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GDIS2017
Welding Solutions for
Advanced High-Strength Steels
Menachem Kimchi1
A. Peer1, Y. Lu1, W. Zhang1, C. Ji2, Y. Park2, T. Abke3, S. Malcom3 1The Ohio State University, 2Dong-Eui University, 3Honda R&D Americas, Inc.
Sponsored Work by: Honda R&D Americas, Inc. & WorldAutoSteel
#GDIS | #SteelMatters 3
Global Formability Diagram for Today’s AHSS Grades
#GDIS | #SteelMatters 4
Outline
• Issues in RSW of AHSS
− Hot Stamped Boron Steel
• Case Study 1:
− Coating Effects
− Parameter Optimization Technique
• Case Study 2:
− Failure Behavior
− Initial Development of Criterion for Crash Simulation
#GDIS | #SteelMatters 5
Failure Modes on Resistance Spot Welds in AHSS
• Destructive inspection
− Chisel test
− Peel test
• Modes of failure
• Implied cooling rates (1000s of oC/s)
• Martensite formation
FB PIF FIF
#GDIS | #SteelMatters 6
Fracture Behaviors of RSW Button 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 fusion Interfacial fracture
with button pull
Interfacial fracture
with partial thickness
fracture
• In AHSS
applications,
the acceptable
strength can be
achieved with
PP or IF failure
#GDIS | #SteelMatters 7
Liquid Metal Embrittlement (LME) in AHSS
• Presence of tensile stress
• Presence of liquid metals(low
melting alloy from coating)
• Susceptible
Microstructure(Austenite in
TRIP,TWIP Steels)
#GDIS | #SteelMatters 8
Possible Solutions to Improve Failure Mode in AHSS
− Long weld time
− Pre/post pulsing
− Down sloping
− Short hold time
− Weld and temper
− Pulsation
− Dilution (AHSS to Mild Steel, HSLA)
− Increased minimum weld size
(0.87 mm, 980 MPa)
#GDIS | #SteelMatters 9
Case Study 1
• Hot Stamped Boron Steel
−Coating Effects
−Parameter Optimization Technique
• C. Ji, M. Kimchi, Y. Kim and Y. Park, "The application of pulsed current
in resistance spot welding of zn-coated hot-stamped boron steels," in
Advances in Resistance Welding, Miami, FL, 2016.
#GDIS | #SteelMatters 10
Previous Research of Hot-Stamped Boron Steel
High speed Camera
Al-Si coated Hot stamped boron steel
#GDIS | #SteelMatters 11
Analysis of Hot-Stamped Boron Steel Coating Layer (Zn)
#GDIS | #SteelMatters 12
Pulsed Current Approach
Coating layer behavior & Contact area
during Initial welding time
Heat Generation Pattern & Nugget
formation
Nugget growth and Expulsion behavior
Based on
#GDIS | #SteelMatters 13
1st Pulse
• Heat generation pattern of 1st Pulse
W
eld
ing
Cu
rren
t (k
A)
Welding Time(Cy)
Wel
din
g
Cu
rren
t (k
A)
Welding Time(Cy)
#GDIS | #SteelMatters 14
1st Pulse
• Coating melting behavior & nugget formation of 1st pulse
#GDIS | #SteelMatters 15
1st Pulse - Results
Molten
B.M
Molten Coating layer
8 kA, 1 cycle 4 kA, 3 cycle
#GDIS | #SteelMatters 16
2nd Pulse
• Comparison of heat generation pattern by 2nd pulse
2mm
4kA 25cy
2mm
5.5kA 10cy
Wel
din
g
Cu
rren
t (k
A)
Welding Time(Cy)
Wel
din
g
Cu
rren
t (k
A)
Welding Time(Cy)
#GDIS | #SteelMatters 17
2nd Pulse
• Comparison of nugget diameter and contact diameter by 2nd pulse
a b
#GDIS | #SteelMatters 18
2nd Pulse - Results
• Comparison of nugget diameter and contact diameter by
2nd pulse
4 6 8 10 12 14 16 18 20 220
2
4
6
8
Dia
me
ter
(mm
)
Welding time (cycle)
a b
a
b
#GDIS | #SteelMatters 19
