Feasibility Study of Laser Welding of Advanced Lightweight Materials (DP980 Butt-Joint)

Transcription

1 Feasibility Study of Laser Welding of Advanced Lightweight Materials (DP980 Butt-Joint) Dechao Lin and Radovan Kovacevic (LAM s team at SMU) Robert Ruokolainen and Xiaohong (Shawn) Gayden (GM s team) NSF/IUCRC-LAM Industrial Board Meeting-May 2007

2 OUTLINE Brief Introduction Objectives Experimental Procedures Results and Discussion Conclusions Future work

3 BRIEF INTRODUCTION Laser welding has higher energy intensity than traditional welding process TIG/MIG welding: W/cm 2 Plasma welding: up to 10 5 W/cm 2 Resistant welding: W/cm 2 Laser welding: up to 10 9 W/cm2 Advantages: small weld bead, full penetration, narrower HAZ Advantages of dual phase steels: High strength Improved formability Capacity to absorb crash energy, and Ability to resist fatigue Reference available at:

4 OBJECTIVES To evaluate the feasibility of achieving satisfactory butt joint welds in DP980 by laser welding To develop a vision system to monitor the welding process To evaluate the welded joint and establish the relationship between process, microstructure and property

5 Experimental Procedures Equipment: a 4-kW fiber laser, Kuka robot, and machine vision system Robot arm Laser head Vision system with CCD Filter Ar-gas nozzle

6 Experimental Procedures 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) Laser beam Ar-gas input Specimen Pressure Clamps Ar-gas flow Back protection Specimen Fixture Clamps

7 Experimental Procedures Joint type and laser beam: (1) butt joint (1.2 mm to 1.2 mm, same gage) Laser beam 1.2 mm 1.2 mm Parameters for this joint: Laser: 1500 W Welding Speed: 20 mm/s, 15 mm/s, and 10 mm/s Ar-gas flow rate: 30 L/min Back Ar-gas flow rate: 20 L/min

8 Experimental Procedures Joint type and laser beam: (2) butt joint (1.5 mm to 1.5 mm, same gage) Laser beam 1.5 mm 1.5 mm Parameters for this joint: Laser: 2200 W Welding Speed: 20 mm/s, 15 mm/s, and 10 mm/s Ar-gas flow rate: 30 L/min Back Ar-gas flow rate: 20 L/min

9 Experimental Procedures Joint type and laser beam: (3) Fillet butt joint (various gages, 1.5 mm to 1.2 mm) Laser beam 1.5 mm 1.2 mm Parameters for this joint: Laser: 2000 W Welding Speed: 20 mm/s, 15 mm/s, and 10 mm/s Ar-gas flow rate: 30 L/min Back Ar-gas flow rate: 20 L/min

10 Experimental Procedures Tensile Test: (1) Base material: DP980 Specimen for test INSTRON tester Strain gage Broken specimen after test

11 Experimental Procedures Tensile Test: (2) Laser welded joint Specimen for test INSTRON tester Strain gage Broken specimen after test

12 Experimental Procedures Metallurgical analysis: Top view Cross-section view

13 Welded specimens: same gages of 1.2 mm to 1.2 mm (1500 W in power) width of the weld (top) HAZ-heat affected zone Top side HAZ Bottom side Cross sections width of the weld (bottom) welding direction top side width of the weld Various welding speed: 10 mm/s 15 mm/s 20 mm/s

14 Widths of the weld bead: same gages of 1.2 mm to 1.2 mm (1500 W in power) 2.83 mm 2.17 mm 2 mm 3 Various widths (top side) Bottom side, mm mm 1.38 mm 0.74 mm Various widths (bottom side) 20 mm Width of the weld Welding speed, mm/s High welding speed produces smaller weld bead

15 Widths of HAZ (average): same gages of 1.2 mm to 1.2 mm (1500 W in power) , mm Width of HAZ Welding speed, mm/min High welding speed produces smaller HAZ

16 Widths of the weld bead: same gages of 1.5 mm to 1.5 mm (2200 W in power) mm 2.54 mm 2.16 mm Top side, mm 3 2 Top side Bottom side Width of the weld Welding speed, mm/s 3.8 mm 2.38 mm 2.08 mm Bottom side

17 Widths of HAZ (average): same gages of 1.5 mm to 1.5 mm (2200 W in power) 3 2, mm 1 width of HAZ Welding speed, mm/min High welding speed produces smaller HAZ

18 Widths of the weld bead: various gages of 1.5 mm to 1.2 mm (2000 W in power) Top side 1.5mm 1.2mm 4 Top side 2.92 mm 2.47 mm 2.14 mm, mm 3 bottom side 2 Bottom side 3.67 mm 2.3 mm 2.01 mm Width of the weld Welding speed, mm/s

