feasibility study of laser welding of advanced …€¢ heat affected zone experiences high...
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Feasibility Study of LaserWelding of AdvancedLightweight Materials
(DP980 Butt-Joint)
NSF/IUCRC-LAM Industrial Board Meeting-May 2007
Dechao Lin and Radovan Kovacevic(LAM’s team at SMU)
Robert Ruokolainen and Xiaohong (Shawn) Gayden(GM’s team)
OUTLINE
• Brief Introduction• Objectives• Experimental Procedures• Results and Discussion• Conclusions• Future work
Laser welding has higher energy intensity than traditional welding process
BRIEF INTRODUCTION
TIG/MIG welding: 104-105 W/cm2
Plasma welding: up to 105 W/cm2
Resistant welding: 104-105 W/cm2
Laser welding: up to 109 W/cm2
Advantages: small weld bead, full penetration, narrower HAZ
Advantages of dual phase steels:High strengthImproved formabilityCapacity to absorb crash energy, andAbility to resist fatigue
Reference available at: http://www.ussautomotive.com/auto/tech/grades/dual_ten.htm
OBJECTIVES
• To evaluate the feasibility of achievingsatisfactory butt joint welds in DP980 bylaser welding
• To develop a vision system to monitorthe welding process
• To evaluate the welded joint andestablish the relationship betweenprocess, microstructure and property
• Equipment: a 4-kW fiber laser, Kuka robot, and machine vision system
Experimental Procedures
Robot arm
Laser headVision system
with CCD
Filter
Ar-gas nozzle
• Material: galvanized steel DP980 (Ferrite matrix + Martensite dispersions)
• Specimen: 7” in length, 3” in width, cut by abrasive water-jet• Fixture: alignment, specimen firm clamps, Ar-gas flow to protect the backside of specimen• Welding process (short movie)
Experimental Procedures
Ar-gas flowBack protection
SpecimenClamps
Pressure
Fixture Clamps
Specimen
Ar-gas inputLaser beam
Experimental Procedures• Joint type and laser beam:
(1) butt joint (1.2 mm to 1.2 mm, same gage)
1.2 mm 1.2 mm
Laser beam
Parameters for this joint:Laser: 1500 WWelding Speed: 20 mm/s, 15 mm/s, and 10 mm/sAr-gas flow rate: 30 L/minBack Ar-gas flow rate: 20 L/min
Experimental Procedures• Joint type and laser beam:
(2) butt joint (1.5 mm to 1.5 mm, same gage)
1.5 mm 1.5 mm
Laser beam
Parameters for this joint:Laser: 2200 WWelding Speed: 20 mm/s, 15 mm/s, and 10 mm/sAr-gas flow rate: 30 L/minBack Ar-gas flow rate: 20 L/min
Experimental Procedures• Joint type and laser beam:
(3) Fillet butt joint (various gages, 1.5 mm to 1.2 mm)
Parameters for this joint:Laser: 2000 WWelding Speed: 20 mm/s, 15 mm/s, and 10 mm/sAr-gas flow rate: 30 L/minBack Ar-gas flow rate: 20 L/min
1.5 mm1.2 mm
Laser beam
• Tensile Test:
Experimental Procedures
INSTRON tester
Strain gage
Specimen for test
Broken specimen after test
(1) Base material: DP980
• Tensile Test:
Experimental Procedures
INSTRON tester
Strain gage
Specimen for test
Broken specimen after test
(2) Laser welded joint
• Metallurgical analysis:
Experimental Procedures
Top view Cross-section view
RESULTS AND DISCUSSION• Welded specimens: same gages of 1.2 mm to 1.2 mm (1500 W in power)
welding direction
20 mm/s15 mm/s10 mm/sVarious welding speed:
width of the weld
top side
Cross sections
width of the weld(top)
width of the weld(bottom)
HAZ
HAZ-heat affected zone
Top side
Bottom side
RESULTS AND DISCUSSION• Widths of the weld bead: same gages of 1.2 mm to 1.2 mm (1500 W in power)
Bottom side
20 mm
2.35 mm 1.38 mm 0.74 mm
2.83 mm 2.17 mm 2 mm
Various widths (top side)
Various widths (bottom side)
Welding speed, mm/s
8 10 12 14 16 18 20 22
