Fundamental investigation into mass transfer process and microstructural transformation pathways in Ti-6Al-4V via underwater wire-laser directed energy deposition

The in-situ analysis of mass transfer behaviours in a novel underwater wire-laser directed energy deposition (ULDED) technique was conducted using in-situ X-ray high-speed imaging. By creating a stable local dry cavity within the water environment, three distinct mass transfer modes were identified: droplet transfer, liquid bridge transfer and spreading transfer modes. The mass transfer behaviours were effectively managed by maintaining the liquid bridge mode to enhance the formability of the metal deposits. The inherent heat treatment effect in ULDED provides a unique opportunity to tailor microstructures and mechanical properties in situ with establishing a thermal environment conducive to the preferred lamellar α+β microstructure instead of the acicular α′ martensite. With increasing heat input, the diffusion-controlled transformation pathway gave rise to the lamellar α+β instead of the martensite decomposition, underpinned by the microstructure morphological characteristics and the crystallographic orientations of constituent phases.