The Influence of Post-Heat Treatment on Microstructure and Mechanical Properties of a Metastable Beta Titanium Alloy Processed by Additive Friction Stir Deposition

Abstract

A near-β titanium alloy with remarkably refined and equiaxed grain structure was produced by a solid-state additive friction stir deposition (AFSD) technique using commercial rod feedstocks of Ti-15Mo-3Nb-3Al-0.2Si (TB8) alloy. The microstructure evolution during post-AFSD heat treatments and associated impacts on mechanical properties were examined through direct aging (DA) and solution treatment followed by aging (STA), respectively. Microstructure analysis revealed that significant refinement of β-grains from 117.5 ± 2.3 µm to 29.1 ± 0.2 µm was achieved by AFSD mainly through continuous dynamic recrystallization (CDRX). Compared with the conventional STA treatment, the precipitation dynamics of α phase during direct aging (DA) at 520 °C was promoted by the stored deformation energy and sub-structures in the as-built (AB) samples and led to higher strength. As a result, the AFSD-processed TB8 near-β titanium alloy exhibited a high yield tensile strength of 1199.5 ± 26.5 MPa, an ultimate tensile strength (UTS) of 1310.4 ± 8.5 MPa, and a total elongation of 9.2 ± 1.7% after 2 h aging at 520 °C, which are comparable to the properties of wrought alloy. The mechanical enhancement was mainly attributed to α precipitation during aging, which also reduced tensile ductility. Compared with the AB sample predominantly deformed by slip and slip bands, the α precipitation in the aged alloys largely suppressed the formation of slip bands whereas strain localization, crack initiation and propagation at β grain boundaries concurrently contributed to reduced ductility. This work clarifies the grain refinement and strengthening mechanisms of the metastable β titanium alloy under AFSD and post-heat treated conditions and unveils the potential of direct aging for optimizing their mechanical performance.

Keywords: Additive friction stir deposition (AFSD), Near-beta titanium alloy, heat treatment, Mechanical properties, Grain refinement