Omega versus alpha precipitation mediated by process parameters in additively manufactured high strength Ti–1Al–8V–5Fe alloy and its impact on mechanical properties

Abstract

The high strength metastable β-Ti alloy, Ti–1Al–8V–5Fe (wt%), also referred to as Ti-185, has been successfully processed using the directed energy deposition (DED) based laser engineered net shaping (LENS) process, obviating the beta fleck problem associated with Fe micro-segregation that has been reported in conventionally processed counterparts. The large solidification range for this alloy resulted in finer scale equiaxed β grains in the as deposited condition for a range of process parameters, unlike the large columnar grains observed in case of AM of other titanium alloys such as Ti–6Al–4V. Furthermore, based on the process parameters, a homogeneous distribution of fine scale ω or α precipitates form within the β grains, which has been rationalized based on quantitative thermo-kinetic modelling of a multi-layered deposition process. Atom probe tomography results indicate early stages of β/ω compositional partitioning, leading to a higher tensile yield strength, close to 1000 MPa, as compared to the solution treated/quenched condition of conventionally processed Ti-185. Homogeneous fine scale α precipitation, with a more pronounced compositional partitioning, resulted in an exceptional yield strength exceeding 1200 MPa in the as-processed condition.