Ultrasonic effects with different vibration positions on gas tungsten arc wire additive manufactured aluminum nanocomposite

Ultrasonically assisted gas tungsten arc based wire additive manufacturing (UA-GTAAM) of TiB2 nanoparticlereinforced AA7075 aluminum metal matrix nanocomposite (MMNC) was conducted with the UA probe directly immersed in the molten pool. This work focuses on the UA effects with different probe positions on the UA-melt interactions and the resulting microstructure. In situ high-speed imaging reveals melt pool surface ripples induced by UA. More refined surface ripples, corresponding to a faster melt flow, are observed with the UA probe immersed deeper into the melt. This condition also leads to a lighter etched color under optical microscope, which is related to a more homogeneous microstructure with less eutectic phase at grain boundaries. Electron backscatter diffraction (EBSD) analysis was employed for grain orientation and nanoparticle distributions, where the lower Confidence Index (CI) served as an indicator for intragranular nanoparticle segregation. The EBSD results show a strong (110) texture in the conventional WAAM builds. High-resolution backscattered electron (BSE) imaging reveals nanoparticles tend to segregate into preferred (001) and (111) grains. UA suppresses the epitaxial growth of (110) grains and promotes a more randomly orientated microstructure. Accordingly, more (001) and (111) grains are available to accommodate nanoparticles and promote a more homogeneous dispersion.