Additive Manufacturing of refractory metals for Nuclear Fusion applications

The research described in this thesis aimed at analysing the processability of pure tungsten and tungsten alloys through the Additive Manufacturing (AM) technology called Laser-Based Powder Bed Fusion (PBF-LB), to develop a manufacturing process suitable for the realization of components used in nuclear fusion and high-energy applications. Indeed, tungsten-based materials are currently considered the best candidates for producing parts that will be placed in direct contact with the plasma inside nuclear reactors, thereby creating the so-called plasma-facing components (PFCs). However, these materials are very difficult to machine through the traditional subtractive manufacturing technologies, due to some of their most peculiar characteristics, like their elevated hardness, the high melting point, and their brittleness at relatively low temperatures. This puts some serious limits on the realization of products with complex shapes. Nevertheless, despite the fact that additive manufacturing allows almost complete freedom in geometry, the AM technique mentioned above does not guarantee the achievement of high-quality parts made of tungsten. Indeed, pure tungsten is well-known for being heavily affected by cracks and porosity when processed with PBF-LB, even when the printing parameters are properly optimized. With this study, great attention was dedicated to the creation and characterization of binary tungsten-tantalum alloys: in addition to pure tungsten, pre-mixed powder blends were also created and processed. Three different chemical compositions were tested, in which the tantalum ponderal content was varied: 2.5%, 7.5%, and 15%. For all the compositions, an initial phase was dedicated to optimizing the process parameters, in order to achieve parts characterized by the least porosity, and, more generally, a minimized presence of internal defects. The powders were processed using two different AM machines: EOS M100, which is provided with a focused infrared (IR) laser that emits a beam with a maximum nominal power of 200 W, and AMCM-EOS M290, which is equipped with a laser having the same wavelength as in the previous system (about 1064 nm), with a maximum power of 1 kW. After having identified the best manufacturing parameters for each material, samples for material characterization tests were built.