Robust Schedules for Spot Welding Zinc Coated Advanced High Strength Automotive Steels

Transcription

1 Robust Schedules for Spot Welding Zinc Coated Advanced High Strength Automotive Steels Gajendra Tawade (Ryerson University) Gary Boudreau (Dofasco Inc. Canada) 1

2 Outline Objective Introduction Experimental Method (Spot Welding Trials) Discussion of Results (Weld Lobe Data Analysis) Summary and Conclusions 2

3 Objective Objective: Present work focuses on developing a robust spot welding process for welding DP600 to itself and to other grades. Different welding pulses and electrode tip designs were used to increase the lobe width 3

4 Introduction Advanced high strength steels have been identified as an affordable solution to satisfy conflicting requirements like Weight reduction & Passenger safety Material characterization data on formability, crash performance and spot welding is still in progress 4

5 Introduction DP600 has excellent mechanical properties. Richer chemistry makes it more difficult to weld Hardenability elements can lead to narrower weld lobes Focus of Current work is to increase lobe width 5

6 Introduction Mechanical properties of materials used Grade Thickness (mm) YS MPa UTS MPa TE(%) n DP HSLA EDDQ 0.7 & All materials are 60G/60G Hot Dipped Galvanized 6

7 Experimental Method 7

8 Method -Equipment and Testing 5 CURRENT TIME Current 1 Second = 60 Cycles Force Current Time 8

9 Method - Materials Tested Develop robust spot welding process for given material combinations 2.0 mm DP600 to 2.0 mm DP mm DP600 to 2.0 mm 350 HSLA 2.0 mm DP600 to 0.7 mm EDDQ 2.0 mm DP600 to 1.0 mm EDDQ All materials were 60G/60G Hot Dipped Galvanized These are the applications from a production vehicle 9

10 Advantages of Robust Welding Schedules Weld Lobes for DP600 & BH 180 (2.0mm to 2.0mm) Welding time (Cycles) D min D max D D max = Electrode dia D min = 4 t Welding Current (ka) DP600 BH Steel DP600 generally have narrower lobes than conventional Low C steels The present work was initiated to increase the lobe width Minimum acceptable lobe width is 2000 Amps 10

11 Discussion of Results Experiment No.1-DP600 to DP600 (2.0/2.0 mm) 11

12 Weld Lobe for DP600 (2.0mm-2.0mm) Welding Time (Cycles) A 600 A 567 A Weld Lobe Welding Current (ka) Sample preparation, electrode installation, stabilization & lobe tests conducted according to A/SP procedures 20% higher Electrode Force for DP600 All other welding parameters selected according to A/SP 12

13 Lobe Widths & Welding Times 2.0mm DP600 welded to 2.0mm DP Acceptable lobe width=2000amps Lobe width with experiments Weld Lobe Width (Amps) Welding Time (Cycles) Weld Lobe Reference weld time for 2.0 mm mild steel 13

14 Weld Lobe for DP600 W eld in g tim e ( c y cles) WELD LOBE DP-600 (2.0mm to 2.0mm) A 1700 A 1300A 767 A Weld Lobe 570 A Welding current (KA) Nature of the curve : wide at the top & narrow extended tail at the bottom 14

15 Nugget Growth Studies for DP600

16 Nugget Growth Electrode-1 5 Sheet-1 Sheet-2 CURRENT TIME Current 1 Second = 60 Cycles Electrode-2 16

17 Nugget Growth in Single Pulse 12 Nugget Dia (mm) /Current (ka) DP600 Welded to DP600 (2.0/2.0mm) Welding Time (Cycles) Nugget Diameter Welding pulse Zinc removed after 13 cycles Rapid Nugget growth after 17 cycles Heat input in the last stage is fast 17

