Evaluating residual stresses and structural changes in L-PBF Scalmalloy® by femtosecond laser peening process

• Fatigue life of L-PBF Scalmalloy® increased by ∼ 1.6 times after femtosecond laser peening (fs-LP). • Fs-LP induced compressive residual stresses up to ∼ 100 µm depth from the surface. • Hardness increased from 2.53 GPa to 2.62 GPa due to an increase in dislocation density after fs-LP. • The interaction mechanism between fs laser pulse and metal surfaces is elucidated. Femtosecond laser peening (fs-LP) has attracted interest as a promising surface enhancement technique due to its ability to induce high pressure shock waves without using a confinement layer. Unlike traditional nanosecond laser peening which introduces compressive residual stresses to depths exceeding 1 mm, fs-LP affects primarily the near-surface region. In this study, we investigate the effects of fs-LP on additively manufactured Scalmalloy® produced via laser powder bed fusion (L-PBF). Residual stress characterizations using X-ray, neutron diffraction and finite element modelling reveal compressive stresses extending up to 100 µm beneath the surface. Cross-sectional analysis suggests the formation of a 2–3 µm nanocrystalline surface layer and evidence of surface oxidation. Hardness increased from 2.53 GPa to 2.62 GPa accompanied by a higher amount of dislocation density. Moreover, fatigue testing demonstrated a ∼1.6 times improvement in fatigue life which is attributed to delayed crack initiation resulting from the combined effects of surface hardening, residual stress and refined grain structure. These findings highlight the unique capabilities of fs-LP process for targeted post-treatment of critical regions in additively manufactured lightweight alloy components, where conventional peening is impractical or overly aggressive.