A Demonstration (DEMO) thermonuclear reactor is the next step after the International Thermonuclear Experimental Reactor (ITER). Designs for a DEMO divertor and the First Wall require the joining of tungsten to steel; this is a difficult task, however, because of the metals’ physical properties and necessary operating conditions. Brazing is a prospective technology that could be used to solve this problem. This work examines a state-of-the-art solution to the problem of joining tungsten to steel by brazing, in order to summarize best practices, identify shortcomings, and clarify mechanical property requirements. Here, we outline the ways in which brazing technology can be developed to join tungsten to steel for use in a DEMO application.

For the manufacture of thin-walled precision structures with complex geometry, the method of brazing is used. It allows to obtain joints with a given structural-phase state, high mechanical characteristics and resistance to corrosion. This overview examines alloys based on copper, precious metals, nickel and iron, which are actively used for joining steels. The data on the operational characteristics of the joints obtained by brazing using nickel-based filler metals are presented and structured. Special attention is paid to nickel braze alloys based on Ni – B, Ni – Si – B, Ni – Cr – B, Ni – Cr – Si, Ni – Cr – Si – B, Ni – Mn, Ni – Cr – P, Ni – Si – Be systems. Such alloys are the most widespread and applicable throughout the world. The overview describes the main phases that are formed in the seams and the heat-affected zones of joints. The dependences of the technological parameters of the process, such as the temperature-time mode of brazing and the size of the gap on the microstructure of the joints and their mechanical characteristics, are shown. The corrosion resistance of steel joints brazed by various nickel-based filler metals is considered. The overview is based on the extensive experience of MEPhI in the production and development of rapidly solidified amorphous-crystalline foils based on nickel. In addition to the world experience, the overview considers works of the research team of the MEPhI laboratory, which has been working on this topic since 1995. This overview will help technologists and designers of energy-intensive equipment to determine the trajectory for the selection of the filler metal composition to obtain the optimal technological parameters, mechanical characteristics and corrosion resistance of the joints applicable to a specific design problem. The reported study was funded by RFBR, project number 20-12-50106.

The knowledge of alloy–process–structure–property relationships is of particular interest for several safety-critical brazed components and requires a detailed characterization. Thus, three different nickel-based brazing filler metals were produced with varying chromium and molybdenum content and were used to braze butt joints of the austenitic stainless steel AISI 304L under vacuum. Two holding times were used to evaluate diffusion-related differences, resulting in six specimen variations. Significant microstructural changes due to the formation and location of borides and silicides were demonstrated. Using X-ray diffraction, alloy-dependent residual stress gradients from the brazing seam to the base material were determined and the thermal-induced residual stresses were shown through simulations. For mechanical characterization, impact tests were carried out to determine the impact toughness, as well as tensile tests at low and high strain rates to evaluate the strain-rate-dependent tensile strength of the brazed joints. Further thermal, electrical, and magnetic measurements enabled an understanding of the deformation mechanisms. The negative influence of brittle phases in the seam center could be quantified and showed the most significant effects under impact loading. Fractographic investigations subsequently enabled an enhanced understanding of the fracture mechanisms.

The investigation results of alloys of the Ni–Si–Be system with a various content of Si and Be are presented. Two low-melting eutectics with melting temperatures of the order of 1100 and 915°C have been found out by the methods of differential thermal and X-ray phase analysis and metallography. Two alloys have been selected for the development of brazing filler metals: Ni–5Si–3Be and Ni–6Si–5Be alloys. Three phases have been shown to be components of the binary and ternary eutectics in the Ni–Si–Be system from the side of the nickel angle of the phase diagram. A Ni–5Si–3Be ribbon and fragments of the Ni–6Si–5Be ribbon have been obtained from the selected alloys by rapid quenching. Filler metal of the Ni–5Si–3Be system has been used for brazing samples of the 12Cr21Ni5Ti austenitic–ferritic corrosion-resistant steel at 1150°C for 30 min.