Study and characterisation of different metal alloys processed through Laser Powder Bed Fusion

Additive Manufacturing (AM) techniques inspired a substantial revolution in the way of concept and produce metal components for industry. Among all the available AM processes, Laser Powder Bed Fusion (LPBF) inspired a noticeable series of investments, studies and standardisation routes since the great interested it acquired in several industrial sectors. In the past years numerous researchers demonstrated how this process produces metal components with innovative and unprecedented microstructures and mechanical properties, disclosing new horizons in the scientific and industrial research. This thesis took under investigation the study and characterisation of three different metal alloys, A357 aluminium alloy, Ti-6Al-4V titanium alloy and pure copper, respectively, all processed by LPBF. Furthermore, the investigation of different post-processing heat treatments was took under study. The processed samples, as well the metal powders used, were characterised by microscopic and macroscopic analyses. The study on A357 aluminium alloy processed by LPBF investigated the process parameters necessary to build full dense components for industrial applications. By correctly combining hatching distance and scanning speed it was possible to fabricate completely dense specimens keeping a good productivity rate. Moreover, the effects of different heat treatments on specimens microstructure and mechanical properties were studied. Particularly, a stress relieving and a subsequent T6 precipitation hardening treatment were performed on the full dense LPBF parts, investigating the effects of different temperatures and durations in the case of T6 treatment. Longer solution treatments enabled to obtain higher hardness values and to reduce the time required to reach peak hardening conditions during ageing. While stress relieving strongly softened the material, a maximum hardness comparable to as-built parts conditions was obtained after subsequent 8 h solution treatment, water quenching and 3 h ageing treatment. Stress reliving treatment slightly modified the as-built microstructure by favouring the diffusion of Si but did not removed the melt pool structures present, furthermore it noticeably increased the elongation at break to detriment of tensile strength. Further T6 treatment modified the tensile properties to values comparable with the as-built conditions eliminating melt pools anisotropic features. The study on Ti-6Al-4V titanium alloy investigated the microstructural, tensile and fatigue properties of the LPBF fabricated parts, produced with two different gas atomised powders. The two powders contained two level of oxygen inside the chemical composition, a low and a high amount, in order to simulate the LPBF processing of Ti-6Al-4V ELI and Ti-6Al-4V grade 5, respectively. Two different building orientations, vertical and horizontal, were chosen for the specimens fabrication and moreover three different testing conditions were considered: after stress relieving, after stress relieving plus heat treatment and after stress relieving plus Hot Isostatic Pressing (HIP). Processing a subsequent heat treatment after stress relieving reduced tensile strength and increased ductility by coarsening α + β lamellar structure while β columnar grains faded. HIP post-processing closed the major part of porosities and defects and enabled to greatly increase both ductility and fatigue resistance. Pores and defects were detected as the most influencing factors upon the fatigue properties, rather than building orientation and oxygen content, which mostly influenced tensile strength. Only stress relieved and HIPped samples resisted more than the chosen endurance limit of 107 cycles at high applied strength than the other specimens. The study carried out on pure copper investigated the feasibility of processing such material with LPBF using a commercial machine equipped with an infrared 200 W fibre laser. The specimens fabricated did not exceed the 83 % of density due to the low absorptivity of copper to infrared radiation, but Diffuse Reflectance Spectroscopy (DRS) analysis demonstrated how modifying the laser radiation from infrared wavelengths to the green ones, the powder bed absorption raised. As-built samples did not present oxides traces inside the microstructure and were constituted by α-Cu phase. The microstructure was constituted by both equiassic and elongated grains depending on the heat fluxes generated inside the material in the horizontal and vertical cross sections.