Wid
th o
f the w
eld
, mm
0
1
2
3
High welding speed produces smaller weld bead
Welding speed, mm/min
8 10 12 14 16 18 20 22
Wid
th o
f HA
Z, m
m
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
RESULTS AND DISCUSSION• Widths of HAZ (average): same gages of 1.2 mm to 1.2 mm (1500 W in power)
High welding speed produces smaller HAZ
RESULTS AND DISCUSSION• Widths of the weld bead: same gages of 1.5 mm to 1.5 mm (2200 W in power)
3.13 mm 2.54 mm 2.16 mm
3.8 mm 2.38 mm 2.08 mm
Top side
Bottom side
Welding speed, mm/s
8 10 12 14 16 18 20 22
Wid
th o
f the w
eld
, mm
1
2
3
4
Top side
Bottom side
Welding speed, mm/min
8 10 12 14 16 18 20 22
wid
th o
f HA
Z, m
m
0
1
2
3
RESULTS AND DISCUSSION• Widths of HAZ (average): same gages of 1.5 mm to 1.5 mm (2200 W in power)
High welding speed produces smaller HAZ
RESULTS AND DISCUSSION•Widths of the weld bead: various gages of 1.5 mm to 1.2 mm (2000 W in power)
2.92 mm 2.47 mm 2.14 mm
3.67 mm 2.3 mm 2.01 mm
Top side
Bottom side
1.5mm 1.2mm
Welding speed, mm/s
8 10 12 14 16 18 20 22
Wid
th o
f the w
eld
, mm
1
2
3
4
Top side
bottom side
Welding speed, mm/min
8 10 12 14 16 18 20 22
wid
th o
f HA
Z, m
m
0
1
2
RESULTS AND DISCUSSION• Widths of HAZ (in 1.2mm side): (1) various gages of 1.5 mm to 1.2 mm (2000 W in power)
RESULTS AND DISCUSSION• Tensile test Results for base material DP980
Tensile stress at yield (offset 0.2%): 648 MPaTensile stress at maximum load: 974 MPaTensile stress at break: 948 MPa
Typical fracture of ductile material
Strain, mm/mm
0.00 0.02 0.04 0.06 0.08 0.10 0.12
Ten
sile
stre
ss
, MP
a
0
200
400
600
800
1000
Strain, mm/mm
0.00 0.02 0.04 0.06 0.08 0.10 0.12
Ten
sile
stre
ss
, MP
a
0
200
400
600
800
1000
RESULTS AND DISCUSSION• Tensile test results for welded specimens of various gages, 1.2 mm and 1.5 mm
DP980
Welded joints
Strain, mm/mm
0.00 0.01 0.02 0.03 0.04
Ten
sile
stre
ss
, MP
a
0
200
400
600
800
1000
10 mm/s
15 mm/s
20 mm/s
Welding Speed, mm/s 10 15 20Tensile stress at yield (offset 0.2%), MPa 620 646 684 648Tensile stress at maximum load, MPa 828 838 847 974Tensile stress at break, MPa 787 797 804 948Extension at maximum, mm 2.09 2.11 2.09 7.18
DP980
Laser welding drastically reduces the toughness and decreases the tensile strength up to 20%
RESULTS AND DISCUSSION
Higher welding speed produces higher strength joint
Welding speed, mm/s
8 10 12 14 16 18 20 22
Ten
sile
stre
ss
, MP
a
600
700
800
900
1000
Tensile stress at maximum load
Tensile stress at yield point (offset 0.2%)
Tensile stress at break
• Tensile test results for welded specimens of various gages, 1.2 mm and 1.5 mm
RESULTS AND DISCUSSION
• Fracture occurs at the thinner sheet and at the HAZ• Weld is stronger than HAZ
10 mm/s 15 mm/s 20 mm/s
Edge of fracture
1.4 mm 1.4 mm 1.3 mm
2 mm
Welding speed:
Sheet of 1.5 mm
Sheet of 1.2 mm
• Tensile test results for welded specimens of various gages, 1.2 mm and 1.5 mm
RESULTS AND DISCUSSION
Welding Speed, mm/s 10 15 20Tensile stress at yield (offset 0.2%), MPa 568 609 622 648Tensile stress at maximum load, MPa 773 811 840 974Tensile stress at break, MPa 735 770 798 948Extension at maximum, mm 1.94 2.24 2.38 7.18
Strain, mm/mm
0.00 0.02 0.04 0.06 0.08 0.10 0.12
Ten
sile
stre
ss
, MP
a
0
200
400
600
800
1000
DP980
Welded joints
Strain, mm/mm
0.00 0.01 0.02 0.03 0.04
Te
ns
ile s
tres
s, M
Pa
0
200
400
600
800
1000
10 mm/s
15 mm/s
20 mm/s
DP980
• Tensile test results for welded specimens of same gages, 1.2 mm
Laser welding drastically reduces the toughness and decreases the tensile strength up to 21%
