Achieving a transition from columnar to mixed equiaxed and short-columnar prior-β grains in LDED TA15

Achieving a columnar-to-equiaxed transition in titanium alloys fabricated by laser directed energy deposition (LDED) remains a challenge without external-field assistance or alloying modification. Here we demonstrate that a transition from columnar to mixed equiaxed and short-columnar in LDED TA15 is achieved through simply tailoring processing parameters. The as-built alloy exhibits a microstructure composed of equiaxed and short columnar prior-β grains instead of conventional coarse columnar grains. Through single-track, multi-track, and multilayer experiments and thermal simulation, we show that this transition is governed by interlayer and especially intertrack thermal accumulation, which reshapes local solidification conditions, restricts sustained epitaxial β growth, and expands the equiaxed growth zone. These results reveal a thermal history mediated mechanism for grain morphology regulation and provide a practical process pathway for microstructure control in LDED TA15.