High power Nd:YAG -laser welding in manufacturing of vacuum vessel of fusion reactor

Laser welding has shown many advantages over traditional welding methods in numerous applications for example in the field of transportation industry. The advantages are mainly based on very precise and powerful heat source of laser light which change the phenomena of welding process when comparing to traditional welding methods, for example GMAW. According to the phenomena of the laser welding, penetration is deeper and thus welding speed higher. Typical welding speeds, with few millimeters thickness of parts to be welded, are metres per minute. Because of the precise power source and high welding speed, the heat input to the workpiece is small and distortions are minimal respectively. Also the shape of laser weld is less critical for distortions than traditional welds. In the case of welding thick sections usability of lasers is not so practical than with thin sheets, because with power levels of present Nd:YAG -lasers depth of penetration is limited up to about 10 mm by single pass welding. One way to overcome this limitation is to use multipass laser welding, in which narrow gap and filler wire is applied. By this process thick sections can be welded with smaller heat input and then smaller distortions and process seems to be very effective comparing "traditional" welding methods not only according to the narrower gap. Another way to increase penetration and fill the groove is using so called hybrid process, in which laser and GMAW are combined. VTT Industrial Systems has been involved several in EU-ITER tasks in which suitable joining method has been considered for manufacturing of vacuum vessel of fusion reactor. Although the walls of vacuum vessel are made from 60 mm stainless steel, Nd:YAG -laser welding with some additions has shown great potential to be the joining method. One reason for using Nd:YAG -laser is the enormous size and weight of the vacuum vessel. So vessel will be constructed at the place of it's final placement. This together with the geometry of vessel set the positional welding requirements for the processes used and Nd:YAG -laser seems to be usable, because of the flexible transmit of laser ligth via optical fibre. In this paper results of several testruns made with Nd:YAG -laser are reported. Filler wire feeding as well as arc welding has been used as an addition with Nd:YAG -laser itself.