challenges and advances in welding of a new generation...

Challenges and Advances in Welding of a New Generation of

High Strength Steels

Presented by T. V. Natale (AK Steel) Written by M. Kimchi (OSU), W. Peterson (EWI), J. Gould (EWI)

CAFE Standard: 54.5 MPG in 2025

New CAFE Standards2012-2016 and 2017-2025

MASS REDUCTION

AISI, www.autosteel.org, 2012

(auto/steel partnership)

Advanced High Strength Steels

(WorldAutoSteel 2011)

Future Steel Vehicle – Phase 2

2013 Ford Escape

(Morgans, GDIS 2013)


Future Steel Vehicle – Phase 2, Steel Portfolio


Future Steel Vehicle – Phase 2Steel Technologies & Joining Technologies

(auto/steel partnership)

Advanced High Strength Steels

(Branagan, GDIS 2013)

AHSS Developments

(Branagan, GDIS 2013)

Growth of AHSS in Automotive Architecture

11

050

100150200250300350400450

2000 2005 2007 2009 2020

Growth of AHSS

AHSS (other)TRIP, CP,RAMartensiteBoronDPlbs

YearSource: Ducker Worldwide

Resistance Spot WeldingAdvanced High Strength Steels AHSS

Chemical composition + steel processing

desired microstructure

desired material properties

Resistance Spot WeldingAdvanced High Strength Steels AHSS

• Dual Phase – ferrite+martensite• Ferritic-Bainitic/Stretch Form - fine f + bainite• Complex Phase – f/b matrix + m, austenite, pearlite• Martensitic• TRIP (Transformation - Induced Plasticity)

– C, Al/Si; f + a ( 5%) (+ m + b)• Hot Formed (Press Hardenable, Hot Stamped)

– C, B; martensitic after heat treatment• TWIP (Twinning – Induced Plasticity) – C, ~ 20% Mn; austenitic• Quench & Partition – C, Mn or Al/Si; f + m + a• Nanosteel – extremely fine grains, unique compositions

[Referen

ce: Re

sistance Welding

Man

ual, RW

MA, p. 1

‐3]

Principal Types of Resistance Welds

Electrodes orWelding Wheels

Seam Weld

Electrodes or Welding Tips

Spot Weld

Electrodes or Dies

Projection WeldAfter Welding

Electrodes or Dies

Butt Weld

Flash Weld

After WeldingHigh‐Frequency Weld

Resistance Spot WeldingResistance Spot Welding AHSSHSLA and AHSS

• Increased electrical resistivity• Heat balance• As strength , composition , and thickness , more

pronounced changes in welding behavior occur. • Increased force and longer welds times typical• Increased weld hardness• Weld fracture modes• Pulsation and/or post weld tempering• Joint fatigue, fracture, stress concentration, crash integrity,

modeling

Cooling Rate as a Function of Gauge, Steel and Welding Process

• Implied cooling rates for different processes– Closed‐form models

• Critical cooling rates– Martensite transformation– Microstructural modeling

• Hardenability of AHSS• Martensite formation

resulting from welding• Hardness of martensite/

effective fracture toughness

10

100

1000

10000

100000

0 0.5 1 1.5 2 2.5

Sheet Thickness (mm)

Coo

ling

Rat

e (C

o /s)

GMAWLBWRMSWRSWIF

AKDQ

M1400

TRIP 600

DP 980

TRIP 800

DP 600

DP 780

RSW Microstructure and Hardness

• 0.8‐mm IF steel– Hardness profile indicates non‐martensitic decomposition– Better fit with model by Li

50

100

150

200

250

300

350

0 1 2 3 4 5

Distance from Edge (mm)

Vick

ers

Har

dnes

s N

umbe

r

| 1mm |

Microstructure of a resistance spot weld on 0.8-mm IF (0.007% C, 0.14% Mn) steel

Hardness profile of a resistance spot weld on 0.8-mm IF (0.007% C, 0.14% Mn) steel

• 1.6‐mm DP 980– “Top Hat” hardness profile– Indicative of isothermal transformation– Martensitic reaction predicted by either model

200.0

250.0

300.0

350.0

400.0

450.0

500.0

0 2 4 6 8 10 12 14 16 18 20 22 24

Location (mm)

Microstructure of a resistance spot weld on 1.6-mm DP980 (0.15% C, 1.4% Mn) steel

