 kA effective current

• Voltage drop at time of peak of first half wave

T t l i t l l t d f lt d t

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• Total resistance calculated from voltage and current

Simulation Procedure: Altered Input Parameters p

12

14

16 RCE curve A RCE curve B

14

16 Resistivity of HTC600X

6

8

10

12

CE

[10-1

0 Ω.m

²]

6

8

10

12

stiv

ity [1

0-7Ω

m]

0 100 200 300 400 500 600 700 800 900 1000

0

2

4RC

0 100 200 300 400 500 600 700 800 900 1000

0

2

4

Res

is

Temperature [°C]

Three Rce curves at electrode/sheet interface were modelled

Temperature [°C]

• Classical Rce curve shape from literature SYSWELD input for uncoated DP600 steel

• Proposed Rce shape with peak in curve Model (Babu, Sci. Tech. Weld. Join. 2004)

• Near zero value

C ass ca ce cu e s ape o te atu e S S put o u coated 600 stee

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• Two material resistivity curves

Simulation Results: SYSWELD

• Strong coupling

2ms iteration time between2ms iteration time between thermal and mechanical analysis

• Electrode sink-in and evolution of contact radiuscontact radius

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only bulk resistance at 9.5 kA at peak first half wave

only Rce plus RT Bulk at 9.5 kA at peak first half wave

Simulation Results: Peak Rce curve

Normal Simulation

Contact Resistance plus RT Bulk R450

500 STR4STR5 Contact Resistance plus RT Bulk R.

Only Bulk Resistance

300

350

400STR5 STR3

ce [μ

Ω]

100

150

200

250

otal

resi

stan

0 2 4 6 8 10 12 14 16 18

0

50

100To

Are these total resistance values the real Rce values?

0 2 4 6 8 10 12 14 16 18Maxium weld current [kA]

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• Back calculate Rce from total resistance and compare to input Rce (Ohm*m2)

Simulation Results

14

16

Input Rce Curve STR4: Normal Simulation STR5: Contact Resistance

10

12

Ω.m

²]

Average interface temperature from electrode contact

4

6

8

RCE curve BRC

E [1

0-10 Ω electrode contact

area

0 100 200 300 400 500 600 700 800

0

2 STR4 STR5

0 100 200 300 400 500 600 700 800 Temperature [°C]

• Normal simulation deviates at higher interface temperatures, represents the first time in simulation that the bulk contribution is measured

2.4 4.2 6.3 8.1Maximum Weld Current (kA)

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time in simulation that the bulk contribution is measured • Current range at which total resistance = Rce is verified from simulation!!

Simulation results: DP600 Rce curve

14

16

RCE curve A STR1 Normal Simulation

8

10

12

10Ω

.m²]

4

6

RC

E [1

0-1

0 100 200 300 400 500 600 700 800

0

2

2.4 4.2 6.3 8.1 10.1 12.2 14 15.7 Temperature [°C]

Maximum Weld Current (kA)

• Valid current range increases for a lower Rce curve!

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Valid current range increases for a lower Rce curve!

• Up to max current of 12 kA the new approach is valid!

Comparison of Simulation and Experimental Results

450

500

STR135

STR1: Normal Simulation Classical Rce shape STR4: Normal simulation Peak Rce Experiment DP600

300

350

400

450 STR1 STR4 experiment

e [μΩ

]

25

30

35 STR1 STR4 experiment

]

150

200

250

300

l res

ista

nce

15

20

E [1

0-10 Ω

.m²]

0

50

100

150

Tota

l

5

10

RC

E

0 2 4 6 8 10 12 14 16 18

0

Maxium weld current [kA]0 1 2 3 4 5 6 7 8 9

0

Maxium weld current [kA]• Peak in Rce curve measured in experiment for cleaned, uncoated DP600 steel

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p ,

• Actual values shifted lower compared to input Rce from SYSWELD

Conclusions

• A new method was proposed to measure the electrical contact resistance at the electrode/sheet interface during thecontact resistance at the electrode/sheet interface during the welding process

• Numerical analysis with SYSWELD verified the accuracy of this method in a defined welding current range acceptable

ldi t d d l f Rwelding current range depends on value of Rce

• The shape of the Rce curve at the electrode/sheet interfaceThe shape of the Rce curve at the electrode/sheet interface for uncoated, cleaned DP600 steel is not the classical exponential decrease, rather there exists a peak in the curve

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Outlook

• Rce vs Imax Rce vs temperature so that it can be used in further RSW simulations

• Influence of zinc coating, organic coating, welding force (i t f i l ) l t d f il f(interfacial pressure), electrode wear, surface oils, surface oxides can be inexpensively, quickly and easily performed with the new method

• Numerical techniques to find Rce at sheet/sheet interface i 2 h t ldiusing 2 sheet welding

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Thanks to k-net JOIN, Network of excellence for joining under the Federal Ministry offor joining under the Federal Ministry of

Economy and Labor, Austria

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Thank you for your attention!!!!

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