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Maree_27361900_2024.pdf
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- This master’s thesis aims to investigate the manufacturing challenges and mechanical properties of Ti-12Mo lattice structures fabricated via SLM by using prealloyed powder as a feed material. Two unit cell topologies are examined in this work to capture the two different types of mechanical behaviour exhibited by lattices structures: octet trusses for stretching-dominated behaviour and gyroids for bending-dominated behaviour. The defects inherent in SLM manufacturing, such as roughness, strut size variation or internal porosity are studied on as-built samples through optical microscope measurements, SEM imaging and X-ray tomography. Both spherical and irregular pores were observed and their origin was discussed. Hardness measurements were conducted to identify the presence of isothermal ω phase leading to crack formation. The optimization of the heat treatment of SLM-manufactured lattices structures was also assessed. XRD and SEM imaging were used to analyze the microstructure of heat treated samples. Different types of heat treatments were tested to retain a fully β-metastable microstructure. However, the precipitation of α phase was observed either due to oxidation or a too slow cooling rate. The mechanical properties of the heat treated SLM-manufactured Ti-12Mo lattice structures were then assessed. Stress-strain curves revealed that octet trusses exhibit higher compressive yield strengths than predicted by the Gibson-Ashby model due to brittle fracture quickly after entering the non-linear deformation regime. DIC analysis was performed, confirming the strain localization and fracture observed at the nodes and in the middle of the struts. Gyroids exhibit the two first stage of deformation but undergo a brittle fracture before the densification stage. The post-rupture analysis revealed that similar brittle features are observed no matter the type of heat treatment conducted. This indicates that the heat treatment is not the primary factor influencing fracture mechanism. Another suggestion could be powder contamination with oxygen or nitrogen leading to α phase precipitation and solid-solution strengthening.