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w w w . a u t o s t e e l . o r g
Weldability of AHSS
Dr. Sree Harsha Lalam, CWEng.Mittal Steel Company
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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
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•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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Effect of cooling rate in a steel
Alloying elements effect
Time
Martensite
Bainite
Ac1
Ac3
Ferrite
Pearlite
1%
12
3
Tem
pera
ture
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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
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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
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SPOT WELDING
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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Impact testing samplesShear
Teflon
Thermocouple
Sample at -20 oC before testing
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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
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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
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ARC WELDING
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Arc welding of AHSS
1.0” 1.6 mm
Filler Wire: ER70-S3
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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
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LASER WELDING
Source: M. Gallagher, et. al., "Laser Welding of AHSS", to be presented at the International Automotive Body Congress (IABC), Sep., 2005.
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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
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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
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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
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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
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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
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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.
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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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