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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