feasibility study of laser welding of advanced …€¢ heat affected zone experiences high...

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 !