Additive manufacturing of a new titanium alloy with tunable microstructure and isotropic properties

Abstract

Conventional titanium alloys have a high propensity in developing columnar grains with strong textures during additive manufacturing (AM), which causes pronounced anisotropy in mechanical properties and hampers their practical applications. In this study, a new metastable β titanium alloy with high Fe addition (Ti-3Al-6Fe-6V-2Zr, wt.%) was designed for AM, and the strategies and underlying mechanisms to eliminating the structural heterogeneity including grain morphology, Fe element segregation and phase constituent distribution were elaborately investigated. It was demonstrated that the alloy has an outstanding intrinsic capability of forming equiaxed grains during direct energy deposition (DED) due to the positive effect of Fe on constitutional supercooling. The final microstructure of bulk sample was determined by the directly deposited microstructure and subsequent microstructure coarsening caused by cyclic heating, and homogeneously equiaxed microstructure without texture could be achieved when the heat-affected zones can fully cover the formerly deposited layer. Despite of very high level of Fe addition, both microscale and macroscale Fe segregation were completely suppressed during DED, by taking the advantages of fast solidification rate and tailoring the heating effect between the adjacent layers. Moreover, the problem related to the heterogeneity in α phase distribution along the building direction was solved by mitigating the heat accumulation during DED. On the basis of these understandings, homogeneously equiaxed Ti3662 alloy with isotropic mechanical properties of high strength (~1200MPa) and decent ductility (~10%) was finally fabricated by DED, which stands a good chance for practical application. This study demonstrates that the special metallurgical process during AM largely expands the design space of titanium alloys on the aspects of composition and microstructure, which can be utilized to fabricate titanium alloys with desirable microstructure and excellent properties.