3rd Pulse
#GDIS | #SteelMatters 20
3rd Pulse
• Optimized welding conditions using three pulsed current
steps
4.5 5.0 5.5 6.0 6.5 7.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
1.5 kA
Bu
tto
n D
iam
ete
r (m
m)
Weld Current (kA)2Cy
8kA-1Cy
2Cy Wel
din
g C
urr
ent
(kA
)
Welding Time(Cy)
5.5kA- 12Cy
15Cy
5.0 -6.5 kA
#GDIS | #SteelMatters 21
Optimized Conditions
Wel
din
g C
urr
ent
(kA
)
Welding Time(Cy)
8kA-1Cy
5.5kA- 12Cy
5.0 -6.5 kA
15Cy
#GDIS | #SteelMatters 22
Case Study 2
• Hot Stamped Boron Steel
−Failure Behavior
− Initial Development of Criterion for Crash
Simulation
• Andrea Peer, Ying Lu, Tim Abke, Menachem Kimchi, and Wei Zhang "Deformation Behaviors of Subcritical Heat
Affected Zone of Ultra-high Strength Steel Resistance Spot Welds." in 9th International Seminar & Conference on
Advances in Resistance Spot Welding. Miami, (3 2016). Paper No. 12
• Ying Lu, Andrea Peer, Tim Abke, Menachem Kimchi, and Wei Zhang "Heat-Affected Zone Microstructure and Local
Constitutive Behaviors of Resistance Spot Welded Hot-Stamped Steel." in Sheet Metal Welding Conference XVII.
Livonia, (10 2016).
#GDIS | #SteelMatters 23
UHSS Performance Testing Approach • Objective: Develop an understanding of the deformation and failure
behavior of resistance spot welds of ultra-high strength steels
#GDIS | #SteelMatters 24
Metallographic Characterization
• The spot weld microstructure is highly inhomogeneous
#GDIS | #SteelMatters 25
Temperature Profile
Martensitic Microstructure
Base Metal
Weld Nugget
Coarse Grain HAZ
Fine Grain HAZ
Unique Microstructure
Subcritical HAZ
#GDIS | #SteelMatters 26
Hardness Profile
• A ring of “softened” material surrounds the
weld nugget
Subcritical Heat Affected Zone (SCHAZ)
• Usibor® BM & WM: > 500 HV
• SCHAZ: ~300 HV
0.7 mm
#GDIS | #SteelMatters 27
Constitutive Behavior Development
YS (MPa) UTS (MPa)
Base Metal 1179 1464
CGHAZ 1342 1811
SCHAZ 618 866
• Ncorr Post-Processing
Extended Stress Strain Curve Example of SCHAZ
tensile test. Virtual
extensometer of 2 mm
used to extract data.
#GDIS | #SteelMatters 28
Single-Sided Wedge Testing
• Observe the localized deformation with the aid of DIC
#GDIS | #SteelMatters 29
• No interfacial failure when
weld diameter > 5.8 mm
− No defined trend for
non-IF failure mode
Interfacial
Weld Metal
WM/CGHAZ
SCHAZ
Weld Size Results
4√t – minimum nugget diameter 5√t – minimum nugget diameter
#GDIS | #SteelMatters 30
FEA Comparison – Wedge Test • Incorporating softened
SCHAZ properties is essential to predict the localized deformation
FEA simulation with SCHAZ flow
No strain localization without
incorporating SCHAZ flow stress
DIC results
Effect of nugget size on local deformation • Small weld (5.5 mm): Concentrated on notch
• Large weld (7.00 mm): Concentrated on SCHAZ
FEA simulation with SCHAZ flow
Small Weld Large Weld
DIC results
#GDIS | #SteelMatters 31
Failure in Hot Stamped Boron Steel
• As-welded Lap Shear
− Difficult to observe
localized
deformation
• Microstructure-
specific properties needed to simulate actual failure behavior
#GDIS | #SteelMatters 32
Summary
• Three common problems associated with welding AHSS:
− Narrow Current Range:
− By implementing the proper welding schedule, the weldability window can be broadened.
