cross tension strength improvement for ahss using post-weld
TRANSCRIPT
Stephen Kelley (Lead Engineer)
Hassan Ghassemi-Armaki (Senior Research Engineer)
ArcelorMittal Global R&D – E. Chicago
May 11, 2016
Cross Tension Strength Improvement
for AHSS Using Post-Weld
Heat Treatment
Strategy
• Select appropriate UHSS to take advantage of high strength
and reduced thickness, achieving light weighting.
• Rapid cooling rate characteristic of RSW applied to UHSS
composition guarantees a fully martensitic weld and
supercritical heat-affected zone (HAZ).
• As-transformed martensite spot welds have high hardness
but low cross tension strength, raising questions about weld
response to a crash event.
• Can spot welds be “modified” in a way to improve crash
response?
• Consider tempering the spot welds while still in the welder to
improve toughness and impact resistance. 2
ST = squeeze time WT = weld time
QT = quench time TT = temper time
HT = hold time WC = weld current
TC = temper current
3
Weld and Temper Firing Pattern
FORCE CURRENT
ST
WT
TT
WC TC
HT
Weld Pulse - Melt
and solidify
Temper Pulse -
Soften martensitic
weld and HAZ
Weld Quench - Cool and transform
completely to martensite
QT
Application of SORPAS® Software
• Use SORPAS® software to determine spot weld time/temperature
profile.
• Notch tip location will have slowest cooling rate during quench.
• To insure a fully martensitic spot weld, determine time for the notch tip
to reach MF temperature; i.e., quench time.
Notch tip Nugget center
4
Quenching - Formation of Martensite
5
0
200
400
600
800
1000
1200
1400
1600
0 10 20 30 40 50 60
Tem
pera
ture
(°C
)
Time (cycles)
Notch tip Temperature
M and
Fully M
Predicted QT min ≈ 42 cy (@ 50 Hz)
MS
MF
42 cy (@ 50Hz) ≈ 50 cy (@ 60Hz)
Martensite Flash Temper Prediction
6
300
350
400
450
500
550
100
200
300
400
500
600
700
800
50 60 70 80 90 100
Ha
rdn
ess (H
v)
Te
mp
era
ture
(°
C)
Post-heat intensity (% Welding intensity)
Temperature
Evolution
Hardness
Evolution
Temperature Range for Noticeable
Martensite Tempering
AC1 limit
Temper Current Range
80 – 95% WC
• Material
− 1.2 mm M1500 EG
− Mechanical properties: 1381 MPa YS, 1620 MPa UTS
• Dome electrode - 6 mm tip face dia.
• Electrode Force - 2.6 kN
• Weld time – 16 cycles
• Minimum weld size – 4.0 mm
• SORPAS®
− Quench time - 50 cycles (@ 60 Hz)
− Temper currents -
80%, 87.5%, 95% AWS WC
7
M1500 Example: Material and Welding Conditions
Nugget Hardness Response to Temper Current
8
• In the as-welded
condition, nugget
hardness relatively flat
and similar to BMH.
• As temper current
increases from 80% to
87.5 % WC, see increasing
nugget softening.
• At 95% WC, nugget
hardness begins to
recover, indicating
reaustenitization &
subsequent.
transformation to
untempered martensite.
Cross Tension Strength vs. Temper Current
9 MWS – Minimum Weld Size AWS – Avg Weld Size Exp – Expulsion
• CTS increases with
average weld
current; i.e., with
weld diameter
• Additional CTS
improvement as
temper pulse current
increases
• Most consistent CTS
increase in the range
of 80% to 87.5% WC
temper pulse
Summary – Temper Pulsing to Improve CTS
10
• Join the material using a suitable welding pulse
• Cool the fusion zone and HAZ by keeping the welded joint
clamped between the water-cooled electrodes
• Employ quench time sufficient to fully transform the fusion
zone and supercritical HAZ to Martensite ; i.e., to reach the
MF temperature
• Apply a subsequent pulse of sufficient current and time to
temper the Martensite without reaustenitizing the weld and
forming fresh untempered Martensite (M M)
Studied Materials and Stackups
• 1.4mm PHS1500 was studied
• Totally, 4 stackups are compared:
− 1.4mm PHS1500 / 1.4mm PHS1500 (With Temper Pulse)
− 1.4mm PHS1500 / 1.4mm PHS1500 (Without Temper Pulse)
− 1.4mm PHS1500 / 1.4mm DP980 (Without Temper Pulse)
− 1.4mm DP980 / 1.4mm DP980 (Without Temper Pulse)
• Welding condition kept unchanged for all stackups. However,
temper pulse cycles were added for first stackup
• 5-mm weld diameter considered for all stackups
• The optimized welding current was used for evaluation
How Crash Performance is Evaluated?
Mechanics of Modeling
• Spot-welds in real components (e.g., B-Pillar, bumper…) are faced with a combination
of loading modes.
• Spot-weld fails if the stress triple of the internal normal, bending and shear stresses
is above the surface.
• Stress-Based Failure Model*:
*Seeger, Feucht, Frank, Haufe, and Keding, LS-DYNA Conf. 2005.
• As long as temper pulse increases the 3D failure surface, crash performance improves
― Combined Tension-Shear & Cross-Tension; KSII (U-Shape Samples)
― Bending Stress; Coach-Peel
• Energy absorption and post-failure damage; Failure Mode
2D Failure Surface for 1.4mm PHS1500
• 2D-failure surface increases more than twice
• Both shear and normal exponents increase, but normal force more.
120% Increase in
Failure Surface
Axial Force
Shear Force
Applied load
KS-0 KS-30 KS-60 U-Shape
Without Temper Pulse:
With Temper Pulse:
2D-Failure Surface after Tempering vs HSS Stackup
• Homogenous 1.4mm DP980 (Without Temper Pulse)
• 1.4 mm PHS1500 / 1.4 mm DP980 (Without Temper Pulse)
2D-Failure surface increases significantly as compared to
heterogeneous stackup and even HSS stackup
Coach-Peel Strength
• Coach peel strength increases with temper pulse.
• Bending stress increases in 3D-failure surface.
65% Improvement
Failure Mode and Post-Failure Damage
• Failure mode changes from full or partial interfacial failure to plug failure.
• Improvement of post-failure damage is more pronounced for coach peel.
Loading
Mode
Without
Temper
Pulse
With
Temper
Pulse
Increase in
area under
curve after
Max. Load
Coach Peel FIF PF 2000%
KSII-30 FIF PF 600%
KSII-60 PIF PF 26%
KSII-90 PIF PF 340%
FIF: Fully Interfacial Failure
PIF: Partial Interfacial Failure
PF: Plug Failure
Coach Peel
Conclusions
18
• Tempering of weld nugget microstructure by using temper pulse
improves not only cross-tension strength, but also strength in
other loading modes.
− 2D failure surface increases significantly.
− Resistance of weld to failure regarding to bending stress
increases, too.
• Failure mode is improved and plug failure is achieved for most
loading modes, which gives benefit of energy absorption.
• Finally, improvement of failure surface can be better than even
heterogeneous and homogenous stackups by using HSS (e.g.,
DP980).