Single-crystal β-Ti–42Nb fabricated via flat-top laser powder bed fusion: melt-pool stability–controlled cellular-dendritic growth

Single-crystal formation in a β-Ti–42wt.%Nb alloy fabricated via flat-top laser powder bed fusion was governed primarily by melt-pool stability, which was considerably influenced by laser scanning speed, without using single-crystal seeds. Under stable melt-pool conditions at low scanning speeds, multilayer single-crystal structures with 〈001〉//build direction (BD) and 〈100〉//scan direction/hatch direction (SD/HD) were obtained using a bidirectional scanning strategy with alternating HDs of 0° and 90°, accompanied by the suppression of both high- and low-angle grain boundaries. Within a single melt pool in the single-crystal region, [001]-oriented cellular-dendritic structures grew epitaxially from the underlying layer in the bottom region. Although 〈010〉- and 〈100〉-oriented structures formed near the melt-pool surface, repeated remelting during multilayer deposition removed these layers. As a result, a lower solidified region dominated by BD-oriented cellular-dendritic structures with short secondary arms extending along the HD remained. Due to the weak lateral thermal gradient along the HD, crystallographic alignment along the HD gradually converged with increasing build height, enabling single-crystal formation. In contrast, at high scanning speeds, unstable melt pools with asymmetric and laterally wandering morphologies disrupted epitaxial cellular-dendritic growth primarily along the HD. This resulted in weak 〈010〉 texture intensities along the SD/HD, thereby inhibiting single-crystal formation.