RESULTS AND DISCUSSION
Higher welding speed produces stronger joint
Welding speed, mm/s
8 10 12 14 16 18 20 22
Ten
sile
stre
ss
, MP
a
500
600
700
800
900
1000
Tensile stress at maximum load
Tensile stress at yield (offset 0.2%)
Tensile stress at break
• Tensile test results for welded specimens of same gages, 1.2 mm
RESULTS AND DISCUSSION
10 mm/s 15 mm/s 20 mm/s
Edge of fracture1.6 mm 1.3 mm 1.0 mm
Welding speeds:
• Tensile test results for welded specimens of same gages, 1.2 mm
• Fracture occurs at the thinner sheet and at the HAZ• Weld is stronger than HAZ
Strain, mm/mm
0.00 0.01 0.02 0.03 0.04
Ten
sile
stre
ss
, MP
a
0
200
400
600
800
1000
Strain, mm/mm
0.00 0.02 0.04 0.06 0.08 0.10 0.12
Ten
sile
stre
ss
, MP
a
0
200
400
600
800
1000
RESULTS AND DISCUSSION
Laser welding drastically reduces the toughness and decreases the tensile strength up to 23%
DP980
Welded joints
10 mm/s
15 mm/s
20 mm/s
Welding Speed, mm/s 10 15 20Tensile stress at yield (offset 0.2%), MPa 577 604 630 648Tensile stress at maximum load, MPa 756 752 807 974Tensile stress at break, MPa 718 714 766 948Extension at maximum, mm 2.22 2.76 2.98 7.18
DP980
• Tensile test results for welded specimens of same gages, 1.5 mm
RESULTS AND DISCUSSION
Higher welding speed produces stronger joint
Welding speed, mm/s
8 10 12 14 16 18 20 22
Ten
sile
stre
ss
, MP
a
500
600
700
800
900
1000
Tensile stress at maximum load
Tensile stress at yield (offset 0.2%)
Tensile stress at break
• Tensile test results for welded specimens of same gages, 1.5 mm
RESULTS AND DISCUSSION
10 mm/s 15 mm/s 20 mm/s
Edge of fracture
2.1 mm 1.8 mm 1.2 mm
Welding speed:
• Tensile test results for welded specimens of same gages, 1.5 mm
• Fracture occurs at the thinner sheet and at the HAZ• Weld is stronger than HAZ
Cross section view
Top view
Broken specimen
Where is the broken line?
Why?
RESULTS AND DISCUSSION
RESULTS AND DISCUSSION
HAZ
Weld zone
• Macrostructure of welded specimen, top surface
RESULTS AND DISCUSSION
FerriteMartensite
In the heat affected zoneIn the base material In the weld zone
Smaller volume fraction of martensite in the area close to the interface between base material and HAZ
RESULTS AND DISCUSSION• Microhardness distribution of welded specimen
Distance, mm
-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10
Microhardness
, Kg/m
m2
150
200
250
300
350
400
450
Center of the weld
Weak point
For 1.5 mm thick sheets at 10 mm/s
Weld zoneHAZ HAZ
CONCLUSIONS
• Laser welding of galvanized dual phase steel DP980 by using 4-kWfiber laser system can produce strong but brittle welded joints.
• Tensile test results show that the fracture for all specimens occurredalong the interface between the heat affected zone and the basematerial, where the hardness reaches the lowest value.
• Smaller fraction of volume of martensite contributes to the lower hardness.• More work is needed to investigate the phase transformation in the laser
welding process for dual phase steels.• Heat affected zone experiences high temperature treatment, in which
martensite phase may transform into other phases.
FUTURE WORK
The following activities are planned:
• Study the relationship between welding process and microstructure and mechanical properties of the weld.
• Investigate the effect of the cooling rate on the welded zone and HAZ.
• Develop techniques to improve the properties of the welded joints.
Acknowledgement
Shanglu YangGustavo QuirogaHosein AtharifarAndrzej Socha
LAM’s team at SMU:
Thank you !