New insights into 3D-printed materials for future fusion reactors

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. These materials must withstand extreme conditions: high temperatures, strong mechanical stresses and intense radiation. Crucial to their ability to do so is the material microstructure, which develops during the printing process. 

Interfaces between tungsten and steel in focus 

For 3D-printed components, tungsten is one of the metals particularly well suited to parts exposed to the hot plasma of nuclear fusion reactors. Steels serve as structural materials. 

Researchers from Paul Scherrer Institute PSI and Deutsches Elektronen-Synchrotron DESY investigated samples made of tungsten in combination with a special stainless steel with a characteristic microstructure. The samples were produced using a metal 3D-printing technique known as laser powder bed fusion. In this technique, metal powder is melted layer by layer using a laser and then rapidly solidified, gradually creating a 3D structure.  

At the interface between the two materials, complex microstructures can form during the printing process and influence the properties of the components. Using high-resolution X-ray techniques, the team could investigate the crystal structure and distribution of chemical elements within these interface regions to micrometre resolution.  

The measurements revealed that an unwanted intermetallic phase made of iron and tungsten forms at the interface. Such phases are detrimental for mechanical stability and potentially also for irradiation resistance of components. The unwanted phase could, they showed, be significantly reduced by adjusting the printing process.  

Large research facilities provide insight into complex microstructures 

Gaining a detailed understanding of what happens inside these materials requires advanced and complementary techniques available only at large research facilities, such as those at PSI and DESY. 

Operando experiments at the microXAS beamline of the Swiss Light Source SLS enabled the microstructures to be studied as they formed during the printing process. At the German synchrotron DESY, X-ray diffraction experiments gave high-resolution information on the crystal structure, whilst X-ray fluorescence experiments revealed the chemical composition of the material. These studies were complemented by neutron imaging at the Swiss Spallation Neutron Source SINQ, which gave insights into the mechanical stresses inside the printed components. 

“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.” 

Earlier study revealed unexpected steel phases 

In an earlier study, the research team discovered unexpected microstructures inside additively manufactured steels. Synchrotron measurements showed small amounts of a high-temperature form of steel known as retained austenite. Its presence is directly linked to the path of the laser during printing, while its amount depends on the energy delivered by the laser.  

The experiments gave 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.