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Minutes of the First Railway Industry Friction Stir Welding Meeting

Held on Thursday 11 April 2002 at TWI, Granta Park, Great Abington, Cambridge, CB1 6AL, UK

Attendees:

Apologies (requesting minutes):

1. Welcome

Bevan Braithwaite (TWI Ltd) welcomed the 22 attendees to this EuroStir® meeting and reviewed how friction stir welding (FSW) had been invented at TWI in 1991. This meeting was part of the Eureka project greek upper sigma!2430 on European Industrialisation of Friction Stir Welding. Photos of the event (PDF file, 118kB)

2. Introduction

Alan Lawton (Railway Safety) mentioned the Ladbroke Grove Accident and that it would be a good idea to investigate FSW. Railway Safety, Angel Trains and HSBC Rail have supplied £30K each to the EuroStir® project and want to supply £50K each in kind. He said the purpose of this meeting was to hear the attendees views about useful work to undertake with the aim of improving railway safety. He intended to investigate FSW from a safety angle as well as to consider some production issues.

3. Presentations

4. Discussion

Mick Roe (Bombardier) said that current production needs were insufficient to justify the capital investment for a FSW machine. The machine concept evaluated could get a pay back in two years but their current through put was limited and therefore break even would be achieved in only eight years.

Mick Roe (Bombardier) mentioned that it was unknown whether there was a real process of 'unzipping' in the heat affected zone of welds or whether this was the predicted failure mechanism of aluminium welds. Stephan Kallee (TWI) expected that friction stir welds would also fail in the heat affected zone. He recommended re-designing the joints, i.e. increasing the workpiece thickness in the joint area.

Mick Roe (Bombardier) asked whether aluminium extrusions failed also in straight lines unlike steel structures where the failure was random.

Torben Lorentzen (DanStir) reported that he had done some very cold welding trials in a EuroStir® Industrialisation study for Corus using aluminium castings and thin sheets. He recommended increasing the ductility by softening the heat affected zone and not to underestimate the difficulties in using the FSW process on the shop floor for the following reasons:

Philip Threadgill (TWI) explained 'kissing bonds' and detection methods.

Martin Ogle (TWI) mentioned that heat treatment after welding was an option. He said it was, however, critical to optimise the joint designs to join heat treated panels. He indicated the research needs regarding material selection and joint designs.

Phil Threadgill (TWI) said that massive grain growth could be the result of heat treatment. Sapa has, however, reported some interesting results with post weld T6 heat treatment of friction stir welded 6082-T4 and 6082-T6.

Stephan Kallee (TWI) said that the blistering effect could cause some problems, e.g. because of contamination by lubricants or as a result of workpieces containing high levels of hydrated oxides or hydrogen. Martin Ogle (TWI) asked whether extrusions could be replaced by FSW of stringers and spars to wrought steels, and recommended rethinking some fundamental concepts. Mick Roe (Bombardier) added that they had considered going back to single sheets but Ray Ford (Railway Safety) warned about condensation issues. Mick Roe (Bombardier) said that they had developed a standard joint which can be used for adhesives but might also be tested with MIG or FSW. He said that applying FSW to fillet welds seemed to be too difficult at this moment in time.

Bevan Braithwaite (TWI) asked whether the tests to assess the performance of joints were sufficient and whether they were identical in different countries.

Martin Ogle (TWI) mentioned that there was a large database available on automobile car crashes. Ray Ford (Railway Safety) replied that the angle of rail collisions was less critical than in automotive accidents. Rail vehicles experience approximately 8g deceleration and car crashes approximately 80g. There is also a big difference to aircraft, which tend to belly flop while rail cars tend to front-end-crash.

5. Preliminary summary

As a result of this discussion, Alan Lawton (Railway Safety) summarised the following:

6. Brainstorming and prioritising of the research needs

John Davenport (TWI Ltd) chaired a brainstorming session in the afternoon, in which the attendees provided research needs on Post-it notes and then prioritised them by attaching a number of red spots.

The output from the Post-it note session is given in the tables further below. These tables should be referred to for a full picture, but some of the key issues which emerged were:

ACTION: Phil Threadgill and Scott Lockyer to issue a proposal for the EuroStir® Rail Car FSW Study by 10 May 2002.

7. Guided Tour Through the TWI Laboratories

A tour through the Laser, Metallurgy and Structural Assessment laboratories was given, which ended with a demonstration of friction stir welding 16mm thick 7000 series aluminium alloy.

8. Date and Venue of next meeting

Wednesday 12 June 2002 at TWI Ltd, Granta Park, Great Abington, Cambridge, CB1 6AL. For further information contact stephan.kallee@twi.co.uk or john.davenport@twi.co.uk


Types of test/effects that we need to understand

Red Spot Score  
20 Dynamic impact performance including such effects as 'unzipping'
11 Fatigue strength and how it compares with other joining methods, both butt and lap welds
9 Ductility of FSW joints
6 Standard tests and test pieces for dynamic properties of joints
  Other topics:
- size effects
- fracture criteria
- corrosion performance of FSW joints
- static and dynamic tensile data
- static and dynamic fracture toughness data
- quantify improvement in static strength

Design

Red Spot Score  
8 Joint design
8 Full scale crash testing (FSW v fusion)
2 Static and dynamic modelling
1 Correlation of small scale tests with analysis
  Other issues included:
- applicability to vehicle zones (eg body tube v body end)
- recycling
- extrusion v fabrication
- use of friction stir welded sheet/plate as an alternative to extrusions
- single skin Al vehicles
- development of joint design best practice
- fatigue strength: how can FSW improve critical details
- arrest cracks in joints
- purposely designed collapsing 'fuse', with or without FSW
- design joints for crashworthiness

Performance requirements

Red Spot Score  
8 Requirements for safer, crashworthy joint construction
8 Testing of crashworthiness effects
1 FE modelling of different crash scenarios
5 Repairability
  Other issues:
- stronger joints
- CEN256/WG2 is considering collision scenarios (output from SAFETRAIN)
- relate weld microstructures to crashworthiness
- surface finish
- cleaner finish

Joining options

Red Spot Score  
1 Tee joints (welded from the tee side)
  Other topics:
- alternative joining methods
- more joint configurations
- fillet welds
- better lap joint performance

Materials

Red Spot Score  
6 Material grades
1 The effect of heat/fire on the material and how to protect it
  Material aspects included:
- alternative alloys
- optimum alloys
- 7xxx series alloys
- pre- and post-weld heat treatments
- post fabrication heat treatment of FSW welded panels
- welding in overaged temper
- development of fatigue data

Integrity and inspection

Total of 12 Red Spot Score  
4 NDT testing of welded joints (defect identification)
3 Process problems
3 Process robustness
2 Quality and inspection of welds in production
  Other topics:
- modelling of Al weld failures
- tests to determine crashworthy joint properties

Standards and codes

Red Spot Score  
2 Design and manufacturing standards
  Other topics:
- quality quantification
- standard tests and samples for good comparison
- comparative collision test samples
- stir tool design and control

Experience in other industries

Red Spot Score  
1 Synergies with other sectors (e.g. automotive, ship)

Economics

Red Spot Score  
4 Economics (FSW compared with alternatives)
  Aspects of economics which were captured include:
- industrialisation
- productivity
- developing process to improve productivity
- tool life
- machine tool costs
- process speed
- energy saving
- whole life costs

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