BY STEPHAN W. KALLEE, JOHN DAVENPORT, AND E. DAVE NICHOLAS

Modern railway carriages are increasingly produced from longitudinal aluminum extrusions with integrated stiffeners. The whole body shell can be made from either single-wall or hollow double-skin extrusions using this concept. This design approach can enhance the crashworthiness of vehicles because of the absence of transverse welds and the high buckling strength of the panels under longitudinal compression (Ref. 1).

Friction stir welding (FSW) was invented and patented (Refs. 2, 3) in 1991 at TWI, Cambridge, United Kingdom. Currently, 72 organizations hold licenses to use the process. Since the initial invention, there has been much activity throughout the world in the development of tool designs, applications, and friction stir welding machines resulting in more than 550 patent applications (Ref. 4).

Fig. 1 - Alstom LHB trains for DSB Danish State Railways during production. Friction-stir-welded roof panels for these trains are made at Hydro Marine Aluminium under a contract with Sapa.

Fig. 2 - Friction-stir-welded roof panel produced at Hydro Marine Aluminium for Sapa for delivery to Alstom LHB in Germany.

An important aspect that generates increasing interest in FSW is its potential to enhance the crashworthiness of aluminum vehicles that can otherwise fail in the heat-affected zone along weld joints. This has been observed in European accidents, notably in Eschede in Germany in June 1998 and Ladbroke Grove in Great Britain in October 1999. It was recommended in the report on the latter that consideration be given "to the use of alternatives to fusion welding" and "the use of improved grades of aluminum, which are less susceptible to fusion weld weakening" (Ref. 5).

Friction Stir Welded Aluminum Panels for Rolling Stock
The Scandinavian aluminum extruders Sapa and Hydro Aluminium were the first in Europe to commercially apply the friction stir welding process for the manufacture of single-wall aluminum roof panels for rolling stock applications. Since 1997, Alstom LHB in Salzgitter, Germany, has purchased these prefabricated panels for Copenhagen suburban trains - Figs. 1, 2. Since early 2001, they have used friction-stir-welded aluminum side walls and, since 2002, FSW floor panels for Munich suburban trains. These panels are made by Sapa.

Fig. 3 - Sapa in Finspång, Sweden, installed an ESAB SuperStir machine with three heads that can be used simultaneously to increase throughput.

Fig. 4 - DanStir in Copenhagen, Denmark, uses a CNC-controlled ESAB SuperStir machine with a work area of 15 x 3 x 1 m.

In March 1999, Alstom LHB engineers considered friction stir welding hollow aluminum profiles for making floor and side panels, but calculated a three-shift operation would be necessary to achieving return on investment in an acceptable time span. They estimated the most significant technical and economic benefits could be achieved by applying FSW to aluminum joints of more than 12 mm thickness. This would replace mechanized gas metal arc welding, which necessitates the associated activities of preheating and grinding of intermediate beads. Additionally, it would lead to improved quality of the welds. Therefore, successful FSW experiments were conducted on up to 23-mm-thick aluminum plates to demonstrate how gas metal arc welds could be replaced in the underframe area of rolling stock.

Bombardier in Derby, United Kingdom, has carried out FSW experiments for butt-joint and lap welds and has conducted fatigue tests at TWI (Ref. 6). It has stated one of the major advantages of FSW is the ability to weld larger joints with reduced distortion. However, it concluded investment in large purpose-built FSW machines is currently difficult to justify, partly due to insufficient volume of work. Using a subcontractor or job shop is now being considered.

Up to 16-m-long SuperStir machines have been designed, built, and commissioned by ESAB in Laxå, Sweden. One has been installed at Sapa and is used for the production of large panels and heavy profiles with a welding length of up to 14.5 m and a maximum width of 3 m - Fig. 3. This machine has three welding heads, which means it is possible to weld from two sides of the panel at the same time or to use two welding heads (positioned on the same side of the panel) starting at the center of the workpiece and welding in opposite directions. ESAB's newest gantry machine has now been installed at DanStir in Copenhagen, Denmark (15 x 3 x 1 m) - Fig. 4.

The EuroStir® Project
A number of European companies requested the provision of job shop services and low cost feasibility studies to share the cost of capital investment, licensing, and R&D efforts. Some of them proposed teaming up in a collaborative project with the overall objective of accelerating the use of friction stir welding in Europe. This EuroStir® project was launched in December 2000 and will last for five years. It is partially funded by EUREKA, which is a pan-European initiative for promoting collaborative research in advanced technology.

Fig. 5 - FSW demonstration using a SMT Tricept 805 robot during a EuroStir® meeting at GKSS in Germany.

Fig. 6 - The ESAB SuperStir machine at TWI, the world's largest laboratory FSW machine for welding prototypes of up to 8 x 5 x 1 m in the EuroStir® project (see www.eurostir.co.uk).

The research and development phase of the 6.8 million Euros project will take 21Ž2 years. Deliverables will be proven welding procedures for test pieces and prototypes in comprehensive detail. Equally important will be the detailed comparison between types of equipment (Figs. 5, 6), which will enable potential users to make informed investment choices. In a EuroStir® case study funded by Railway Safety, United Kingdom, and two rolling stock leasing companies, it is planned to compare the quality of friction stir and gas metal arc welding in appropriate aluminum alloys. It has been proposed to undertake small-scale static and dynamic tests, i.e., high strain rate tensile and drop-weight tests.

The dissemination phase of the EuroStir® project will be funded mainly by industry for two and one half years. It will involve seminars, workshops, and demonstrations. Manufacturing economics will feature strongly in this phase. A vital project achievement will be the establishment of at least 25 user organizations across Europe within five years. The project currently has 34 collaborators and is open to additional participants from EUREKA countries.

Friction Stir Welding in Japan
Hitachi of Japan uses the double-skin design of the car, which is constructed from friction-stir-welded aluminum extrusions. One of the reasons for this is the exceptionally low distortion of the FSW process. This contrasts markedly with the distortion that can occur when arc welding thin-gauge aluminum and eliminates the need for straightening and filling. To date, Hitachi has delivered a range of vehicles for both commuter and express use (Figs. 7, 8). These efforts have been recognized in Japan by the presentation of the prestigious Okouchi Award jointly to Hitachi and TWI.

Fig. 7 - Commuter train built by Hitachi with full-length friction stir welds of double-skin side and roof panels (welded from one side).

Fig. 8 - Express train built by Hitachi containing full-length friction stir welds of double-skin side and roof panels (welded from both sides).

Nippon Sharyo and another Japanese company have been using friction-stir- welded panels produced by Sumitomo Light Metal Industries (Ref. 7) for the floor panels of the new Shinkansen - Figs. 9, 10. Some of these trains operate at speeds up to 285 km/h. Nippon Light Metals has also made use of friction stir welding for subway rolling stock. By 1998, it reported more then 3 km of welds had been produced. The weld quality was confirmed to be excellent based on microstructural, X-ray, and tensile test results.

Fig. 9 - Trainsets with FSW floor panels of Sumitomo Light Metal operate on the Shinkansen in Japan.

Fig. 10 - Friction-stir-welded floor panel produced by Sumitomo Light Metal for Shinkansen trains.

Conclusions