Friction Stir Processing of Beta C and Ti-185: A Unique Pathway to Engineer Microstructures for Exceptional Properties in β Titanium Alloys

Metastable β titanium alloys offer a wide range of attractive property combinations. Conventional processing options have practical limitations in achieving maximum attainable properties and utilizing them to their fullest potential. In the current study, friction stir processing (FSP) of β alloys is explored as a unique path for microstructural engineering. Two metastable β titanium alloys, Ti-1Al-8V-5Fe (Ti-185) and Ti-3Al-8V-6Cr-4Mo-4Zr (Beta C), subjected to FSP with two different tool rotation rates at a constant traverse speed were analyzed in different microstructural conditions. Fully retained β grain structures with grain sizes in the range 4 to 7 µm in Ti-185 and 10 to 15 µm in Beta C were obtained in the as-FSP condition. Post-FSP duplex aging treatment of the low heat input condition resulted in better tensile properties compared to those of high heat input condition, attributable to high number density of nucleation sites generated during FSP. Transmission electron microscopy observations of high-strength Ti-185 sample revealed fine α platelets of length in the range 50 to 130 nm and an aspect ratio in the range 3 to 10, providing significant strengthening contribution. Approximate nose temperatures and nose time calculations of the continuous cooling transformation of β to α were made for various commercial Ti alloys to assess the potential of achieving ultra-high strength levels via FSP.