Ultrasonic nanocrystal surface modification for post-processing of additively manufactured AISI 440C stainless steel

• UNSM induced surface hardening, compressive residual stress, and reduced roughness • Strain induced martensite caused by UNSM characterized by EBSD and XRD analysis • UNSM led to the formation of nanograin structure and precipitation of cementite Laser-based powder bed fusion (L-PBF) method has been shown as a promising method in additive manufacturing (AM) of the AISI 440C stainless tool steel. Nevertheless, the as-printed material typically exhibits a soft austenitic structure, poor surface finish, and tensile residual stresses, all of which limit its mechanical performance. In this study, ultrasonic nanocrystal surface modification (UNSM) was applied as a post-processing treatment to improve the mechanical and surface properties of as-built AISI 440C L-PBF samples. The UNSM treatment resulted in a 26% increase in surface hardness, a 50% reduction in surface roughness, and a transition of tensile residual stress (57 MPa) into compressive residual stress (−364 MPa). Microstructural analysis revealed that nano-grain formation, increased dislocation density, strain-induced martensitic transformation, and carbide precipitation were the key mechanisms responsible for surface hardening. These findings demonstrate the potential of UNSM as an effective post-processing method for enhancing the performance of L-PBF-manufactured AISI 440C components.