Laser welding train-coach wall panels

Jukka Siltanen, Ilpo Maaranen, and Ville-Matti Nurmela

New standard for the manufacturing of railway applications

Nowadays, the manufacturing of railway applications in Europe is done according to the EN 15085-2 standard. It relieves the DIN 6700 series of German standards used widely in Europe in the manufacturing of railway applications. The EN 15085-2 standard defines a certification level for parts and components of railway applications. Four certification levels (CL1-CL4) are laid down for the certification of a welding manufacturer. The certification level set for this project was CL1. The certification level CL1 requires that the manufacturer has a valid welding quality system according to the ISO 3834-2 standard and furthermore there are some additional requirements. The EN 15085-2 standard also defines the weld performance class, which takes into account the stress and safety category, weld performance, and inspection class.


Project development was challenging, but successful

The development work on a new wall panel that complies with certification level CL1 (see sidebar "New standard for the manufacturing of railway applications" on facing page) was done in close cooperation between Ruukki Metals (Uusikaupunki, Finland), Rautaruukki Oy (Hämeenlinna, Finland), and Alstom Transport (Saint-Ouen, France) rail transport systems. Ruukki Metals and Alstom Transport worked together to develop a new solution for the assembly of wall panels for train coaches. The steels used for the panels were supplied by Rautaruukki, and the laser-welding of the wall panels was performed at the Ruukki Metals service center. The goal of the cooperative effort was to reduce the weight of train coaches, strengthen some of the critical parts of the panel and shorten the production lead time, and to produce a more accurate and flatter wall panel.

Manufacturing laser-welded panels

The production chain of the wall panel has the following phases and inspections:
• arrival of material (visual inspection and verification of material)
• laser cutting (visual inspection and measuring of cut quality if necessary)
• laser welding (visual, penetrant and radiographic inspection and reporting)
• repair welding of the laser-welded panel if needed (inspection)
• dimension measuring of the panel (measuring of the dimensions and reporting)
• packaging and shipping to the customer

Steels used for the wall panels are the thermo-mechanically hot-rolled, high-strength steels Optim 500 MC and Optim 700 MC with 2 mm thicknesses. Optim is a trademark for the steel manufactured by the Finnish steel mill Ruukki Metals Oy. The base and main material of the wall panel is a high strength steel, Optim 500 MC, but in certain places on the panel, calculated to be critical, some steel is cut away and replaced with a higher strength steel, Optim 700 MC. The joint type used in the laser welding of panels is a butt joint. The average size of the panel is 3000 x 18,000 mm with a ± 1 mm tolerance for the main dimensions. The total length of the weld in one panel is typically more than 30 meters. The length of the laser weld in a ready-made panel is smaller because some material is cut away from the places for doors and windows openings. The laser welding and the inspection of the welds are done according to standards ISO 15614-11, ISO 13919-1, and EN 15085-2.

Welding of wall panels

The quality target for the welding was class C (intermediate, ISO 13919-1) corresponding to the weld quality CPC2 according to the EN 15085-2 standard. Two laser welding stations (MC1 and MC2) equipped with CO2 lasers were used (FIGURE 1). The maximum laser power is 5 kW for welding station MC1 and 8 kW for welding station MC2. The edges of all the steel plates were laser-fusion cut before welding.

Laser welding with the laser system MC2.
FIGURE 1. Laser welding with the laser system MC2.

The welded joint type was a butt joint, and different material combinations were welded. A close to zero gap was needed between the plate edges in laser welding. At the welding station MC1, laser welding was used for tack welding, and at the welding station MC2 laser welding and also GTAW (gas tungsten arc welding) without filler material were used for tack welding. At the welding station, MC1, a continuous weld, was welded in one pass. The welding lines for the parts joined at the welding station MC2 were complex, and to make sure that there was a continuous and high-quality weld along the whole length of the joint, three welds were made next to each other.

The welding parameters used were almost the same for both welding systems. The average laser power used was 3 kW, and travel speed was 2.3 m/min. The welding energy for one weld was 0.08 kJ/mm.

Weld quality testing

Destructive and non-destructive tests were included for the test pieces of the welding procedure specifications record (WPQR) and non-destructive tests for laser-welded wall panels. The inspection of the wall panel was done according to inspection class CT2, which means 100% extent for visual examination and 10% extent for surface crack detection (penetrant testing) and radiographic examination. For some critical parts of the panel, the extent of surface crack detection and radiographic examinations was set higher than 10%. For the first panel of a new series, the extent for all the examinations was 100%.

Visual, penetrant, and radiographic examinations

The EN 15085-2 certificate requires that there are some qualified and trained visual inspectors in-house. These engineers were qualified according to standard EN 473 for visual examination to NDT level 2. The laser operators were trained to control the welding quality independently during their daily work. This procedure decreased the costs of inspection notably because most of the welding imperfections were noticed and fixed in the early phase. If imperfections are noticed by an inspection authority and defective parts are returned for fixing and finally inspected again, increased inspection costs are incurred. Despite this kind of procedure, some imperfections were found during visual examination. The typical imperfections present were incomplete penetration and single pores. Pores were typically open to the surface, so it was possible to find these visually. The main reason for these imperfections was a gap that was too large between the edges of the steel plates.

The surface crack detection was made by a penetrant test according to the EN 15085-2 standard for the inspection of wall panels. The penetrant testing of the wall panels did not reveal any new weld defects in addition to what had already been found during the visual examination. Reported imperfections in the penetrant testing were incomplete fillet groove, incomplete penetration, and single gas pore.

The radiographic examinations of wall panels were performed by a new digital radiographic examination system. The usability of the system was verified by a comparative test with the conventional radiographic examination system. The new type of system was used because the number of pictures taken was huge and the time available for the inspections was limited. The total number of pictures taken was impressive; in some cases more than 60 were taken. The following imperfections were reported for the laser-welded wall panels: excess weld metal, incomplete fillet groove, undercut, weld spatter, single gas pore and uniformly distributed porosity.

Repair welding

The repair welding of the wall panel took place with a GTAW, and it was outsourced. The repair welding was performed by welders qualified according to the EN 287-1 standard. The filler wire used for the welding was Esab Tigrod OK 13.13 (Mn3NiCrMo) with a diameter of 1.0 mm. The repair welding was done manually, and a copper plate as a backing was used to cool down the place of repair. Argon (99.99%) was used as a shielding gas.

Assembled laser-welded wall panel.
FIGURE 2. Assembled laser-welded wall panel.

Dimension measuring

The main dimensions of the welded panels were measured. Some deviations of dimensions were reported, but none of the panels were rejected because of these. The main reason for the deviations of some dimensions was the repair welding of the panels by GTAW. Generally speaking, to stay inside the tolerance range of dimensions in the laser welding of the coach was easy if repair welding of the wall panel was not needed.

Conclusions

The project for the development of the new type of laser-welded train coach wall has been long, difficult, and challenging. However, step-by-step processing has been improved, and the quality level is good and stable. The properties of the wall panels have proved to be excellent, and the customer is satisfied. The panels are light and even, and the dimensions are accurate, which makes them easy to assemble in later working phases.

The new type of laser-welded train coach wall panel seems to have fulfilled the promise of a success story in the use of laser technology (FIGURE 2).


Jukka Siltanen (jukka.siltanen@ruukki.com) is an application expert at Rautaruukki Oy Hämeenlinna, Finland; and Ilpo Maaranen is a plant director and Ville-Matti Nurmela is a development and quality manager at the Ruukki Metals service centre in Uusikaupunki, Finland.

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