Microstructure of a resistance spot weld on 1.6-mm DP980 (0.15% C, 1.4% Mn) steel

RSW Microstructure and Hardness

(Tumuluru, SMWC II, 2006)

RSW Hardness Profiles for Dual Phase Steels

Fracture Modes for RSW (AWS D8 and A/SP)

Peel and Chisel Criteria - Failure ModesButton pulled without evidence of interfacial fracture

Partial thickness fracture with button pull

Partial thickness fracture with no button pull

Interfacial fracture with button pull and partial thickness fracture

Full interfacial Fracture No fusionInterfacial fracture with button pull

Interfacial fracture with partial thickness fracture

Potential Solutions to Improving Peel Behaviorof AHSS Spot Welds

• Passive methods– Long weld time– Pre/post pulsing– Short hold time– Increased minimum weld size

• Active methods– Weld and temper– Dilution (patented)

WeldDown Slope Quench Temper Hold

Electrode Force

Dilution Weld


Force and Current Profiles for RSW of AHSS

(Tumuluru, 2006)

1.6 mm HDGA DP steels

RSW Current Range Behavior of Dual Phase Steels

(Matsushita, JFE Tech. Report, 2013)

TSS & CTS Behavior of Spot Weld Joints

Tensile Shear Strength of High‐Strength Steels vs. Thickness

Static‐Cross Tension Strength of HSS vs. Carbon, Thickness

W/2

W L

WELD IS CENTERED

HS

20 mm; +0.2,- 0

AT W/2 AND L/2

Effect of Fracture Appearance on Absorbed Energy

0 50

100

200

300400

500

600

Fracture Appearance Rating

Ene

rgy

(in-lb

f)

Tensile Shear Test

0 1 3 4 50

100200300400500600

Fracture Appearance Rating

Ene

rgy

(in-lb

f)

Dynamic Cross Tension Test

Effect of Steel Strength on Peak Load in Weld Cross‐Tension Impact Behavior of RSW

Cracking

Cracking

Effect of Steel Strength on Absorbed Energy in Weld Cross‐Tension Impact Behavior of RSW

Temper Diagram for 0.9 mm DP980

General Characteristics of In‐Situ Tempering of Spot Welds

• Overall J‐shaped curve• Two distinct regions

– Steady state– Transient

• (Short time, high current)

• Correlations between hardness and weld failure morphology

Temper Time (cycles)0 50 100 150 200

0

20

40

60

80

100

120

140

30 Rc32 Rc34 Rc36 Rc38 Rc40 Rc42 Rc

024 8 16 32 64 128 200

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

125

130

135

140

Button Peel Interfacial Failures

0 16 32

656565

Zero, Mid, Full Temper Locations

Tem

per C

urre

nt (%

Exp

ulsi

on L

imit)

Spike Temper Characteristics of 1.0mm DP780 HDG

Temper Time (Cycles)

0 1 2 3 4 5 6 7 8 9 1090

100

110

120

130

140

150

160

170

180

190

200

28 Rc30 32 34 36 38


0 1 2 3 4 5 6 7 8 9 10

100

120

140

160

180

200

Tem

per Current (kA

)

10

11

12

13

14

15

16

17

18

19

20

21

29.730.0 26.836.7 36.836.5 37.530.731.7

36.5 32.033.3

28.326.531.7 28.036.038.0

31.2 33.329.328.237.038.5 36.7

39.0 27.531.739.0 37.2

39.0 27.530.535.839.5 39.239.239.0 29.228.7

34.738.0 39.029.338.7 26.732.237.5

38.236.0

35.526.8

32.0

38.2 37.030.028.0

35.837.3

26.8

36.8

36.227.3

33.538.8

29.3

35.336.2

28.2

0 1 2 3 4 5 6 7 8 9 10

10

11

12

13

14

15

16

17

18

19

20

21

D71

D72D73 D74

D75

D76

D77 D78

D79

D70M0.0

0.5

1.0

1.5

2.00 100 200 300 400 500

HV ( 1Kg, 15sec. )

Dis

tanc

e, m

m

D757 - 3M0.0

0.5

1.0

1.5

2.00 100 200 300 400 500

HV ( 1Kg, 15sec. )

Dis

tanc

e, m

m


0 1 2 3 4 5 6 7 8 9 1090

100

110

120

130

140

150

160

170

180

190

200

28 Rc30 32 34 36 38


0 1 2 3 4 5 6 7 8 9 10

100

120

140

160

180

200

Tem

per Current (kA

)

