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Weldability of AHSS

Dr. Sree Harsha Lalam, CWEng.Mittal Steel Company


Overview

• AHS Steels & Weld Joint• Spot welding

– Spot Welding Window– Effect of Coating – AC vs. Mid-Frequency DC– Weldmetal Hardness– Mechanical Properties– Fracture Mode– Effect of Paint Baking– Impact Testing at -20 oC and 20 oC

• Arc welding• Laser welding• Induction welding • Summary


•The optimum resistance welding parameters of AHSS are different from the mild steel parameters.

•AHSS can be welded using Resistance, Arc, Laser, Induction welding processes.

Summary


Effect of cooling rate in a steel

Alloying elements effect

Time

Martensite

Bainite

Ac1

Ac3

Ferrite

Pearlite

1%

12

3

Tem

pera

ture


Weld Microstructure

Ferrite Ferrite + Martensite

HSLA350 Arc weld

• All weld joints including AHSS, will have similar microstructures.

• The hardness of each region depends on the alloying elements.

Martensite


Chemical composition of steels

TRIP Steels : High C, Mn, Al/Si, etc.

C (wt%)

Mn (wt%)

Si (wt%)

S (wt%)

P (wt%)

DQSK CR 0.04 0.19 0.012 0.015 0.011HSLA350 CR 0.08 0.73 0.022 0.005 0.01

DP500 CR 0.07 1.71 0.025 0.006 0.011DP600 CR 0.09 0.95 0.300 0.008 0.01

DiForm140T CR (DP965) 0.14 1.45 0.300 0.007 0.015

M220 CR (M1500) 0.25 0.45 0.026 0.015 0.01


SPOT WELDING


Influencing parameters on weldability

Mild Steel AHSSAlloying Elements Low HighMicrostructure Simple Ferrite/DP/Martensite

Resistivity Lower HigherCurrent Range Higher LowerElectrode Life Longer Shorter

Fracture Location in HAZ in HAZ/ Nugget / Interface

Electrode Force Lower HigherCurrent Higher Lower

Using same welding parameters


Similar current range

3.701.87DiForm 140T (DP965) CR2.601.90HSLA350 HDG

3.401.50M900 CR

1.790.83BH210 EG

4.761.80DP600 CR2.631.60TRIP590 CR2.751.60HFT440 GA

2.101.50HSLA 350 GA1.901.40DP600 HDG

2.741.40DP600 CR

1.210.81DQSK EG1.280.65DP500 EG

Current range (kA)

Gauge( mm)

Material(Min. Button = 4 x √t)

• With optimum welding parameters the AHSS will produce current range similar to mild steel.


Effect of coating on weld time and current

• Similar to mild steel the weld time, current depends on coating

TRIP590 1.6mm, Nugget Size 6.67mm

5.5

6.0

6.5

7.0

7.5

8.0

8.5

8 13 18 23 28Weld Time (Cycles, 60 Hz)

Cur

rent

(kA

)TRIP590 CR TRIP590 GA

GACR


Effect of current mode

• Mid-Frequency DC will increase the current range

6 7 8 9 10 11

2

3

4

5

6

7

8

9MFDC

AC

Min. Button Size

Elec. Force: 3.8 kNDP600 CR 1.4 mm

But

ton

size

(mm

)

Current (kA)6 7 8 9 10 11

4

5

6

7

8

9 DiForm140T(DP965) 2.0 mm, Dome Electrode

Min. Button

AC MFDC

But

ton

Size

(mm

)Current (kA)

MFDCAC

DP600 CR 1.4 mm DP965 CR 2.0 mm


Weld metal hardness depends on steel chemical composition

Weld metal hardness depends on base material chemical composition, not on weld time.

191 HV 355 HV 436 HV 469 HVHeat Affected ZoneBase Material

Weld Metal

100

150

200

250

300

350

400

450

500

0 2 4 6 8 10 12

Indentation Distance (mm)

Vick

er's

Har

dnes

s (V

HN

)

DP600HDG 1.4mm, 12 cyclesDP600HDG 1.4mm, 26 cyclesHSL350HDG 1.9mm, 28 cyclesTRIP590GA 1.6 mm, 28 cyclesTRIP590CR 1.6 mm, 18 cycles

HSLA


Weld Time Vs Load

• Optimum weld time depends on the type of coating• Pulsing has little effect of tension shear load.

TRIP590 CR 1.6 mm, Nugget Size 6.67 mm

5

7

9

11

13

15

17

19

21

10 12 16 18 26 28Weld Time (cycles, 60 Hz)

Load

(kN

)

TSCT

TRIP590 GA 1.6 mm, nugget size 6.67 mm

5

7

9

11

13

15

17

19

21

14 16 20 24 24p 28 30

Weld Time (cycles, 60 Hz)

Load

(kN

)

TSCT

TS- Tension Shear, CT-Cross Tension

TRIP 590 1.6 mm


Fracture mode may not be an indication of weldability

Below min. button Size

Button Pullout

Partial Interfacial Fracture

2267 lbf

2258 lbf 2117 lbf

Cross Tension

Tension Shear

DP965 CR


Paint Baking improves the ductility

• Paint baking increases the cross tension load. • Paint baking improves the weld ductility and energy.

