Abstract
Ultrasound is a potential method to enhance the mechanical properties during directed energy deposition (DED), however, its intrinsic strengthening mechanism has not been completely uncovered. In this study, the ultrasound-induced strengthening mechanism has been explored from perspectives of microstructure evolution (i.e., grain size and phase composition), and it has been compared with that of the heat treatment after building without ultrasound. The results indicate that the columnar grain (γ) underwent obvious transition and refinement by inducing ultrasound, along with the changes in the morphology and size of Laves phase. Yet the second phase (i.e., γ′ + γ″, Laves phase, and δ) of heat-treated samples was different than that of the samples obtained by ultrasound. Moreover, combined with grains changes and the quantitative analysis of Laves phase, the tensile properties and fracture mechanism were analyzed. The experimental results demonstrate that the tensile properties were improved by both ultrasound and heat treatment. Among them, the average ultimate tensile stress of samples treated with ultrasound increased by 10% compared to the as-built samples along the scanning direction. Consequently, the main strengthening mechanism of samples by ultrasound was found to be grain refinement, while a large number of strengthening phases (γ′ + γ″) precipitated to strengthen the matrix in heat-treated samples. Thus, clarifying strengthening mechanism is crucial for understanding the mechanical properties of as-built samples.
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