19 Widths of HAZ (in 1.2mm side): (1) various gages of 1.5 mm to 1.2 mm (2000 W in power) 2, mm 1 width of HAZ Welding speed, mm/min

20 Tensile test Results for base material DP , MPa Tensile stress Strain, mm/mm Typical fracture of ductile material Tensile stress at yield (offset 0.2%): 648 MPa Tensile stress at maximum load: 974 MPa Tensile stress at break: 948 MPa

21 Tensile test results for welded specimens of various gages, 1.2 mm and 1.5 mm mm/s 800 DP , MPa 600 Welded joints, MPa mm/s mm/s Tensile stress Strain, mm/mm Tensile stress Strain, mm/mm Laser welding drastically reduces the toughness and decreases the tensile strength up to 20% Welding Speed, mm/s Tensile stress at yield (offset 0.2%), MPa Tensile stress at maximum load, MPa Tensile stress at break, MPa Extension at maximum, mm DP980

22 Tensile test results for welded specimens of various gages, 1.2 mm and 1.5 mm Tensile stress at maximum load, MPa 800 Tensile stress at break Tensile stress Tensile stress at yield point (offset 0.2%) Welding speed, mm/s Higher welding speed produces higher strength joint

23 Tensile test results for welded specimens of various gages, 1.2 mm and 1.5 mm 1.4 mm 1.4 mm 1.3 mm Edge of fracture Sheet of 1.2 mm 2 mm Sheet of 1.5 mm Welding speed: 10 mm/s 15 mm/s 20 mm/s Fracture occurs at the thinner sheet and at the HAZ Weld is stronger than HAZ

24 Tensile test results for welded specimens of same gages, 1.2 mm mm/s 800 DP , MPa Welded joints, MPa mm/s Tensile stress Strain, mm/mm Tensile stress mm/s Strain, mm/mm Laser welding drastically reduces the toughness and decreases the tensile strength up to 21% Welding Speed, mm/s Tensile stress at yield (offset 0.2%), MPa Tensile stress at maximum load, MPa Tensile stress at break, MPa Extension at maximum, mm DP980

25 Tensile test results for welded specimens of same gages, 1.2 mm Tensile stress at maximum load, MPa Tensile stress at break Tensile stress Tensile stress at yield (offset 0.2%) Welding speed, mm/s Higher welding speed produces stronger joint

26 Tensile test results for welded specimens of same gages, 1.2 mm 1.6 mm 1.3 mm 1.0 mm Edge of fracture Welding speeds: 10 mm/s 15 mm/s 20 mm/s Fracture occurs at the thinner sheet and at the HAZ Weld is stronger than HAZ

27 Tensile test results for welded specimens of same gages, 1.5 mm mm/s 800 DP mm/s, MPa Welded joints, MPa mm/s Tensile stress Strain, mm/mm Tensile stress Strain, mm/mm Laser welding drastically reduces the toughness and decreases the tensile strength up to 23% Welding Speed, mm/s Tensile stress at yield (offset 0.2%), MPa Tensile stress at maximum load, MPa Tensile stress at break, MPa Extension at maximum, mm DP980

28 Tensile test results for welded specimens of same gages, 1.5 mm Tensile stress at maximum load, MPa Tensile stress at break Tensile stress Tensile stress at yield (offset 0.2%) Welding speed, mm/s Higher welding speed produces stronger joint

29 Tensile test results for welded specimens of same gages, 1.5 mm Edge of fracture 2.1 mm 1.8 mm 1.2 mm Welding speed: 10 mm/s 15 mm/s 20 mm/s Fracture occurs at the thinner sheet and at the HAZ Weld is stronger than HAZ

30 Cross section view Top view Where is the broken line? Broken specimen Why?

31 Macrostructure of welded specimen, top surface HAZ Weld zone

32 Martensite Ferrite In the base material In the heat affected zone In the weld zone Smaller volume fraction of martensite in the area close to the interface between base material and HAZ

33 Microhardness distribution of welded specimen Weld zone 450 HAZ HAZ , Kg/mm Microhardness 200 Center of the weld Distance, mm For 1.5 mm thick sheets at 10 mm/s Weak point

34 CONCLUSIONS Laser welding of galvanized dual phase steel DP980 by using 4-kW fiber laser system can produce strong but brittle welded joints. Tensile test results show that the fracture for all specimens occurred along the interface between the heat affected zone and the base material, 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.

35 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.

36 Acknowledgement LAM s team at SMU: Shanglu Yang Gustavo Quiroga Hosein Atharifar Andrzej Socha

37 Thank you!