The Effect of Radiation on Laser Welds for Fusion Applications

Abstract

A prime candidate for the future of low-carbon energy technology, nuclear fusion faces many materials and engineering challenges from radiation damage to maintenance. Concerns surrounding radiationinduced activation have lead to the development of EUROFER97 steel as a reduced-activation structural material, to be welded and joined in-service as part of a future fusion device. This thesis examines the changes in mechanical and thermal properties of previously unexplored laser-welded EUROFER97, hereafter referred to as Eurofer. A recently-developed non-destructive photothermal technique accessing thermal diffusivity properties, transient grating spectroscopy, has shown promise for use in the thin damaged layers generated using ion-irradiation. Firstly, this thesis presents an investigation into the use of this technique with low-activation singlecrystal elements iron, chromium, vanadium and tungsten under selfand silicon-ion irradiation at room temperature. No change in thermal diffusivity was observed in iron, chromium or vanadium up to damage levels of c. 1 dpa. Tungsten exhibited a drop in thermal diffusivity from 0.78(7) cm2 s −1 in the undamaged state to 0.29(2) cm2 s −1 at 1.668 dpa. Transient grating spectroscopy was found to be a good measure of single-crystal thermal diffusivity and can clearly distinguish changes due to irradiation from those due to grain boundaries. This technique was also assessed for use with polycrystalline Eurofer, comparing the thermal diffusivity of transient grating spectroscopy to that of laser-flash analysis; a systematic shift downward in transient grating spectroscopy by 15% was observed. Properties of laser-welded Eurofer in the as-welded condition and subsequently in a 1.9 MeV, 450 ◦C proton-irradiated state are then exam- ined. Weld sections heat-treated at 760 ◦C for 4 hours were similarly irradiated. Irradiated welded Eurofer was examined using nanoindentation, transient grating spectroscopy and X-ray diffraction with convolutional multiple whole profile fitting. A nanoindentation study found the centre of the weld to have a pile-up-corrected nanohardness of 4.0(4) GPa, decreasing to 2.1(3) GPa in the as-welded parent material. Irradiation temperature and post-weld heat treatment were both found to have a recovery effect on weld hardness, with the latter being entire. Proton-irradiation damage was not found to contribute to nanohardness at the temperature investigated. X-ray dislocation analysis found 1-dimensional defect-density to drop in the weld region upon irradiation from 66(9)×1014 m−2 to 15(6) – 5.8(8)×1014 m−2 . Irradiation was not found to be a strong influence on dislocation density at 450 ◦C, observable weld defects were annealed along with those generated by irradiation. Transient grating spectroscopy found the thermal diffusivity in parent Eurofer to be higher than in the weld by 16(8)%. Proton-irradiation facilitates recovery to the parent level in the weld. No observable change occurred in the parent. In sum, transient grating spectroscopy is able to distinguish both radiation damage and microstructure related changes to thermal diffusivity in single crystal materials; microstructure is distinguishable in EUROFER97. Further work is required to expand this young technique and understand the effects of microstructure on surface acoustic waves.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Preuss (Supervisor) & Ed Pickering (Supervisor)