INFLUENCES OF SECONDARY PULSE ON RESISTANCE SPOT

WELDING OF HOT FORMING STEELS

Jeff HouD.Saha,  S.  Nayak,  N.Zhou,  A.Gerl ich (UofWaterloo)

2014   International  Symposium  on  Advances   in  Resistance  Welding ,  Atlanta

K.R.Chan,  N.Scotchmer (Huys Industr ies)

OUTLINE

• Motivation• Objectives• Equipment and Material Specification• Simulation Work (Swantec SORPAS)• Experimental Work• Summary• Future Work

MOTIVATION - APPLICATION

Reference: ArcelorMittal

BUMPER BEAMS

WINDSCREEN FRAME (UPRIGHT)

DOOR REINFORCMENTS

B‐PILLER

MOTIVATION –USIBOR 1500P

Reference: ArcelorMittal

AS RECEIVED MICROSTRUCTURE

HOT STAMPED MICROSTRUCTURE

Physical & Mechanical Properties of USIBOR 1500P

Coating: Aluminized

YS (MPa) UTS (MPa) Coating Phases

AsDelivered 350‐550 500‐700 Al‐Si

Hot Stamped 1100 1500 Al‐Si‐Fe

PROBLEM

• High base metal strength promotes interfacial failure mode for resistance spot welded components.

• For energy absorption during impact point of view, plug failure mode may be more beneficial.

OBJECTIVE

• To simulate a welding window and secondary pulsing schedules for welding.

• Explore the importance of intermission cooling time prior secondary pulsing.

• To investigate the resulting mechanical and metallurgical properties of the simulated welding schedules.

EQUIPMENT AND MATERIAL SPEC.• Equipment:

144/180-kVA MFDC RSW• 60kA Current• 25kN Force

150kN Instron Tensile Tester

• Electrode:RWMA Female B Style CLASS II

• 6mm Diameter

• USIBOR 1500P Coupons:1.5x60x140mm as per AWS D8.9:2012Heat treated at 930oC for 5-10mins and oil quenched (rate>50oC/s)

SORPAS- SINGLE PULSE WELD WINDOW

SORPAS Simulation

Weld Time (Cycles)

32.0 3.77 5.04 6.37 7.08 7.57 8.18 8.49 8.9028.0 3.75 4.95 6.21 7.00 7.45 8.06 8.34 8.7324.0 3.75 4.83 5.92 6.72 7.31 7.68 8.18 8.5220.0 3.74 4.68 5.58 6.45 7.09 7.44 7.88 8.2816.0 3.74 4.55 5.20 6.10 6.68 7.17 7.51 7.9212.0 3.74 4.40 4.86 5.47 6.19 6.62 7.02 7.358.0 3.18 4.05 4.59 4.90 5.33 5.83 6.28 6.624.0 1.77 3.07 3.51 4.19 4.55 4.79 4.99 5.240.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0

Weld Current (kA)Weld Force: 4.0 [kN]

Minimum Required Nugget Diameter (mm):   4.90

Legend:

Undersized nugget diameterSufficient nugget diameter (No Expulsion)Nugget overgrowth (Risk of Expulsion)

Electrode Melting

SORPAS – SINGLE PULSE WELD

• Potential Welding Schedules6kA – 32 Cycles7kA – 20 Cycles8kA – 12 Cycles

0

1

2

3

4

0

1

2

3

4

5

6

7

8

9

0 10 20 30 40

Weld Force (kN)

Weld Cu

rren

t (kA

)

Process Time (Cycles)

6kA 7kA 8kA Force (kN)

SORPAS – SECONDARY PULSE WELD

0

1

2

3

4

0

2

4

6

8

10

12

0 20 40 60

Weld Force (kN)

Weld Cu

rren

t (kA

)

Process Time (Cycles)

20 Cycle Cooling

8kA Main 6kA 2nd 8kA 2nd

10kA 2nd Force (kN)

0

1

2

3

4

0

2

4

6

8

10

12

0 20 40 60 80 100

Weld Force (kN)

Weld Cu

rren

t (kA

)Process Time (Cycles)

60 Cycle Cooling

8kA Main 5kA 2nd 7kA 2nd

9kA 2nd Force (kN)

SORPAS – 20 CYCLE COOLING

0

250

500

750

1000

1250

1500

1750

2000

2250

0 500 1000 1500

Tempe

rature ( C )

Process Time (ms)

