Published in European Railway Review Magazine, October 2002
Synopsis
Aluminium rolling stock manufacturers are making increasing use of friction stir welding, a joining technique that operates below the melting point of the workpieces. The process has production, cost and joint performance benefits which were first exploited by the Japanese railway industry. Europe is fast catching up.
Introduction
Passenger rail vehicles are fabricated out of steel, stainless steel or aluminium. Fusion welding has traditionally been the preferred joining technique. Often this is MIG (metal inert gas) welding but laser welding has been used more recently for some joints. Lasers have a lower heat input, which produces less distortion, and when coupled with their faster welding speeds, this makes them particularly useful for long welds. Rail vehicles are complex structures witha range of material thicknesses and joint types to be joined reliably and speedily at low cost. It is therefore not surprising that a range of technologies is needed to meet these requirements.
Aluminium rail vehicles are now frequently produced from longitudinal aluminium extrusions with integrated stiffeners. Using this approach the whole body-shell can be made from either single-wall or hollow double-skin extrusions.
One of the difficulties with fusion welding these structures is that the high heat input leads to distortion. Manufacturers learn how to control and correct this to some extent, but extra costs can be incurred and there may remain the need to use filler to ensure a satisfactory cosmetic appearance to the rolling stock. Manufacturers report low distortion as one of the key benefits for switching to friction stir welding, a non-fusion joining technology which has reduced heat input.
Friction stir welding (FSW) was invented and patented in 1991 [1,2] by TWI in Cambridge, United Kingdom. The process has seen a great deal of development activity and now 78 organisations around the world hold licences for its use. Users are found across a range of industries including rail, shipbuilding, automotive and aerospace. For any new process to be of interest it must offer cost or technical benefits. Manufacturers report a range of benefits from using FSW:
- Reduced cost, although manufacturers are naturally reluctant to publish their quantified savings. Manufacturers are either investing in their own FSW facilities or, where the volume does not justify this, are buying in prefabricated panels from an increasing number of specialist FSW suppliers.
- Less distortion.
- A single FSW pass may be all that is needed to replace multiple MIG welding passes required for thicker sections.
- No weld spatter. The surface and root of the joint are clean and their cosmetic appearance so good they need not even be painted. This provides potential for further cost reduction.
- No weld fume during manufacture, which is increasingly important as health and safety standards tighten.
- The process can operate in any orientation, because gravity has no influence.
- Energy efficient.
- Consumables like gas or filler wire are not needed.
- No porosity in the welds.
- Excellent mechanical properties.
On this last point, one aspect which is generating increasing interest in FSW is its potential to contribute to the crash worthiness of aluminium vehicles that could otherwise fail in the heat affected zone along weld seams. Such afailure mode has been seen in accidents such as Eschede (Germany) in 1998 and Ladbroke Grove (United Kingdom) in 1999. In the report on the latter [3] it was recommended that consideration should be given 'to the use of alternatives to fusion welding' and 'the use of improved grades of aluminium which are less susceptible to fusion weld weakening.' The need for strength of vehicle roofs and walls was also highlighted in one of the interim recommendations of the Investigation Board of the Hatfield Derailment Investigation (United Kingdom). The recommendation was that 'train sets should be designed, built and maintained to maximise the chance of their remaining upright and intact during high speed derailment. Particular aspects of rolling stock design which should be reviewed are [...] strength of vehicle roofs and walls'. [4]
A part-European-funded project called ALJOIN has been initiated to investigate the crashworthiness of joints in aluminium rail vehicles. This project will quantify the performance of various joining technologies, including FSW, and point the way towards the use of optimum processes and joint designs. The project participants are Alcan (Switzerland), ARRC (UK), Bombardier Transportation (Sweden), DanStir (Denmark), D'Appollonia (Italy) and TWI (UK). [5]
Despite the many benefits of FSW, there are some limitations which manufacturers need to take into account. FSW welding speeds can still be slower than those of some fusion welding processes. However, attention to process development, particularly tool design, is continually speeding up FSW. The process also requires the workpieces to be rigidly clamped and a backing bar is generally needed. Fortunately, the workpieces do not have to be in complete contact. Trials have shown that a gap of approximately 10% of the workpiece thickness can be tolerated before the quality of the weld starts to suffer.
Process description
In many ways FSW has more in common with machine tool processes than it does with fusion welding processes. Friction stir welding uses a rotating tool, which moves along the joint between two components (
Fig.1). The resulting frictional heating softens the aluminium which flows plastically to produce a solid-state weld.
