Assessment of in-situ ultrasonic treatment effects along the build direction in the PBFLB/M process

To achieve the desired mechanical properties from additively manufactured (AM) parts, post-processing is often required to modify the unconventional microstructure in the as-built configuration. However, this can undermine the benefits of AM as a limited-supply-chain and time-critical manufacturing method. To alleviate this situation, in situ grain modification methods, such as scanning strategy and field assist, are investigated as alternatives to post-processing. Among other field assist methods, ultrasonic treatment (UT) offers a low-cost, highly transferable, and effective solution. However, for extended builds, it suffers from limited understanding of ultrasonic wave attenuation and its interaction with the material subgrain structure. In this study, grain structure evolution induced by low-intensity UT across the build direction is investigated using high-magnification imaging and microhardness analysis. It is found that while low-energy UT, below previously reported levels, can cause beneficial modification of the microstructure. The attenuation of wave propagation is transient along the build direction and depends on spatial orientation, such as upstream and downstream locations relative to the shielding gas flow. However, it depends on the wave attenuation, and the effect becomes negligible after 25% of the total build height. It was found that UT reduced the area fraction of the Laves phase by 7% in the bottom region. Despite this limitation, it was found that low-power UT, at 2.1% of the industry-standard (persistently used) value, can be used to achieve favorable grain-structure modification in the PBF-LB/M process.