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

The high strength metastable beta-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 beta 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 omega or alpha precipitates form within the beta 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 beta/omega 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 alpha precipitation, with a more pronounced compositional partitioning, resulted in an exceptional yield strength exceeding 1200 MPa in the as-processed condition.

Automated summary

The Ti-1Al-8V-5Fe alloy (Ti-185) was successfully processed using the directed energy deposition (DED) based laser engineered net shaping (LENS) process to eliminate the beta fleck problem. The alloy exhibited finer scale equiaxed beta grains in the as deposited condition, with a homogeneous distribution of fine scale omega or alpha precipitates within the beta grains contributing to increased tensile yield strength. Early stages of beta/omega compositional partitioning were observed in atom probe tomography results, leading to exceptional yield strength exceeding 1200 MPa in the as-processed condition.