Achieving columnar to equiaxed transition in the as-deposited condition via coaxial wire feeding during laser directed energy deposition of Ti-6Al-4V

Abstract

Titanium alloy components fabricated by high-deposition-rate wire-based directed energy deposition (DED) often exhibit coarse prior β grains with a strong solidification texture, which results from the intrinsic melting and solidification conditions experienced by the deposited material. In this work, the columnar to equiaxed β grain transition of Ti-6Al-4V alloy was achieved via a coaxial wire-feeding laser DED process in the as-deposited condition. The coaxial wire laser deposition process was achieved using a coaxial laser head with a vertically fed wire surrounded by an annular beam. Defect-free Ti-6Al-4V parts can be fabricated under both stable liquid bridge and wire penetration metal transfer modes. The optical microscope and electron backscatter diffraction results showed that the thin-walled part fabricated with stable wire penetration mode exhibited a near-fully equiaxed β-grain structure of 200–300 μm size. A 3D multi-physics thermal-fluid model was developed to compute the melting and solidification conditions of the molten pool, revealing the refinement mechanism for the prior β grains. The calculated solidification parameters at the solid/liquid interface predicted mixed columnar + equiaxed grains for the stable wire penetration mode. The results implied that a certain volume of the mushy zone inside the molten pool for the stable wire penetration mode, attributed to the annular-shaped laser heat source with reduced heat input and the specific relative position of the wire to the laser. The unmelted coaxially fed wire in the mushy zone inside the melt pool can serve as heterogeneous nucleation particles, triggering the columnar-to-equiaxed transition. This work provides a method to refine the prior β grains in the as-deposited condition during the wire-based additive manufacturing of titanium alloy without post-processing or alloy modification.