Influence of HFMI Treatment on Fatigue Performance of Hybrid-WAAM Stiffeners

Kurzfassung

High-frequency mechanical impact (HFMI) treatment is widely used to improve the fatigue performance of welded structures and is incorporated into modern fatigue design recommendations. However, its effectiveness depends strongly on local weld geometry and treatment quality. This is particularly relevant for hybrid wire arc additively manufactured (WAAM) structures, where sharp weld toe geometries and small notch radii may restrict treatment penetration and cause the formation of crack-like defects due to material overlap during HFMI treatment. This study investigates the fatigue performance of hybrid-WAAM stiffeners representative of shipbuilding applications. Fatigue tests were performed on as-welded specimens and specimens treated by both robot-assisted and manual HFMI in the fatigue-critical transition between the base plate and WAAM stiffener. Despite the formation of HFMI-induced crack-like defects, generally considered detrimental to fatigue performance, significant fatigue strength improvements were observed compared with the as-welded condition. These findings are not fully captured by current HFMI literature and design recommendations. To identify the governing mechanisms, fatigue testing was combined with surface, residual stress, and material characterization. Three-dimensional profilometry quantified HFMI-induced geometric modifications, while X-ray diffraction mapping characterized the near-surface residual stress distribution. Hardness mapping, microstructural analyses, and fractographic investigations were further conducted to assess local material response and correlate crack initiation and failure behavior with local surface conditions. The results provide new insights into the applicability of HFMI treatment for hybrid-WAAM structures with challenging weld geometries.