16 Cycles, Flat Sample, Gray:As-welded, Pink: Baked

0

200

400

600

800

1000

1200

1400

1600

1800

5.6

5.7

5.5

5.5

5.6

6.9

6.9

7.5

6.9

6.9

8.5

8.5

8.6

8.5

8.6

8.5

9.7

9.6

9.7

Current (kA)

CT

(lbf)

16 Cycles, Flat Sample, Gray:As-welded, Pink: Baked

0

20

40

60

80

100

120

140

5.6

5.7

5.5

5.5

5.6

6.9

6.9

7.5

6.9

6.9

8.5

8.5

8.6

8.5

8.6

8.5

9.7

9.6

9.7

Current (kA)

Ener

gy (J

)

DP590 GA, 1.1 mm


Decreased hardness in baked weld

• Paint baking decreases the coarse grain HAZ hardness and improves the fracture mode.

150

200

250

300

350

400

450

500

0.0 0.1 0.2 0.3 0.4 0.5

Distance (in)

Vick

ers

Har

dnes

s (V

HN

)

Paint Baked, 350F30minAs-welded

DP590 GA, 1.1 mm


Impact testing samplesShear

Teflon

Thermocouple

Sample at -20 oC before testing


AHSS higher impact load

• AHSS has higher impact load bearing capacity

3000

4000

5000

6000

7000

8000

9000

10000

11000

3 4 5 6 7 8 9 10

Button Size (mm)

Impa

ct L

oad

(lbf)

HSLA350 DP600 DiForm140T Cold Rolled 1.4 mmDP965

DP600

HSLA350


Low-Temperature impact load

Spot weld performance under impact load depends on button size

Button Shear Double Button

68.5 J 51.0 J 111.1 J

0

20

40

60

80

100

120

140

4.71 4.71 4.71 7.63 7.63 7.63 8.35 8.35 8.35

Button Size (mm)

Impa

ct E

nerg

y (J

)

25 C -20C

Fracture Mode

Shear Mixed Button


ARC WELDING


Arc welding of AHSS

1.0” 1.6 mm

Filler Wire: ER70-S3


AHSS has higher arc weld load

0

100

200

300

400

500

600

700

800

0 2 4 6 8 10 12 14Elongation (mm)

Wel

d St

reng

th (M

Pa)

DP965

DP600M1300

HSLA350


LASER WELDING

Source: M. Gallagher, et. al., "Laser Welding of AHSS", to be presented at the International Automotive Body Congress (IABC), Sep., 2005.


Laser welding of AHSS

“bottom”shim

“top” shim30mm100mm

500mm 25mm

• TRUMPF 4.5 kW Nd:YAG laser• Power used: 3.8 kW• Focal distance : 200mm• No Purging Gas• Shims : 0.004” (0.1mm)• Location of Shims: 1.5” long with 1.5” spacing between

shims

WELD JOINT


Laser weld: Effect of penetration

M900 1.5/1.5

DP600 1.2/1.2

DP600 1.2/2.0

DP600 2.0/2.0

HSLA350 1.9/1.90

10

20

30

40

50

60

70

80

90

2 3 4 5 6 7 8 9

Pene

trat

ion

(%)

Weld Speed m/min


Laser weld: Failure locations

Interfacial Failure

HAZ Failure

Base Metal Failure

• Interfacial failure usually occurs when penetration/weld width is small

• When HAZ failure occurs, almost always through bottom sheet. • Weld rotation occurs


Effect of penetration on load

0

1000

2000

3000

4000

5000

6000

0 20 40 60 80 100

% Penetration

Load

(lbs

)

DP600 2/2DP600 1.2/1.2HSLA350 1.9/1.9M130 1.5/1.5

DP600 2/2

M900 1.5/1.5

HSLA 1.9/1.9

DP600 1.2/1.2


Strength of laser weld

Failed in Base material

0

100

200

300

400

500

600

DP6001.2/1.2

DP6002.0/2.0

HSLA1.9/1.9

M900-DP6001.5/2.0

DP600-M9001.2/1.5

Stre

ss (M

Pa)

Laser weld strength increases with base material strength


Summary

• AHSS need higher electrode force and optimized weld schedule.

• AHSS welds have higher load bearing capacity than mild steels.

• Paint baking increases the weld ductility.• AHSS has higher impact load bearing

capacity.• Weld strength depends on the alloying

elements and base material strength.• The maximum strength in laser lap weld joints

can be achieved at ~40% penetration.


Thank you

•The optimum resistance welding parameters of AHSS are different from the mild steel parameters.

•AHSS can be welded using Resistance, Arc, Laser, Induction welding processes.

Summary

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