Quantitative relationship between anisotropic strain to failure and grain morphology in additively manufactured Ti-6Al-4V

The aim of this study was to identify processing-microstructure-mechanical property links in additively manufactured Ti-6Al-4V. First, the microstructure and mechanical properties of Ti-6Al-4V produced via two laser powder bed fusion (LPBF) additive manufacturing (AM) methods, one using a pulsed laser (P-LPBF) and the other a continuous-wave laser (CW-LPBF), were investigated and compared. Second, existing data from the literature were integrated with the present data in order to identify a general quantitative relationship between anisotropic ductility and grain morphology in additively manufactured Ti-6Al-4V. This revealed that an exponential relationship exists between the anisotropic grain morphology and anisotropic elongation to failure in Ti-6Al-4V. In particular, this relationship shows that a prior-β grain aspect ratio (grain height to grain width) exceeding 6 results in significant anisotropy in elongation. Namely, the columnar grains dominate the fracture mechanics by furnishing significant damage accumulation paths for tension in the longitudinal direction, resulting in higher ductility in the build direction than that in the longitudinal direction. With respect to processing, it was shown that as-built CW-LPBF samples had nearly equiaxed grains while those made by P-LPBF had elongated columnar grains. This resulted in greater yield strength, ultimate tensile strength, and ductility in the CW-LPBF samples compared to P-LPBF samples.