10

11

12

13

14

15

16

17

18

19

20

21

29.730.0 26.836.7 36.836.5 37.530.731.7

36.5 32.033.3

28.326.531.7 28.036.038.0

31.2 33.329.328.237.038.5 36.7

39.0 27.531.739.0 37.2

39.0 27.530.535.839.5 39.239.239.0 29.228.7

34.738.0 39.029.338.7 26.732.237.5

38.236.0

35.526.8

32.0

38.2 37.030.028.0

35.837.3

26.8

36.8

36.227.3

33.538.8

29.3

35.336.2

28.2

0 1 2 3 4 5 6 7 8 9 10

10

11

12

13

14

15

16

17

18

19

20

21

D71

D72D73 D74

D75

D76

D77 D78

D79

D70M0.0

0.5

1.0

1.5

2.00 100 200 300 400 500

HV ( 1Kg, 15sec. )

Dis

tanc

e, m

m

D757 - 3M0.0

0.5

1.0

1.5

2.00 100 200 300 400 500

HV ( 1Kg, 15sec. )

Dis

tanc

e, m

m

Tem

per C

urre

nt (%

Exp

ulsi

on L

imit)

(Shi, TWI 2005)

Effect of In‐Process Tempering on TRIP 700 Spot Welds


Tensile Shear Fatigue Behavior of Steel Spot Welds

(Sigler, SMWC XV 2012)

‐Weld cracks within weld indentation area had no effect on fatigue life.‐ FGHAZ orientation had a large effect on fatigue performance.

Fatigue Performance of AHSS Spot Welds –Microstructural Effects

Yurioka CE

(Khan, Mat. Trans.,2008)

Fusion Zone Hardness in AHSS Spot Welds

(den Uijl, WiW 2008)

Resistance Butt Welding of AHSS

• Steel grades commonly resistance butt welded

– 600 MPa– 800 MPa– 1000 MPa

• Process requirements– Short cycle times– High currents– Fast reacting force systems

• Metallurgical response– Constrained heat affected zones– Minimized extent of HAZ softening– Mechanical constraint to provide

properties• Effect of parent material strength• Typical scrap levels today for AHSS:

0.2 – 2%

Developing temperature distribution during resistance butt welding mild steel

Distribution of effective strain through a cross section of a resistance butt welded mild steel


Weld hardness of a high frequency weld in a DP 280/600 tube.


Hardness variation across induction welds for various types of steel

(Ichiyama, Nippon Steel Tech. Report, 01/2007)

Flash‐Butt Welding of High Strength Steels

X80 steel, 12mmIncreasing upset length and preheating reduced weld defects.

(Ichiyama, Nippon Steel Tech. Report, 01/2007)

Flash‐Butt Welding of High Strength Steels

980HT steel, 13mmIncreasing upset current (and hence upsetting displacement)improved weld toughness by removal of bondline oxides and steel inclusions.

(Biro, SMWC XV, 2012)

Higher than Expected Strengths from Dissimilar ConfigurationAdvanced High Strength Steel Spot Welds

Example of dissimilar configuration with CTS matching the “minimum rule”

For AHSS/AHSS configurations, cross‐tension strength is always higherthan the strength expected from the lower strength material in the joint.

Cross‐tension strength for TRIP800configurations, showing “positive deviation”

(Yu, Mat. Trans 53 (11),2012)

Resistance Spot Welding of 1.4 mm DP980 & TWIP980 Steels

‐ TWIP microstructure fully austenitic inBM, HAZ & FZ

‐ Current range much narrower for TWIP vs. DP

(Yu, WJRS, 03/2014)

CCC vs. CPC Spot Welding of 1.4 mm TWIP980

Constant Power Control produced wider current range and slower nugget growth rate

(Matsushita, JFE Tech. Report 2013)

Pulse SpotTM Welding of 1180 MPa AHSS

Resistance Spot Welding of a 1.6mm, Q&P 980 Steel

(Wang & Speer, Metallogr. Microstruct. Anal., 2013)

(den Uijl, Tata Steel online report)

HSLA 300, 2.2 mm

DP800 GI, 1.6 mm

IF GI, 0.8mm

Resistance spot welding of a complicated joint in newadvanced high strength steel

challenges and advances in welding of a new generation...

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