Microstructural evolution in laser melted boron alloyed Ti-6Al-4V
H. Hizli, Zhiyi Zou, James W. Murray, Adam T. Clare, Marco Simonelli
Abstract
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.