ResearchBrief

Friction stir welding in rail vehicle manufacture (EU Project EuroStir®)

T035July 200

BackgroundIn his report after the Ladbroke Grove accident in 1999 Lord Cullen recommended that consideration should be given 'to the use of alternatives to fusion welding'. It had been noticed in the accident that the aluminium rail vehicles had come apart at weld seams without there being signs of significant plastic deformation in the vicinity of the welds. This effect has been called 'weld unzipping'. During the last decade, the engineering industry has made increasing use of a non-fusion joining technique called friction stir welding (FSW), Figure 1.

Figure 1: The FSW process has more similarities to machine tool technology than it does to fusion welding techniques.

It was therefore thought appropriate to compare this fabrication approach with the traditional approach of metal inert gas (MIG) welding (Figure 2) for the manufacture of aluminium rail vehicles.

The MIG welding process melts the metal and creates a weld bead at the join, where filler material has been added (Figure 3).

Figure 2: The MIG welding process: a common fabrication method for aluminium rail vehicles

In the case of FSW (Figure 1) a rotating non-consumable tool is plunged between two clamped plates. Friction between the tool and the plate generates heat, which softens the material around the tool. The rotating FSW tool is then traversed, heating, softening, and forging material as it moves, along the joint line. As the tool moves, a consolidated solid-phase joint is formed. The shoulder on the tool ensures that the thickness at the joint is the same as that of the starting aluminium (Figure 3).

RSSB Research and Development ProgrammeFloor 6, Central HouseUpper Woburn PlaceLondonWC1H 0HYwww.rssb.co.uk/r_and_d.asp / research@rssb.co.uk

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Figure 3: Cross sections through welds: on the left is a MIG weld, on the right the process used was FSW.

AimsThe main aim of this work was to explore the crashworthiness performance of welds made by the MIG and FSW processes in typical aluminium alloys which are used for vehicle body shells.

MethodologyIt was recognised from the outset that the scope of this project would preclude the testing under crash conditions of major structures. The approach taken was therefore to focus on small scale testing to provide insight into the performance of larger structures under crash conditions. The scope of the work formed part of a major European project called EuroStir®, which was being coordinated by TWI Ltd. The purpose of EuroStir® was to explore potentially beneficial applications for FSW and ensure that the European Union derived maximum benefit from the FSW process. FSW had been invented and patented by TWI in 1991 and is now in global use in many industries.However, despite widespread use of FSW, there was very little public domain data that allowed a comparison to be made between FSW and MIG welded aluminium alloys of the type used in rail vehicle manufacture, particularly so in the case of dynamic test data. Therefore, the objectives of this work were to:• Produce good quality MIG and FSW

welds in aluminium alloys 6005-T6 and 6082-T6 for evaluation.

• Undertake small-scale static and dynamic tests (tensile test and crush) to assess weld performance and com-pare the methods.

To ensure that the work was entirely relevant to conditions that were used in the rail industry:• The MIG welds were made with two

types of filler wire (Al-Si 4043 and Al-Mg 5356 alloys), both of which are used in the industry.

• 3mm thick aluminium sheet and box sections in alloys 6082-T6 and 6005-T6 were used.

• MIG and FSW welds were made with varying heat inputs, so as to assess the likely consistency of these proc-esses when used by the industry.

The decision was taken in this work to test the samples of MIG weld with the bead left on, rather than machining the samples back to the original thickness of the aluminium.

FindingsThe tensile, hardness, and metallographic evaluations undertaken all showed the MIG and FSW welds to be of typical industrial standard. The FSW specimens tested in tension had somewhat higher proof and ultimate tensile strength values than comparative MIG welds.For both MIG and FSW specimens the heat input during welding softened the hardened parent aluminium in the weld area. This is an important point, because if such welds in a rail vehicle are put under tensile load during a crash, the tendency is for deformation and subsequent fracture to occur in the weld area. If a large difference exists between the weld zone and parent material strength, then crash deformation is more likely to be concentrated in the welded zone, providing conditions that may precipitate so called 'weld unzipping'. It is for this reason that there is merit in using designs where the joints have a thickness that over-matches that of the parent aluminium. In this way tensile failure in the weld is suppressed.During this work it was found that conventional drop weight impact tests (Figure 4) were not appropriate for testing.

