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)

How Temper Pulse Can Improve the Crash

Performance?

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).

Questions?

#GDIS

Presentations will be available May 16

at www.autosteel.org