Additive manufacturing of 316L stainless steel by electron beam melting for nuclear fusion applications

A feasibility study was performed to fabricate ITER In-Vessel components by one of the metal additive manufacturing methods, Electron Beam Melting (R) (EBM)(R). Solid specimens of SS316L with 99.8% relative density were prepared from gas atomized precursor powder granules. After the EBM (R) process the phase remains as austenite and the composition has practically not been changed. The RCC-MR code used for nuclear pressure vessels provides guidelines for this study and tensile tests and Charpy-V tests were carried out at 22 degrees C (RT) and 250 degrees C (ET). This work provides the first set of mechanical and microstructure data of EBM (R) SS316L for nuclear fusion applications. The mechanical testing shows that the yield strength, ductility and toughness are well above the acceptance criteria and only the ultimate tensile strength of EBM (R) SS316L is below the RCC-MR code. Microstnicture characterizations reveal the presence of hierarchical structures consisting of solidified melt pools, columnar grains and irregular shaped sub-grains. Lots of precipitates enriched in Cr and Mo are observed at columnar grain boundaries while no sign of element segregation is shown at the sub-grain boundaries. Such a unique microstructure forms during a non-equilibrium process, comprising rapid solidification and a gradient 'annealing' process due to anisotropic thermal flow of accumulated heat inside the powder granule matrix. Relations between process parameters, specimen geometry (total building time) and sub-grain structure are discussed. Defects are formed mainly due to the large layer thickness (100 mu m) which generates insufficient bonding between a few of the adjacently formed melt pools during the process. Further studies should focus on adjusting layer thickness to improve the strength of EBM (R) SS316L and optimizing total building time.(C) 2017 Elsevier B.V. All rights reserved.