18 Nugget Growth in Double Pulse (DP600) RYERSON 12 Nnugget Dia (mm)/ Current (ka) Welding Time (Cycles) Welding Pulse NuggeDiameter All Zinc removal after 13 Cycles Nugget growth without zinc can be ensured at 20 cycles Nugget growth with double pulse is slower than single pulse Can this effect be enhanced with lower current intensity on 2 nd pulse? 18

19 Designing of Enhanced Welding Pulse Double Pulse Different Intensity Welding Current (ka) Welding Time (Cycles) Welding Pulse Experiments were conducted to find appropriate current intensity on the second pulse 19

20 Nugget Growth With Different Welding Pulses Nugget Diameter (mm) Nugget growth Vs welding time for different pulses Single pulse Double Pulse 100%-87.5% 100%-75% Enhanced Pulse Slope of lines Single Pulse = 5.61 Double Pulse = 5.41 Enhanced Pulse = 3.82 Welding Time (Cycles) 20

21 38% increase in the lobe width with proper welding pulse (2.0mm DP600 t0 2.0mm DP 600) 2 No. of Pulses 1800 Amps Enhanced Pulse Amps Single Pulse Lobe width (Amps) Lobe width Weld lobes at 26 cycles For same Welding parameters Enhanced pulse showed wider lobe than Single pulse 21

22 Experiment No 2- HSLA to DP600 (2.0/2.0 mm)

23 Achievement of Acceptable Lobe Width Lobe Width 2.0mm HSLA-2.0mm DP600 Weld Lobe Width (Amps) Acceptable Lobe Width Single Pulse Enhanced Pulse Welding Time (Cycles) Reference weld time for 2.0mm Mild steel 23

24 Heat Balance Unequal Thickness 24

25 Heat Balance Tip Diameter Electrode MM Sheet MM Sheet-2 EDDQ 0.7 MM Electrode MM 25

26 Results & Discussion Experiment No-3 DP600 to EDDQ (2.0/0.7mm) 26

27 Lobe Widths for DP600 to EDDQ (2.0/0.7 mm) Weld Lobe Width (Amps) Welding Time (Cycles) Single Pulse Reference weld time for welding 0.7mm to 0.7 mm Mild steel is 11 cycles 27

28 Results & Discussion Experiment No-4 4 DP600 to EDDQ (2.0/1.0 mm) 28

29 Lobe Widths for DP600 to EDDQ (2.0/1.0 mm) Lobe Widths (Amps) Weld Lobe Welding Time (Cycles) Reference weld time for welding 1.0mm to 1.0mm Mild steel is 13 cycles Experiments were conducted with Double pulse, enhanced pulse and triple pulse. Single pulse was found to be the most beneficial 29

30 Summary 30

31 Summary- Equal Thickness Material A welded to Material B (2.0mm) Lobe with Single Pulse at 26 Cycles Lobe with Enhanced Pulse at 26 cycles % Increase in the Lobe Width DP600 to DP Amps 1800 Amps 38% DP600 to 350 HSLA 1800 Amps 2233 Amps 24% Maximum lobe width for HSLA to DP (2.0/2.0 mm) combination was achieved with Enhanced Pulse = 2587 Amps 31

32 Summary- Unequal Thickness Material A welded to Material B DP600 to EDDQ (2.0/0.7 mm) DP600 to EDDQ (2.0/1.0 mm) Maximum Lobe with Single Pulse 1633 Amps 1425 Amps Single pulse found to be more beneficial than Double Pulse or Enhanced Pulse for welding unequal thickness combination 32

33 Conclusion Lobe width can be improved by experimenting with weld pulse design Acceptable lobe width can be achieved using Enhanced Welding Pulse for DP600 joints and DP-HSLA joints Welding Process is stable with Enhanced Pulse Lobe data with unequal tip diameters are available for further discussions 33

34 Future work Mechanical testing of joints with Single Pulse & Enhanced Pulse Design Designing of Enhanced pulse for unequal thickness combinations Welding unequal thickness combinations with equal diameter tips Metallographic study of welds 34

35 Thank you! 35