Loading Orientation Effects on the Strength Anisotropy of Additively-Manufactured Ti-6Al-4V Alloys under Dynamic Compression

The microstructure of additively-manufactured metals depends on the direction of build, and is distinctly different from those of conventional metals. This work examines the effect of strain rate, heat treatment, and loading orientation relative to the build direction of Ti-6Al-4V samples that have been additively manufactured by direct metal laser melting to determine how the microstructure affects overall mechanical properties. The effect of rate dependence on additively manufactured Ti-6Al-4V was investigated by compressing cylinders of the material both quasi-statically in a screw-driven load frame (10⁻⁴ s⁻¹ to 10⁻¹ s⁻¹), and dynamically in a split Hopkinson (Kolsky) pressure bar system (375 s⁻¹ to 6000 s⁻¹). The yield strength of the additively manufactured Ti-6Al-4V was observed to monotonically increase with increasing strain rates and the samples failed along a 45° direction through the thickness regardless of loading orientation. As in the case of traditionally forged metals, annealed additively manufactured Ti-6Al-4V samples exhibited lower yield strengths than their non-annealed counterparts at similar strain rates. For quasi-static loads, a clear dependence of response on loading orientation angle with respect to the material layering direction was seen, with the yield strength being greatest when loading was applied parallel to the build direction – a notable contrast to what is observed in tensile results in which the yield strength is lowest when tension is applied along the build direction. No clear relationship between the yield strength and loading orientation was observed in the dynamic tests, likely because the differences were within the measurement uncertainty of the method.