Fibre laser hybrid welding of aluminium alloys for the rail sector

Transcription

1 Fibre laser hybrid welding of aluminium alloys for the rail sector Chris Allen 1, Pak Chong 2, Paul Hilton 1 and Yoshitomo Watanabe 3 1 TWI Ltd, 2 Formerly TWI, now Subsea7, 3 Nippon Sharyo Ltd

2 Contents Interest in alternative welding technologies from the rail sector Fibre lasers Hybrid laser-mig welding Welds in 3mm thick plate Joint geometry development Welds in 3mm thick extrusions Gap tolerance

3 Interest in alternative welding technologies from the rail sector For use in new type of high speed train body manufacture

4 Interest from the rail sector Aluminium alloys (extruded sections and rolled plates) used in high speed train railcar bodies Image from 'JAPAN RAILFAN MAGAZINE' Vol.45 No.532 Aug., 2005 p86

5 Interest from the rail sector MIG welding has been conventionally used in past Extrusion design for MIG welded joint

6 Interest from the rail sector In modern designs, joint lengths up to 25m High heat input of MIG can lead to build up of distortion over weld length High heat input also leads to loss of strength and wide HAZ Area around joint requires thickening, to maintain strength, adding weight to rail carriage

7 Interest from the rail sector Two alternative welding processes are being considered Friction stir welding (FSW) Hybrid laser-mig (combination of laser and MIG welding) Both have a lower heat input than MIG welding Lower distortion Lower degree of strength loss in joints Higher minimum strength Narrower HAZ

8 FSW in the rail sector FSW still requires that the joint area be thickened, due to factors including Resistance to tooling forces during FSW Wide HAZ Thicker section in joint region Weight saving potentially greater with hybrid laser-mig welded structure Welding speeds higher Extrusion design for FSW joint

9 Hybrid welding at TWI TWI is developing hybrid fibre laser-mig welding for Nippon Sharyo, a Japanese railcar manufacturer Weld procedure development on close fitting butt joints 3mm thickness rolled plates, extruded plates and hollow section extrusions A6N01S-T5 (~AA6063, or Al~0.6Mg~0.6Si) ER5356 (Al~5Mg) MIG consumable used Weld profile from cross-sections Weld quality from radiographic inspection Optimum joint geometry trials Gap tolerance of welding conditions developed

10 Why Fibre lasers? Fibre delivered Flexible welding Multi-kW power Higher efficiency (20~25%) Diode pumped, solid state design Reduced servicing Smaller footprint More portable Higher beam quality

11 Why Fibre lasers? Fibre delivered Flexible welding Multi-kW power Higher efficiency (20~25%) Diode pumped, solid state design Reduced servicing Smaller footprint More portable Higher beam quality Laser output power, W CO2 Nd:YAG Yb-fibre Jan-70 Jan-75 Jan-80 Jan-85 Jan-90 Jan-95 Jan-00

12 Why hybrid welding? So what is hybrid welding? MIG Laser

13 Benefits include Why hybrid welding? Greater penetration Higher welding speed Improved fit-up tolerance Improved weld quality and profile Filler metal addition control of weld microstructure control of hot cracking Laser weld Laser weld Hybrid weld Hybrid weld

14 Hybrid welding set up Jig Laser beam axis MIG torch axis Air-knife Welding head MIG torch

15 Hybrid welds in 3mm plate Laser / MIG powers typ. 7.0 / 3.0 kw Maximum welding speed for full penetration 5.0 m/min (d = 0.6mm) Grade 1 welds max. pore size typ. 0.5mm

16 Optimum extrusion geometry trials First trials on joints between rolled plates and extruded plates Without joint space Grade 4 welds 21 pores in diameter range 1-2mm With joint space Grade 2 welds 2 pores in diameter range 1-2mm Joint space essential But, notch defect in root Extrusion #1 Extrusion #2 Joint space 3mm

17 Optimum extrusion geometry trials Two further types of self jigging joint configurations between extrusions tried with joint spaces Asymmetric joint space Symmetric joint space

18 Optimum extrusion geometry trials Asymmetric joint space leads to Grade 3 quality welds 5 pores in diameter range 1-2mm in 300mm test length Notch defects still present in root Asymmetric joint space Original joint line

19 Optimum extrusion geometry trials Symmetric joint space leads to Grade 1 or Grade 2 quality welds 2 pores in diameter range 1-2mm Notch defects no longer present Joint is essentially full penetration butt weld Underlying material machined away (to facilitate radiographic inspection) Original joint line

20 Optimum extrusion geometry Hybrid weld between two close fitting hollow extruded sections

21 Hybrid welds in thicker materials High quality welds can also be achieved in thicker materials Hybrid weld in Al alloy plate up to 12mm thick Laser power at workpiece = 7kW 0.8 m/min (d = 0.6mm) PF welding position (vertical up) Weld profile acceptable if cap and root machined

22 Gap tolerance: 3mm plates Continuously tapering gap joint (0-2mm) bridged to 0.7mm gap before under fill exceeded 0.3mm (customer requirement) Excess 0.5mm at 0mm gap Excess 0.3mm at 0.4mm gap Under fill 0.3mm at 0.7mm gap

23 Gap tolerance: 3mm extrusions Greater gap tolerance has been developed, through further modification of extrusion geometry

24 Gap tolerance: 3mm extrusions 0.7mm gap 1.0mm gap 1.5mm gap Weld beads would be machined down after welding Customer would now accept welds made across a 1.5mm gap

25 Conclusions Fibre lasers proven suitable for hybrid welding Low distortion, high quality hybrid fibre laser-mig welds can be made in 3mm 6xxx aluminium alloys at >5m/min Design of joint geometry essential for low porosity Fully penetrating welds in to a symmetric joint space procedures tolerant to joint fit-up, meeting customer requirements on under fill gap tolerance to 1.5mm possible, if weld cap can be machined subsequently gap tolerance to 0.7mm possible, if weld cap not machined self-jigging