Multiple beams Yb:fibre laser welding process for Grade 2205 duplex stainless steel

Abstract

This thesis describes the development of a Yb:fibre laser welding process for Grade 2205 duplex stainless steel (DSS) using a multiple beam approach. Grade 2205 DSS can be found in many structural applications, and in particular in the UK, for the manufacturing of intermediate level nuclear waste (ILW) containers made of 3 and 6 mm thick plates. To ensure good weld integrity, Yb:fibre laser welding was the choice of welding process used for these nuclear containers. The existing welding practice requires filler wire feed to maintain the microstructure in the weld. However, filler wire adds cost and complexity to the process which often leads to high failure rate. For this reason, the possibility of removing the requirement for filler wire is very attractive. One possible way of doing this is to tailor the laser beam’s energy distribution delivered to the workpiece to influence the material’s cooling rate and therefore the resulting microstructure in the weld. In this study, tailored energy distributions were achieved by impinging multiple beams onto the work-piece, which were produced using either a beam splitter module (dual beam) or a diffractive optical element (DOE). Experiments included welding trials using a single beam, dual beam and diffracted beam. In all experiment, the temperature during welding was recorded, and the resulting weld microstructure was analysed through metallography assessment, point counting, Vickers hardness testing and corrosion testing. In addition, electron backscatter diffusion (EBSD) imaging and energy-dispersive X-ray spectroscopy (EDX) have also been carried out on selected samples. These techniques offer in-depth analysis of the crystal size, orientation, shape, and phase which define the weld properties. Process optimisation using single beam at welding speed of 1 to 1.5 m/min showed that the use of pure nitrogen shielding gas during welding improved the austenitic microstructure by up to 15% when compared with welding shielded with pure argon gas. The weld nevertheless resulted in ferrite phase volume fraction which was over the acceptable threshold of 70% necessary for its long-term corrosion performance. Dual beam welding of 3 mm plates achieved the reduction of the ferrite phase volume fraction within the desired range (30-70%). These results were supported by the improvement of the corrosion test outcome in comparison with single beam results. The DOE used in this study was characterised and a bespoke welding head allowing quick access to the optics was designed and manufactured. Results with 6 mm plates showed a local reduction of the ferrite volume fraction around 60% at the weld cap. In conclusion, the investigation in this doctorate study showed that autogenous single beam welding of grade 2205 DSS is possible providing a slow welding speed and that the undesirable ferrite volume fraction excess can be reduced to an acceptable amount by using a tailored energy distribution.