Analysis of Mechanical Properties and Fatigue Resistance in Laser Welded WAAM Ultra-High-Strength Steel

The examination of WAAM UHS steel laser welds revealed effective material penetration, with desirable geometry showcased by a nearly I-shaped structure. Minor deficiencies were observed at the weld face, while excessive penetration was evident at the weld's root. Cross-sectional analysis indicated no discernible porosity or defects within the weld. Microstructural analysis highlighted fine-grained structures with dispersed precipitates in the WAAM UHS steel base material. Laser welding induced changes in the grain structure, resulting in finer grains and a mixture of ferrite and martensite in the weld zone. Significant increases in hardness were observed in the weld metal and HAZ near the fusion line, attributed to martensite prevalence induced by rapid cooling rates. The hardness of the base material measured around 294 HV, significantly rising in the weld metal, exceeding 401 HV. Mechanical properties altered post-welding, with yield strength decreasing from 749 MPa to 732 MPa. Laser welded WAAM UHS steel had 4% higher tensile strength compared to base material. However, ductility reduced from 27% to 22.5%. Bending fatigue tests revealed a considerable reduction in fatigue limit for laser-welded samples (80 MPa) compared to the base material (419 MPa), with fractures originating from the fusion line between the HAZ and the base material. Notably, the notch sensitivity of ultra-high-strength steels significantly reduces fatigue resistance.