Titanium alloys have a high strength to weight ratio and are commonly found in aerospace applications. The Ti-6Al-4V alloy is the most frequent alloy utilized from the titanium alloys, which is available in the α/β-processed and β-processed condition. However, fusion welding (such as GTAW) of titanium alloys may be required for joining components, which can reduce mechanical properties of the weldment (toughness and ductility). Friction stir processing of high melting temperature metals, such as titanium alloys, has become of particular interest, due to the metallurgical enhancements produced (fine equiaxed grains). The α+β Ti-6Al-4V alloy has shown to produce a range of microstructures for processing both above and below the β-transus, which correlates to specific properties. The purpose of this study is to characterize the resulting stir zone microstructures, process forces, and thermal profiles for processing both above and below the β-transus for Ti-6Al-4V in three initial conditions: α/β-processed and β-processed base metal, and a GTAW weld overlay. In addition, this work utilizes hot torsion testing as a physical simulation for friction stir processing.
Friction stir processing of Ti-6Al-4V below the β-transus was characterized as a microstructure consisting of equiaxed-α grains 0.5 – 2μm in diameter. Processing below the β-transus for the α/β-processed condition and a GTAW weld overlay produced a center stir zone of full equiaxed-α grains (sub-micron). Conversely, processing below the β-transus for the β-processed condition resulted in a center stir zone microstructure of elongated equiaxed-α grains (1.6 ±0.7μm) and regions of transformed-β (2.2 ± 1μm). The α/β-processed condition is a Grade 5 (β-transus of 990°C), while the β-processed conditions a Grade 23 (β-transus is 979°C). This difference in transformation temperatures explains the difference in resulting center stir zone microstructures. No clear evidence of tungsten-lanthanum tool debris was found for processing either above or below the β-transus in the stir zone, however, slight tool degradation was found for processing below the β-transus.
Friction stir processing above the β-transus was microstructurally evident by a stir zone composed of 10 - 40μm recrystallized β-grains with either a basketweave or lamellar structure and a continuous network of α at the grain boundary. Moreover, processing below the β-transus provided a dimensionless heat input of Q = 37.5, while processing above the β-transus was Q less than 50, suggesting this dimensionless heat input does not correlate well with the resulting microstructures.
Hot torsion testing of α/β-processed Ti-6Al-4V was utilized as a method for physically simulating the stir zone microstructure produced from friction stir processing. This method was found to produce various microstructures at the center gauge section, with several experiments resulting in fracture. For increasing strain rate and decreasing deformation temperature, the grain size was reduced. Hot torsion testing of Ti-6Al-4V produced similar equiaxed-α grain sizes at a deformation temperature of 925°C and strain rate of 2.469 s-1 (250 RPM), as was observed during sub-transus friction stir processing (150 RPM/2 ipm).