Abstract
Nuclear fusion is a potential source of electricity which can address the environmental problems posed by fossil fuels. Eurofer97 steel is a primary structural material for vacuum vessels and test blanket modulus in fusion Tokamaks, designed to contain self-sustaining tritium breeding. Maintaining the structural integrity of such fusion reactor components remains a severe challenge. Laser welding, performed by robotic devices, is one of the most promising joining techniques for assembly and maintenance, although it induces a large amount of residual stress. The interaction of the residual stress and the heterogeneous microstructure degrades the mechanical performance of fusion components.
The present study investigates the residual stress distribution in laser-welded Eurofer97 joints. Efforts have been made to establish a novel time-resolved, multi-scale residual stress measurement using a plasma focused ion beam and digital image correlation (PFIB-DIC) ring-core method. The results have been cross-validated with neutron diffraction, high-resolution neutron imaging, and nanoindentation techniques. The residual stress derived from the different techniques was consistent. Neutron tomography was employed to inspect the volumetric strain distribution and visualise the strain state inside the joint.
The mechanistic relationships between microstructure, mechanical properties, and residual stress were also established using microstructural characterisation and nanoindentation techniques. A 30% hardening effect caused by residual stress was found. The underpinning deformation mechanism was further explored using in situ neutron diffraction and lab-based tensile-DIC experiments. Residual stress was identified as the primary strengthening mechanism during the initial deformation stage, whilst microstructural strengthening predominates as deformation grows. The time-resolved and multi-scale residual stress, and the underpinning understanding of its impact on mechanical properties, provide critical insight into developing predictive tools for lifetime analysis of the structural integrity of fusion components.