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The coarse prior-β grain structure in titanium alloys produced by additive manufacturing is associated to mechanical anisotropy and limited fatigue life. Here we report a novel methodology to refine such structure by rapid heat treatment of Ti–6Al–4V produced by laser powder-bed fusion. The refinement was analysed using high-temperature EBSD that showed, for the first time, how high angle boundary β grains nucleated and grew with quasi-equiaxed morphology by epitaxial recrystallization. These findings show the potential for such heating regime to be applied to control grain size, morphology and distribution in a wider category of structural alloys produced by additive manufacturing. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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-4 V 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-4 V-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-4 V 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-4 V alloys under rapid solidification conditions. • A powder-free methodology to mirror the thermal history encountered in PBF-LB. • Higher boron levels lead to larger melt pool depth due to the exothermic reaction. • Finer grain and the TiB precipitates enhance the mechanical properties. • Lack of β phase in the obtained microstructure, regardless of B concentrations. • A minimum of 0.2 wt% B is required to achieve noticeable grain refinement. [ABSTRACT FROM AUTHOR]