A model for the future of fusion energy: the JET tokamak in the United Kingdom. The materials studied are closely related to those expected to be used in future reactors such as ITER. (Copyright: UKAEA, courtesy of EUROfusion)
An international research team from Hamburg and Villigen (Switzerland) has investigated how microstructures form in additively manufactured materials for future fusion reactors using modern synchrotron and neutron techniques. The researchers first discovered unexpected phases inside 3D-printed steels and now show in a second study which processes occur at the interfaces between different materials – and how these can be influenced by the printing process.
Additive manufacturing is considered a promising technology for producing components for future fusion reactors. Metal 3D printing makes it possible to create complex structures, for example breeding blankets or divertors – key components of a fusion power plant. However, the materials must withstand extreme conditions: high temperatures, strong mechanical stresses and intense radiation. A crucial factor is the microstructure of the materials which develops during the printing process.
The two studies were carried out in collaboration with the Paul Scherrer Institute (PSI) and DESY and have been published in the journals Additive Manufacturing and Materials & Design.
Interfaces between tungsten and steel in focus
The current study focused on samples made of tungsten in combination with a special stainless steel (AISI 415) whose microstructure mainly consists of the steel phases ferrite and martensite. The samples were produced using a metal 3D-printing technique known as laser powder bed fusion (PBF-LB/M), in which metal powder layer is melted using a laser and then rapidly solidified. This way, that material is added layer by layer building 3D structures. Tungsten is considered a key candidate for components directly exposed to the hot plasma of nuclear fusion reactors while steels serve as structural materials.
Of particular interest is the interface between the two materials where complex microstructures can form during the printing process and influence the properties of the components. Using high-resolution X-ray techniques – micro-X-ray diffraction (μXRD) to analyse the crystal structure and micro-X-ray fluorescence (μXRF) to determine the distribution of chemical elements – the team investigated these interface regions with micrometre resolution. The measurements revealed that the intermetallic phase Fe₇W₆ forms at the interface.
The study also shows that the formation of the unwanted Fe₇W₆ phase can be significantly reduced by adjusting the energy input during the printing process using a layer-wise energy-grading strategy. In addition to phase formation, the team also investigated mechanical stresses inside the printed components using neutron Bragg Edge Imaging at PSI.
Synchrotron techniques provide insight into complex microstructures
Many of the structures inside additively manufactured metals are difficult to study using conventional techniques. Experiments at the PETRA III beamline P06 at DESY provided high-resolution information on the crystal structure and chemical composition of the material. Formation of such microstructure was then observed using operando experiments at microXAS beamline at PSI: “Only by combining several advanced techniques can we track how microstructures develop during the additive manufacturing process,” says Malgorzata Grazyna Makowska from PSI. “This allows us to analyse chemical composition and crystal structure simultaneously.”
Measurements at the PETRA III light source at DESY form a central part of the study and enable a detailed analysis of the phase structure within the materials. “Using high-energy micro X-ray diffraction, we can look directly inside such materials and map their phase structure in three dimensions,” explains Gerald Falkenberg, head of the PETRA III beamline P06. “This allows us to reveal structures that remain hidden when using conventional techniques.”
Earlier study revealed unexpected steel phases
In an earlier study, the research team had already discovered unexpected microstructures inside additively manufactured steels. Synchrotron measurements showed that small amounts of a high-temperature phase can remain in certain regions of the material in the form of retained austenite. The distribution of the unexpected crystalline phase is directly linked to the path of the laser used for manufacturing of this material, while its amount depends rather on amount of the energy that is delivered to the material by laser processing.
The experiments at DESY and PSI gave us insight into the presence of the retained austenite which could not be detected with any other technique due to its small amount and metastability.
“The microstructure of additively manufactured metals is often more complex than expected,” says Ken Vidar Falch, DESY scientist and co-author of the studies. “By combining several high-resolution techniques, we can now examine these structures much more precisely and understand how they are influenced by processing conditions.”
New insights for fusion energy materials
The results provide important insights into how microstructures and stresses develop in additively manufactured materials for fusion reactors. This knowledge could help optimise the 3D printing of metals and improve the reliability of such components.
“In the future, we would like to investigate minor phases forming at the interface region in three dimensions also for other 3D-printed multi-materials which will require higher spatial resolution,” says Malgorzata Grazyna Makowska. “With PETRA IV we will be able to investigate such microstructures much faster and with significantly higher sensitivity and resolution,” elaborates Gerald Falkenberg. “This will allow us to analyse additive manufacturing processes and their effects on material structures systematically and in three dimensions.
(partly from DESY News)
References:
Garrivier et al., Multimodal synchrotron characterization of the formation and spatial distribution of retained austenite in PBF-LB/M-manufactured ferritic–martensitic steel, Additive Manufacturing (2025), DOI: 10.1016/j.addma.2025.105055
Garrivier et al., Microstructural Effects of Tungsten Deposition on 415 Steel During PBF-LB/M Additive Manufacturing of Plasma Facing Components, Materials & Design (2026), DOI: 10.1016/j.matdes.2026.116057
