Microstructural evolution in laser melted boron alloyed Ti-6Al-4V

Grain refinement in cast titanium alloys has been demonstrated through the addition of boron (B) in hypoeutectic concentration. However, there is still a poor understanding of how effective B is towards microstructural refinement under rapid solidification conditions. This study investigates the impact of boron (B) addition on the microstructure formation in Ti-6Al-4V alloys under rapid solidification conditions, particularly relevant to Laser Powder Bed Fusion (PBF-LB), a leading technique in metal additive manufacturing. By examining a range of Ti-6Al-4V-xB alloys with boron contents varying from 0 wt% to 2.5 wt%, prepared through arc melting and subsequent laser surface melting to mimic PBF-LB conditions, our empirical analysis focuses on identifying the threshold B concentration for effective in-situ grain refinement in these alloys. Results indicate that a boron concentration of 0.2 wt% or higher significantly refines the microstructure of Ti-6Al-4V alloy, leading to the formation of a quasi-continuous TiB network. Crucially, our comprehensive analysis across all samples reveals a consistent microstructural composition, characterized by the presence of martensitic alpha and titanium boride (TiB) phases, with an absence of the beta-titanium (β-Ti) phase. This underlines the significant role of boron in stabilizing these specific phases, thus contributing to our understanding of phase formation dynamics in boron-modified Ti-6Al-4V alloys under rapid solidification conditions.