Improving microstructure and mechanical properties of thin-wall part fabricated by wire arc additive manufacturing assisted with high-intensity ultrasound

The coarse columnar grains in wire arc additive manufacturing (WAAM) usually lead to performance anisotropy and greatly degrade the mechanical properties. Herein, a novel approach was developed by synchronously introducing a high-intensity ultrasound into the WAAM process. The effect of ultrasound on microstructure and mechanical properties of AM part along the build direction was explored for the first time. Experimental results show that the columnar grains of each layer are replaced by equiaxed grains under the effect of high-intensity ultrasound, and the grain morphology varies along the build height. Due to the acoustic cavitation induced by ultrasound in the molten pool, the grain size is significantly reduced, and the grain refinement strengthening enhances the mechanical properties. The yield strength and ultimate tensile strength (UTS) increase in the vertical direction by 16.4% and 15.8%, respectively. The in-plane anisotropy of UTS declines from 9.39% to 5.12%. The propagation law of ultrasound in large-size AM part is revealed. When the build height is less than 74 mm, the ultrasonic intensity gradually decreases with the height, leading to a reduction in the degree of grain refinement, which is consistent with the experimental observation. The research provides a theoretical basis and technical solution for controlling the grain structure during the WAAM process assisted with high-intensity ultrasound.