Microstructure and mechanical properties of Ti-6Al-4V alloy processed by additive friction stir deposition over a wide temperature range

The present study examines the effect of temperature on the deposition of Ti-6Al-4V by additive friction stir deposition (AFSD). Microstructure evolution and mechanical properties are characterized for deposition temperatures of 700°C, 800°C, 900°C, and 1000°C. Processing below the β-transus temperature was expected to yield fine equiaxed α grains due to dynamic recrystallization within the (α + β) phase region. However, this study revealed that even processing in the (α + β) phase region at 800°C and 900°C, the as-deposited microstructure exhibited a fully transformed β microstructure with (α + β) lamellae nucleating from the prior β grain boundaries and closely resembled the microstructure observed at 1000°C. These striking microstructural similarities suggested a complete dynamic transformation from α to β, driven by the high strain and strain rate inherent to the AFSD processing. In contrast, at 700°C, the deposit displayed a bimodal structure comprising equiaxed α and (α + β) lamellae. The mechanical properties of the 800°C, 900°C, and 1000°C processed materials were similar, with minor variations associated with the differences in α lath size and prior β grain size. Finally, the tensile properties of as-deposited Ti-6Al-4V were compared with the forging standard requirements outlined in ASTM and other fusion-based additive manufacturing processes.