FSW tools are manufactured from a wear resistant material with good properties at elevated temperature. State-of-the-art tools permit >1km of weld to be produced in 5mm thick aluminium extrusions without the need for tool changes. Tool wear during FSW of steel is much more significant, and a solution to this has to be found before FSW becomes commercially viable for steel. Development continues in this area.
The FSW tool has a profiled pin and a shoulder with a diameter larger than that of the pin. The pin length is similar to the required weld depth. The pin is traversed along the joint line, while the shoulder is in intimate contact with the top surface of the workpiece to provide consolidation. Many pin geometries are in use and under development. Fig.2 shows a prototype Whorl TM tool superimposed on a transverse section taken from75mm thick aluminium alloy AA 6082-T6 which has been welded from both sides.
Nippon Sharyo use friction stir welded panels produced by Sumitomo Light Metal Industries [6] for the floor panels of the new Shinkansen ( Fig.5 and 6). Some of these trains operate at speeds up to 285km/hour. Nippon Light Metal Co have also made use of friction stir welding for subway rolling stock. As early as 1998 they reported that over 3km of welds had been produced. The weld quality was confirmed to be excellent based on microstructural, X-ray and tensile test results.
In March 1999, Alstom LHB engineers considered friction stir welding of hollow aluminium profiles for making floor and side panels, but calculated that a three-shift operation would be necessary to achieve a return on investment inan acceptable time span. They estimated that the most significant technical and economic benefits could be achieved by applying FSW to aluminium joints of more than 12mm thickness. [7] This would replace mechanised MIG welding which necessitates the associated activities of pre-heating and grinding of intermediate beads. Additionally, it would lead to improved quality of the welds. Therefore, successful FSW experiments were conducted in up to 23mm thick aluminium plates, to demonstrate how MIG welds could be replaced in the underframe area of rolling stock.
Following certification by Deutsche Bahn AG, Sapa are now delivering to Alstom LHB floor panels which are 2.7m wide x 14.4m long. With the previous MIG welded product, Alstom were seeing problems with distortion. These created difficulties during robotic welding and added costs through the need for skilled rework. Floor panels for 97 train sets are being supplied, totalling 80km of FSW joined extrusions. In addition there are deliveries of side panels for150 train sets totalling 45km of FSW joined extrusions. Use of Sapa FSW extrusions has also been reported for the Alstom METROPOLIS underground coaches for the metros in Singapore, Shanghai and Warsaw.
Bombardier Transportation in Derby (United Kingdom) has carried out FSW experiments on butt and lap welds and has conducted fatigue tests at TWI. [8] They have stated that one of the major advantages of FSW is the ability to weld larger joints with reduced distortion. However, they concluded that investment in large purpose-built FSW machines was difficult to justify - partly due to insufficient volume of work. It has recently been announced that KMT-tekniikka of Finland is to supply Bombardier with FSW components up to 10mm thick. This Finnish firm is to buy its own FSW machine in 2003.
There are a number of FSW machine manufacturers around the world. ESAB has supplied the majority of machines in Europe, most recently a CNC controlled SuperStir TM gantry machine to DanStir (Denmark). This machine ( Fig.9) has a work area of 15 x 3 x 1m and is used for sub-assemblies in rail and other industry sectors.
SAPA's SuperStir TM machine has three welding heads, which makes it possible to weld from two sides of the panel at the same time, or to use two welding heads on the same side of the panel, starting in the centre of the workpiece and welding in opposite directions. Fabrications up to 14.5 x 3m may be accommodated.
These examples show that the European rolling stock industry is increasingly aware of FSW and the benefits it can bring. In December 2000 EuroStir ® was launched, a EUREKA part-funded project to promote the adoption of FSW and provide a forum for companies to investigate the potential of FSW without committing to the purchase of capital equipment and licences. This €8.5Mproject [9] which will last five years covers all industry sectors including rail. A EuroStir ® case study funded by Railway Safety, Angel Trains and HSBC Rail is planned to compare the quality of friction stir and MIG welds in appropriate aluminium alloys, with emphasis on dynamic performance. EuroStir ® currently has 39 collaborators including also CAF (Spain) and Alstom Transport (France) and is keen to see additional organisations join from EUREKA countries. Trials may be carried out on various types of FSW equipment, for example the state-of-art equipment shown in Fig.10 which iscapable of welding prototypes up to 8 x 5 x 1m.