Process-structure-property analysis of Ti-6Al-4V alloy printed using electron and laser beam additive manufacturing

Additive Manufacturing is increasingly being used for the fabrication of customised, geometrically complex metallic parts, particularly for use in the biomedical device and aerospace industries. This thesis focuses on Ti-6Al-4V alloy, which accounts for more than half of the total titanium production, due to its superior combination of unique properties such as excellent high strength-to-density ratio, corrosion resistance and biocompatibility. The objective of this PhD project is to gain a fundamental understanding on the process-structure-properties relationship of Ti-6Al-4V alloy printed using the electron beam (EB) and laser beam (LB), additive manufacturing (AM) processes. The AM processing factors investigated were divided into two conditions: process design factors and process parameters, in order to enhance the Ti-6Al-4V alloy properties. The process design factors were evaluated based on the impact of build height, build plate type, and build position. It was demonstrated that a microstructural gradient with part height (up to 170 mm) was obtained, for samples printed with the EB system. A significant factor influencing this change in microstructure with height, was found to be the heat sink properties of the stainless-steel build plate. An investigation of the effect of sample positioning on the LB build plate, demonstrated a small increase (0.01% to 0.09%) in printed part porosity, obtained on the side of the print bed, away from the argon gas inlet. However, no significant microstructural differences were observed indicating that the mechanical properties of the alloy were homogenous across the build area. For parts printed under the process design factors, these were also subjected for post-processing treatment, particularly using hot isostatic pressing (HIP), to examine the effect on part microstructure and mechanical performance. The influence of HIP treatment demonstrated coarsening of microstructure and pore closure, improving the ductility of the printed parts. The effect of process parameters were investigated based on systematically changing electron beam current, laser beam power and scan speed, while maintaining a constant input energy density. It was found that despite what appeared to be identical levels of EB energy input (1.967 J/mm2), obtained by systematically altering both the beam current (28 to 20 mA) and scan speed (4270 to 2440 mm/s), the resulting printed alloys exhibiting variations in both microstructures and porosity. This indicated that there were differences in how the alloy material absorbs the energy, as these two process parameters were varied relative to each other to maintain a constant input energy. An investigation into the impact of individual parameters revealed that the changes in the microstructural properties were particularly influenced by the beam current, while the scan speed had a greater impact on porosity generation. A further study examined the effects of process parameters with melt pool changes based on print line scans on the LB system. In this study, both numerical modelling and in-situ optical emission monitoring were carried out, to facilitate a greater understanding of the melt pool characteristics. It was demonstrated based that there was a direct correlation between the melt pool emission intensity obtained using the in-situ monitoring system and the temperature-time profile obtained from numerical modelling. The modelling output also provides an indication of the melt pool temperatures, as well as that of the dwell time of the melt pool at higher temperatures, both of which help explain changes in alloy phase composition. Overall, the findings of this study contribute to a better understanding of the AM process-structure-properties relationship, allowing for a more accurate parameters selection and further improving the properties of printed Ti-6Al-4V alloy.