Haibiao Ji

Addressing the requirement for low magnetic permeability in fusion reactor structural materials, this study fabricated 316 L austenitic stainless steel via selective laser melting (SLM). Systematic characterization of microstructural and property features was conducted through metallographic observation, mechanical testing, and magnetic permeability measurements. Microstructural analysis reveals that SLM-formed specimens exhibit typical melt pool stacking morphology with columnar grains growing preferentially along the build direction, while rapid solidification induces localized precipitation of $\delta $ -ferrite phase. Mechanical tests demonstrate significant anisotropy, where both tensile strength and impact toughness in the horizontal plane ( $Y$ -direction) are superior to those along the build direction ( $Z$ -direction). Magnetic permeability measurements indicate that the relative magnetic permeability of SLM-formed samples is slightly higher than conventionally forged 316 L, which correlates closely with the presence of $\delta $ -ferrite. The research confirms SLM technology’s potential for manufacturing fusion reactor structural components, though process parameter optimization or postbuild heat treatment are required to balance mechanical properties with permeability requirements.

With the progress of the large-scale scientific facilities, the vacuum system and the stability of materials have shown the need for a higher level of reliability to ensure the stable operation of the beam and provide an ultrahigh vacuum environment. The key component such as the vacuum vessel of the Tokamak fusion experimental reactor, has a high requirement for its relative magnetic permeability, and the steel used must meet the relative magnetic permeability of less than 1.05 or even 1.03; therefore, the control of the magnetic permeability is of great significance for the development of nuclear fusion testing devices and various accelerators. This article conducts research on the issue of magnetic permeability exceeding standards in engineering applications of 316L (N) austenitic stainless steel plates, bolts, and other components. It is found that nickel element composition plays a decisive role in controlling magnetic permeability. In addition, bolts with relative magnetic permeability exceeding standards were demagnetized at 1050 $^{\circ}$ C, and it was found that their magnetic permeability was controlled, but their sealing performance decreased.

The vacuum vessel (VV) as an ultrahigh vacuum pressure vessel is one of the essential components of the Chinese Fusion Engineering Test Reactor (CFETR) device. VV has strict position tolerance. The VV port stub is a typical large-scaled welded structure manufactured by electron beam welding (EBW) technique. Welding has the most significant influence on deformation. Therefore, it is essential to analyze welding deformation. A systematic welding simulation method aimed at welding deformation controlling for a large EB welded structure is proposed in this article. First, the dynamic process of EBW of 50-mm-thick ultralow carbon austenitic stainless steel was simulated based on the thermal elastoplastic theory. The weld forming mechanism of high energy density welding of the thick plate was revealed. Then the inherent strain extracted from the results of the simulation of local welding was introduced to the simulation of tailor welding of the port stub. Finally, optimal welding sequence and clamping conditions were obtained, and the reliability of the simulation was verified by the profile measurement results of the inner shell of the upper port stub.

The 316L thick plate electron beam welding (EBW) has been widely used in fusion test reactor manufacturing. Therefore, the numerical simulation of the 50 mm 316L austenitic stainless steel by two heat sources and experimental on microstructure and residual stress have been studied in this article. In the simulation study, the traditional heat source model (3D Gaussian heat source) and composite heat source (double ellipsoid heat source superimposed on the 3D Gaussian heat source) were proposed to simulate the welding of local joint. Weld cross-section, temperature curve, and residual stress after welding obtained by simulations were investigated. The experimental study involved residual stress tests and microstructure analysis. It turned out that the result of the composite heat source was closer to the actual joint. The residual stress distribution of simulation was validated and in accordance with experimental measurement. Moreover, the microstructures were studied by electro backscattered diffraction (EBSD) and compared with the temperature curve. The formation mechanism of microstructural heterogeneity was caused mainly by different thermal cycles at different positions of the thick plate. The top of the joint was more prone to stress concentration.

The China Fusion Engineering Test Reactor vacuum vessel has strict tolerances requirements and a large number of fully penetrated joints in the manufacturing and assembly processes. As the most complicated component of vacuum vessel, port stubs have stricter tolerance requirements, to guarantee the high assembly accuracy with the vacuum vessel. Due to the small deformation of electron beam welding, this method is adopted for the welding of port stub. However, welding deformation cannot be avoided, thus it is necessary to carry out finite element analysis to analyze and control the welding deformation of the port stub. In this paper, based on thermo-elastic–plastic theory, the double ellipsoid and cone-shaped hybrid heat source can accurately outline the electron beam weld pool contour of the 316L stainless steel with 50 mm thickness, which is further verified by the welding process experiment. The above hybrid heat source is applied to analyze the welding deformation of the lower port stub, and according to the deformation simulation results, the welding fixture is designed to reduce the welding deformation. The welding deformation analysis can effectively provide theoretical data support and practical guidance for the actual electron beam welding process of port stub.

Laser trackers are widely used in product inspection in the manufacturing process of the vacuum vessel (VV) for the China Fusion Engineering Test Reactor (CFETR). Due to the influence of temperature change on the geometrical properties of metallic materials, temperature change seriously affects the measurement accuracy of the workpiece. Therefore, temperature compensation is important to reduce the measurement errors caused by temperature change. This study uses 1/32 VV mockups as the research object. According to the results of finite element analysis and the onsite measurement results of the 1/32 VV at different temperatures, a method of temperature compensation for the 1/32 VV at different temperatures is summarized.