ADDITIVE MANUFACTURING OF COPPER AND COPPER ALLOYS FOR HIGH ENERGY APPLICATIONS

In this research activity, the additive manufacturing technology called Laser Powder Bed Fusion (LPBF) was employed to assess the processability of a copper-based alloy and pure copper powders to create components for nuclear fusion reactors. In particular, the research aimed to characterize the materials for building the accelerating grids for a Neutral Beam Injector (NBI) of a tokamak. Since the printing of pure copper is still challenging, the copper alloy has been preferred and investigated more in detail. The CuCrZr alloy was chosen for the study since its composition makes it suitable for its application in nuclear fusion. Additionally, the alloy is mechanically superior to pure copper, and many suppliers can produce the material. The study began with optimizing the printing process, seeking the most appropriate parameters to provide the lowest porosity in the built parts. Then several heat treatments were performed on the CuCrZr alloy to enhance the material’s performance, either in terms of mechanical properties or thermal and electrical conductivity. Two machines were employed: the EOSint M280 was initially used, provided with a low-power IR laser (400 W of nominal power), and then the research moved to the AMCM EOS M290, with a 1 kW IR laser of the same wavelength. From the study, only a laser power above 500 W can successfully print the CuCrZr alloy to obtain parts without defects. However, after the process optimization, the porosity inside the pure copper parts was still 10 times higher than in the CuCrZr material (0.222 % and 0.028%, respectively). The alloy was revealed to be promising since the heat-treated material reached conductivities equivalent to the as-manufactured pure copper, keeping the mechanical properties higher than the pure copper ones. The study proceeded with optimizing the surface of the as-built samples, to obtain the best surface finish in the case of inclined surfaces or internal cooling channels that cannot be inspected otherwise. The analyses carried out during the research were several, such as the microstructure observations with optical and electron microscopy. The investigations of some samples with a Transmission Electron Microscope (TEM) were also carried out, to understand the precipitation mechanism of the alloying elements inside the CuCrZr alloy after the heat treatments. Subsequently, hardness was measured and, in some cases, microhardness was investigated, too, both in as-built conditions and after treatments for CuCrZr and pure copper. Electrical conductivity was assessed through eddy current measurements, and other tests were carried out to complete the characterization, such as tensile tests, thermal conductivity measurements, Differential Scanning Calorimetry (DSC), and X-Ray Diffraction (XRD). The thermal conductivity was measured through an in-house-made setup developed at the University of Padova to recreate a 1-D steady-state thermal flow.