Main 6ka 8ka 10ka AC1 AC3 MS Melt

SORPAS Exp B FZ Thermal Profile

MELT

AC1AC3

MS

20 CYCLE COOLING– NUGGET GROWTH2ndPulse Initial Nugget Reheat Peak Temp Temp 

Legend

6kA

5.68mm

5.68mm

8kA

5.71mm

10kA

6.88mm

SORPAS Exp A Nugget Growth

WELD PROPERTIES & FAILURE MODES –20 CYCLE COOLING

Minimum Required Strength: 14kN(AWS D8.9:2012)

Single Pulse Welds

Single Pulse TSS(kN) Failure Mode

8kA – 12 Cycle 19.45 Interfacial

Two Pulse Welds

2nd Pulse TSS(kN) Failure Mode

6kA 21.28 Interfacial

8kA 19.04 Interfacial

10kA 24.40 Mix

19.45 21.28 19.04 24.40

0

5

10

15

20

25

30

Tensile

 She

ar Stren

gth (kN)

Process Parameters

SPC‐ 8kA TPC ‐ 6kA TPC ‐ 8kA TPC ‐ 10kA

14kN Min Shear Strength AWS D8.9M

20 CYC COOL- MICROSTRUCTURES

8kA Main Nugget

5.743mm

6kA TPC 5.740mm

50x Magnification Macro-structure

20 CYC COOL- MICROSTRUCTURES

8kA TPC 5.727mm

10kA TPC 6.797mm

50x Magnification Macro-structure

HARDNESS PROFILE – 20 CYC COOLFZ Boundary HAZ Boundary

250

300

350

400

450

500

550

600

0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8 5.2 5.6

Vickers H

ardn

ess (HV5

00g)

Distance From Nugget Center (um)

8kA Main Nugget 6kA TPC 8kA TPC 10kA TPC FZ Boundary HAZ Boundary

HAZ BMFZ

SORPAS – 60 CYCLE COOLING

0

250

500

750

1000

1250

1500

1750

2000

2250

0 500 1000 1500 2000

Tempe

rature ( C )

Process Time (ms)

Main 5ka 7kA 9kA AC1 AC3 MS Melt

SORPAS Exp B FZ Thermal Profile

MELT

AC1AC3

MS

60 CYCLE COOLING – NUGGET GROWTH2ndPulse Initial Nugget Reheat Peak Temp Temp 

Legend

5kA

5.68mm

5.68mm

7kA

5.68mm

9kA

5.69mm

SORPAS Exp B Nugget Growth

WELD PROPERTIES & FAILURE MODES –60 CYCLE COOLING

Minimum Required Strength: 14kN(AWS D8.9:2012)

Single Pulse Welds

Single Pulse TSS(kN) Failure Mode

8kA – 12 Cycle 19.45 Interfacial

Two Pulse Welds

2nd Pulse TSS(kN) Failure Mode

5kA 11.16 Interfacial

7kA 20.20 Mix

9kA 21.56 Mix

19.45 11.16 20.20 21.56

0

5

10

15

20

25

30

Tensile

 She

ar Stren

gth (kN)

Process Parameters

SPC ‐ 8kA TPC ‐ 5kA TPC ‐ 7kA TPC ‐ 9kA

14kN Min Shear Strength AWS D8.9M

60 CYC COOL - MICROSTRUCTURES

8kA Main Nugget

5.743mm

5kA TPC 5.742mm

50x Magnification Macro-structure

60 CYC COOL - MICROSTRUCTURES

7kA TPC 5.970mm

9kA TPC 6.432mm

50x Magnification Macro-structure

HARDNESS PROFILE – 60 CYC COOLFZ Boundary HAZ Boundary

250.00

300.00

350.00

400.00

450.00

500.00

550.00

600.00

0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8 5.2 5.6

Vickers H

ardn

ess (HV 5

00g)

Distance From Nugget Center (um)

8kA Main Nugget 5kA TPC 7kA TPC 9kA TPC FZ Boundary HAZ Boundary

HAZ BMFZ

SUMMARY• Tensile Shear Strengths (TSS):

No significant change in TSS unless nugget is overgrown (20 cycle cool)Extreme drop in TSS for 5kA secondary pulsing for (60 cycle cool).

• Failure Modes:Nugget overgrowth promotes plug failure (20 cycle cool).Recrystallization and nugget re-melt promote plug failure (60 cycle cool).

• Microstructure:Minimal microstructural differences for (20 cycle cool).Columnar feature in FZ disappears with 7kA secondary pulse. (60 cycle cool)

• Hardness Profiles:Prolonged heating creates broadening of HAZ (20 cycle cool).Tempering secondary pulse results in nugget softening (60 cycle cool).

FUTURE WORK

• Continuous optimization of welding operation.

• Investigation of failure initiation sites with different temper pulses – partial pull tests.

• Explore effects of secondary pulsing on electrode durability.

Thank You! Any Questions?

Acknowledgements