An acoustic emission study of anisotropy in additively manufactured Ti-6Al-4V

This investigation applies acoustic emission (AE) to study the microstructural features of additively manufactured (AM) Ti-6AL-4V parts fabricated through direct energy deposition (DED). Microstructural features including anisotropy, porosity, and grain size were tailored by processing parameters such as laser power, powder feed rate, and scan speed. Additionally, rolled and annealed Ti-6Al-4V specimens were used as a baseline. AE features were measured for specimens subjected to cyclic loading via three-point bending tests in two orientations. Higher levels of AE counts and energy were observed in the specimens fabricated with higher laser power while lower levels of AE counts and energy were observed in the specimens fabricated with lower laser power. For AM specimens fabricated with low laser power, the AE features were similar in two different testing orientations where microstructures were columnar for both orientations. However, for AM specimens fabricated with high laser power, the AE were different along different testing orientations, with higher indications being along the growth direction. This may be due to the microstructural anisotropy where AM specimens exhibited columnar grains aligned along the growth direction and a mixed columnar-equiaxed grains in a direction perpendicular to the growth direction. The AE features in the baseline rolled and annealed Ti-6Al-4V were considerably higher than those in AM samples. This may be related to fine recrystallized (α + β) microstructure for rolled and annealed Ti-6Al-4V compared to coarse microstructure in AM samples. The AE observations were further studied by AE waveform analysis.