IN-SITU ALLOYING BASED ADDITIVE MANUFACTURING OF HIGH-PERFORMANCE BETA Ti-Mo ALLOYS

β Titanium (Ti) alloys have shown outstanding commercial importance, mainly due to their high specific strength, low modulus, and excellent fatigue and corrosion resistance. Considering the difficulties and high cost of manufacturing β Ti alloys, this work applied the novel cost-effective method, in-situ alloying based laser powder bed fusion (LPBF) processing, to fabricate β Ti-Mo alloys, and explored the processing-microstructure-property relations. For the densification and homogenisation of β Ti-Mo alloys, LPBF parametrical studies were firstly performed to β Ti-12Mo alloy from low-cost elemental powders. When the high laser area energy density (AED) was applied, the refractory Mo powder melted and dissolved into the Ti matrix due to the high temperature of the melt pool and increased cycles of remelting. Additionally, keyhole-induced defects emerged under such high AED, which could be eliminated by optimising laser beam delay (LBD) and island spacing (IS) settings. After that, the metastable β Ti-12Mo and Ti-12Mo-6Zr-2Fe (TMZF) alloys with highly dense and largely chemical homogeneous structures were successfully fabricated by in-situ alloying based LPBF. Furthermore, it is noticed that applying varied AED during LPBF can significantly control the microstructure and mechanical properties of Ti-Mo alloys. In view of this, a Ti-Mo functional gradient composite (FGC), with great compressive behaviour, was fabricated via alternating AEDs layer-wise, which paved a new opportunity to design and engineer highperformance biomimetic FGCs components. Moreover, detailed investigations have been performed to understand the influence of scanning strategies (simple and chessboard scan) and post heat treatment on the microstructureproperty relations of β TMZF alloys processed by in-situ alloying based LPBF. The specimens processed via simple scanning strategy showed considerably high strength but were brittle. While the TMZF alloys, manufactured using chessboard scanning strategy, possessed an excellent combination of high yield strength of 1026 MPa, low modulus of 85.7 GPa, and good ductility of 12.7 %. After solution heat treatment, the specimens exhibited a lower modulus of 70.9 GPa, while maintaining high yield strength and ductility. The study revealed that different scanning strategies and post heat treatment could regulate the grain structures, texture, and secondary phases of β Ti alloys, which offered a viable strategy for optimising their mechanical properties. The metastable β Ti-12Mo alloys typically show low yield strength (< 500 MPa) but good ductility, resulting from twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP). The metastable β Ti-12Mo alloy, fabricated by LPBF, showed a significant improved tensile behaviour of high yield strength (725 MPa), excellent strain hardening rates (SHR) and good ductility. The strengthening and deformation mechanisms were understood through the combined usage of in-situ and ex-situ characterisations. The high yield strength mainly resulted from the specific isothermal ω nanoparticles, strengthening the alloy through the dislocation shearing mechanism. The high SHR and good ductility could be attributed to the hierarchical TWIP/TRIP effects, which were operative during the plastic deformation. At the very early stage of plastic deformation (strain < 1.9 %), {332}<113> twinning was the dominant deformation mechanism, while the deformation-induced α" phase was not yet activated. At the later stage of deformation, various primary and secondary twins, and martensite α" phases formed hierarchical structures, contributing to the dynamic Hall-Petch effect. In this work, the in-situ alloying based LPBF technology is demonstrated to be a promising manufacturing route for the fabrication of high-performance β Ti-12Mo alloy, TMZF alloy and Ti-Mo FGCs, that are hard to process via conventional ways. The novel method can be the desirable candidate for some applications in the biomedical and aerospace industries.