Printability and grain refinement in stainless steel processed by non-contact ultrasound-assisted additive manufacturing

Additive manufacturing of laser directed energy deposition (L-DED) is flexible in fabricating and repairing metal parts. However, the mechanical property of the fabricated part is compromised by the coarse and epitaxial grains. Although L-DED assisted by ultrasound in contact-transmission mode can promote grain refinement, it simultaneously brings side effects such as poor printability and unstable ultrasonic energy in melt pool. This work proposes an ultrasound-assisted L-DED in non-contact mode, using 316L stainless steel as the mode material. Due to the dissipated and moderate energy of non-contact ultrasound in the melt pool, smooth and regular melted tracks can be obtained without fusion defects, showing excellent printability to the sample. Meanwhile, the non-contact ultrasound can effectively refine the grain structure and reduce the texture intensity of the 316L stainless steel, leading to notable improvements in both the magnitude and uniformity of hardness of the fabricated sample. The effects of ultrasonic parameters, such as the incidence angle and energy intensity on the morphology of single tracks, microstructure, and hardness of the alloy were investigated systematically. The L-DED assisted by non-contact ultrasound simultaneously achieved excellent printability and refined microstructure, resolving the trade-off between printability and grain refinement in conventional contact-ultrasound-assisted additive manufacturing.