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Although the material microstructure attributes play an important role in the mechanical properties of the additively manufactured (AM) part, their impact is largely ignored in the mechanics modeling of AM processes. A physics-based mechanical threshold stress (MTS) model is proposed to consider the material internal attributes, such as grain size and dislocation to dislocation barrier during simulation of the material flow stress and deformation. This model is based on the dislocation dynamics and thermal activation theory with a combination of three main components of athermal stress, thermal stress, and threshold stress. The MTS flow stress model is embedded into a fully coupled thermo-mechanical incremental plasticity model to predict the residual stress during AM processes from the prediction of temperature field and associated thermal stresses. The experimental measurements of residual stress are conducted on additively manufactured Ti-6Al-4 V samples for the model validation. The proposed model provides insight into the impact of microstructure on residual stress prediction in the additive manufacturing process modeling. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]