Figure 4: (a) and (b) Tests such as these did not put the weld zone into tension and so an alternative test was devised, as shown in Figure 5.

Figure 4(a) shows a sample with the weld at the centre of a box section. When impacted from above with a flat weight, samples such as these either simply crumpled, or crumpled and cracked at the corners, Figure 4(b). Such specimens did not put the weld region into tension.Therefore, special drop weight specimens were prepared (Figure 5) which had a part-circular piece removed from the top. A cylindrical weight impacting this specimen forced the material in the region of the weld into tension so that, if the weld showed any tendency to 'unzip', this test would be well able to detect this behaviour.

Figure 5: Drop weight test designed to force the material in the vicinity of the central weld into tension.

Results of these tests demonstrated good behaviour of all the welds. Typical samples after test are shown in Figures 6 and 7.

Figure 6: Typical MIG welded specimens after the modified drop weight test. Note the lack of extensive weld 'unzipping'.

Although FSW is a non-fusion process, a key finding of this work was that during welding a similar degree of softening occurs as for MIG welding but in a narrower zone. This means that when designers over-match the thickness of the joint to achieve parent metal properties, a similar extent of over-matching will be required. (For an example of over-matching, see figure 8.)Differences in performance between the MIG and FSW samples were linked to the extent of heat affected zone softening and the fact that the MIG samples were aided by the reinforcement of the weld metal cap.In summary, this project has shown that for the tests carried out, both the MIG and FSW processes give welded joints that perform similarly. Neither joining technique appears overly sensitive to heat input variations likely to be seen in the rail industry. The data produced during this project is available in the public domain (on the RSSB website) as benchmark data and as an aid to rail vehicle designers.

Figure 7: Typical FSW welded specimen after the modified drop weight test. Note the lack of extensive weld 'unzipping' as with MIG.

Next StepsManufacturers of rail vehicles are increasingly making use of FSW for long lengths of welds because of its cost competitiveness, low distortion, and absence of welding fumes and spatter. This project has suggested that, when correctly applied, both MIG and FSW are suitable processes from the static and dynamic weld performance point of view. However, it must be remembered that this work did not include fatigue and was limited to small scale test pieces. Large- or full-scale testing is required to throw additional light on the comparison between MIG and FSW fabrication. Such testing has been carried out within the partly EC funded project ALJOIN: The Crashworthiness of Joints in Aluminium Rail Vehicles. The ALJOIN project also highlighted the importance of using over-matched thickness joint designs to avoid a tendency towards 'unzipping' of welds.

Industry reviewThis work was reviewed by RSSB, Angel Trains Ltd, HSBC Rail (UK) Ltd, and Bombardier Transportation. Two key findings of the work were that:• Welded samples fabricated by MIG

and FSW performed similarly for the tests carried out. It is important to know that FSW and MIG processing

both induce a similar level of soften-ing on the parent aluminium.

• Tests carried out at both low and high strain rates did not identify any marked tendency towards weld 'unzipping'. Prior to the start of this project there was very little data avail-able on this aspect.

Stakeholders agreed with the importance of over-matching the thickness of the joint to obtain equivalent strength of the parent material, Figure 8.

Figure 8: An example of a FSW joint with over-matched thickness.

Although small scale testing throws light on the likely behaviour of large structures such as rail vehicles, RSSB accepts the importance of carrying out larger-scale testing. It was for this reason that RSSB also became involved in the ALJOIN project in which some larger components have been tested.It was the view of the review group that there was merit in pulling together the various threads of the work on crashworthiness. There was a need to present an overview which would inform the rail industry and ensure that appropriate messages were fed into guidelines and standards that may be developed in the future. This is being progressed as part of research project T118 Whole train dynamic behaviour in collisions and improving crash worthiness.

AcknowledgementsThanks are due to Bombardier Transportation and Sapa for the supply of material.

ContactHead of Engineering ResearchR&D ProgrammeRail Safety and Standards Boardresearch@rssb.co.uk