| Summary: | This PhD thesis investigates the role of boron-modified Ti6Al4V alloys fabricated using Powder Bed Fusion – Laser Beam in achieving in-situ grain refinement and enhanced mechanical properties. Through the incorporation of both amorphous and crystalline boron, this research explores the resulting microstructural evolution, phase transformation, and mechanical performance. Key findings demonstrate that even a small addition of boron (0.1 wt.%) significantly refines the grain structure of Ti6Al4V alloys, transitioning from coarse, columnar grains to a more desirable equiaxed morphology. This refinement is particularly critical for overcoming the anisotropic mechanical properties commonly associated with additively manufactured titanium alloys.
Crystalline boron was found to be more effective than amorphous boron in promoting uniform grain refinement, densification, and improved mechanical properties. During solidification, the edge-to-edge matching mechanism between Boron particles and the β-phase played a crucial role in these improvements, enabling the crystalline boron-modified Ti6Al4V to consistently outperform its amorphous counterpart, especially at higher energy densities and longer laser exposure times.
The study also developed a novel Schmid factor algorithm to evaluate the influence of grain orientation and active slip systems on mechanical behaviour. This analysis revealed that boron-modified alloys, particularly those with crystalline boron, exhibited improved mechanical anisotropy, with more isotropic properties observed under 0° orientation.
Furthermore, the research emphasizes the importance of optimizing PBF-LB/M process parameters, with laser power and exposure time identified as key factors in controlling grain refinement and phase distribution. Crystalline boron-modified alloys demonstrated higher relative density, hardness, and more consistent mechanical properties, especially under higher power settings.
The strengthening mechanisms in boron-modified Ti6Al4V alloys were primarily attributed to grain refinement and structural strengthening, while the load-bearing effect of TiB particles played a minimal role. These findings offer a pathway for improving the mechanical performance of PBF-LB/M produced titanium alloys through controlled microstructural refinement.
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