Deciphering the microstructural development and excellent ductility in electron beam wire-fed additive manufacturing of Ti-6Al-3Nb-2Zr-1Mo alloys based on high deposition rate

Electron beam wire-fed (EBWF) additive manufacturing has higher energy density and greater deposition efficiency than powder bed and is suitable for the large-sized parts, with giving rise to larger melt pool size and thermal accumulation, consequently easily forming greater microstructural heterogeneity. This work investigates marine Ti-6Al-3Nb-2Zr-1Mo (Ti6321) alloys based on the intrinsic deposition characteristics of EBWF to better understand the nature of heterogeneous microstructure in high deposit rate processes. Meanwhile, these microstructures are replicated by the cyclic heat treatment to in situ observe the microstructural evolution. The results indicate that the heterogeneous nucleation and α/β boundaries migration primarily govern the formation of fine colony band and coarsening basket-weave microstructure, respectively. And the repeated α↔β transformation creates an opportunity for α globularization during deposition, involving in activity of high-density dislocations. Besides, the varying capacity of resistance to deformation at the onset of plastic deformation, resistance to crack initiation and propagation at the non-uniform deformation stage contribute to the different tensile responses when loading along Z and X direction. As part of this work, the stress-induced martensite transformation behavior in β phase is discovered for the first time even though the minor β volume fractions in as-deposited Ti6321 alloys, which plays an innegligible contribution to enhancing ductility. Overall, the work provides valuable insights into the microstructural evolution, globalization mechanisms and excellent ductility in the as-deposited Ti alloys fabricated by high deposit rate process.