− More complex welding schedules can allow for more control over heat generation and current
density, thus minimizing the possibility of expulsion to grow the nugget further.
− Hardness Values:
− The increased hardness values seen in AHSS weld nuggets can induce interfacial failure.
− Acceptable strengths can be achieved with interfacial failure.
− Nonhomogeneous Microstructures:
− The nonhomogeneous microstructures seen in AHSS welds cannot be universally classified as a
beneficial or detrimental effect of welding.
• Solutions need to be made on a case by case study and cannot be blanketed over all advanced high-strength
steels. Different solutions will need to be taken for different grades as well as different stack-up
configurations and sheet thicknesses.
#GDIS | #SteelMatters 33
For More Information
Andrea Peer
The Ohio State University
614-716-9692
Menachem Kimchi
The Ohio State University
614-270-4296
34
• Unaffected Base Metal: • Fully martensitic microstructure with fine martensite laths
• Hot stamping heat treatment prior to welding
• Heated above the austenitization temperature (Ac3)
• Austenite completely transformed into martensite, which is supersaturated in carbon
Base Metal Characterization
35
Weld Metal Characterization
• Solidified Weld Metal: • Fully martensitic microstructure with fine martensite laths
• Molten weld nugget solidifies rapidly during welding
• Austenite completely transformed into martensite, which is supersaturated in carbon
36
CGHAZ Characterization
• Coarse-Grained Heat Affected Zone: • Fully martensitic microstructure with fine martensite laths
• Heated above the austenitization temperature (Ac3) for an extended period of time
• Large austenite grains completely transform into martensite
37
SCHAZ Characterization
• Subcritical Heat Affected Zone: • Ferrite grains and cementite precipitates
• Cementite precipitates decorate along the prior austenite grain boundaries and along the inter-lath regions of martensite
• “Over-tempering” of martensite decomposition of metastable martensite into ferrite and cementite.
• Heated to peak temperatures below the Ac1
Ferrite Cementit
e
Microstructure Simulation • SCHAZ span < 1 mm
• CGHAZ span < 0.6 mm • Very difficult to measure
local mechanical properties of this region
• Gleeble Simulation: • Creates a bulk
homogeneous SCHAZ microstructure to extract mechanical property data
38 Y. Adonyi. Heat-Affected Zone Characterization by Physical Simulations, Welding Journal, 2006
INP
UT
• Heating Rate • Holding Time • Holding
Temperature • Cooling Rate
CGHAZ SCHAZ
Weld Size Study
• Objective: Determine effect of weld size on failure mode.
• Weld Sizes: • Below Min 4.0-4.1 kA
• Small 4.8-4.9 kA
• Medium 5.6-5.7 kA
• Large 6.3-6.4 kA
• Near Exp 6.7-6.8 kA
39
Weld Metal
40
Interfacial Failure SCHAZ
0.13 0 -.13
Peak Load Failure Peak Load End of Test
Peak Load End of Test
Key Challenges with AHSS - Joining
Selection of Joining Technique Body-in-White Joining Processes
• Resistance spot
• Projection welding
• GMAW
• MIG brazing
• Laser welding (TWB)
• Mechanical fastening
• Magnetic pulse welding
• Deformation resistance welding
10
100
1000
10000
100000
0 0.5 1 1.5 2 2.5
Sheet Thickness (mm)
Co
olin
g R
ate
(C
o/s
)
GMAW
LBW
RMSW
RSWIF
AKDQ
M1400
TRIP 600
DP 980
TRIP 800
DP 600
DP 780
Key Challenges with AHSS - Joining
Selection of Joining Technique Body-in-White Joining Processes
• Resistance spot
• Projection welding
• GMAW
• MIG brazing
• Laser welding (TWB)
• Mechanical fastening
• Magnetic pulse welding
• Deformation resistance welding
10
100
1000
10000
100000
0 0.5 1 1.5 2 2.5
Sheet Thickness (mm)
Co
olin
g R
ate
(C
o/s
)
GMAW
LBW
RMSW
RSWIF
AKDQ
M1400
TRIP 600
DP 980
TRIP 800
DP 600
DP 780
