Laser-based additive manufacturing of refractory metals and their alloys: A review

The system of refractory metals and their alloys appear to be promising candidate materials for additive manufacturing. These materials are challenged by a combination of high raw material costs, complex geometries and applications as well as machining difficulties, which additive manufacturing can help alleviate. Pure tungsten produced by laser-based additive manufacturing suffers from cracking due to its brittle nature. The cracking could be improved but not eliminated through optimisation of the build process parameters and scan strategies. Alloying to improve ductility and reduce oxygen contamination at grain boundaries proved more successful. Pure molybdenum faces similar challenges with cracking due to brittleness, which could be limited by process optimisation, but more effectively optimised by alloying and grain refinement. Tantalum has almost solely been produced as the pure material and significant research has focussed on manufacturing porous lattice structures for implant applications. Research on niobium and niobium-based alloys has been more limited, but their higher inherent ductility makes cracking less likely. Only a single study produced pure rhenium by additive manufacturing. This contribution will review and provide an overview of the state of the research field regarding laser-based additive manufacturing with a specific focus on the resulting microstructures, densification levels, cracking behaviour and mechanical performance of these materials. Furthermore, this review will provide suggestions for the direction of research for each refractory metal alloy system within the field of additive manufacturing.