Dynamic α globularization in laser powder bed fusion additively manufactured Ti-6Al-4V

As a dual-phase α+β titanium alloy broadly used in the aerospace and biomedicine sectors, Ti-6Al-4V offers a variety of attractive mechanical properties imparted by its tailorable microstructures typically developed via thermomechanical processing. Such a capability in microstructural control, however, is still beyond the reach of laser powder-bed fusion (LPBF) additive manufacturing (AM) of Ti-6Al-4V. Here we demonstrate for the first time the development of near-equiaxed α microstructure in situ via dynamic globularization in the as-built state of LPBF Ti-6Al-4V. Akin to globularization during thermomechanical processing, the globularization process during LPBF is also governed by a series of mechanisms such as sub-grain boundary formation, boundary splitting, thermal grooving, lamellar termination migration and epitaxial growth. This is largely driven by continuous introduction of excess dislocations at the mismatched α/β interphase boundaries while the α↔β phase transformation takes place repeatedly during LPBF. The findings in this study provide new knowledge towards the development of a holistic microstructural control strategy in additively manufactured Ti-6Al-4V.