Microstructure and mechanical properties of ultra-high strength steel fabricated by dual ultrasonic impact treatment-assisted wire arc additive manufacturing

Samples of 14Ni3Cr3Mo2Mn ultra-high strength steel were successfully fabricated using the dual ultrasonic impact treatment-assisted wire arc additive manufacturing (DUIT-WAAM). The influence mechanism of DUIT on the microstructure evolution and mechanical properties of the WAAM components was systematically investigated. Microstructural test results indicate that the introduction of dual ultrasonic impact significantly altered the solidification behavior of the material. The microstructure of the single-bead single-layer specimen transitioned from coarse, strongly textured columnar dendrites to fine, texture-free equiaxed grains. The grain refinement was most pronounced in the weld toe region, reaching 72.0%. Furthermore, in the WAAM thin-wall components, dual ultrasonic impact effectively fragmented the epitaxially growing dendrites, interrupted the growth of columnar grains across the fusion line, and increased the proportion of high-angle grain boundaries and the dislocation density. Mechanical property test results demonstrate that the introduction of dual ultrasonic impact significantly enhanced the mechanical properties of the as-built thin-wall parts: the average microhardness of the impacted specimens increased from 301.8 HV to 322.1 HV compared to the non-impacted specimens; the horizontal ultimate tensile strength increased from 971.5 MPa to 1165.3 MPa, and the vertical strength increased from 943.5 MPa to 1122.0 MPa. Although the vertical elongation decreased slightly due to interlayer melt pool misalignment, the horizontal elongation was maintained at 11.45%. This study achieved grain refinement and texture weakening in WAAM samples through dual ultrasonic impact, providing new insights for addressing the issues of coarse microstructure and anisotropy in the additive manufacturing of ultra-high strength steels.